Understanding progressive cavity pumps

Key concepts Cavities in the pump gently move many types of fluids and solids without disturbance. Elastomeric stators provide good sealing and resist abrasion.


Key concepts&/HEADLINE>

Cavities in the pump gently move many types of fluids and solids without disturbance.

Elastomeric stators provide good sealing and resist abrasion.

Dry running is the most common cause of pump failure.

A progressive cavity pump moves most any fluid, even one that includes delicate or abrasive pieces more than 4 in. in diameter, without damage to the inclusions or the pump. How does it do that?

The progressive cavity pump was developed and patented by Dr. Rene Moineau in France more than 70-yr ago. The interaction of a single-threaded, helical rotor rolling eccentrically within a solid, helical stator, internally double-threaded with double the rotor pitch length, produces a series of cavities, 180-deg apart, progressing from inlet to outlet. The cavities follow each other and produce a pulsation-free, positive displacement flow without the need for valves (Fig. 1). This basic configuration is called a 1:2 profile.

Progressive cavity pumps with other profiles, from 2:3 up to 9:10 are possible. The 2:3 profile can either increase flow by 150% at the same speed or reduce speed and wear while still maintaining the original flow.

A second patent used an elastomeric stator to achieve a compression fit with the rotor, eliminating any clearance requirements. The effect was high volumetric efficiencies with fluids of high and low viscosities, as well as abrasion resistance superior to all other positive displacement pumps.

The displacement of a progressive cavity pump depends on three design features:

  • Rotor diameter


      • Rotor eccentricity


          • Stator pitch.

            • The pump's pressure rating depends on the number of stages. By repeating the identical, single-helical con-figuration, increasing the length of the stator and rotor increases the number of stages (Fig. 2).

              Flow through the elements is not far removed from the straightest distance between suction and discharge. The result is relatively low velocity and shear for a given displacement and excellent capabilities for handling highly viscous and sensitive slurries. A good example is the ability to pump oil and water mixtures without emulsification.

              Another feature that gives this pumping principle its advantage for slurry handling is the use of elastomers as the stator. Since the clearance between the elements is eliminated, the pump is capable of pumping low-viscosity and gaseous fluids as well as high-viscosity fluids.

              The elastomeric stator adds abrasion resistance beyond that of conventional rotary pumps. Particles tend to imbed rather than abrade. It also allows deformation to partially accommodate large solids, such as rocks, rags, and tramp metal.

              Almost every conceivable fluid-like material can be pumped, including sandy crude oil, coal slurries, waste sludge, polymers, grease, tallow, sealant, asphalt emulsions, paper stock, refractory mortar, gypsum roof deck, and caulking compound.

              Drive components

              In a progressive cavity pump, the rotor rolls in an eccentric path opposite the direction of rotation. Torque from the inline drive shaft must be transmitted through universal joints, flexible shafting, or flexible stator mounts. Improper drive designs result in low operating efficiencies, seal leakage, increased stator wear, and short pump operating life.

              Types of eccentric drives

              • Ball and pin-type universal joint is the earliest design and widely used today (Fig. 3). It is a low-friction device that runs without lubrication and is easy to maintain. Compared to other types, it is the least expensive and used for lighter duties. It can be sealed to prevent abrasive and corrosive wear.


                  • Geared universal joints are rugged and reliable (Fig. 4). The universal joint is almost totally encapsulated in a heavy metal shell that protects the elastomeric seal.

                    It has multiple gear tooth contact, as opposed to the single line contact of the ball and pin joint design. Lubrication and seal design are critical. Maintenance is difficult but infrequent.


                      • Flex shafts are a simple design. The shaft must be extremely uniform in diameter, smooth over its full length, and terminated in a radiused, rigid connection. The pump must be operated at or below design conditions, or the shaft ends up as a large corkscrew. One of the disadvantages of a flexible shaft is that it makes a long pump longer.


                          • Flexible shafting in the form of rubber covered wire cable and reinforced hose has been used for light-duty, unidirectional applications. The cables and hose are usually in tension and the lay of the cable must be tightened, not unwound, by the direction of rotation.

                            • Types of joints

                              • Cardan universal joints allow for great angularity, thus reducing the length of connecting rods and eliminating the need for hollow tubes (Fig. 5). The joint is simple, inexpensive, rugged, accessible, and easy to service. Lubrication and sealing are critical. This style joint can be used with any fluid.


                                  • Rubber universal joints are used where the pumped fluid removes lubrication and embrittles the hardened metal parts used in mechanical joints.


                                      • Oldham or sliding block couplings replace two universal joints. This action, while saving space, means the sealing device goes through extreme flexing, shortening its life. It is best to use this coupling with fluids that lubricate.

                                        • Maintenance

                                          Progressive cavity pumps are designed for a minimum of maintenance, which usually includes routine lubrication and packing adjustment. If mechanical seals are used, cooling/flushing water flow should be checked. The pump is one of the easiest to work on. The main elements are very accessible and require few tools to disassemble and reassemble on-site.

                                          During the last 25 yr, there has been a trend towards the use of smaller diameter rotors and longer sealing lines in an attempt to reduce wear and improve pump efficiency. Some manufacturers have changed to this geometry, producing rotor/stator designs that maintain pump capacities and pressure capabilities while reducing the rotor eccentricity sliding velocity and increasing the length of the sealing line, providing a more "straightline" flow through the pump.

                                          Thrust loads on the universal joints and bearings are reduced due to the smaller rotor cross-section and smaller eccentricity (Fig. 6). Flow rates are identical at the same speed, since the longer pitch compensates for the smaller rotor eccentricity and diameter.

                                          In comparison to large rotor/stator configurations, the smaller rotor diameter has a sliding velocity about 20% lower, which has an impact on the longevity of the rotor and stator, which are the main wearing items. The longer sealing line tends to improve volumetric efficiency, particularly in the lower speed and upper pressure ranges.

                                          Since thrust load is a direct function of system pressure acting against the rotor cross-section, the older design transmits about 50% more thrust load on universal joints and bearings.


                                          Smooth rotor surfaces increase life, reduce starting and running torque, improve performance, and provide smooth operation.

                                          For applications with abrasive fluids, chromium surface coatings can be applied. Modern methods include an electrolytic procedure that produces fissure-free, nonporous coatings that fuse deep into the base metal so it cannot lift or peel, a common fault with standard chrome plating.


                                          Progressive cavity pump stators are usually molded to size for a custom fit. Some stators are molded in long tubes and cut to length. Other forms of stators are a two-part design. The casing is generally made of steel and the elastomer, with seals at both ends, vulcanized to it. The inner shape of the elastomer comes from a metal core placed in the center of the casing before vulcanizing. Stators of this type usually have tighter dimensions at the ends.

                                          A wide variety of elastomers are necessary to meet application demands: natural rubber of various hardnesses and abrasion resistant qualities, synthetic rubbers of various compositions, plastics such as PTFE, filled PTFE, nylon, polypropylene, high molecular polyethylene polyacetal, and fabric-based laminates.


                                          Destruction of the pump stator by dry running occurs when loss of lubrication of the pumped fluid causes excess friction and temperature on the surface of the stator. It is the most common cause of pump failure.

                                          Common methods of preventing dry running conditions are presence/ absence sensors, stator temperature probes, and nonintrusive pressure sensors.

                                          Presence/absence sensors protect pumps by detecting the absence of liquid flowing into the suction port. If supply fluid is not detected for a preset period of time, the system deactivates the pump motor starter or other critical process equipment. This sensor is reliable protection against dry running because it can stop the pump before dry running occurs (Fig. 7).

                                          Stator temperature probes have a thermistor sensor and sleeve inserted into the stator to measure the temperature between the rotor and stator. An electronic temperature regulator deactivates the motor starter when a preset temperature is reached. On abrasive applications, a few minutes of running dry results in accelerated wear of the stator. Tests have shown that a rotor and stator can run dry for several minutes before an increase in heat is detected.

                                          Nonintrusive pressure sensors at the pump outlet are an effective way to prevent dry running. Pressure detection over the entire pipe circumference ensures that coatings, settled material, or bridging does not affect pressure readings. As an added feature, a liquid-filled gauge allows visual monitoring of pressure.

                                          Not only does a pressure sensor protect a pump from running dry, it also prevents failure caused by high pressure.

                                          Plant Engineering magazine extends its appreciation to Bornemann Pumps, Inc., and Moyno, Inc. for their assistance in the preparation of this article. The cover picture was taken with the cooperation of Shanley Pump & Equipment, Inc.

                                          &HEADLINE>More info&/HEADLINE>

                                          Joe Foszcz may be reached at 630-320-7135; jfoszcz@cahners.com.

                                          For additional material on related topics, see the “Fluid handling” channel on Plant Engineering Online: www.plantengineering.com.

                                          &HEADLINE>Examples of pumped product&/HEADLINE>

                                          Adhesive with titanium dioxide

                                          Animal feed

                                          Apricots, raw, sliced

                                          Baby food

                                          Bentonite mixture with water, sand,

                                          and coal dust

                                          Bleach, chromate based

                                          Brewery sludge

                                          Cast resin

                                          Ceramic waste slurry

                                          Cherry juice with whole cherries

                                          Chocolate masses

                                          Clay sludge

                                          Coconut masses


                                          Egg whites

                                          Filter cake from vacuum filters

                                          Fish, whole and pieces

                                          Fruit mashes, dejuiced/rape

                                          Fuel with coal dust

                                          Grain mashes

                                          Grease-water mixtures

                                          Gypsum slurry

                                          Hand cleanser

                                          Knifing filler

                                          Latex sludge

                                          Lime sulfate with gypsum

                                          Magnesium hydroxide paste

                                          Malt grains

                                          Meat/bone mixtures

                                          Methyl cellulose pulp

                                          Mine sludge, thick



                                          Mud sludge

                                          Nickel hydroxide sludge

                                          Noodle dough


                                          Paint paste

                                          Paper stock

                                          Peat slurry

                                          Petroleum residue slurry

                                          Phosphate slurry

                                          Plant extracts

                                          Plastisol pastes, up to

                                          1,000,000 cps

                                          Potato pulp

                                          Rape mashes with oil

                                          Sewage sludge, dewatered

                                          Sludge from dust scrubbers

                                          Sludge from sugar factories

                                          Slurry with butanol

                                          Soft soap

                                          Spinach puree


                                          Sugar beets

                                          Sulfur sludge

                                          Titanium dioxide

                                          Tooth paste


                                          Stator material properties

                                          MaterialGenerally resistant to:Generally attacked by:Max. allowable temperature, F

                                          Natural rubber

                                          Most moderate chemicals, wet or dry organic acids, alcohols, ketones, aldehydes

                                          Ozone, strong acids,fats, oils, greases, most hydrocarbons


                                          Nitrile synthetic rubber

                                          Many hydrocarbons ,fats, oils, greases

                                          Ozone, strong acids, ketones, esters, aldehydes, chlorinated and nitro hydrocarbons



                                          Animal and vegetable fats, oils, greases, ozone, strong and oxidizing chemicals

                                          Petroleum solvents, coal tar, solvents, aromatic hydrocarbons



                                          All aliphatic, aromatic, and halogenated hydrocarbons

                                          Ketones, low molecular weight esters and nitro containing compounds


                                          &HEADLINE>Pump selection steps&/HEADLINE>