In steam generating equipment the buildup of scale and other materials such as sludge on the surface areas of a boiler can produce overheating and even corrosion. Both corrosion and overheating can result in unscheduled downtime of your equipment.
Typical contaminants found in boiler feed water that form scale or sludge include calcium, magnesium, iron, copper, aluminum, silica, and sometimes even silt and oil. These materials either harden as scale on the boiler tube surfaces or as sludge deposits that form elsewhere in the system and travel to the metal surface by way of the flowing water.
Scale is formed when salts, which have restricted solubility, find their way into the boiler water. When in a soluble form these salts will precipitate when compounded by evaporation. Scales forms because when precipitates form they usually have a homogeneous composition and crystal-like structure.
High evaporation is caused by high heat, which serves to concentrate the unevaporated water in the area of evaporation. The concentrated water creates the perfect environment for the precipitation of scale-forming compounds. The quality of the scale is determined by the composition of the concentrated water. Typical chemical composition of scale includes calcium, magnesium, silica, aluminum, iron, and occasionally sodium.
The hardening of chemicals into scale does not happen overnight. It’s a slow process. Because these scale deposits form slowly they tend to be extremely dense which creates two problems:
- Scale creates a high level of insulation, which reduces the efficiency of the system. In fact, a 1/8 inch thick layer of scale can cause as much as 25 percent loss in efficiency, and
- Scale can be so dense that any attempt to remove it is ineffective
Sludge deposits can also be hard, dense, and stubborn to remove. Sludge is a buildup of solids that precipitate in bulk boiler water or which enter the boiler in the form of suspended solids. When exposed to high heat levels, which is the case when the boiler is drained, sludge deposits can become baked on.
In addition to reducing boiler efficiency, scale and other deposits formed in the heat transfer path from the boiler flame to the water restrict boiler water circulation. This happens because the surface of the tube the water flows through becomes coarse which increases drag in the boiler circuit. Reduced circulation creates additional problems such as accelerated deposition of minerals, overheating, and untimely steam-water separation.
Because the same amount of steam is being generated as in a boiler system without scale, the steam-water ratio in the scale covered generating tube is increased. The water in a tube effected by scale increases in mineral concentration which in turn increases the chance of more scale forming from the boiler water salts.
All of this can amount to a total failure of a furnace tube due to overheating.