3.3.2 Wear in displacement pumps

Wear in displacement pumps

Wear in displacement pumps is first detected through internal leakage, which varies as the cube (third power) of the positive clearance between the sealed components. Displacement pumps are thus dependent upon increased clearances due to wear. Displacement pumps are traditionally limited to use with non abrasive fluids and to viscous fluids which preferably have some lubricating value. Exceptions to this can be made by utilizing principles of  construction which eliminate leakage by the use of an elastic element diaphragm pumps and peristaltic pumps, for example, by guiding the pump working elements and by operating at extremely low speeds.

Considering elementary principles, wear is a function of:

  • Surface pressure between components = pg
  • Relative sliding speeds between components = vg
  • A dependence upon the frictional co-efficient = μ

or with more usual wear theory association, with the pg · vg value:

  • wear ~ pg · vg
  • the heat produced per unit of surface = μ · pg · vg

The technique of creating wear-resistant displacement pumps is concerned with reducing the product of pg · vg. As previously indicated, low values of  vg results in unwieldy constructions, the only solution then is to reduce pg.

The driving force for displacement pumps can be transmitted to the various components in different ways. In the case of gear pumps the external drive is normally transmitted to only one of the gears, while the other is driven by  engagement with the gear teeth of the first. However, if both the gears are driven externally, the relationship is completely different and it is necessary to synchronize the shafts by means of a separate synchronizing gear in order that the teeth shall pass within each other when rotating. The surface pressure between the interactive gear components is thus reduced to nil. The shape of the interactive gear surfaces can now be chosen with regard to the pumped fluid. Instead of gear teeth, the pump elements can now be given smoother shapes, as in the case of lobe rotor pumps.

Among displacement pumps there is a special family of pumps whose interactive components are guided by external devices as in the case of the lobe rotor pump mentioned above. Even pumps where the sliding pressure is eliminated in other ways also belong to this group. Examples of such displacement pumps are:

  • Rotating piston pumps, also called lobe rotor pumps, where the drive is by means of an external synchronizing gear.
  • Piston pumps where the piston rod is guided in external bearings in contrast to the case where the piston itself absorbs the side forces.
  • Screw pumps where a central externally driven screw inter-meshes with two sealing idler screws (sometimes known as Imo pumps, name from the original manufacturer). By means of sophisticated screw profiles, the idler screws are driven basically by fluid pressure, which results in very low surface contact pressure between the screws.

With the sliding pressure eliminated or greatly reduced the duty range is increased. Depending upon other design features, this range of application can be extended to contaminated fluids, low viscosity, dry-running capability or
high speed.

Important design factors to consider to minimize wear, leakage problems and friction losses in displacement pumps:

  • Clearance between mating surfaces
  • Mating surface seal length
  • Fluid viscosity
  • Fluid purity and lubrication value
  • Pressure (fluid)
  • Pressure and sliding pressure between mating surfaces
  • Sliding speed between machine elements
  • Valve or port size

Displacement pumps suitable for abrasive fluids should be constructed according to the following:

  • The interactive components should be positively guided by devices located outside the pumped fluid or have greatly reduced sliding pressure.
  • The forces induced on the pump element by the fluid pressure should be absorbed by bearings which are not in contact with the fluid.
  • Large displacement volumes should be sought after when handling contaminated fluids in order to reduce the fluid’s area of surface contact with the pump material
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