Piston pumps volumetric efficiency

The slip of a pump is fraction or percent loss of capacity relative to theoretical. Slip is (1 — Cvoi), w here e oi is the volumetric efficiency. Volumetric efficiency is the actual liquid pumped (usually considered water) relative to that which should theoretically be pumped based on piston displacement. 

The theoretical delivery of a piston pump is equal to the total swept volume of the cylinders. The actual delivery may be less than the theoretical value because of leakage past the piston and the valves or because of inertia of the valves. In some cases, however, the actual discharge is greater than theoretical value because the momentum of the liquid in the delivery line and sluggishness in the operation of the delivery valve may result in continued delivery during a portion of the suction stroke. The volumetric efficiency, which is defined as the ratio of the actual discharge to the swept volume, is normally greater than 90 per cent. 

The theoretical fluid displacement of a piston pump equals the total volume swept by the piston on each delivery stroke. Because of leakage past the piston and the valves and failure of the valves to close instantly, this theoretical displacement is not attained in actual practice. The volumetric efficiency, defined as the ratio of the actual displacement to the theoretical displacement, is usually in the range of 70 to 95 percent. 

For well designed pumps X is normally close to zero, i.e. the pump is of rigid construction so tjE can be calculated directly from pump and fluid data. This calculation shows that the relative dead spaces df and 8dh should be minimised to achieve good volumetric efficiency. Furthermore as the slip factor should approach tjs 1 (valves should be tight and operate with no slip [15]) the volumetric efficiency can be obtained directly from the indicator diagram (Figure 9.12) and the table beneath it. The diagram shows the relation between the decompression stroke h (from 3 to 4) and the compression stroke /i2 (from 1 to 2) and the table enables Joukowsky-shock, volumetric efficiency, shock factor and piston velocity to be calculated. 

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