BOILER WATER CHEMISTRYSESSION 2
WATER SOURCES
• Water source can be
River/Lake water, Well water, Ground
water, Industry treated, Municipal
water, Process treated water and Sea
Water
• Element or species in water
• Cations (21+), Anions (6+) and
Dissolves solids
• Most important/predominant
elements are in the table
Cation​
Anion​
Calcium
Chloride
Magnesium
Sulphate
Barium
Bicarbonate
Boron
Carbonate
Iron
Nitrate
Manganese
Fluoride
Potassium
Phosphate
Sodium
Copper
Zinc
Strontium
Silica
Critical Parameters in Water​
Conductivity
The conductivity can be defined as the ability of a substance to conduct electric
current.
The more conductive a substance, the more electrical current it conducts.
pH
–pH is the logarithm (base 10) of the inverse of the concentration of H + ions in a
given solution.
– pH measures the amount of H + ions and / or OH- ions present in solution.
–The more hydrogen ions (H + ), the more acidic the medium. The more hydroxyl
ions (OH- ), the more basic (alkaline) is the medium.
Alkalinity
1. Alkalinity can be defined as the responsiveness of water sample to the addition
of an acid.
2. It is the neutralizing capacity of a medium, when subjected to a strong acid
3. There are three types of alkalinity: i) Total Alkalinity or M Alkalinity ii) Partial
(Phenolphthalein) Alkalinity or P Alkalinity iii) Hydroxyl Alkalinity or OH Alkalinity
Hardness
Hardness is the sum of ions that can cause scale in waters. We can represent the
total hardness as:
Total Hardness = Calcium Hardness + Magnesium Hardness
Iron
– Iron is present in water as ferric (Fe+3 ) or ferrous (Fe+2 ).
–Ferric iron has a very limited solubility, the oxidation from ferrous to ferric occurs
at pH between 7.0 to 8.5.
–Boiler systems will have pH values greater than 8, all iron ions will be present in
the ferric state.
Copper
–Normally copper in water appears due to corrosion of some equipment that have
some copper alloy in their materials.
– As the solubility of copper is not high, it will precipitate on the metal surfaces.
Silica
– Silica is found in natural waters in the forms of soluble silicates and in the
colloidal form.
–Silica can produce hard and adherent scales in the internal boiler surfaces when
its residual in the water is not controlled
Deaerator
–The objectives of deaeration are:
– i) Increase the temperature of the feedwater through the contact with low pressure steam
and to a lesser degree from the return of condensate
–Create water droplets to improve heat transfer and gas diffusion
–Continuously remove the non-condensable gases from the scrubber section by venting steam
to atmosphere (vent)
–Mechanical deaeration involves the physical removal of dissolved gases from
boiler feedwater, this includes oxygen, carbon dioxide and ammonia.
–Oxygen dissolved in the outlet should be less than 10 ppb (0.010 ppm) and ideally
< 7 ppb
Spray type Deaerator
–When a deaerator is not working, the most
common reasons that it is not obtaining < 7
ppb oxygen are:
–i) Inadequate venting
–ii) Wide fluctuations in feedwater flow or
steam pressure.
– iii) Feedwater flow rates outside design
specifications
–iv) Deaerator operation outside design
temperature range
–v) Plugged or broken spray nozzles
–vi) Missing or broken trays
Economizer:
–Large amounts of heat may be wasted in the exhaust gases leaving via the stack.
–An economizer is a heat exchanger located somewhere in the gas passage
between the boiler and the stack.
Superheater:
–Superheaters and reheaters are specially designed in-line tube bundles that
increase the temperature of saturated steam.
– They are simple single-pass heat exchangers with steam flowing inside the tubes
and the flue gas passing outside, generally in crossflow.
CONDENSATE POLISHING
–The condensate polisher must fulfill two simultaneous duties:
1. It must remove the "crud", which is suspended solids (mostly metal oxides) resulting from
corrosion and erosion;
2. It must also remove any dissolved solids originating from the make-up water, possible leaks of
the condenser, or from regeneration of the ion exchange resins.
–The polisher must perform filtration and demineralization at the same time.
–Bag Filter , Carbon Filter , Cartridge Filters and Electromagnetic Filters are
examples of the type of filtration systems that are commonly used to treat and
recover condensate
SOFTENERS
–Softeners are required where natural
hardness (calcium and magnesium
compounds) exists in the water.
–In the ion exchange softening process,
the scale-forming cations, such as
Ca2+ and Mg2+, are removed and
replaced by sodium cations.
REVERSE OSMOSIS
–Reverse osmosis (RO) membrane treatment is a method of reducing the dissolved
solids in raw water by the use of a semi-permeable membrane
–Reverse osmosis involves applying pressure that is greater that this equilibrium
pressure and forces the water to flow from a higher concentration of salts to a
lower concentration.
–An RO system is typically fed with low pressure feedwater (approx 30 psi)
–This process requires approximately 300 psi for a brackish water application and
900 psi for a saltwater application.
–A single-pass brackish water RO system, where feedwater is converted into
permeate only once, typically removes 99% of impurities (TDS) from the feed
water with typical recovery rates ranging from 40-90%
–A single-pass seawater RO system typically removes 99% of impurities with
recovery rates ranging from 20- 60%.
DIMENERALIZATION
–Demineralization, or deionization as it
is sometimes called, is the process of
removing dissolved ionic impurities
from water.
–The positively charged cations and
negatively charged anions allow
conductance of electricity and are
called electrolytes.
In series operation, the water is usually first passed through a column of strong acid
cation exchange resin which is in the hydrogen form (R-H + ) to exchange the cations
in solution for hydrogen ions.
ION EXCHANGE
–Ion exchange usually describes a
process of purification of aqueous
solutions using solid polymeric ion
exchange resin.
–Ion exchangers are either cation
exchangers, which exchange
positively charged ions (cations),
or anion exchangers, which
exchange negatively charged ions
(anions).
CLARIFICATION
–Clarification is an essential step
in a water or wastewater
treatment process to remove
suspended solids through gravity
settling, providing a clarified
liquid effluent.
–Primary and secondary
clarification is needed to
determine the appropriate
technology to meet treatment
requirements.
Primary Clarification
–Primary clarification, also known as sedimentation, is the first step in the
water treatment process for removing suspended solids (TSS), oil and
grease
–Sludge is settled to the bottom of the clarifier basins and collected by a
rake and removed by a sludge removal system.
–A typical primary clarifier removes 60 percent of suspended solids and 30
to 40 percent of Biological Oxygen Demand (BOD).
Secondary Clarification
–During the secondary clarification process the biomass from
microorganisms settles to the bottom in the form of activated sludge.
– Waste sludge is removed and thickened prior to the digestion process.