3.2.14 Slurry pumps

Slurry pumps

Slurry pumps are pumps used for pumping suspensions of various solid particle sizes in industries such as mining, dredging, and steel. They are often designed to be suitable for heavy wear since slurry is very abrasive, some slurries are corrosive as well which presents a challenge because corrosion-resistant materials like stainless steel are generally softer than high iron steel. The most common metal alloy used are high chrome steel alloys, which is basically white iron with 25% chromium added to make it less brittle. For particle sizes of up to approx 5 mm the components in contact with the pumped fluid are coated with a rubber lining, whilst hard metal materials are used in the case of larger particles and higher fluid temperatures. Despite the use of extremely wear resistant material the operational life is relatively short, might be as low as in the order of a few months and depending greatly upon the speed of flow.

For delivery heads of above 30 to 50 m it is therefore normal to employ several pumps in series. Pump impellers are of the closed or semi-open type with suitably thick sides and blades. The number of blades are relatively few. For closed impellers the sealing clearance to the pump casing is axially shaped to reduce wear. Shafts and bearings must be designed for heavy duty due to the out-of-balance forces figure 3.214a.

Selection of slurry pumps is difficult and correction factors and wear factors must be taken into consideration. Density corrections due to to the concentration of solids, wear considerations due to abrasion caused by slurry properties are usually classified and determined according to the concentration of solids, particle size, shape and weight. The peripheral speed of the impeller is one of the main features that has to be taken into account when selecting slurry pumps in order to maintain a reasonable life in service due to high abrasion of solids. Normal water pump selecting criteria, that is, flow capacity, head, efficiency and power and NPSH has to be considered as well.

Slurry pumps are usually V-belt driven so as to be able to adjust the volume flow rate as required. Throttling is particularly difficult when transporting solids suspended in fluids.

Figure 3.214a Solids pump with replaceable rubber lining

Due to the abrasive nature of the fluid particular attention has to be paid to the design of the shaft seals. Design solutions such as stuffing box containing pure sealing fluid, see pages under Pump Seals section about hydrodynamic seals in centrifugal pump seals with back vanes and as seal less pumps with overhung impellers for vertical models, as in figure 3.214b, are available. The pump in this figure is particularly suitable for frothing fluids, where air can rise and not block the impeller intake. A certain degree of self-regulation is obtained by the air mixing at low level.