Auxiliary Boiler
Survey
Boiler survey
through here!
MTPNO 867 Machinery SiO
Høvik, 2006.08.26
1
Table of Content
Preface
Page 004
Introduction
Page 006
Chapter 1: Boilers, Understanding the Basics
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Introduction
Steam Fundamentals
Heat Transfer
Circulation
Feed and Boiler water Treatment
Material applications in marine boilers
Conclusion Page 49
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035
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Chapter 2: Guide to Boiler Failure Modes
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Introduction
Deposit or Scale Formations, Water Side
Long Term Overheating
Short Term Overheating
Caustic Corrosion, Water Side
Low pH Corrosion, during service, Water Side
Low pH Corrosion, during Acid Cleaning, Water Side
Oxygen Corrosion, Water Side
Oil Ash Corrosion, Fire Side
Cold End Corrosion, Fire Side
Corrosion Fatigue Cracking
Stress Corrosion Cracking
Chapter 3: Auxiliary Boiler Survey
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Introduction
Survey Preparation
Survey Safety Measures
Shell Type Boilers
Horizontal Shell Type Boilers
Vertical Shell Type Boilers
Water Tube Boilers
Types of Horizontal Shell Boilers
Types of Vertical Shell Boilers
Types of Vertical Composite Boilers
Types of Two Drum Water Tube Boilers
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Chapter 4: Combustion and Atomizers
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Introduction
Combustion
Atomizers
Spill Type Pressure Jets Atomizers
Spinning Cub Atomizers
Steam assisted Pressure Jets Atomizers
Ignition Burner
Burner Safety Systems
Oil Fired Combustion System Survey
Visual Survey
Function Test
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Chapter 5: Refractories and insulation
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Introduction
Refractories
Survey of Refractory
Insulation
Survey of Insulation
Chapter 6: Boiler Mountings and Fittings
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Introduction
Safety Valves
Boiler Valves
Water Level Gauges
Pressure Gauges
Boiler Plate
Soot Blowers
Chapter 7: Boiler Control and Monitoring
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Introduction
Automated Feed Water Regulation
Automated Combustion Control
Monitoring of Auxiliary Boilers
Testing of Control and Monitoring System
3
Preface
Det Norske Veritas has experienced a number of incidents where different types
of steam boilers have caused fatal accidents and material damages, especially
involving oil-fired auxiliary boilers older than ten years.
Considering the severe consequences that equipment failure may have on crew’s
safety and ship operations, steam boilers may represent a potentially high risk
factor if improperly maintained. These poorly maintained boilers can result in
furnace explosions, as well as rupturing of the pressurized parts.
We have noted that some of our Surveyors need to increase their competence
related to performance of boiler surveys. Based on this and the impression that
competence related to boiler operation and maintenance is decreasing now days
among seafarers, it was decided to develop this course.
We hope the content will be of interest to you as a surveyor. As a reminder of
the possible catastrophic consequences of boiler failure, please see examples
below.
Result of an exhaust gas boiler failure, capacity 1.5 Ton/Hr, steam pressure 5 bars.
Result of an exploded smoke tube boiler of 14 years old, the remains of the boiler are indicated by the red circle.
4
As you see the consequences are huge. That's why it is so important that you as a DNV
surveyor know what you are dealing with, and are able to take the correct decisions based
on your observation.
Good luck with the course!
Kim Rolfsen
Head of Section
Machinery Ships in Operation
Maritime Technology and Production Centre
On behalf MTPNO 867 Machinery SiO I like t o convey our grat it ude t o all t he Surveyors at
Høvik, and at t he st at ions who support ed us in realizing t he course by reviewing t he
cont ent , supplying pict ures, and giving valuable com m ent s.
Frans Paardekooper
Proj ect Manager, Auxiliary Boiler Survey Course
MTPNO 867 Machinery SiO
Høvik, 2006.08.26
5
Introduction
Scope of Course
The purpose of this course is to equip the Surveyor with the basic, minimal
amount of knowledge necessary in order to competently survey a marine boiler.
The content of this course is focussed on oil fired auxiliary boilers for marine use,
since this is the type we most frequently encounter for survey.
Nevertheless the material presented is also applicable for other types of marine
boilers, such as main and exhaust gas boilers.
Boiler History
In 200 B.C. a Greek named Hero designed a simple machine that used steam as
a power source, named aelopile meaning rotary steam engine. It took many
centuries before this invention was put into practical use.
Hero´s aelopile.
Steam generation as an industry began in the 17th century and the development
was sparked off by the rising demand for ore, minerals, and coal. In order to
satisfy this demand mines became deeper and as a result were often flooded
with ground water.
The first commercially successful steam engine, including boiler was patented by
Thomas Savery in 1698, and its purpose was to pump water from the mines.
These early boilers were made of copper and riveted construction, they delivered
steam just above atmospheric pressure.
6
Savery´s engine, 1700.
Developments in material, production, engine technology, and the ever
increasing demand for higher power output and efficiency led to boiler designs
with higher steam production and pressures.
Turning our attention now to marine engineering, it took until 1803 before the
first steam propulsion plant was installed on the paddle wheeled vessel Charlotte
Dundas. This was quickly superseded by the passenger vessel Chermont in
1807, and in 1811 by the famous Comet.
The paddle wheel vessel Charlotte Dundas.
7
As a result of many boiler explosions with fatal consequences the first safety
regulations were issued in 1817. Boilers had to be made of wrought iron or
copper (no cast iron), and were subjected to pressure testing and inspections.
Successful introduction of the screw propeller in 1837 gave a great impetus to
steam propulsion. The vertical compound engine appeared in 1854, and this
engine required higher steam pressures. Improved boiler designs permitted
working pressures of 1.7 bars, and with the introduction of the triple expansion
engine in 1871 this was raised to 4 bars.
The Scotch boiler (Tank type boiler).
By the end of the nineteenth century (1880) it was realised that a new type of
boiler had to be used due to:
1. Introduction of the steam turbine which required higher steam production
and pressures
2. The maximum working pressure for a tank (Scotch) type boiler was at
that time considered to be 11 bar, this in view of plate thickness and
associated weight.
3. To be able to rise steam pressure more quickly, important for warships.
4. Limit the consequences of pressure part rupture. In the year 1880 it was
reported that there were 170 boiler explosions in the US, with a loss of
259 lives, and 555 people injured.
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Sectional header main boiler (water tube boiler)
The way forward was considered to be the water tube boiler design. The earliest
patent is from William Blakey in 1766, but the first successfully used types are
from James Rumsey in 1788. The first designs suffered from circulation
deficiencies, inadequate water treatment, and poor tube arrangement. It took
until 1889 before the water tube boiler was first tried on the yacht Reverie, and
its success caused a rapid development of this concept for naval and merchant
vessels. The drum type water tube boiler came into practical being in the
1890`s, this was made possible by the availability of rolled steel plates making
economical drum production feasible.
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Two drum, D Type main boiler.
Further developments have lead to the two and three drum water tube boilers so
that ever rising steam temperature could be controlled in an economical way.
Modern use of Steam
The ability of mankind to generate steam in a safe and dependable manner is
one of those few technologies that initiated a series of events. It started the
industrial revolution in the late 17th century and is still shaping today’s world.
Most of the electricity we consume today is produced by steam, it’s also used in
numerous production processes. At present we operate land based water tube
boilers for electricity production of 1300 MW, with a steam production of 1232
kg/s, 265 bars, and 543˚C.
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A steam boiler of a modern power plant, make VGB Power Tech.
Focusing on the maritime industry we can divide the boilers as follows:
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•
Main boilers, supplying steam for the propulsion and auxiliary turbines.
These are two drum water tube boilers, an example is the B&W Radiation
boiler type MRR, steam production 24.5 kg/s, 104 bar, and 513˚C. Today
we find steam propulsion mostly on LNG tankers and some older VLCC’s.
Auxiliary boilers supplies steam for heating of fuel and cargo. On oil
tankers the generated steam is used to drive cargo pump turbines.
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•
Exhaust gas boilers or economisers are installed on almost all vessels with
oil fired auxiliary boilers and increase the plants overall efficiency by
utilising the waste heat in the main engine exhaust. The produced steam
is generally used for heating purposes.
We can also divide the boilers according to their construction, shell, horizontal,
vertical type, and water tube type, this will be addressed later in the course.
B&W Radiation main boiler type MRR, steam production 24.5 kg/s, 104 bars, and 513˚C.
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Chapter 1: Boilers, Understanding the Basics
Introduction
Many people unfamiliar with boilers have the impression that they are basically
just large water kettles, however a boiler is complex and there are many
comprehensive books written on the subject.
The scope of this chapter is to summarize much of that vast amount of
information about boilers into the important, basic facts relevant to Surveyors.
Steam Fundamentals
Steam maintained its dominant position as a working fluid in thermodynamic
cycles because of its unparalleled combination of high thermal capacity, high
critical temperature, wide availability, and nontoxic nature.
Key properties of a working fluid are:
• Pressure and temperature.
• Enthalpy, which can be described as the internal stored energy per unit of
mass.
• Entropy, which can be explained as a measure of the thermodynamic
potential of a system in units of energy, per units of mass.
• Specific volume.
In a steam process or cycle we may find steam in the following conditions:
• Sat urat ed st eam or sometimes called dry steam. In this condition there is
a unique relationship between pressure and temperature as tabulated in
the steam tables. When one property (temp. or Pressure) is known one
can find the corresponding enthalpy, temp. / pressure in the steam tables.
Most auxiliary boilers generate saturated steam which is utilised for
heating of cargo, fuel, accommodation, and other utilities. Chosen steam
pressures are usually between 6 to 18 bars, this represents the most
optimal combination of the steam thermal capacity (enthalpy) and
necessary material thickness of boiler and system.
• Superheat ed st eam (sometimes named live steam) is created by heating
saturated steam of a given pressure, above the saturation temperature.
Superheated steam is found on vessels equipped with steam turbines. The
advantage of this is an increased thermal efficiency of the installation,
higher thermal capacity of the steam, and steam expansion can be
continued longer in the turbine. This due to the delayed formation of
water droplets in the steam, which starts at the saturation temperature.
• Wet or finished st eam is a mixture of steam and water. It is found in the
last stages of the turbine before the condenser.
Taking a closer look at figure 1, temperature / enthalpy diagram in which the
steam generation process is set out. The following areas are distinguished.
• Line A to B: Water is in the liquid phase, left of the line is the liquid region.
• Line B to C: Liquid and vapour phases coexist (wet steam), water is
evaporating at constant temperature. In point C we have 100% vapour,
saturated steam.
• Line C to D: Steam is superheated in superheat region.
• Line A, B, C, and D is a line of constant pressure.
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•
•
The total enthalpy can be split in a liquid enthalpy (A-B), and enthalpy for
evaporation (A-C). In the lower pressure regions of the diagram the
evaporation enthalpy is a bigger part of the total enthalpy as in the higher
pressure regions. This evaporation energy is released during condensation
of the steam in heating coils, this is why high pressure steam (40-100
bars) is not used for heating purposes.
The saturated liquid and vapour line meet at the indicated critical point,
221 bars and 374 ˚C. At this point water no longer exhibits boiling
behaviour, it changes instantly to steam. The difference in density
between liquid and vapour phase is zero, one cubic meter of water weighs
the same as one cubic meter of steam. Therefore natural circulation is not
possible, this is expanded upon in chapter on circulation. An important
point to remember is, the closer the boiler is operated near the critical
point the more problematic it becomes to achieve a good natural
circulation.
Fig. 1 Temperature / Enthalpy diagram of steam generation process.
Although the process of boiling water is a familiar phenomenon, in general terms
it may be described as a heat transfer process where heat addition to a liquid no
longer raises its temperature, but heat is absorbed as the liquid becomes a gas.
If the boiling process in a simple water cooker is examined, the following stages
can be differentiated (see Fig. 2).
1. I ncipient boiling: The temperature of the water adjacent to the heated
surface slightly exceeds the local saturation temperature of the water
while the bulk of the water remains sub cooled. Very small bubbles are
formed adjacent to the heated surface, which periodically collapse as they
come in to contact with the cooler water.
2. Nucleat e boiling: As head transfer rate increases the temperature of the
water reaches saturation temperature and the bubbles are no longer
confined to the heated surface, they move into the fluid. Steam generation
has started.
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3. Film boiling: Further increase of the heat flux causes larger surface
evaporation rates which eventually restrict the liquid flow to the surface.
It will cover the heated surface with an insulating layer of steam and the
ability of the surface to transfer heat drops.
Fig. 2 Transition from heating to boiling (ebullition) as wall temperature increases.
In designing boilers care must be exercised to control film boiling. In high heat
input locations such as furnaces it is important to maintain nucleate boiling in
order to adequately cool the surface and prevent material failure.
Film boiling may occur in existing boilers caused by a disturbance of the
circulation, resulting in insufficient cooling. Or an increase of the head input due
to flame impingement.
Figure No. 3 below illustrates a boiling curve of a heated wire in a pool, although
the characteristics are similar for most situations. The heat transfer rate per unit
area (heat flux) is plotted versus the temperature difference between metal
surface and bulk fluid.
Incipient boiling is called subcooled nucleate boiling in this illustration, the
following points are noted.
• At point C film boiling has commenced. This transition is referred to as
the “critical heat flux” CHF, “departure from nucleate boiling” DNB, burn
out, dry out, or boiling crisis.
• Line C-D: This is the onset of film boiling, more of the heated surface is
blanketed with steam.
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•
•
Line D-D´: In fossil fuel boiler furnaces the head input is effectively
independent of the surface temperature. Therefore a reduction in the
heat transfer rate results in a corresponding increase in surface
temperature from point D to D`. In some cases the elevated metal
temperature is so high that the metal surface may melt.
Line D-E: If the heat transfer rate is dependent upon the surface
temperature, typically for a nuclear steam generator, the average local
temperature of the surface increases as the local heat transfer rate
declines. This region (D-E) is referred to as unstable film boiling or
transition boiling. Because a large surface temperature increase does not
occur, the main consequences are a decline in heat transfer performance.
Line E-D´-F: The surface is effectively blanketed by an insulating layer of
steam or vapour. The energy is transferred from the solid surface
through this layer by radiation, conduction, and micro convection to the
vapour interface. From this interface, evaporation occurs and bubbles
depart. This heat transfer region is called stable film boiling.
Fig. 3 Boiling curve-heat flux versus applied temperature difference.
From the above it is clear that an oil fired boiler must not be operated beyond
point C, once point D is reached there may be the possibility to jump to D´ while
the head flux remains unchanged. This will result in a substantial rise of the
surface temperature which will most likely lead to damages.
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Heat Transfer
I nt r oduct ion
Heat transfer deals with the transmission of thermal energy and plays a central
role in boilers. The following are the three basic modes of heat transfer.
1. Conduct ion: Transfer of thermal energy due to a temperature difference
between adjacent molecules in a solid, for example a steel plate or tube.
2. Convect ion: Transfer of thermal energy within a liquid or gas by a
combination of molecular conduction and macroscopic fluid motion. In
boilers it occurs adjacent to heated surfaces as a result of fluid motion
(water or gas) passing that surface.
3. Radiat ion: Transfer of thermal energy between bodies by electromagnetic
waves. This transfer requires no intervening medium as with conduction
and convection.
One or more of these modes may takes place simultaneously in a boiler, at one
location, and controls the amount of heat transferred.
Conduct ion
The laws of physics concerning heat flow by conduction are.
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•
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Heat flows in the direction of decreasing temperature.
The flow of heat per unit of time is proportional to change of temperature
in the direction of the heat flow, and the dimensions of exposed area.
The heat transferred per unit of time is inversely proportional to the wall,
material thickness.
Heat flow also depends on a material property named thermal conductivity
which differs for various materials.
Con ve ct ion
In boilers convection occurs during heat transfer between flue gas/tube on the
gas side, and tube/feed water on the steam side. Convection has two forms.
1. Nat ural convect ion: Fluid motion is due to local density differences alone,
heated lighter fluid rises and is replaced by cooler fluid.
2. Forced convect ion: A mechanical force from a fan or circulation pump
gives motion to the fluid.
Natural convection takes place on the steam / water side and forced convection
on the gas side in marine auxiliary boilers.
As with conduction the heat flows in the direction of descending temperature,
and is proportional to the temperature change and area exposed.
Also here the heat flux depends on a fluid property called “convection heat
transfer coefficient”, which is a function of the thermal and hydrodynamic
properties (pressure, temp., flow) of the liquid or gas and surface geometry.
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Ra dia t ion
The amount of radiant energy emitted by a body is determined by its
temperature and the nature of the surface. In order for a body to absorb radiant
energy its absolute temperature needs to be lower than that of the emitting
body.
Two forms of radiation are encountered in a boiler.
1. Flam e radiat ion is found in the furnace, this is mainly caused by glowing
carbon particles which are created during the combustion.
2. Flue gas radiat ion is encountered outside the furnace in the convection
part of the boiler, and is contributed by the large presence of CO and
H O in the flue gases.
Contamination by soot of the heated surface will result in a lesser amount of
radiant energy being absorbed, leading to higher flue gas temperatures in other
parts of the boiler and the funnel.
Con t a m in a t ion of h e a t e d su r fa ce
In boilers we experience fouling of the heated surface by soot on the flue gas
side, and by scale and deposits on the steam / water side. Occasionally we are
confronted with steam / water side contamination by oil as a consequence of a
leaking fuel tank heating coil.
Contamination of flue gas side
Fouling by soot of the heated surface will reduce the heat transfer rate, and
thereby less steam will be generated. It will however not lead to higher tube
wall temperatures since an insulating soot layer is formed.
In practice it will normally cause an increase of wall temperature since a certain
amount of steam is necessary for the vessel’s operation, therefore this steam
reduction will be compensated by burning more fuel.
Contamination of steam / water side
Fouling of the steam / water side will also decrease the transferred heat flux and
result in less steam production. But more importantly it will lead to substantial
higher tube wall temperatures.
Contamination by oil is especially dangerous, since oil isolates 20 times better
than a layer of scale of the same thickness. This leads to overheating and
reduced material strength. An oil deposit of only 0.5 mm is calculated to lead to
a 1/3 strength reduction of its original design value.
Also here one should keep in mind that reduced steam generation is commonly
compensated by combusting more fuel, adding an additional temperature
increase.
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Fig. 1 Heat transfer in a furnace wall by conduction.
Table 1 Calculated surface temperatures (T1, T2, T0) for clean and contaminated tubes.
The above is illustrated in figure No. 1, in conclusion the furnace wall
temperature is increased from 256˚C to 633˚C with an oil film of just 0.5 mm.
Furthermore the transferred heat is reduced from 137 kW/m to just 82.3
kW/m , a reduction of 40%.
Consequently the thermal efficiency of a fouled boiler will be reduced, fuel
consumption is multiplied, and the risk for damages significantly increased.
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Aalborg AQ 3 boiler, damaged area in pictures below is indicated by red circle.
Furnace tope plate deformed and weld to flue gas uptake pipe fractured, due to
contamination of heated surface.
Result of oil contamination, deformed furnace plate viewed from in side the furnace.
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Circulation
I nt r oduct ion
For a system to generate steam continuously and keep material temperatures
within design limits, water must circulated through the tubes. Two different
approaches are commonly used.
1. Nat ural or t herm al circulat ion, encountered in marine boilers.
2. Forced or pum ped circulat ion, utilised in land based power plant and
exhaust gas boilers.
N a t u r a l Cir cu la t ion
In an unheated downcomer no steam is present (Fig. 1). Heat addition
generates a steam water mixture in the riser. Because the steam water mixture
in the riser is less dense than the water in the downcomer, gravity will cause the
water to flow downward in the downcomer and will cause the steam water
mixture to move upwards into the steam drum.
Fig. 1 Natural circulation loop.
The rate of circulation depends upon the difference in average density between
the unheated water and the steam water mixture.
The total circulation rate potentially depends primarily upon four factors.
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1. Height of t he boiler: Taller boilers result in a larger total pressure
difference between the heated and unheated legs and therefore produce
larger total flow rates
2. Operat ing pressure: Higher operating pressures provide higher density
steam and steam water mixtures. Thus reducing the total weight
difference between the two and reducing the flow rate.
3. Heat input rat e: Higher heat input typically increases the amount of steam
in the heated riser and reduces the average density of the water steam
mixture, increasing total flow rate.
4. Free flow area of t he com ponent s: An increase in cross sectional (free
flow) areas for the water and water steam mixture may increase the
circulation rate.
For ce d Cir cu la t ion
As illustrated below (see Fig. 2), a mechanical pump is added to the flow loop
and the pressure difference created by the pump controls the water flow rate.
Fig. 2 Simple forced circulation loop.
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Unlike natural circulation, forced circulation does not enjoy an inherent flow
compensation effect when heat input changes. Flow rate does not increase
significantly with increasing heat flux.
Two different systems are distinguished.
1. Re- circulat ing syst em : The circulation pump suction is supplied by gravity
from the drum and forces water through the heated riser, a water steam
mixture is generated and discharged in to the steam drum. Steam is
separated from the mixture and the water re-circulates.
2. Once t hrough syst em : This system provides continuous evaporation of
slightly sub cooled water to 100% steam, without steam water separation,
a steam drum is not required.
Forced circulation is mainly used where boilers are designed to operate near or
above the critical pressure of 221.3 bars. The forced re-circulation system is
also utilised in exhaust gas boilers.
Cir cu la t ion in M a r in e Boile r s
Natural circulation is predominately found in today’s marine boilers. The figure
(Fig. 3) below illustrates a circulation loop for a two drum boiler. Blue, yellow,
and red coloured downcomers (arrows) supply the water drum and headers with
relative cool feed water (sub saturation temperature), via the generating and
furnace tubes a steam water mixture is returned to the steam drum.
Fig. 3 Circulation loop, two drum boiler.
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Because of the high constant heat flux with these boilers an uninterrupted
cooling especially of the furnace tubes is essential. Inadequate cooling can result
in rapid overheating, cycling thermal stress failures or material failures from
different tube expansions. Circulation can be locally disturbed by tube blockage
due to deposits or flow interruption.
Also in tank or shell type boilers a natural circulation is generated within the
water content of the boiler. The circulation in these boilers is less critical on
account of a lower constant heat flux and operating pressure.
Also in vertical shell type boilers natural circulation takes place.
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Feed and Boiler Water Treatment
I nt r oduct ion
In boilers, water is converted into steam, which leaves the boiler in a relatively
pure state. Impurities (other than gases) which enter with the feed water are
retained and concentrated in the boiler water.
If left unattended this may result in the following:
•
Form at ion of hard scales, these are formed by certain constituents in
zones of high heat input leading to a retardation of heat flow, and raising
the metal temperature above normal operating temperatures. This can
cause overheating and ultimately failure of pressure parts.
Two examples of poor boiler water treatment, hard scale and sludge deposits.
•
•
Sludge, or solid part icles normally carried in suspension, may settle locally
and restrict the flow of cooling water, or in some cases, may deposit in the
form of insulating layers with an effect similar to that of hard scale.
Oil and grease prevent adequate wetting of the internal surface, and in
areas of high heat input causes overheating. Or the oil / grease may
carbonize and form a tightly adherent insulating coating.
Two examples of oil contaminated boilers, both need chemical cleaning.
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•
•
•
Dissolved gases and acidic conditions result in corrosion which can weaken
the boiler by removal of metal. This usually occurs in localized areas in
the form of cavities and pits.
Cert ain chem icals if present in specific concentrations may produce
intergranular attack on the metal, leading to embrittlement and failure.
High concent rat ions of foam producing solids in the boiler water results in
water carry over and contaminate the steam.
From the above it is evident that the ultimate purpose of feed and boiler water
treatment is to keep the internal surfaces free of scale or sludge, and prevent the
corrosion of these surfaces, thereby maintaining the integrity and performance of
the boiler.
Magnetite layer on internal surface of a steam drum (left) and split water tube (right), sign of good
water treatment.
The permissible amount of contaminants and treatment chemicals entering the
boiler decreases with rising boiler pressures and heat transfer rate, therefore the
required boiler water quality level increases with higher steam pressures. Boiler
water quality has a significant influence on deposition, of which the insulating
effects become less tolerable as pressures rises, because overheating is more
likely.
Fe e d w a t e r
The total feed water flow to a boiler normally comprises of a small quantity of
make up water, to replace water lost from the system due to leakage or by blow
down, together with the condensate recovered from the system.
Make up water
Virtually all ocean going vessels use make up water evaporated from seawater,
thus contaminants / feed water treatment is minimized. Some contaminants
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may be encountered in the distillate due to carry over of water particles with the
vapor, and re-absorption of non-condensable gases.
Additional solids (scale formers) removal is not required, however dissolved
gases (oxygen, CO2) must be removed to prevent corrosion.
Condensate
In a well maintained feed system the amount of make up water required will be
minimal and the bulk of the feed water will be returned condensate. The main
problem arising with the use of condensate is the possible pick up of copper from
copper alloys used for condenser tubes. Corrosion of aluminium-brass or cupronickel may take place, with the result that copper corrosion products will be
returned to concentrate in the boiler. This problem is aggravated by any ingress
of sea water to the system.
The copper oxides deposit on the heated surfaces and cause galvanic corrosion.
Sca le
Scale formation in boilers leads to lower efficiency because of a reduction of heat
transfer rate. Overheating and tube failure may result, and often high cost of
chemical cleaning may be entailed.
Accumulation of scale deposits will reduce heat transfer and boiler efficiency.
The salts of calcium and magnesium are the main source of scale problems. It is
possible to eliminate these contaminants from the make up water before entry
into the system, but for most marine boilers the alternative is to use chemicals to
modify the scale formers so that they are precipitated as a relative non adherent
sludge, which can be blown out of the boiler before any scale is formed.
The common chemicals used to prevent formation of scale are:
• Sodium phosphat e. This is used to precipitate the calcium (lime) salts
from the solution as calcium phosphate sludge.
• Sodium Hydroxide. This is also known as caustic soda and it precipitates
magnesium salts from the solution as magnesium hydroxide sludge.
These chemicals are normally added as a dilute solution, fed to the boiler either
by means of a proportioning pump, or by injection from a pressure pot direct into
the boiler.
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Accumulation of sludge has a comparable effect as scale build up, reduced heat transfer and
overheating.
Cor r osion
The presence of dissolved gases such as oxygen and carbon dioxide in feed and
boiler water will cause corrosion. However, it does not always occur in the form
of general wastage, but often as localized deep pitting which can readily lead to
tube failure.
Oxygen
One of the most common reasons for boiler corrosion remains the action of
dissolved oxygen in make up and feed water. Generally, oxygen pitting will
occur near or above the waterline in the steam drum of an operated boiler, or
very close to the feed water entry point.
An oxygen corrosion pitting damaged a
Sunrod pin tube.
28
The oxygen content in feed and boiler water can be reduced by the following
means.
• Therm al deaerat ion: The solubility of gases such as oxygen and carbon
dioxide in water reduces with increasing water temperatures. Oxygen is
removed in a vented deaerating heater where steam and condensate are
mixed, or by heating the cascade /hotwell to approximately 90 ˚C.
Thermal deaeration will remove up to 75% of the unwanted oxygen, the
remaining oxygen needs to be absorbed chemically.
• Chem ical deaerat ion ( scavenging) : The following chemicals are added to
the boiler water to remove the remaining oxygen.
1. Sodium Sulphite: This will combine with oxygen to form sodium
sulphate, which results in the formation of additional dissolved
salt.
2. Hydrazine: This will react chemically with oxygen to form nitrogen
and water but will not form dissolved solids.
The oxygen content in water decreases with rising temperature, keep cascade or hotwell tank at a
minimum of 85 °C.
Both chemicals are toxic, and hydrazine is considered to be carcinogenic to
humans.
Carbon dioxide
As a result of the chemical reaction between sodium hydroxide (caustic soda)
and magnesium (scale former) carbon dioxide is formed, this will combine with
water to form carbonic acid. This acid can dissolve ferrous metals both in the
boiler and the condensate system. The most common method used to eliminate
carbon dioxide is by adding chemicals to the feed water such as hydrazine, and
volatile amine.
29
Metal passivation
The water/steam side metal surface of a boiler is passivated when the right
conditions are created (pH 9.5 to 11), this will inhibit further corrosion.
Metal passivation is the process by which a base metal surface forms a protective
oxide film. For boilers this means that the loose non protective film of hematite
(Fe2O3) readily formed when an excess of oxygen is present, is reduced to
magnetite (Fe3O4) or black iron oxide. This is a dense, tight protective oxide film
which inhibits corrosion because it is a less reactive iron oxide. Over time, this
thin mono-molecular film formed by passivators becomes self-repairing and its
growth is self-limited because corrosion products necessary for the process are
unavailable, as corrosion is inhibited.
Graph is showing the attack of steel at 310 °C by water of varying degrees of acidity and alkalinity.
Ca r r y Ove r a n d Pr im in g
The term “Carry Over” is the phenomenon of water droplets being carried over
with the steam into the steam system. Priming relates to contamination of the
steam by injection of gross quantities of water.
The effects of the above can be serious, in that water droplets containing
suspended and dissolved solids can evaporate later in the steam system, and
deposit their entrained solids in the superheater section, or perhaps eventually
on the turbine blades.
30
This phenomenon is caused by high concentration of impurities in the boiler
water which causes foaming to occur.
Correct boiler water treatment, and regular blowdown to reduce boiler water
impurities will prevent this from happening.
W a t e r Tr e a t m e n t
Sometimes auxiliary boilers are regarded as “kettles”, and no corrective water
treatment for scale prevention or blowdown for sludge ejection is considered
necessary. This of course is a fallacy, and adoption of a feed / boiler water
treatment procedure will pay dividends in the long run.
Treatment for low pressure boilers
Suitable feed and boiler water treatment for small low pressure boilers (6-30
bars) can be provided by so called combined (multi) chemical treatment
products. This entails one product being added to the boiler water which,
precipitates hardness, providing the water with the necessary alkalinity, and
scavenges dissolved oxygen.
In order to maintain feed and boiler water within the desired quality levels the
following tests may be carried out daily.
•
•
•
•
Phenolpht halein ( P) alkalinit y t est ( 100- 300 ppm CaCo 3 ) : The dosage level
of combined treatment product is based on the P alkalinity value.
Chloride value t est ( 200 ppm m ax) : This is a reference point for
controlling the rate of blowdown, and an indication of seawater
contamination.
Boiler w at er pH t est : Recommended limits are 9.5 to 11 in order to
prevent corrosion attack.
Condensat e pH t est : Recommended limits are 8.3 to 9.0 to control
corrosion after the boiler.
Depending on the water analysis results a certain quantity of treatment product
is supplied to the boiler via a potfeeder, proportioning pump, or directly in to the
hotwell. Chloride values will determine the rate and amount of blowdown
necessary to bring the boiler water within recommended levels.
Treatment for Medium & High pressure Boilers
The use of combined chemical treatment products for these boilers is not
adequate, since higher pressures and temperatures increase the tendency for
scale and corrosion, making it necessary to have the possibility of changing the
chemical conditions and test parameters individually. A coordinated treatment
program including single function chemical dosage and monitoring is essential.
Feed and boiler water testing are carried out more frequently with these boilers.
Normally, this is done twice to four times a day in order to maintain the required
water quality level. Also the extent of testing has increased as can be seen in
below example.
•
•
•
•
Phenolpht halein ( P) alkalinit y t est ( 100- 130 ppm CaCo 3 ) : Alkalinity
control.
Tot al ( M) alkalinit y t est ( below 2 x P alkalinit y) : Alkalinity control.
Phosphat es t est ( 20–40 ppm PO4 ) : precipitates hardness.
Hydrazine t est ( 0.03- 0.15 ppm N2 H 4 ) : Oxygen control.
31
•
•
•
Chloride t est ( < 30 ppm ) : Controlling rate of blowdown.
Boiler wat er pH t est ( 9.5- 11) : Corrosion control.
Condensat e pH t est ( 8.3- 9) : Corrosion control.
Depending on the test results, different chemicals are added to the feed and
boiler water.
Solid concentrations in the boiler water may also be determined by a conductivity
meter, it displays a visual readout of the ability of the water to conduct an
electrical current. The greater the solid concentration the higher the reading will
be. Once a threshold value is reached or exceeded (<2000 microΏ, 300μS/cm)
the boiler has to be blown down to reduce solid content to within acceptable
limits.
Test results, administered chemical dosages, make up water quantity, and blow
down rate are normally recorded in a feed / boiler water treatment log. Most
major suppliers (Nalco, Drew Asland, Unitor) of treatment chemicals provide the
additional service of reviewing these treatment logs and revert back with
comments and advice.
Boile r Cle a n in g
New Boilers, Initial Water Side Cleaning
The initial cleaning of new boilers for service, or that of older equipment after
major alterations or repairs entails the boiling out of the unit with a caustic
solution, to remove grease and other deposits, which may be present in the
steam generating part. During boil out the unit is fired at a low rate to maintain
50 % of the normal operating pressure. The boiling out period is usually from 12
to 36 hours, during which the boiler is blown down periodically through all blow
down connections. If necessary, the boil out may be supplemented by an
inhibited acid cleaning to remove mill scale.
Following the boil out it is general practice to drain the boiler, and as soon a
possible start flushing the boiler with hot fresh water. After this operation the
boiler is cooled down and thoroughly inspected. If the results are satisfactory
the boiler is fired up, if not the cleaning process is repeated.
Boiler in Operation
For satisfactory and efficient operation a boiler must be kept clean on both the
waterside and fireside. With adequate attention to the pre boiler feed system
and by maintaining the boiler water chemistry within prescribed limits, there
should be little need to clean the waterside. The fireside, on the other hand
requires daily attention if the steam temperature and boiler efficiency are to be
maintained at their optimum values.
Wat er Side Cleaning
If conditions are allowed to deteriorate to the point that scale or baked on sludge
are found during waterside inspections, chemical analysis of the deposits will
indicate the cleaning method best suitable for their removal.
Tubes may be cleaned by passing air turbine driven brushes and scale cutters
through each tube, and flushing with high pressure water hose. Scale may be
removed from internal surfaces by pneumatic chipping hammer, mechanical wire
brush, and washing down with high pressure water hose.
32
It may be necessary to de-scale the boiler by acid cleaning if access for
mechanical cleaning is limited, or the unit is heavily contaminated. A specialized
firm should be consulted to accomplish this process, which entails the use of acid
and neutralizing rinsing agents. The acid strength, neutralizer, and the
temperature at which they are used are of vital importance if the cleaning
process is to be kept within safe limits. Excessive acid strength or un-neutralized
acid remaining after cleaning will attack the metal, possibly to the point that
parts need to be renewed.
The acid cleaning operation normally takes 8 to 36 hours, and is concluded with
a thoroughly inspection of the waterside.
Fire Side Cleaning
Soot blowers are used to clean the boiler fire side, and air or steam is used as
the blowing medium. Depending on the boiler type and fuel burned the boiler is
at least soot blown every 24 hours, but usually every 4 or 12 hours.
Most oil fired auxiliary boilers are not equipped with soot blowers, these boilers
need to be periodically washed down with high pressure water hose.
Degreasing
In the event the waterside is found contaminated with oil, it needs be removed
with the help of degreasing chemicals. First the boiler is completely drained and
as much oil mopped out as possible, subsequently it is filled with fresh water and
degreasing chemicals added in the prescribed quantities. The degreasing period
is usually 12 hours after which the boiler is drained and flushed with fresh water.
After satisfactory inspection the boiler is fired up and a small dosage of
degreasing chemicals is added to the boiler water in order to remove the last
traces of oil during operation.
Appendix
Reference is made to the below appendixes regarding:
•
•
Boil out procedure from Asland
Boiler Acid Cleaning procedure from Asland
33
Drew Marine
Boiling Out Newly Constructed and Retubed Marine Boilers
LAC™ liquid alkaline cleaner is a combination of fast-acting detergents, wetting agents
and alkaline cleaners blended in a water-based carrier. LAC liquid alkaline cleaner can
be used in boiling out new or retubed boilers as well as in cleaning contaminated boilers
and associated systems.
Application
Any specific recommendations and procedures given by the boiler manufacturer should
be followed regarding initial boil out of new boilers. The procedure is usually necessary
to remove initial oil and grease from tube storage and expansion if applicable, plus mill
scale formed during construction.
Dosage
Dosage is normally 2-6% of LAC liquid alkaline cleaner by volume.
Procedure
NOTE: READ MATERIAL SAFETY DATA SHEET BEFORE HANDLING CHEMICALS
AND MAKE SURE THAT GOOD VENTILATION IS PROVIDED.
1. Drain and flush out any loose material with a high pressure water hose. Remove as
much oily matter as possible from the boiler by wiping with clean rags.
2. Replace any gauge glasses having mica backing with another type for the boil out or
valve off the water column gauge cocks so that no cleaning solution enters the
gauge. Important: Make sure that the cocks are tagged “closed” and that they are
returned to the normal “open” position after cleaning and flushing cycles.
3. Start filling the boiler with a 2-6% solution by volume of LAC liquid alkaline cleaner
and fresh water at 60-70°C.
4. Close manhole openings using plain material gaskets. Fill the boiler to the normal
steaming level.
5. Open drum vents and drains on superheater outlets if superheaters are fitted.
6. Start firing the boiler. No steam is to be generated. When steam comes out of
vents, indicating a definite pressure, close the vents. Close the superheater inlet
drain. Leave outlet drain or outlet vent slightly open. Take care that this highly
alkaline solution does not flood over into superheaters if fitted as this can cause high
alkaline concentrations on service firing and could be detrimental to superheater
material.
7. Raise boiler pressure at a rate not greater than 7 kg/cm2 (100 psig) per hour to half
of normal operating pressure.
8. Maintain the above condition for 24 hours. During the 24-hour period, make short
blowdowns from drums and headers. Add water as needed to maintain the
necessary level.
34
MAKE SURE THAT ALL OPERATING PERSONNEL UNDERSTANDS AND FOLLOWS SAFETY PROCEDURES CAREFULLY MENTIONED IN THE MATERIAL SAFETY DATA SHEETS.
MODULE 2.1.4
REMOVAL OF SCALE AND CORROSION PRODUCTS FROM OIL
FIRED BOILERS
CHEMICALS:
DESCALE-IT (DESCALANT)
GC (NEUTRALIZATION)
AMERZINE (PASSIVATION)
EQUIPMENT:
1.
2.
3.
SOLUTION STRENGTH:
SOLUTION STRENGTH:
SOLUTION STRENGTH:
20 %
1%
0.1%
CHEMICAL CIRCULATION TANK
CHEMICAL CIRCULATION PUMP
HEATING EQUIPMENT
CLEANING PROCEDURE:
PLEASE NOTE: DEPOSITS SUCH AS OIL OR ORGANIC MATERIALS SHOULD BE REMOVED PRIOR TO
ACID CLEANING. REFER TO MODULE 2.1.1 OR 2.1.2!
• SECURE EQUIPMENT TO BE CLEANED FROM SERVICE, SEGREGATE OR BLANK OFF FROM SYSTEM AS
A WHOLE AND COOL BEFORE DRAINING.
• OPEN ALL ACCESS PORTS, MANHOLE COVERS AND AS MANY HANDHOLE CAPS AS PRACTICAL.
• REMOVE AS MUCH DEBRIS AND DEPOSIT ACCUMULATION AS POSSIBLE BY FLUSHING WITH HIGH
VELOCITY WATER FLOW OR MANUALLY.
• MAKE NECESSARY CONNECTIONS FOR FILLING AND ATMOSPHERIC VENT LINES.
• FILL EQUIPMENT WITH DISTILLATE UNTIL THE TOP ROW IS COVERED WHILE ADDING DESCALE-IT TO
ESTABLISH RECOMMENDED STRENGTH. SECURE REMAINING OPENING IN EQUIPMENT MAKING
CERTAIN THAT VENT IS FULLY OPEN (HYDROGEN GAS DEVELOPS).
• APPLY HEAT FROM AN EXTERNAL SOURCE. TEMPERATURE RANGE TO BE KEPT 50 - 70 °C. DO NOT
EXCEED ACID SOLUTIONS OVER 70 °C.
• OVERALL CLEANING TIME WILL BE IN THE ORDER OF 4 - 12 HOURS, BUT DO NOT EXCEED 24
HOURS CONTACT TIME WITH ACID SOLUTIONS.
• WHEN THE CLEANING IS COMPLETE, COOL AND DRAIN THE EQUIPMENT. THOROUGHLY FLUSH
WITH DISTILLATE.
• DRAIN THE ACID WASTE SOLUTION TO A TANK OR BILGE WHERE NEUTRALIZATION CAN BE
ACCOMPLISHED BEFORE DISCHARGE.
• REMOVE ALL LOOSENED DEPOSITS BY FLUSHING OR MANUAL CLEANING.
• REFILL THE UNIT WITH DISTILLATE TO THE TOP ROW.
• FOR NEUTRALIZATION ADD THE PRECALCULATED AMOUNT OF GC TO THE EQUIPMENT. HEAT TO
50 - 70 °C AND CIRCULATE AS MENTIONED ABOVE FO R 1 - 2 HOURS OR UNTIL THE PH OF THE
SOLUTION IS NEUTRAL OR HIGHER.
• COOL DOWN AND DRAIN THE SOLUTION TO THE SAME TANK OR BILGE HOLDING THE WASTE ACID
FROM THE CLEANING AND FLUSH THE UNIT WITH DISTILLATE.
• SECURE OPENING, REFILL SYSTEM WITH WATER, AND CIRCULATE FOR 30 MINUTES TO 1 HOUR.
DRAIN AND FLUSH SYSTEM AGAIN.
• FOR PASSIVATION REFILL THE BOILER WITH DISTILLATE AND ADD THE PRECALCULATED AMOUNT
OF AMERZINE TO THE BOILER. FIRE THE BOILER IN IDLE CONDITION FOR 12 HOURS. THE AMERZINE
SOLUTION CAN REMAIN IN THE BOILER.
• INITIAL DOSE BOILER WATER TREATMENT CHEMICALS.
• EQUIPMENT IS NOW READY FOR RETURN IN SERVICE.
35
Material applications in marine boilers
I nt r oduct ion
The vast majority of materials used in the construction of boilers are carbon
steels. Carbon steels may be defined by the amount of carbon retained in the
steel or by the steelmaking practice.
These steels are commonly divided into four classes by carbon content.
1.
2.
3.
4.
Low carbon steels:
Medium – low carbon steels:
Medium – high carbon steels:
High carbon steels:
0.15 % carbon maximum
between 0.15 and 0.23 % C.
between 0.23 and 0.44 % C.
more than 0.44 % C
However, from a design viewpoint, high carbon steels are those over 0.35%
because these can not be used as welded pressure parts. Medium to Low carbon
steels see extensive use as pressure parts particularly in the low pressure
application where strength is not a significant design issue.
Carbon steels are also referred to as killed, semi killed, rimmed, and capped
depending on how the carbon – oxygen reaction of the steel refining process was
stopped. During the steel making process, oxygen, introduced to refine the
steel, combines with carbon to form a gas. If the oxygen introduced is not
removed or combined prior to or during casting by the addition of Si, Al, or some
other deoxidizing agent, the gaseous products continue to evolve during
solidification of the metal in the mold. The amount of gas evolved during
solidification determines the type of steel and the amount of carbon left in the
steel. If no gas is evolved and the liquid lies quietly in the mold, it is known as
killed steel. With increasing degrees of gas evolution, the products are known as
semi killed and rimmed steels. Virtually all steels used in boilers today are fully
killed because these steels have the lowest number of volumetric defects, giving
a high quality steel.
Steel can be altered by modifying its microstructure through heat treatment.
Various heat treatments may be used to meet hardness or ductility
requirements, improve machinability, refine grain structure, remove internal
stresses, or obtain high strength levels or impact properties.
The most common heat treatments used during steel fabrication are the
following.
•
•
Annealing, or also called full annealing is done by heating ferritic steel
above the upper critical transformation temperature (A3, 870 - 700 °C),
holding it there long enough to fully transform the steel to austenite and
then cool it at a controlled rate in the furnace below 316 °C. A full anneal
refines the grain structure and provides a relatively soft, ductile material
that is free of internal stresses.
Process annealing, better known as stress relieving and some times called
sub-critical annealing is performed at a temperature just below the lower
critical temperature (A1), usually between 510 – 704 °C. Stress relieving
neither refines grains nor re-dissolves cementite, but it improves the
ductility and decreases residual stresses in work hardened or welded steel.
36
Equilibrium diagram showing phase solubility limits, carbon in iron.
•
•
Norm alizing is a variation of full annealing. Once it has been heated above
the upper critical temperature, normalized steel is cooled in air rather than
in a controlled furnace. Normalizing is sometimes used as a
homogenization process; it assures that any prior fabrication or heat
treatment history of the material is eliminated. Normalizing relieves the
internal stresses caused by previous hot or cold working and, while it
produces sufficient softness and ductility for many purposes, it leaves the
steel harder and with a higher tensile strength than full annealing. To
remove cooling stresses, normalizing is often followed by tempering.
Tem pering is applied after normalizing or quenching of steels. These
preliminary heat treatments impart a degree of hardness to the steel but
also make it brittle. The object of tempering, a secondary treatment, is to
remove some of that brittleness by allowing certain transformations to
proceed in the hardened steel. It involves heating to a predetermined
point below the lower critical temperature (A1) and is followed by any
desired cooling rate. Some hardness is lost by tempering, but toughness
37
•
is increased, and stresses induced by quenching are reduced or
eliminated. Higher tempering temperatures promote softer and tougher
steels.
Hardening or quenching occurs when steels of higher carbon grades are
heated to produce austenite and then cooled rapidly (quenched) in a liquid
such as water or oil. Upon hardening, the austenite transforms into
martensite. Martensite is formed at temperatures below about 204 °C,
depending on the carbon content and the type and amount of alloying
elements in the steel.
Normalizing and tempering are frequently used in the production of boiler
materials. Stress relieving (process annealing) is carried out after cold forming,
rolling and welding of boiler parts.
Pure iron lacks the required mechanical properties to be usable in modern
boilers, therefore alloying elements are deliberately added in a controlled
quantity to modify the material properties to match a particular specification.
The most important elements used in boiler materials with their specific effects
are mentioned below.
•
Carbon is the most important alloying element in steel. In general an
increase in carbon content produces higher ultimate strength and hardness
but lowers the ductility and toughness of steel alloys. In low alloy steels
for high temperature application the carbon content is usually restricted to
a maximum of about 0.15% to assure optimum ductility for welding,
expanding and bending operations. But the carbon content should not be
lower than 0.07% for optimum creep strength.
General effect of carbon on the mechanical properties of hot rolled carbon steel.
38
•
•
•
Molybdenum , when added to steel increases its strength, elastic limit,
resistance to wear, impact qualities and hardenability. Mo contributes to
high temperature strength and is the most effective single additive to
increase creep strength.
Chrom ium raises the yield and ultimate strength, hardness, and toughness
of steel at room temperature. It is virtually irreplaceable in resisting
oxidation at elevated temperatures and also increases high temperature
strength. The optimum chromium content for creep strength in annealed
alloy steels is about 2.25%.
Nickel increases toughness when added to steel and improved resistance
to corrosion. The most important use of nickel as an alloying element in
steel is its combination with chromium. The various combinations of
chromium and nickel in iron produce alloy properties that can not be
obtained with equivalent amounts of a single element. These steels are
resistant to corrosion and oxidation at high temperature. In addition they
offer greatly enhanced creep strength.
Steels of different properties are used in boilers, each selected for one or more
specific purposes. Each steel must have properties for both manufacturing and
satisfactory service life. When evaluating the material properties of steel grades
intended for the construction of boilers, it is not only the ambient temperature
properties such as, tensile, yield strength, hardness, and toughness that needs
to be reviewed. But also the material properties at high temperatures (operation
temp.) need to be considered.
Tensile and yield strength data determined at ambient temperature, can not be
used as a guide to mechanical properties of metals at higher temperatures. Even
though such tests are made at the higher temperature, the data is inadequate
for designing equipment for long term services at these temperatures. This is
true because, at elevated temperatures, continued application of load produces a
very slow continuous deformation, which can be significant and measurable over
a period of time and may eventually lead to fracture, this phenomenon is called
creep. The maximum allowable working stresses for these ferrous materials, to
be used for high temperature application are based partially on long term creep
rupture tests.
Ele va t e d t e m pe r a t u r e m a t e r ia l pr ope r t ie s
At elevated temperatures, the service life of a metal component subjected to
either vibratory or non vibratory loading, is predictably limited. In contrast, at
lower temperatures and in the absence of a corrosive environment, the life of a
component in non vibratory service is unlimited, providing the operational loads
do not exceed the yield strength of the material. Elevated temperature
behaviour begins, approximately at the following temperatures for the below
listed alloys:
•
•
•
•
•
Aluminium alloys
Titanium alloys
Low carbon steels
Austenitic iron based high temperature alloys
Nickel based high temperature alloys
205
315
370
540
650
°C
°C
°C
°C
°C
Tensile and Yield Strength
39
Higher temperatures have a significant effect on the mechanical properties of
metals. Ferrous materials lose much of there tensile / yield strength at these
elevated temperatures. At a temperature of 430 °C plain carbon steel sees a
rapid decline of its yield strength. As an example, at 300 °C the yield strength of
plain carbon steel is approximately 450 MPa, this has decreased to approximately
100 MPa at 600 °C. This temperature increase results in a yield strength
reduction of 77.8 %, it will be obvious that the material will yield, deform, and
rupture if the operational stresses remain constant. The above described occurs
in boilers that have experienced sudden increase in operating temperature, for
instance by starvation of feed water causing deformation of the overheated
parts.
Creep
Stress imposed at elevated temperatures, produces a continuous strain in the
component and results in a phenomenon called creep. Creep, by definition, is a
time dependent strain, occurring under stress. After a period of time, creep will
terminate in fracture by stress rupture, also called creep rupture.
The conditions of temperature, stress, and time under which creep and stress
rupture failures occur, depend on the metal or alloy, its microstructure, and on
the service environment. At lower temperatures, a steel grade with very small
grains (fine grain size) may be stronger than the same steel grade with fewer
large grains (coarse grain size) because the grain boundaries act as barriers to
slip. At elevated temperatures, where thermally activated deformation can
occur, a fine grain structure material may be weaker, because the irregular
structure at the grain boundary promotes local creep. This allows grains to
rotate by grain boundary sliding. Creep deformation and creep strength are
grain size sensitive properties. Thus a larger grain size improves creep strength.
Most creep curves consist of three distinctive stages, see figure below.
Schematic creep curve showing the three stages of creep.
40
1. Prim ary creep, also known as transient creep, represents an initial elastic
strain resulting from the immediate effect of the applied load. This is a
region of increasing plastic strain at a decreasing strain rate.
2. Secondary Creep, also known as steady state creep is usually
characterized by extremely small variations in rate of deformations. This
period is essentially one of constant creep rate and represents the
predictable service life of a component, therefore being of particular
interest to designers.
3. Tert iary creep, refers to the region of increasing rate of extension, that is
followed by fracture. In service tertiary creep may be accelerated by a
reduction of cross section, resulting from cracking or localized necking.
Environmental effects such as oxidation, that reduces cross section, may
also initiate tertiary creep or increase the tertiary creep rate.
Under certain conditions some metals may not exhibit all three stages of plastic
extension. For example, at high stresses or temperatures, the absence of
primary creep is not uncommon. At the other extreme, notably in cast alloys, no
tertiary creep can be observed, and fracture may occur with only minimum
extension.
Creep curves showing no primary creep and no tertiary creep.
A component under creep loading will eventually fracture, if the strain occurring
under creep does not relief the stress. Depending on the alloy, the appearance
of the stress rupture fracture may be microscopically brittle or ductile. A brittle
fracture occurs with little or no elongation or necking, and a ductile fracture is
typically accompanied by discernible elongation and necking.
Stress rupture ductility is an important factor in boiler material selection. As
shown by the schematic creep curves in below figure, a higher rupture ductility
for the same load and temperature conditions means a higher safety margin.
Creep damage occurs at the grain boundaries, by the formation of internal micro
fracture and voids. The process of creep damage is different form fatigue
41
damage, in that the former starts inside the material, whereas fatigue damage
occurs due to the formation and propagation of a surface micro crack.
Schematic creep curves for alloys having low and high stress rupture ductility.
The first micro structural evidence of creep damage will be noted somewhere
along the linear portion of the secondary creep period. The microstructure of a
metal (grain boundaries) including evidence of creep damage can be made
visible by etching a smooth polished surface and examining the specimen under
high magnification in a microscope. Alternatively one can take a replica of the
etched surface and view this under the microscope.
Voids that form on the grain boundaries at the early stages of creep with little deformation visible.
When considering suitability of a metal alloy for use in steam boiler construction, creep
data of the applicable alloy needs to be evaluated to inshore satisfactory service life. To
simplify the practical application of creep data it is customary to establish two values of
42
stress (for a material at a temperature) that will produce two corresponding rates of creep
(elongation): 10% per 10000 h and 100000 h, respectively.
Creep rate curves for 2-1/4Cr-1 Mo steel.
For any specified temperature, several creep rupture tests must be run under different
loads. The creep rate during the period of secondary creep is determined from these
curves and is plotted against the stress. When this data is plotted on logarithmic scale,
the points of each specimen often lie on a line with a slight curvature.
Typical creep rupture curves for 2-1/4Cr-1 Mo steel.
43
The minimum creep rate for any stress level can be obtained from this graph, and the
curve can also be extrapolated to obtain creep rates for stresses beyond those for which
data is obtained.
Because tests are not normally conducted for more than 10000 h, the values for rupture
times longer than this are determined by extrapolation.
Elevated Temperature Fatigue
In service, the steady loads, or strains, to which components are subjected, are
often accompanied by mechanically induced cyclic loads that are responsible for
failure by fatigue.
The effect of temperature on fatigue strength is marked: fatigue strength
decreases with increasing temperature. However the precise relationship
between temperature and fatigue strength varies widely, depending on the alloy
and the temperature to which it is subjected.
At high temperatures, the fatigue strength often depends on the total time the
stress is applied rather than solely on the number of cycles. This behaviour
occurs because of continuous deformation under load at high temperatures.
Under fluctuating stress, the cyclic frequency affects both the fatigue life and the
amount of creep. This is shown in the figure below, at room temperature the
curves converge at the tensile strength plotted along the mean stress axis. At
high temperature, the curves terminate at the stress rupture strength along the
mean stress axis.
Effect of temperature on the fatigue life of S-816 alloy tested under a fluctuating axial load at a
frequency of 216000 cycles per hour.
Combined creep and fatigue loads result in substantially decreased life at
elevated temperatures as compared with that of anticipated simple creep
loading.
44
Thermal Fatigue
Mechanical vibration is not the only source of cyclic loads. Transient thermal
gradients within a component can induce plastic strain, if these gradients are
repeatedly applied, the resulting cyclic strain can induce component failure. This
process is known as thermal fatigue. The two conditions necessary for thermal
fatigue are:
1. Some form of mechanical constraint.
2. A change in temperature.
Thermal fatigue crack in weld of tube plate and boiler shell.
Thermal expansion or contraction caused by a temperature change, acting
against a constraint, causes thermal stress. A constraint can be imposed by for
example a rigid pipe mounting or tube plate. In thick sections, temperature
gradients are likely to occur both along and through the material causing highly
triaxial stresses and reducing material ductility, even though the uniaxial ductility
often increases with increasing temperature.
In the event a component is exposed to creep and thermal fatigue at the same
time, than creep strains are superimposed on thermal strains and thus account
for a further reduction in life expectancy.
General Oxidation
Oxidation (also called metal burning) as well as all other reactions of metals with
gaseous environments, has long been recognized as a severe limitation to the
use of metals at high temperature. The temperature (scaling temperature) at
which carbon steel appreciably oxidizes is approximately 550 °C. It can be
45
recognized as a thick, brittle, dark oxide layer which often contains longitudinal
fissures and cracks. In other areas, patches of oxide may have exfoliated.
Cracks and exfoliated patches result from component expansion and contraction.
Thermally deteriorated metal on a failed wall tube. Note the spalled and cracked oxide resembling
tree bark caused by expansion of the tube during bulging and thermally induced stresses.
Metallurgical Instabilities
Stress, time, temperature, and environment may change the metallurgical
structure during service. They may thus contribute to failure by reducing
strength, although some changes may enhance strength. These structural
changes are also referred to as metallurgical instabilities. The elevated
temperature at which creep occurs also leads to micro structural changes, and
metallurgical instabilities in the metal. Creep damage and micro structural
degradation of the metal occur simultaneously, resulting in a further reduction of
the service life.
a) Section through the failure lip showing a complete spheroidization of the carbide phase in ferrite.
46
(b) Section in the same plane as the failure, but 180° around the circumference of the tube
structure is nearly normal pearlite and ferrite.
(c) Section taken 205 mm (8 in.) away from the failure is also normal ferrite and pearlite.
These three structures indicate the elevated temperature is confined to a small area of the failure.
All etched with nital. 500 X. Material is exposed to a temperature of 650 °C.
Material Hardness
Hardness may be defined as resistance to indentation under static or dynamic
loads, and also as the resistance to scratching, abrasion, cutting or drilling. To
the metallurgist, hardness is important as an indicator of the effect of heat
treatment, fabrication process, or service exposure. Hardness values are also
roughly indicative of the ultimate tensile strength of steels.
Hardness testing is the simplest of the mechanical tests, can easily be preformed
on location, and is often the most versatile tool available in the field. Among its
many applications hardness testing can be used to assist in evaluating the
combined effects of creep and micro structural degradation, in order to
determine whether the material is fit for further service. Comparing the
measured hardness values of the failed / damaged component with that
prescribed by the material specification provides an approximation of the tensile
strength changes, and also indicates extent of softening or hardening caused by
overheating.
M a t e r ia l a pplica t ion
Det Norske Veritas, along with other Classification Societies and National Boiler
Authorities have the responsibility for identifying and approving material
specifications, for those metals deemed suitable for boiler construction and for
development of the allowable design values for these metals as function of their
service temperature.
For Det Norske Veritas these steel grades are mentioned in Part 2, Chapter 2,
Section 2 “Rolled Steel Plates”, and Section 4 “Tubes” (January 2005 publication)
of the DNV Rules. Material supplied for construction or repair of boilers needs to
be delivered with the following documentation:
• Plates with a NV Material Certificates issued by DNV.
• Tubes / pipes and flanges with a Material Works Certificate, which is issued
by the manufacturer.
In both cases the material manufacturer of the plate and tubes needs to be DNV
Approved Manufacturer for the applicable products.
The DNV designation of boiler steel grades is not widely used, it is more common
to be confronted with a steel grade according to an industry standard, as for
example DIN, ISO; BS; ASTM, or JIS. Reference is made to enclosed table with
comparable steel grades.
In general the following materials are used in auxiliary boilers encountered on
board merchant vessels.
• Furnaces are usually made of carbon steel suitable for high temperature
with carbon content of 0.15 to 0.20%. Frequently used steel grades for
these parts are NV 410, H ll (DIN), P 265GH, or (ASTM) A 516GR60. In
regions of high heat input and more fluctuating temperatures such as flue
gas pipes, burner mouth, and furnace crown, low alloyed steels (19Mn6)
are sometimes chosen.
47
•
•
•
The boiler shell and tube plates are also usually made of carbon steel of
medium carbon content. In the 80`s some manufacturers (Sunrod) used
high strength steel grades for the boiler shell but, this is not practiced at
present (Several boiler codes restrict the us of high tensile steels).
Flame and water tubes for auxiliary boilers are usually made of carbon
steel, for example grades St. 37.8I or St. 35.8I. Most employed tubes are
still seamless tubes, although today’s electric resistance welded tubes are
of equal quality.
The highest metal temperatures occur in the superheater and reheater.
Consequently, materials need to have superior high temperature
properties and resistance to oxidation. Mild steel is considered permissible
for superheaters up to steam temperatures of 399 °C, above this
temperature alloy steel grades are used. The alloying elements are
usually molybdenum and chromium.
When for some reason the original material can not be obtained during a boiler
repair, and one is confronted with the situation of choosing a comparable steel
grade. Please exercise the necessary prudence as will be evident by the
following example:
Part of a boiler shell needs to be cropped and renewed. The
original boiler shell is made of steel grade NV 1Cr /2Mo, and since
this material could not be supplied the decision has been made to
use steel grade NV D36 for the boilers shell repair.
Material grade
NV 1Cr /2Mo
Original boiler shell
material
NV D36
Proposed steel
grade for repair
Tensile strength
470 / 620 MPa
490 / 630 MPa
Yield strength
305 MPa
355 MPa
Elongation
20%
21%
Charpy value
20 J at 20 °C
34 J at -20 °C
The novice may conclude from the above data that steel grade NV D36 is even
superior to the original steel grade and perfectly suitable for the intended shell
repair.
As mentioned earlier mechanical properties determined at ambient temperature
can not be used as a guide to mechanical properties at higher temperatures.
Also NV D36 is only suitable for structural application and not for boiler /
pressure vessel application. We may conclude NV D36 is not a comparable steel
grade to NV 1Cr /2Mo and should not be used, its use will lead to early
unexpected failure of the shell plate by creep. Comparable steel grades to NV
1Cr /2Mo are for example 14CrMo45, or 13CrMo44.
In general one may state that apart from the obvious differences in chemical
composition between structural steel and boiler steel grades, boiler steels have
been tested with regard to their elevated temperature behaviour, creep rupture
48
tests. Comparable steel grades should therefore be found in the appropriate
sections of steel grades for boiler pressure vessel application of the different
industrial standards.
Pr in ciple st r e sse s in boile r s
Stress is defined as the internal force between two adjacent elements of a body,
divided by the area over which it is applied. The main significance of a stress is
its magnitude, however, the nature of the applied load and the resulting stress
distribution are also important. The designer must consider whether the loading
is mechanical or thermal, whether it is steady state or transient, and whether
the stress pattern is uniform.
The principle stresses during boiler operation may be divided as follows.
• Pressure stresses (steady state load) which are classified as primary
membrane stresses since they remain as long as the pressure is applied.
• Thermal stresses result, from restricting a member that is attempting to
expand or contract, due to a temperature change.
• Alternating stresses (transient load) resulting from cyclic pressure vessel
operation, this may lead to fatigue cracks at high stress concentration.
When the wall thickness is small compared to other dimensions, the stress
acting over the thickness of the vessels wall and tangential to its surface, can be
represented by mathematical formulas for a common shell form.
The basic equations for the longitudinal stress σ1 and hoop or circumferential
stress σ2 in a cylindrical vessel with a wall thickness tw, diameter D, and subject
to a pressure p are:
σ1 = p.D / 4.tw
σ2 = p.D / 2.tw
From the above formulas, it is evident that the hoop stress has twice the
magnitude of the longitudinal stress. This translates in to the fact that
longitudinal welds are twice as highly loaded as circumferential welds. Therefore
they should receive extra attention during inspections, and be a main candidate
for any NDT examination after welding repairs.
Longitudinal weld seam in a steam drum.
49
Con clu sion
In concluding this chapter on “Boilers, Understanding the Basics” please find
below some considerations, which may be worth keeping in mind during boiler
surveys.
•
Most auxiliary boilers we inspect produce saturated steam, mainly used
for heating of fuel oil and cargo. The steam pressures involved are
relatively low (6 to 13 bars) and therefore some people underestimate the
energy contained in a boiler, and eventual consequences incase of sudden
release of the water steam mixture. The frequently heard fraise “it is only
6 bars steam pressure” should not be taken seriously. Bear in mind that
even if a ruptured boiler shell does not result in an explosion, sudden
release of the water content will flood the engine room space with steam.
This will among other consequences (steam temperature, visibility loss),
lead to oxygen depletion of the engine room space resulting in a life
threatening situation for the engine room crew.
Dramatic failure of an exhaust gas boiler shell
•
•
Contamination of the heat transfer surface by only a thin layer (0.5 mm)
of oil raises the furnace wall temperature to dangerously high values (633
°C). Operating the boiler in this condition for any prolonged time will lead
to overheating, material degradation, and failure of these pressure parts.
When discovered during inspections imminent remedial actions are called
for, to prevent future disasters.
Theoretically the effect of water side contamination will be a raise in
furnace wall temperature, and reduction of heat transfer rate which
translates in less steam being produced. The steam consumption system
still requires the original steam output and therefore the loss of steam
50
•
•
•
•
•
•
•
•
•
•
production is compensated by burning more fuel. Practically therefore,
contaminated heat transfer surfaces will lead to an additional wall
temperature raise and increased fuel consumption.
Feed and boiler water treatment is necessary for every type of boiler. It
will keep the water / steam surfaces free from scale, sludge, and prevents
corrosion. If water treatment is carried out properly, the waterside of a
20 year old boiler looks just as clean as that of a 5 year old boiler. High
pressure boilers are less forgiving with regard to negligent water
treatment, than low pressure boilers. Also, the boiler water quality
requirements become more stringent, with raising pressures and heat
transfer rates.
A general rule as to what extent of scale contamination a boiler can safely
operate with can not be given, due to the complexity of the subject. First
of all, the weight and thickness of the scale layer alone does not always
accurately indicate the tendency to overheating. Scale composition and
morphology also influence heat transfer. Basically, a boiler should be
without scale in order to operate at peak performance. The negative
effects of scale also largely depend on its location, high heat transfer
surfaces such as furnace crown are very susceptible to scale formations.
Scale also prevents a proper visual inspection, since it hides defects, such
as corrosion pits and cracks.
Marine auxiliary boilers are generally made of carbon steel grades. These
materials start to be affected by elevated temperature behavior at 370
°C. Affects such as creep and metallurgical instabilities deteriorate the
micro structure of the material, resulting in reduced strength. The
material will have a predictable time to failure depending on the stress,
time, and temperature of exposure. The areas around the furnace in
marine auxiliary boilers are generally affected by elevated temperature
behaviour, due to scale or oil contamination.
General oxidation of carbon steels occurs when they are exposed to
temperatures of 550 °C or above. It can be recognised by longitudinal
fissured scale on the exposed surface or completely cracked surface, so
called crazy cracking.
Plain carbon steel sees a rapid decline of its yield strength at a
temperature above 430 °C.
Look for thermal fatigue cracks at any point of restraint such as, stay
connections, internal supports, and tube plate connection.
The stress in longitudinal welds is twice as high as that in circumferential
welds of a cylindrical object. Therefore longitudinal welds of the boiler
shell deserve extra attention during inspections.
Very small deformations in boiler plating may be the result of stress redistribution by plastic deformation, setting of the material.
Larger deformations in boiler plating are always the result of overheating.
Most probably this will be long term overheating in which case the heated
surface will be contaminated by scale, mud, soot, or you find evidence of
flame impingement or missing brickwork. If the heated surfaces are
completely clean and burner is operating properly then it is short term
overheating, frequently caused by feed water starvation.
Boiler parts exposed to long term overheating are affected by creep and
possibly other elevated temperature phenomena. Deformation of the
exposed parts clearly indicates that creep is in it’s 3rd stage (tertiary
51
•
•
•
creep) and failure is imminent. It is not known when failure will occur (a
week, a year), or how much the material strength has decreased.
As a general rule deformed boiler parts should be renewed as soon as
possible. A deformed furnace crown or cylindrical plate is not as strong as
when it had its original shape. Stresses in the material at the
deformation have increased, especially at the edges of the deformation.
When dealing with deformed boiler parts, the following questions needs to
be answered in the order listed below.
1. Is the boiler structurally still sound? If deformations are too
extensive, sharp corners or large areas, it needs to be repaired
before boiler operations are resumed.
2. Is the deformed material fit for further use? This question has to
be answered if one considers the boiler structurally fit for further
operation, until repairs are carried out.
The deformed material can be evaluated with help of the following
techniques.
1. Measuring the material hardness, this can be done by a portable
hardness measurer. Take several hardness readings of the
deformed area and also of not affected material. Compare the
values with the material specification. Too low readings indicate
loss of material strength, and too high readings imply brittleness of
the material. Please keep in mind that measurements should be
taken of the base material. Sometimes overheated material has a
hard oxide layer, or the surface has hardened, this hard layer has
to be removed by grinding, before accurate hardness readings can
be obtained.
Portable hardness measurement set from Equotip.
52
2. Make a replica of the materials microstructure and examine this
under the microscope. An area of the deformed plate needs to be
polished and etched to produce a replica, this is not always easy in
a boiler. Creep starts from within the material, if creep damage is
seen on the surface, the material has to be renewed immediately.
A replica of the surface (polished and etched) is made by applying a softened plastic foil to the
surface. This foil moulds itself to the metal surface when pressed. After its removal from the metal,
53
the plastic replica provides an exact copy of the etched surface microstructure, which can then be
examined under our laboratory’s high-quality and very high-resolution microscopes.
•
•
•
•
•
In case the boiler is considered fit for a short time of operation until
repairs can be implemented, it is important to clean the boiler. Removal
of scale, mud and soot will drastically decrease the material operation
temperature of the damaged areas. One should also accurately record
the dimensions of the deformed area, length, breath, and depth. It will
be possible then to monitor the damage, progressing creep will be
indicated by increased dimensions of the deformed area.
Welded connections in way of deformations should be carefully inspected
during damage surveys. Often these weld seams are overstressed by the
deformation and develop cracks. Cleaning of the area and examination
by Magnetic Particle Inspection (MPI) is a wise course of action.
Hydraulic pressure test of damaged boiler only guarantees us that it can
withstand the test load at room temperature. The effect of long term
operation at service temperatures is not taken into account, therefore if
the damage is too extensive a successful hydraulic pressure test is not a
guarantee for safe operation.
When performing a hydraulic pressure test, the test pressure should
remain on the boiler for approximately 60 minutes before final inspection.
It was customary in the past to heat the test water to approximately 80
°C in order to prevent leaking of the expanded tube connections. At
present with all welded boilers this is not applicable any more, water at
room temperature is in order.
Caution should be exercised when accepting pressure down grading of
damaged boilers. Reducing the working pressure with 1 or 2 bars will
reduce the material stresses due to pressure, but the thermal and
alternated stresses are not considered. Also the strength reduction of the
overheated material due to creep and other elevated temperature
phenomena is not known. In general pressure down grading looks good
on paper, but it will not give the desired operational safety.
Appendix
Reference is made to below tables of comparable boiler steel grades.
54
55
56
Chapter 2: Guide to Boiler Failure Modes
Introduction
This chapter is a field guide to the most commonly encountered boiler failures. It
will help the novice and the more experienced observer to identify the failure and
pinpoint its likely locations. The failures discussed are found in boilers of
virtually all pressures and construction.
Deposit or Scale Formations, Water side
Loca t ions
Basically deposits can occur at any location in the boiler where water or steam is
present. Scale formation tends to concentrate in the hottest steam generation
regions, high heat flux areas. Therefore combustion chamber plates, wall and
screen tubes are usually more heavily fouled. Also deposition often occurs
immediately down stream from circumferential-weld backing rings, which
disturbs the flow and are favored sites for steam blanketing.
Tube sections virtually plugged with deposits. The tube on the right is from a low pressure boiler
and is fouled with almost pure calcium carbonate. The centre tube contains silicates, phosphates,
and other components. And the left tube section is rendered almost 20 % copper.
Superheater deposits are caused by carry over of boiler water. Scale formation
will usually be concentrated near superheater inlets or in nearby pendant U –
bends.
Severely corroded
blades from a steam
turbine.
57
Ge n e r a l D e scr ipt ion
Boiler deposits originate from four sources: water borne minerals, treatment
chemicals, corrosion products, and contaminants. Deposits from these sources
may interact to increase deposition rate, which produces a more tenacious layer,
and serves as nucleation site for new deposit formation.
One deposition process involves the concentration of soluble and insoluble
substances in a thin film bordering the metal surface during steam-bubble
formation.
Five instants in the life of a steam bubble.
Material segregates at the steam / water interface, moves along the interface,
and is deposited at the bubble base as the bubble grows. Other deposit
mechanisms involve precipitation from solution and settling of large particulate
matter. Inverse-temperature solubility leads to deposition where heat transfer is
great. The tendency to form deposits is related to localized heat input, water
turbulence, and water composition at or near the tube wall. When the steam
bubble becomes dislodged from a tube wall, the deposits are washed with water
(re-dissolved). The rate at which the deposit builds depends on the rate of
bubble formation and the effective solubility of the deposit. In case of high heat
input a stable steam blanket (film boiling) can be formed and cause
concentration of water soluble material.
Cross section of bulged tubes cause by overheating due to heavy deposits.
58
Steam-blanket deposits do not re-dissolve, because the surface cannot be
washed while blanketed with steam.
Prolonged operation above the maximum deposit loadings may produce serious
corrosion, overheating failures, and increase fuel consumption. However, the
weight and thickness of the deposit layer alone does not always accurately
indicate the tendency to overheat. Deposit composition and morphology also
influence heat transfer.
Elim in a t ion
All deposits are undesirable, and ultimately result from water-chemistry
properties and boiler operation. Proper water treatment can reduce depositions.
The most important boiler operating characteristic influencing deposition is firing
practice. Also elimination of hot spots, correct monitoring of water levels, proper
burner position, and appropriate blowdown practices contribute to reduced
deposition.
Long Term Overheating
Loca t ions
Failures resulting from long term overheating occur in combustion chamber
plates, wall, screen, and superheater tubes. Tubes and plates especially subject
to overheating often contain significant deposits, have reduced coolant flow,
experience excessive fire side heat input, or are near or opposite burners. Tubes
adjacent to restricted or channeled furnace gases suffer from long term
overheating. Other tubes and areas subjected to overheating include sections in
which refractory has been spalled. Slanted tubes, such as nose arches, are
particularly susceptible to long term overheating due to steam channeling.
Tube failure in a nose arch in the furnace (left) and a massive thick walled tube fracture caused by
creep (right).
Fire tubes are rarely affected.
Failures usually occur in relatively broad areas and involve many tubes that are
either ruptured or bulged.
59
Ge n e r a l D e scr ipt ion
Long-term overheating is a condition in which metal temperatures exceed design
limits for days, weeks, months, or longer. This type of overheating is the cause
of more boiler failures than any other mechanism. Because steel loses much
strength at elevated temperatures, rupture caused by normal internal pressure
becomes more likely as temperatures rises.
Yield stress of plain carbon steel as function of temperature. Please note the rapid strength
reduction above 430 °C.
The maximum allowable design temperature is primarily a function of tube /
plate metallurgy. As the amount of alloying element, particularly chromium and
molybdenum, is increased, higher temperatures can be tolerated. Long term
overheating depends on temperature, length of time at temperature, and tube /
plate metallurgy.
A mild steel tube subjected to temperatures above 454˚C for more than a few
days may experience long-term overheating. If temperatures remain elevated
for a prolonged period, overheating will certainly occur.
Furnace and gas temperatures often exceed 1093 ˚C, if the heat transfer rate on
the water side is markedly influenced by deposits, film boiling, and coolant flow
restriction this will result in increased metal temperatures above design
condition.
Thermal oxidation (metal burning)
One sign of long term overheating can be a thick, brittle, dark oxide layer on
both internal and external surfaces. If the metal temperature exceeds a certain
60
value for each alloy, thermal oxidation will become excessive. Often the
thermally formed oxide layer contains fissures and cracks.
Thermally deteriorated metal on a failed wall tube, note the spalled and cracked oxide
resembling tree bark caused by expansion of the tube during bulging.
In other areas, patches of oxide may have exfoliated. Cracks and exfoliated
patches result from tube expansion and contraction caused by deformation
during overheating and / or thermal stressing. Tube wall and plate thinning can
result from cyclic thermal oxidation and spalling.
Creep rupture (stress rupture)
Stress rupture usually produces a thick-lipped rupture at the apex of the bulge
and a deformation. Creep produces slow plastic deformation and eventual
coalescence of micro voids in metal during overheating. Often a small
longitudinal fissure will be present at the apex of a heavily oxidized bulge.
Small, ragged creeps rupture at the apex of a bulge, note thick rupture edges.
61
Elim in a t ion
Long term overheating is a chronic, rather than a transient problem. Therefore it
requires removal of a chronic system defect. Excess deposits should be removed
by chemical or mechanical cleaning and recurring prevented by proper water
treatment. Improper boiler operation, and excessive heat input should be
avoided.
Short Term Overheating
Loca t ions
Failures caused by short term overheating are confined to steam/water cooled
tubes (wall, screen, roof tubes) and combustion chamber plates. When low
water level is the cause, failure will occur near the top of water tube wall
headers, near steam drums. Also roof tube walls and top of furnace plates are
usually found deformed.
Longitudinal tube rupture (left), large fish mouth rupture of rifled nose arch tube (right). Note
the absence of any deposits.
Ge n e r a l D e scr ipt ion
Short term overheating occurs when material temperature rises above design
limits for a brief period. In all instances, metal temperatures are at least 454 ˚C
and often exceed 730 ˚C. Failure is usually caused by a boiler operation upset.
Conditions leading to short term overheating are partial or total tube pluggage,
and insufficient coolant flow due to upset conditions, or excessive fire side heat
flux.
Several factors which often present the failures caused by short term
overheating are uniform tube expansion, absence of significant internal deposits,
absence of large amounts of thermal formed magnetite, and violent rupture.
62
Elim in a t ion
The solution of short term overheating, which is often caused by a brief upset
condition, is to eliminate the upset. If restricted coolant flow due to tube
pluggage is suspected, drums, headers, and other areas should be inspected and
cleaned.
Caustic Corrosion, Water side
Loca t ions
Generally, caustic corrosion is confined to;
1. Water cooled tubes in regions of high heat flux.
2. Slanted and horizontal tubes.
3. Locations beneath heavy deposits.
4. Heat transfer regions at or adjacent either to backing rings at welds or to
other devices that disrupt flow.
Ge n e r a l D e scr ipt ion
The term caustic and ductile gouging refers to the corrosive interaction of
sufficiently concentrated sodium hydroxide with a metal to produce distinct
hemispherical or elliptical depressions. At times a crust of hard deposits and
corrosion products will surround and / or overlie the attacked region.
Patch of hard iron oxides on internal surface (left) and cratered region beneath (right).
The susceptibility of steel to be attacked by sodium hydroxide is based on the
amphoteric nature of iron oxide; that is, oxides are corroded by both low pH and
high pH environments. High pH substances, such as sodium hydroxide will
63
dissolve magnetite. When this happens sodium hydroxide may react directly
with the iron.
Two critical factors contribute to caustic corrosion.
1. The availability of sodium hydroxide or of alkaline producing salts.
2. A mechanism of concentration.
Sodium hydroxide is often intentionally added to the boiler water at non
corrosive levels. Alkaline producing salts may contaminate the condensate by
in-leakage through the condenser. Poorly controlled water treatment may also
cause excessive alkalinity. Because sodium hydroxide and alkaline producing
salts are rarely present at corrosive levels in the bulk environment there are
basically three concentration mechanisms.
1. Depart ure from nucleat e boiling ( DNB) : During nucleate boiling a minute
concentration of boiler water solids will form at the metal surface. As the
steam bubble separates from the metal surface the water will re-dissolve
soluble solids as sodium hydroxide. At the onset of film boiling, the rate of
bubble formation exceeds the rinsing rate. Under these conditions,
sodium hydroxide, as well as other dissolved solids or suspended solids
will begin to concentrate.
2. Deposit ion: A similar situation occurs when deposits shield the metal from
the bulk water. Steam that forms under these thermally insulating
deposits escapes and leaves behind a corrosive residue that can deeply
gouge the metal surface.
3. Evaporat ion at a wat erline: Where a waterline exists, corrosives may
concentrate by evaporation, resulting in gouging along the waterline.
Elim in a t ion
When the availability of sodium hydroxide or alkaline producing salts and the
mechanism of concentration exist simultaneously, they govern susceptibility to
caustic corrosion. Caustic corrosion is counteracted by reducing or eliminating
the availability of sodium hydroxide and alkaline producing salts.
Caustic gouging beneath deposits.
64
Low pH Corrosion, during service, Water side
Loca t ions
Generally, in-service acid corrosion is confined to water cooled tubes in regions
of high heat flux; slanted or horizontal tubes, location beneath heavy deposits;
and heat transfer regions adjacent to backing rings at welds, or other devices
that disrupted flow.
Appearance of deposits covering groove (left), and contour of groove after deposit removal (right).
Ge n e r a l D e scr ipt ion
Although relatively rare, a general depression of bulk water pH may occur if
certain contaminants gain access to the boiler. Boilers using water of low
buffering capacity can realize a bulk pH drop to less than 5 if contaminated with
sea water, hydrochloric acid, or sulfuric acid.
The concern of this chapter, however, is with the more common creation of
localized pH conditions. Two circumstances must exist simultaneously to
produce this condition.
1. The boiler must be operated outside of normal, recommended water
chemistry parameters. This may happen in case of a leaking condenser,
in-leakage of sea water takes place.
2. A mechanism for concentrating acid producing salts. This condition exists
where boiling occurs and adequate mixing is hindered by the presence of
porous deposits or crevices. Where deposits and crevices are present, a
65
concentration of acid-producing salts may induce hydrolysis to produce
localized low pH conditions, while the bulk water remains alkaline.
There are basically three concentration mechanisms, and they are similar to high
pH corrosion (caustic corrosion).
1. Departure from nucleate boiling, film boiling.
2. Deposition.
3. Evaporation at the waterline.
Where low pH conditions exist, the thin film of iron oxide is dissolved and the
metal is attacked.
It is very difficult to distinguish localized attack by low pH substances, from those
by high pH substances simply by visual examination. Distinguishing between the
two may require a metallographic examination.
Low pH gouging
Elim in a t ion
When the availability of free acids or acid producing salts and the mechanism of
concentration exist simultaneously, they govern susceptibility to localized low pH
corrosion. Low pH corrosion is counteracted by reducing or eliminating the
availability of free acids or acid producing salts.
66
Low pH Corrosion, during Acid Cleaning, Water side
Loca t ions
In general any surface exposed to acid is susceptible. One of the first areas to
be effected is the tube ends inside mud and steam drums. Hand-hole covers,
drum manholes, and shell welds may also be affected. Heat-transfer surfaces
and weldments may experience vigorous attack. Shielded regions within
crevices, behind backing rings, and under remaining deposits may prevent
proper neutralization of the cleaning acid. This result in vigorous localized
attacks of the metal once the boiler is returned to service.
Acid corrosion on the internal tube surface (left) and jaggedness associated with severe acid corrosion (right).
Ge n e r a l D e scr ipt ion
Attack of a metal surface by strong acid is generally unmistakable. The surface
usually has a rough or jagged appearance, depending on the severity of the
attack.
Corrosion of steel by acids is a natural consequence of steel’s thermodynamic
instability in these environments. Steel will corrode spontaneously in most acids.
During the corrosion reaction, iron displaces hydrogen from the solution. That is,
iron is oxidized and iron ions go into the solution. Hydrogen ions are reduced
and form hydrogen bubbles at the metal surface.
To shift this corrosion process, inhibitors are added to acid-cleaning solutions
used in boilers.
Uncontrolled acid corrosion of the boiler during cleaning generally results from an
unanticipated deviation from the standard conditions or practices. Many
67
deviations are possible and may include events such as thermally induced
brackdown of the inhibitor, inappropriate selection of cleaning agent or cleaning
strength, excessive exposure times and temperatures, and failure to neutralize
completely.
Elim in a t ion
Mitigation of low pH corrosion of boiler equipment during acid cleaning requires
close monitoring of the entire cleaning procedure. The following are a few
examples of parameters to be monitored and evaluated during the procedure.
1. Deposit weight det erm inat ion; Deposit weight measurements at a number
of locations will aid in determining the proper acid strength, exposure
time, and total quantity required to adequately clean the boiler.
2. Deposit analyses; This will help in determining the appropriate cleaning
agent and the sequence in which the agent should be used.
3. Tem perat ure of cleaning; Both the solution and metal temperature should
be safely below the thermal breakdown point of the inhibitor.
4. Monit oring; Chemistry of neutralizer should be monitored following the
boilers exposure to the acid.
5. Visual inspect ion; Tubes, mud drums, steam drum should be inspected
after cleaning.
Oxygen Corrosion, Water side
Loca t ions
Although relatively uncommon in an operating boiler, oxygen attack is a problem
frequently found in idle boilers. In an operating boiler the first areas to be
affected are the economizer and feed water headers. In cases of severe oxygen
contamination other areas of the boiler may be affected, such as the surface
along the waterline in the steam drum, and the steam separation equipment. In
all cases, considerable damage can occur even if in a short period of oxygen
contamination.
Oxygen pits inside a tube section (left and right), external surface pits on fire tube (middle).
68
Ge n e r a l D e scr ipt ion
Since the oxides of iron are iron’s natural, stable state, steels will spontaneously
revert to this form if conditions are thermodynamically favorable. Generally,
conditions are favorable if steel, which is not covered by a protective form of iron
oxide, is exposed to water containing oxygen.
The corrosiveness of water increase as temperature and dissolved solids
increases, and pH decreases. Aggressiveness generally increases with an
increase in oxygen.
Fractures in the protective magnetite are caused by thermal or mechanical
stresses during operation, there fractures furnish anodic regions where oxygen
containing water can react with the bare, unprotected metal.
Oxygen corrosion often occurs as pitting which is covered by non protecting iron
oxides. In addition to wall perforation, oxygen pits can act as stress
concentration sites, thereby fostering the development of corrosion fatigue
cracks, caustic cracks, and other stress related failures.
Elim in a t ion
Since water is always present in an operating boiler, and the protective
magnetite coating exists in a state of continuous breakdown and repair,
mitigation of oxygen corrosion is achieved by sufficient diminishing dissolved
oxygen. This is achieved by proper operating dearators, and admission of
adequate quantities of oxygen scavenging chemicals to the feed water.
Oil Ash Corrosion, Fire side
Loca t ions
Oil ash corrosion is a high temperature, liquid phase corrosion phenomenon
generally occurring where metal temperatures are in the range of 593 to 816 ˚C.
It may affect superheater tubes, water cooled tubes, and support / attached
equipment which operate at a higher surface temperature than the tubes.
Wall thinning as a result of oil ash corrosion.
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Ge n e r a l D e scr ipt ion
Oil ash corrosion occurs when molten slag containing vanadium compounds
forms on the tube wall according to the following sequence:
1. Vanadium and sodium compounds present in the fuel are oxidized in the
flame to V2O5 and Na2O.
2. Ash particles stick to the metal surface, with Na2O acting as a binding
agent.
3. V2O5 plus Na2O react to the metal and form a liquid (eutectic).
4. The liquid formed dissolves the magnetite, exposing the underlying metal
to rapid oxidation.
It is believed that corrosion occurs by catalytic oxidation of the metal by
vanadium pentoxide (V2O5) or complex vanadates.
This corrosive slag may develop when fuels containing high levels of vanadium,
sodium, sulfur, or a combination of these elements are used; when excessive
amounts of excess air is available for the formation of V2O5; where metal
temperatures exceeding 593 ˚C are achieved.
As the metal temperature increases, the range of compositions of Na2O and V2O5
that form the liquids expands considerably. Hence, in units with relatively thick
layers of internal scale, the metal temperature will increase and may exceed
temperatures at which sodium–vanadium complexes form liquids. If this occurs,
sudden, unexpected problems with oil ash corrosion may appear, even though
operating parameters and fuel chemistry remain unchanged.
High temperature corrosion at the base of an attachment.
Elim in a t ion
Oil ash corrosion is eliminated by controlling the critical factors that govern it.
First, if fuel containing very low quantities of vanadium, sodium, and sulfur
cannot be specified one has the second option of firing the boilers with a low
excess of air to retard V2O5 formation. Third possibility is to prevent metal
temperatures from exceeding 593 ˚C.
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Cold End Corrosion, Fire side
Loca t ions
Cold end corrosion occurs wherever the metal temperature drops below the
sulfuric acid dew point of the flue gas. It occurs in relatively low temperature
sections of the boiler such as economizers and air heaters.
Corrosion and perforations of a finned economizer tube resulting from exposure to sulfuric acid.
Ge n e r a l D e scr ipt ion
In general the problem is associated with the combustion of fuels containing
sulfur or sulfur compounds. Sulfur in the fuel is oxidized to sulfur trioxide which
as the flue gas cools reacts with water vapor to form vapor phase sulfuric acid.
If the sulfuric acid vapor contacts a relatively cool metal surface, it may
condensate as liquid sulfuric acid. The temperature at which sulfuric acid
condenses varies from 116 to 166 ˚C, or higher depending on sulfur trioxide and
water vapor concentrations in the flue gas.
The critical factors governing cold end corrosion include the presence of corrosive
quantities of sulfur trioxide, the presence of moisture in the flue gases, and the
presence of metals whose surface temperature is below the sulfuric acid dew
point.
Elim in a t ion
Cold end corrosion is eliminated by gaining control of the critical factors
governing it. The oxidation of sulfur trioxide is prevented by specifying low
sulfur fuel and operating the boiler with a low air excess. Also, by raising the
metal temperature above sulfur acid dew point will prevent occurrence of cold
end corrosion.
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Corrosion Fatigue Cracking
Loca t ions
Corrosion fatigue occurs in any location where cyclic stresses of sufficient
magnitude are operative. Rapid boiler start up and shutdown can greatly
increase the susceptibility to corrosion fatigue. Common locations are at points
of attachment or rigid constraint, such as connections to inlet or outlet headers,
tie bars, and buckstays.
Ge n e r a l D e scr ipt ion
The term refers to cracks propagating though a metal as a result of cyclic tensile
stresses operating in an environment that is corrosive to the metal. The term
and definition above are somewhat misleading in the case of boilers, since
normal oxidation of metal to magnetite is sufficient to induce corrosion fatigue in
the presence of sufficient cyclic tensile stresses. Cracks develop according to the
following sequence;
1. During the first phase of cyclic stress, the tube wall undergoes expansion.
Since the oxide layer is brittle relative to the tube wall, the oxide layer
may fracture, opening microscopic cracks through the oxide to the metal
surface.
2. The exposed metal surface at the root of the crack oxidizes, forming a
microscopic notch in the metal surface.
3. During the next expansion cycle, the oxide will tend to fracture along this
notch, causing it to deepen.
4. As this cyclic progress continues, a wedge shaped crack propagates
though the wall, until rupture occurs or the tube wall is penetrated.
Developing corrosion fatigue crack formed at base of cracked layer of iron oxide (magnification 400X) (left),
Mature corrosion fatigue crack (mag.200), (right).
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The crack always propagates in a direction perpendicular to the direction of the
principal stress. Hence, if the principal cyclic stress is produced by fluctuations in
internal pressures, longitudinal cracks are produced. If the principal stress is a
bending stress produced by thermal expansion and contraction of the tube,
cracks will be transverse. Corrosion fatigue cracks commonly occur adjacent to
physical restrains, and are often associated with pits which serve as stress
concentrating notchs.
Longitudinal cracks resulting from internal pressure (left) and transverse cracks due to bending stresses (right).
Elim in a t ion
Corrosion fatigue cracking is eliminated by controlling cyclic stresses, and
environmental factors. Reducing or limiting cyclic stresses by extended start up
and shutdown times. Controlling pH and excessive levels of dissolved oxygen
can be useful in eliminating pitting corrosion, which will eliminate a common
point of initiation for corrosion fatigue cracking.
Stress Corrosion Cracking
Loca t ions
In principle, stress corrosion cracking could occur wherever a specific corrodent
and sufficient tensile stress coexist. Because of improved water treatment
programs and boiler designs the occurrence of caustic stress corrosion cracking
(caustic embrittlement) is much less frequent than in the past.
Ge n e r a l D e scr ipt ion
The term stress corrosion cracking refers to a metal failure resulting from a
synergistic interaction of tensile stress and a specific corrodent to which the
metal is sensitive. The tensile stresses may be either applied, such as those
caused by internal pressures, or residual, such as those introduced by welding.
In boilers, carbon steel is specifically sensitive to concentrations of sodium
hydroxide, while stainless steel is specifically sensitive to both sodium hydroxide
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and to chlorides. The combination of concentrated sodium hydroxide, some
soluble silica, and tensile stresses will cause continuous intergranular cracks to
form in carbon steel. As the cracks progress the strength of the remaining intact
metal is exceeded, and a brittle, thick walled fracture will occur.
Instances of caustic stress corrosion cracking in boiler metal operating below 149
˚C are rare.
Crack on the internal surface, note the proximity to the weld.
Elim in a t ion
To eliminate problems with stress corrosion cracking it is necessary to gain
control of either tensile stresses or concentrations of corrodents.
1. Tensile stress can be either applied or residual. Applied stresses are
service generated stresses and can only be partly influenced by proper
operation of the boiler. Residual stresses are the result of manufacturing
and construction processes such as welding or tube bending. These
stresses can be relieved by proper annealing techniques.
2. Avoiding concentrated corrodents is generally the most successful means
of reducing or eliminating stress corrosion cracking. This is accomplished
by avoiding departure from nucleate boiling and keeping internal surfaces
free from deposits, in other words proper water treatment.
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Chapter 3: Auxiliary Boiler Survey
Introduction
Det Norske Veritas in common with all other Classification Societies require all
boilers, whether main or auxiliary boilers to be surveyed periodically. The scope
of this inspection is to confirm that the boiler can be safely operated for the
duration of the upcoming period, and identify the need for immediate repairs or
maintenance.
At these surveys, the boiler, superheater, economizer, and air heater are to be
examined internally and externally, and principal boiler mountings are to be
opened up and inspected. The survey is finalized with an examination of the fuel
oil burning system under operation, and testing of the safety functions.
Survey Preparation
For the quality of the survey it is of paramount importance that the boiler is
prepared properly. In preparing a boiler for survey there are certain
fundamentals which should be observed, the most important ones are listed
below.
•
•
The boiler is to be taken off-line, completely drained, and cooled down.
All mountings should be isolated safely from any live feed or steam ranges
so that these can be opened up for inspection and overhaul. Blanks
should, if necessary, be fitted to secure safe isolation. The ships side blow
down valve should be shut.
Deposits on the steam / water side need to be removed in order to carry out a proper survey.
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•
•
In multi-boiler installations extra vigilance should be observed in safely
isolating the off line boiler, from the online boiler. Common pipe
connections such as blowdown lines and drain pipes from safety valves
chests must be taken into consideration.
All water /steam side manhole covers must be opened in order to facilitate
internal survey. Also handhole covers of headers are to be removed for
inspection.
The boiler water side is not properly cleaned, deficiencies may be overlooked when inspecting a boiler in this
condition.
•
•
•
Access to the fire side / furnace should be provided by removing the
burner and opening doors to smoke boxes.
The boiler should not be entered until it is sufficiently ventilated, and
cooled down.
Before a boiler can be surveyed it should be thoroughly cleaned. Failure
to do this may result in serious defects being overlooked. In well
maintained boilers some wire brushing and a good hose-down of the water
steam side is sufficient. If hard scale has been allowed to form mechanical
cleaning by chipping hammer, or chemical cleaning may be necessary.
Removal of loose soot and deposits on fire side tube walls and furnace
floor is normally adequate. If the fire side is very dirty then mechanical
cleaning and water washing may be needed.
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Smoke tubes are blocked by soot, this has to be cleaned before commencing the survey.
Survey Safety Measures
For personal safety reasons, the following must be considered before entering a
boiler for internal inspection.
• It is advisable to start with the external boiler survey, during which it can
be confirmed that the boiler is safely isolated.
• A boiler is a confined space and therefore applicable safety instructions
and policies need to be adhered to. Make certain the boiler space is well
ventilated and the atmosphere is safe. No person should enter the boiler
without authority. Some responsible person should always be standing by
the manhole door when another person is in the boiler.
• Before entering a boiler empty all coverall pockets, sometimes it can be
extremely difficult to recover lost property in a boiler.
• Make sure internal boiler parts are cooled down sufficiently for entry.
Apart from the discomfort while doing the survey, boiler exit may be more
difficult or impossible due to body expansion and sweating.
• Since auxiliary boiler spaces are normally very small, we recommend that
only one person enters the boiler.
• If feeling claustrophobic before entry, we strongly advice not to enter the
boiler. Before entry, give the necessary consideration on how to exit the
boiler.
• Once in a boiler and experiencing difficulties to come out, it is essential to
stay calm and not to panic. Take a small break and try again, remember
when you managed to enter, it is possible to get out.
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•
Goggles and filtration masks may be worn during fire side inspection. This
in order to protect one self against soot inhalation and eye irritation by
soot.
When the boiler is of a type that can not be satisfactory internally examined due
to inaccessibility, a hydrostatic pressure test may be called for. Please, take
account of the fact that a liquid pressurizing medium is far less dangerous than a
pneumatic pressurizing medium. Therefore, pressure testing with steam or
compressed air should not be attempted.
Shell Type Boilers
These boilers are of moderate steam capacity and have been evolved to work
with feed water of medium quality. They are suitable for relatively simple steam
installations. Tank type boilers have relatively large water content in relation to
its volume, heated surface, and steam production. Normally the heated surface
consists of a cylindrical furnace or fire box located in the lower part of the shell.
Shell or tank type boilers can be classified by their construction in two groups.
1. Horizontal shell type boilers.
2. Vertical shell type boilers.
Horizontal shell type boiler, Steambloc packaged boiler.
Today we mostly find vertical shell type boilers installed onboard, which are
utilized as auxiliary boilers.
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Vertical tank type boiler, Mission OS
H or izon t a l Sh e ll Type Boile r s
The famous Scotch boiler has by now been replaced by modern designs such as
the Steambloc package boilers, and Mission 3 phase boilers. However, the basic
design of these boilers is still the same as the Scotch boiler.
Figure 1 represents a horizontal shell type boiler and indicates the various points
were defects may be commonly found. These defects will now be discussed.
Fig. 1, position of defects.
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1. Distortion of combustion chamber tube plate (Fig. 1, b): This defect, if
slight, can be detected by placing a straight edge across the tube ends and
sighting the face of the tube plate. The pushing of the tube plate inwards
is caused by overheating of the tubes, usually attributable to excessive
scale or oil deposits. In extreme cases, signs of leakage of the welded
tube ends may be evident and when this is found, cracks in welded tube
connections may have occurred.
Distortion of tube plate.
2. Wastage of combustion chamber or furnace stays (Fig. 1, d): This common
defect, often referred to as “necking”, is accelerated by the straining
action imposed by continual expansion and contraction of the combustion
chamber, or furnace, by temperature fluctuations. When dealing with
wasted stays it should be remembered that their strength varies with the
square of their diameter.
Wastage and fracture of stays.
3. Overheated chamber top (Fig. 1 e): The first part to suffer from water
shortage is the combustion chamber crown.
Overheated top plate.
An accumulation of mud, scale, or other insulating material such as oil can
also be the cause of overheating. When a distortion of a combustion
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chamber top occurs, special attention should be given to the welded
connections (possible cracks).
4. Wastage of door flanges and landings (Fig. 1, l): This type of defect
remains all too common. Manhole and handhole door spigot clearance
should receive special attention at each boiler survey. Leakage from man
/ hand hole doors can cause serious shell wastage. Particular attention
should always be given to the fit of the doors and to obtaining a good
joint. A careful check should be made for strained door studs, slack fitting
or stripped nuts, and distorted manhole door dogs.
Picture of leaking hand hole on the left and wastage of door flange on the right.
5. Grooving of end plates (Fig. 1, n): Grooving has been found in the boiler
shell plating adjacent to the welded connection of the end plate to the
shell.
Grooving in way of weld of boiler shell plate.
6. Cracking of furnace at welded connection to end plate (Fig. 1, s): Serious
furnace failures have occurred in the past when circumferential corrosion
fatigue cracking resulted in the rupture and collapse of the furnace. In
view of the serious consequences of such a defect particular attention
should be given to this weld connection during boiler surveys.
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Cracks in furnace at connection to end plate.
7. Uniform furnace distortion (Fig. 1, u): Overheating and subsequent
deformation is always caused by the presence of some insulating medium,
scale, mud, or oil contamination. This effect may be further increased by
faulty combustion. The usual way of getting some idea as to whether a
furnace is possibly round, is to sight along the corrugations with a torch
from inside the combustion chamber.
Checking furnace for distortion drawing on the left, and a collapsed furnace on the right.
8. Unidirectional thermal cracking (Fig. 1, v): Thermal cracking sometime
occurs in furnaces of package boilers. This can be attributed to flame
impingement from a faulty burner, which was allowed to operate in this
condition for some time.
Thermal cracks on peaks of corrugations right and local bulge in furnace left.
9. Local bulge in furnace (Fig. 1, w): Local bulges in furnaces are caused by
overheating and are remedied by cutting out the bulged piece and welding
in a new piece. On no account should repair by fitting doubler plates be
contemplated.
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10.General internal wastage of shell bottom (Fig. 1, y): In neglected boilers
corrosive deposits may have accumulated at the bottom for a long period.
This may result in pitting and wasted areas. In general the only
satisfactory method of repair consists of fitting a new boiler shell insert.
The building up of wasted shell areas by welding should not be permitted.
11.Overheated combustion chamber back plate (Fig. 1, z): It is quite
common, especially in boilers which are not kept clean, to find the back
plate bulged between the stays. A bulged plate accumulates scale and
mud, and promotes further overheating and extension of the bulge.
Provided the bulging of the plate between the stays is not very extensive,
and has not stretched the material in way of the stay holes to cause
leakage. The obvious remedy is to keep the waterside as clean as
possible, in order to prevent further overheating.
Overheated combustion chamber back plate.
Vertical shell type boiler AQ 3.
Ve r t ica l Sh e ll Type Boile r s
The original vertical boilers were riveted and were either of cross tube design
with central uptake, or of the horizontal smoke tube design. The present day all
welded vertical boilers with fireboxes and smoke tubes are basically
developments of the earlier designs. A type different from the foregoing is the
one fitted with water tubes, which has gained in popularity. The modern vertical
shell type boilers nowadays incorporate membrane tube walls (mono walls)
around the furnace.
The following contains some common defects found in vertical shell tube boilers.
1. In general vertical boilers are big enough for internal access, although the
lower parts around the fire box (red circle at bottom, see sketch below)
are often very restricted and resort has to be made to the best possible
examination through small hand holes.
2. In case of poor water treatment, pitting in firebox and to a lesser extent
the shell can be expected. In case of smoke tubes boilers, the tubes,
shell, and furnace crown may be affected. It is also important to ascertain
in such a case that the stay tubes which tie the flat tube plates together
are still in a good condition.
83
Pitting on top of the firebox (left), area affected by pitting and deformation as mentioned below
are marked by a red ellipse in the sketch (right).
3. While concentrating on the internal examination it is always advised to
make a special point of looking for distortions of the heated surfaces.
Especially, the flatter parts where mud or scale can accumulate leading to
overheating. Overheating through water shortage usually results in
serious deformations of the furnace or firebox.
Deformed furnace plate seen from the water side (left) and the furnace side (right).
4. The firebox or furnace of vertical boilers is usually connected to the bottom
of the cylindrical shell by what is known as an “ogee” ring. In service this
ring is located in a zone of little if any circulation and it forms a receptacle
for sludge and corrosive sediments. As it is subject to considerable
straining due to pressure and temperature fluctuations, it is prone to
“grooving” which can be of a serious nature. The presence of this groove
can, it will be appreciated, only be found by careful examination through
bottom shell hand hole doors.
84
Grooving of shell and firebox plate in way of ogee and foundation ring.
5. As mentioned previously in this chapter, one type of vertical boiler is
utilizing water tubes in lieu of smoke tubes. These boilers consist of an
upper and lower cylindrical section joined to another by straight water
tubes, enclosed in the lower section of the firebox, the gases from which
pass through the water tubes on route to the uptake. Several fatal
accidents have occurred involving the detachment of the top half shell of
such boilers. While making internal examination of this type, special
attention should be paid to the circumferential welded seam between
upper tube plate and shell.
Crack in circumferential weld between upper and lower section, sketch on the right is indicating the
position.
85
Fatigue cracking of boiler shell and tube plate connection may result in top half being
detached from the boiler as illustrated above.
6. The flue gas pipe is an area where heat concentrations are likely to occur,
which may result in thermal cracking. Therefore, this area should have
special attention during internal survey.
Cracks found in the lower and upper weld of the flue gas seen from the steam / water side. The
position of the cracks is indicated in the sketch on the right.
7. Boilers having hemispherical furnace crowns incorporate sometimes bar
stays between the crown and the flat lower tube plate. It will be readily
seen that any distortion of the furnace crown may result in overstressing
the welded connections of the stays. Ultimately, this may lead to failure of
the weld.
8. With regard to the external examination, it is sometimes found that
wastage exists beneath damp lagging around leaking boiler mountings.
Therefore any signs of boiler mountings leakage should be further
investigated if the boiler shell is affected by corrosion. Also soot stains on
the lagging, or flaked off paint is an indication of flue gas leakage and
must be investigated.
86
Crack in weld of stay between furnace crown plate and lower tube plate, position is indicated in the sketch
on the right.
Deformation of the crown plate overstresses the stay weld which results in cracks, this is also applicable
for the welds of the flue gas pipe.
9. Foundation and boiler supports should be examined at every survey.
Sometimes these are exposed to overheating caused by defective
brickwork. Or when it is situated below the engine room platform, it may
be subjected to the corrosion effects of occasional bilge water and a
general damp atmosphere. It should be remembered that the combined
weight of a boiler and its content, which may be as much as 30 tons, is
supported by this structure.
10.Some vertical boilers are designed with a toroidal header which is situated
at the bottom of the combustion chamber. This header forms a ring and
supports the weight of the boiler. Normally, it is welded to a T or L section
support ring and further secured by a number of triangular shaped
brackets. Cracks have been found at the toes of these brackets.
87
Leaking valves as shown above may lead to serious damage and dangerous situations as illustrated in the
left picture where the extension tube and boiler shell are affected by corrosion.
Fig. 1 Tube plug to be used on fire tubes and as a general rule not more than 10 % of tubes should be plugged
due to unfavorable head distribution.
11.When isolated tube failures occur in service it is the practice to fit
stoppers. Such stoppers should be removed and the defective tube
renewed as soon as possible. Figure 1 shows a stopper with bar which is
used for plain smoke tubes, some manufactures supply these stoppers
88
with a chain for curved or bent tubes. Smoke tube stoppers should always
be removed during boiler surveys, whether or not the defective tubes are
to be renewed. This in order to examine that the tread and the rod are in
a good condition. Failure in service of a smoke tube stopper could result
in serious injuries to the personnel. On no account should welded blanks
or, worse still, driven in tapered plugs be fitted in plain smoke tubes.
12.It sometimes happens that thermal cracks develop in the ends of plain
smoke or stay tubes, at their combustion chamber or firebox ends. This
may be accelerated by the use of over-long tubes, resulting in protruding
(un-cooled) ends. When this defect is recurring, even after the protruding
ends have been cut off, it has sometimes been effective to fit protective
heat resistant ferrules after the tubes have been renewed.
Heat resisting ferrules or simply cutting off the uncooled portion is protecting the fire
tube ends.
Water Tube Boilers
Water tube boilers came into extensive use in the merchant navy as main boilers
during and immediately following the 1914-1918 war. Today we only find them
utilized as auxiliary boilers on vessels in need of large steam quantities of a
certain pressure and temperature, normally not produced by shell type boilers.
The generated superheated steam is used to drive cargo pump turbines and
turbo generators.
The main reasons for adapting water tube boilers instead of shell type boilers
are:
• Saving weight: The relative weight of a Scotch boiler to a water tube boiler
installation for equivalent heating surface area is approximately 3:1.
• The possibility of using higher temperatures and pressures: The limit of
working pressures for scotch boilers, for practical reasons, such as shell
thickness and lack of flexibility was 21 bars. Water tube boilers did not
have this constraint, and therefore due to higher pressures and
temperatures machinery size and weight for a given output was reduced
and thermal efficiency increased.
• Greater mechanical flexibility: The water tube boiler is not so sensitive to
fluctuating pressures.
• Higher steam outputs: The good circulation and the ability to withstand
higher pressures have enabled higher steam outputs compared to a scotch
boiler.
89
•
Wider safety margin in event of explosions: The possibility of a serious
explosion is considered to be far more remote with a water tube boiler
than with a shell type boiler. In the former, tube diameters are wisely
limited and drums are protected from direct radiation and flame
impingement. If a tube fails, the content of the boiler escapes at a rate
determined by the tube bore. Whereas in the latter, serious rupture of an
overheated furnace can almost instantaneously release the content into
the engine room.
A thorough conscientious examiner in any walk of life knows the values of
working to a definite routine and, in case of a boiler survey, where it is of the
utmost importance that nothing is missed, this is essential. The total heating
surface of each individual boiler embodies generating, superheating, feed and /
or air heating surfaces. The boiler design varies from one installation to the
next. Initially, therefore, it is practically essential that the Surveyor makes a
brief scrutiny of the boiler arrangement plan, noting in particular the super
heater design and the method of steam temperature control.
The layout of the boiler unit having been ascertained, a convenient survey route
should be planed as suggested in figure 2.
Fig. 2 Suggested survey route for a D type water tube boiler.
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St e a m dr u m
Access to the steam drum is often rendered extremely difficult due to the
presence of internals such as cyclone separators, feed troughs, perforated plates,
etc., and the extent of removal of these can be the cause of controversy at
survey times.
Steam drum interior of a Kawasaki main boiler, equipment installed depends on steam production
and size of steam drum.
The primary purpose of the drum internals (separators, scrubbers, baffle plates)
is to permit separation of the saturated steam from the water-steam mixture
leaving the boiling heat transfer surface. The steam free water is then recirculated with the feed water to the heat absorbing surface for further steam
generation. Other internals are feed pipes to mix the feed water with the
saturated water, and blow down pipes to remove solids from the water surface.
Below some common defects found in steam drums will be discussed.
1. Internal pitting of the steam drum surface, if not deemed serious on
account of the drum thickness, should not be ignored, but taken as an
indication of what may exist in the tube bores, where it could well be
serious. When pitting is present in the short length of the bore normally
visible in a bent boiler tube, the first consideration should be, “is it active
or not?”. If the pitting is of shallow depth and inactive (no corrosion
products) no further action is needed. On the other hand, visible active
pitting of substantial depth (approximately 40 % tube thickness) definitely
requires investigation further down the tubes. Generally, the most
seriously affected tubes are those in close proximity to the furnace.
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Above corrosion in a water drum (right) may indicate severe corrosion in tubes, The left picture of a steam
drum shows the longitudinal weld which should receive particular attention due to higher stress levels then the
circumferential welds.
2. While examining the internal steam drum attention should be paid to the
condition and fastening of any fittings. Cracks have been experienced in
the welds to the drum of such internal equipment supports.
3. All drum openings to mountings should be sighted; some times tools are
forgotten and left in passages leading to gauge glasses or other
mountings.
4. Drum welds, in particular the longitudinal welds should be carefully
examined, since corrosion / cracks have been found in way of the welds
heat affected zone. The tangential stresses in a boiler drum are twice the
magnitude of the axial stresses. Therefore the longitudinal welds are
more critical than the circumferential welds.
Fu r na ce
After entering the furnace, it is prudent to pause in the middle of the floor and
get an overall impression of the general condition. Look at the screen, water
wall, roof tubes, and the refractory.
Bear in mind that a shortage of cooling as a rule results in a general distortion of
the furnace tubes. The roof tubes are the once first affected by this in case of
water shortage.
Membrane or mono water walls are nowadays almost without exception used in
all modern water tube boilers. They have resulted in great savings in refectory,
and provided a gastight furnace wall.
Different methods of welding a membrane or mono tube wall.
92
However, there are also disadvantages, repair of a failed tube is more difficult. If
not repaired immediately the un-cooled tube will burn and gases can escape into
the engine room. Secondly, in the event of a furnace explosion the damage is
likely to be more serious since the pressure built is much greater before release.
In the following some points for consideration will be discussed.
1. Screen tubes as their name implies screens the superheater from the
radiant heat from the furnace. Overheating of these tubes is usually
shown by distortion or occasionally by swelling. By shining with a torch
sideways across the face of the bank it is easy to see which screen tubes
are distorted. The tube bank should also be inspected for cleanliness,
gasses should have free passage through the bank.
Gas passage is blocked by soot at the bottom of screen tubes, position is indicated in the sketch on the
left.
Checking for distorted screen and wall tubes with the help of a flash light.
93
2. Water leakages in the furnace are usually shown by white stains down the
outside of the pipe.
White stains are indicating a leaking fin tube bent of a Sunrod CHS boiler, item 2 applies for all
boilers.
3. The tubes in the proximity of the burner flame envelope should be
examined for flame impingement. Which can lead to distortion, tube
bulging, and thermal fatigue cracks.
4. The same observations made regarding screen tubes apply equally to
water wall tubes (side, roof, and floor tubes).
5. Openings in the tube wall for soot bowers, flame peep holes, need to be
given special attention since they find themselves more exposed and fail
more frequently.
Failed tube in way of a so called nose arch.
94
6. Refractory is part of the boiler heat retaining envelope, and protects the
underneath steel parts against overheating. Probably the most important
refractory material in the furnace is that installed to protect parts of the
steam and water drum against direct heat exposure from the furnace.
Failure of this refractory has on occasions resulted in circumferential
thermal fatigue cracking of the drums, which necessitated renewal.
7. Heaters must be surveyed through handholes for corrosion, pitting and
cleanliness. Sometimes deposits accumulate in the middle of bottom
headers which disturbed the flow through the tubes.
Deposits have accumulated at mid length of water wall header.
8. When outside the boiler a quick glance should be given to the flatness of
the boiler casing, any distortion may be caused by a minor furnace
explosion. Soot spots or peeled off paint may indicate leakages which are
in need of further investigation.
W a t e r dr u m
Normally, the internal examination of water drums produces little to note in way
of defects. If however pitting, corrosion or deposits were observed in the tubes
ends in the steam drum or headers then the lower parts of the tube bores need
special attention from inside the water drum.
Su pe r h e a t e r s
Due to the fact that the superheater and its supports are exposed to high steam
and flue gas temperatures, it is most likely to find defects in this part of the
boiler. Mild steel is used for superheater tubes with steam temperatures up to
399 ˚C, above this temperature alloy steels are used. The alloying elements
used being molybdenum and chromium in various proportions.
The following contains some common defects found in superheaters.
1. Distortion of superheater banks as a result of overheating. This can be
caused by reduced flow during start up or shut down, or the built up of
deposits on the internal tube surface.
2. Cracked and burned supports, these parts are not cooled and therefore
subjected to high temperatures.
95
3. The safe working of the superheater is to some extent allied to its external
cleanliness and this must be considered during survey. In case of
operating the superheater in dirty condition, special attention should be
paid to those parts through which there is still a gas path, as these have
probably been operated at excessive metal temperatures.
Newly installed superheater on the right and on the left a picture of the supports.
4. Failure of severely overheated superheat tubes has in several cases lead
to iron burns which completely destroyed the boiler.
Steam drums, water drums, and headers are usually stress relieved upon
completion of the welding work. Several incidents have occurred where
attachments have been welded to the drums after stress relief, and eventually
cracks have developed in way of these welds of such a serious nature that
renewal of the drum was necessary.
Therefore the above should be kept in mind when welding work on drums or
headers is considered.
96
Types of Horizontal Shell Boilers
Different configurations
Horizontal shell boilers have several different combinations of tube layout. This
involves the number of passes the hot flue gasses make through the boiler
before they are discharged. Also they are distinguished in dry and wet back
configurations.
Dry back configuration.
Above is shown a dry back boiler where the hot gasses are reversed by a
refractory lined chamber on the outer plating of the boiler.
Wet back configuration.
A more efficient method of reversing the hot gases is obtained in the wet back
configuration. The reversal chamber is contained entirely within the boiler.
Modern packaged boilers operate therefore according to the wet back principle.
97
Early horizontal shell type boilers had compared to today’s standard, a very poor
efficiency. The flue gases were moving from the furnace more or less straight in
to the chimney. This lead to the development of two passes dry back boilers
which had a considerably improved thermal efficiency.
Two pass, dry back boiler.
A further development of the two pass boiler was the three pass boiler, with a
wet back. This improved the thermal efficiency again, and therefore is the
standard configuration in use today.
Three pass, wet back boiler.
98
Cochran Chieftain Packaged Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Cochran
Type:
Chieftain
Steam capacity range:
2000 - 15000 kg/h
Steam pressure:
17 bar maximum
D e scr ipt ion
This is a three pass semi wet back design of a package boiler which has a large
heating surface in relationship to its volume. As indicated above it is made in a
wide range of sizes varying in output and working pressure.
Cut-away view of a Cochran Chieftain packaged boiler.
The first pass consists of a single plain furnace which is wasted at about two
thirds of its length. This in order to accelerate the furnace gases before they
enter the combustion chamber, and also provides structural flexibility. The
furnace is self supporting due to the thick (21 mm) furnace plate used.
There are two passes of the smoke tubes. The first pass consists entirely of plain
tubes which are curved at one end so they may be received radially into the
holes in the hemispherical combustion chamber tube plate. These tubes are
lightly expanded and then seal welded at their combustion chamber ends, whilst
at the front tube plate they are attached by expanding only.
99
Section through the pressure shell of a Chieftain boiler, 17.2 bars, 3068 kg/h
The second pass of tubes is entirely convectional and contains a proportion of
stay tubes which are supporting the front and rear tube plate. Plain tubes are
expanded into the tube plate whilst stay tubes are expanded and afterwards
welded.
Cochran Wee Chieftain Packaged Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Cochran
Type:
Wee Chieftain
Steam capacity range:
1000 - 5000 kg/h
Steam pressure:
10.34 bar (available up to 28 bars)
D e scr ipt ion
The range of smaller boilers based on the Chieftain boilers is generally referred
to as Wee Chieftain boilers. Its three pass wetback design is renowned for
reliability, durability and its robust design. The construction of the pressure shell
is very similar to that of the Chieftain although scantlings will generally be found
to be lighter. Various designs of furnace and combustion chamber are adopted
depending on the rating and size of the particular boiler.
Nowadays these boilers are delivered as a total package, incorporating
combustion equipment, feed water pump and controls, control panel, all
necessary valves and fittings. Packaged boilers are available for immediate use
once they are fastened down to the ships structure and connected to the various
systems.
100
Cochran Wee Chieftain boiler.
Cochran Borderer Package Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Cochran
Type:
Borderer
Steam capacity range:
5000 - 6800 kg/h
Steam pressure range:
6.9 – 10.34 bar
D e scr ipt ion
The Cochran Borderer features a three pass, full wet back design for maximum
efficiency. Key elements are that the boiler is compact, and its robust design.
Cochran Borderer Package Boiler.
101
Steambloc Package Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Babcock & Wilcox Ltd
Type:
Steambloc
Steam capacity range:
590 - 10000 kg/h
Steam pressure:
17 bar
D e scr ipt ion
Designed as a dry back, single furnace, return tube, horizontal boiler with a high
efficiency rating, it is available in a variety of evaporation capacities and
pressures. In its most simple form, it may be encountered as a two pass unit
having a plain furnace. It should be noted that the pressure shell plating
thickness is considerably less than that of the furnace, while the tube plates are
the heaviest plates. Larger units of the design incorporate a part corrugated
furnace and are manufactured with three passes of gases.
Steambloc 3 pass Packaged boiler, skid mounted.
In the above depictured boiler the rear smoke box is constructed with a division
wall or baffle, to separate the second and third passes of the smoke tubes. This
baffle forms an inner smoke box, and is fitted with a separate door to completely
102
seal the flue gases, which have traversed the first pass from the cooler
exhausting from the third pass.
Rear smokebox of a Steambloc 3 pass boiler.
Parat Halvorsen B 5, Smoke Tube Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Parat Halvorsen AS
Type:
B 5 Smoke Tube
Steam capacity range:
1000 - 15000 kg/h
Steam pressure:
12 bar
Parat B5 smoke tube boiler.
103
Types of Vertical Shell Boilers
Clayton Steam Generator
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Clayton Industries
Number of sizes:
15 different sizes
Power output range:
100 – 6800 kW
Steam capacity range:
144 - 10944 kg/h
Steam pressure range:
1.08 – 204.84 bar
D e scr ipt ion
All Clayton boilers employ the following principles:
1. Controlled or forced circulation.
2. Counter flow heat transfer.
A pump continuously supplies feed water to a helical coil heat exchanger, which
transfers its heat to the water. The flow of feed water is counter to the flow of
combustion gases, an engineering principle that contributes to high fuel-to-steam
efficiency. Water leaving the heat exchanger passes through a mechanical
separator where the liquid and vapour are separated. Steam exits the separator
to the steam header.
Flow diagram through a Clayton steam generator.
The employed principles of forced circulation and counter flow and the resultant
low water content result in many advantages. Quick start and response
capabilities result primarily due to the low water content, forced circulation, and
helical coil design. Another benefit of this design is reduced blowdown. The
amount of water removed from the system to maintain an acceptable level of
total dissolved solids (TDS) is greatly reduced compared to conventional boilers.
Further, a Clayton steam generator provides an automatic indication of scale
build-up: indicated by an increase in feed pump pressure.
The Clayton design is inherently safe with no possibility of a hazardous steam
explosion. The heated pressure part steam generator is a spiral spring coil.
104
Clayton steam generator, burner is in the bottom. Right schematic drawing of the steam separator.
The feed water flow is provided by a heavy duty positive displacement diaphragm
pump. A single high-grade carbon steel, continuous coil heat exchanger employs
a staggered configuration and spacing of coil sections, to help ensure turbulent
and high velocity gas flows that facilitate high rates of heat transfer.
Staggering and spacing of the coil sections, on the right Clayton circulation pump.
105
Sunrod CPDB Boiler
Pa r t icu la r s
Make:
Type:
Steam capacity:
Steam pressure:
Sunrod
CPDB
1500 kg/h
12 bar
D e scr ipt ion
This type of boiler is constructed with a complete water cooled furnace, no
refractory liner whatsoever being required. The patent pin tube elements are
laced in a large diameter uptake tubes. These elements claim to increase the
efficiency of the boiler and at the same time enable a compact design to be
achieved.
It should be noted that these types of boilers have relatively thin shell plating in
comparison with the furnace and crown plates. Such boiler shells are
sometimes referred to as pressure envelopes, the total weight of the full boiler is
taken by the furnace structure rather than the shell.
Upper right, new pin tube elements in factory. See arrow for corresponding area in below drawing, of
furnace support brackets (yellow), Pin tubes waterside (red), pin tube from furnace (blue).
106
Sunrod CPDB boiler.
107
Sunrod CPH Boilers
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Sunrod
Type:
CPH
Steam capacity range:
700 - 35000 kg/h
Steam pressure:
up to 18 bar
D e scr ipt ion
The Sunrod CPDB type was further developed into the CPH type, one of the
incentives was to cope with the increasing demand for boilers with larger steam
capacity.
Sunrod CPH boiler
108
This boiler incorporates a water cooled furnace formed by a fusion welded tube
panel, membrane or mono tube walls. The tubes are connected at their lower
ends to a toroidal heater, and their upper ends are attached to the steam
chamber. A number of large downcomers ensures good circulation. The fire
tube (burner tube) is water cooled by a separate heater. Each of the uptake
tubes contains a pin tube element. As an example the CPH 140 type has 39
uptake tubes and can produce 30000 kg steam per hour.
Su r ve y poin t s a n d de fe ct s
The following depicts typical defects and areas for attention of the Sunrod CPH
and CPDB type boilers.
The locations of these defects are indicated by corresponding coloured circles in
the above drawing of the CPH boiler.
Picture of defect
Description
Pin t ube elem ent
The lower connection between
the pin tube element and uptake
tube is found leaking. Indicated
on the picture by white debris.
Sometimes this weld connection
is found corroded. Close up
inspection of these areas is
advised.
Cracks at down com ers
Cracks have been experienced in
way of the connection between
the down comers and the steam
chamber.
Also cracks have been found in
way of the weld between the U
section and shell plating.
109
Deform ed upt ake t ube
An overheated and deformed
uptake tube which needs to be
renewed. As a temporarily
repair the uptake tube can be
closed by inserting a cylindrical
plate at the top and bottom
(right side in picture). If
possible the pin tube element
should be removed, and holes
cut in the uptake tube for
cooling, otherwise the fitted
closing plates will overheat.
New uptake tube.
Grooving weld upt ake t ube
Corrosion and grooving is found
at the weld connection between
the uptake tube and the furnace
crown plate.
Deform at ion of furnace crown
Deformation is found in way of
outer ring of uptake tubes. In
severe cases cracks between
uptake tubes are found.
110
Aalborg AQ 3 Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
AQ 3
Heated surface range:
15 – 175 m
Steam capacity range:
800 – 8750 kg/h
Steam pressure range:
7 - 12 bar
Boiler weight range:
2900 – 180000 kg
D e scr ipt ion
The principle heated surface of the AQ 3 boiler is a cylindrical furnace, or
firebox, enclosed within the water space in the lower part of the boiler shell.
Basically, these boilers consist of a lower, or water chamber and an upper, or
steam/ water chamber. These two chambers are connected by a large number
of vertical water tubes and two large down comers. These down comers are
essential to ensure a high rate of circulation when maximum steam production is
required.
Aalborg AQ 3 boiler
111
About one third of the tubes are stay tubes, most of these are situated in a ring
as near as possible to the periphery of the tube plates because any outward
deflections of these plates, when under pressure, results in stress concentrations
in this area. The flue gases ascend through the elliptical flue pipe into the smoke
box.
Aalborg AQ 3 boiler
112
Where they are by means of baffle plates which are attached to the first row of
water tubes, evenly dispersed throughout the smoke box before escaping to the
chimney. The boiler is fitted with a large stay bar between the upper tube plate
and the shell crown plate. A further stay bar is fitted between the lower tube
plate and the furnace crown plate.
Su r ve y poin t s a n d de fe ct s
The following depicts typical defects and areas for attention of the Aalborg AQ 3
boilers.
The locations of these defects are indicated by corresponding coloured circles or
arrows in the above drawing of the boiler.
Picture of defect
Description
Deform ed furnace crown plat e
Furnace crown plate is the first
area affected by overheating.
Indents accumulate mud and
sludge which result in further
overheating.
Deformation seen from furnace
Cracking of welds
Deformation of the furnace crown
plate may cause cracking of stay
and flue pipe welds. Check these
welds with MPI if indents are
found.
113
Crack flue pipe w elds
Since this is an area of high heat
input cracks are often found in the
weld connections of the elliptical
flue gas pipe.
Crack in w eld upper t ube plat e
Cracks have been found in the
weld between the shell plate and
the upper tube plate. If allowed
to progress they may lead to very
dangerous situations.
Previous repair
Crack
Cracks in burner t ube
When the brickwork, quarl around
the burner is missing or in poor
condition the fire tube gets
overheated and cracks.
A number of manufacturers produce boilers of similar design to the Aalborg AQ 3
type. One of these types of boilers is the Osaka OVE.
Also the Hitachi Zosen HV, and the Helsingoskibs vertical boiler are very similar to
the AQ 3 boiler from Aalborg.
114
Osaka boiler, type OVE.
Aalborg AQ 9 Broiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
AQ 9
Thermal output range:
5.7 – 32.3 MW
Steam capacity range:
8000 – 45000 kg/h
Standard steam pressure range:
9 - 12 - 18 bar
Boiler weight range:
24500 – 90500 kg
D e scr ipt ion
The AQ 9 is widely used in diesel powered ships in conjunction with an exhaust
gas boiler installation.
Flow diagram of an oil fired AQ 9 boiler working in conjunction with an AV 6N exhaust gas boiler.
115
The oil fired boiler automatically supplements the steam supply, when the
demand exceeds the production of the exhaust gases. Furthermore the AQ 9 may
be used as steam space for the exhaust gas boiler.
Aalborg AQ 9 boiler
116
The boiler consists of a cylindrical furnace, a convection section and a water and
steam space.
In this boiler the furnace is surrounded by a closely placed row of membrane wall
tubes. The furnace tubes are welded at the bottom into a ring (toroidal) heater.
At the top the furnace tubes are welded into the lower tube plate of the
intermediate water chamber surrounding the cylindrical uptake through which the
hot flue gases pass to the convection chamber above.
The flue gases are drawn through the central uptake and, by means of baffle
plates attached to the vertical water tubes in the smoke box, are caused to flow
spirally across the tubes to the chimney.
Baffle plates
Flue gas flow in the convection tube bundle, flow is directed by baffle plates, black dots are stay
tubes.
The top of the intermediate water chamber acts as the lower tube plate for the
vertical water tubes forming the convection heating surface. The upper tube
plate, together with the cylindrical shell and flat top, form the steam / water
space of the boiler.
A gastight membrane wall encloses the convection heating surface and this wall is
fitted with a sufficient number of cleaning doors and a flanged exhaust gas outlet.
117
Effective natural circulation is secured by means of eight external unheated down
comers, connecting the steam / water chamber and the bottom ring heater.
Aalborg AQ 9 boiler
Su r ve y poin t s a n d de fe ct s
The following depicts typical defects and areas for attention of the Aalborg AQ 9
boilers.
118
The locations of these defects are indicated by corresponding coloured circles or
arrows in the above drawing of the boiler.
Picture of defect
Description
Cracks underside upt ake t ube
The under side of the uptake tube
is exposed to hot flue gases,
cracks have been found at this
location.
It is very difficult to see a possible
crack from the furnace floor, close
up inspection of the uptake tube
is absolutely necessary.
From the bottom all is in order
Deform at ion of t ube bends
These tube bend should be
inspected for bulging /
deformation, since they are
exposed to the hot flue gases.
119
Picture of defect
Description
Baffle plat es in convect ion space
In time these baffle plates are
found damaged and sometimes
burned away since their cooling is
very limited.
Missing or damaged baffle plates,
causes the flue gases to go
directly into the exhaust channel.
Baffle plate
Baffle plates
Toriodal heat hers
Ring heaters to be inspected via
hand holes. The pictures are
taken during an inspection with an
endoscope.
Aalborg AQ 12 Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
AQ 12
Thermal output range:
0.89 – 4.50 MW
Steam capacity range:
1250 – 6300 kg/h
Standard steam pressure range:
9 - 12 - 16 bar
Boiler weight range:
6800 – 24400 kg
120
Aalborg AQ 12 boiler
121
D e scr ipt ion
The AQ 12 has a cylindrical furnace which is completely water cooled, except for
the furnace bottom which is ready cast with refractory. Short stays are fitted
between the shell plate and furnace side wall for support on the larger designs.
Also a number of long stays are fitted between the furnace crown plate, and the
shell crown plate.
From the furnace the hot flue gases enter the convection section, which consists
of a rectangular bank of water tubes between the furnace and shell crown plates.
This convection bank is fitted inclined, in order to secure strong water circulation.
Illustration shows a sectional view of an Aalborg AQ 12 boiler.
Su r ve y poin t s a n d de fe ct s
The following depicts typical defects and areas for attention of the Aalborg AQ
12 boilers.
The locations of these defects are indicated by corresponding coloured circles or
arrows in the above drawing of the boiler.
122
Picture of defect
Description
Cracked, leaking st ays
Due to the inaccessibility it is not
possible to inspect these stays
during inspection of the water
space.
In order to detect a cracked stay,
a tell tale hole is drilled on the
ends of the stay.
Tell tale hole
Furthermore these tell tale holes
are normally covered with
insulation material. The first sign
of a cracked stay is water leakage
through the insulation material.
New stay
Corrosion furnace crown plat e
Mud and sludge accumulates on
the flat furnace crown plate, when
left unattended this may cause
corrosion pitting. Especially in way
of the welds, the heat affected
zone.
123
Picture of defect
Description
Cracks in convect ion sect ion
Cracks have been found at the
entry to the rectangular
convection section. Especially at
the four corners of the box. This
part of the convection section is
exposed to the hot flue gases and
cooling is limited.
New convection section
Blocked gas passage
The flue gas passage through the
convection section is blocked with
soot.
Aalborg AQ 18 Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
AQ 18
Thermal output range:
4200 – 3210 kW
Steam capacity range:
6300 – 45000 kg/h
Standard steam pressure range:
9 - 18 bar
124
Boiler weight range:
Fuel consumption range:
Thermal efficiency:
Flue gas flow:
Flue gas outlet temp (max/min load)
14200 – 63800 kg
450 – 3210 kg/h
84 % at 100% MCR
7200 – 52000 kg/h
370/220 – 390/250 °C
D e scr ipt ion
The AQ 18 boiler comprises many of the design details of the well known AQ 9
boiler. The water drum and the steam drum are cylindrical vessels with flat top
and bottom plates.
The new features of the AQ 18 are that it has two drums, the burner is top
mounted and the furnace is located in the centre of the generating tube bank.
The furnace consists of membrane wall panels.
Flow diagram for a steam atomizing burner plant.
The boiler is designed for steam atomizing oil burners, and as a standard the AQ
18 is supplied with a KBSA burner from Vesta AS. The KBSA utilizes the Y jet
atomizing principle in the burner nozzle. The layout of the top mounted burner
combined with the furnace which fits the flame makes it possible to obtain a
complete combustion of even the lowest fuel grades.
The advantages of the design with cylindrical vessels that have flat plates of equal
thickness are that stress concentrations in corners are minimized. As opposed to
designs with different plate geometry and thickness, cracks are avoided in
connections after many starts and stops of the burner during the boilers lifetime.
125
Aalborg AQ 18 boiler
126
The furnace and outer casing walls are made of polygonal membrane wall panels.
This gives a very sturdy construction which is gastight.
Sketch of the AQ 18 boiler
The generating bank is located between the furnace wall and the casing wall. The
generating bank consists of vertical tubes arranged in a staggered configuration.
The large size water drum supports the boiler. The advantage of the design with
a water drum is that headers are avoided, and therefore there is a safe circulation
to all tubes with no risk of blocking and subsequent burning of the furnace tubes.
127
The drum gives optimal space for the heating coil, this ensures a quick start up
and eliminates the risk of corrosion on the water and fire side. The upper drum is
furnished with the necessary internal fittings to ensure even distribution of feed
water and a sufficient dryness of the steam. The upper drum carries all essential
boiler mountings.
Su r ve y poin t s a n d de fe ct s
The following depicts typical defects and areas for attention of the Aalborg AQ
18 boilers.
The locations of these defects are indicated by corresponding coloured circles or
arrows in the above drawing of the boiler.
Picture of defect
Description
Deform ed lower st eam drum Plat e
The bottom plate of the steam
drum is exposed to the hot flue
gasses and deformations are
found in between the stay tubes.
The deformations also cause
overstressing of the stay tube
welds and the burner tube welds.
128
Deform ed leaking t ubes
Bulged and leaking tubes are
sometimes found in the
membrane wall.
Leaking tube
Check tube walls with torch.
Mission OS Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
Mission OC
Steam capacity range:
1600 – 6500 kg/h
Steam pressure:
10 bar
Boiler weight range:
7000– 17200 kg
D e scr ipt ion
Mission OS is a high performance oil fired auxiliary boiler in the low capacity
range up to 6.5 t/h. It is designed as a vertical cylindrical boiler. The boiler shell
surrounds the cylindrical furnace and the convective section consisting of pintube elements.
The pin tube elements are an integrated part of the boiler design. The pin tube
elements support the furnace and boiler top plate. The design has been
significantly optimised to achieve lower weight and improved strength. The result
is a longer lifetime of the pin tube by a factor of 8.
The boiler pressure part is made of well proven mild steel with elevated
temperature properties.
The burner housing is mounted on the boiler front, angled 15 degrees downward
against the furnace bottom. This design allows for a long flame and gives better
129
utilisation of the furnace and the flow of the combustion particles becomes
optimal. The result is high performance combustion even with the lowest grades
of fuel.
Mission OS Boiler
Mission OM Boiler
Pa r t icu la r s 1 1 ba r ve r sion
Make:
Type:
Steam capacity range:
Thermal output range:
Steam pressure:
Boiler weight range:
(Various steam production size ranges are offered within a design.)
Pa r t icu la r s 1 8 ba r ve r sion
Make:
Type:
Steam capacity range:
Thermal output range:
Steam pressure:
Boiler weight range:
(Various steam production size ranges are offered within a design.)
Aalborg
Mission OC
8000 – 20000 kg/h
5600 – 14000 kW
11 bar
19000– 35000 kg
Aalborg
Mission OC
14000 – 45000 kg/h
9800 – 31500 kW
18 bar
27300– 62700 kg
130
Sketch of the Mission OM boiler, 11 bar version.
Mission OM is a vertical cylindrical medium size steam boiler in the 8 - 45 t/h
capacity range. The boiler is constructed from standard modules and comes
preassembled in order to minimise installation time.
Sketch of the Mission OM boiler, 18 bar version.
131
The furnace features gas tight membrane walls. A sufficient number of down
comers ensure safe natural circulation.
The boiler is equipped with integrated pin-tube elements in the convection
section. Furthermore, the pin tube elements support both the furnace and the
boiler top plate.
Above improved pin tube design used in the Mission OS and OM, right burner opening in furnace wall.
The standard boiler is delivered with an Aalborg Industries rotary cup or steam
atomizing burner and a control system, ensuring fully automatic operation and
control. The burner operates on diesel oil, heavy fuel oil or gas and is designed
for a high turn down ratio with complete combustion at low oxygen levels.
Mission OL Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
Mission OL
Steam capacity range:
12500 – 55000 kg/h
Thermal output range:
8800 – 38800 kW
Steam pressure:
18 bar
Boiler weight range:
25400– 78000 kg
D e scr ipt ion
The slim top-fired design makes the boiler perfect for use in e.g. tankers.
132
The Mission OL is a lightweight boiler, requiring a minimum of deck space.
Aalborg Mission OL boiler
The two drum cylindrical boiler design, features straight tubes directly connecting
the water and steam drums. This ensures safe circulation with no risk of
overheating and subsequent burnout of the tubes. Water circulation is further
enhanced by external non heated down comers.
On the left tube with extended pins used in the Mission OC, and right a Mission OC boiler under construction.
133
Thanks to gastight, polygonal membrane wall panels the sturdily constructed
furnace is resistant to gas pulsations. The integrated burner design reduces
refractory to a protective layer at the bottom of the furnace and around the
access doors.
The convective section consists of straight tubes. These tubes have been
extended by pins bent to create a unique flue gas flow, enhancing the heat
transfer and lowering the pressure loss across the convective section. The result
is a compact and efficient heat transfer surface.
The Mission OC is designed with a high flue gas velocity in the convection tube
bank, which causes a self cleaning effect on the heat transfer surface.
Soot blowing equipment is supplied with the boiler in a standard delivery. Water
washing of the heating surface can easily be carried out from the top of the tube
bank.
The advanced KBSD steam atomising burner is used. It has been specially
designed to fit the Mission OL boiler, and providing excellent burner
performance.
Operation and control of the
Mission OL boiler plant are facilitated
by a reliable and user friendly
microprocessor based control
system. A PC, conveniently placed in
the control room, with a graphical
user interface enables remote control
and monitoring of the boiler and
burner plant. From the system all
kinds of diagnostics and statistical
data may be retrieved, which is an
invaluable help in fault tracing
situations.
Parat Halvorsen MW Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Parat Halvorsen AS
Type:
MW Water Tube Boiler
Steam capacity range:
8000 – 25000 kg/h
Steam pressure:
7 bar
Boiler weight range:
21000– 45000 kg
D e scr ipt ion
The Parat MW is designed for chemical tankers, small to medium size crude oil
tankers, FPSO`s, and cruise liners.
It is a fully automated steam boiler, with a well dimensioned steam drum which
makes the boiler suitable for evaporating steam from other exhaust gas boilers.
The boiler is supplied with a circular furnace, has a convection section and is
designed for forced draught.
It operates with natural circulation, and a sufficient number of down comers are
installed ensure proper circulation.
As insulation material, rock wool is used jacketed with galvanized sheet metal
plates.
134
Parat MW water tube boiler
135
Types of Vertical Composite Boilers
A composite boiler is able to be operated on either diesel engine exhaust gases or
oil, or both, when necessary. They normally consist of an exhaust gas fired and
oil fired section, built within one boiler shell.
Aalborg AQ 5 Boiler
Aalborg AQ 5 Composite boiler.
136
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
AQ 5
Steam pressure range:
7 – 18 bar
Heated surface oil fired section:
75 m²
Heated surface of exhaust gas section:
200 m²
Steam production oil fired section:
up to 4000 kg/h
Steam production exhaust section:
up to 2500 kg/h
The steam production of exhaust gas section depends on the amount and
temperature of the exhaust gases.
D e scr ipt ion
This boiler is a composite version of the AQ 3 type, in which the pressure shell is
in the form of three cylindrical drums. The lower drum contains the oil fired
furnace, the centre drum, which is made with two tube plates, is essentially a
water space, whilst the upper drum is a steam and water space.
Sectional view of an AQ 5 boiler
137
The tube plate of the lower drum and the lower tube plate of the centre drum
form the boundaries of the smoke box and oil fired section, whilst the upper tube
plate of the centre drum and the tube plate of the upper drum form the
boundaries of the exhaust gas section. The three sections are connected by
straight plain and stay tubes, and two large down comers which connect the
upper and lower drums to promote efficient circulation.
Su r ve y poin t s a n d de fe ct s
The following depict typical defects and areas for attention of the Aalborg AQ
boilers.
The location of these defects is indicated by corresponding coloured circles in the
above boiler drawing.
Picture of Defect
Description
Cracks in down com ers
Differential expansion between
the stay plate, generating tubes,
and the down comers, result in
cracks in the down comers.
These cracks are frequently
found in way of the welds.
Crack in down comer
138
St ay arrangem ent
The stay arrangement between
the shell sections and tube
plates requires careful attention.
A number of manufacturers produce boilers of similar design as the Aalborg AQ 5.
One of these types of boilers is the Osaka Composite boiler type OEVC.
Osaka OEVC Boiler
Aalborg AQ 16 Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
AQ 16
Steam production oil fired section:
up to 6300 kg/h
D e scr ipt ion
The AQ 16 is a combination of two well known boiler types out of the QA series.
In principle the AQ 16 is a combination of the AQ 12 and AQ 7 exhaust gas
139
boiler. The oil fired part is of the AQ 12 design, and the exhaust part of the AQ 7
design which is installed adjacent to this.
As an option the boiler can be combined with a compact silences. Since the
exhaust part is equipped with smoke tubes no circulation pumps are necessary.
Schematic drawing of a AQ 16 boiler with silencer
140
Aalborg Mission OC Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
Mission OC
Steam pressure:
9 bar
Steam production oil fired section:
750 – 5000 kg/h
Steam production exhaust section:
up to 3000 kg/h
D e scr ipt ion
MISSION™ OC is a vertical boiler with an exhaust gas section consisting of
smoke tube. The cylindrical shell surrounds the smoke tubes, the furnace, the
steam space and the convective section consisting of pin-tube elements. These
pin tube elements support both the furnace and the boiler top plate. The boiler
pressure part is made of well-proven mild steel with elevated temperature
properties. Stress concentrations in corners are minimised by the simple design
of cylindrical shells with flat plates of equal thickness.
It operates with a mono bloc type pressure-jet burner. The burner housing is
mounted on the boiler front, angled 15 degrees downwards against the furnace
bottom. This allows for a long flame and offers better utilisation of the furnace.
The flow of the combusting particles becomes optimal, and this results in a high
performance combustion, even with the lowest grades of fuel. The MISSION™
OC is also available with a rotary-cup burner.
The composite boiler can be supplied with a compact silencer to suit any type of
diesel engine.
141
Schematic drawing of a Mission OC boiler
Mission OC boiler at different stages during building.
142
Su r ve y poin t s a n d de fe ct s
The following depicts typical defects and areas for attention of the Mission OC
boilers. The location of these defects is indicated by corresponding coloured
arrow in the above drawing.
Picture of Defect
Description
Blocked sm oke t ubes
Smoke tubes blocked by soot
needs to be cleaned.
Parat MC Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Parat Halvorsen AS
Type:
MC Combined Oil / Exhaust fires
Steam production oil fired section:
5000 kg/h
Steam production exhaust section:
3500 kg/h
D e scr ipt ion
The principle of the boiler is based on a common water
and steam space and separate sections for the oil fired
and exhaust gas. If several engines are utilized, the
boiler can be delivered with several exhaust gas
sections.
The boiler comprises of smoke tubes for both the oil fired
and exhaust gas sections.
Parat MC Boiler
143
Schematic view of a Parat MC boiler
144
Types of Two Drum Water Tube Boilers
Mission D Type Boiler
Pa r t icu la r s (Various steam production and pressure size ranges are offered within a design.)
Make:
Aalborg
Type:
Mission D Type
Steam capacity range:
25000 – 120000 kg/h
Thermal output range:
17600 – 84700 kW
Steam pressure:
18 bar
Thermal efficiency:
84% at 100% MCR
Boiler weight range:
39600 – 120900 kg
D e scr ipt ion
The D-type boiler is designed especially for tanker operation, it produces steam
for:
• Cargo pump turbines
• Heating
• Tank cleaning, as well as the production of inert gas.
Schematic drawing of a Mission D Type boiler.
The boiler consists of a steam and water drum connected by a generating tube
bank. The furnace is made of membrane walls forming a fully water cooled gastight furnace. Refractory is limited to a protective layer on the deflected tubes
around the inspection and access doors, and around the burner opening.
Large unheated external down comers, welded to the drums, secure natural
water circulation with a large circulation ratio.
145
The convective section is arranged between the drums. The heating surface in
the centre of the convective section has been extended with pins, enhancing heat
transfer and creating a unique lengthwise flue gas flow. The result: a compact,
highly effective heat transfer surface. High flue gas velocity in the convection
tube bank causes a self cleaning effect on the heat transfer surface. Soot
blowing equipment is supplied with the boiler in a standard delivery.
The Mission D-type boiler comes with the advanced KBSD steam atomising
burner. This burner has been designed to fit large capacity Mission boilers,
including the Mission D-type boiler.
Thorough analysis with the CFD (Computational Fluid Dynamics) program, a
unique flame structure and geometrically optimum wind box and register was
designed. This ensures even air distribution, minimum pressure loss and a large
turn down ratio.
Right, Used steam generating tube with pins, Left a CFD analysis of the burner.
Mission D Type boiler ready for shipping
146
Chapter 4: Combustion and Atomizers
Introduction
A boiler requires a source of heat at a sufficient temperature to produce steam.
Fossil fuels, heavy fuel oil or diesel oil are burned directly in the furnace to
provide this heat, although waste energy from another source (e.g. exhaust
gases) may also be used.
Combustion
Combustion is defined as the rapid chemical combination of oxygen with the
combustible elements of a fuel. There are just three combustible elements of
significance in fossil fuels used onboard ships: carbon, hydrogen and sulfur.
Sulfur is of minor significance as a heat source, but a major contributor to
corrosion and pollution problems.
The objective of good combustion is to release all of the energy in the fuel while
minimizing losses from combustion imperfections and excess air. The
combination of the combustible fuel elements and compounds in the fuel with all
the oxygen requires t em perat ures high enough to ignite the constituents, mixing
or t urbulence to provide intimate oxygen / fuel contact, and sufficient t im e to
complete the process. This is sometimes referred to as the three Ts
(temperature, turbulence and time) of combustion.
In contrast to diesel engines, boilers operate with a continuous combustion
process. Consequently, ignition performance of the fuel is irrelevant, and due to
the non cyclic performance there is no need to sweep away the products of
combustion before more fuel can be introduced, as the whole process is one of
continuous supply of air and fuel. Combustion can therefore take place with
much lower air to fuel ratios than with diesel engines.
Although only a limited level of fuel oil preheating (when operating on heavy fuel
oil) is necessary with boilers plants compared to diesel engines, it is of equal
importance to satisfactory combustion performance. With boiler plants the
intended level of atomization is relatively coarse with a droplet size of around 50
to 100 μm. This slightly increases the allowable viscosity, usually in the range of
15 to 30 cSt depending on the burner design, with a corresponding reduction in
pre-heat temperature.
Irrespective of the burner type, the overall intention is that the finely divided and
swirling fuel oil spray produced by the atomizer, will be thoroughly mixed with
the turbulent primary air (motion imparted by the air register swirl or diffuser
plate) to give a short wide flame, enveloped initially by a equally turbulent
secondary air supply. When the fuel is sprayed into the combustion chamber, its
temperature rises as it approaches the previously introduced fuel oil, which is
now burning. The outer shell of an individual fuel oil droplet evaporates to form
a surrounding cloud of vapour. When this vapour reaches its auto ignition
temperature, combustion commences at those points where the ratio is
stoichiometric, surrounding the droplets with a zone of combustion. Further
evaporation of the droplet (now accelerated by the heat from the combustion
zone) will supply combustible material to one side of this zone, while the inward
diffusion of air supplies the necessary oxygen to the other. The supply of this air
147
changes from primary to secondary air as the kinetic energy imparted on
injection propels the droplets through the furnace.
Schematic diagram of an atomizer with air register showing air flow, primary and secondary flame.
If the viscosity of the oil leaving the burner unit is too high, there will be an
increase in droplet size, leading to unstable combustion control, poor
combustion, and smoke and deposit formation. Too low viscosity results in poor
distribution (penetration) of the flame within the furnace.
Flame blow off and flashback restrict the maximum and minimum fuel oil flow
rates through any given burner. In case of flame blow off, the air / fuel mixture
velocity exceeds the flame speed, while in flashback the converse applies.
With boilers, other than some small package boilers the combustion air is
normally pre-heated, this can be up to 130˚C. Pre-heating the air raises the
boilers overall thermal efficiency, gives a higher specific steam generation
capacity, enhanced load control and improves combustion performance by
ensuring a more complete utilization of the fuel oil and decreases fouling.
Increased pre-heating also acts to decrease the tendency of SO2 to further
oxidize to SO3 in the exhaust gas stream, reducing the cold corrosion potential.
Boiler tube fouling, whether a result of incorrect pre-heating or the use of fuel
oils with a high carbon residue value, not only inhibits tube surface heat transfer,
making regular soot blowing necessary, but can also accumulate on the air swirl
plates and atomizer leading to poor air / fuel mixture and increasing the
probability of yet further fouling.
In the most extreme case of poor combustion or failure to ensure regular and
thorough soot blowing, the accumulation of carbonaceous deposits particularly in
way of the cooled sections of the uptake can result in soot and hydrogen fires.
148
Conventional multi boiler fuel oil system from Saacke.
Atomizers
The following atomizers are commonly employed on auxiliary boilers.
•
•
•
Spill type pressure jets.
Spinning cub atomizer.
Steam assisted pressure jets.
Spill t ype pr e ssu r e j e t s a t om ize r
The pressure head of the fuel oil is converted into a velocity head as it passes
through the small tangential holes at the atomizer tip. In addition, the holes
impair a swirling motion to the oil, the discharge from the nozzle being broken up
into a fine mist by centrifugal force. The burner is provided with a leak off from
the swirl chamber. By increasing the amount of leak off, the amount of fuel oil
that is delivered to the furnace is reduced without seriously impairing the
atomization.
149
Aalborg KBPJ spill type pressure jet burner, capacity range 0.35 to 5.25 MW.
Spill type burner, the oil enters centrally and spill oil leaves through annular passage in burner
body.
150
Spin n in g cu b a t om ize r
The working principle of rotary cub burners is based on atomising by centrifugal
force. The atomising cub is driven at high speed via a heavy-duty belt drive.
The oil is gently positioned at low pressure into the spinning cub where
gradually, and forced by the centrifugal action of the cub, it moves forward until
it is thrown off the cub rim as a very fine, uniform film. The high velocity
primary air discharged around the cub strikes the oil film, breaks it up and
converts it into a mist of fine particles which are introduced into the combustion
zone, and burner. The secondary air necessary for complete combustion is
supplied by a forced-draught fan through the wind box and burner air register.
Aalborg KB rotary cub burner, capacity range 0.55 to 38 MW.
Aalborg Rotary cub burner.
151
Rotary cup burner from Saacke used for fuel oil.
Front and back of Aalborg KB Rotary cub burner.
St e a m a ssist e d pr e ssu r e j e t s At om ize r s
Low pressure steam is used in this type of atomiser to increase the effectiveness
of fuel oil pressure as a means of obtaining atomization. These atomisers have
several advantages, it is claimed that their use results in a cleaner boiler and
they require lower fuel pressures. But their main disadvantage is the absorption
of up to 1% of the steam output and this of course is loss of fresh water.
152
Aalborg KBSA Steam atomised burner, capacity range 1.7 to 46.9 MW.
Burner tip of a skew jet atomizer.
153
The KBSD steam atomising burner is use on top-fired boilers, capacity range 1.7 to 46.6 MW.
I gn it ion bur n e r
Starting initial combustion or lighting the main burner requires an independent
source of ignition. As a minimum the ignition burner assembly consist of a fuel
nozzle, spark ignition source and an energy source to produce the spark. The
ignition burner is operated on gas oil, and the arrangement can be stationary or
equipped with a retracting mechanism for protection from furnace radiation.
Programmable ignition controls are installed that automatically sequence all
functions including on / off of the fuel, atomizing and purge medium, lighting and
spark probe insertion and retraction. A manual ignition control is normally fitted
for emergencies.
Ignition burner seen
from the furnace (right)
and from the burner
front (left).
154
Burner Safety Systems
Automated sequence of operation, interlocking and alarm systems are of prime
importance in safe and reliable operation of the oil fired combustion system. In
observance of the DNV rules and recommended practice of operation, an
automated control system should include the following.
• Purge interlock requiring a specified minimum air flow for a specific time
period to purge the furnace before the fuel trip valve is opened.
• A spark producing device must be in operation before introducing any fuel
to the furnace (fuel trip valve is opened). The ignition source must
continuously provide a flame or spark of adequate size until combustion of
the main burner is self sustaining or a specified time period has elapsed,
resulting in ignition failure alarm and fuel trip valve closure.
• Flame detector connected to an alarm (flame failure) and interlocked to
shut off the burner fuel valve upon loss of flame.
• After flame failure or unsuccessful ignition automatic restart is not to take
place until local reset on the control panel has taken place.
• The burner door or front is equipped with a limit switch which shuts off the
fuel supply and gives an alarm when the burner is swinged out or
retracted off its position.
Right, limit switch of burner door. Left, flame detector.
•
•
•
A positive air flow through the burner into the furnace and up the stack
must be maintained at all times. Therefore failure of the forced draft fan
will cause an alarm and shutoff of fuel supply.
Adequate fuel pressure and temperature for proper atomization must be
maintained at all times. Shutoff of fuel supply and applicable alarm must
be raised in case of low fuel pressure and temperature.
Apart from the above items which are directly connected to the oil firing
combustion system, the following conditions will also result in closure of
the fuel trip valve and sounding of the applicable alarm
¾ Water level low.
¾ Steam pressure high.
155
•
¾ Forced circulation failure.
Automatic start and stop of the oil fired combustion system controlled by a
set upper and lower value of the steam pressure.
Oil Fired Combustion System Survey
The complete inspection can be divided into the following two separate parts.
1. Visual survey of the combustion system.
2. Function test of the burner unit and its safety functions.
Visu a l Su r ve y
The following contains some common defects and points to be taken in to
consideration while inspecting oil fired combustion systems.
•
Inspect the burner front for signs of fuel oil leakages at flanges and filters,
fuel oil soaked insulation material should be renewed. Also drip pans
underneath burner, fuel filters and fuel pumps should be clean and free of
fuel oil.
Check burner front, fuel pipes, and drip tray for fuel leakages.
•
Fire detection and extinguishing equipment in way of burner front to be
inspected. Above the burner unit there is normally a fire detector and a
fixed fire extinguishing system (CO2 or foam nozzle) installed. Also in the
vicinity of the burner there should be a red sand box, check the content
because sometimes this is used to store other items or the sand is used
and contaminated in which case it has to be renewed.
156
•
Swing out or retract the burner unit and inspect the atomiser tip, it must
be free from carbon residues which will impair fuel atomization leading to
poor combustion. Also the air register swirl vans and tip plates (swirler)
are prone to carbon residue clogging, sometimes swirl vanes and tip plates
are found partly or completely burned away. Needless to say that this will
result in a deficient air / fuel mixture causing further fouling of the burner
unit and heated surfaces. And incase the flame is deflected there is the
risk of flame impingement, overheating of furnace walls.
Air register with swirl vans and tip plates, inspect for clogging, bent and burned vanes.
•
Survey the refractory quarl around the burner from inside the furnace,
sometimes it is found cracked or big pieces of refractory material are
missing. This refractory quarl serves two purposes, firstly to protect the
steel underneath the refractory against radiation heat and secondly to
improve combustion by deflecting the heat back into the combustion zone.
157
Refractory quarl around the burner which is due for renewal.
Fu n ct ion Te st
During function test of the oil fired combustion system the following to be
observed.
•
•
Start the burner unit and check that the automated start up sequence is
followed, purging the furnace and ignition of main burner, in case of
unsuccessful ignition an alarm should be raised and fuel trip valve closed.
Most furnace explosions are caused by improper purging of the furnace
before ignition, therefore this is an important safety function and it should
not be by-passed as sometimes seen.
Once the burner is in operation watch the flame through the peep hole
opposite the burner. The flame should burn stable without flickering, or
producing black smoke. It should have a bright yellow colour, a dark
yellow flame indicates too high CO emission levels meaning reduced
combustion efficiency. Flame blow off or blow back are caused
respectively by too high or too low combustion air flow.
158
Peep hole to observe the flame.
•
The following safety function can be tested as described below.
¾ Flame failure alarm and fuel valve trip by removing the flame
detector from its housing.
Ignition burner with flame detector.
¾ Emergency stop burner by operating the stop button on the local
control panel or near the burner unit.
¾ Burner door open alarm by opening the door, while this alarm is
activated it should not be possible to start the burner.
¾ Low combustion air flow alarm by closing off (if valve is fitted) the
air to the pressostat and drain it, fuel trip valve should close.
¾ High / low fuel pressure and temperature by closing the valves to
the pressostat and thermostat and drain them, alarm should be
activated. Set points of the pressostat and thermostat can be
checked with calibration equipment.
159
Chapter 5: Refractories and insulation
Introduction
There is perhaps the tendency when surveying auxiliary boilers to think too much
of potential dangers through deterioration of pressure parts and not pay enough
attention to the heat retaining envelope which consists of refractory in the
furnace and an insulation system on the outside.
Refractories
Refractory materials are used for their insulating and erosion resisting properties.
It is a material that will retain its solid state even at very high temperatures,
which in boiler furnaces can be as high as 1300 °C.
Although in recent years the amount of refractory in boilers has been drastically
reduced by the introduction of membrane tube walls, heat resistant steel grades
and better designs. Still in every boiler one finds a small amount of refractory in
the furnace which fulfils one or all of the following purposes.
•
•
Protect the underneath boiler material from overheating and distortion.
Refractory is found in way of headers, boiler drums, floors and usually
around the burner.
From baffles to direct the gas flow, this application is mainly used in case
of water tube boilers.
Refractory materials are made of naturally occurring clay composed of alumina,
silica and quartz. The material properties vary considerably and are largely
dependent on the proportion of alumina present.
In present day auxiliary boilers we find firebricks only on the furnace floor.
Mouldable or plastic refractory is nowadays widely used, it must be pounded into
place during installation and is found in way of headers, floors, burners etc.
During start up care must be taken with the fire rate to allow the newly installed
mouldable refractory to properly dry and cure.
Su r ve y of r e fr a ct or y
When visually inspecting refractory inside the furnace, look for large cracks and
missing pieces, small hairline cracks are to be expected. For safe and reliable
boiler operation the refractory should be in a good condition, this in order to
prevent overheating and deformations which may result in costly repairs.
Insulation
Heat loss from a boiler is reduced by fitting insulation material on the outside, it
is an economical balance between the value of heat loss and cost of insulation
work. Insulation systems are designed to provide both safety for the personnel
and minimum heat loss.
A much used material is mineral wool which is comprised of molten slag, glass or
rock blown into fibers or spun. It is supplied in the form of bocks or blankets and
when installed covered with a sheet metal lagging for protection.
160
Su r ve y of I n su la t ion
Upon inspecting the boiler insulation take note of the following.
• Damaged or missing insulation or sheet metal lagging should be repaired
or placed back. This is particularly important where exposed temperatures
are over 220 °C and there is a risk of fuel impingement of the insulation
material.
• Soot spots on the sheet metal lagging indicate a flue gas leakage and
should be investigated.
• Corrosion of the sheet metal lagging indicate a water leakage underneath
the insulation, this should be further investigated. Also pay particular
attention too leaking valve glands, if this is allowed to continue for some
time it may lead to sever corrosion of the boiler shell in way of the
penetrations. In the past this has lead to very dangerous situations since
this corrosion was not noticed due to the fact it is covered by insulation
material.
Leaking valves or manholes as illustrated above can cause serious corrosion of the boiler shell.
•
Burned and flaked off paint from the sheet metal lagging may indicate a
hot spot caused by steam leakage which must be investigated.
161
Chapter 6: Boiler Mountings and Fittings Boiler
Introduction
All marine boilers are required to be fitted with certain essential mountings with
the objective to improve boiler, operation, efficiency, and safety. In this chapter
these required mountings, their inspection and testing will be discussed.
Det Norske Veritas, other Classification Societies, and National Boiler Authorities
have rules and standards for these mountings. These rules and Standards
govern the type, number, construction, and certification of the required
mountings. Nowadays these governing rules are, to a large extent, harmonized
between the different Authorities, especially with regard to type and number of
required mountings.
For Det Norske Veritas these requirements can be found in Part 4, Chapter 7,
Section 6 of the DNV Rules (January 2003 publication), and are the basis for this
chapter.
Safety Valves
The most critical valve on a boiler is undoubtably the safety valve. Its primary
function is to protect life and property by limiting the internal boiler pressure to a
point below its safe operating level.
Proper maintenance, adjustment, and testing of these valves is essential to
guarantee their correct operation when needed. It is literally the last safety
before a boiler explosion. Popping of the safety valves means that all other
safety systems such as burner control, burner shut down, high steam pressure
alarm, have failed to stop the steam pressure from rising above normal operation
levels.
Ba sic ope r a t ion of a sa fe t y va lve
The basic spring loaded safety valve also called standard or conventional type is
a simple and reliable self acting overpressure protecting device.
Typical safety valve designs for
steam application according to
DIN and ASME Standards.
162
When the steam pressure rises above the set pressure of the safety valve, the
disc begins to lift from its seat. This however causes a contraction of the spring
and results in an increase of spring force, meaning that the pressure has to
continue to rise before any further lift is possible. The additional pressure rise
required before the safety valve will discharge at its rated capacity is called
overpressure. All spring loaded safety valves make use of a shroud (lip) on the
periphery of the disk (valve lid) for the purpose of giving the disk additional
uplift once raised from their seat by the steam.
Typical disk and shroud arrangement used on rapid opening safety valves.
163
The volume contained within this shroud is known as the control or huddling
chamber. As lift begins the steam enters this chamber and a larger area of the
shroud is exposed to the steam pressure. This incremental increase in opening
force overcompensates for the increase in spring force, causing rapid opening.
Simultaneously, the shroud reverses the flow direction of the stream which
provides a reaction force, which further enhances the lift.
Operation of a conventional safety valve.
Once normal steam pressure is restored the valve is required to close again, but
due to the increased lift this will not happen until the pressure has dropped
under the original set pressure. The difference between the set pressure and
the reseating pressure is known as blowdown.
Relationship between pressure and lift for a safety valve.
The blowdown rings found on most ASME type safety valves are used to make
fine adjustments to the overpressure and blowdown values of the safety valve.
The lower blowdown ring is a common feature on many high capacity direct
loaded spring valves and its adjustment governs the blowdown value, thereby
limiting usable steam losses. The upper ring is usually factory set and
essentially negates manufactures tolerances which affect the geometry of the
huddling chamber.
164
The blowdown rings on a ASME type safety valve.
Industry standards and rules that govern the design of safety valves, generally
only define the following three dimensions that relate to the discharge capacity
of the safety valve.
1. Flow area: The minimum cross sectional area between the inlet and the
seat, at its narrowest point (d in below figure).
2. Curt ain area: The area of the cylindrical or conical discharge opening
between the seating surfaces created by the lift of the disk above the seat
(d1 in below figure).
3. Discharge area: This is the lesser of the curtain and flow area, which
determines the flow through the valve.
Illustration of the standard defined areas.
Type s of sa fe t y va lve s
The valve inlet design can be either a full nozzle or a semi nozzle type.
165
A full nozzle valve (a) and a semi nozzle valve (b).
Full nozzles are usually incorporated in safety valve designs for high pressures.
The approach channel and disk are the only part of the valve exposed to high
steam pressures and temperatures when in closed position. This makes it
possible to use less expensive materials for the body. The advantage of semi
nozzle design is that the seat can be easily replaced, without renewing the
whole inlet.
The terms full lift, high lift and low lift valves refer to the amount of travel the
disc undergoes as it moves from its closed position to the position required to
produce the full discharge capacity, and how this affects the discharge capacity
of the valve.
1. Full lift : The disk lifts sufficiently so that the curtain area no longer
influences the discharge area. Therefore the discharge capacity is
subsequently determined by the bore diameter. This occurs when the
disks lifts a distance of at least a quarter of the bore diameter.
2. High lift : The disk lifts a distance of at least /12th of the bore diameter.
This means that the curtain area, and ultimately the position of the disk,
determines the discharge area.
3. Low lift : The disk lifts only a distance of /24th of the bore diameter. The
discharge area is determined entirely by the disk position.
The discharge capacity of high lift valves tends to be significantly lower than
those of full lift valves. For a given capacity therefore one can usually select a
full lift valve that has a nominal size several times smaller than the
corresponding high lift valves. The size of the waste steam pipe and that of the
apertures in the boiler shell are also correspondingly reduced.
This ability of the valve to lift fully is achieved by allowing the steam from initial
lift to impinge on additional lifting surfaces, either in the form of a lip, shroud, or
pistons in guided cylinder.
For auxiliary boilers of moderate pressures the conventional full lift safety valve
is today commonly encountered in service.
166
167
I n st a lla t ion
As per rule requirement each boiler has to be equipped with two safety valves.
Sometimes these are fitted in one chest which is directly connected to the boiler
shell. When the heated boiler surface is less than 10 m², only one safety valve
is required, this is not often encountered in the field. If the boiler is equipped
with a superheater then at least one safety valve must be fitted on the outlet
side. The later also applies for an exhaust gas boiler (economizer), at least one
safety valve is necessary with the discharge capacity equal to the maximum
steam production.
Easing gears need to be fitted on all safety valves which can be operated from
the boiler control position. The operating handles are normally found in way of
the burner, or steam pressure gauge.
Examples of safety valve installation.
Each safety valve has its own waste steam pipe discharging to the atmosphere,
it has to be suitably supported and where necessary fitted with expansion joints
to prevent undue loading of the valve chest. In the past accidents have
happened as a result of safety valves connected to a common waste steam pipe.
The blowdown of one safety valve will cause a back pressure in the common
waste steam pipe, and this will influence the functioning of the other connected
safety valves, resulting in delayed popping (or not opening at all) of the valve.
The rules permit the use of a common waste steam pipe but have stipulated the
installation of so called balanced safety valves. Balanced safety valves are those
that incorporate means of eliminating the effect of backpressure. On board
ships we do not see many balanced safety valves since it is common practice to
install individual waste seam pipes.
168
A drain pipe is fitted on the lowest part of each valve chest on the discharge
side, which is usually led by a continuous fall to the bilge or below the floor
plate. This prevents accumulation of condensate in the valve chest causing
corrosion, and in large amounts may influence opening of the valve. It is
obvious that these drain lines should be kept free of blockage and no valves or
chocks must be fitted.
Ce r t ifica t ion a n d st a n da r ds
Most countries have their independent authorized approval bodies who examine
the design and performance of safety valves to confirm conformity with the
relevant standard or code. For steam boiler application there are very specific
requirements for safety valve performance in the different standards and codes.
Examples of standards related to safety valves are, BS 6759, JIS B 8210, ASME
I, DIN 3320, ISO 4126.
Safety valve standards are normally very specific about the information that
must be carried on the valve. Marking is mandatory on both the shell, usually
cast or stamped, and the name plate which must be securely attached to the
valve. A general summary of the information required is listed below.
On the shell:
• Size, pressure designation
• Material designation
• Direction of flow by arrow
On identification plate:
• Manufacturer, model type.
• Date of manufacturer and serial number.
• Certified discharge capacity.
• Number of relevant standard.
On the left marking on the valve body, and on the right an example of an identification plate.
In the event a safety valve has to be exchanged with one of another brand or
type, is of utmost importance to confirm that the new valve has the same or
greater discharge capacity as the previous one. Also the inlet and outlet
connections should be checked that they by no means restrict the steam flow.
169
According to the DNV Rules (July 2004), the safety valves should be provided
with the following documentation.
• All valves with Dn> 100 mm and p>16 bar need to be supplied with a
DNV Product Certificate.
• The manufacturer has to provide a certificate stating the rated valve
capacity at the approved boiler pressure and temperature.
• All valves need to be hydrostatically pressure tested at 1.5 times the
nominal pressure. If not witnessed by a Surveyor this is confirmed by a
Test Report from the maker.
• Material Certificates are required to be provided as follows.
1. Steel valves Dn > 100 mm, Td > 400 °C – NV Material Certificate.
2. Steel valves Dn<= 100 mm, Td>400 °C – Material Test report.
3. Steel or nodular cast iron valves Dn > 100 mm, Td <= 400 °C –
Material Work Certificate.
4. Steel or nodular cast iron valves Dn <= 100 mm, Td <= 400 °C –
Material Test report.
5. .Copper alloy valves Dn > 50 mm, – Material Test report.
6. Copper alloy valves Dn <= 50 mm, – Material Work Certificate.
The pressure and temperature ratings of the safety valve should be in
accordance with a recognized national standard, as for instance DIN 2401 or
ANSI 16.5-1958, API STD 600-1957. Reference is made to the enclosed
pressure / temperature ratings according to mentioned standards at end of this
chapter.
Su r ve y of sa fe t y va lve s
Part of the boiler survey is opening up and inspection of the individual parts of
the safety valves. Occasionally it happens that people feel it’s not necessary to
disassemble the spring house assembly. A reason given for this is to save the
original setting, so adjustment of the valve can be omitted. From the above it is
clear that inspection of these critical parts (spring, spindle) is absolutely
necessary and therefore the complete valve is to be disassembled.
When overhauling safety valves it is good engineering practice not to mix parts
of different valves. All parts need to be cleaned and inspected and as applicable
measured to manufactures instructions. The following general guide lines may
be of use during these inspections.
• Spindles to be checked for straightness, erosion or corrosion, and
damages. Bent valve spindles are frequently the cause of sluggish
operation.
• Springs to be checked for any permanent set or corrosion. Ideally the
length of the spring should be compared with the length of a spare spring.
Alternatively with the spring length of other safety valves, or sometimes
acceptance criteria is mentioned in the instruction manual.
• The valve or disk to be checked for damages to the shroud / lip and
sealing surface. If valves are of the winged type, and with the narrow
seating recommended by the maker for steam tightness, very little wear
is permissible on the valve wings if effective sealing is to be maintained.
The makers recommendations should be followed, if these specified
dimensions are not maintained it may lead to feathering of the waste
170
•
•
steam pipe considerably below the blow off pressure. Feathering safety
valves result in damaged valve seats.
Seat to be checked for damage. Normally the valve and seat are ground,
and when properly done, a light gray continuous line on the circumference
of the seat and disk is displayed.
Valve body to be checked for corrosion and erosion. If after visual
inspection reduction of the body thickness is suspected one may
determine the remaining thickness with a pair of outside calipers.
Adjustment and testing of safety valves
Before attempting to adjust the safety valves it is essential to verify the
accuracy of the boiler pressure gauge.
According to the DNV Rules and other standards the safety valves must be set
at a pressure not exceeding 3% above the approved design pressure of the
boiler. Normally the approved design pressure of piping and equipment in the
steam system is equal to that of the boiler, if this is not the case than the lowest
design pressure has to be set. The approved design pressure can be found on
the boiler plate which is permanently attached in a prominent place on the
boiler, which is generally around the burner. Normally the approved design
pressure is taken approximately 1 bar higher than the maximum allowable
working pressure, rule wise the design pressure is not to be less than the
maximum working pressure.
Sometimes we see the boiler is operated below the approved design pressure,
and then one may adjust the safety valves at any value above the operating
pressure as long as it does not exceed 3% of the approved design pressure.
Example: Approved design pressure,
Maximum Setting of safety valves,
Operating pressure,
13 bars
13.39 bars
9 bars
Safety valves can be set at any pressure between 9 and 13.39 bars.
The hydraulic test pressure is 19.5 bars so the fear of some Chief
Engineers to bring the pressure up to 13.39 bars is unfounded.
The principal that the safety valves are the last safety against an over pressure
failure of the boiler, should be kept in mind when adjusting the overpressure
safety system. The correct sequence of alarms and shut down is as follows:
1. Burner stops at its normal set pressure for start and stop of burner.
2. High steam pressure alarm.
3. High high steam pressure alarm with simultaneous burner shut down.
4. Lifting of safety valves.
For boilers equipped with a super heater, it is a recognized practice to set the
safety valves on the superheater at a stipulated value below the set pressure of
the safety valves on the steam drum. This is done to ensure that the
superheater is circulated with `cooling steam´ at all times. Adjustment in this
manner results in the superheater safety valve first lifting, and thus preventing
overheating of the superheater.
The DNV Rules and other standards prescribe that the discharge capacity of the
safety valves has to be such that the pressure does not rise more than 10 %
171
above the design pressure with closed boiler stop valve and maximum firing
condition. This may be confirmed at new building by an accumulation test, with
a duration of 15 minutes for smoke tube boilers, and 7 minutes for water tube
boilers.
With regard to the above example this means that the pressure should not rise
above 14.3 bars.
Normally the only time it is necessary to adjust a safety valve is immediately
after a boiler survey. The initial setting may be performed by water or air on a
test bench, but the final adjustment and function test has to be done on the
boiler with steam. Since the safety valve is such a critical safety component it is
essential function tests are performed under realistic operating conditions.
Occasionally great reluctance is encountered in performing safety valve function
test on the boiler. The arguments given for this reluctance are completely
unjustified, and when the following is observed it should not create a dangerous
situation.
• Before any adjustment / test, accuracy of the pressure gauge to be
confirmed.
• Internal inspection of water / steam and flue gas side, confirmed
structural soundness of the boiler.
• Steam pressure gauge to be constantly observed during the test.
Remember that the hydraulic test pressure is far above set pressure of
the safety valves and allowable accumulation test pressure (10 % pd).
• The test should be properly prepared, adequately manned, and conducted
in a controlled and calm manner.
Below some general guide lines are listed when conducting safety valve testing.
• Set points of burner stop and shut down to be raised above safety valve
pressure. Alternatively one can close the valve to the relevant pressure
switches.
• During steam pressure rise closely watch the pressure gauge, popping of
the valve is clearly indicated by slight decrease in pressure followed by an
increase indicating full lift, completed by blowdown at a pressure below
the set pressure. Full lift of the valves is indicated by the spindle raise for
some valve designs, or the free play of the easing gear. Raising boiler
pressure is best done by closing boiler stop valve and manually controlling
the burner while monitoring the steam pressure.
• During the test the safety valve must lift smartly and fully at its adjusted
pressure, and after it has relieved excess pressure, shuts with equal
smartness. Sluggish or feathering valves indicate malfunction of the
moving parts, also feathering valves result in seat damage.
• Only popping of the safety valves is not advisable since this may result in
dirt from the boiler being trapped in-between the sealing surfaces, leading
to leaking valves. To avoid this it is better to allow full lift and blowdown
of the valve so that any possible dirt is blown out. Claims that safety
valves are of a design that can not be tested (or only a number of times)
due to leaking afterwards, should not be accepted as an excuse to omit
testing under steam. If subject valves are of such poor quality they must
be renewed since they are not fit for normal service. This means after a
pressure release in service one has to shut down and depressurize the
boiler to exchange the safety valve, because the original valve no longer
closes properly.
172
•
•
•
While the safety valves are blowing off excessive pressure, inspect the
waste steam pipes and drain pipes leading to the bilge. The drain pipes
are sometimes found blocked or damaged.
After reseating of the safety valves, test the easing gear from the burner
position. It is not uncommon to find them wrongly assembled or seized.
It is good engineering practice to measure the distance between spring
compression nut and upper face of spindle and record this value in the
engine log book for later reference.
Boiler valves
The main valves fitted on a boiler as per DNV Rules and relevant codes are listed
below.
• Steam stop valve: Each boiler is to be fitted with a stop valve as close to
the shell as possible. On multi boiler installation connected to common
steam header an additional stop valve needs to fitted in series with the
first one. The additional valve is generally a globe valve of the screw
down, non return type which prevents one boiler pressurizing another.
• Feed water check valve: This may be a globe valve of the screw down,
non return type or a separate check and stop valve placed as near to each
other as possible. The check valve prevents the boiler content blowing
into the engine room, in the event of a feed line failure.
Location of the feed water check valve and on the right a boiler check valve.
•
•
Blow-down valves: Restrictions are in place for the use of cocks with
tapered plugs. They are to be bolted type with separate packing gland
which may only be used up to 13 bars. As with steam stop valves on
multi boiler plants, an additional valve needs to be fitted in series with the
first one.
Test valve: Every boiler has at least one test valve, sometimes combined
with a sampling cooling tank. This arrangement makes it possible to
obtain boiler water test samples in a safe manner.
Steam stop valves fitted to ordinary shell type boilers are normally right angle
globe valves made of nodular or spheroidal graphite cast iron or cast steel. All
other valves are generally conventional globe valves.
173
Ce r t ifica t ion
The certification requirements for valves according to the DNV Rules (July 2004)
are similar to those of safety valves, they should be provided with the following
documentation.
• All valves with Dn> 100 mm and p>16 bar need to be supplied with a
DNV Product Certificate.
• All valves need to be hydrostatically pressure tested at 1.5 times the
nominal pressure. If not witnessed by a Surveyor this is confirmed by a
Test Report from the maker.
• Material Certificates are required to be provided as follows.
1. Steel valves Dn > 100 mm, Td > 400 °C – NV Material Certificate.
2. Steel valves Dn<=l to 100 mm, Td>400 °C – Material Test report.
3. Steel or nodular cast iron valves Dn > 100 mm, Td <= to 400 °C –
Material Work Certificate.
4. Steel or nodular cast iron valves Dn <= to 100 mm, Td < equal to
400 °C – Material Test report.
5. .Copper alloy valves Dn > 50 mm, – Material Test report.
6. Copper alloy valves Dn <= to 50 mm, – Material Work Certificate.
Also valves should be used in accordance with an internationally recognized
standard of pressure and temperature rating, for instance DIN 2401 or ANSI
16.5-1958, API STD 600-1957. Reference is made to the enclosed pressure /
temperature rating tables at end of this chapter.
The material grade and pressure rating is generally cast in the valve body and
bonnet.
DN 15, PN 40 angel steam stop valve and spring loaded feed water stop valve.
174
General use globe valve design which is typically suitable for any on / off, and throttling services.
Su r ve y of va lve s
All valves adjacent to the boiler shell need to be opened up in connection with
the survey. It is common practice to leave the valve body in place and remove
the bonnet which is completely disassembled in the workshop. After all the
parts are cleaned, inspected, and overhauled they are presented to the
attending Surveyor. Commonly the bonnets are presented assembled after new
stuffing box packing rings are fitted and the disk sealing surfaces are ground.
Some general inspection hints can be found below.
• Stem, spindle to be checked for straightness and erosion wear in way of
the stuffing box packing.
• Valve body to be checked for corrosion and erosion.
• Disk and seats sealing surface must be in good condition to obtain a tight
valve.
Water level gauges
The DNV Rules prescribe that every boiler needs to have two independent
means of indicating the water level. In practice this is usually two water gauge
glasses, but one could be replaced by an approved equivalent device. This may
be an approved electronic level gauge indicator.
The water gauge glasses must be so positioned on the boiler that the lowest
visible water level corresponds with the lowest, safe working water level. Also
the location of combustion chamber top or furnace crown of tank type boilers
175
needs to be marked adjacent to the water gauge glass. Cocks or valves need to
be fitted on each end of the gauge glass which can be closed from a safe
position in the event a glass brakes.
Water gauge glass with round glass for low pressures.
For boiler pressures up to about 20 bars it is normal practice to use round glass
tubes. Above 20 bars the glass tube is replaced by what is in effect a built up
rectangular section box with a thick glass plate on the front and back. Since
gauge glasses are prone to impact damage a protector is often fitted around
them. Cock handles should always be fitted in such a manner that they are
pointing vertically downwards in normal working, open position. With the cock
handles disposed in this way, it can be seen in a glance that all are correct, and
there is no danger of vibration causing one to shut.
Su r ve y of ga u ge gla sse s
The following general points may be considered during survey.
•
•
The water level should be easy to read in both glasses. It is not
uncommon that the glasses become cloudy or discoloured with age and in
such case renewal is required.
The water level in both gauge glasses should be the same, if not this
indicates a blockage. Testing of the gauge glasses to be performed by
blowing them through with water and steam, by opening and closing the
valves on each end.
176
Reflective water level gauge, DN 26, PN 25, the body is made of carbon steel.
•
The double plate glass type of gauge is normally illuminated from the rear
by an ordinary lamp. This arrangement must be in good working
condition otherwise water level readings are not possible.
Pressure gauge
Per rule requirement every boiler must be equipped with a pressure gauge at an
easy readable position. The highest permissible working pressure is to be
marked on the gauge in red.
A pressure gauge of the Bourdon tube type is usually fitted. Sometimes it’s
connected to the steam space by a ring type siphon tube, which fills with
condensate and protects the dial mechanism from high temperatures.
177
Pressure gauge mounted on a ring siphon and right two coiled Bourdon tubes, C shaped (a) and coiled
(b).
Su r ve y of pr e ssu r e ga u ge
The survey consists of confirming the gauge accuracy by sending it ashore for
calibration, or using the pressure calibration equipment on board.
Boiler plate
Each boiler is required to have a boiler plate permanently attached to the boiler
in a prominent location, usually around the burner. This plate displays the
following information which is hard stamped on the plate.
Example of a boiler name plate.
178
•
•
•
•
•
•
•
Name and domicile of the manufacturer
Manufactures type destination and serial number
Year of manufacture
Design pressure
Design temperatures
Hydraulic test pressure
DNV identifying mark
All the major standards and codes require fitting of a boiler plate, some
standards require the maximum working pressure and output to be displayed.
Soot blowers
Soot blowers are mechanical devices used for periodical on line cleaning of the
boiler flue gas side to remove ash and slag deposits. They direct a cleaning
medium through nozzles against the soot or ash accumulations on the heated
surface. Furthermore they prevent plugging of the gas passages. Mostly
compressed air is used as a cleaning medium on auxiliary boilers, but also
steam can be used.
There are many different types of soot blowers in use, for example, manual
fixed position blowers, fully automated blowers and retractable blowers. On
most auxiliary boilers they are in general not installed, but we usually find them
fitted on exhaust gas boilers. In general it is the manual fixed position type,
which uses compressed air as a cleaning medium.
Su r ve y of soot blow e r s
Survey of soot blowers entails verifying that they are in workable condition.
Check that nozzles are not blocked, blow pipe properly supported, and operating
mechanism not seized. Operate the blower rotation hand wheel without air, it
should move easily.
On the right a picture of the blower rotation wheel and left the nozzle tube with support.
179
Appendix
Inspection Documents
The surveyor will meet different kinds of inspection documents specified by
international and national standards. In order to be able to compare these with
the certificate types defined by DNV, a comparison of the international standards
for inspection documents is given. The documents are listed in increasing order,
i.e. from 1 where only a confirmation from the manufacturer is necessary to 7
where the highest level of documentation is required. Note that whereas ISO
10474 still include 3.1B and 3.1C type inspection documents, the latest edition of
EN 10204 does not. ISO 10474 is seldom (if ever) used by purchasers.
Consequently, DNV shall not issue 3.1C inspection certificate unless ISO 10474 is
explicitly referenced by the purchaser.
180
Pressure and temperature ratings for valves.
Pressure / Temperature-Rating
Druck und Temperaturabstufung nach DIN 2401 / ANSI B 16.5
Description
GS-CK 14 V
TT-St 35 V
GS-26 Cr Mo 4
26 Cr Mo 4
GS-10 Ni 14
10 Ni 14
GG-20
GG-25
R-St 37-2
group
TT-Stahl
3,5 Ni
GG
C-Stahl
GS-C 25 N
C 22.3
C 22.8
C-Stahl,
warmfest
GS-22 Mo 4
15 Mo 3
Mo
0,5
GS-17 Cr Mo
55
13 Cr Mo 44
Cr Mo
1,0 0,5
GS-12 Cr Mo
9.10
10 Cr Mo 9.10
1.4408
Cr Mo
2,25 1,0
Cr Ni Mo
bar / - -60 -50
°C
196 110
10
10 10 10
16
16 16 16
25
25 25 25
40
40 40 40
63
63 63 63
100
100 100 100
160
160 160 160
250
250 250 250
10
16
10
16
25
40
63
100
160
250
320
25
40
63
100
160
250
320
400
25
-10
+20
10
16
25
40
63
100
160
250
10
16
10
16
25
40
63
100
160
250
320
25
40
63
100
160
250
320
400
25
10
16
25
40
63
100
160
250
10 10 8 7 6
16 16 13 11 10
10 10 8 7 6 4 2
16 16 14 13 11 8 6
25 25 22 20 17 13 8
40 40 35 32 28 21 12
63 63 50 45 40 32 19
100 100 80 70 60 50 30
160 160 130 112 96 80 49
250 250 200 175 150 125 89
320 320 250 225 192 160
25 25 25 25 22 19 17
40 40 40 40 35 30 28
63 63 63 63 56 47 45
100 100 100 100 87 74 70
160 160 160 160 139 118 112
250 250 250 250 217 185 174
320 320 320 320 278 236 222
400 400 400 400 348 296 278
25 25 25 25 25 23 21 18
40
63
100
160
250
320
400
160
40
63
100
160
250
320
400
160
40 40 40 40 40 36 34 29 15
63 63 63 63 63 58 56 47 25
100 100 100 100 100 91 87 74 38
160 160 160 160 160 146 139 118 62
250 250 250 250 250 227 227 184 97
320 320 320 320 320 292 279 237 124
400 400 400 400 400 364 348 295 155
160 160 160 160 160 160 130 90 70 52
250
320
400
16
250 250 250 250 250 250 250 200 150 108 81
320 320 320 320 320 320 320 230 180 139 104
400 400 400 400 400 400 400 300 215 174 130
16 16 14 13 11 9 7 5 4 3
16 16
50 120 200 250 300 400 450 500 530 550 600
9
181
18 10 2
1.4401
1.4571
1.4308
1.4301
1.4541
Cr Ni
18 8
25
40
63
100
160
250
320
16
25
40
63
100
160
250
320
PN nach DIN 2401
Äquivalent PN ~API 600
16
25
40
63
100
160
250
400
16
150
10,5 bar
260 0C
15,0 bar
120 0C
16
25
40
63
100
160
250
400
25 25
40 40
63 63
100 100
160 160
250 250
400 400
16 16
25 25
40 40
63 63
100 100
160 160
250 250
400 400
25/40
300
21,1 bar
450 0C
48,5 bar
120 0C
25
40
63
100
160
250
400
16
25
40
63
100
160
250
400
63
400
28,1 bar
450 0C
64,7 bar
120 0C
25 24 20 18 16 12 10 8 6
40 39 38 36 34 30 28 26 24 21 20
55 53 50 48 45 39 37 36 33 29 25
82 79 76 73 69 59 56 54 50 43 40
124 119 115 109 96 88 85 80 74 65 60
206 199 191 182 170 151 140 136 124 108 100
320 315 310 300 280 249 235 228 208 181 170
16 15 12 11 9 7 5 4 3
25 22 18 15 14 10 8 7 5
40 36 30 30 26 23 22 21 20 16 14
54 48 44 40 35 31 29 28 26 22 20
81 72 66 60 53 48 44 40 38 34 32
122 110 99 90 80 71 67 64 60 53 50
204 182 165 150 133 119 111 107 100 88 80
320 303 275 250 222 199 186 179 175 150 140
100
160
250
320/400
600/800
900
1500
2500
43,4 bar 63,3 bar 105,5 bar 175,8 bar
450 0C 450 0C
450 0C
4500C
97,0 bar 145,5 bar 242,6 bar 404,3 bar
120 0C 120 0C
120 0C
120 0C
top
182
Chapter 7: Boiler Control and Monitoring
There are three major reasons to install control and monitoring equipment on an
auxiliary boiler.
1. Safety: Automation of combustion and feed water control, and the
implementation of automated safety protocols have greatly contributed to
increase overall operational safety of steam plants.
2. Stability: The boiler operates more steadily, and predictable, without
fluctuations and shutdowns when control systems are in place.
3. Accuracy: Automated processes are more accurate than those that are
manual controlled. The process can be optimized with increased economic
efficiency.
Boiler controls in general consist of three variably interconnected systems,
combustion, feed water, and superheated steam temperature control. From a
historic perspective the feed water regulation was the first one to be automated,
quickly followed by combustion controls.
Superheated steam controls are rarely fitted on superheated steam producing
auxiliary boilers, but they are essential for propulsion boilers. Nowadays all
auxiliary boilers are equipped with a form of feed water and combustion control
and a monitoring system.
Automated feed water regulation
The boiler maker has designed the boiler to safely operate within an upper and
lower normal water level. Sometimes these normal water levels (NWL) are
marked in the vicinity of the gauge glass.
A feed water control system therefore has the flowing tasks.
1. Monitor and control the water level by supplying more or less feed water
to the boiler.
2. Detect the water level, and when an unsafe level is reached take
appropriate action. Depending on the detected level this action may be
sounding an alarm (high, low level), or shutting down the feed water
supply or burner.
In order to be able to monitor and control the boiler water level, the level must
first be accurately detected. The following principal types of level detection
devices are appropriate to steam boilers.
Con du ct ivit y pr obe s
When considering a tank filled with water in which a probe is immersed.
This probe is connected by electric cable via a voltage source and ampere meter
to the tank. As long as the probe is immersed in water there will be current flow
through the circuit. When the water level is lowered and the probe tip is out of
the water the current flow will stop.
This point measurement (on / off of current) when the probe tip touches water
can be used to trigger an action through an associated controller, for example
start / stop pump or open / close valve. A single probe can only provide a single
action, therefore four or more probes are built into a common housing, which are
cut to their appropriate lengths on installation.
183
Operating principle of a conductivity probes, single tip.
On the left a conductivity probe with controller, on the right a probe arrangement to switch a feed pump on
and off.
Ca pa cit a n ce pr obe s
The capacitance level probe consists of a conducting, cylindrical probe which acts
as the first capacitor plate and is covered with a dielectric material. The second
capacitor plate is formed by the boiler shell together with the water content.
Therefore, by changing the water level, the area of the second capacitor plate is
changed, which changes the overall capacitance of the system.
The change in capacitance is however small, so the probe is used in conjunction
with a amplifier, which feeds the amplified signal to a suitable controller.
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On the left capacitance in water, and on the right a typical level control.
Floa t con t r ol
This is one of the simplest forms of level measurement, but still widely used on
auxiliary boilers.
Float control directly in the boiler and with external chamber.
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On the left typical mounting arrangements and left different types of Mobrey level controls.
The float moves up and down as the boiler water level changes. At the end of the
float rod is a magnet which operates the magnetic switches. A more sophisticated
type uses a coil wrapped around a yoke inside the cap and can be used for
modulating control systems.
The float control can be mounted in an external chamber or directly within the
boiler shell.
Sketch of a level control system using a differential pressure cell.
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D iffe r e n t ia l pr e ssu r e ce lls
The differential pressure cell is installed with a constant head of water on one
side, and the other side is so arranged that the water head varies with the boiler
water level. Variable capacitance, strain gauge, or inductive techniques are used
to measure the deflection of the diaphragm which is converted and fed to a
controller.
Au t om a t ic on / off le ve l con t r ol syst e m s
All the methods of level detection described above can be used for on / off level
control. The signal produced is used to start the feed pump at a predetermined
low boiler water level and allow it to run until the set high level is reached.
This type of control system is only found on small boilers with a steam generation
rate of below 5000 kg/h. The disadvantages of this system are frequent start and
stops of feed pump, temperature cycling of the boiler due to relatively high flow
rate of “cold” feed water.
Sketch of an on / off level control system.
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Aut om a t ic m odula t e d le ve l con t r ol syst e m
With this type of control the feed pump runs continuously, and an automatic
valve controls the feed water flow rate to match the steam demand.
Sketch of a modulated level control system.
When operating correctly, modulating control can dramatically smooth the steam
flow and ensures greater water level stability.
To protect the feed pump from overheating when pumping against a closed
modulation valve, a recirculation or spill line is provided to ensure a minimum flow
rate through the pump. For modulated level control only floats with a continuous
signal output, capacitance probes, and differential pressure cells can be used as
water level sensor.
Most auxiliary boilers on board have a form of modulated feed water control
system installed.
Sin gle e le m e n t w a t e r le ve l con t r ol
The standard single boiler level control system, with proportional control, gives
excellent feed water regulation for the majority of marine auxiliary boiler plants.
However with single element proportional control, the water level must first fall
before the feed water control valve opens. This means that the water level must
be higher at low steaming rates, and lower at high steaming rates. This
translates in a falling level control characteristic.
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The limitation of single element level controls is its inability to react quickly onto
sudden large load changes. In water tube boilers this may lead to unstable steam
pressures and water levels.
Tw o e le m e n t w a t e r le ve l con t r ol
Two element control systems reverse the falling water level control characteristic,
and ensure the water level is made to rise during high steaming rates.
Level control characteristics.
This strives to ensure that the quantity of water in the boiler stays constant
during all loads, and during periods of increased, sudden steam demand, the
feed water control valve opens. The system works by using the signal from a
steam flow meter installed in the steam line to increase the level controller set
point during high steam loads. The two elements of the signal are:
1. First element: Boiler water level signal from level detection device.
2. Second element: Steam flow signal from flow meter in discharge line.
Any boiler installation which experiences frequent, sudden load changes may
operate better with a two element feed water control system.
On / off control mode
Proportional plus integral mode
Proportional control mode
Proportional plus derivative control
Typical system responses for different control modes. The above example is for a temperature control, but same
applies for level control system.
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Sketch of a two element boiler water level control system.
Automated combustion control
The basic requirement for a combustion control system is to provide efficient
combustion during all power levels. This is achieved by supplying the correct
relationship of air and fuel to the furnace at the requested steam generation rate.
In addition to the chapter “Combustion and Atomizers” we will broadly outline the
basic burner control systems in use on marine auxiliary boilers.
On / off bur n e r con t r ol
This is the simplest combustion control system, and it means that either the
burner is firing at full rate, or it is shut off. The major disadvantage of this control
method is that the boiler is subjected to large and frequent thermal shocks, every
time the boiler fires. It is also not able to cope with large and sudden load
increases and therefore this control system is limited to small boilers up to a
steam production of 500 kg/h.
H igh / low / off bur n e r con t r ol
This is a slightly more complex system where the burner has two firing rates. The
burner first operated at a lower firing rate and than switches to full firing rate on
increased steam demand, thereby overcoming the worst thermal shock. Also it
switches from full, to intermediate firing rate, and burner stop upon reducing
loads. This type of burner control is usually fitted on boilers with a steam
production of up to 5000 kg/h.
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M odu la t in g com bu st ion con t r ol
Modulated burner control will alter the firing rate to mach the boiler load over the
whole steam production range. Every time the burner shuts down and re-starts,
the furnace must be purged, this wastes energy and reduces efficiency. Full
modulation, however, means that the boiler keeps firing over the whole steam
generation range to maximise thermal efficiency and minimize thermal stresses.
This type of combustion control is fitted on most auxiliary boilers with a medium
to large steam production.
A mechanical and electronic modulated combustion control units for Saacke burners, a steam
pressure signal moves the servo motors.
Monitoring of auxiliary boilers
Different rules and standards vary in their monitoring requirements, but the
minimum extent of monitoring according to the DNV Rules (January 2003
publication) is found below.
•
•
•
•
Water level low alarm: This signal has to come from an independent level
detection device. Therefore boilers must have two sets of level detection
devices, one for controlling the water level and shutting down the burner
(Level low low alarm), and the other set for low and high level alarms.
Water level lower (level low low): This alarm is raised after water level low
alarm and also shuts down the burner automatically. After normal water
level (NWL) is restored one has to manually reset this alarm in order to
start the burner.
Water level high alarm: Although not required but installed on most boilers
this alarm stops the feed water pump, or closes the feed water control
valve.
Circulation failure alarm: This alarm is found when the boiler has forced
circulation. The signal usually comes from a pressure sensor, if the
circulation pressure reaches the lower set point the alarm is raised and the
burner shut down. Also here manual reset is required before boiler start up
is possible.
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•
•
•
•
•
•
•
Combustion fan failure alarm: The signal comes from a pressure sensor, if
the combustion air pressure reaches its lower set point the alarm is raised
and the burner tripped. Manual reset is required before boiler start up is
possible.
Heavy fuel oil temperature high alarm: A temperature sensor in the fuel
supply line is supplies the signal for this alarm.
Heavy fuel oil temperature low alarm: It is very uncommon to find a
viscosity meter in boiler fuel oil systems. Mostly the fuel oil viscosity is
controlled by setting a certain temperature on the controller of the heater.
Steam pressure high alarm: A pressure sensor fitted on the boiler raises
this alarm.
Steam pressure higher (pressure high high) alarm: Usually a second
pressure sensor raises this alarm and trips the boiler after steam pressure
high alarm has sounded. Also here manual reset is required before start up
of the plant is possible. According to the DNV Rules this alarm is only
necessary “When the automatic control system does not cover the entire
load range from zero load”, practically all boilers are fitted nowadays with a
burner trip.
Superheated steam temperature high alarm: This is only applicable when
superheated steam above 350 °C is produced. Therefore this alarm is very
seldom encountered on auxiliary boilers.
Flame failure alarm: One some installations this alarm is split into an
ignition failure alarm, only active during start up, and a flame failure alarm
active during operation. Both alarms will trip the burner and manual reset
of the alarm is required before restart of the plant.
Existing installations still extensively use thermostats and pressostats to control
and trigger the various alarms and safety functions. Modern installations tend to
favor central electronic cabinets which control and monitor the complete boiler
operation.
Central control cabinet on the right and alarm / control panel on the left from Saacke.
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Testing of control and monitoring system
The last part of a boiler survey is generally testing of the control and monitoring
system. Also these tests should be properly prepared, adequately manned, and
conducted in a controlled and calm manner.
Below you find some general guidelines listed, which might be helpful during
execution of these tests.
•
•
•
By simply observing water level gauge glass, steam pressure, and burner
operation, the proper functioning of control systems can be confirmed.
Load changes can be simulated by closing, throttling, and opening of the
steam stop valve. After an initial load change the boiler parameters should
return to a stabile condition. Feed water control valves or burner controls
should not hunt.
Steam pressure high, steam pressure high high, and burner shut down can
best be tested during steam pressure rising for safety valves testing
(popping).
Water level low, level low low alarm, and boiler trip can be tested in the
following ways.
1. If the boiler is fitted with an external chamber in which the level
detection sensors are fitted, it is a simple matter of draining this
chamber after isolating valves are closed. It is a prudent cause of
action to open this chamber during the course of the survey.
Inspection of the float and cleaning of the chamber is then made
possible.
Low water level test on a boiler with an external chamber.
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Internally mounted level sensor.
•
•
•
2. If the level detection sensors are fitted within the boiler shell, then
the only proper way of testing is to lower the water level. This may
best be done by, closing the steam stop valve, manually control the
burner and set it at minimal firing rate, close the feed water control
valve or stop the feed pump, and while closely observing the level
gauge glass, slowly open the blow down valve until low level alarms
sound and the boiler trips. Remember that level alarms and boiler
trip should activate, with water in the lower visible regions of the
level gauge glass.
Combustion fan failure can be easily tested by stopping the fan, and incase
of a pressostat by draining the line to it.
Incase the boiler has forced circulation this alarm can be tested by tripping
the circulation pumps.
Pressure and temperature alarms may also be tested by use of calibration
equipment found on board vessel with unmanned engine rooms.
Please be aware that wherever DNV Rules are mentioned in this course hand out
we have not quoted the exact complete rule text, and have used the publication
at the time of writing this course. Therefore we refer to the DNV Rules for
complete and exact text and possible updates.
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