By Allan Budris
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Casing type can have a large impact on pump reliability (the mean time between repairs) and, to a lesser extent, pump efficiency when pump flow rates are above or below the best efficiency point (BEP). Although the objective of many pump applications may be to operate the pump at its BEP, this is often not achieved due to:
The three basic casing types primarily used for centrifugal pumps are the single volute, double volute and vaned diffuser. Of these, the most common - at least for low- to medium-flow, single-stage pumps - is the single volute, with double volutes used more for larger, single-stage pumps handling liquids without solids, and vaned diffusers primarily applied to multistage pumps.
Each of these casing types has its advantages and disadvantages regarding bearing and shaft seal life, pump efficiency and pump cost. Although pump users do not always have a choice of the casing type, there are applications where at least some vendors may provide the pump user with this decision. It is, therefore, advantageous for pump users to be knowledgeable regarding the impact of these potential casing selections.
Single volute pumps have been in existence for several years. With the exception of vertical pumps of the turbine type, the majority of single-stage pumps built in the United States are of the single volute type. The main advantage of single volute casing is its simplicity: it is less difficult to cast, and therefore less expensive to produce given the open areas surrounding the impeller periphery. At flow rates near the pump BEP, single volute casings are more efficient than double volute casings. Further, the inherent greater throat areas can also handle larger solids, as found in wastewater.
The volute scroll is designed for constant velocity near the best efficiency flow rate, which yields a uniform static pressure around the periphery of the impeller but only at the BEP (see Fig. 1). This pressure equilibrium is, however, destroyed when the pump is operating at flow rates below (see Fig. 2) or above the BEP. Figure 2 shows the pressure distribution around the impeller in a single volute at zero flow rate, where A = total reaction, B = reaction on side shrouds, C = reaction on impeller periphery, and D = non-uniform fluid momentum. This off-BEP, non-uniform pressure distribution around the impeller results in increased net radial loads on the rotor (see Fig. 3). Likewise, this increased load deflects the pump shaft and can result in excessive deterioration at the wearing rings, seals or bearings. In extreme cases, shaft breakage due to fatigue can result.
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