Multistage Pump Casings

Although the majority of single-stage pumps are of the volute-casing type, both volute and diffuser casings are used in multistage pump construction. Because a volute casing gives rise to radial thrust, axially split multistage casings generally have staggered volutes so that the resultant of the individual radial thrusts is balanced out (see Figures 16 and 17). Both axially and radially split casings are used for multistage pumps. The choice between the two designs is dictated by the design pressure, with 2000 lb/in2 (138 bar)° being typical for 3600-rpm axially split casing pumps.

FIGURE 16 An arrangement of a multistage volute pump for radial-thrust balance
FIGURE 17 The horizontal flange of an axially split casing, six-stage pump (Flowserve Corporation)

Axially Split Casings Regardless of the arrangement of the stages in the casing, it is necessary to connect the successive stages of a multistage pump. In the low and medium pressure and capacity ranges, these interstage passages are cast integrally with the casing proper (see Figures 18 and 19). As the pressures and capacities increase, the desire to maintain as small a casing diameter as possible, coupled with the necessity of avoiding sudden changes in the velocity or the direction of the flow, leads to the use of external interstage passages cast separately from the pump casing. They are formed in the shape of a loop, bolted or welded to the casing proper (see Figure 20).

Interstage Construction for Axially Split Casing Pumps A multistage pump inherently has adjoining chambers subjected to different pressures, and means must be made available to isolate these chambers from one another so that the leakage from high to low pressure will take place only at the clearance joints formed between the stationary and rotating elements of the pump. Thus, the leakage will be kept to a minimum. The isolating wall used to separate two adjacent chambers of a multistage pump is called a stage piece, diaphragm, or inter-stage diaphragm. The stage piece may be formed of a single piece, or it may be fitted with a renewable stage piece bushing at the clearance joint between the stationary stage piece and the part of the rotor immediately inside the former. The stage pieces, which are usually solid, are assembled on the rotor along with impellers, sleeves, bearings,

FIGURE 18 Two-stage axially split casing volute pump for small capacities and pressures up to 450 lb/in2 (31 bar) (Flowserve Corporation)
FIGURE 19 Two-stage axially split casing volute pump for pressures up to 500 lb/in2 (34 bar) (Flowserve Corporation
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