Control of Boiler Feedwater and Cooling Water

Abstract

A water that is not scale-forming will not precipitate minerals that would adhere to the boiler wall and reduce the heat transfer from the hot gas to the water. Such a reduction in rate of heat transfer would not only result in loss of heat to the flue gases, with consequent reduction of efficiency, but might also cause excessive increase in the temperature of the boiler metal. The water temperature in a boiler is determined by the pressure at which it operates. At 100 psiof pressure, the temperature is 328 F ; at 200 psi, 382 F ; and at 400 psi, 444 F. As the water temperature cannot decrease, the metal temperature must increase. The practical temperature limit of boiler steel currently used is approximately 900 F. If the scale is thick enough, the metal temperature may exceed this limit, and a failure will result. In extremely high-pressure boiler work, a scale thickness of 0.010 in. is sufficient to cause a failure. Noncorrosive water is water that will not destroy metal. In general, there is little trouble with corrosion in the boiler itself, but if steam is returned as corrosive condensate, condensate lines can be severely damaged. Water that will not carry over is water that has no solids which can be entrained in the steam bubbles and pass out of the boiler with the steam. If solids are carried over, they deposit on valves, piping, and turbine blades. A heavy deposit on turbine blades can reduce the efficiency of the turbine and may even cause an unbalance that can destroy the machine. Caustic embrittlement is similar to corrosion, but acts in the boiler and causes intercrystalline cracking of the metal. This condition will develop more readily on riveted joints and small cracks such as those around the tube holes or around a leaking manhead. If not checked, it can cause complete failure of the boiler. The present approach to the problem consists of maintaining a certain ratio of sulfate to carbonate, depending on the operating pressure of the boiler. A ew approach the maintenance of simultaneous control of the pH and phosphate content has been suggested.