25 May 2005
are usually iron and copper, resulting from aggressive condensate. Microbiological
deposits may form in cooling systems from bacterial or algae growths, or from
decomposition products of various microorganisms.
Boiler Deposits. Boiler deposits may take various forms. In low-pressure
boilers using a relatively hard feedwater, deposits are essentially calcium and
magnesium, silicates, sulfates, carbonates, phosphates and hydroxides, plus some
organics. Deposits may also contain considerable amounts of silica, iron, and copper.
These deposits can be spongy or porous or relatively hard and glass-like. Deposits of
the latter characteristic occur where silica is present in appreciable quantities in the
boiler water. Deposits in medium-pressure to high-pressure boiler systems usually are
mixtures of iron and copper oxides and phosphates. Dense deposits may tend to form in
high-heat transfer areas. Considerable quantities of sludge-type accumulations may be
found in downcomers, mud drums, waterwall headers, crossover tubes, and areas of
low water circulation in the boiler.
REMEDIAL CLEANING PROCEDURES. Cleaning procedure information
and procedures presented in this Chapter are general in nature and must be modified to
fit specific applications. Because contractors perform most cleanings, these procedures
are provided only for general information (see paragraphs 9-1.1 and 9-1.2).
Mechanical Methods. Mechanical methods are the oldest techniques
used for removing deposits. To perform an adequate mechanical-type cleaning, the
equipment to be cleaned may need to be partially or entirely dismantled. Even when
equipment is dismantled, some areas may be extremely difficult to reach and clean.
Chemical cleaning has largely replaced mechanical process equipment cleaning as the
most satisfactory method of removing deposits; however, mechanical methods such as
wire brushing, tumbling, scraping, and abrasive blasting with sand and grit are still
employed in special applications.
Cleaning Agents. Cleaning agents may be broadly classified as being
acid, alkaline, organic, or solvent cleaners. There is no general or universal cleaner that
removes all deposits. The selection of a solvent or cleaning agent is based on the
material's ability to remove or dissolve the deposit, as well as on cost considerations,
safety hazards, and the effect of the cleaning material on the metals involved.
General Guidance and Procedures for Preparing Cleaning Solutions.
General guidance and procedures for preparing cleaning solutions of inhibited
hydrochloric (muriatic) acid and inhibited sulfamic acid are provided in paragraphs 9-2.2
and 9-2.3. Inhibited acid contains special chemical inhibitors that prevent the acid
cleaner from attacking the base metal while allowing the acid to remove the unwanted
corrosion product or scale deposit.
Hydrochloric (Muriatic) Acid. Inhibited hydrochloric (muriatic) acid in
strengths of 5 to 20% is very effective for removing calcium scale and iron oxide;