Water is the working fluid for steam generation and boilers, which are among the most costly and vital pieces of equipment in an industrial complex. Proper treatment and conditioning of water can increase boiler performance, energy efficiency, maintain production capability and reduce operating costs while extending its operational life. Boiler water treatment avoids scaling and corrosion, insuring safe and reliable operation. Lack of appropriate treatment can cause a series of problems to develop, ranging from loss of productivity, accelerated wear and, in extreme events, its destruction.
Boilers are the heart of the steam system and ultimately, they receive all residual contaminants that remain in feed water, after external treatment. And when water enters the boiler, elevated temperatures and pressures cause water contaminants to behave differently. Like in cooling water, since there is a continuous water make-up to compensate evaporation, a concentration cycle occurs in boiler water. Under existing conditions inside the boiler, most water-soluble components reach their solubility limit and may leave solution as particulate solids, in crystallized or amorphous forms, developing scale and deposits over boiler heat transferring surfaces. These deposits have insulation properties and can impair heat transfer.
Large amounts of deposits along the boiler can reduce heat transfer and boiler efficiency significantly and, in the meantime, they may provoke corrosion and eventually tube failure by overheating at a point. The presence of dissolved gases is also common when water enters a boiler. Certain gases in solution, like CO 2 and O 2' are released when heated and react with water to form carbonic acid (H2CO3), greatly increasing corrosion.
In order to control these corroding processes, two types of boiler water treatment are necessary: internal and external. External boiler water treatment is usually done immediately after industrial water treatment, deeper removal of dissolved solids, particularly major participants in scale formation, like calcium and magnesium, and to some extent silica. Silica is a chemical compound that also forms scale and may bring specific problems to high pressure steam turbines and superheating areas of the boiler. Since no treatment can completely remove all contaminants and their amount keeps growing, because of the concentration cycle, so supplementary internal treatment is needed. This is done by addition of chemicals that convert scale-forming compounds into a sludge, which can be withdrawn by bottom purge or blowdown. Chemicals that are generally used in this treatment, are sodium salts of carbonate, aluminate, phosphate, tri- and polyphosphate, sulfite and special compounds as polymers. Materials of vegetable and animal origin can be also injected to remove scale, but this is an outdated practice. Application of such methods relies on some conditions, like boiler design type, steam pressure; feed water contaminant concentration, and maximum allowable operating concentration in boiler water. Usually, depending on the quality of the makeup water, low blowdown flow rates and periodicity indicate well-treated boilers. Elevated blowdown rate is usually uneconomical, because of heat and water losses.
Silica removal is supplemented on high pressure water-tube boiler designs by installation of top drum internal accessories, and a continuous skimming or surface blowdown from this drum is necessary. Heat recovery from continuous blowdown is an energy efficiency option.
Removal of dissolved gases is accomplished by external and internal treatments. Externally to the boiler, dissolved O2 and CO2 are expelled by preheating the feed water before it enters the boiler. This process, called mechanical deaeration, is normally performed under vacuum, in a vessel external to the boiler. Boiler feed water enters this drum passing a kind of liquid-vapor contact element, like a small distillation tower, where it crosses counter current with steam. Vacuum and heat reduces the solubility of gases, which are vented to the atmosphere. One good energy efficiency practice here is to use low pressure steam, especially if it is available in excess. Deaeration operating pressure can be specified to use this steam and help steam balance, by absorbing a stream in surplus, hence contributing to fuel savings. Efficient mechanical deaerators can reduce oxygen concentration to very low levels, but even trace amounts can cause corrosion. Consequently, supplementary removal is necessary, and this is done by addition of a chemical oxygen scavenger, such as sodium sulfite or hydrazine. When reacting with oxygen, sodium sulfite forms soluble sodium sulfate, which increases total dissolved solids in boiler water, increasing blowdown and internal treatment requirements.
Hydrazine reacts to form nitrogen and water that do not increase solid concentration and this is of mandatory use in high pressure boilers which require low solids. The approach presented here for internal boiler water treatment is traditional and well-established by use for many years. But technology is always advancing and chemical companies and boiler manufacturers update their products constantly. Disregarding the specific chemical products mentioned here, all water quality requirements remain, whatever newer product may be considered.
Well-managed boiler water quality is critical for an energy efficient utility system. This parameter must be considered in boiler design and selection. Maintaining its expected specifications can propitiate conditions to achieve maximum boiler efficiency and since the main energy conversion happens in the boiler, this efficiency can be propagated throughout the whole plant.
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