Water Treatment General Considerations
Water treating requirements vary significantly with incoming water quality and the final uses. Water must be treated to economically satisfy the following types of users:
1. Water for cooling in the middle of a superheater: When reducing the temperature in the middle of superheater coils, water of extreme purity (50 parts per billion) must be used. Any dissolved solids in the water will plate out in the superheater.
2. Water for desuperheating at the superheater outlet: This is conventional desuperheating and the water used should be the best quality water in the plant. Typically, this is condensate or demineralized water at 10 to 20 ppm or less.
3. Water for injecting into the gas turbine burner for NOX control: The gas turbine manufacturer should be consulted. Normally, this must be extremely pure water. Frequently steam from the HRSG is condensed for this service. If the steam is injected into the gas turbine, a much larger quantity of steam is required (compared to water) for the same NOX control.
4. Water for boiler feedwater make-up to:
a. Heat Recovery Steam Generators of the dry, superheated type: use the American Boiler Manufacturers Association (ABMA), Guidelines. The more stringent limitations set by the ASME in 1979 are rarely met in industry. As a design guideline, assume that if your existing water treating is adequate for your existing power boilers, it is good enough for the HRSG.
b. Once-through 60% to 80% quality steam generators: Because the tubes are never run dry, the only treatment for this brackish water is softening.
c. High-pressure boilers: Evaluate the merits of operating at the more economic guidelines of the ABMA rather than adopting (and installing the necessary equipment to meet) the more stringent requirements of drum concentration, found in the ASME guidelines.
5. Water to process steam generators, all pressure levels: For all waste heat recovery projects, should be guided by what the plant presently does in this area.