Progressive Cavity Pumps: Working Principle, Stator Selection and Sizing

Quick Answer

A progressive cavity pump (also called a progressing cavity, eccentric screw or single-screw pump) is a rotary positive displacement pump in which a single-helix metal rotor turns eccentrically inside a double-helix elastomer stator with twice the rotor's pitch. The interference fit seals a chain of cavities that travel from suction to discharge without changing shape, so the pump gives smooth, near pulsation-free flow proportional to speed and handles viscous, shear-sensitive and solids-laden liquids such as sludge, slurries, food pastes and crude oil. Displacement per revolution is 4 × eccentricity × rotor diameter × stator pitch. Each stage (one stator pitch) holds a limited pressure: practitioners quote about 6 bar (85–90 psi) per stage for clean liquid with an elastomer stator, falling to as little as 1.4 bar (20 psi) for abrasive slurry, so higher pressures are reached by adding stages. Choose the stator elastomer for chemistry, temperature and swell (nitrile typically to about 100 °C, fluoroelastomer to about 160–200 °C), pump abrasive slurries with a larger pump at lower speed, and never let it run dry or against a closed valve: fit dry-run and overpressure protection.

A typical enquiry reads: "Sludge pump, 10 m³/h, 6 bar, same as the old one." The old one was a small progressive cavity (PC) pump running at around 600 rpm, and the plant has been changing its stator every few months because the thickened sludge carries grit from the inlet works.

Most short stator lives trace back to selection, not to the pump: speed, pressure per stage, elastomer and protection. This guide explains how the pump works and how to choose those four things with numbers.

How a Progressive Cavity Pump Works

The principle was described by René Moineau in 1930. The pump has two pumping parts:

• Rotor: a single-start metal helix, often chromium-plated or stainless steel. Every cross-section is a circle of diameter d whose centre sits a distance e, the eccentricity, from the rotor axis
• Stator: an elastomer lining moulded and bonded inside a steel tube, with a two-start helical bore whose pitch is twice the rotor's. Every cross-section of the bore is a slot of width d and length d + 4e

The rotor is a slight interference fit in the stator. As it turns, its circular section slides back and forth along the slot while the rotor axis orbits the stator axis in the opposite direction at radius e. The contact lines form a chain of sealed cavities that travel from suction to discharge without changing size or shape: no valves, very low pulsation and gentle shear, so the pump moves sludge, pastes, fruit pieces and crude oil that would be damaged by, or would damage, other designs.

Because the rotor moves eccentrically, a coupling rod with two universal joints (pin, geared or cardan types) or a flexible shaft carries torque from the concentric drive shaft. The joints are wear parts, sealed against the product.

Most transfer pumps use this 1:2 geometry; higher-lobe geometries such as 2:3 give more flow at the same speed. Even-wall stators, with a uniform elastomer layer, hold more pressure per stage and shed heat better but cost more; conventional thick-wall stators pass larger solids.

Displacement and flow

The free area between rotor and stator is the slot area minus the rotor circle: (πd²/4 + 4ed) − πd²/4 = 4ed, the same at every rotor position. Multiply by the distance a cavity travels per revolution, one stator pitch Ps, to get displacement per revolution:

V = 4 × e × d × Ps

Delivered flow is Q = V × n × ηv, where n is speed and ηv is volumetric efficiency. The loss is slip: liquid leaking back across the seal lines. Slip rises with pressure per stage, falls with viscosity and interference, and is roughly a fixed volume per minute at a given pressure, so it takes a bigger share of the flow at low speed. Otherwise flow follows speed and barely changes with discharge pressure, which is why PC pumps meter well and are normally controlled by a variable-frequency drive.

Stages and pressure per stage

One stage is one stator pitch. Pressure capability adds with stages: with more seal lines in series, each carries a smaller pressure difference, so slip, elastomer flexing and heat all fall.

The familiar "6 bar per stage" is a ceiling for clean liquid, not a design figure for slurry.

Choosing the Stator Elastomer

Stators usually wear out before anything else, so the compound is the most important line on the data sheet. Three things decide it: chemical resistance, temperature and swell.

The temperatures are typical figures from trade and oilfield sources. Compounds differ, and the pump maker's rating for its own compound governs.

Temperature and swell. An elastomer expands roughly ten times as much as steel for the same temperature rise, and many liquids swell it further. Either way the bore closes on the rotor: interference, torque and heat rise, and the stator wears or tears. For hot duty, makers fit a rotor and stator with less interference, sized for the operating temperature; in oil wells, rotors of different sizes are swapped as temperatures change. Give the maker the full temperature range and ask for the compound's swell data in your liquid.

Three field rules: fit the rotor with a non-petroleum lubricant so the elastomer does not swell before start-up; buy rotor and stator from the same maker; and do not stockpile stators, because elastomer ages and spares over three to five years old can be expected to give shorter lives.

Speed, Abrasion and Wear

Wear in a PC pump rises with speed because every revolution drags the rotor along the full seal line under load; one practitioner's guide says speed matters even more for wear in a PC pump than in a centrifugal pump. Hard particles tend to press into the elastomer rather than cut it, which is why PC pumps handle abrasive slurries better than other rotary positive displacement pumps, but each revolution loads the seal line against those particles again, so speed still sets stator life.

• Abrasive slurry: a larger pump at lower speed. The extra capital buys stator life (see the worked example)
• Viscous liquid: lower speed so the cavities fill. One published guideline suggests about 150 rpm at 1,000 mPa·s (cP) and up to about 700 rpm at 100 mPa·s
• Expect the stator to wear first. A common rule is one rotor for every two stators; replace stators that are pitted, gouged or worn

Dry Running and Overpressure

Two events destroy PC pumps faster than any slurry.

Dry running. The pumped liquid lubricates the rotor–stator contact. Without it, friction overheats the elastomer and destroys the stator; practitioners name dry running as the most common cause of failure. Protection options:

• Product-presence or flow sensors at the pump suction
• Low-level interlocks on the feed tank or hopper
• Suction pressure monitoring
• A temperature probe in the stator, but not on its own: tests have shown a rotor and stator can run dry for several minutes before the temperature rise is detected

Overpressure. A closed discharge valve or a line blocked by settled sludge barely changes the flow of a positive displacement pump; pressure simply rises until the stator, a joint, the pipe or the motor gives way. Fit a pressure switch or relief device on the discharge, upstream of the first valve. Our positive displacement pumps guide covers relief arrangements.

Configurations and Applications

Common configurations

• Horizontal transfer pumps on a baseplate or close-coupled to a gear motor, with packing or a mechanical seal
• Open-hopper pumps for semi-solids that will not flow on their own, such as dewatered sludge cake: an auger feed screw pushes product from a wide hopper into the first stage, and some designs add a separately driven bridge breaker to stop the product arching over the screw
• Metering pumps: small PC pumps dose polymer solutions, slurries and viscous additives, because flow follows speed and shear is low; see our dosing pumps guide
• Hygienic pumps for food, with food-grade elastomers
• Downhole PCP systems for oil wells: the stator runs on the bottom of the production tubing and the rotor on the sucker-rod string, turned by a surface drive; ISO 15136-1 covers the pumps. SPE's PetroWiki puts large pumps at up to about 800 m³/d (roughly 5,000 bbl/d) and lift at up to about 3,000 m (9,840 ft), with special elastomers to about 180 °C. Gas lowers volumetric efficiency, and changing the pump usually means pulling the tubing. Drilling mud motors use the same geometry in reverse

For refinery and petrochemical process duties, buyers often specify API 676, which covers rotary positive displacement process pumps for petroleum, petrochemical and gas industry services; pump makers offer PC designs to it. Check the current edition and the maker's exceptions.

Typical applications

Wastewater sludge (primary, thickened, digested and dewatered cake) and polymer dosing, covered in our wastewater treatment equipment guide; mining tailings and mineral slurries; foods such as fruit with whole pieces, fish and chocolate masses; resins, pastes and chemical slurries; crude oil and produced fluids; and pulp and paper process streams.

Progressive Cavity vs Twin-Screw vs Lobe Pumps

Screw pumps suit large flows of clean or lubricating liquids at temperatures an elastomer cannot take; three-screw designs, whose idler screws float in the pumped liquid, need a lubricating one such as fuel or lube oil. A lobe pump suits compact, frequently cleaned installations; our gear pumps guide compares it with gear and vane pumps. The PC pump wins where the liquid carries abrasive or fragile solids, must be metered or must not be sheared.

Worked Example: Sizing for an Abrasive Sludge

Duty: 10 m³/h (44 US gpm) of thickened sewage sludge carrying fine grit, at 6 bar (87 psi) discharge pressure.

Step 1: the small, fast option. Pump A has d = 50 mm, e = 10 mm and Ps = 150 mm.

• V = 4 × 10 × 50 × 150 = 300,000 mm³ = 0.30 l per revolution
• At 600 rpm: 0.30 × 600 = 180 l/min = 10.8 m³/h theoretical, or 9.7 m³/h at 90% volumetric efficiency
• To deliver 10 m³/h it must run at 10,000 l/h ÷ (0.30 l × 60 × 0.9) ≈ 617 rpm

Step 2: apply an abrasive-duty speed limit. Suppose the pump maker's guidance for this grit content limits speed to 200 rpm. The displacement needed is 10,000 l/h ÷ (200 rpm × 60 × 0.9) = 0.93 l per revolution. Pump B is geometrically similar to A but 1.5 times larger (d = 75 mm, e = 15 mm, Ps = 225 mm):

• V = 4 × 15 × 75 × 225 = 1,012,500 mm³ = 1.01 l per revolution (1.5³ × 0.30)
• Speed for 10 m³/h at 90% volumetric efficiency: 10,000 l/h ÷ (1.0125 l × 60 × 0.9) ≈ 183 rpm

Pump B turns at 30% of pump A's speed. In geometrically similar pumps, rubbing speed at the seal line scales with rotational speed times size, so pump B's is roughly 0.30 × 1.5 ≈ 0.44 of pump A's, less than half.

Step 3: stages. If the maker rates the stator at 3 bar (44 psi) per stage for this abrasive duty, 6 bar needs 6 ÷ 3 = 2 stages.

Step 4: power and torque.

• Hydraulic power: Q × Δp = (10 ÷ 3,600) m³/s × 600,000 Pa = 1.67 kW
• Torque to displace against pressure: V × Δp ÷ 2π = 0.0010125 m³ × 600,000 Pa ÷ 2π ≈ 97 N·m, before the friction torque of the interference fit
• If the maker's curve shows 55% overall efficiency at this duty, shaft power is 1.67 ÷ 0.55 ≈ 3.0 kW; a 4 kW motor covers the higher break-away torque at start-up

Confirm the result against the maker's performance curve, which gives the real slip and friction torque for the chosen stator fit and liquid.

Ordering Checklist

• Liquid: name, viscosity at pumping temperature, density, pH and chemical composition
• Solids: content, maximum particle size, abrasiveness (grit, sand) and fibres
• Duty: minimum, normal and maximum flow; discharge pressure including peaks; suction conditions and NPSH available; continuous or intermittent running
• Temperature: normal, maximum and cleaning (CIP or steam-out) temperatures
• Materials: stator elastomer (or ask for a proposal), rotor material and coating, casing material
• Shaft seal: packing, single or double mechanical seal, flush or quench
• Drive: variable-frequency drive or fixed speed, gear motor, supply voltage and frequency, hazardous-area classification
• Protection: dry-run sensing, discharge pressure switch or relief device
• Configuration: horizontal or vertical, open hopper with feed screw, baseplate or close-coupled
• Standards and documents: API 676 where specified, performance test, material certificates
• Spares: stators, rotors, joint kits and seals

Common Mistakes

1. Buying the smallest pump at maximum speed. It costs less, but on abrasive slurry it wears out stators far sooner.

   Prevention: State the solids content and abrasiveness, and accept a larger pump running slower.

2. Loading each stage as if the liquid were clean. Abrasive slurry at full pressure per stage wears the stator quickly.

   Prevention: Give the discharge pressure including peaks, and let the maker add stages for abrasive duty.

3. No dry-run protection. One empty tank or blocked suction line can wreck a stator.

   Prevention: Fit a product-presence, flow or level interlock; do not rely on a stator temperature probe alone.

4. No overpressure protection. A closed valve or a plug of settled sludge builds pressure until something breaks.

   Prevention: Fit a pressure switch or relief device on the discharge, upstream of any valve.

5. Choosing the elastomer on chemistry alone. Heat and swell close the stator on the rotor, raise torque and trip the motor.

   Prevention: Give the full temperature range and the liquid's composition, and ask for a stator fit sized for the operating temperature.

6. Mixing rotors and stators from different makers. Geometry and fit differ, so the pair may slip or bind.

   Prevention: Buy matched rotor and stator spares from the original pump maker, and fit them with the lubricant it specifies.

7. Stockpiling spare stators. Elastomer ages on the shelf, and old stators give short lives.

   Prevention: Hold spares for one or two changes and rotate the stock; treat stators more than three to five years old as suspect.

Supply from Kasko Makine

Kasko Makine supplies positive displacement pumps for viscous and difficult liquids:

• Progressive cavity pumps — also called eccentric screw pumps: the rotor and stator form fixed-shape cavities that carry viscous, shear-sensitive, abrasive or solids-containing fluids with smooth, low-pulsation flow, for wastewater and sludge handling, chemical and petrochemical plants, food and beverage processing, oil and gas, and pulp and paper
• Screw pumps — one or more rotating screws giving steady, low-pulsation, low-shear flow of viscous and lubricating fluids, for oil and fuel transfer, chemical and petrochemical processes, hydraulic systems, and marine and industrial pumping

Engineering support: Send the liquid, solids content, viscosity, temperature range, flow and pressure. We will review pump type, size, speed, number of stages and stator elastomer against your data, and quote the pump with its drive and protection.

Send your fluid data, flow and pressure for a progressive cavity pump selection and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Positive Displacement Pumps: Types, vs Centrifugal & Selection Guide
• Sewage & Wastewater Pumps: Types, Impellers & Selection Guide
• Wastewater Treatment Equipment: Screens, Clarifiers, Aeration and Dewatering
• Diaphragm Pumps (AODD): How They Work, Types & Selection Guide
• Mechanical Seals Explained: API 682 Types, Arrangements and Piping Plans
• Industrial Mixers and Agitators: Impeller Selection, Baffles and Scale-Up

Frequently Asked Questions

Q: What is a progressive cavity pump?
A: A progressive cavity pump is a rotary positive displacement pump in which a single-helix metal rotor turns eccentrically inside a double-helix elastomer stator with twice its pitch. Sealed cavities travel from suction to discharge without changing shape, giving smooth, low-pulsation flow proportional to speed. Also called an eccentric screw or single-screw pump, it handles viscous, abrasive, shear-sensitive and solids-laden liquids.

Q: How do you calculate the flow of a progressive cavity pump?
A: Displacement per revolution equals four times the eccentricity times the rotor diameter times the stator pitch. Multiply by speed and volumetric efficiency to get delivered flow. A 50 mm rotor with 10 mm eccentricity in a 150 mm pitch stator displaces 0.30 litres per revolution, or 10.8 cubic metres per hour at 600 rpm before slip. Slip rises with pressure and falls with viscosity.

Q: How much pressure can one stage of a progressive cavity pump develop?
A: A stage is one stator pitch. With an elastomer stator on clean, non-abrasive liquid, practitioners quote about 6 bar, or 85 to 90 psi, per stage. For abrasive slurry the figure can fall to about 1.4 bar, or 20 psi. Higher pressures are reached by adding stages, which lowers the pressure across each seal line and reduces slip and wear.

Q: Which stator elastomer should I choose for a progressive cavity pump?
A: Choose for chemical resistance, temperature and swell. Natural rubber suits oil-free, water-based slurries to about 85 °C. Nitrile, the general choice for sludge, mineral oils and fuels, is typically limited to about 100 °C. EPDM suits hot water and water-based chemicals to about 127 °C, but not mineral oils or fuels. Fluoroelastomer handles hot oils and aggressive chemicals to about 160 to 200 °C, depending on the maker's compound.

Q: Why should abrasive slurries be pumped at low speed?
A: Wear rises with speed, because the rotor rubs along the whole seal line on every revolution. For abrasive slurry, choose a larger pump running slower and add stages to lower the pressure per stage. In a typical example, a pump 1.5 times larger delivers the same flow at 183 rpm instead of 617 rpm, and the rubbing speed falls by more than half.

Q: Can a progressive cavity pump run dry?
A: No. The pumped liquid lubricates the rotor and stator; without it, friction overheats the elastomer and destroys the stator, and dry running is the most common cause of failure. Fit a product-presence, flow or level interlock, because a stator temperature probe alone may react only after several minutes of dry running. Discharge overpressure protection is also needed.

Q: What is the difference between a progressive cavity pump and a twin-screw pump?
A: A progressive cavity pump uses one metal rotor in an elastomer stator with an interference fit, so it handles abrasive and fragile solids well, but the elastomer limits its temperature and pressure is added with stages. A twin-screw pump uses two metal screws driven by timing gears without contact, suiting large flows of clean or lubricating liquids at higher temperatures, but it tolerates abrasives poorly.