Pump Types Explained: Centrifugal vs Positive Displacement, Submersible and Progressive Cavity Pumps

Every pump belongs to one of two families. Centrifugal (rotodynamic) pumps spin an impeller to speed the liquid up and convert that speed into pressure, so the flow they deliver falls as the head they work against rises. Positive displacement pumps — progressive cavity, gear, lobe, vane, piston, plunger, diaphragm and peristaltic — trap a fixed volume and push it out with every turn or stroke, so their flow is set by their speed and they will build whatever pressure the pipe demands. A submersible pump is not a third family: it is (usually) a centrifugal pump with a sealed motor built to run under the liquid, which also cools it — a 4-inch well motor of 3–10 hp needs at least 0.25 ft/s (7.62 cm/s) of water flowing past it.

This guide covers how the two families differ, the submersible types (well, borehole, sump, effluent, sewage and grinder), how a progressive cavity pump works and why it must never run dry, the other common types, and how to read a pump curve. The figures come from the U.S. Department of Energy and Hydraulic Institute sourcebook, manufacturer manuals and the original patent, listed under Sources.

Centrifugal vs positive displacement

The Hydraulic Institute (HI), which writes the U.S. pump standards, splits pumps into two classes. Rotodynamic pumps “impart energy to the pumped fluid by means of a rotating impeller, propeller or rotor” and include centrifugal, mixed-flow and axial-flow pumps. Positive displacement pumps “trap and move a volume of fluid with each shaft rotation”, and come in reciprocating types (pistons, plungers, diaphragms) and rotary types (screws, gears, lobes, vanes, flexible tubes).

• Centrifugal pumps are the most common because they are simple, cheap, need little maintenance and last a long time. Their flow depends on the back-pressure: on the flat curve in the DOE/HI sourcebook’s example, a pump running at 160 ft of head and 250 gpm loses 100 gpm of flow when the system back-pressure rises by just 10 ft. They suit high flow at moderate head with thin liquids.
• Positive displacement pumps have a fixed displacement, so flow is proportional to speed and the pressure is set by the system’s resistance. The sourcebook lists where they win: highly viscous liquids, high pressure at low flow, self-priming, liquids that must not be sheared, and metering. The catch is that they keep pushing against a closed valve (“deadheading”) until a relief valve lifts, the motor stalls or the pipe bursts, so they are installed with relief valves — often built into the pump.

Submersible pumps: well, borehole, sump, effluent, sewage

A submersible pump has its motor sealed into the same unit as the pump end and sits below the liquid it pumps. Being submerged means it never has to lift liquid by suction and needs no priming, and the surrounding liquid carries the motor’s heat away. Sewage and sump models from Zoeller, for example, use a hermetically sealed, oil-filled motor in a cast-iron housing to pass that heat to the liquid. The types differ mainly in what they are allowed to swallow.

Well and borehole pumps

A well pump (called a borehole pump outside North America) is a long, narrow multistage centrifugal pump on top of a motor. University of Nebraska Extension describes the domestic unit as about 3½ in in diameter and 2–3 ft long, needing a casing of at least 4 in (some fit 3-in wells); it needs no frost protection because it sits below the water, but it has to be pulled out of the well for repairs. The motor sits below the pump intake so that the water being drawn in flows up past it and cools it. Franklin Electric’s 2026 AIM manual sets the minimum speed of that flow:

• 0.25 ft/s (7.62 cm/s) past 4-inch motors of 3–10 hp, and 0.50 ft/s (15.24 cm/s) past 6-inch (5–60 hp) and 8-inch encapsulated motors, in water up to 86 °F (30 °C). 4-inch motors of 2 hp and smaller only need a flow greater than zero.
• The flow past the motor should not exceed 10 ft/s (3.05 m/s).
• Hotter water (86–122 °F, 30–50 °C) needs more flow; from 122 to 140 °F (50–60 °C) the motor must also be de-rated.
• A flow-inducer sleeve (a tube around the motor that forces the water past it) is needed when the well is too wide to reach that speed, when the pump is in an open body of water (always), set in or below the well screen, in a rock well below the casing, or in a well fed from the top.

The required flow grows quickly with casing diameter, because the same speed through a wider gap is more water. Franklin’s table:

Minimum flow for motor cooling in water up to 86 °F (30 °C), from Franklin Electric’s AIM manual (2026), page 9. The 6-inch column covers 5–60 hp encapsulated and < 40 hp MagForce motors; larger MagForce motors need 1.5–1.6 ft/s.

Sump, effluent, sewage and grinder pumps

These sit in a basin (pit) and pump drainage up to a sewer, drain field or the surface. The Sump and Sewage Pump Manufacturers Association (SSPMA), whose members include Zoeller, Liberty, Little Giant and Goulds, separates them by solids size:

A sewage ejector passes whole solids through a non-clog (usually vortex) impeller and so needs a 2-inch or larger discharge. A grinder pump chops the solids with a cutter first, so it can push through a small pipe (Zoeller’s Shark grinders use a 1¼ in discharge) against a much higher head — up to 107 ft (33 m) for Zoeller’s centrifugal grinder and 240 ft (73 m) for its progressing cavity grinder — which is why grinders serve pressure sewers and long uphill runs. Sump and sewage systems must be kept apart: the Illinois Plumbing Code, for instance, forbids sewage sumps from receiving subsoil or footing drainage and vice versa. Choosing and sizing a sewage pump for a building is covered step by step in How to Choose a Sewage Pump for a Commercial Building.

Progressive cavity pumps

A progressive cavity pump (also called a progressing cavity, eccentric screw or Moineau pump; HI lists it as a single-screw rotary pump) has only two working parts: a rotor, a single-start helical screw, usually steel, and a stator, a tube with a helical bore that has one more lobe than the rotor, usually moulded from an elastomer. Because of that extra lobe, the rotor touches the stator along continuous sealing lines and leaves a chain of separate, sealed cavities between them. As the rotor turns — rolling eccentrically inside the stator, which is why a jointed coupling rod connects it to the drive — the cavities travel from the suction end to the discharge end without changing shape or volume, carrying the liquid with them.

The Moineau principle. The geometry was patented by René Joseph Louis Moineau of Paris. His U.S. patent 1,892,217, “Gear mechanism”, filed on 27 April 1931 and granted on 27 December 1932, claims priority from his French filing of 13 May 1930 and describes a pump, motor or transmission made of “two helical members, disposed within one another, the outer member having one helical thread or tooth more than the inner member”. That one-tooth difference is still the defining feature.

What it handles well. Because the cavities don’t change volume, the flow is steady (almost pulsation-free), proportional to speed, and the liquid is not churned — so it suits liquids a centrifugal pump cannot handle. NETZSCH, one of the largest makers, states that its NEMO pumps cover:

• viscosities from 1 cSt to 10 million cSt, including thixotropic and dilatant products, shear-sensitive or not;
• abrasive and non-abrasive, lubricating and non-lubricating, corrosive (pH 0–14), sticky and toxic media;
• solids up to 6 in (150 mm);
• flows from a few gallons per hour up to 4,400 gpm (1,000 m³/h), with 1 to 8 stages for pressures up to 720 psi (50 bar) standard or 4,350 psi (240 bar) in high-pressure versions; in its standard geometries each stage is good for about 90 psi (6 bar);
• suction lift up to 30 ft (9 m) of water, flow in either direction, and temperatures from −5 to 570 °F (−20 to +300 °C) with suitable stator materials.

NETZSCH’s own application lists include thickened and dewatered sewage sludge (up to 35% dry solids in its hopper pumps), biogas substrates, food, paper, mining and chemical products, and chemical metering, where the output is proportional to speed. Zoeller also builds a small progressing cavity grinder pump for pressure sewers, mentioned above.

Why it must not run dry. The rotor seals against the stator along continuous contact lines, and the pumped liquid is what lubricates and cools that contact. With no liquid, friction heats the elastomer until the stator is destroyed — NETZSCH’s accessory literature describes its dry-running protection as avoiding “thermal destruction of stators”. Its protection units either measure the temperature between rotor and stator and shut the pump down when friction pushes it over a set point, or (for solid stators) sense the flow in the pipe and stop the pump when it drops. Like any positive displacement pump it also needs protection against a closed discharge valve, because it will keep building pressure.

Other common pump types

• Gear pumps (rotary positive displacement): two meshing gears carry liquid around the casing in the spaces between their teeth and push it out where the teeth mesh again. Like other positive displacement pumps they handle viscous liquids such as oils well, and the DOE/HI sourcebook warns that positive displacement pumps are “very susceptible to wear from abrasives”.
• Diaphragm pumps (reciprocating positive displacement, often air-operated): a flexing diaphragm and two check valves move the liquid. The DOE/HI sourcebook notes that diaphragm and peristaltic pumps need no shaft seals and so do not leak, which is valuable for corrosive or hazardous liquids.
• Peristaltic (hose or tube) pumps (HI: “tube, flexible”): rollers squeeze a flexible tube along its length, so the liquid only ever touches the inside of the tube and there is no seal to leak.
• Piston and plunger pumps (reciprocating positive displacement): the classic high-pressure, low-flow pump, from pressure washers to chemical injection. The flow pulses, which can shake pipework unless dampened, and the internal valves wear and need periodic replacement.
• Jet pumps: a centrifugal pump at the surface drives water through a venturi (the jet) that creates a partial vacuum and draws well water up. UNL Extension gives shallow-well jets for water less than 25 ft (7.6 m) down and deep-well jets, with the jet down in the well, for 25 to 250 ft (7.6–76 m). They must be primed and lose prime if the suction pipe leaks air, and they produce less pressure and flow than a submersible.

Pump types compared

PD = positive displacement. Every PD type needs over-pressure protection (a relief valve) on the discharge.

How to read a pump curve

A centrifugal pump’s performance is published as a set of curves against flow on the horizontal axis:

• Head vs flow. Head (ft or m of liquid) is highest at zero flow — the shut-off head — and falls as flow increases. Head is used instead of pressure because a pump lifts any thin liquid to the same height; the pressure that height produces depends on the liquid’s density.
• Efficiency and power. Efficiency is fluid power divided by shaft power. Fluid power in horsepower is head (ft) × flow (gpm) × specific gravity ÷ 3,960. Pump efficiencies range from 35% to more than 90%. For a radial centrifugal pump the brake-horsepower curve rises steadily with flow, so the pump draws least power at shut-off.
• Best efficiency point (BEP). The flow at which efficiency peaks and radial bearing loads are lowest. The sourcebook describes a preferred operating region (POR) around it, in which the pump’s service life is not significantly affected by hydraulic loads, vibration or flow separation, and a wider allowable operating region (AOR) that sets the minimum and maximum flows. Bearing life varies with the inverse cube of the load, so running near BEP matters.
• Minimum flow. At very low flow the liquid recirculates inside the impeller, which damages it much as cavitation does; manufacturers list a minimum flow to stay above.
• NPSH required. Usually drawn as a separate line rising with flow (see below).

Where the pump actually runs. Overlay the system curve: the static head (the height to lift, plus any pressure difference) plus friction losses, which rise roughly with the square of flow — double the flow and the friction quadruples. The pump operates where its curve crosses the system curve. Pick a pump whose crossing point lands near its BEP. To estimate friction losses for the system curve, use the Pipe Pressure Drop Calculator.

NPSH: required vs available. If the pressure at the impeller inlet falls below the liquid’s vapour pressure, bubbles form and then collapse violently (cavitation), eroding the impeller and casing. NPSH required (NPSHR) is a property of the pump at each flow; HI defines it as the point where cavitation cuts the head by 3% and now calls it NPSH3. NPSH available (NPSHA) is a property of the installation — atmospheric or tank pressure plus the height of liquid above the pump, minus suction-line friction and the vapour pressure. The DOE/HI sourcebook’s rule is NPSHA at least 25% above NPSHR at all expected flows. Its worked example: a 4×1.5-6 pump (4 in suction, 1.5 in discharge, 6 in impeller) at its design point is 74% efficient on a 5 hp motor and needs 6 ft of NPSH, so the system should supply at least 7.5 ft.

Changing the curve. The sourcebook gives the affinity laws for trimming an impeller from diameter D₁ to D₂: flow scales with D₂/D₁, head with its square and power with its cube. The same proportions apply to a change of speed; the Pump & Fan Affinity Laws Calculator works both. Remember that slowing a pump moves it along a parabola through zero flow, so in a system with high static lift the flow falls much faster than the laws alone suggest.

Positive displacement pumps have no falling head curve: their flow is proportional to speed and, in the sourcebook’s words, “essentially independent of backpressure”, so they are chosen by displacement and speed for the flow, then checked for the pressure and power the system demands. To convert flow units on any curve, use the flow rate converter.

Sources

• U.S. Department of Energy (Industrial Technologies Program) and Hydraulic Institute, Improving Pumping System Performance: A Sourcebook for Industry, 2nd ed., May 2006 (DOE/GO-102006-2079) — pump classes, PD applications and relief valves, BEP, POR/AOR, NPSH and the 25% margin, the 4×1.5-6 example, fluid-power formula, impeller-trim affinity laws, friction ∝ flow². https://www.energy.gov/sites/prod/files/2014/05/f16/pump.pdf
• Hydraulic Institute, Pump Types (HI Data Tool), accessed 2026 — rotodynamic and positive displacement definitions; “Screw, single (progressing cavity)” and “Tube, flexible (peristaltic)”. https://datatool.pumps.org/introduction-definitions-references/pump-types
• Franklin Electric, AIM Manual: 60 Hz Submersible Systems — Application, Installation, Maintenance, 2026 edition, pages 8–9 (cooling flow, flow-inducer sleeves, 10 ft/s limit, minimum-gpm table). https://franklinwater.com (copy read from https://dwee.nebraska.gov/sites/default/files/water/2026_60Hz_AIM_Manual_Submersible_Systems.pdf)
• R. J. L. Moineau, U.S. Patent 1,892,217, Gear Mechanism, filed 27 April 1931 (France 13 May 1930), granted 27 December 1932. https://patents.google.com/patent/US1892217A/en
• NETZSCH Pumps & Systems, NEMO Progressing Cavity Pumps brochure 305 (EN-US edition, accessed October 2026) — performance ranges, geometries, applications. https://pumps-systems.netzsch.com/_Resources/Persistent/8/7/2/a/872a6245cbaf61322b7a4dd7c59d206f2548260e/NETZSCH-305-NEMO-Progressing-Cavity-Pump-Brochure-EN-US.pdf
• NETZSCH, Original Accessories (dry-running protection STP2A/STP2D, flow-sensing unit). https://www.motralec.com/telecharger/Netzsch_Original_Accessories.pdf
• Sump and Sewage Pump Manufacturers Association (SSPMA), Sizing for New or Replacement Sewage Pumps, 2011 (presented by M. Rousseau, NOWRA) — sump 1/2 in, effluent 3/4 in, sewage 2 in solids. https://www.nowra.org/Customer-Content/www/CMS/files/22Mega_Rousseau.pdf
• Zoeller Company, Pumps and Systems catalog FM0532 (rev. 11/2016) — oil-filled motors, solids ratings, Shark grinder heads and 7/32 in shredding. https://www.cooneybrothers.com/userfiles/ad/documents/Zoeller_2640003_Catalog.pdf
• Illinois Department of Public Health, Illinois Plumbing Code, 77 Ill. Adm. Code 890.1360, Sanitary Wastes below Sewer (amended 2014). https://www.ilga.gov/ftp/JCAR/AdminCode/077/077008900J13600R.html
• J. R. Hygnstrom, W. Woldt and S. O. Skipton, Private Drinking Water Wells: The Distribution System, University of Nebraska–Lincoln Extension G2152, July 2012 — submersible and jet pumps. https://extensionpubs.unl.edu/publication/g2152/pdf/view/g2152-2012.pdf