3.64 Cavitation positive displacement pump
Cavitation positive displacement pump
The onset of cavitation in positive displacement pump causes a reduction in volume flow rate since part of the displacement becomes filled with vapour. The NPSH concept applies as for centrifugal pumps although there are no distinct criteria for the pump required NPSH.
For liquids which are completely de-aerated or de-gassed the required NPSH can be based upon a criterion amounting to a small percentage reduction of volume flow rate. Generally speaking however the concentrations of entrained gas are too high it is therefore common practice, when calculating the required NPSH to assume that cavitation or gas separation will not adversely affect the performance, noise-level or life of the displacement pump. NPSH criteria, for displacement pumps are therefore very much a question of interpretation, which is well worth remembering when specifying.
For displacement pumps with varying (pulsating) volume flow, an additional pressure head is required in order to accelerate the liquid column in the suction line. This pressure head requirement is calculated in association with the system and not the pump. The obvious reason for this being that the pump manufacturer does not supply the suction line.
This pressure head, required for the acceleration of the liquid column, hacc appears as a negative term in the equations 3.61a>>> and 3.62a>>>. The term hacc depends upon the displacement pump’s volume flow variations which could, to advantage, be quoted by the pump manufacturers. Pressure head required for acceleration of liquid column is calculated from the product of liquid mass and acceleration, where the acceleration is determined from the first derivative of the volume flow.
For piston pumps an assessment can be made by applying the equation 3.64a which, to a certain extent, can also be used for other types of displacement pump.
where
hacc = pressure head, required for the acceleration of the liquid column (m)
g = acceleration of gravity 9,806 (m/s²)
Ls= Length of suction line (m)
n = number of strokes or speed of displacement pump (r/min)
kacc = dimensionless constant dependent upon the design and construction of the pump
The following guide applies for kacc
0.4 for single acting single cylinder piston pump
0.2 for double acting single cylinder piston pump
0.12 for double acting twin cylinder piston pump
0.07 for a three cylinder piston pump
0.03 for a seven cylinder piston pump