ROTARY POSITIVE DISPLACEMENT PUMPS :

Positive displacement rotary screw or gear pumps rely on fine clearance between moving parts for their efficient operation. Excessive wear or erosion of parts, due to friction contact or the presence of abrasives, it is avoided by employing this type of pump for specialised rather than general duties. Contact between elements in some screw pumps is made unnecessary by gear drives. When used for lubricating oil and hydraulic systems, rotary displacement pumps benefit from the sealing effect and provision of lubrication between parts.

PUMPED VOLUME AND POWER :

For a rotary positive displacement pump operating at zero pressure the volumetric efficiency should be 100% but as the differential pressure increases, the amount of leakage (slip) through clearances will increase. This'slip will naturally be less the more viscous the pumped liquid is, for any given pressure. It should also be noted that, slip being a function of the clearance, viscosity and differential pressure, is constant irrespective of running speed. In pratice, changes in flow conditions affect this slightly. Slip and leakage can cause erosion and increase of clearance, particularly when the liquid being pumped contains abrasives.

Pressure is limited by the torque available from the drive motor, the strength of parts, the amount of slip or leakage are considerations of overall efficiency.

Power requirement of the pump may be split into two components, mainly hydraulic power and f rictional power. The hydraulic power is that required for the pumped medium.

ACCELARATION FORCES:

Accelaration forces are by far the greatest losses in a rotary pump and the liquid properties are dependent on the distance of movement of the dynamic unit and their speed (accelaration is dependent on both).

Depth and form of tooth influence this in a gear pump, the pitch of the screw is the major factor. Thus high helix angle screws can be used up to a relatively high speed on small pumps and lower helix angles on larger pumps, to keep the pitch within limits set by the field requirements for suction performance.

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