Difference between revisions of "Progressive Cavity Pumps"

[[Category:Pumps]]{{Knoppen}}

[[File:Progressive cavity pump.jpg|thumb|right|Progressive Cavity Pumps]]

'''Progressive Cavity Pumps''' are of [[Positive Displacement Pumps]] and are also known as [[Progressive Cavity Pumps]], eccentric screw pump or cavity pump. It transfers fluid by means of the progress, through the pump, of a sequence of small, fixed shape, discrete cavities, as its rotor is turned. This leads to thevolumetric flow rate being proportional to the rotation rate 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. 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 rotor takes a form similar to a corkscrew, and this, combined with the off-center rotary motion, leads to the alternative name: eccentric screw pump.Different rotor shapes and rotor or stator pitch ratios exist, but are specialized in that they don't generally allow complete sealing, so reducing low speed pressure and flow rate linearity, but improving actual flow rates, for a given pump size, and/or the pump's solids handling ability.

==Operation of Progressive Cavity Pumps==

In operation progressive cavity pumps are fundamentally fixed flow rate pumps, like [[Piston Pumps]] and [[Peristaltic Pumps]], and this type of pump needs a fundamentally different understanding to the types of [[Pumps]] to which people are more commonly first introduced, namely ones that can be thought of as generating 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 pressures being generated. To prevent this, pumps are often fitted with cut-off pressure switches, burst disks , or a 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.

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 . 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 can be pumped reliably if the medium is viscous enough to maintain a lubrication layer around the particles and so protect the stator.

==Typical design of Progressive Cavity Pumps==

Specific designs involve the rotor of the pump being made of a steel, coated with a smooth hard surface, normally chromium, with the body made of a molded elastomer inside a metal tube body. The elastomer core of the stator forms the required complex cavities. The rotor is held against the inside surface of the stator by angled link arms, bearings allowing it to roll around the inner surface . Elastomer is used for the stator to simplify the creation of the complex internal shape, created by means of casting, which also improves the quality and longevity of the seals by progressively swelling due to absorption of water and/or other common constituents of pumped fluids. Elastomer or pumped fluid compatibility will thus need to be taken into account.

Two common designs of stator are the equal-walled and the unequal-walled. The latter, having greater elastomer wall thickness at the peaks allows larger-sized solids to pass through because of its increased ability to distort under pressure. The former have a constant elastomer wall thickness and therefore exceed in most other aspects such as pressure per stage, precision, heat transfer, wear and weight. They are more expensive due to the complex shape of the outer tube.

==Sources==

[http://en.wikipedia.org/wiki/Progressive_cavity_pump Wikipedia Progressive Cavity Pump]