to 212° F 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 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 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
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, 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
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.