The casing is the major stationary component of the pump and mainly provides two functions:
- Converting velocity head from the impeller into pressure head and guidance of the flow to the discharge connection
- Creating a pressure boundary for the pumped liquid
The two main types of casings in centrifugal pumps are volute casings and diffuser casings.
A very common classification of pump casings is the diffuser construction. A diffuser is actually a series of vanes surrounding the impeller which accepts the discharge of liquid from the impeller. It efficiently reduces the velocity and, in the case of a multistage pump, directs this lower velocity fluid into vaned return channels which guide the liquid to the inlet or eye of the next stage impeller.
In the case of a single stage pump, the discharge fluid from the diffuser is collected in a surrounding casing, which guides the liquid out of the pump through the discharge nozzle.
Diffuser casings are commonly used for vertical turbine and multistage, high-head pumps for maximum pump reliability and where high efficiency is of major importance. High efficiency is not the only advantage for a diffuser pump. In the diffuser type, the impeller discharges into a uniform ring of nozzles, and consequently the radial loading on the impeller is fully balanced (meaning zero radial load over the full flow range). Keeping the radial load on the impeller at a minimum value over its full flow range through the selection of a certain casing type can have a significant impact on improving the pump life, especially since bearing failures are the second most common reason for pump failures.
Other reasons to choose for a diffuser casing is to be able to select a pump that operates at its best efficiency point (BEP), in order to achieve a higher pump efficiency or to avoid possible damage from low-flow operation in suction recirculation. To achieve the same efficiencies with a volute casing, you would need hundreds of volute designs to complete a range for every possible duty, which is often too expensive. That’s the reason most of the times a volute type of pump is not operating on its BEP.