ISGEC John Thompson

SECTION-8

CHEMICAL BOIL OUT

PROCEDURE
 ISGEC John Thompson

SECTION - 8

Chemical boil out procedure

8.1 GENERAL

The efficiency of a boiler is dependent on the heat transfer

capacity of the heating surfaces, and this is seriously impaired
by the internal surfaces of tubes if they get coated with deposits,
as occurs naturally in the course of normal running.

Also, a newly erected boiler, or one on which extensive repairs to

pressure parts have been carried out, may have an oil or grease
film on internal areas which is impossible to wholly remove by
manual cleaning. Since oil and grease have a very low rate of
heat transfer, tubes contaminated by them can become
overheated when subjected to high temperatures. This will lead
to blistering or burning of the tube metal with consequent risk of
rupture. Chemical cleaning of boiler pressure parts is carried out
to over come the above problems. This is to be carried out at
following stages.
a) Prior to commissioning a newly erected boiler.
b) Following extensive repairs to boiler pressure parts.
c) When it is suspected that the internal heating surfaces are
fouled with deposits from normal running. Due to poor
quality of feed water.
NOTE:
Water quality should be the prime concern for operator to have
long life of pressure parts.

8.1 (a) Initial Preparation

a) Boiler hydraulic is complete.

b) Power supply is available to run the drives.
c) Water & Chemicals are available.
d) Boilers filling arrangements are complete.
e) Fuel is available
It is essential to have spare gaskets for upper and lower drums
as drum inspection has to be carried out after alkali boil and
after chemical cleaning to ensure effectiveness of the alkali boil
out and chemical cleaning.

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 ISGEC John Thompson

8.2 ALKALI CLEANING

This is the most commonly used method of boiler cleaning and,

case of a newly erected boiler, it is carried out on completion of
drying out operation.

Chemicals used must never be introduced in solid form but

must first be dissolved in water and introduced in to the boiler
system through either the chemical injection equipment (if
available), or through the drum manhole. In the latter case the
water level must first, lowered by draining and then resorted
again afterwards.

All phosphate formula is used for alkali boil out of boiler to

minimise the potential for stress corrosion cracking of stainless
components in the event that some chemicals were spilled into
the superheater. If the phosphate solutions enter the
superheater, they must, of course be flushed out. But by
omitting caustic forming chemicals, the impact of trace residuals
on the super heater is reduced. TRISODIUM PHOSPHATE AND
DISODIUM Phosphate are used for alkali boil out. These are to
be mixed with warm / hot water.

8.2.1 Boiling Out (Atmospheric)

a) Open all air vents open, steam drum manhole doors. Fill
boiler with properly treated demineralized water upto first
port of drum level gauge glass.

b) Prepare the boil out chemical solution in hot demineralised

water in the chemical mixing tank. The solution is
formulated by dissolving the following proportionate
quantities of chemicals, depending on the volume of the
system to be cleaned.
0.05% Na2PO4.7H2O (500 PPM disodium phosphate)
0.1% Na3PO4.12H2O (1000 PPM trisodium phosphate)

c) Mix dissolved chemicals and introduce solution into drum

while at the same time slowly feeding water to effect a more
even distribution along the drum length.

d) Close manhole door and secure tightly.

e) Temperature within boiler is now raised by following the start

up procedure to bring water upto atmospheric boiling point.

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 ISGEC John Thompson

f) Increase slowly the heat input to boiler to raise the pressure.

g) At a pressure of approximately 30 psig (2.1 kg /cm2. g) a

good flow of steam from air release valves will be noticed.
Upon seeing this the air release valves are to be shut.

h) Continue increasing boiler pressure to 15% of normal

working pressure.

i) Maintained 15% of normal working pressure for a period of

12 hours. This will ensure a rapid circulation of water
through boiler.

j) Give blow down at every 2 hrs interval from water wall and
evaporator drains for One Minute each.

k) Extinguish fire and blowdown boiler manifolds individually by

giving blowdown for a period of 2 to 3 minutes each.

l) Allow boiler to cool down naturally and when pressure comes

to 2.5 kg / cm2, with manifold blowdowns closed, blowdown
steam drum until water level falls below gauge glass.

m) Following this the entire boiler can be blown down as quickly

as possible, using all manifold and drum blowdown and
drains, at the Engineers discretion.

This way evacuate the water before any dirt or oil has time to
settle down.

NOTE:
When blowing down through all drains and blowdowns
simultaneously, due regard must be exercised for the
capacity of the blowdown system.

n) When pressure falls to 30 psig (2.1 kg/ cm2. g.) open-air

release valves.

o) With blowdown and drain valves still open, wash through

drums. Manifolds and tubes, with high pressure clean water
to wash out any remaining sediment.

p) Close all manhole doors, using new joints as necessary. If

chemicals for pressure boil out are to be introduced through
Steam Drum manhole, one of these must be left open.

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 ISGEC John Thompson

q) Close all blowdown and drain valves except superheater drain

valve, which must be left open.

r) The boiler is now ready for the pressure boil out.

8.2.2 Pressure Boil Out:

a) Fill Boiler with water in the same manner as for atmospheric

boil out (See above).

b) Dissolve following chemicals in hot/warm water

Trisodium Phosphate
Disodium Phosphate

Concentration of above chemicals to be maintained is 500

ppm during pressure boil out.

c) Introduce solution into drum while at the same time slowly

feeding water to effect a more even distribution along drum
length.

d) Close manhole doors and secure tightly.

e) Continue to fill boiler up to 1/3 glass. Do not further raise

level initially as water will swell while heating and any
blowdown for level control will lead to chemical loss.

CAUTION: Never allow temperature of gases entering

superheater zone to rise above normal working
steam temperature of superheater until a positive
flow of steam is established through open
superheater drain valve.

f) Water level in gauge will rise to glass as heat is absorbed and

water expands. This level must be maintained.

g) Continue to raise pressure. At a pressure of approximately 30

psig (2.1 kg. s/ cm2. g.) steam will be seen freely issuing from
open-air release valves. These may be closed.

h) Raise gradually pressure to 75 % of the normal working

pressure limited to a maximum of 40 kg/cm2 (Refer Technical
data sheet for working pressure). While raising pressure give
blow down at 15%, 25%, 35%, 50%, pressure from blow down
valves for one minute each.

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 ISGEC John Thompson

i) Raise pressure to 75% (limited to a maximum of 40 kg/cm2)

of normal working pressure and hold it for 12 hrs.

j) Blow down from water wall drains every 2 hrs for One Minute
each.

k) During this period of gradual pressure raising the plant

should be under constant observation, and the following
inspections made.

 Check all bolted flanges, manhole doors and

inspection caps for leaks and tightness.

 Check amount of movement due to thermal

expansion against witness marks. Restriction to free
movement of pressure parts can result in over-
stressing and rupture and any such restriction
observed must be investigated.

 Check dampers under hot conditions to ensure that

they do not bend throughout their extend of travel
and isolating dampers seal effectively when closed.

 Check all combustion equipment and auxiliaries for

correct functioning. Take a note of any irregularity,
which cannot be rectified without shutting down.

l) Boiler water samples are obtained from CBD and analysed for
alkalinity, phosphate content, dissolved silica and oil content.
Repeat every 2 hours till the oil level in the sample falls below
5 PPM. Firing is to be stopped after oil level falls below 5 PPM
or 12 hours boil out whichever is later.

m) Operate each blowdown and drain valve successively for a few

seconds to discharge any accumulated sludge. Repeat several
times meanwhile maintaining drum water level at glass.

n) Allow boiler to cool down naturally and when pressure comes

down to 2.5 kg/cm2. Open all blowdown and drain valves
(exercising due regard for capacity of blowdown system) to
completely blowdown the boiler.

o) When pressure comes down to 30 psig. (2.1 kg /cm2 g) open

all air release valves.

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 ISGEC John Thompson

8.3 FINAL INSPECTION

a) When boiler has cooled down open all manhole doors and
remove all manifold inspection caps to allow inspection of all
internal surfaces.

b) Accumulated dirt and sludge should be scraped out and all

drums, manifolds and tubes washed through with high-
pressure clean water. Any sediment can be washed though
blowdown and drain valves.

c) Examine all internal baffles and check security and the

tightness of all joints and fittings, also the internal pipe work
and cyclones, if provided.

d) If water gauge glass has become fouled in boiling out these

must be dismantled and cleaned to makers instructions.

e) Refit all manhole doors and inspection caps with new joints
and secure tightly.

f) Examine all external heating surfaces and gas passes of boiler

for excessive soot deposits resulting from low firing rates
employed in boiling out. Clean manually the soot deposits.

g) Check all brick work and refractory. Repair as necessary.

h) Check for any ash accumulation in any area.

i) Check that expansion joints are set correctly and that

expansion clearances are adequate. This will particularly apply
to the baffles and walls of brickset boilers.

NOTE: Boiler water contains chemicals, which are hazardous

for human safety, and hence safe disposal is necessary.

Section # 8 Page 6 of 6