Pump

PUMP, a machine to raise or move fluids. The method to be adopted in raising the elevation or the pressure of a fluid de pends largely on the volume to be handled and the magnitude of the lift. For large volumes the centrifugal pump is almost uni versally used. In its modern forms this type is capable of pump ing against any pressure up to about 1,500 lb. per sq.in. Where small volumes are to be handled, under high heads, the recipro cating pump is more suitable, while where corrosive liquids are to be pumped, other types of air-lift pumps often offer advantages. (See VACUUM-PUMP.).

Centrifugal Pump.

In its simplest form the Centrifugal Pump consists of an impeller fitted with vanes and rotating in a closed casing. Water is supplied to the centre of the impeller. Due to its rotating in the impeller its pressure is increased by centrifugal force, and it is delivered at the periphery with an increased pressure and a high velocity. The kinetic energy in volved in this exit velocity would be wasted if no means were available for converting it into pressure energy before leaving the pump, and various means are adopted for affecting this con version. In the majority of pumps the impeller is surrounded by a spiral volute chamber (fig. I) whose area gradually increases is the direction of flow, so that the velocity is gradually reduced before the water leaves the pump. Such a device is capable of converting at most about 40% of the kinetic energy into pressure energy. Where higher efficiencies are required a ring of guide vanes is used, surrounding the impeller ( fig. 2). These vanes are so de signed as to receive the water without shock on leaving the wheel and to direct it through passages having gradually diverg ing walls into the collecting volute from which it is fed to the dis charge pipe. In these passages the pressure increases as the ve locity is gradually reduced, and under favourable circumstances such an arrangement is capable of converting some 75% of the kinetic energy into pressure energy.

The necessary peripheral speed of a centrifugal pump increases with the working head, and since fluid frictional losses increase rapidly with the speed, the efficiency begins to fall off appreciably if the working head per impeller is more than about i5o feet. Where very high heads have to be handled, multi-stage pumps are used in which the water passes through one impeller after another in series. Pumps of this type are extensively used for boiler feed work and for hydraulic power pumping stations.

The characteristics of a centrifugal pump depend essentially on the vane angles of its impeller, and under suitable conditions its overall efficiency may attain a value as high as 85%. At the inner periphery the vane angle (fig. 3) should be such that the water enters the impeller without shock. This condition is satisfied if tan (3=f2/u2 where is the radial component of the velocity of the water, and is the peripheral velocity of the impeller at this point. At the point of discharge the vanes may either be radial, or may be curved forwards, or backwards from the direction of rotation depending upon the duty for which the pump is designed. If 7 be the vane angle at this point, and if be the peripheral velocity and the radial component of velocity of the water, the gain of pressure head in the impeller is, neglect ing frictional losses, equal to while if K represents the proportion of the kinetic energy of discharge which is converted into pressure energy in the volute chamber or guide vanes, the total gain of pressure in the pump is given by In the majority of pumps the angle y is between 3o° and 75°, the value increasing with the height of lift.

Reciprocating Pumps.

These may be either of the single acting or double acting type. In the former, water is drawn into the pump through a suction valve on one stroke, and is forced out through the delivery valve on the return stroke. By fitting suction and delivery valves on both sides of the plunger, the pump becomes double acting.