Progressive cavity pump

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A progressive cavity pump, also known as a progressing cavity pump or eccentric screw pump, is a kind of pump which transfers fluid by means of a sequence of small, fixed shape, discrete cavities, that move through the pump as its rotor turns. This leads to the volumetric flow rate being proportional to the rotation rate ( by-directionally ) and to low levels of shearing being applied to the pumped fluid. Hence these pumps have application in fluid metering and pumping of viscous or shear sensitive materials. It should be noted that the cavities taper down toward their ends and overlap with their neighbours, 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.

The mechanical layout that causes the cavities to move through the pump, without changing shape at all, is hard to visualise, but manages to accomplish this feat due to the special nature of the gap formed between a helical shaft and a two start, twice the wavelength and double the diameter, helical hole, and how that gap changes as the shaft is "rolled" around the inside surface of the hole, with a motion the same as planetary gears.

The rotor also has to have a circular cross-section, and the hole an oval one, in-order to form a seal between cavities. The rotor so takes a form similar to a corkscrew.

Different rotor shapes and rotor/stator pitch ratios exist.

At a high enough pressure the sliding seals between cavities will leak some fluid rather than pumping it, so when pumping against high pressures a longer pump with more cavities is more effective, since each seal has only to deal with the pressure difference between adjacent cavities. Pumps with between two and a dozen or so cavities exist.

In operation progressive cavity pumps are fundamentally fixed flow rate pumps, like gear pumps and peristaltic pumps, and this type of pump needs a fundamentally different understanding to the types of pumps people are more commonly first introduced too, namely ones that can be thought of as generating a pressure. This can lead to the mistaken assumption that all pumps can have their flow rates adjusted by using a valve attached to their outlet, but with this type of pump this assumption is a problem, since such a valve will have practically no effect on the flow rate and completely closing it will involve very high, probably damaging, pressures being generated. In order to prevent this pumps are often fitted with cut-off pressure switches, burst disks (deliberately weak and easily replaced points), or a throttleable bypass pipe that allows a variable amount a fluid to return to the inlet. With a bypass fitted, a fixed flow rate pump is effectively converted to a fixed pressure one.

Specific designs involve the rotor of the pump being made of a steel, coated in a smooth hard surface, normally chromium, with the body ( the stator) made of a rubber lined steel tube. The rubber core of the stator forms the required complex cavity. The rotor is held against the inside surface of the stator by angled link arms, bearings (which have to be within the fluid) allowing it to roll around the inner surface (un-driven). Rubber is used for the stator to simplify the creation of the complex internal shape, created by means of casting, and also to allow good sealing, but the interface between rotor and stator has to be lubricated by the fluid being pumped (Hydrodynamic lubrication) so can require extra torque to start, and if allowed to '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 successfully pumped as long as the medium they are in is viscous enough to provide protection to the stator.

Two common designs of stator are the "Equal-walled" and the "Unequal walled". The latter, having greater rubber wall thickness at the peaks allows larger-sized solids to pass through because of its increased ability to compress under pressure.