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ACID CLEANING PROCEDURESBoiler units can be acid cleaned by either the "circulation" or "fill and

soak" method. The circulation method (fig. 1-35) can be used to clean units with positive liquid flow

paths, such as forced circulation boilers. The inhibited acid solution is circulated through the unit at the

correct temperature until test analyses of samples from the return line indicate that the acid strength has

reached a balance and no further reaction with the deposits is taking place. Because the strength of theacid solution can be determined frequently during the cleaning process, this method can be more

accurately controlled and can use lower strength solutions than the fill-and-soak method. The fill-and-soak method (fig. 1-36) is used for cleaning units with natural circulation. The boiler unit is filled

with the inhibited acid solution at the correct temperature and allowed to soak for the estimated

time. It is not possible to obtain accurate representative samples of the cleaning solution during the

soaking period.

FLUSHING AND NEUTRALIZINGAfter acid cleaning, drain and then flush the unit with clean, warmwater until the flushing water effluent is free of acid and soluble iron salts. Next, a neutralizing solution

is circulated through the unit until the effluent shows a definite alkaline reaction. The types of

neutralizing solutions used are as follows: soda ash, trisodium phosphate. sodium tripolyphosphate, or

other nontoxic chemicals. After circulation of the neutralizing solution, the water level can be dropped

to the normal level and the boiler fired at 50 psig with open vents to permit the escape of liberated gases.

Finally. the boiler is again drained and flushed with clean, warm water.Boiling Out New boilers. or boilers

that have been fouled with grease or scale, should be boiled out with a solution of boiler compound. New

boilers must be washed out thoroughly. The steps required for one method of boiling out are as follows:

1. Dissolve 5 pounds of caustic soda and 1 1/2 pounds of sodium nitrate or 10 pounds oftrisodium phosphate for each 1,000 gallons of water the boiler holds at steaming level. Put the

mixture into the boiler as solution a

Figure 1-

35.Acid cleaning by circulation method.1-40

2. .In multiple-drum boilers, divide the charge and put equal amounts in each of the lower drums.

2. Fill the boiler with hot feedwater to the level of the bottom of the steam drum. Turn the steam

into the boiler through the usual boiling out connections, or bottom blow, and allow the boiler to

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fill gradually to the top of the gauge glass. 3. Steam pressure in the boiler should be kept

between 5 and 10 pounds. The boiling out should continue for 48 hours. Immediately after boiling

out, give a series of bottom blows to remove the bulk of the sludge. The boiler should be cooled,

washed out immediately, and given the usual mechanical cleaning. You may not always want to

use the above method for boiling out. The steps for a second satisfactory method for boiling outare as follows: 1. Clean out all loose scale and any scale adhering to the boiler that can be

removed manually. 2. Place about 15 pounds of caustic soda or soda ash and 10 pounds ofmetaphosphate for each 100-boiler horsepower (hp) of the boiler. 3. Seal the boiler openings but

OPEN ALL VENTS. Fill the boiler about three-quarters full with water. 4. Start the burner and

raise the temperature of the water in the boiler to about 200F. Maintain this temperature

for about 24 to 48 hours. Add makeup water as required during this period to fill the boiler to

the base of the safety valve. 5. Analyze the boiler water during the boiling out period and add

enough caustic soda and metaphosphate to maintain the following concentrations: Causticity asppm OH 300 to 500 Phosphate as ppm PO3 100 to 150 6. Open the boiler at the end of the

boiling-out period and clean out the sludge and loose scale. Pay particularattention to removing

scale and sludge from water legs infire-tube boilers. 7. Flush the boiler thoroughly. 8. If a lot of

corrosion is exposed when the scale is removed, notify your superior so a boiler inspection can be

made. When the boiler is operated, any residual scale may cause faulty operation. The boiler

should be taken out of service at frequent intervals to remove sludge formed from disintegrated

scale. As soon as personnel can work in the boilers, wire brush the drums and ends of all tubes