Boiler feedwater entrainment

The amount of hardness deposits in steam is a function of entrainment of water, as well as the TDS of the boiler feedwater. A well-designed boiler, then, often is equipped with a mesh-type demister pad to remove entrainment from the produced steam. 

To compensate for this problem, the top tube of the tube bundle should be kept well below the top of the kettle. This also will help retard entrainment of boiler feedwater in the steam. 

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. 

Feedwater heater A steam-to-water heat exchanger that heats the boiler feedwater, generally with steam extracted from a steam turbine. In a closed feedwater heater the two fluids are separated by the use of shell and tube construction. In an open feedwater heater the fluids are mixed. A feedwater heater that separates entrained gases from the feed-water by vigorous agitation with steam. 

Although boiler feedwater (BFW) has been treated to remove minerals, residual contaminates (carbonates, silicates) are still present. These minerals, especially the silicates, are bad, as to some extent they are going to be entrained into the generated steam. The biggest problems with entrained silicates (TDS— Total Dissolved Solids) are ... 

Entrainment of high TDS boiler feedwater into steam from a boiler is promoted by excessive pH. Caustic is often added to the feedwater to reduce corrosion. However, if the pH increases much above 10.5, then the boiler tends to foam and hardness deposits are carried over into the steam. Hardness deposits degrade the performance of steam ejectors, steam superheating coils, and steam turbines. 

Low temperature catalysts typically contain copper and zinc and are effective over a temperature range of about 350° to 650°F. They are used where very low carbon monoxide concentrations are required in the product gas. These catalysts are extremely sensitive to poisoning by sulfur compounds, and the feed gas has to be thoroughly desulfurized before contacting the catalyst. Chlorides and silica from entrained boiler feedwater in the steam can also poison the low temperature catalyst. Since the equilibrium for the shift reaction is more favorable at lower temperatures, almost complete conversion of carbon monoxide—on the order of 99%—is possible with low temperature catalysts. 

The amount of hardness deposits in steam is a function of entrainment of water, as well as the TDS of the boiler feedwater. 

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