2.3 Pump selection for suspensions

Pump selection for suspensions

Pump selection for suspensions needs special attention since the service life of a pump can be severely affected already at particle contents exceding about 0.5%. For light particles, e.g. paper pulp, the reader is referred to Chapter 3 and the section including pulp pumps. In the case of heavy particles or slurry, pumps of special design should be used for particle of more than 3% by weight. Establishing the limit is of course dependent on size, hardness and shape of the particles. Gypsum sludge and chalk are easily handled, but sand presents difficulties. The zone between pumps for pure liquids and special pumps for the transportation of solids is spanned by special designs of clean liquid pumps, e.g. semi-open impellers for centrifugal pumps and special materials for displacement pumps.

Gentle pumping

Gentle pumping is used when the solids to be transported by the Iiquid are to be damaged as little as possible. For this purpose the goods can be anything from delicate bacteria cultures to foodstuffs and root crops. The basis for a systematic choice of pump is seldom available. In the case of centrifugal pumps, the delivery head per stage is a measurement of the stirring of the Iiquid, while the speed of the pump should not exercise any influence. In order to avoid damage to the transported particles the passages through the pump should be large. This means that among centrifugal pumps the group non-clogging pumps should be used and among displacement pumps lobe rotor type of pumps are first and foremost considered for this purpose. For further information see chapter 3 under technical features of positive displacement pumps.

Figure 2.31 shows a comparison between damage to goods caused by two different types of non-clogging pumps. The figure demonstrates that in this case the channel impeller type pump is superior to the free-flow pump. On the other hand if bacteria cultures are pumped as active sludge in a purification plant other test results indicate that the free-flow pump is clearly superior to the channel impeller type. These examples illustrate the difficulties in a general assessment of the property gentle pump handling.

– – – free flow pump

___ channel impeller pump

Figure 2.31 Results of pumping test made with a free-fiow pump and a channel impeller pump. For black coal the damage criterion was when the size of grain was less than 12 mm