Progressive cavity pump
231301
222157181
2008-06-27T20:53:58Z
69.178.130.114
/* Manufacturers */
A '''progressive cavity pump''' is also known as a '''progressing cavity pump''', '''eccentric screw pump''' or even just '''cavity pump''' and, as is common in engineering generally, these pumps can often be referred to by using a [[genericized trademark]]. Hence names can vary from industry to industry and even regionally; examples include: ''Mono'' pump, ''Moyno'' pump, ''Mohno'' pump and ''Nemo'' pump.
This type of [[pump]] transfers fluid by means of the progress, through the pump, of a sequence of small, fixed shape, discrete cavities, as its [[Rotor (turbine)|rotor]] is turned. This leads to the [[volumetric flow rate]] being proportional to the rotation rate ( bidirectionally ) and to low levels of [[shearing (physics)|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 principle of this pumping technique is frequently misunderstood, often it is believed to occur due to a dynamic effect caused by drag, or friction against the moving teeth of the screw rotor. However in reality it is due to sealed cavities, like a [[piston pump]], and so has similar operational characteristics, such as being able to pump at extremely low rates, even to high pressure, revealing the effect to be purely positive displacement (see [[pump]]).
== Theory ==
The mechanical layout that causes the cavities to, uniquely, be of fixed dimensions as they move through the pump, is hard to visualize (its essentially 3D nature renders diagrams quite ineffective for explanation), but it is accomplished by the preservation in shape of the gap formed between a helical shaft and a two start, twice the wavelength and double the diameter, helical hole, as the shaft is "rolled" around the inside surface of the hole. The motion of the rotor being the same as the smaller gears of a [[planetary gears]] system. This form of motion gives rise to the curves called [[Hypocycloid]]s.
In order to produce a seal between cavities, the rotor requires a circular cross-section, and the stator an oval one. The rotor so takes a form similar to a [[corkscrew]], and this, combined with the off-center rotary motion, leads to the name; eccentric screw pump.
Different rotor shapes and rotor/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 pumps solids handling ability.
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.
== Operation ==
In operation progressive cavity pumps are fundamentally fixed flow rate pumps, like [[piston pump]]s and [[peristaltic pump]]s, 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 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 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 travelling 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 as long as the medium is viscous enough to maintain a lubrication layer around the particles and so provide protection to the stator.
== Typical design ==
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 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 (which have to be within the fluid) allowing it to roll around the inner surface (un-driven). Elastomer is used for the stator to simplify the creation of the complex internal shape, created by means of [[casting]], and 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/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.
==Typical application areas==
*Food and drink pumping
*Viscous chemical pumping
*Oil pumping
*Sewage [[sludge]] pumping
*Slurry pumping
==Specific uses==
*Lubrication oil pump
*Mining slurry pump
*Oilfield [[Mud motor]]s
*Marine Diesel fuel pump
*Grout/Cement Pump
*Winery use
==External images==
*[http://www.lifetime-reliability.com/images/013_helical_rotor_pump.jpg abrasive material pump cutaway] ( two start rotor and so three start stator.)
*[http://www.pcmdelasco.com/img/compact_design.gif low pressure pump cutaway]
*[http://www.dfdistrib.com/gallery/images/11_jpg.jpg small pump with motor and gearbox]
*[http://www.orbitpumps.co.uk/images/axialmen.gif large pump]
*[http://www.rotomacpump.com/Rmain2.jpg rotor and stator 3D visualisation]
==External links==
detailed pump development description [http://www.moyno.com/website/ffs.php?subdir=catalogs%5C%5Csalesman%5C%5Cmisc%5C%5C&docfile=cempcp_article.pdf COMPENSATING ECCENTRIC MOTION IN PROGRESSING CAVITY PUMPS]
Alternative description [http://www.animatedsoftware.com/pumpglos/progrssv.htm glossary of pumps]
Alternative description [http://www.liquidcontrol.com/etoolbox/howitworks.html Graco Inc] (bottom of page)
Magazine entry/description for a Pump from Netzsch, [http://www.eurekamagazine.co.uk/article/6473/Design-progression-for-cavity-pumps.aspx Eureka magazine article]
==Manufacturers==
* [http://www.westcommpump.com Vigor] - Vigor Progressive Cavity Pumps
* [http://www.pcm-pump.com PCM pump manufacturer] (technology originator)
* [http://www.allweiler.de/10966/1000/Spezialnavigation/Home/awr_start.asp] - Allweiler Pumps
* [http://www.sydexpump.com] - Sydex Pumps
* [http://www.fluidresearch.com/pump_solutions.php Fluid Research Corp and ViscoTec] - Fluid Research Corporation
* [http://www.pc-pump.com PC-Pump] - PCP System Modelling Software
* [http://www.rmenergy.com/ R&M Energy Systems] - Moyno Pumps
* [http://www.mono-pumps.com/web/mono/monohome.nsf Mono]- Pumps
* [http://www.netzsch-pumps.com Netzsch] - Nemo Pumps
* [http://www.weatherford.com/ Weatherford Artificial Lift Systems]
[[Category:Pumps]]
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