Feedwater Contamination from Makeup Water

Many different types of external treatment processes are available to deal with typical MU water contaminants such as undesirable dissolved solids, colloidal particles, or noncondensable gases. 

in any boiler plant, the installed treatment processes are unsuitable or inadequate for the needs or if operational problems persist, downstream operational problems certainly will develop in a short time. The extent of these problems and their associated costs are likely to be serious if the external treatment problem is not rectified at the source. In almost all cases, internal chemical treatment cannot totally and economically compensate for lack of pretreatment. 

Where downstream boiler plant problems do result, these may often be traced back to inappropriate FW quality. This does not mean that the FW is necessarily contaminated in some way, but rather that the most suitable or necessary type or capacity of pretreatment equipment (such as a water softener, dealkalization plant, or deaerator) has most probably not been installed. 

A lack of much-needed MU water softening, dealkalization, or similar external treatment capability is quite common in many small to midsized industrial facilities. It seldom produces a complete boiler failure or anything quite so dramatic, but it is nonetheless very costly in terms of additional fuel costs and maintenance. Personnel may learn to live with the resultant poor quality of FW for many years, accepting the position as normal until the facts are pointed out and alternative proposals are considered. 

The use of FW that has been inadequately pretreated makes the mechanical operation of a boiler and the control of boiler water conditions innecessarily difficult. Additional quantities of internal treatment chemicals and higher rates of blowdown (BD) are usually required, which reduces boiler plant efficiency and raises the cost of generating steam. 

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Feedwater contamination From makeup water (Corit.)... 

Condensate, however, may not always be usable as boiler feedwater, due to the presence of certain process contaminants (see Table 1). When steam is used for heating, to power equipment, and for varied process applications, condensate comes into contact with oil and other impurities. Contamination from fuel-oil heaters or leaking seals in steam-driven equipment, such as pumps, turbines or compressors, is common. Also, the raw water used as plant makeup, may contain significant levels of solvents, uel oils, gasolines and greases. In addition to these general contamination sources, each industry has its unique one. (Table II). If any of these contaminants become entrained in the returned condensate stream, numerous and sometimes unpredictable problems in the boiler and related systems cs.n occur. 

These contaminants can enter the steam supply system with the makeup water or in process heat exchange equipment. They can exist in the boiler drum in relatively high concentrations without causing problems. It is only when they are carried over into the exiting steam that they enter the turbine. Efficient boiler drum separators can limit total dissolved solids to as little as 0.5 to 1.0 ppm. Silica is difficult to separate from the steam and must be controlled in the boiler feedwater. Care must also be taken with the fluid used to attemperate the steam exiting the superheater. Contaminated process returns can bypass the steam separation in the drum as attemperator fluid. This should be maintained at less than 1.0 ppm total dissolved solids. 

As the water evaporates into steam and passes on to the superheater, soHd matter can concentrate in a boHer s steam dmm, particularly on the water s surface, and cause foaming and unwanted moisture carryover from the steam dmm. It is therefore necessary either continuously or intermittently to blow down the steam dmm. Blowdown refers to the controHed removal of surface water and entrained contaminants through an internal skimmer line in the steam dmm. FHtration and coagulation of raw makeup feedwater may also be used to remove coarse suspended soHds, particularly organic matter. 

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