to 212F as possible to accelerate the liberation of hydrogen absorbed in the metal. After opening the unit, place air blowers at the open drum manholes to circulate air through the unit. Use a reliable combustible gas indicator to test the boiler atmosphere for explosive mixtures.

ACID CLEANING PROCEDURES - Boiler 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 the acid 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 NEUTRALIZING - After acid cleaning, drain and then flush the unit with clean, warm water 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 of trisodium phosphate for each 1,000 gallons of water the boiler holds at steaming level. Put the mixture into the boiler as a

Figure 1-35. - Acid cleaning by circulation method.