Centrifugal pumps are highly susceptible to variations in process parameters, such as suction pressure, specific gravity of the pumped liquid, backpressure induced by control valves, and changes in demand volume. Therefore, the dominant reasons for centrifugal-pump failures are usually process-related.
Several factors dominate pump performance and reliability: internal configuration, suction condition, total dynamic pressure or head, hydraulic curve, brake horsepower, installation, and operating methods. These factors must be understood and used to evaluate any centrifugal pump-related problem or event.
All centrifugal pumps are not alike. Variations in the internal configuration occur in the impeller type and orientation. These variations have a direct impact on a pump’s stability, useful life, and performance characteristics.
Impeller Type. There are a variety of impeller types used in centrifugal pumps. They range from simple radial flow, open designs to complex variable-pitch, high-volume enclosed designs. Each of these types is designed to perform a specific function and should be selected with care. In relatively small, general-purpose pumps, the impellers are normally designed to provide radial flow and the choices are limited to either enclosed or open design.
Enclosed impellers are cast with the vanes fully encased between two disks. This type of impeller is generally used for clean, solid-free liquids. It has a much higher efficiency than the open design. open impellers have only one disk and the opposite side of the vanes is open to the liquid.Because of its lower efficiency, this design is limited to applications where slurries or solids are an integral part of the liquid.
Impeller Orientation. In single-stage centrifugal pumps, impeller orientation is fixed and is not a factor in pump performance. However, it must be carefully considered in multistage pumps, which are available in two configurations: in-line and opposed.
In-Line. In-line configurations have all impellers facing in the same direction. As a result, the total differential pressure between the discharge and inlet is axially applied to the rotating element toward the outboard bearing. Because of this configuration, in-line pumps are highly susceptible to changes in the operating envelope.
Because of the tremendous axial pressures that are created by the in-line design, these pumps must have a positive means of limiting endplay, or axial movement, of the rotating element. Normally, one of two methods is used to fix or limit axial movement: (1) a large thrust bearing is installed at the outboard end of the pump to restrict movement, or (2) discharge pressure is vented to a piston mounted on the outboard end of the shaft.
The first method relies on the holding strength of the thrust bearing to absorb energy generated by the pump’s differential pressure. If the process is reasonably stable, this design approach is valid and should provide relatively trouble-free service life. However, this design cannot tolerate any radical or repeated variation in its operating envelope. Any change in the differential pressure or transient burst of energy generated by flow change will overload the thrust bearing, which may result in instantaneous failure.