Evaluating Boiler Circulating Pumps

What information is available regarding boiler circulating pumps for combined-cycle power plant service? Boiler circulating pumps circulate water within the boiler to enhance boiler operation. They take suction from a header connected to the bottom of the boiler drum and discharge through additional tube circuits. This means the water pumped is at boiler temperature and pressure. For this reason, standard boiler circulating pumps must be designed for high temperature (usually between 300 and 600 F [150 and 315 C] depending on boiler size and rating) and high pressure (corresponding to boiler temperature and water vapor pressure). For small boilers that have relatively low temperatures and pressures, conventional overhung pump designs may be suitable for boiler circulating service. Boiler circulating pumps must develop enough head to overcome the friction of the tube circuits; however, the combination of high temperature and pressure results in sealing conditions that require special sealing systems. Because of the relatively low head requirements, the pumps are single-stage with single-suction impellers and a single seal chamber. This creates a problem of unbalanced axial thrust, which may require special pump bearing systems or balancing arrangements. An alternate solution is to use pumps of wet-motor construction, where the pump and the motor are inside the pressure vessel, eliminating sealing and unbalanced axial thrust issues. Such special pumps are welded into the boiler piping. For higher temperatures to 685 F (365 C) and pressures from 1,800 to 2,800 pounds per square inch (psi) (124 to 193 bar), special pump designs are required. For more information about pumps used in combined-cycle power plants, please see HI’s guidebook Power Plant Pumps: Guidelines for Application and Operation to Maximize Uptime, Availability, and Reliability. Read more HI Pump FAQs by clicking here.