Boiler Circulating Pumps used in coal, combined cycle and nuclear plants
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.
Because of the relatively low head requirements, the pumps are single stage with
single-suction impellers and a single seal chamber. This creates an unbalanced
axial thrust problem, which may require special pump bearing systems or
balancing arrangements. An alternate solution is to use wet motor construction
pumps, in which the pump and the motor are inside the pressure vessel,
eliminating sealing and unbalanced axial thrust issues. These special pumps are
welded into the boiler piping. For higher temperatures up to 365 C (685 F) and
pressures from 124 to 193 bar (1,800 to 2,800 psi), special pump designs are
required.