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 The principle of using a pump to move quantities of a fluid is simple enough. What is less straight forward is the decision about what type of pump to use, particularly when the application involves pumping slurries or sludges.

With all the attendant drawbacks of repair costs, downtime, loss of production and lack of efficiency, the penalties for selecting the wrong pump for an application can be severe. This is especially true when choosing equipment to pump liquids containing solids, where the nature of the slurry or sludge being pumped plays a more significant role in the selection process than in the case of a simple, benign liquid.

In general terms, by slurry or sludge we're referring to two-phase liquids, namely solids in suspension - but in reality these can be anything from ultra-fine particles in a colloidal suspension, for example dirty water or latex, through to thick dough-like pastes with far higher solids concentration. Each type of product has its own particular flow and handling characteristics and any such product is likely to have some abrasive qualities. Slurries drawn from mining and quarrying operations are a good example as they may contain large angular solids, they may also be corrosive, or they could be shear sensitive, or perhaps a combination of all of these.

The degree of abrasive wear that the solid particles are likely to cause is a critical factor in pump selection. Any type of pump is going to be vulnerable to abrasion as each design has its own potentially vulnerable components: for example, erosive wear to the impeller and casing of centrifugal pump, particles becoming lodged in the meshing components of gear and lobe pumps, and between the rotor and stator of a progressing cavity pump.

The level of abrasion is directly linked to the specific nature of the solids passing through the pump, namely their size, shape and hardness, and to their level of concentration in the product.

Concentration: the greater the quantity of solids present, the greater the wear that will take place. A 20% concentration of abrasive solids by volume will generate proportionately more abrasive wear than a 5% concentration of the same solids.

Size: large solids, especially if jagged and hard, will cause much more scoring than smaller and/or softer solids, in addition to their tendency to jam between the pump elements.

Shape: solids with sharp corners and edges, or of irregular shape, will cause considerably more wear than rolling spherical solids.

It's a hard life

And how hard are those solids suspended in the liquid? Under the standard Mohs scale for mineral hardness (with talc being the softest at 1 and diamond the hardest at 10), a particle of china clay may have a hardness in the region of 2.5, whereas quartz or silica sand, which are also likely to be present to some extent within a clay slurry, will be rated at around 7 (Table1).

The way materials with different hardness values interact with the pump must be considered, as this will have a direct bearing on the speed at which the pump components wear. In general, standard metal pump components are usually suitable for use with softer, less gritty solids, whereas harsher abrasives, i.e. hard angular particles, will require either hardened metals, special wear resistant coatings or, conversely, softer, more-resilient, materials e.g. rubber. To reduce wear, the general rule is that metallic pump components need to be harder than the solids that are to be pumped.

The weight of the solid particles in suspension can also have an influence on pump selection. Heavier particles settle and drop out of suspension faster than lighter ones. When suspended in water, particles less than 75 microns in size, (e.g. fine silt or sand) must be pumped at 1 m/s to prevent settling, whereas larger particles up to 800 microns will need to be pumped at around 2 m/s. the majority of commonly encountered solids in water can usually be pumped at 1.5 - 2 m/s (Table 2). If solids are pumped too slowly and sedimentation occurs in the system or in the pump this can have cost implications due to increased system maintenance and
pump downtime. Loose solids that are easily dispersed may cause a sudden influx of wear in the pump on start up, whereas compacted solids can lead to clogging within the pump (if the internal clearances are not sufficient) and, of course, blockages in the system. On the suction side of the pump this could cause pump starvation leading to cavitation problems, and in extreme cases dry running. Blockages in the discharge will tend to raise the system pressure if the same flow rate is maintained.

Assessing the different pump types

So how do the different types of pumps measure up against these various considerations?

Centrifugal pumps

These pumps are excellent for fluid transfer at low pressures; giving a uniform non-pulsating flow. Centrifugal pumps are used to give high flows with low viscosity abrasive slurries, the wear being contained with either rubber lining or hard facing materials. The pump is generally not self-priming, and is a very high shear pump and therefore will damage shear sensitive products. The pumps can be damaged by cavitation. 
 
 

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Shijiazhuang Mets Machinery Co., Ltd. Shijiazhuang Hi-tech Industry Development Zone Tel:0086-311-68058177 Fax:0086-311-68058178