Q. What information is available regarding boiler circulating pumps for combined-cycle power plant service?
A. Boiler circulating pumps circulate water within the boiler to enhance boiler operation. They take suction from a header that is connected to several downcomers from the bottom of the boiler drum and discharge through additional tube circuits. This means the water pumped is at boiler temperature and pressure. Boiler circulating pumps are designed for high temperatures (usually between 150 C and 315 C [300 F and 600 F], depending on boiler size and rating) and high pressure (corresponding to boiler temperature and water vapor pressure). For small boilers with relatively low temperatures and pressures, conventional overhung pump designs—such as Hydraulic Institute (HI) pump type OH2 (see Image 3.11)—may be suitable for boiler circulating service. Boiler circulating pumps must develop only enough head to overcome the friction of the tube circuits. However, the combination of high temperature and pressure results in conditions that require special sealing devices.
Q. How are auxiliary cooling water pumps used in combined-cycle power plant service, and what pump types can be used?
A. Auxiliary cooling water pumps (see Figure 6.15) usually operate in a closed-loop system using high-quality water. The water received from the circulating water pumps moves through the tube side of the auxiliary cooling water heat exchanger. The auxiliary cooling water circulates through the shell side and is cooled. The auxiliary cooling water then proceeds to small heat exchangers used for cooling the pump and motor bearing frames, mechanical seals and other equipment. The fluid pumped from auxiliary cooling water pumps is clean, high-quality water, unlike the main cooling system, which takes water directly from the cooling tower or other sources. Clean, high-quality water is used to cool pump, fan and motor bearing frames; mechanical seals; and heat exchangers because they could be easily plugged if the cooling water contains high levels of suspended or dissolved solids. The fluid temperature usually ranges from 10 C to 27 C (50 F to 80 F).
Q. How should a mounting base be designed and installed for a sealless rotodynamic pump?