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Circulating pumps are centrifugal pumps designed to generate a forced circulation in a closed system. Examples are heating systems for hot water or high-temperature hot water at temperatures above 120 °C, heat transfer systems, forced circulation boilers and reactor circuits. They are also used in open systems (e.g. swimming pool filtering systems).
The design of a circulating pump is often determined by the frequently high temperature of the fluid handled and the relatively low head (in relation to the system pressure), which corresponds to the head loss in the circulation system. Different types of circulating pumps are used in closed pump circuits: circulating pumps with or without a shaft seal, and special designs.
Circulating pump with shaft seal
Circulating pumps with a shaft seal are often horizontal pumps, driven by electric motors or steam turbines. Their pump shafts are sealed against the full pressure of the closed system by cooled gland packings or mechanical seals. The thrust bearing has to be particularly robust to absorb the high static axial thrust.
This type of pump is usually an economical solution for system pressures of up to 100 bar. For higher system pressures circulating pumps without shaft seal are used.
Circulating pump without shaft seal
Seal-less circulating pumps are also referred to as glandless circulating pumps. They are often vertical pumps, driven by wet rotor motors with or without a can.
The pump and the electric motor are contained in a common, pressure-tight casing with a heat barrier between the pump section and the motor section. The heat barrier, which can either be an active or a passive component, enables the pump set to handle fluids with temperatures of up to 420 °C.
The bearings are lubricated by the fluid handled. The system pressure is virtually unlimited. The completely leak-free design makes these pumps suitable for handling hazardous or valuable fluids.
Special designs of circulating pumps
Circulating pumps can also be specially designed for specific applications:With special impellers for liquid/gas mixtures or suspensionsWith sealing systems incorporating a gas cushion or meeting other special requirements of nuclear reactor technologyWith heating devices for liquids which solidify rapidlyWith protective lining preventing erosionExplosion-proof designs
