Centrifugal Slurry Pump Hydraulic Design

In order to reduce wear, the hydraulic design of a slurry pump should be such that the rotating wetted component and fluid velocities must be kept as low as practical while hard metal or elastomer wetted-section thicknesses are increased. Impeller meridional sections are also usually kept close to rectangular and with front sealing on a vertical face to minimize the axial rotating surfaces. Collectors (casings) are usually semi-volute (or even

FIGURE 2 Dredge pump impeller, 105 in (2.67 m) diameter

annular) to give better wear over a wider range of best-efficiency-point quantity (flow rate) (BEPQ).

Where larger solids are to be pumped, the inside shroud impeller widths are usually oversized, the number of vanes is reduced to two or three, and the vane overlap may be reduced. All of these distortions modify or reduce the hydraulic performance. Given the variety of types (and severity) of services, the design solution that provides the best compromise between wear life and efficiency and provides the end user with the overall lowest total cost of ownership is not simple to achieve. Although specific to a particular slurry type, Kasztejna et al. (1986) and Cooper et al. (1987) provide a good insight into some of the design tradeoffs and special problems involved in the design of a centrifugal slurry pump.

In general, slurry pumps are of low specific speed (as defined in Section 2.1), in the range of 750 to 2000 for Ns, (14.5 to 39 for nq and 0.27 to 0.73 for 0,). They employ three to five vanes, have impeller inside shroud widths that are 75-200% wider than those a normal water pump, and use impeller vane and shroud thicknesses two to three times that of a water pump. The results of these distortions flatten the constant-speed head-flow curves and reduce efficiency by five or so percent. Typical head coefficients (as defined in Section 2.1) and efficiencies are shown in Figures 5 and 6.

In order to minimize wear over different ranges of percent of BEPQ, operation slurry pump casings vary from the true volute water pump T type (shown schematically on Figure 7) through the semi-volute C type to the essentially annular A type. There are also a few examples of what is called the OB type with recessed tongue and special extended

FIGURE 3 Typical single-wall hard iron slurry pump
FIGURE 4 Typical double-wall elastomer-lined slurry pump
FIGURE 5 Head coefficient versus specific speed nqf based on m3/s, m, and rpm (from Addie and Helmly, 1989). [Note: for specific speed Ns, based on gpm, ft, and rpm, multiply nri by 51.65; for universal specific speed Hs, divide nq by 52.92.]
FIGURE 6 Pump efficiency as a function of specific speed nqf based on m3/s, m, and rpm (from Addie and Helmly, 1989). [Note: for specific speed Ns, based on gpm, ft, and rpm, multiply nq by 51.65; for universal specific speed Hs, divide nq by 52.92.]
FIGURE 7 Types of shells (casings) and impellers
FIGURE 8 Trends in location of casing belly wear

neck intended for limiting wear in the tongue area due to excessive recirculation (which itself is symptomatic of misapplication). The general applicability of each of these is shown in Figure 8, taken from Addie and Helmly (1989).

Impeller types may be roughly categorized also as being of the old-style square meridional section radial vane RV type (still common in rubber-lined pumps where it is difficult to mold a twisted vane), the most common rounded rectangular meridional section with twisted vanes ME type, and the HE type which is closer to a water pump.

Each combination of the types illustrated in Figure 7 has its own hydraulic performance and wear characteristics. The HE/T combination generally has the highest performance, but is not necessarily the most forgiving for wear, whereas the ME/C combination is capable of respectable efficiency while at the same time having more predictable wear performance. In extremely heavy-duty wear service, operation must be at low discharge flow rates—below BEPQ. The A type shell, or even the OB type, could be best for wear where a pump has been misapplied badly. For additional information on the selection of different hydraulic types, see Addie et al. (1996).

Impeller and shells using elastomers such as rubber, neoprene, and urethane tend to be limited to impeller tip speeds less than about 75 ft/s (23 m/s), although this can rise with stiffer elastomers, at some expense to wear life. Impellers employing elastomers require higher available NPSH because of the thicker impeller vane sections needed, and this condition may limit their use, for example in pumping flue-gas desulphurization slurries.

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