3.4 Positive displacement pump

Positive displacement pump

Positive displacement pump is a category of pumps which all have in common that they are moving a fluid by repeatedly enclosing a fixed volume mechanically, sometimes with help of seals or valves. Positive displacement pumps, also known as PD pumps, are named after the enclosed volume, the displacement. There are two types of moving action, reciprocating or rotary, the fluid are driven with a cyclic movement through the pump and the system by a fixed displacement consisting of pistons, screws, gears, lobes, diaphragms or vanes depending on pump design (there are other machine elements used for other pump types).

All positive displacement pumps are more or less self priming i.e. they can begin pumping with a dry suction pipeline and pump casing. They can also pump a wide variety of fluids of varying characteristics, from very low viscosity fluids to very high viscosity pastes. The construction, application and pumping characteristics of different types of displacement pumps varies considerably however. It is therefore necessary to thoroughly understand the specific characteristics of each type of pump in order to be able to determine the suitability of a particular pump for a particular application. Gear pumps are not suitable for abrasive fluids for example, where an eccentric screw pump with its flexible stator is more suitable. If the back-pressure in the pipeline is relatively high a vane pump with flexible vanes is unsuitable, a gear pump or piston pump being a better choice.

The specified volume flow per working revolution is relatively constant and reduces insignificantly for moderate pressure increases. Control of the volume flow should be carried out by increasing or decreasing the pump speed. The flow can also be controlled by arranging a by-pass between the delivery and suction lines. Do not, however, attempt to throttle the delivery line in order to reduce the volume flow, as in the case of centrifugal pumps. This results in increased pressure, increased power consumption and increased loads on the working components, without significantly reducing the flow. Serious damage can occur if the delivery line should become completely blocked, by the closing of a valve for example. The pressure will increase rapidly and can become so great that it causes failure of seals, pipelines and other vital components. Where there is a risk of the pump operating against closed valves a pressure-relief valve should be fitted as a safety precaution. The opening pressure of which should be selected so as to provide a good margin of safety for the protection of the system and its components.

A certain amount of internal leakage (slip) is always present in displacement pumps, this is due to the clearance between the working elements. This clearance can be in the order of 0.01 to 0.9 mm and is necessary in order to counteract galling and to reduce friction within the pump. Leakage occurs from the delivery side back to the suction side, i.e. against the direction of flow. The amount of slip, which reduces volumetric efficiency, varies greatly according to the type of pump construction. An eccentric screw pump with its long rotor and effective sealing rubber stator has less slip than a conventional gear pump having only line contact between the gears. The pumped fluid, if it is sufficiently viscous, can help to seal the clearances and reduce internal leakage. The amount of internal leakage is not affected by speed variations, the amount of slip being the same whether the pump is running at 200 r/min or 400 r/min, providing that the back-pressure remains constant.

As previously mentioned all displacement pumps are more or less self-priming (can evacuate air from their own suction pipe and start pumping fluid). The evacuating capabilities being dependent upon the pump construction and the inner clearances necessary between the working components. Pumps which can operate with smaller clearances have better evacuating capabilities. This property is also improved by increasing speed. Displacement pumps should not be installed so as to utilize their maximum suction capacity. Pump wear is unavoidable and this means that in time, the maximum suction performance will be reduced. This means that the pump loses its priming ability with subsequent loss of its pumping function. Unnecessary service costs are thus induced in order to constantly maintain the pump in “as new” condition.

Classification of positive displacement pump by design

There are many. different types of displacement pump construction or design. Figure 3.4a comprises a summary of various types arranged basically in accordance with their technical type.

*Normally non-guided working element

** Always with synchronizing gear

*** Normally for hydraulic pumps only.

**** See also under section Metering pumps

This section does not deal with so-called hydraulic pumps. Hydraulic pumps are used mainly for transferring power in hydro-static transmissions. The working fluid is normally used as the power transmitting (usually oil) media. Hydraulic pumps operate at pressures of 10-70
MPa (100-700 bar) and are usually piston, gear or screw pumps.