Because the cleaning is more complete, it is
possible to examine the unit thoroughly for
defects, such as cracks and corrosion pitting.
A more thorough job is accomplished because
the acid reaches areas inaccessible to mechanical
Less dismantling of the unit.
Lower cost and labor.
ACIDS FOR CLEANING.The following acids
are used to clean boilers: hydrocholoric acid, phosphoric
acid, sulfamic acid, citric acid, and sulfuric acid.
HYDROCHLORIC ACID is most frequently used
for boiler cleaning because it has a relatively low cost and
satisfactory inhibitors are available. Also, the chemical
reactions of the hydrochloric acid with the boiler deposits
usually result in soluble chlorides.
PHOSPHORIC ACID can remove mill scale from
With this acid, the boiler can be fired
directly without producing noxious or corrosive fumes.
Direct firing produces good circulation and distribution
of the cleaning solution.
Another advantage of
phosphoric acid cleaning is that the metal surfaces resist
corrosion after cleaning.
When cleaned with
phosphoric acid, you must protect metal surfaces from
surface corrosion during draining and before
SULFAMIC ACID is available in powder that must
be placed in solution. The powdered acid is easier and
safer to handle than liquid acids in carbons. It does not
produce noxious fumes as it dissolves and it is less
corrosive than hydrochloric acid, especially at higher
concentrations and temperatures.
CITRIC ACID AND SULFURIC ACID are used for
removing boiler waterside deposits. Sulfuric acid is
economical and easily inhibited. However, a danger is
that the sulfuric acid can form insoluble salts, such as
INHIBITORS.Without inhibitors, acid solutions
attack the boiler metal as readily as they attack the
deposits. With the addition of suitable inhibitors, the
reaction with the boiler metal is greatly reduced.
Inhibitors used include arsenic compounds, barium salts,
starch, quinolin, and pyridin. Commercial inhibitors are
sold under trade names by various chemical concerns.
Other inhibitors are manufactured by companies that
furnish complete acid cleaning services.
SAFETY PRECAUTIONSWhen acid cleaning
a boiler installation, you must observe SAFETY
precautions as follows:
Slowly pour the acid into water when mixing the
Use goggles, rubber gloves, and rubber aprons
when handling acids.
After acid cleaning, be sure to thoroughly flush out
all of the tubes that are horizontal or slightly
sloping. Obstructions in these tubes can cause
poor circulation, overheating, and failure of tubes
when the unit is placed in service.
Do not exceed the specified acid and inhibitor
allowable temperature. The inhibiting effect
decreases with the temperature rise and the
probability of acid attack of the boiler metal
Provide competent chemical supervision for the
Close all valves connecting the boiler with other
piping or equipment.
Provide adequate venting for safe release of acid
Before acid cleaning, replace all brass or bronze
parts temporarily with steel or steel alloy parts.
NEVER POUR WATER INTO ACID.
Do not chemically clean boilers with riveted joints.
During acid cleaning, hydrogen gas can develop
through the reaction of the acid on the boiler metal.
Some of the generated gas becomes part of the
atmosphere inside the boiler, and the remainder is
absorbed by the boiler metal, then liberated
gradually. Because hydrogen air mixtures are
potentially explosive, be careful when opening a
unit for inspection after acid cleaning. Until the
atmosphere within the boiler pressure parts has
been definitely cleared of explosive gases, do NOT
use open flames. flashlights, lighting equipment.
or anything that might produce a spark near the
openings to the pressure parts. Do NOT enter the
boiler. The unit can be cleared of explosive gases
by thoroughly flushing the unit with warm water
with a positive overflow from the highest vent
openings. The water temperature should be as near