1. PREPARATION FOR BOILER FEED WATER
Let it be a fire tube boiler or water tube boiler, boiler feed water (BFW) is the basic
raw material for producing steam. For meeting the requirement of BFW for the boiler, the
feed has to be processed.
BFW is prepared by simple raw water which is processed to form demineralized
water and that in the later stages is converted to BFW. Raw water is filled up with lot many
minerals and compounds which have to be removed for proper usage.
DEMINERALIZED WATER (DM water)
It is obtained by removal of dissolved minerals present in raw water by using the
ion exchange process. A DM water plant consists of two units and an additional unit is
needed only for extra purity of the DM water.
First unit consists of high capacity strong acidic cation (SAC) exchange resin which
is responsible for converting positive ions of salts and replacing them with hydrogen ions
to form respective acids. The produced acid is then absorbed in the next unit which
consists of strong basic anion (SBA) exchange resin. The factors needed to be considered
for the desired quality of product are conductivity and total dissolved solids present in the
DM water after processing. Extra unit that might be considered for improved product
quality is the mixed bed deionizers which is a single unit filled with SAC and SBA exchange
resin mixed together.
BOILER FEED WATER (BFW)
The diagram displayed adjacent gives a much better
insight into how BFW is prepared. The deaerator is
responsible for preparation of BFW water in a continuous
operational plant. The processed DM water is the feed into
the deaerator which is tagged as make up. The low pressure
steam preheats and separates the dissolved gases in the
incoming DM water. The trapped returns are recycled
condensate outlets from the steam traps, coils and reboilers.
While considering recycling the condensate, make sure the
systems which generate the same are in no way responsible
for contamination of the BFW water for a longer run period.
The purpose of the vent is to release the unwanted inerts
from the system. Normally the operating pressure of a
deaerator will be slightly positive than the atmospheric
pressure.

2. CONDITIONING OF DM WATER IN DEAERATOR
Deaeration is a necessary part to avoid the following conditions-
• CORROSION- It is destruction of metal by chemical or electromechanical reaction
with its surrounding environment. It enhances with increase in levels of oxygen
carbon dioxide content as well as at low pH values.
• FOULING- It occurs because of restriction or blockage in piping and equipment
passageways that results in inefficient water flow.
• FOAMING- It occurs when high concentration of soluble salts (aggravated by grease,
organic matter and suspended solids) create frothy bubbles/foam in steam space of
the system. It is responsible for degrading the quality of steam.
• SCALE FORMATION- The corroded/eroded/inert particles adhere to the heating
surface forming a layer of insulation and hence reducing the heat transfer efficiency.
Increase in the thickness of scale is responsible for increase in fuel consumption for
achieving the desired output.
• CAUSTIC EMBRITTLEMENT- It occurs with high concentration of alkaline salts
(pH>11). Higher pH value will liberate hydrogen absorbed by iron in steel. It is
visible and evident in high temperature areas of boiler system (waterside). It
manifests in the form of hairline cracks.
Sodium carbonate is used for softening water by lime soda process. If the
concentration of sodium carbonate increases, it undergoes hydrolysis to form
sodium hydroxide. Sodium hydroxide makes water alkaline. Due to capillary action,
alkaline water enters the cracks. The water starts evaporating leading to increase in
amount of hydrogen. Sodium hydroxide attacks and dissolves iron in boiler as
sodium ferrate. It results in embrittlement of boiler parts under stress like rivets,
bends and joints.
Cathodic area is iron surrounded by dilute sodium hydroxide solution and
anodic area is iron surrounded by comparatively concentrated sodium hydroxide
solution. It can be prevented by
o Replacing sodium carbonate with sodium phosphate as a softening agent.
o Blocking hairline cracks of the boiler with lignin and addition of sodium
sulfate is also helpful.

3. Chemical dosing is preferred for removal of dissolved oxygen in water.
Hydrazine (N2H4) is an inorganic compound, colorless flammable liquid with
ammonia odor. It acts as oxygen scavengers. It does not contribute to solids,
carbon dioxide and organics.
To maintain a steady quality of boiler feed water to the boiler system, some
specifics have been discussed below for achieving a stable steam system.
 Continuous blowdown is necessary for a steam drum to minimize the
concentration of impurities when steam is continuously evaporating
from the system. This targets the surface impurities on water. It has to
be optimized to continuous operation as energy loss involved is also
to be looked after. It is also possible to extract low pressure steam
when the blowdown is flashed into a blowdown vessel.
 Intermittent blowdown is necessary to remove the sludge & insoluble
particles which are forced to accumulate at the bottom in mud drum.
Addition of ammonia is also recommended for maintaining the pH of the
water.