Internal Cleaning-—Boilers

Steam-drum internals and the ID surface of the drum should be inspected before washing to determine any problems, including poor circulation, poor water quality, and low steam purity.

The inside of shells, drums, and tubes should then be washed down thoroughly to remove mud, loose scale, or similar deposits before they dry and become more difficult to remove. The washing operation should be carried out from above if possible, to carry the material downward to the blowoff or handholes. A hose with sufficient water pressure or hand tools should be used to remove soft scale and sludge. The blow-off line should be disconnected prior to the washing procedure to keep mud and scale out of the blowdown drum. The tubes of horizontal-return-tube boilers should be washed from below and above. It is especially important to ensure that all tubes and headers are clear of sludge after the wash is completed. Water should be passed down each individual tube and observed to exit from below. Each header should be opened sufficiently to give clear view so that it can be ascertained that all sludge has been removed. Precautions should be taken to ensure that the water does not come into contact with the brickwork of the combustion chamber. If contact cannot be avoided, the brickwork should be dried out carefully when the boiler is fired up.

The use of an inhibited acid solution on the inside of the boiler is a common method of cleaning the interior surfaces. Prior to cleaning, samples of sludge and deposits should be analyzed to ensure the cleaning solution can adequately remove the material. During the cleaning operation, corrosion probes/coupons are often used to monitor the corrosivity of the circulating solution. After acid cleaning, the interior of the boiler must be neutralized, washed down, and refilled with water. If a nitrogen purge is used after acid cleaning, drums should be checked for oxygen content before entry. Acid cleaning should not be used on superheaters or other equipment, which contains pockets that cannot be thoroughly flushed out. Precautions must be taken to make sure that all sludge is removed after an acid wash.

It is normal practice to fill pendent-type superheaters with condensate or demineralized water and to keep the superheater full of this water while the remainder of the boiler is acid cleaned. During chemical cleaning, all phases of the operation should be closely supervised by experienced, responsible individuals. During chemical cleaning, all electric power and other ignition sources in the near boiler must be turned off to prevent explosion of the hydrogen and other hazardous gases that are normally given off during the cleaning.

Another common method of cleaning uses chelates. The chelates are added to the boiler water, and the boiler is fired to create circulation and thereby facilitate cleaning of the internal surfaces. Insufficient removal of chelates after cleaning is a common cause of boiler tube cracking and subsequent failure.

See the ASME Boiler and Pressure Vessel Code, Sections VI and VII, for more information on the care and cleaning of boilers.