STEAM DISTRIBUTION AND UTILIZATION
1. INTRODUCTION ................................................................................................................. 1
1.1 Why do we use steam? ...................................................................................................... 1
1.2 What is steam? .................................................................................................................... 3
1.3 Steam quality...................................................................................................................... 8
2. STEAM DISTRIBUTION SYSTEM ............................................................................... 8
2.1 What is the steam distribution system? ........................................................................... 8
2.2 Pipes................................................................................................................................... 10
2.3 Drain points....................................................................................................................... 16
2.4 Branch lines ...................................................................................................................... 17
2.5 Strainers ............................................................................................................................. 19
2.6 Filters ................................................................................................................................. 23
2.7 Separators .......................................................................................................................... 25
2.8 Steam traps ........................................................................................................................ 27
2.9 Air vents ............................................................................................................................ 36
2.10 Condensate recovery ..................................................................................................... 40
2.11 Insulation of steam pipelines and hot process equipments ....................................... 43
3. ASSESSMENT OF STEAM DISTRIBUTION SYSTEM ....................................... 47
3.1 Assessment of steam traps .............................................................................................. 47
3.2 Assessment of heat losses from un-insulated surfaces ................................................ 49
3.3 Assessment of savings from condensate recovery....................................................... 52
4. ENERGY EFFICIENCY OPPORTUNITIES .............................................................. 54
5. OPTION CHECKLIST ...................................................................................................... 65
6. WORKSHEETS................................................................................................................... 66
7. REFERENCES ..................................................................................................................... 67
1. INTRODUCTION
This chapter explains what steam is, its properties and why we use steam.
1.1 Why do we use steam?
Steam1 has come a long way from its traditional associations with locomotives and the
Industrial Revolution. Steam today is an integral and essential part of modern technology.
Without it, our food, textile, chemical, medical, power, heating and transport industries could
not exist or perform as they do. Steam provides a means of transporting controllable amounts
of energy from a central, automated boiler house, where it can be efficiently and
1
This section is a summary of Module 1.1 Steam –The Energy Fluid, In: Spirax Sarco Learning Centre, Block 1,
‘Introduction’.www.spiraxsarco.com
economically generated, to the point of use. Therefore as steam moves around a plant it can
equally be considered to be the transport and provision of energy.
For many reasons, steam is one of the most widely used commodities for conveying heat
energy. Its use is popular throughout industry for a broad range of tasks from mechanical
power production to space heating and process applications. Reasons for using steam
include:
§ Steam is efficient and economic to generate
§ Steam can easily and cost effectively be distributed to the point of use
§ Steam is easy to control
§ Energy is easily transferred to the process
§ The modern steam plant is easy to manage
§ Steam is flexible
The alternatives to steam include water and thermal fluids such as high temperature oil. Each
method has its advantages and disadvantages, as shown in Table 1.
Table 1. Comparison of heating media with steam1
Steam
High heat content
Latent heat approximately
2 100 kJ/kg
Inexpensive
Some water treatment costs
Good heat transfer
coefficients
High pressure required
for high temperatures
No circulating pumps required
Small pipes
Easy to control with
two way valves
Temperature breakdown is
easy through a reducing valve
Steam traps required
Condensate to be handled
Flash steam available
Boiler blowdown necessary
Water treatment required
to prevent corrosion
Reasonable pipework
required
No fire risk
System very flexible
Hot water
Moderate heat content
Specific heat
4.19 kJ/kg°C
Inexpensive
Only occasional dosing
Moderate coefficients
High temperature oils
Poor heat content
Specific heat often
1.69-2.93 kJ/kg°C
Expensive
High pressure needed
for high temperatures
Circulating pumps required
Large pipes
Relatively poor
Coefficients
Low pressures only
to get high temperatures
Circulating pumps required
Even larger pipes
More complex to control three way valves or
differential pressure valves
may be required
Temperature breakdown
more difficult
No steam traps required
No condensate handling
No flash steam
No blowdown necessary
Less corrosion
More complex to control three way valves or
differential pressure valves
may be required.
Temperature breakdown
more difficult
No steam traps required
No condensate handling
No flash steam
No blowdown necessary
Negligible corrosion
Searching medium,
Very searching medium,
welded or flanged joints usual welded or flanged joints usual
No fire risk
Fire risk
System less flexible
System inflexible
1.2 What is steam?
A better understanding of the properties of steam may be achieved by understanding the
general molecular and atomic structure of matter and applying this knowledge to ice, water
and steam. 2
A molecule is the smallest amount of any element or compound substance still poss essing all
the chemical properties of that substance which can exist. Molecules are made up of even
smaller particles called atoms, which define the basic elements such as hydrogen and oxygen.
The specific combinations of these atomic elements provide comp ound substances. One such
compound is represented by the chemical formula H2O, having molecules made up of two
atoms of hydrogen and one atom of oxygen. The reason water is so plentiful on the earth is
because hydrogen and oxygen are amongst the most abundant elements in the universe.
Carbon is another element of significant abundance, and is a key component in all organic
matter.
Most mineral substances can exist in the three physical states (solid, liquid and vapour),
which are referred to as phases. In the case of H2O, the terms ice, water and steam are used
to denote the three phases respectively.
The molecular structure of ice, water, and steam is still not fully understood, but it is
convenient to consider the molecules as bonded together by electrical charges (referred to as
the hydrogen bond). The degree of excitation of the molecules determines the physical state
(or phase) of the substance
1.2.1 Triple point
All the three phases of a particular substance can only coexist in equilibrium at a certain
temperature and pressure, and this is known as its triple point. The triple point of H2 O, where
the three phases of ice, water and steam are in equilibrium, occurs at a temperature of 273.16
K and an absolute pressure of 0.006112 bar. This pressure is very close to a perfect vacuum.
If the pressure is reduced further at this temperature, the ice, instead of melting, sublimates
directly into steam.
Ice
In ice, the molecules are locked together in an orderly lattice type structure and can only
vibrate. In the solid phase, the movement of molecules in the lattice is a vibration about a
mean bonded position where the molecules are less than one molecular diameter apart. The
continued addition of heat causes the vibration to increase to such an extent that some
molecules will eventually break away from their neighbors, and the solid starts to melt to a
liquid state (always at the same temperature of 0°C whatever the pressure). Heat that breaks
the lattice bonds to produce the phase change while not increasing the temperature of the ice,
is referred to as enthalpy of melting or heat of fusion. This phase change phenomenon is
reversible when freezing occurs with the same amount of heat being released back to the
surroundings. For most substances, the density decreases as it changes from the solid to the
2
This section is taken fromModule 2.2 What is Steam?, In: Spirax Sarco Learning Centre, Block 2, ‘Steam Engineering
Principles and Heat Transfer’. www.spiraxsarco.com
liquid phase. However, H2 O is an exception to this rule as its density increases upon melting,
which is why ice floats on water.
Water
In the liquid phase, the molecules are free to move, but are still less than one molecular diameter
apart due to mutual attraction, and collisions occur frequently. More heat increases molecular
agitation and collision, raising the temperature of the liquid up to its boiling temperature.
Steam
As the temperature increases and the water approaches its boiling condition, some molecules
attain enough kinetic energy to reach velocities that allow them to momentarily escape from the
liquid into the space above the surface, before falling back into the liquid. Further heating causes
greater excitation and the number of molecules with enough energy to leave the liquid increases.
As the water is heated to its boiling point, bubbles of steam form within it and rise to break
through the surface. Considering the molecular structure of liquids and vapours, it is logical that
the density of steam is much less than that of water, because the steam molecules are further
apart from one another. The space immediately above the water surface thus becomes filled with
less dense steam molecules.
When the number of molecules leaving the liquid surface is more than those re-entering, the water
freely evaporates. At this point it has reached boiling point or its saturation temperature, as it is
saturated with heat energy. If the pressure remains constant, adding more heat does not cause the
temperature to rise any further but causes the water to form saturated steam. The temperature of the
boiling water and saturated steam within the same system is the same, but the heat energy per unit
mass is much greater in the steam.
At atmospheric pressure the saturation temperature is 100°C. However, if the pressure is increased,
this will allow the addition of more heat and an increase in temperature without a change of phase.
Therefore, increasing the pressure effectively increases both the enthalpy of water, and the
saturation temperature. The relationship between the saturation temperature and the pressure is
known as the steam saturation curve (Figure 1).
Figure 1: Steam Saturation Curve
(Spirax Sarco)
Water and steam can coexist at any pressure on this curve, both being at the saturation
temperature. Steam at a condition above the saturation curve is known as superheated steam:
§ Temperature above saturation temperature is called the degree of superheat of the steam
§ Water at a condition below the curve is called sub-saturated water.
If the steam is able to flow from the boiler at the same rate that it is produced, the addition
of further heat simply increases the rate of production. If the steam is restrained from
leaving the boiler, and the heat input rate is maintained, the energy flowing into the boiler
will be greater than the energy flowing out. This excess energy raises the pressure, in turn
allowing the saturation temperature to rise, as the temperature of saturated steam correlates
to its pressure.
1.2.2 Enthalpy
Enthalpy of water, liquid enthalpy or sensible heat (h f) of water
This is the heat energy required to raise the temperature of water from a datum point of 0°C to its
current temperature. At this reference state of 0°C, the enthalp y of water has been arbitrarily set to
zero. The enthalpy of all other states can then be identified, relative to this easily accessible
reference state. Sensible heat was the term once used, because the heat added to the water produced
a change in temperature. However, the accepted terms these days are liquid enthalpy or enthalpy of
water. At atmospheric pressure (0 bar g), water boils at 100°C, and 419 kJ of energy are required to
heat 1 kg of water from 0°C to its boiling temperature of 100°C. It is from these figures that the
value for the specific heat capacity of water (CP ) of 4.19 kJ/kg °C is derived for most calculations
between 0°C and 100°C.
Enthalpy of evaporation or latent heat (hfg)
This is the amount of heat required to change the state of water at its boiling temperature,
into steam. It involves no change in the temperature of the steam/water mixture, and all the
energy is used to change the state from liquid (water) to vapour (saturated steam). The old
term latent heat is based on the fact that although heat was added, there was no change in
temperature. However, the accepted term is now enthalpy of evaporation. Like the phase
change from ice to water, the process of evaporation is also reversible. The same amount of
heat that produced the steam is released back to its surroundings during condensation,
when steam meets any surface at a lower temperature. This may be considered as the useful
portion of heat in the steam for heating purposes, as it is that portion of the total heat in the
steam that is extracted when the steam condenses back to water.
Enthalpy of saturated steam, or total heat of saturated steam
This is the total energy in saturated steam, and is simply the sum of the enthalpy of water
and the enthalpy of evaporation.
hg = hf + hf g
Where:
hg = Total enthalpy of saturated steam (Total heat) (kJ/kg)
hf = Liquid enthalpy (Sensible heat) (kJ/kg)
hfg = Enthalpy of evaporation (Latent heat) (kJ/kg)
The enthalpy (and other properties) of saturated steam can easily be referenced using the
tabulated results of previous experiments, known as steam tables. The steam tables list the
properties of steam at varying pressures. They are the results of actual tests carried out on
steam.
1.2.3 Dryness fraction
Steam with a temperature equal to the boiling point at that pressure is known as dry saturated
steam. However, to produce 100 percent dry steam in an industrial boiler designed to
produce saturated steam is rarely possible, and the steam will usually contain droplets of
water. In practice, because of turbulence and splashing, as bubbles of steam break through
the water surface, the steam space contains a mixture of water droplets and steam. If the
water content of the steam is 5 percent by mass, then the steam is said to be 95 percent dry and
has a dryness fraction of 0.95. The actual enthalpy of evaporation of wet steam is the product
of the dryness fraction (x) and the specific enthalpy (hf g) from the steam tables. Wet steam
will have lower usable heat energy than dry saturated steam.
Actual enthalpy of evaporation = hfg x
Therefore:
Actual total enthalpy = hf
+ hfg x
Because the specific volume of water is several orders of magnitude lower than that of steam, the
droplets of water in wet steam will occupy negligible space. Therefore the specific volume of
wet steam will be less than dry steam:
Actual specific volume =
vg x
Where: vg is the specific volume of dry saturated steam
1.2.4 The Steam phase diagram
The data provided in the steam tables can also be expressed in a graphical form. Figure 2
illustrates the relationship between the enthalpy and the temperature at various different
pressures, and is known as a phase diagram.
Figure 2. Temperature Enthalpy Phase Diagram
(Spirax Sarco)
As water is heated from 0°C to its saturation temperature, its condition follows the saturated
liquid line until it has received all of its liquid enthalpy, hf, (A - B).If further heat continues to
be added, it then changes phase to saturated steam and continues to increase in enthalpy while
remaining at saturation temperature, hfg, (B - C). As the steam/water mixture increases in
dryness, its condition moves from the saturated liquid line to the saturated vapour line.
Therefore at a point exactly halfway between these two states, the dryness fraction (x) is 0.5.
Similarly, on the saturated vapour line the steam is 100 percent dry. Once it has received all of
its enthalpy of evaporation, it reaches the saturated vapour line. If it continues to be heated
after this point, the temperature of the steam will begin to rise as superheat is imparted (C - D).
The saturated liquid and saturated vapour lines enclose a region in which a steam /water
mixture exists - wet steam. In the region to the left of the saturated liquid line only water
exists, and in the region to the right of the saturated vapour line only superheated steam exists.
The point at which the saturated liquid and saturated vapour lines meet is known as the critical
point. As the pressure increases towards the critical point the enthalpy of evaporation
decreases, until it becomes zero at the critical point. This suggests that water changes directly
into saturated steam at the critical point.
Above the critical point only gas may exist. The gaseous state is the most diffuse state in which
the molecules have an almost unrestricted motion, and the volume increases without limit as the
pressure is reduced. The critical point is the highest temperature at which a liquid can exist. Any
compression at constant temperature above the critical point will not produce a phase change.
Compression at co nstant temperature below the critical point however, will result in liquefaction
of the vapour as it passes from the superheated region to the wet steam region. The critical point
occurs at 374.15 o C and 221.2 bara for steam. Above this pressure the steam is termed
supercritical and no well-defined boiling point applies.
1.3 Steam quality
Steam should be available at the point of use:3
§ In the correct quantity to ensure that a sufficient heat flow is provided for heat transfer
§ At the correct temperature and pressure, or performance will be affected
§ Free from air and incondensable gases which act as a barrier to heat transfer
§ Clean, as scale (e.g. rust or carbonate deposit) or dirt have the effect of increasing the rate
of erosion in pipe bends and the small orifices of steam traps and valves
§ Dry, as the presence of water droplets in steam reduces the actual enthalpy of evaporation,
and also leads to the formation of scale on the pipe walls and heat transfer surface.
2. STEAM DISTRIBUTION SYSTEM
This section describes the steam distribution system and its various components.
2.1 What is the steam distribution system?
The steam distribution system4 is the essential link between the steam generator and the steam
user. There are various methods to carry steam from a central source to the point of use. The
central source might be a boiler house or the discharge from a co-generation plant. The boilers
may burn primary fuel, or be waste heat boilers using exhaust gases from high temperature
processes, engines or even incinerators. Whatever the source, an efficient steam distribution
system is essential if steam of the right quality and pressure is to be supplied, in the right
quantity, to the steam using equipment. Installation and maintenance of the steam system are
important issues, and must be considered at the design stage.
An understanding of the basic steam circuit or ‘steam and condensate loop’ is required (see
Figure 3). As steam condenses in a process, flow is induced in the supply pipe. Condensate has a
very small volume compared to the steam, and this causes a pressure drop, which causes the
steam to flow through the pipes.
The steam generated in the boiler must be conveyed through pipework to the point where its heat
energy is required. Initially there will be one or more main pipes, or 'steam mains', which carry
steam from the boiler in the general direction of the steam using plant. Smaller branch pipes can
then carry the steam to the individual pieces of equipment.
When the boiler main isolating valve (sometimes referred to as the ‘
crown’valve) is opened,
steam immediately passes from the boiler into and along the steam mains to the points at lower
pressure. The pipework is initially cooler than the steam, so heat is transferred from the steam to
the pipe. The air surrounding the pipes is also cooler than the steam, so the pipework will begin
to transfer heat to the air.
3
More details on steam quality criteria can be found in Module 2.4 Steam Quality, In: Spirax Sarco Learning Centre, Block 2,
‘Steam Engineering Principles and Heat Transfer’. www.spiraxsarco.com
4
Section 2.1 is a summary of Module 10.1 Introduction to Steam Distribution, In: Spirax Sarco Learning Centre, Block 10,
Steam Distribution. www.spiraxsarco.com
Figure 3. A Typical Steam Circuit (Spirax Sarco)
Steam on contact with the cooler pipes will begin to condense immediately. On start-up of the
system, the condensing rate will be at its maximum, as this is the time where there is maximum
temperature difference between the steam and the pipework. This condensing rate is commonly
called the ‘starting load’. Once the pipework has warmed up, the temperature difference between
the steam and pipework is minimal, but some condensation will occur as the pipework still
continues to transfer heat to the surrounding air. This condensing rate is commonly called the
‘running load’.
The resulting condensation (condensate) falls to the bottom of the pipe and is carried along by the
steam flow and assisted by gravity, due to the gradient in the steam main that should be arranged to
fall in the direction of steam flow. The condensate will then have to be drained from various strategic
points in the steam main.
When the valve on the steam pipe serving an item of steam using plant is opened, steam flowing
from the distribution system enters the plant and again comes in contact with cooler surfaces. The
steam then transfers its energy in warming up an equipment and product (starting load), and, when up
to temperature, continues to transfer heat to the process (running load).
There is now a continuous supply of steam from the boiler to satisfy the connected load and to
maintain this supply more steam must be generated. In order to do this, more water (and fuel to heat
this water) is supplied to the boiler to make up for the water which has previously been evaporated
into steam. The condensate formed in both the steam distribution pipework and in the process
equipment is a convenient supply of useable hot boiler feedwater. Although it is important to
remove this condensate from the steam space, it is a valuable commodity and should not be allowed
to run to waste. Returning all condensate to the boiler feedtank closes the steam energy loop, and
should be practiced wherever practical.
The distribution pressure of steam is influenced by a number of factors, but is limited by:
§ The maximum safe working pressure of the boiler
§ The minimum pressure required at the plant
As steam passes through the distribution pipework, it will inevitably lose pressure due to:
§ Frictional resistance within the pipework
§ Condensation within the pipework as heat is transferred to the environment.
Therefore allowance should be made for this pressure loss when deciding upon the initial
distribution pressure.
A kilogram of steam at a higher pressure occupies less volume than at a lower pressure. It
follows that, if steam is generated in the boiler at high pressure and also distributed at a high
pressure, then the size of the distribution mains will be smaller than that for the same heat load.
Generating and distributing steam at higher pressure offers three important advantages:
§ The thermal storage capacit y of the boiler is increased, helping it to cope more efficiently
with fluctuating loads, minimizing the risk of producing wet and dirty steam.
§ Smaller bore steam mains are required, resulting in lower capital cost, for materials such as
pipes, flanges, supports, insulation and labour.
§ Smaller bore steam mains cost less to insulate.
Having distributed at a high pressure, it will be necessary to reduce the steam pressure to each
zone or point of use in the system in order to correspond with the maximum pressure required by
the application. Local pressure reduction to suit individual plant will also result in drier steam at
the point of use.
The most important components of a steam distribution system are described in the next sections:
§ Pipes (2.2)
§ Drain points (2.3)
§ Branch lines (2.4)
§ Strainers (2.5)
§ Filters (2.6)
§ Separators (2.7)
§ Steam traps (2.8)
§ Air vents (2.9)
2.2 Pipes
This section describes the pipework of a steam system. 5
2.2.1 Pipe material
Pipes for steam systems are commonly manufactured from carbon steel to ANSI B 16.9 Al06.
The same material may be used for condensate lines, although copper tubing is preferred in some
industries. For high temperature superheated steam mains, additional alloying elements, such as
5
Section 2.2 is a summary of information in Module 10.2 Pipes and Pipe Sizing, and Module 10.3 Steam Mains and
Drainage. In: Spirax Sarco Learning Centre, Block 10, ‘Steam Distribution’
. www.spiraxsarco.com
chromium and molybdenum, are included to improve tensile strength and creep resistance at high
temperatures. Typically, pipes are supplied in 6- meter lengths.
2.2.2 Pipeline sizing
The objective of the steam distribution system is to supply steam at the correct pressure to the
point of use. Pipeline sizing is an important factor.
Oversized pipework means:
§ Pipes, valves, fittings, etc. will be more expensive than necessary.
§ Higher installation costs will be incurred, including support work, insulation, etc.
§ For steam pipes a greater vo lume of condensate will be formed due to the greater heat
loss. This in turn means that either more steam trapping is required or wet steam is
delivered to the point of use.
Undersized pipework means:
§ A lower pressure may only be available at the point of use. This may hinder equipment
performance due to only lower pressure steam being available.
§ There is a risk of steam starvation.
§ There is a greater risk of erosion, water hammer and noise due to the inherent increase in
steam velocity.
The require pipeline size can be calculated based on pressure drop and velocity described below.
a) Pipeline sizing based on pressure drop
Pressure drop through the distribution system is an important feature. In practice whether for
water pipes or steam pipes, a balance is drawn between pipe size and pressure loss. Pressure
drop as a general rule, should not exceed 0.1 bar/50 m. Pipe sizing can be computed using
the chart in Figure 4. Those who prefer tables instead of graphs can use Table 2 to determine
the pipeline size.
An example calculation is as follows:
Given:
§ Inlet pressure P1 = 7 bar g
§ Steam flowrate = 286 kg/h
§ Minimum allowable P2 = 6.6 bar g
§ Length of pipeline = 165 m
Calculate the maximum pressure drop per 100 m
Maximum pressure drop per 100 m = P1 –P2 x 100
L
= (7.0 –6.6) x 100
165
= 0.24 bar
Determine the pipeline size based on the pressure drop using the nomogram in Figure 4:
§ Select the point on the saturated steam line at 7 bar g, and mark Point A.
§ From point A, draw a horizontal line to the steam flowrate of 286 kg/h, and mark Point B.
§ From point B, draw a vertical line towards the top of the nomogram (Point C).
§ Draw a horizontal line from 0.24 bar/100 m on the pressure loss scale (Line DE).
§ The point at which lines DE and BC cross will indicate the pipe size required. In this case, a
40 mm pipe is too small, and a 50 mm pipe would be used.
Figure 4. Steam Pipeline Sizing Chart – pressure drop approach (Spirax Sarco)
Table 2. Saturated Steam Pipeline Capacities in kg/h for Different Velocities, schedule 40
pipe (Spirax Sarco)
Thermal Equipment: Steam distribution and utilization
Figure 5. Steam Pipeline Sizing Chart –velocity approach (Spirax Sarco)
b) Pipeline sizing based on velocity
Velocity is an important factor in sizing pipes. As a general rule, a velocity of 25 to 40 m/s is
used when saturated steam is the medium. 40 m/s should be considered an extreme limit, as
above this, noise and erosion will take place particularly if the steam is wet. Even these velocities
can be high in terms of their effect on pressure drop. In longer supply lines, it is often necessary
to restrict velocities to 15 m/s to avoid high pressure drops. It is recommended that pipelines over
50 m long are always checked for pressure drop, no matter what the velocity.
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14
Thermal Equipment: Steam distribution and utilization
Superheated steam can be considered as a dry gas and therefore carries no moisture.
Consequently there is no chance of pipe erosion due to suspended water droplets, and steam
velocities can be as high as 50 to 70 m/s if the pressure drop permits this.
Pipe sizing based upon the velocity approach for saturated and superheated steam can be
done using the nomogram as shown in Figure 5. Those who prefer tables instead of graphs
can use Table 2 to determine the pipeline size.
An example calculation is as follows:
Given:
§ Inlet pressure: 7 bar g
§ Steam flowrate: 5000 kg/h
§ Maximum velocity: 25 m/s
Calculate the pipeline size based on velocity using the nomogram in Figure 5
§ Draw a horizontal line from the saturation temperature line at 7 bar g (Point A) on the
pressure scale to the steam mass flowrate of 5 000 kg/h (Point B).
§ From point B, draw a vertical line to the steam velocity of 25 m/s (Point C). From point C,
draw a horizontal line across the pipe diameter scale (Point D).
§ A pipe with a bore of 130 mm is required; the nearest commercially available size, 150 mm,
would be selected.
2.2.3 Piping layout
The European Standard EN45510, Section 4.12 states that whenever possible, steam mains
should be installed with a fall of not less than 1:100 (1 m fall for every 100 m run), in the
direction of the steam flo w. This slope will ensure that gravity, as well as the flow of steam, will
assist in moving the condensate towards drain points where the condensate may be safely and
effectively removed (Figure 6).
Figure 6. Typical Steam Piping Installation (Spirax Sarco)
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Thermal Equipment: Steam distribution and utilization
2.3 Drain points
The drain point6 must ensure that the condensate can reach the steam trap. Careful consideration
must therefore be given to the design and location of the drain points. Consideration must also be
given to condensate remaining in a steam main at shutdown, when steam flow ceases. Gravity
will ensure that the water (condensate) will run along sloping pipework and collect at low points
in the system. Steam traps should therefore be fitted to these low points.
The amount of condensate formed in a large steam main under start-up conditions is sufficient to
require the provision of drain points at intervals of 30m to 50m, as well as natural low points
such as at the bottom of rising pipework. In normal operation, steam may flow along the main at
speeds of up to 145 km/h, dragging condensate along with it. Figure 7 shows a 15 mm drain pipe
connected directly to the bottom of a main.
Figure 7. Trap Pocket Too Small (Spirax Sarco)
Although the 15 mm pipe has sufficient capacity, it is unlikely to capture much of the condensate
moving along the main at high speed. This arrangement will be ineffective. A more reliable
solution for the removal of condensate is shown in Figure 8. The trap line should be at least 25 to
30 mm from the bottom of the pocket for steam mains up to 100 mm, and at least 50 mm for
larger mains. This allows a space below for any dirt and scale to settle. Such dirt and scale can
easily be removed if the bottom of the pocket is fitted with a removable flange or blowdown
valve.
Figure 8. Properly Sized Trap Pocket (Spirax Sarco)
6
Section 2.3 is taken fromModule 10.3 Steam Mains and Drainage. In: Spirax Sarco Learning Centre, Block 10, ‘Steam
Distribution’. www.spiraxsarco.com
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©UNEP
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Thermal Equipment: Steam distribution and utilization
Recommended drain pocket dimensions are shown in Figure 9 below.
Figure 9. Recommended Drain Pocket Dimensions (Spirax Sarco)
2.4 Branch lines
Branch lines 7 are normally much shorter than steam mains. As a general rule, therefore, provided
the branch line is not more than 10 metres in length, and the pressure in the main is adequate, it
is possible to size the pipe on a velocity of 25 to 40 m/s, and not to worry about the pressure
drop.
Figure 10. A branch line (Spirax Sarco)
7
Section 2.4 is taken fromModule 10.3 Steam Mains and Drainage. In: Spirax Sarco Learning Centre, Block 10, Steam
Distribution. www.spiraxsarco.com
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©UNEP
17
Thermal Equipment: Steam distribution and utilization
2.4.1 Branch line connections
Branch line connections taken from the top of the main line, carry the driest steam (Figure 10). If
connections are taken from the side, or even worse from the bottom (as in Figure 11a), they can
accept the condensate and debris from the steam main. The result is very wet and dirty steam
reaching the equipment, which will affect performance in both the short and long term. The
valve in Figure 11b should be positioned as near to the off-take as possible to minimize
condensate lying in the branch line, if the plant is likely to be shutdown for any extended
periods.
Figure 11a. Incorrect Steam Off-take
(Spirax Sarco)
Figure 11b. Correct Steam Off-take
(Spirax Sarco)
2.4.2 Drop leg
Low points will also occur in branc h lines. The most common is a drop leg close to an isolating
valve or a control valve (Figure 12). Condensate can accumulate on the upstream side of the
closed valve, and then be propelled forward with the steam when the valve opens again consequently a drain point with a steam trap set is good practice just prior to the strainer and
control valve.
Figure 12. Drop Leg Supplying Steam to a Heater (Spirax Sarco)
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Thermal Equipment: Steam distribution and utilization
2.4.3 Rising Ground and Drainage
There are many occasions when a steam main must run across rising ground, or applications
where the contours of the site make it impractical to lay the pipe with the 1:100 fall. In these
situations, the condensate must be encouraged to run downhill and against the steam flow. Good
practice is to size the pipe on a low steam velocity of not more than 15 m/s, to run the line at a
slope of no less than 1:40, and install the drain points at not more than 15 meter intervals (see
Figure 13). The objective is to prevent the condensate film on the bottom of the pipe increasing
in thickness to the point where droplets can be picked up by the steam flow.
Figure 13. Reverse Gradient on Steam Main (Spirax Sarco)
2.5 Strainers
This section provides an overview of strainers. 8
As the marketplace becomes increasingly competitive, more emphasis has been placed on reducing
plant downtime and maintenance. In steam and condensate systems, damage to plant is frequently
caused by pipeline debris such as scale, rust, jointing compound, weld metal and other solids,
which may find their way into the pipeline system. Strainers are devices which arrest these solids
in flowing liquids or gases, and protect equipment from their harmful effects, thus reducing
downtime and maintenance. A strainer should be fitted upstream of every steam trap, flow meter
and control valve.
Strainers can be classified into two main types according to their body configuration; namely the
Y-type and the basket type. Typical examples of these types of strainers can be seen in Figure 14.
8
Section 2.5 is taken fromModule 12.4 Strainers. In: Spirax Sarco Learning Centre, Block 12, ‘Pipeline Ancillaries’.
www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
A
Figure 14a. Y-Type Strainer
(Spirax Sarco)
Figure 14b. Basket Type Strainer
(Spirax Sarco)
2.5.1 Y-type strainers
For steam, a Y-type strainer is the usual standard and is almost universa lly used. Its body has a
compact cylindrical shape that is very strong and can handle high pressures. It is literally a pressure
vessel, and it is not uncommon for Y-type strainers to be able to handle pressures of up to 400 bar
g. The use of strainers at these pressures is however complicated by the high temperatures
associated with the steam at this pressure; and subsequently exotic materials such as chromemolybdenum steel have to be used.
Although there are exceptions, size for size, Y-type strainers have a lower dirt holding capacity
than basket strainers, which means that they require more frequent cleaning. On steam systems,
this is generally not a problem, except where high levels of rust are present, or immediately after
commissioning when large amounts of debris can be introduced. On applications where significant
amount of debris are expected, a blowdown valve can usually be fitted in a strainer cap, which
enables the strainer to use the pressure of the steam to be cleaned, and without having to shut down
the plant.
Y-type strainers in horizontal steam or gas lines should be installed so that the pocket is in the
horizontal plane (Figure 15a). This stops water collecting in the pocket, helping to prevent water
droplets being carried over, which can cause erosion and affect heat transfer processes. On liquid
systems however, the pockets should point vertically downwards (Figure 15c).
Although it is advisable to install strainers in horizontal lines, this is not always possible, and they
can be installed in vertical pipelines if the flow is downwards, in which case the debris is naturally
directed into the pocket (Figure 15b). Installation is not possible with upward flow, as the strainer
would have to be installed with the opening of the pocket pointing downwards and the debris would
fall back down the pipe.
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Thermal Equipment: Steam distribution and utilization
(a). Steam or gas applications
(b) Flow
vertically
downwards
(c) Liquid applications
Figure 15. Correct Orientation of Strainers (Spirax Sarco)
2.5.2 Straight and angle type strainers
In addition to Y-type strainers, straight and angle type strainers are used when the geometry
of the steam pipework does not suit a Y-type strainer being used.
2.5.3 Basket type strainers
The basket type or pot type strainer is characterized by a vertically orientated chamber,
typically larger than that of a Y- type strainer. Size for size, the pressure drop across a basket
strainer is less than that across the Y-type as it has a greater free straining area, which makes
the basket type strainer the preferred type for liquid applications. As the dirt holding capacity
is also greater than in Y-type strainers, the basket type strainer is also used on larger
diameter steam pipelines. Basket type strainers can only be installed in horizontal pipelines,
and for larger, heavier basket strainers, the base of the strainer needs to be supported.
When basket type strainers are used on steam systems, a significant amount of condensate
may be formed. Consequently, strainers designed for use in steam systems usually have a
drain plug, which can be fitted with a steam trap to remove the condensate. Basket type
strainers are commonly found in a duplex arrangement. A second strainer is placed in
parallel with the primary strainer, and flow can be diverted through either of the two
strainers. This facilitates cleaning of the strainer unit whilst the fluid system is still
operating, reducing the downtime for maintenance.
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Thermal Equipment: Steam distribution and utilization
Figure 16. A Duplex Basket Strainer (Spirax Sarco)
2.5.4 Strainer Screens
There are two types of screens used in strainers:
§ Perforated screens. These are formed by punching a large number of holes in a flat sheet of
the required material using a multiple punch. The perforated sheet is then rolled into a tube
and spot welded together. These are relatively coarse screens and hole sizes typically range
from 0.8 mm to 3.2 mm. Consequently, perforated screens are only suitable for removing
general pipe debris.
§ Mesh screens. Fine wire is formed into a grid or mesh arrangement. This is then commonly
layered over a perforated screen, which acts as a support cage for the mesh. By using a mesh
screen, it is possible to produce much smaller hole sizes than with perforated screens. Hole
sizes as small as 0.07 mm are achievable. Subsequently, they are used to remove smaller
particles, which would otherwise pass through a perforated screen. Mesh screens are usually
specified in terms of ‘mesh’
; which represents the number of openings per linear inch of
screen, measured from the centerline of the wire. Figure 17 shows a 3- mesh screen.
Figure 17. Examples of a 3-mesh screen (Spirax Sarco)
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Thermal Equipment: Steam distribution and utilization
2.5.5 Strainer Options
In addition to standard strainers, there are several other options available.
Magnetic Inserts
A magnetic insert may be placed in a basket type strainer in order to remove small iron or steel
debris. Small particles of iron or steel may be present in a fluid where there is wear of iron or
steel parts. These particles will pass through even the finest mesh screens, and it is necessary to
use a magnetic insert. The insert is designed so that all the fluid passes over the magne t at
relatively low velocity and the magnetic element is powerful enough to catch and hold all the
metal particles present. The magnetic material is usually encased in an inert material such as
stainless steel to prevent corrosion.
Self Cleaning Strainers
There are number of different types of self-cleaning strainer, which enable the build up of debris
on the screen to be removed without shutting down the plant. The cleaning process can be
initiated either manually or automatically; furthermore, strainers that are automatically cleaned
can usually be set to clean either on a periodic basis, or when the pressure drop across the
strainer increases. The most common types are:
§ Mechanical type self-cleaning strainers, which use some form of mechanical scraper or
brush, which is raked over the screen surface. It dislodges any debris that is trapped in the
screen, causing it to fall down into a collection area at the bottom of the strainer.
§ Backwashing type strainers, which reverse the direction of flow through the screen. A set of
valves is changed over so that water is directed across the screen in the reverse direction and
out through a flush valve. The fluid dislodges any debris entrained in the screen and carries it
out in the backwash fluid to a waste drain.
Temporary Strainers
Temporary strainers are designed for protection of equipment and instrumentation during startup periods. The strainer is usually installed between a set of flanges for an initial period after a
new plant has been installed.
2.6 Filters
Filters are used to remove smaller particles.9 Whilst strainers remove all visible particles in
the steam, it is sometimes necessary to remove smaller particles, for example, in the
following applications:
§ When there is direct injection of steam into a process, which may cause contamination of
the product. Example: In the food industry, and for the sterilization of process equipment
in the pharmaceutical industry.
§ Where dirty steam may cause rejection of a product or process batch due to staining or
visible particle retention. Example: Sterilizers and paper/board machines.
§ Where minimal particle emission is required from steam humidifiers. Example:
Humidifiers used in a “clean”environment.
9
Section 2.6 is taken fromModule 12.4 Strainers. In: Spirax Sarco Learning Centre, Block 12, ‘Pipeline Ancillaries’.
www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
§
For the reduction of the steam water content, ensuring a dry, saturated supply.
In such 'clean steam' applications, strainers are not suitable and filters must be used. A filter
used in a steam system typically consists of a sintered stainless steel filter element. The
sintering process produces a fine porous structure in the stainless steel, which removes any
particles from fluid passing through it. Filters capable of removing particles as small as 1
/gym are available, conforming to the good practice needs of culinary steam.
The fine porous nature of the filter element will create a large pressure drop across the filter
than that associated with the same size strainer; this must be given careful consideration
when sizing such filters. In addition filters are easily damaged by excessive flow rates, and
the manufacturers’limits should not be exceeded.
When the filter is used in steam applications, a separator should be fitted upstream of the
filter to remove any droplets of condensate held in suspension. In addition to improving the
quality of the steam, this will prolong the life of the filter. A Y-type strainer should also be
fitted upstream of the filter to remove all larger particles which would otherwise rapidly
block the filter, increase the amount of cleaning required and reduce the life of the filter
element. By installing pressure gauges either side of the filter, the pressure drop across the
filter can be measured, which can then be used to identify when the filter requires cleaning.
An alternative to this is to install a pressure switch on the downstream side of the filter.
When the downstream pressure decreases below a set level, an alarm light can be switched
on in a control room alerting an operator who can then clean the filter.
Figure 18. A Horizontal In-line Filter (Spirax Sarco)
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Thermal Equipment: Steam distribution and utilization
2.7 Separators
Separators 10 are used to remove suspended water droplets from steam. Wet steam is steam
containing a degree of water, and is one of the main concerns in any steam system. It can
reduce plant productivity and product quality, and can cause damage to most items of plant
and equipment. Whilst careful drainage and trapping can remove most of the water, it will
not deal with the water droplets suspended in the steam. To remove these suspended water
droplets, separators are installed in steam pipe lines.
The steam produced in a boiler designed to generate saturated steam is inherently wet.
Although the dryness fraction will vary according to the type of boiler, most shell type steam
boilers will produce steam with a dryness fraction of between 95 and 98 percent.
The water content of the steam produced by the boiler is further increased if priming and
carryover occur. There is always a certain degree of heat loss from the distribution pipe,
which causes steam to condense. The condensed water molecules will eventually gravitate
towards the bottom of the pipe forming a film of water. Steam flowing over this water can
raise ripples that can build up into waves. The tips of the waves tend to break off, throwing
droplets of condensate into the steam flow.
The presence of water in steam can cause a number of problems:
§ As water is an extremely effective barrier to heat transfer, its presence can reduce plant
productivity and product quality.
§ Water droplets traveling at high steam velocities will erode valve seats and fittings, a
condition known as wiredrawing. The water droplets will also increase the amount of
corrosion.
§ Increased scaling of pipework and heating surfaces from the impurities carried in the
water droplets.
§ Erratic operation of control valves and flow meters.
§ Failure of valves and flow meters due to rapid wear or water hammer.
Although there are a number of different designs of separator, they all attempt to remove the
moisture that remains suspended in the steam flow, which cannot be removed by drainage
and steam trapping.
There are three types of separator in common use in steam systems:
2.7.1 Baffle type separators
A baffle or vane type separator consists of a number of baffle plates, which cause the flow to
change direction a number of times as it passes through the separator body. The suspended
water droplets have a greater mass and a greater inertia than the steam; thus, when there is a
change in flow direction, the dry steam flows around the baffles and the water droplets collect
on the baffles.
10
Section 2.7 is taken fromModule 12.5 Separators. In: Spirax Sarco Learning Centre, Block 12, ‘Pipeline Ancillaries’.
www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
Furthermore, as the separator has a large cross-sectional area, there is a resulting reduction in
the speed of the fluid. This reduces the kinetic energy of the water droplets, and most of them
will fall out of suspension. The Condensate collects in the bottom of the separator, where it is
drained away through a steam trap.
Figure 19. Baffle Type Separator
(Spirax Sarco)
2.7.2 Cyclonic Type
The cyclonic or centrifugal type separator uses a series of fins to generate high-speed cyclonic
flow. The velocity of the steam causes it to swirl around the body of the separator, throwing
the heavier, suspended water to the wall, where it drains down to a steam trap installed under
the unit.
Figure 20. Cyclonic Type Separator (Spirax Sarco)
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Thermal Equipment: Steam distribution and utilization
2.7.3 Coalescence Type
Coalescence type separators provide an obstruction in the steam path. The obstruction is
typically a wire mesh pad (sometimes referred to as a demister pad), upon which water
molecules become entrapped. These water molecules tend to coalesce, producing droplets
that are too large to be carried further by the gas system. As the size of the droplets
increases, they become too heavy and ultimately fall into the bottom of the separator.
It is common to find separators, which combine both coalescence and cyclonic type
operations. By combining the two methods, the overall efficiency of the separator is
improved.
Figure 21. Coalescence Type Separator
(Spirax Sarco)
2.8 Steam traps
2.8.1 What are steam traps?
No steam system is complete without that crucial component 'the steam trap' (or trap) 11 . This is
the most important link in the condensate loop because it connects steam usage with condensate
return. A steam trap quite literally 'purges' condensate, (as well as air and other incondensable
gases), out of the system, allowing steam to reach its destination in as dry a state/condition as
possible to perform its task efficiently and economically.
The quantity of condensate a steam trap has to deal with may vary considerably. It may have to
discharge condensate at steam temperature (i.e. as soon as it forms in the steam space) or it may
be required to discharge below steam temperature, giving up some of its 'sensible heat' in the
process.
11
Section 2.8.1 is a summary of Module 11.1 Introduction –Why steam traps? In: Spirax Sarco Learning Centre, Block 11,
‘Steam Traps and Steam Trapping’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
The pressures at which steam traps can operate may be anywhere from vacuum to well over a
hundred bar. To suit these varied conditions there are many different types, each having their
own advantages and disadvantages. One type of steam trap cannot possibly be the correct choice
for all applications. Considerations for steam trap selection include the ability of the steam trap
to:
§ Vent air at 'start- up', i.e. the beginning of the process when the heater space is filled with air,
which unless displaced, will reduce heat transfer and increase the warm- up time
§ Remove condensate but not the steam
§ Maximize plant performance. Simply put, unless specifically designed to waterlog, for a heat
exchanger to operate at its best performance, the steam space must be filled with clean dry
steam. The type of steam trap will influence this.
There are three basic types of steam trap into which all variations fall. All three are classified by
International Standard ISO 6704:1982. These are shown in Figure 22 and include:
§ Thermostatic (operated by changes in fluid temperature). The temperature of saturated
steam is determined by its pressure. In the steam space, steam gives up its enthalpy of
evaporation (heat), producing condensate at steam temperature. As a result of any further
heat loss, the temperature of the condensate will fall. A thermostatic trap will pass
condensate when this lower temperature is sensed. As steam reaches the trap, the temperature
increases and the trap closes.
§ Mechanical (operated by changes in fluid density). This range of steam traps operates by
sensing the difference in density between steam and condensate. These steam traps include
'ball float traps' and 'inverted bucket traps'. In the 'ball float trap', the ball rises in the presence
of condensate, opening a valve, which passes the denser condensate. With the 'inverted
bucket trap', the inverted bucket floats when steam reaches the trap and rises to shut the
valve. Both are essentially 'mechanical' in their method of operation.
§ Thermodynamic (operated by changes in fluid dynamics). Thermodynamic steam traps rely
partly on the formation of flash steam from condensate. This group includes
'thermodynamic', 'disc', 'impulse' and 'labyrinth' steam traps.
Steam Traps
Thermostatic
1.
2.
3.
Liquid expansion
Balance pressure
Bimetallic
Mechanical
1.
2.
Ball floating
Inverted bucket
Thermodynamic
1.
2.
3.
Impulse
Labyrinth
Fixed orifice
Figure 22. Types of steam traps
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Thermal Equipment: Steam distribution and utilization
Also loosely included in this type are 'fixed orifice traps', which cannot be clearly defined as
automatic devices as they are simply a fixed diameter hole set to pass a calculated amount of
condensate under one set of conditions. All rely on the fact that hot condensate, released under
dynamic pressure, will flash-off to give a mixture of steam and water.
Because mechanical steam traps are most commonly used, only these will be described in more
detail. For more details on all types of steam traps please refer to the Spirax Sarco Learning
Centre (www.spiraxsarco.com).
2.8.2 Mechanical steam traps
Mechanical steam traps include ball float steam traps and inverted steam traps, which are
described below.12
a) Ball float steam trap
The ball float type trap operates by sensing the difference in density between steam and
condensate. In the case of the trap shown in Figure 23a, condensate reaching the trap will cause
the ball float to rise, lifting the valve off its seat and releasing condensate. As can be seen, the
valve is always flooded and neither steam nor air will pass through it. Modern traps use a
thermostatic air vent, as shown in Figure 23b. This allows the initial air to pass whilst the trap is
also handling condensate.
Figure 23a. Float Trap with Air Cock
(Spirax Sarco)
Figure 23b. Float Trap with Thermostatic
Air Vent (Spirax Sarco)
The automatic air vent uses the same balanced pressure capsule element as a thermostatic steam
trap, and is located in the steam space above the condensate level. After releasing the initial air, it
remains closed until air or other non-condensable gases accumulate during normal running and
cause it to open by reducing the temperature of the air/steam mixture. The thermostatic air vent
offers the added benefit of significantly increasing condensate capacity on cold start- up.
12
Section 2.8.2 is taken from Module 11.3 Mechanical Steam Traps. In: Spirax Sarco Learning Centre, Block 11, ‘Steam
Traps and Steam Trapping’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
In the past, the thermostatic air vent was a point of weakness if water hammer was present in the
system. Even the ball could be damaged if the water hammer was severe. However, in modern
float traps the air vent is a compact, very robust, all stainless steel capsule, and the modern
welding techniques used on the ball makes the complete float-thermostatic steam trap very
robust and reliable in waterhammer situations (for information on waterhammer see item 8 in
section 4).
In many ways the float-thermostatic trap is the closest to an ideal steam trap. It will discharge
condensate as soon as it is formed, regardless of changes in steam pressure.
Advantages of the float-thermostatic steam trap
§ The trap continuously discharges condensate at steam temperature. This makes it the first
choice for applications where the rate of heat transfer is high for the area of heating surface
available.
§ It is able to handle heavy or light condensate loads equally well and is not affected by wide
and sudden fluctuations of pressure or flowrate.
§ As long as an automatic air vent is fitted, the trap is able to discharge air freely.
§ It has a large capacity for its size.
§ The versions which have a steam lock release valve are the only type of trap entirely suitable
for use where steam locking can occur.
§ It is resistant to water hammer.
Disadvantages of the float-thermostatic steam trap
§ Although less susceptible than the inverted bucket trap, the float type trap can be damaged by
severe freezing and the body should be well lagged, and /or complemented with a small
supplementary thermostatic drain trap, if it is to be fitted in an exposed position.
§ As with all mechanical type traps, different internals are required to allow operation over
varying pressure ranges. Traps operating on higher differential pressures have smaller
orifices to balance the buoyancy of the float.
b) Inverted bucket steam trap
The inverted bucket steam trap is shown in Figure 24. As its name implies, the mechanism
consists of an inverted bucket, which is attached by a lever to a valve. An essential part of the
trap is the small air vent hole in the top of the bucket. Figure 24 shows the method of operation.
In (i) the bucket hangs down, pulling the valve off its seat. Condensate flows under the bottom of
the bucket filling the body and flowing away through the outlet. In (ii) the arrival of steam causes
the bucket to become buoyant, it then rises and shuts the outlet. In (iii) the trap remains shut until
the steam in the bucket has condensed or bubbled through the vent hole to the top of the trap
body. It will then sink, pulling the main valve off its seat. Accumulated condensate is released
and the cycle is repeated.
In (ii), air reaching the trap at start-up will also give the bucket buoyancy and close the valve.
The bucket vent hole is essential to allow air to escape into the top of the trap for eventual
discharge through the main valve seat. The hole, and the pressure differential, is small so the trap
is relatively slow at venting air. At the same time it must pass (and therefore waste) a certain
amount of steam for the trap to operate once the air has cleared. A parallel air vent fitted outside
the trap will reduce start-up times.
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Thermal Equipment: Steam distribution and utilization
Figure 24. Operation of an Inverted Bucket Steam trap
(Spirax Sarco)
Advantages of the inverted bucket steam trap
§ The inverted bucket steam trap can be made to withstand high pressures.
§ Like a float-thermostatic steam trap, it has a good tolerance to waterhammer conditions.
§ Can be used on superheated steam lines with the addition of a check valve on the inlet.
§ Failure mode is usually open, so it is safer on those applications that require this feature, for
example turbine drains.
Disadvantages of the inverted bucket steam trap
§ The small size of the hole in the top of the bucket means that this type of trap can only
discharge air very slowly. The hole cannot be enlarged, as steam would pass through too
quickly during normal operation.
§ There should always be enough water in the trap body to act as a seal around the lip of the
bucket. If the trap loses this water seal, steam can be wasted through the outlet valve. This
can often happen on applications where there is a sudden drop in steam pressure, causing
some of the condensate in the trap body to 'flash' into steam. The bucket loses its buoyancy
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Thermal Equipment: Steam distribution and utilization
§
§
§
and sinks, allowing live steam to pass through the trap orifice. Only if sufficient condensate
reaches the trap will the water seal form again, and prevent steam wastage.
If an inverted bucket trap is used on an application where pressure fluctuation of the plant
can be expected, a check valve should be fitted on the inlet line in front of the trap. Steam
and water are free to flow in the direction indicated, while reverse flow is impossible, as the
check valve would be forced onto its seat.
The higher temperature of superheated steam is likely to cause an inverted bucket trap to lose
its water seal. A check valve in front of the trap should be regarded as essential under such
conditions. Some inverted bucket traps are manufactured with an integral check valve as
standard.
The inverted bucket trap is likely to suffer damage from freezing if installed in an exposed
position with sub-zero ambient conditions. As with other types of mechanical traps, suitable
lagging can overcome this problem if conditions are not too severe. If ambient conditions
well below zero are to be expected, then it may be prudent to consider a more robust type of
trap to do the job. In the case of mains drainage, a thermodynamic trap would be the first
choice.
2.8.3 Steam trap selection and installation
Table 3 summarizes the selection of steam traps for different applications.
Table 3. Selection of suitable steam traps for different process applications (BEE, 2004)
Application
Steam mains
§
§
§
§
§
§
§
Equipment
Reboiler
Heater
Dryer
Heat exchanger etc.
Tracer line
Instrumentation
Feature
§ Open to atmosphere, small
capacity
§ Frequent change in pressure
§ Low pressure - high pressure
§ Large capacity
§ Variation in pressure and
temperature is undesirable
§ Efficiency of the equipment is a
problem
§ Reliability with no over heating
Suitable trap
Thermodynamic,
Mechanical:
Float
Mechanical:
Float
Bucket
Inverted bucket
Thermodynamic,
Thermostatic: Bimetallic
When selecting and installing a steam trap, the following sho uld be considered:13
a) Waterhammer
Waterhammer is condensate in the steam system that is picked up by moving steam and can
cause damage to pipelines, fittings and steam traps. Symptoms of waterhammer are often
attributed to malfunction of the steam trap. A more likely explanation is that a faulty steam trap
has been damaged by waterhammer. Waterhammer can be caused in a number of ways,
including:
§ Failure to remove condensate from the path of high velocity steam in the pipework.
13
Section 2.8.3 is a summary of (a) Module 11.5 Considerations for Selecting Steam Traps. In: Spirax Sarco Learning
Centre, Block 11, ‘Steam Traps and Steam Trapping’. www.spiraxsarco.com, and (b) Energy Efficiency in Thermal Utilities.
Book 2, by the India Bureau of Energy Efficiency, 2004
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Thermal Equipment: Steam distribution and utilization
§
§
From an application, which is temperature controlled and where condensate has to lift to a
return line, or return to a pressurized system.
The inability of condensate to properly enter or travel along an undersized return line, due to
either (a) flooding, or (b) overpressurisation with the throttling effects of flash steam.
The problem of waterhammer can be eliminated by positioning the pipes so that there is a
continuous slope in the direction of flow. A slope of at least 12 mm in every 3 metres is
necessary, as also an adequate number of drain points every 30 to 50 metres.
For more information on waterhammer see item 8 in section 4.
b) Dirt
Dirt is another major factor which must be considered when selecting traps. Although steam
condenses to distilled water, it can sometimes contain trace products of boiler feed treatment
compound and natural minerals found in water. Pipe dirt created during installation and the
products of corrosion also need to be considered.
c) Strainers
These devices are frequently forgotten about in steam systems, often, it seems, in an effort to
reduce installation costs. Pipe scale and dirt can affect control valves and steam traps, and reduce
heat transfer rates. It is extremely easy and inexpensive to fit a strainer in a pipe, and the low cost
of doing so will pay dividends throughout the life of the installation. Scale and dirt are arrested,
and maintenance is usually reduced as a result.
Selection is simple. The strainer material is selected to match the type of installation and the
system pressure up to which it is expected to operate. Different filter screen sizes may be
considered for differing degrees of protection. The finer the filter, the more often it may need
cleaning. One thing is certain, strainers are far easier and cheaper to buy and maintain than
control valves or steam traps.
d) Steam locking
The possibility of steam locking can sometimes be a deciding factor in the selection of steam
traps. It can occur whenever a steam trap is fitted remotely from the plant being drained. It can
become acute when condensate is removed through a syphon or dip pipe. To relieve this problem
a trap is needed with a 'steam lock release' valve. This is an internal needle valve which allows
the steam locked in the syphon pipe to be bled away past the main valve. The float trap is the
only type of trap with this facility and is the correct choice on rotating machinery such as drying
cylinders.
e) Group trapping
Group trapping describes the use of one trap serving more than one application (Figure 25). The
original reason for group trapping was that there used to be only one kind of steam trap. It was
the forerunner of the present day bucket trap, and was very large and expensive. Steam traps
today are considerably smaller and cost effective, allowing individual heat exchangers to be
properly drained. It is always better for steam using equipment to be trapped on an individual
basis rather than on a group basis.
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The only satisfactory arrangement is to drain each steam space with own trap and then connect
the outlets of the various traps to the common condensate return main (Figure 25).
Figure 25. Group Trapping and Individual Trapping
with Common Condensate Return (BEE, 2004)
f) Diffusers
With steam traps draining to atmosphere from open ended pipes, it is possible to see the
discharge of hot condensate. A certain amount of flash steam will also be present relative to the
condensate pressure before the trap. This can present a hazard to passers by, but the risks can be
minimized by reducing the severity of the discharge. This may be achieved by fitting a simple
diffuser (Figure 25) to the end of the pipe which reduces the ferocity of discharge and sound.
Typically, sound levels can be reduced by up to 80%.
Figure 26. Diffuser (Spirax Sarco)
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g) Drain point
The drain point should be large enough and should be located to allow the condensate to flow
into the trap easily. For example, a 150 mm steam main will require a drain of at least 100 mm
diameter and 150 mm deep located at the bottom of the main. Table 4 can be used to select the
drain point dimensions.
Table 4. Drain Pocket Dimensions (Spirax Sarco)
h) Pipe sizing
The pipes leading to and from steam traps should be of adequate size. This is particularly
important in the case of thermodynamic traps, because their correct operation can be disturbed
by excessive resistance to flow in the condensate pipework. Pipe fittings such as valves, bends
and tees close to the trap could also cause excessive back pressures and should be avoided.
i) Air venting
When air is carried into the trap space by the steam, the trap function can be affected unless
adequate provision is made for removing air either through the steam trap or a separate air vent.
If air is not vented properly, the plant can take a long time to warm up and may operate below its
potential output.
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Thermal Equipment: Steam distribution and utilization
2.9 Air vents
This section explains the use of air vents in a steam distribution system. 14
2.9.1 Effect of air on steam
If air is mixed with steam and flows along with it, pockets of air will remain at the heat exchange
surfaces where the steam condenses. Gradually, a thin layer builds up to form an insulating
blanket, hindering heat transfer as shown in Figure 27.
The thermal conductivity of air is 0.025 W/m °C, while the corresponding value for water is
typically 0.6 W/m °C, for iron it is about 75 W/m °C and for copper about 390 W/m °C. A film
of air only 1 mm thick offers about the same resistance to heat flow as a wall of copper some
15 metres thick!
It is unlikely that the air exists as an even film inside the heat exchanger. More probably, the
concentration of air close to the condensing surface is higher, and lower further away. It is
convenient however, to deal with it as a homogenous layer when trying to show its resistance to
heat flow.
When air is added to steam, the heat content of a given volume of the mixture is lower than the
same volume of pure steam, so the mix temperature is lowered. Hence, the presence of air has a
double effect:
§ It offers a resistance to heat transfer via its layering effect
§ It reduces the temperature of the steam space thus reducing the temperature gradient across
the heat transfer surface
Figure 27. Effect of Air on heat Transfer (Spirax Sarco)
14
Section 2.9 is a summary of Module 11.12 Air Venting Theory. In: Spirax Sarco Learning Centre, Block 11, ‘Steam Traps
and Steam Trapping’.www.spiraxsarco.com
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The overall effect is to reduce the heat transfer rate below that which may be required by a
critical process, and in worst cases may even prevent a final required process temperature being
reached. In many processes, a minimum temperature is needed to achieve a chemical or physical
change in a product, just as a minimum temperature is essential in a sterilizer. The presence of
air is particularly problematic because it will cause a pressure gauge to mislead. It follows that
the temperature cannot be inferred from the pressure.
2.9.2 Air in the system
Air is present within steam pipes and steam equipment at start-up. Even if the system were filled
with pure steam when used, the condensing steam would cause a vacuum and draw air into the
pipes at shutdown. Air can also enter the system in solution in the feedwater. At 80°C, water can
dissolve about 0.6 percent of its volume, of air.
Signs of air are:
§ A gradual fall off in the output of any steam heated equipment
§ Air bubbles in the condensate
§ Corrosion
2.9.3 Air removal
The most efficient means of air venting is with an automatic device. Air mixed with steam
lowers the mix temperature. This enables a thermostatic device (based on either the balanced
pressure or bimetallic principle) to vent the steam system. An air vent fitted on the steam space
of a vessel (Figure 28) or at the end of a steam main (Figure 29) will open when air is present.
For maximum removal of air, the discharge should be as free as possible. A pipe is often fitted to
carry the discharge to a safe location, preferably not a condensate return line, which could
restrict the free release of air and may also encourage corrosion.
Figure 28. Jacketed Pan with
Automatic Air Vent (Spirax Sarco)
Figure 29: End of Main Automatic
Air Vent (Spirax Sarco)
When an air vent is fitted to bypass a steam trap (Figure 29), it will act as a steam trap after the
air is vented, and may from time to time discharge condensate. In such cases it is necessary to
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Thermal Equipment: Steam distribution and utilization
reconnect the air vent to the condensate line after the trap.
If the condensate discharge line from a trap rises to high level, the flooded line imposes a
backpressure on the trap and the air vent. The ability of the air vent to discharge air is reduced,
especially at start-up. This applies equally when the air vent is incorporated within a steam trap.
When the shape of the application steam space and the location of the steam inlet mean that most
of the air leaves through the condensate outlet, it is preferable if discharge lines from the steam
trap and air vent do not rise to high level.
Figure 29. Inverted Bucket Trap with Parallel Air Vent
(Spirax Sarco)
2.9.4 The air vent location
When a coil or a vessel has a relatively small cross-section, the steam admitted to it will act like
a piston, pushing the air to a point remote from the steam inlet. This 'remote point' is usually the
best location for the air vent. In the case of a steam user of the shape shown in Figure 29, some
of the air will pass through the condensate outlet, according to the provision made in the trap, or
in a bypass, for handling air. The rest of the air might collect as indicated, forming a cold spot on
the heating surface. The unit cannot warm up evenly, and distortion may be caused in some
equipment, such as the beds of laundry ironers.
As an air/steam mixture is denser than pure steam at the same pressure, it is usually sufficient to
provide air venting capability within the low- lying steam trap. However, the mode of operation
of the trap means that condensate forms a water seal at the trap inlet sometimes preventing air
from reaching the trap.
There may be the need to consider an automatic air vent connected to the steam space above the
level of any condensate. Often it is convenient and sufficiently effective to connect it to the top
of the steam space, as in Figure 30.
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Thermal Equipment: Steam distribution and utilization
Figure 30. Air Vent at the opposite end of the Steam Inlet (Spirax Sarco)
However, in the case of two steam spaces of the same size and shape but with different steam
inlet positions, the location of the air vent could be different. In Figure 31 and Figure 32,
condensate drains from the bottom of the vessel but with the bottom steam inlet, at start-up, air
would tend to be pushed to the remote point which is at the top. It may be best to locate an air
vent at the top whilst a float-thermostatic steam trap will handle any residual air which has
collected at the bottom of the vessel.
With top steam entry, the air will tend to be pushed to the bottom at start- up, and provision
should be made for venting it at low level. Usually, a trap with a high air venting capability such
as a float-thermostatic trap will do the job. However, in practice, to ensure complete removal of
air during running conditions, a separate air vent fitted at the top of the vessel (as shown in
Figure 32) may again often prove beneficial, especially on irregularly shaped vessels .
Figure 31. Air Vent located opposite
low level steam inlet (Spirax Sarco)
Figure 32. Air Vent (in steam trap)
located opposite high level steam
inlet (Spirax Sarco)
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Thermal Equipment: Steam distribution and utilization
2.10 Condensate recovery
This section explains condensate recovery in a steam system.15
2.10.1 What is condensate recovery
When a kilogram of steam condenses completely, a kilogram of condensate is formed at the
same pressure and temperature. An efficient steam system will reuse this condensate. Failure to
reclaim and reuse condensate makes no financial, technical or environmental sense.
Saturated steam used for heating gives up its latent heat (enthalpy of evaporation), which is a
large proportion of the to tal heat it contains. The remainder of the heat in the steam is retained in
the condensate as sensible heat (enthalpy of water) as shown in Figure 33.
Figure 33. After giving up its latent heat to heat the process, steam turns
to water containing only sensible heat (Spirax Sarco)
As well as having heat content, the condensate is basically distilled water, which is ideal for use
as boiler feedwater. An efficient steam system will collect this condensate and either return it to
a deaerator, a boiler feedtank, or use it in another process. Only when there is a real risk of
contamination should condensate not be returned to the boiler. Even then, it may be possible to
collect the condensate and use it as hot process water or pass it through a heat exchanger where
its heat content can be recovered before discharging the water mass to drain.
Condensate is discharged from steam plant and equipment through steam traps from a higher to a
lower pressure. As a result of this drop in pressure, some of the condensate will re-evaporate into
‘flash steam’. The proportion of steam that will ‘flash off’in this way is determined by the
amount of heat that can be held in the steam and condensate. A flash steam amount of 10% to
15% by mass is typical. However, the percentage volumetric change can be considerably more.
Condensate at 7 bar g will lose about 13% of its mass when flashing to atmospheric pressure, but
the steam produced will require a space some 200 times larger than the condensate from which it
was formed. This can have the effect of choking undersized trap discharge lines, and must be
taken into account when sizing these lines.
15
Section 2.10 is taken from Module 14.1 Introduction to Condensate Recovery. In: Spirax Sarco Learning Centre, Block 14,
‘Condensate Recovery’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
2.10.2 Condensate recovery system
An effective condensate recovery system, collecting the hot condensate from the steam using
equipment and returning it to the boiler feed system, can pay for itself in a remarkably short
period of time. Figure 34 shows a simple steam and condensate circuit, with condensate
returning to the boiler feedtank.
Figure 34. A Typical Steam and Condensate Circuit (Spirax Sarco)
2.10.3 Reasons for condensate recovery
Reasons for condensate recovery include:
§ Financial reasons: Condensate is a valuable resource and even the recovery of small
quantities is often economically justifiable. The discharge from a single steam trap is often
worth recovering. Un-recovered condensate must be replaced in the boiler house by cold
make-up water with additional costs of water treatment and fuel to heat the water from a
lower temperature.
§ Water charges: Any condensate not returned needs to be replaced by make- up water,
incurring further water charges from the local water supplier.
§ Effluent restrictions: In the UK for example, water above 43°C cannot be returned to the
public sewer by law, because it is detrimental to the environment and may damage
earthenware pipes. Condensate above this temperature must be cooled before it is discharged,
which may incur extra energy costs. Similar restrictions apply in most countries, and effluent
charges and fines may be imposed by water suppliers for non-compliance.
§ Maximizing boiler output: Colder boiler feedwater will reduce the steaming rate of the
boiler. The lower the feedwater temperature, the more heat, and thus fuel needed to heat the
water, thereby leaving less heat to raise steam.
§ Boiler feedwater quality: Condensate is distilled water, which contains almost no total
dissolved solids (TDS). Boilers need to be blown down to reduce their concentration of
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Thermal Equipment: Steam distribution and utilization
dissolved solids in the boiler water. Returning more condensate to the feedtank reduces the
need for blowdown and thus reduces the energy lost from the boiler.
2.10.4 Layout and sizing of condensate return lines
Layout and sizing of condensate return lines are important when installing a condensate recovery
system. 16 No single set of recommendations can cover the layout of condensate pipework. Much
depends on the application pressure, the steam trap characteristics, the position of the condensate
return main relative to the plant, and the pressure in the condensate return main. For this reason it
is best to start by considering what has to be achieved, and to design a layout which will ensure
that basic good practice is met. The prime objectives are that:
§ Condensate must not be allowed to accumulate in the plant, unless the steam using apparatus
is specifically designed to operate in this way. Generally apparatus is designed to operate
non- flooded, and where this is the case, accumulated condensate will inhibit performance,
and encourage the corrosion of pipes, fittings and equipment.
§ Condensate must not be allowed to accumulate in the steam main. Here it can be picked up
by high velocity steam, leading to erosion and waterhammer in the pipework.
The subject of condensate piping will divide naturally into four basic types where the
requirements and considerations of each will differ. These four basic types are defined and
illustrated in Figure 35.
Sizing of all condensate lines is a function of:
§ Pressure. The difference in pressure between one end of the pipe and the other. This pressure
difference may either promote flow, or cause some of the condensate to flash to steam.
§ Quantity. The amount of condensate to be handled.
§ Condition. Is the condensate predominately liquid or flash steam?
16
Section 2.10.4 is a summary of Module 14.2 Layout of Condensate Return Lines and of Module 14.3 Sizing of Condensate
Return Lines. In: Spirax Sarco Learning Centre, Block 14, ‘Condensate Recovery’
. www.spiraxsarco.com. More detailed
explanations can be found in these modules.
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Thermal Equipment: Steam distribution and utilization
Figure 35. Four Types of Condensate Lines (Spirax Sarco)
2.11 Insulation of steam pipelines and hot process equipments
Insulation is an important part of conserving energy in the steam system. 17
2.11.1 Purpose of insulation
A thermal insulator is characterized by a low thermal conductivity and is therefore able to keep
heat confined within or outside a system by preventing heat transfer to or from the external
environment. Insulation materials are porous and contain a large number of dormant air cells. A
large amount of heat energy may be lost without insulation or if insulation is inefficient or
improperly installed.
Thermal insulation, by reducing heat loss, delivers the following benefits:
§ Reduction of fuel consumption
§ Better process control by maintaining process temperatures at a constant level
§ Corrosion prevention by keeping the exposed surface of a refrigerated system above dew
point
17
Section 2.11 is an edited version of Energy Efficiency in Thermal Utilities. Book 2, by the India Bureau of Energy
Efficiency, 2004, page 97 -99
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Thermal Equipment: Steam distribution and utilization
§
§
Fire protection of equipme nt
Absorbing of vibration
In addition, staff working conditions are improved because insulation protects them from
exposure to hot surfaces and radiant heat and because insulation reduces noise levels.
2.11.2 Types of insulation
Insulators can be classified based on the three temperature ranges for which they are used:
§ Low Temperature Insulators (up to 90o C), which are used for refrigerators, cold and hot
water systems, storage tanks, etc. The most commonly used materials are cork, wood, 85
percent magnesia, mineral fibres, polyurethane and expanded polystyrene
§ Medium Temperature Insulators (90 – 325 o C), which are used in low-temperature heating
and steam generating equipment, steam lines, flue ducts, etc. The most commonly used
materials include 85 percent magnesia, asbestos, calcium silicate and mineral fibres
§ High Temperature Insulators (325oC and above), which are typically used for boilers,
super-heated steam systems, oven, driers and furnaces. The most commonly used materials
are asbestos, calcium silicate, mineral fibre, mica, vermiculite, fireclay, silica and ceramic
fibre.
The table below describes the applications, advantages and disadvantages of various insulating
materials. Insulation materials can also be obtained in bulk in the form of moulded sections, for
example, semi-cylindrical for pipes and slabs for vessels, flanges, valves etc. The main advantage
of moulded sections is the ease of installation of new insulation and replacement or repair of
existing insulation.
2.11.3 Selecting insula ting materials
Important factors that should be considered when choosing insulating materials are:
§ Operating temperature of the system
§ Type of fuel being fired
§ Resistance of the materials to heat, weather and adverse conditions
§ Thermal conductivity of the material
§ Thermal diffusivity of the material
§ Ability of the material to withstand the various conditions, such as thermal shock, vibration
and chemical attack
§ Resistance of the material to flames/fire
§ Permeability of the material
§ Total cost, including material purchase, installing and maintenance
Table 6. Insulating Materials for Different Applications
TYPE OF INSULATION
APPLICATION
Polystyrene
An organic insulator made by
polymerizing styrene
Suitable for low temperatures
(-167o C to 82 oC). Mainly
used in cool rooms,
refrigeration piping and
concrete retaining structures
Suitable for low temperatures
Polyurethane
ADVANTAGES &
DISADVANTAGES
Advantages: rigid and lightweight
Disadvantages: combustible, has a
low melting point, is UV
degradable, and susceptible to
attacks by solvents
Advantages: closed cell structure,
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TYPE OF INSULATION
APPLICATION
Made by reacting isocyanides
and alcohols. Made in
continuous slab or foamed in situ
(-178o C to 4o C). Mainly used
in cool rooms, refrigerated
transports, deep freezing
cabinets, refrigeration piping
and floor and foundation
insulation
Suitable for temperatures up to
820o C. Mainly used to
insulate industrial ovens, heat
exchangers, driers, boilers and
high temperature pipes
Rockwool (mineral fibre)
Manufactured by melting basalt
and coke in a cupola at about
1500o C. Phenol-based binders
are used. Is available as mats,
blankets, and loose form or
preformed for pipe insulation
Fibreglass
Formed by bonding long glass
fibres with a thermo setting resin
to form blankets and bats, semirigid boards, high density rigid
boards and preformed pipe
sections
Calcium Silicate
Made from anhydrous calcium
silicate material reinforced with a
non-asbestos binder. Available
in slab form of various sizes.
Ce ramic fibre
Made from high purity alumina
and silica grains, melted in an
electric furnace and blasted by
high velocity gases into light
fluffy fibres. Made in various
forms, including cloth, felt, tape,
coating cements and variform
castable (fire brick)
Suitable for temperatures up to
540o C. Mainly used to
insulate industrial ovens, heat
exchangers, driers, boilers and
pipework
Suitable for temperatures up to
1050o C. Mainly used to
insulate furnace walls, fire
boxes, back-up refractory, flue
lining and boilers
Suitable for temperatures up to
1430o C. Mainly used to
insulate furnace and kiln backup refractories, fire boxes,
glass feeder bowls, furnace
repair, induction coil
insulation, high temperature
gaskets and wrapping
materials
ADVANTAGES &
DISADVANTAGES
low density and high mechanical
strength
Disadvantages: combustible,
produces toxic vapors and has a
tendency to smolder
Advantages: has a wide density
range and is available in many
forms. It is chemically inert, noncorrosive and maintains mechanical
strength during handling
Advantages: will not settle or
disintegrate with ageing.
Disadvantages: Fibreglass products
are slightly alkaline –pH9 (neutral
is pH7). It should be protected
against external contamination to
avoid acceleration of steel corrosion
Advantages: small air cell structure,
a low thermal conductivity, and will
retain its size and shape in its
useable temperature range. It is
lightweight, but has good structural
strength so it can withstand
mechanical abrasion. It will not
burn or rot, is moisture resistant and
non-corrosive
Advantages: suitable for many
applications because of the variety
of forms
2.11.4 Insulation of steam and condensate lines
It is essential to insulate steam and condensate pipelines because they are a major source of heat
loss through heat radiation from the pipelines. Suitable ins ulation materials are cork, glass wool,
rock wool and asbestos. Flanges should also be insulated because an uncovered flange is
equivalent to leaving 0.6 meter of pipeline un- insulated (SEAV, 2005). Flanges are often not
insulated to make it easier to check the condition of the flange. A solution is to install detachable
insulation covers, which can be removed when checking the flange.
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Thermal Equipment: Steam distribution and utilization
Figure 37 gives an indication of the amount of heat loss from un-insulated pipelines. An
explanation of how to calculate the required thickness of insulation is given in section 3.
Figure 37. Heat loss from a 1-metre un-insulated pipe at various pipe
diameters (SEAV, 2005)
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Thermal Equipment: Steam distribution and utilization
3. ASSESSMENT OF STEAM DISTRIBUTION SYSTEM
This section explains how to carry out an assessment of steam traps, of heat losses from uninsulated surfaces and of savings from condensate recovery.
3.1 Assessment of steam traps
Steam traps themselves do not use a lot of energy. But malfunctioning steam traps can result in
large energy losses of the steam system. The assessment of steam trap performance is therefore
concerned with answering the following two questions:18
§ Is the trap working correctly or not?
§ If not, has the trap failed in the open or closed position?
Traps that fail in the ‘open’position cause energy losses. Any condensate not returned into the
steam system causes the boiler to heat up new water to make more steam. The steam heating
capacity may also be reduced, resulting in indirect energy losses. Traps that have failed open will
also pressurise the condensate discharge lines and affect the discharge efficiency of other traps.
Traps that fail ‘closed’do not result in energy or water losses, but can result in significantly
reduced heating capacity and damage to steam heating equipment.
Four steam trap performance tests exist: visual, sound, temperature and integrated. Worksheet 2 in
section 6 of this chapter can be used to carry out a steam trap audit.
3.1.1 Visual testing
Visual testing of the flow or variation of flow of steam traps makes use of sight glasses (Figure
41), sight checks, test tees and three-way test valves. This method works well with traps that have
an on/off cycle or low flows. This method becomes less viable when the flow, and thus the volume
of water and flash steam, is high. One solution is to divert condensate before it reaches the trap and
if the trap continues to flow then this is a sign of leakage.
Figure
18
Section 3.1 is an edited version of Energy Efficiency in Thermal Utilities. Book 2, by the India Bureau of Energy
Efficiency, 2004, page 63-64
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Thermal Equipment: Steam distribution and utilization
For testing a steam trap, there should be an isolating valve provided downstream of the trap and a
test valve in the trap discharge. When the test valve is in the open position, the following should
be observed:
§ Condensate discharge. Inverted bucket and thermodynamic disc traps should have intermittent
condensate discharge. Float and thermostatic traps should have a continuous condensate
discharge. Thermostatic traps can have either continuous or intermittent discharge depending
upon the load. If inverted bucket traps are used for extremely small load, it will have a
continuous condensate discharge.
§ Flash steam. This should not be mistaken for a steam leak in the trap. It is difficult to visually
identify whether a trap is blowing flash steam or live steam but, generally, if steam blows out
continuously in a blue stream, it is leaking live steam. If steam floats out intermittently in a
white cloud, it is flash steam.
§ Continuous steam blow and no flow indicate that there is a problem in the trap. Whenever a
trap fails to operate and the reasons are not immediately obvious, check for any lack of
discharge from the trap, steam loss, continuous flow, sluggish heating, to find out whether it is
a system problem or the mechanical problem in the steam trap.
3.1.2 Sound testing
Sound testing makes use of ultrasonic leak detectors (see Monitoring Equipment chapter),
mechanics stethoscopes, screwdriver or a metal listening rod. These methods use the sound
created by flow to determine if the trap is functioning properly. This method works well with
traps that have an on/off cycle or low flows, but not so well with traps with varying and/or high
flows. Similar to visual checking, trap leaks can be detected by diverting the flow before it reaches
the trap
3.1.3 Temperature testing
Temperature testing makes use of infrared guns (see Monitoring Equip ment chapter), surface
pyrometers, temperature tapes, and temperature crayons. These equipment measure the discharge
temperature on the outlet side of the trap, with high temperatures indicating leaks and low
temperatures indicating blocked, undersized, or faulty traps. Temperature tapes or crayons are set
at a chosen temperature level and if the temperature on the trap outlet exceeds this level. Infrared
guns and surface pyrometer can detect temperatures on both sides of the trap. To obtain reliable
readings, the pipe must be bare, clean and without scale, as all of these reduce heat transfer and
thus lower the recorded temperature. Some of the more expensive infrared guns can compensate
for wall thickness and material differences.
3.1.4 Integrated testing
Spirax Sarco mentions the development of an integrated steam trap testing device due to the
limitations of the above methods. 19 This consists of a sensor, fitted inside the steam trap, which is
capable of detecting the physical state of the medium at tha t point by conductivity. This device has
the following benefits:
§ It is not affected by flash steam disturbance.
§ The result is finite and not subject to interpretation.
19
Module 11.14 Testing and Maintenance of Steam Traps. In: Spirax Sarco Learning Centre, Block 11, ‘Steam Traps and Steam
Trapping’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
§
Monitoring can be done locally, remotely, manually or automatically, and can detect
immediate failure, thus minimizing waste and maximizing investment.
3.2 Assessment of heat losses from un-insulated surfaces
Heat losses from un-insulated surfaces can be substantial and should therefore be assessed. 20
3.2.1 Economic Thickness of Insulation (ETI)
The effectiveness of insulation follows the law of decreasing returns. This means that insulation
results in energy and cost savings, but with increasing thickness of insulation the additional amount
of energy and cost saved is going down. At a certain level, any additional insulation is no longer
economically justifiable. The point where the amount of insulation gives the greatest return on
investment is called the “economic thickness of insulation”(ETI) and is shown in Figure 40. The
ETI is calc ulated based on the following factors, which differ between companies:
§ Cost of fuel
§ Annual hours of operation
§ Heat content of fuel
§ Boiler efficiency
§ Operating surface temperature
§ Pipe diameter/thickness of surface
§ Estimated cost of insulation.
§ Average exposure ambient still air temperature
I+H
Cost
I
H
M
Insulation Thickness
I : Cost of Insulation
I + H : Total Cost
H : Cost of Heat Loss
M : Economic Thickness
Figure 40. Determination of the Economic Thickness of Insulation Material (BEE, 2004)
20
Section 3.2 is an edited version of Energy Efficiency in Thermal Utilities. Book 2, by the India Bureau of Energy
Efficiency, 2004, page 99-102
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Thermal Equipment: Steam distribution and utilization
3.2.2 Calculating heat loss - methodology
Various charts, graphs and references are available to calculate the amount of heat loss.
Worksheet 3 in section 6 of this chapter can be used to carry out an assessment of heat losses
through insulation. The heat loss can be calculated using the following equations:
Total heat loss (Hs in kCal/hr) = S x A
S = [10+(Ts-Ta)/20] (Ts-Ta)
A (m2 ) = 3.14 x diameter (m) x length (m)
Where:
S
A
Ts
Ta
= Sur face heat loss in kCal/hr m2
= Surface area in m2
= Hot surface temperature in o C
= Ambient temperature in oC
Note: This equation can be used for surface temperatures up to 2000C. Factors like wind
velocities, and conductivity of insulating material have not been considered.
The additional energy costs associated with heat loss can be calculated with the following
equations:
Equivalent fuel loss (Hf) (kg/year)
Annual costs of heat loss ($)
Hs x yearly hours of operation
= -----------------------------------------GCV x ? b
= Hf x Fuel cost ($/kg)
Where:
GCV = Gross Calorific Value of fuel kCal/kg
?b
= Boiler efficiency in percent
3.2.3 Calculating heat loss - example
Question: A 100 m steam pipe line with a 100 mm diameter is not insulated and supplies steam at
10 kg/cm2 to the equipment. Calculate the fuel savings if the steam pipeline would be insulated
with 65 mm glass wool with aluminum cladding.
Assumptions:
Boiler efficiency = 80 percent
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Thermal Equipment: Steam distribution and utilization
Fuel oil cost = US$ 300/tonne
Gross calorific value of fuel oil = 10300 kCal/kg
Surface temperature without insulation (Ts) = 170o C
Surface temperature after insulation (Ts) = 65 o C
Ambient temperature (Ta) = 25 o C
Step 1: calculate the surface heat loss and total heat loss of the un-insulated pipeline (S1 and
Hs1)
S1 = [10+ (Ts-Ta)/20] x (Ts-Ta)
Ts = 170oC
Ta = 25o C
S1 = [10+(170-25)/20] x (170-25)
= 2500 kCal/hr m2
A (m2 ) = 3.14 x diameter (m) x length (m)
Diameter = 0.1 m
Length = 100 m
A1 = 3.14 x 0.1 x 100 = 31.4 m2
Total heat loss (Hs1) = S1 x A1
= 2500 x 31.4
= 78850 kCal/hr
Step 2: calculate the surface heat loss and total heat loss of the insulated pipeline (S2)
S2 = [10+ (Ts-Ta)/20] x (Ts-Ta)
Ts = 65o C
Ta = 25o C
S2 = [10+(65-25)/20] x (65-25)
= 480 kCal/hr m2
Total heat loss/hr (Hs2) = S2 x A2
Diameter = 0.23 m (= 100 mm + 65 mm + 65 mm)
Length = 100 m
A2 = 3.14 x 0.23 x 100 = 31.4 m2
Total heat loss (Hs2) = S2 x A2
= 480 x 72.2
= 34656 kCal/hr
Step 3: calculate the yearly fuel savings and cost savings (Hf and US$)
Total reduction in heat loss Hs = Hs1 –Hs2
= 78860 –34656 = 44194 kCal/hr
Equivalent fuel loss (Hf) (kg/year) = Hs x yearly hours of operation
GCV x ? b
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Thermal Equipment: Steam distribution and utilization
Annual hours of operation = 8400 hrs
Total heat loss reduction = 44194 kCal/hr
Gross calorific value of fuel oil = 10300 kCal/kg
Boiler efficiency = 80 percent (0.8)
Fuel oil costs = US$ 300/tonne (US$ 0.3/kg)
Hf = 44194 x 8400 = 45052 kg/year
10300 x 0.8
Annual costs of heat loss (US$) = Hf x Fuel cost (US$/kg)
= 45052 x 0.3
= US$ 13516
3.3 Assessment of savings from condensate recovery
The procedure for calculating the energy and cost savings that can be achieved by condensate
recovery is explained in Figure 41.
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Thermal Equipment: Steam distribution and utilization
Figure 41. Calculations of Energy and Cost Savings from Condensate Recovery
(UNEP, 2004)
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Thermal Equipment: Steam distribution and utilization
4. ENERGY EFFICIENCY OPPORTUNITIES
This section describes the main opportunities to improve the energy efficiency of the steam
distribution system.
1. Manage steam traps
Energy losses can be reduced using steam traps, provided attention is giving to the following
areas:21
§ Testing of steam traps is explained in section 3.1
§ Routine maintenance, which depends on the type of trap and its application. The balanced
pressure steam trap for example, has an element which is designed for easy replacement.
Changing these on a regular basis, maybe once every three years or so, might seem wasteful
in time and materials. However, this practice reduces the need for trap checking and should
ensure a trouble free system with minimal losses through defective traps. Routine
maintenance which involves cleaning and re-using existing internals uses just as much labour
but leaves an untrustworthy steam trap. It will have to be checked from time to time and will
be prone to fatigue. Any routine maintenance should include the renewal of any suspect parts,
if it is to be cost effective.
§ Replacement of internals. The renewal of internal parts of a steam trap makes good sense.
The body will generally have as long a life as the plant to which it is fitted and it is only the
internal parts which wear, depending on system conditions. There are obvious advantages in
replacing these internals from time to time. It depends on the ease with which the new parts
can be fitted and the reliability and availability of the refurbished trap. The elements of
thermostatic traps can generally be changed by removing a screwed in seat. Replacement is
simple and the remade trap will be reliable assuming the maintenance instructions are
correctly carried out. Always check with the manufacturer regarding the correct technique for
any maintenance work required on steam traps. A reputable manufacturer will always be able
to supply appropriate literature, advice, and spare parts.
§ Replacement of traps. On occasions, it will be easier and cheaper to replace traps rather than
repair them. In these cases it is essential that the traps themselves can be changed easily.
Flanged connections provide one solution, although the flanged trap is more expensive than
the equivalent screwed trap. Mating flanges are an additional expense.
2. Avoid steam leaks
Steam leaks are a source of energy loss and must be avoided. It is estimated that a 3 mm
diameter hole on a pipeline carrying 7kg/cm2 steam would waste 33 kL of fuel oil per year.
Steam leaks on high-pressure mains are prohibitively costlier than on low pressure mains. Any
steam leakage must be quickly attended to. In fact, the plant should consider a regular
surveillance programme for identifying leaks at pipelines, valves, flanges and joints. Indeed, by
plugging all leakages, one may be surprised at the extent of fuel savings, which may reach up to
5 percent of the steam consumption in a small or medium scale industry or even higher in
installations having several process departments.
To avoid leaks it may be worthwhile considering replacement of the flanged joints which are
rarely opened in old plants by welded joints. Figure 42 provides a quick estimate for steam
21
Item 1 is a summary of Module 11.14 Testing and Maintenance of Steam Traps. In: Spirax Sarco Learning Centre, Block 11,
‘Steam Traps and Steam Trapping’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
leakage based on plume length. For example, if the plume length is 700 mm then the steam loss
is 10 kg/hr.
Figure 42: Steam Losses VS Plume Length
(Spirax Sarco)
3. Provide dry steam for process
The best steam for industrial process heating is the dry saturated steam. Wet steam reduces total
heat in the steam. Also water forms a wet film on heat transfer and overloads traps and
condensate equipment. Superheated steam is not desirable for process heating because it gives up
heat at a rate slower than the condensation heat transfer of saturated steam. It must be
remembered that a boiler without a superheater cannot deliver perfectly dry saturated steam. At
best, it can deliver only 95 to 98 percent dry steam. The dryness fraction of steam depends on
various factors, such as the level of water in the boiler. Indeed, even as simple a thing as
improper boiler water treatment can become a cause for wet steam. Since dry saturated steam is
required for process equipment, due attention must be paid to the boiler operation and lagging of
the pipelines.
Wet steam can reduce plant productivity and product quality, and can cause damage to most
items of plant and equipment. Whilst careful drainage and trapping can remove most of the
water, it will not deal with the water droplets suspended in the steam. To remove these
suspended water droplets, separators are installed in steam pipelines. The steam produced in a
boiler designed to generate saturated steam is inherently wet. Although the dryness fraction will
vary according to the type of boiler, most shell type steam boilers will produce steam with a
dryness fraction of between 95 and 98 percent. The water content of the steam produced by the
boiler is further increased if priming and carryover occur. A steam separator may be installed on
the steam main as well as on the branch lines to reduce wetness in steam and improve the quality
of the steam going to the units.
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Thermal Equipment: Steam distribution and utilization
4. Utilize steam at the lowest acceptable pressure for the process
A study of the steam tables would indicate that the latent heat in steam reduces as the steam
pressure increases. It is only the latent heat of steam, which takes part in the heating process
when applied to an indirect heating system, such as a heat exchanger. Thus, it is important that
its value be kept as high as possible. This can only be achieved with lower steam pressures. As a
guide, the steam should always be generated and distributed at the highest possible pressure, but
utilized at as low a pressure as possible since it then has higher latent heat. However, it may also
be seen from the steam tables that the lower the steam pressure, the lower will be its temperature.
Since temperature is the driving force for the transfer of heat at lower steam pressures, the rate of
heat transfer will be slower and the processing time greater. In equipment where fixed losses are
high (e.g. big drying cylinders), there may even be an increase in steam consumption at lower
pressures due to increased processing time. There are, however, several equipment in certain
industries where one can profitably go in for lower pressures and realize economy in steam
consumption without materially affecting production time. Therefore, there is a limit to the
reduction of steam pressure. Depending on the equipment design, the lowest possible steam
pressure with which the equipment can work should be selected without sacrificing either on
production time or on steam consumption
5. Proper utilization of directly injected steam
The heating of a liquid by direct injection of steam is often desirable. The equipment required is
relatively simple, cheap and easy to maintain. No condensate recovery system is necessary. The
heating is quick, and the sensible heat of the steam is also used up along with the latent heat,
making the process thermally efficient. In processes where dilution is not a problem, heating is
done by blowing steam into the liquid, i.e. direct steam injection is applied. If the dilution of the
tank contents and agitation are not acceptable in the process, i.e. direct steam agitation is not
acceptable, indirect steam heating is the only answer. Ideally, the injected steam should be
condensed completely as the bubbles rise through the liquid. This is possible only if the inlet
steam pressures are kept very low— around 0.5kg/cm2 –and certainly not exceeding 1 kg/cm2 . If
pressures are high, the velocity of the steam bubbles will also be high and they will not get
sufficient time to condense before they reach the surface. Figure 46 shows a recommended
arrangement for direct injection of steam by sparge pipe. A large number of small diameter holes
(2 to 5mm), facing downwards, should be drilled on the separate pipe. This will help in
dissipating the velocity of bubbles in the liquid. A thermostatic control of steam admitted is
highly desirable.
A more efficient way to inject steam into water is to use a properly manufactured steam injector.
Good injectors are designed such that the steam creates a venturi action to coincidentally draw
cool water through the injector. The heat in the steam is transferred much faster and more
completely than by a sparge pipe, ensuring that all injected steam is absorbed by the water before
it is able to escape from the water surface, thus reducing heat losses.
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Thermal Equipment: Steam distribution and utilization
Figure 43: Typical Steam Injector Installation
(Spirax Sarco)
6. Minimize heat transfer barriers 22
The metal wall may not be the only barrier in a heat transfer process. There is likely to be a film
of air, condensate and scale on the steam side. On the product side there may also be baked-on
product or scale, and a stagnant film of product. Agitation of the product may eliminate the effect
of the stagnant film, whilst regular cleaning on the product side should reduce the scale. Regular
cleaning of the surface on the steam side may also increase the rate of heat transfer by reducing
the thickness of any layer of scale, however, this may not always be possible. This layer may
also be reduced by careful attention to the correct operation of the boiler, and the removal of
water droplets carrying impurities from the boiler.
Filmwise condensation
The elimination of the condensate film is not quite as simple. As the steam condenses to give up
its enthalpy of evaporation, droplets of water may form on the heat transfer surface. These may
then merge together to form a continuous film of condensate. The condensate film may be
between 100 and 150 times more resistant to heat transfer than a steel heating surface, and 500 to
600 times more resistant than copper.
22
Item 6 is taken from Module 2.5 Heat Transfer. In: Spirax Sarco Learning Centre, Block 2, ‘Steam Engineering Principles and
Heat Transfer’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
Dropwise condensation
If the droplets of water on the heat transfer surface do not merge immediately and no continuous
condensate film is formed, ‘dropwise’condensation occurs. The heat transfer rates which can be
achieved during dropwise condensation are generally much higher than those achie ved during
filmwise condensation.
As a larger proportion of the heat transfer surface is exposed during dropwise condensation, heat
transfer coefficients may be up to ten times greater than those for filmwise condensation. In the
design of heat exchangers where dropwise condensation is promoted, the thermal resistance it
produces is often negligible in comparison to other heat transfer barriers. However, maintaining
the appropriate conditions for dropwise condensation has proved to be very difficult to achieve.
If the surface is coated with a substance that inhibits wetting, it may be possible to maintain
dropwise condensation for a period of time. For this purpose, a range of surface coatings such as
Silicones, PTFE and an assortment of waxes and fatty acids are sometimes applied to surfaces in
a heat exchanger on which condensation is to be promoted. However, these coatings will
gradually lose their effectiveness due to processes such as oxidation or fouling, and film
condensation will eventually predominate.
As air is such a good insulator, it provides even more resistance to heat transfer. Air may be
between 1 500 and 3 000 times more resistant to heat flow than steel, and 8 000 to 16 000 more
resistant than copper. This means that a film of air only 0.025 mm thick may resist as much heat
transfer as a wall of copper 400 mm thick! Of course all of these comparative relationships
depend on the temperature profiles across each layer.
Figure 44 illustrates the effect this combination of layers has on the heat transfer process. These
barriers to heat transfer not only increase the thickness of the entire conductive layer, but also
greatly reduce the mean thermal conductivity of the layer. The more resistant the layer to heat
flow, the larger the temperature gradient is likely to be. This means that to achieve the same
desired product temperature, the steam pressure may need to be significantly higher. The
presence of air and water films on the heat transfer surfaces of either process or space heating
applications is not unusual. It occurs in all steam heated process units to some degree.
To achieve the desired product output and minimize the cost of process steam operations, a high
heating performance may be maintained by reducing the thickness of the films on the condensing
surface. In practice, air will usually have the most significant effect on heat transfer efficiency,
and its removal from the supply steam will increase heating performance
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Thermal Equipment: Steam distribution and utilization
.
Figure 44. Temperature Gradients across Heat Transfer Barriers
(Spirax Sarco)
7. Proper air venting 23
When steam is first admitted to a pipe after a period of shutdown, the pipe is full of air. Further
amounts of air and other non-condensable gases will enter with the steam, although the
proportions of these gases are normally very small compared with the steam. When the steam
condenses, these gases will accumulate in pipes and heat exchangers. Precautions should be
taken to discharge them. The consequence of not removing air is a lengthy warming up period,
and a reduction in plant efficiency and process performance.
Air in a steam system will also affect the system temperature. Air will exert its own pressure
within the system, and will be added to the pressure of the steam to give a total pressure.
Therefore, the actual steam pressure and temperature of the steam/air mixture will be lower than
that suggested by a pressure gauge.
Of more importance is the effect air has upon heat transfer. A layer of air only 1 mm thick can
offer the same resistance to heat as a layer of water 25 µm thick, a layer of iron 2 mm thick or a
layer of copper 15 mm thick. It is very important therefore to remove air from any steam system.
Automatic air vents for steam systems (which operate on the same principle as thermostatic
steam traps) should be fitted above the condensate level so that only air or steam/air mixtures can
reach them. The best location for them is at the end of the steam mains. The discharge from an
air vent must be piped to a safe place. In practice, a condensate line falling towards a vented
receiver can accept the discharge from an air vent. In addition to air venting at the end of a main,
air vents should also be fitted:
§ In parallel with an inverted bucket trap or, in some instances, a thermodynamic trap. These
traps are sometimes slow to vent air on start-up.
§ In awkward steam spaces (such as at the opposite side to where steam enters a jacketed pan).
23
Item 7 is taken from Module 10.5 Air Venting, Heat Losses and Summary of Various Pipe Related Standards. In: Spirax Sarco
Learning Centre, Block 10, ‘Steam Distribution’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
§
Where there is a large steam space (such as an autoclave), and a steam/air mixture could
affect the process quality.
8. Minimize waterhammer24
Waterhammer is the noise caused by slugs of condensate colliding at high velocity into pipework
fittings, plant, and equipment. This has a number of implications:
§ Because the condensate velocity is higher than normal, the dissipation of kinetic energy is
higher than would normally be expected.
§ Water is dense and incompressible, so the ‘
cushioning’ effect experienced when gases
encounter obstructions is absent.
§ The energy in the water is dissipated against the obstructions in the piping system such as
valves and fittings.
Figure 45. Formation of a ‘solid’slug of water (Spirax Sarco)
Indications of waterhammer include a banging noise, and perhaps movement of the pipe. In
severe cases, waterhammer may fracture pipeline equipment with almost explosive effect, with
consequent loss of live steam at the fracture, leading to an extremely hazardous situation. Good
engineering design, installation and maintenance will avoid waterhammer; this is far better
practice than attempting to contain it by choice of materials and pressure ratings of equipment.
Commonly, sources of waterhammer occur at the low points in the pipework (Figure 46). Such
areas are due to:
§ Sagging in the line, perhaps due to failure of supports.
§ Incorrect use of concentric reducers (see Figure 10.3.7) - Always use eccentric reducers with
the flat at the bottom.
§ Incorrect strainer installation - They should be fitted with the basket on the side.
§ Inadequate drainage of steam lines.
§ Incorrect operation - Opening valves too quickly at start- up when pipes are cold.
24
Item 8 is taken from Module 10.4 Steam Mains and Drainage. In: Spirax Sarco Learning Centre, Block 10, ‘Steam
Distribution’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
Figure 46. Possible Sources of Waterhammer (Spirax Sarco)
To summarize, the possibility of waterhammer is minimized by:
§ Installing steam lines with a gradual fall in the direction of flow, and with drain points
installed at regular intervals and at low points.
§ Installing check valves after all steam traps, which would otherwise allow condensate to run
back into the steam line or plant during shutdown.
§ Opening isolation valves slowly to allow any condensate, which may be lying in the system
to flow gently through the drain traps, before it is picked up by high velocity steam. This is
especially important at start-up.
9. Insulate steam pipelines and hot process equipments
Insulation is needed to avoid the loss of heat through radiation from steam pipes as explained in
section 2.11.
10. Improve condensate recovery 25
Condensate recovery was already explained in section 2.10. Figure 47 compares the amount of
energy in a kilogram of steam and condensate at the same pressure. The percentage of energy in
condensate to that in steam can vary from 18 percent at 1 bar g to 30 percent at 14 bar g. Clearly
the liquid condensate is worth reclaiming. If this water is returned to the boiler house, it will
reduce the fuel requirements of the boiler. For every 6o C rise in the feed water temperature, there
will be approximately 1 percent saving of fuel in the boiler.
25
Item 10 is taken from Module 14.1 Introduction to Condensate Recovery. In: Spirax Sarco Learning Centre, Block 14,
‘Condensate Recovery’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
Figure 47. Heat Content o f Steam and Condensate at the same
Pressures (Spirax Sarco)
11. Recover flash steam26
Flash steam is released from hot condensate when its pressure is reduced. As an example, when
steam is taken from a boiler and the boiler pressure drops, some of the water content of the boiler
will flash off to supplement the ‘live’steam produced by the heat from the boiler fuel.
If use is to be made of flash steam, it is helpful to know how much of it will be available. The
quantity is readily determined by calculation, or can be read from simple tables or charts.
Example: Calculate available flash steam
The condensate enters the steam trap as saturated water, at a gauge pressure of 7 bar g and a
temperature of 170°C. The specific amount of heat in the condensate at this pressure is 721
kJ/kg. After passing through the steam trap, the pressure in the condensate return line is 0 bar g.
At this pressure, the maximum amount of heat each kilogram of condensate can hold is 419 kJ
and the maximum temperature is 100°C. There is an excess of 302 kJ of heat which evaporates
some of the condensate into steam. The heat needed to produce 1 kg of saturated steam from
water at the same temperature, at 0 bar gauge, is 2257 kJ. The percentage of flash steam per kg
of condensate is calculated as follows:
Percentage of flash steam available =
S1 — S2
X 100%
L2
Where: S1 = the sensible heat of higher pressure condensate
S2 = the sensible heat of the steam at lower pressure (at which it has been flashed)
L2 = the latent heat of flash steam (at lower pressure).
26
Item 11 is a summary of part of Module 14.6 Flash Steam. In: Spirax Sarco Learning Centre, Block 14, ‘Condensate
Recovery’. www.spiraxsarco.com
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Thermal Equipment: Steam distribution and utilization
In this example, the percentage of flash steam evaporated therefore is:
= [(S1 — S2 ) / L2 ] X 100%
= [(721 –419)] / 2257] X 100%
= 13.4%
Example: Determine available flash steam using a chart
Alternatively, the chart in Figure 48 can be read directly for the moderate and low pressures
encountered in many plants. The example is depicted in Figure 48 and shows that 0.134 kg of
flash steam is produced per kg of condensate passing through the trap.
Figure 48: Quantity of Flash Steam Graph (Spirax Sarco)
Typical applications of flash steam include heating (with air heater batteries and space heating
installations using either radiant panels or unit heaters), steam heated hot water calorifier and a
steam to water calorifier. Boiler blowdown can also be recovered as flash steam, as is explained
in the separate Boiler Chapter.
The Spirax Sarco Learning Centre provides more information on flash steam recovery vessels,
requirements for successful recovery applications, and typical applications of flash steam.
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Thermal Equipment: Steam distribution and utilization
12. Use a thermo-compressor to reuse low pressure steam
In many cases, very low pressure steam is reused as water after condensation when there is no
better reuse option. In many cases it becomes feasible to compress this low pressure steam with
very high pressure steam and reuse it as a medium pressure steam. The major energy in steam is in
its latent heat value and thus thermo-compression would give a large improvement in waste heat
recovery. The thermo-compressor (Figure 50) is a simple equipment with a nozzle where HP steam
is accelerated into a high velocity fluid. This entrains the LP steam by momentum transfer and
then recompresses in a divergent venturi. It is typically used in evaporators where the boiling steam
is recompressed and used as heating steam.
DISCHARGE
STEAM
M.P.
MOTIVE
STEAM
H.P.
SUCTION STEAM
L.P.
Figure 49. Thermocompressor
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Thermal Equipment: Steam distribution and utilization
5. OPTION CHECKLIST
This section includes most important energy efficiency options
§
§
§
§
§
§
§
§
§
§
§
§
§
§
§
§
§
§
§
§
Fix steam leaks and condensate leaks (a 3 mm diameter hole on a pipe line carrying 7 kg/cm 2
steam would waste 33 kilolitres of fuel oil per year)
Accumulate work orders for repair of steam leaks that cannot be fixed unless the plant is shut
down. Provide each leak with tag.
Use back pressure steam turbines to produce lower steam pressures
Use more efficient steam desuperheating methods
Ensure process temperatures are correctly controlled
Maintain lowest acceptable process steam pressures
Reduce hot water wastage to drain
Remove or isolate all redundant steam piping
Ensure condensate is properly removed from process and heating equipment
Ensure condensate is returned or re- used in the process. (60 C raise in feed water temperature
by economizer/condensate recovery corresponds to a 1 percent saving in fuel consumption,
in boiler)
Preheat boiler feed-water with condensate and flash steam wherever possible. Where not
possible, heat with live steam from the boiler by steam injection method.
Recover the heat from continuous (TDS) boiler blowdown
Check operation of steam traps
Minimize waterhammer
Remove air from indirect steam using equipment (a 0.25 mm thick air film offers the same
resistance to heat transfer as a 330 mm thick copper wall)
Inspect steam traps regularly and repair malfunctioning traps promptly
Consider recovery of vent steam (e.g. on large flash vessels
Use waste steam for water heating
Use an absorption chiller to condense exhaust steam before returning the condensate to the
boiler
Establish a steam efficiency- maintenance programme. Start with an energy audit and follow
up, then make a steam efficiency-maintenance programme a part of your continuous energy
management programme
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Thermal Equipment: Steam distribution and utilization
6. WORKSHEETS
This section includes following worksheets:
• Technical Specifications of Steam Traps
• Steam Trap Audit
• Insulation Losses
Trap size
Trap type
Trap ref.
(No)
Section. No
Worksheet 1: TECHNICAL SPECIFICATIONS OF STEAM TRAPS
Location re f.
(plant dept/block)
Type of discharge
(continuous/
semi continuous/
intermittent)
Trap capacity
(kg
Condensate/hr)
Worksheet 3: INSULATION LOSSES
No.
Location
Equipment
reference
Outer
diameter
Energy Efficiency Guide for Industry in Asia –www.energyefficiencyasia.org
Surface
temperature
Remarks
Status of trap fittings
Diagnosis of situation
Functional
status of trap
Application of trap
Trap
location
Trap pressure
kg/cm2
Trap size
Trap type
Trap ref. (no.)
Section no.
Worksheet 2: STEAM TRAP AUDIT
Insulation
thickness (if
any)
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Thermal Equipment: Steam distribution and utilization
7. REFERENCES
Bureau of Energy Efficiency. Energy Efficiency in Thermal Utilities. Book 2, 2004
US Department of Energy (DOE), Fedral Energy Management Programme. Steam Trap
Performance Assessment, Doe/Ee -0193, Page 8. 1999. www.eren.doe.gov/femp
Spirax Sarco. Learning Centre. www.spiraxsarco.com/learn
Sustainable Energy Authority of Victoria (SEAV). Fact sheet: Boiler Optimisation. 2005.
www.seav.sustainability.vic.gov.au/manufacturing/sustainable%5Fmanufacturing/
United Nations Environment Programme (UNEP). Cleaner Production –Energy Efficiency
Manual –Guidelines for the integration of Cleaner Production and Energy Efficiency. 2004.
www.uneptie.org
Many sections of this chapter were taken from, based on or are a summary of modules featured
in Spirax Sarco’
s web-based Learning Centre with the kind permission of Spirax Sarco. For
more detailed information please refer to www.spiraxsarco.com/learn for the following
modules:
1. Introduction
2. Steam Engineering Principles and Heat Transfer
3. The Boiler House
4. Flow metering
5. Basic Control Theory
6. Control Hardware: Electric/Pneumatic Actuation
7. Control Hardware: Self-acting Actuation
8. Control Applications
9. Safety Valves
10. Steam Distribution
11. Steam Traps and Steam Trapping
12. Pipelines Ancillaries
13. Condensate Removal
14. Condensate Recovery
15. Desuperheating
16. Equations
Copyright:
Copyright © United Nations Environment Programme (year 2006)
This publication may be reproduced in whole or in part and in any form for educational or non-profit purposes without special
permission from the copyright holder, provided acknowledgement of the source is made. UNEP would appreciate receiving a
copy of any publication that uses this publication as a source. No use of this publication may be made for resale or any other
commercial purpose whatsoever without prior permission from the United Nations Environment Programme.
Disclaimer:
This energy equipment module was prepared as part of the project “Greenhouse Gas Emission Reduction from Industry in Asia
and the Pacific” (GERIAP) by the National Productivity Council, India. While reasonable efforts have been made to ensure that
the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the
accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or
indirectly through the use of, or reliance on, the contents of this publication.
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Thermal Equipment: Steam distribution and utilization
Spirax Sarco copyright and disclaimer:
Spirax Sarco cannot be held responsible for any mishap, or misinterpretation of this technical material, or out-of-date technical
material, or any claim by any person or persons or organisations as a result of this information as printed in this document,
either expressed or implied, and whether in hard copy or electronic copy. The Spirax Sarco technical material used in this
document is copyright of Spirax Sarco and remains the full and exclusive intellectual property of Spirax Sarco at all times.
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