Gear Pumps: External vs Internal Types, Slip, Sizing and Selection

Quick Answer

A gear pump is a rotary positive displacement pump that carries liquid from inlet to outlet in the spaces between gear teeth and the casing; where the teeth mesh again, the liquid is squeezed out and cannot return. External gear pumps use two meshing gears side by side. Internal gear pumps run a small idler inside a larger rotor, with a fixed crescent sealing suction from discharge; a gerotor is the crescent-less version, with an inner rotor one tooth short of the outer. Flow is displacement × speed × volumetric efficiency. The shortfall is slip, leakage back through the clearances, which rises with differential pressure and with the cube of the clearance, and falls as viscosity rises. Gear pumps therefore suit oils, fuels, resins, polymers and bitumen; abrasives wear them quickly, and very thin liquids raise slip. Viscous liquids need lower speeds so the tooth spaces can fill. A gear pump has no shut-off head, so a relief valve is mandatory and the motor is usually sized at the relief valve's full-bypass pressure. Shaft power is flow × differential pressure ÷ overall efficiency. API 676 covers rotary positive displacement pumps for petroleum, petrochemical and gas services.

A typical complaint: a gear pump that moved fuel oil at full rate in winter delivers noticeably less on a hot afternoon, while the same model on a cold, heavy oil rattles and starves at start-up. Neither pump is faulty. The first is losing flow to slip as the oil thins; the second is turning too fast for its tooth spaces to fill.

Both problems trace back to a specification that gave a flow and a pressure but no viscosity range. This guide covers how gear pumps work, what controls their real output, and how to size and protect one.

How Gear Pumps Work

External gear pumps

Two gears mesh inside a close-fitting casing; one is driven, the other turns with it. As the teeth unmesh at the inlet, the opening space draws liquid in. The liquid travels round the outside of both gears, trapped between the teeth and the casing, and is forced out where the teeth mesh again. Tight clearances at the tooth tips and gear faces hold leakage down; hydraulic designs add side wear plates and, according to one university fluid-power course, run with gaps typically under 10 µm.

The weakness is an unbalanced side load on the bearings, which limits speed, pressure rating and bearing life. In the usual design all four shaft bearings run in the pumped liquid, so it must be clean and lubricating. External gear pumps are widely used in hydraulic fluid power and for lubricating oil, fuel and clean chemicals.

Internal gear pumps (crescent type)

A large outer gear, the rotor, drives a smaller inner gear, the idler, which turns on an off-centre pin. A fixed crescent fills the gap between them and seals the inlet from the outlet. Liquid fills the tooth spaces of both gears as they separate, travels either side of the crescent and is forced out as they mesh again.

Process internal gear pumps run slower than external designs. They are non-pulsing and self-priming, can run dry for short periods and can be reversed for loading and unloading, and their clearance can be adjusted for high temperature, high viscosity or wear. Process designs are not built for high speed or high pressure: one distributor quotes typical ratings up to 14–17 bar (about 200–250 psi), higher for special models, and viscosities from about 1 cP to 1,000,000 cP. Gap-compensated hydraulic internal gear pumps are a separate class, rated to about 300 bar at up to 3,000 rpm. With one bearing in the liquid, sometimes two, they tolerate dirtier liquids than external designs.

Gerotor pumps

A gerotor is an internal gear pump without a crescent. The inner rotor has N teeth and the outer rotor N + 1, so the teeth form sealed chambers that grow and shrink as both rotors turn. Gerotors are compact and smooth, and suit lubrication systems and low to moderate pressure hydraulics; their fixed clearances mean they cannot pass solids.

Flow, Slip and Volumetric Efficiency

Theoretical flow is displacement times speed. Real flow is lower by the slip:

Q = V × n × ηv

Here V is the displacement per revolution, n the speed and ηv the volumetric efficiency. Slip is liquid leaking back from discharge to suction through the tooth-tip, face and mesh clearances. Flow in those narrow gaps is laminar, and leakage through a gap of width w, length L and height h follows the slot-flow law:

q = w × h³ × Δp ÷ (12 × μ × L)

Three practical rules follow:

• Slip rises with the cube of the clearance. Wear that doubles a gap multiplies the leakage through it by eight; a 25% increase multiplies it by 1.95.
• Slip rises in proportion to differential pressure. Going from 8 to 10 bar adds 25% to the slip.
• Slip falls as viscosity rises. Halving the viscosity roughly doubles the leakage.

Slip depends little on speed, so it is a larger share of the flow at low speed. A pump maker's application engineer, writing in a trade journal, puts typical gear pump volumetric efficiency at around 80–90%, but the real figure depends on viscosity, pressure and wear, and the maker's performance curves govern.

Flow hardly changes with discharge pressure: a gear pump has no shut-off head, and the system resistance, not the pump, sets the pressure. See our guide to pump curves and system curves.

Viscosity and Speed

Thick liquids reduce slip but cannot flow into the tooth spaces quickly. At too high a speed the spaces do not fill, the pump cavitates or rattles, and output falls. Makers therefore publish maximum speed against viscosity. As an illustration, one distributor's guide steps gear pump speed down from 1,725 rpm at 50 SSU to 1,000 rpm at 5,000 SSU and 200 rpm at 100,000 SSU; the maker's curve for the actual pump governs. The same guide calls for suction and discharge lines one or two sizes larger than the pump ports on viscous duties, and the net positive inlet pressure should be checked at the coldest, thickest condition.

Heating works the other way: it allows higher speed, more capacity and lower power, which is why pumps for bitumen, heavy fuel oil and resins are usually jacketed or heat-traced. Thin liquids such as solvents and light fuels sit at the opposite extreme, where slip is high and the liquid does little to lubricate bushings and gear faces.

Worked Example: Sizing a Gear Pump for Fuel Oil

A plant needs 12 m³/h (200 l/min, 52.8 US gpm) of fuel oil against 8 bar (116 psi) differential. At the pumping temperature the oil is about 100 cSt, and the maker's chart allows 600 rpm for this pump size at that viscosity. Assume a volumetric efficiency of 0.90 and an overall efficiency of 0.75 at this duty; the maker's curves govern both.

1. Displacement: V = Q ÷ (n × ηv) = 200 ÷ (600 × 0.90) = 0.370 l/rev. Choose the next model up, 0.40 l/rev.
2. Speed: to deliver exactly 200 l/min, n = 200 ÷ (0.40 × 0.90) = 556 rpm, through a reducer or a variable-frequency drive. Theoretical flow is 0.40 × 556 = 222 l/min, of which 22 l/min slips back.
3. Power at duty: hydraulic power = flow (l/min) × Δp (bar) ÷ 600 = 200 × 8 ÷ 600 = 2.67 kW (the same as 12 m³/h × 8 bar ÷ 36). Shaft power = 2.67 ÷ 0.75 = 3.56 kW (4.8 hp).
4. Power at relief: with the relief valve passing the full theoretical flow at 10 bar (145 psi), its full-bypass pressure, hydraulic power is 222 × 10 ÷ 600 = 3.70 kW and shaft power about 4.9 kW. Size the motor on this figure, not on the 3.56 kW duty.
5. Sensitivity: if the oil runs hotter and its viscosity halves, slip doubles to 44 l/min and delivery falls to 178 l/min (10.7 m³/h), 11% short; about 611 rpm restores 200 l/min. A rise to 10 bar adds 25% to the slip (194 l/min delivered), and wear that opens the clearances by 25% costs about as much as halving the viscosity (179 l/min).

So a gear pump enquiry needs the viscosity range and the relief setting, not just flow and pressure.

Relief Valves and Overpressure Protection

A gear pump produces flow; the system resistance produces the pressure. Against a closed or blocked discharge the pressure rises until the weakest component fails: casing, seal, coupling, gearbox or pipework. Protection comes in two layers:

• Integral relief valve: built into most gear pumps, it passes flow back to the suction side inside the pump. It protects the pump only.
• System relief valve: a separate valve that protects the piping and equipment downstream and returns liquid to the tank. See our guide to pressure relief and safety valves.

An integral valve is not a flow control. When it recirculates, all of the shaft power, about 4.9 kW in the worked example, ends up as heat in the small volume of liquid circulating inside the pump, and the temperature climbs quickly. Control flow with speed, not by throttling the discharge, and never fit an isolating valve between the pump and its relief valve.

Materials, Wear and Sealing

Close clearances make gear pumps efficient and vulnerable. Abrasive particles open the clearances, and because slip rises with the cube of the gap, output falls away quickly once wear starts. Where some abrasive content cannot be avoided, choose a larger pump and run it slower, use hardened parts and fit a suction strainer; internal gear designs, with fewer bearings in the liquid, cope better than external ones. For true slurries, use a progressive cavity, lobe or peristaltic pump.

Gear vs Lobe vs Vane vs Centrifugal Pumps

Vane figures come from one industry handbook and the internal gear pressure from one distributor; makers' ratings govern.

Lobe pumps often need speed reductions of 25% or more on high-viscosity liquids. For the wider family, see our guides to positive displacement pumps and centrifugal pumps.

API 676 and Applications

API 676 sets minimum requirements for rotary positive displacement process pumps in petroleum, petrochemical and gas industry services. It does not cover controlled-volume pumps, which come under API 675 (see our guide to dosing pumps), hydraulically driven pumps, or reciprocating pumps, which come under API 674. Its abstract notes that for auxiliary services such as lube oil systems, a maker's standard pump with demonstrated experience may be acceptable. Check the current edition before writing it into a specification.

Typical gear pump duties:

• Fuel oil transfer and burner feed, with heated lines for heavy fuel oil
• Lube oil systems for turbines, compressors and gearboxes
• Resins, polymers, adhesives, paints and inks
• Bitumen and asphalt, in jacketed pumps
• Chemicals, alcohols, solvents and glycol, in suitable materials
• Hydraulic power units
• Food products such as syrups and chocolate

Common Mistakes

1. Giving flow and pressure without the viscosity range. A pump that suits warm oil can starve on cold oil and lose flow to slip on hot oil.

   Prevention: State the viscosity at minimum, normal and maximum temperature, and size for both ends.

2. Running at motor speed on a viscous liquid. The tooth spaces do not fill; the pump cavitates, rattles and wears.

   Prevention: Use the maker's speed–viscosity limit, a reducer or drive, and suction lines larger than the ports.

3. Sizing the motor at duty pressure. The motor trips or stalls when the relief valve lifts.

   Prevention: Size the motor at the relief valve's full-bypass pressure, which for a pump-mounted valve is above its cracking pressure.

4. Relying on the integral relief valve alone. It protects the pump, not the piping and equipment downstream.

   Prevention: Fit a system relief valve, with no isolating valve between the pump and either relief valve.

5. Using the integral relief valve as a flow control. Continuous recirculation turns the full shaft power into heat inside the pump.

   Prevention: Control flow with speed, and return any continuous bypass to the tank.

6. Pumping abrasive slurries. The clearances open quickly, and slip rises with their cube.

   Prevention: Use a progressive cavity, lobe or peristaltic pump, or a larger internal gear pump at low speed with hardened parts and a strainer.

7. Throttling the discharge to reduce flow. Flow hardly changes, while pressure, power and slip rise.

   Prevention: Change speed with a variable-frequency drive or reducer.

8. Starting dry. Gears, bushings and seal run without lubrication.

   Prevention: Prime before start-up and protect against loss of suction.

Ordering Checklist

• Liquid: name, viscosity at minimum, normal and maximum temperature, density, vapour pressure, solids or abrasives, corrosivity
• Duty: normal and maximum flow; suction and discharge pressures; net positive inlet pressure available or suction lift
• Drive: power supply, speed control by reducer or variable-frequency drive, hazardous-area classification
• Construction: external or internal gear, casing and gear materials, heating jacket, shaft seal
• Protection: integral and system relief valves and their set pressures, dry-run protection
• Standard: API 676 or the maker's standard
• Documents: hydrostatic and performance test records and material certificates; see our guide to material test certificates

Supply from Kasko Makine

Kasko Makine supplies rotary positive displacement pumps:

• Gear pumps: positive displacement pumps that transfer fluid by the meshing of rotating gears, used in hydraulic systems and industrial applications, especially for high-viscosity fluids with steady, precise flow. The product page lists constant flow output, a simple mechanical design and reliable continuous operation, for hydraulic fluid power systems, chemical and petrochemical installations, lubrication systems, oil and fuel transfer, and industrial processes that need precise flow.
• Lobe pumps: non-contacting lobes for viscous, abrasive and sensitive fluids with gentle, low-shear flow; listed for food and beverage, pharmaceutical and biotechnology, chemical and petrochemical, and cosmetics and personal care applications.
• Vane pumps: sliding vanes in a rotating rotor for smooth, low-pulsation flow at low to medium pressure with clean or lightly contaminated liquids, in unbalanced (fixed displacement) and balanced (reduced bearing load) types; listed for hydraulic systems, fuel transfer and injection, lubrication, chemical and process industries, and automotive and industrial machinery.

Engineering support: Send the liquid, its viscosity at the minimum and maximum operating temperatures, the flow, the suction and discharge pressures and any solids content. We will review pump type, speed, materials and relief protection against your data, and reply with pricing, lead time and engineering details.

Send the liquid, its viscosity range, the flow and the pressures for a gear pump proposal 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
• Centrifugal Pumps: How They Work, Types, Specifications & Selection Guide
• Pump Curves & System Curves: How to Find the Operating Point
• Mechanical Seals Explained: API 682 Types, Arrangements and Piping Plans
• Industrial Gearboxes and Couplings: Service Factor, AGMA Ratings and Alignment
• Dosing Pumps: Metering Pump Types, Accuracy and Chemical Injection Sizing

Frequently Asked Questions

Q: How does a gear pump work?
A: A gear pump traps liquid in the spaces between the gear teeth and the casing, carries it from the inlet side to the outlet side, and forces it out where the teeth mesh again; the mesh stops it flowing back. Each revolution displaces a fixed volume, so flow is set by speed and stays nearly constant as pressure changes, less the slip that leaks back through the internal clearances.

Q: What is the difference between external and internal gear pumps?
A: An external gear pump uses two gears meshing side by side, usually with four bearings in the liquid; it suits clean liquids and, in hydraulic designs, high pressures. An internal gear pump runs a small idler gear inside a larger rotor, with a crescent sealing suction from discharge. A process internal gear pump runs slower, primes well, handles very viscous liquids and can be reversed. A gerotor is an internal design without a crescent.

Q: What is slip in a gear pump?
A: Slip is liquid that leaks back from discharge to suction through the clearances at the tooth tips, gear faces and mesh. It rises in proportion to differential pressure and with the cube of the clearance, and falls as viscosity rises: halving the viscosity roughly doubles the slip. Slip is why gear pumps lose flow on thin or hot liquids and as they wear, and why volumetric efficiency is below 100 percent.

Q: How do you calculate gear pump flow and power?
A: Flow equals displacement per revolution times speed times volumetric efficiency: 0.40 litres per revolution at 556 rpm and 90 percent volumetric efficiency gives 200 litres per minute. Hydraulic power in kilowatts equals flow in litres per minute times differential pressure in bar, divided by 600. Shaft power is that figure divided by the overall efficiency. Size the motor at the relief valve set pressure.

Q: Why does a gear pump need a relief valve?
A: A gear pump has no shut-off head. It keeps displacing liquid whatever the pressure, so against a closed or blocked discharge the pressure rises until the casing, seal, coupling or piping fails. An integral relief valve protects the pump, and a separate system relief valve protects the piping. Neither should be used to control flow, because recirculation turns the full hydraulic power into heat.

Q: Can gear pumps handle viscous or abrasive liquids?
A: Gear pumps handle viscous liquids well, and internal gear designs are quoted up to about a million centipoise, but speed must fall as viscosity rises so the tooth spaces can fill. Abrasives are different: they wear the close clearances, and slip rises with the cube of the gap. For slurries choose a progressive cavity, lobe or peristaltic pump, or run a larger internal gear pump slowly with hardened parts.

Q: Which standard covers gear pumps for refinery service?
A: API 676 covers rotary positive displacement process pumps, which include gear pumps, for petroleum, petrochemical and gas industry services. It excludes controlled-volume metering pumps, which fall under API 675, and reciprocating pumps, which fall under API 674. For auxiliary duties such as lube oil systems, a maker's standard pump with demonstrated experience may be acceptable. Check the current edition before quoting it in a specification.