A progressive cavity pump or cavity pump transfers fluid by means of the progress of a sequence of small, fixed-shaped, discrete cavities through the pump as its rotor rotates. These pumps have applications in fluid metering and pumping of viscous or shear-sensitive materials. The cavities taper down toward their ends and overlap with their neighbors, so that, in general, no flow pulsing is caused by the arrival of cavities at the outlet, other than that caused by compression of the fluid or pump components.
Read also: Rotary Vane Pump, its parts, working, and Applications
Working
Progressive cavity pumps are fixed-flow rate pumps. This can lead to the mistaken assumption that all pumps can have their flow rates adjusted by using a valve attached to their outlet. However, with this type of pump, this assumption is a problem since such a valve will have practically no effect on the flow rate. Completely closing it will involve very high pressures being generated. To prevent this, pumps are often fitted with cut-off pressure switches, rupture discs (deliberately weak and easily replaced), or a bypass pipe that allows a variable amount of fluid to return to the inlet. With a bypass fitted, a fixed flow rate pump is effectively converted to a fixed pressure one.
At the points where the rotor touches the stator, the surfaces are generally traveling transversely, so small areas of sliding contact occur. These areas need to be lubricated by the fluid being pumped (hydrodynamic lubrication). This can mean that more torque is required for starting, and if allowed to operate without fluid, called ‘run dry’, rapid deterioration of the stator can result.
While progressive cavity pumps offer long life and reliable service transporting thick or lumpy fluids, abrasive fluids will significantly shorten the life of the stator. However, slurries (particulates in a medium) can be pumped reliably if the medium is viscous enough to maintain a lubrication layer around the particles and so protect the stator.
Application
- Food and drink pumping
- Oil pumping
- Coal slurry pumping
- Sewage and sludge pumping
- Viscous chemical pumping
- Storm flow screening
- Downhole mud motors in oilfield directional drilling (it reverses the process, turning hydraulic into mechanical power)
- Limited energy well water pumping
- Grout or cement pumping
- Lubrication oil pumping
- Marine diesel fuel pumping
- Mining slurry pumping
- Oilfield mud motors