Liquid pumps

A wide variety of liquids are now being used in chemical plants, and these usually have to be pumped through pipelines. In choosing the type of pump most suitable for any specific operation, a number of problems have to be taken into account. In the first place the quantity of liquid, and the pressure at which it is required to be pumped, must be considered. Neglecting all other considerations, the reciprocating pump is ideal for pumping small quantities of liquid at high pressure, the amount of fluid delivered depending on the volumetric displacement of the pistons. The delivery however is rather uneven a defect which can only be remedied by compounding a number of cylinders, thus making the machine rather large and expensive.

The viscosity of the liquid is another factor which must be taken into account, in that it largely determines the frictional losses which will occur. Rotary- type pumps are widely used in the handling of highly viscous liquids. They differ from reciprocating pumps in that they deliver an even flow of liquid, but they are unsuitable for pumping liquids of low viscosity, which tend to leak past the tips of the gear teeth. They are mainly used for the pumping of oils and similar liquids of high viscosity, which are less liable to leakage and which moreover provide the necessary lubrication for the moving parts of the pump, thus obviating the need for a separate lubricant. A further consideration involved in the choice of pump is- whether or not the liquid is corrosive or contains solid particles in suspension. In such cases, precautions have to be taken to avoid damage to the mechanism. With regard to suspensions, the clearances in the pump must be large enough to permit the particles to pass, and from this point of view, the rotary pump is not suitable, clearances necessarily being small to reduce leakage. The centrifugal type of pump is more commonly used when suspensions are present, since various types of impeller can be fitted, thus enabling the pump to handle a wide variety of liquids, including those with suspensions.