A Heppel Rotary Pump is a positive displacement pump that uses two intermeshing curved rotors turning in opposite directions inside a close-fitting casing to trap and transfer fluid from inlet to outlet. It solves the problem of moving viscous, particle-laden, or shear-sensitive liquids at low to medium pressures without valves or reciprocating parts. Each rotor revolution sweeps a fixed volume of fluid through the casing, so output is proportional to shaft speed. The design is self-priming and tolerates suction lift up to about 6 m, which is why it became a standard marine bilge and feed pump through the late 19th century.
Heppel Rotary Pump Interactive Calculator
Vary displacement, speed, efficiency, and pressure to see positive-displacement flow, slip loss, power, and an animated twin-rotor pump diagram.
Equation Used
This calculator uses the positive-displacement pump relation described for the Heppel rotary pump: each revolution sweeps a fixed volume, so theoretical flow is displacement times shaft speed. Volumetric efficiency reduces that value to estimate actual delivered flow, while pressure rise converts the delivered flow into hydraulic power.
- Displacement is the swept fluid volume per shaft revolution.
- Actual flow equals theoretical flow corrected by volumetric efficiency.
- Slip loss is the difference between theoretical and actual flow.
- Hydraulic power ignores mechanical, gear, and bearing losses.
How the Heppel Rotary Pump Actually Works
Two shaped rotors sit on parallel shafts inside a figure-of-eight casing, geared together externally so they rotate in opposite directions without ever touching each other or the casing wall. As each rotor turns, the cavity between the rotor lobe and the casing wall fills with fluid at the inlet, carries that trapped volume around the outside of the rotor, and then discharges it at the outlet as the lobes mesh and squeeze the cavity closed. There are no inlet or outlet valves — the rotor geometry itself does the timing. Output volume per revolution is fixed by geometry, so flow scales linearly with RPM. That is the defining property of any positive displacement pump.
The critical engineering issue is clearance. The gap between rotor tips and the casing wall — and between the two rotors at the mesh point — must sit in the 0.05 to 0.15 mm band for water service. Open it up to 0.3 mm through wear and you double slip flow, which is the leakage of pumped fluid back from the high-pressure side to the low-pressure side through those clearances. Volumetric efficiency falls from around 92% on a fresh pump to under 70% on a worn one, and self-priming starts to fail because the pump cannot generate enough vacuum to lift water against atmospheric pressure. If you notice the discharge pressure dropping at constant RPM, or the pump losing prime after a shutdown, worn rotor tips or a scored casing are almost always the cause.
The timing gears that synchronise the two rotors live in an oil bath outside the wetted casing. They carry no torque from the fluid — they exist purely to keep the rotors phased so the lobes never collide. If those gears develop backlash beyond about 0.1° at the rotor tip, the rotors will rub at the mesh, gall, and seize within minutes. Heppel's original 1860s patent specifications called out rotor concentricity within 2 thou of an inch, and that figure is still the right target for a modern restoration build.
Key Components
- Curved Rotors (pair): The two intermeshing lobed rotors that trap fluid against the casing wall and carry it from suction to discharge. Each rotor typically carries 2 or 3 lobes machined to a cycloidal or involute profile. Concentricity must hold within 0.05 mm and the rotor tip-to-casing clearance must sit between 0.05 and 0.15 mm for clean water service.
- Figure-of-Eight Casing: The cast iron or bronze housing machined as two intersecting cylindrical bores that the rotors sweep. The bore finish must be 0.8 µm Ra or better — rougher and you accelerate slip flow as turbulence kicks up at the tip clearance. Inlet and outlet ports sit at the top and bottom of the figure-of-eight.
- Timing Gears: External spur or helical gears keyed to the rotor shafts that hold the two rotors in correct phase so the lobes never touch. They run in a separate oil bath. Backlash above 0.1° at the rotor tip leads to mesh contact and rapid galling.
- Shaft Seals: Gland packing or mechanical seals where the rotor shafts exit the casing. Packing rings need to be retorqued every 200 hours of service to maintain a controlled drip rate of around 5 to 10 drops per minute — fully dry packing burns out the shaft sleeve in under a shift.
- Drive Shaft and Coupling: Only one shaft is driven externally; the second rotor takes its torque through the timing gears. The drive coupling must be flexible enough to absorb the small torque pulses you get every time a pair of lobes meshes — typically 4 or 6 pulses per revolution depending on lobe count.
Real-World Applications of the Heppel Rotary Pump
Heppel Rotary Pumps fill the niche where you need a self-priming positive displacement pump that handles moderately viscous or solids-bearing liquids without the pulsation of a piston pump or the shear of a centrifugal. They appear most often in marine, food and beverage, and process transfer service where the fluid cannot tolerate the high tip speeds of a centrifugal impeller. You will still find original Heppel and Heppel-pattern pumps running in heritage installations a century after they were built, which speaks to how forgiving the geometry is when you keep the clearances honest.
- Marine: Bilge and ballast service on Victorian and Edwardian steam vessels — the SS Great Britain restoration in Bristol uses a period-correct Heppel-pattern bilge pump driven off the engine room auxiliary shaft.
- Brewing: Wort and fermented beer transfer at heritage breweries — Hook Norton Brewery in Oxfordshire runs a rotary lobe pump on this pattern for low-shear yeast handling between fermenter and conditioning tank.
- Chemical Process: Transfer of mid-viscosity organics like glycerine and light tar fractions at specialty chemical plants, where the absence of valves means no clogging on suspended solids.
- Food: Chocolate, honey, and condensed milk transfer at facilities like Cadbury's Bournville heritage line, where the pump's low-shear action protects product texture.
- Heritage Steam: Boiler feed duty on preserved traction engines and stationary mill engines — the Kew Bridge Steam Museum in London runs Heppel-pattern auxiliaries on several of its working Cornish engines.
- Water and Drainage: Low-head drainage service in fenland pumping stations, replacing original scoop wheels where head exceeds 3 m but flow remains modest.
The Formula Behind the Heppel Rotary Pump
The theoretical flow rate of a Heppel Rotary Pump is set entirely by the swept volume per revolution and the shaft speed, minus slip flow through the internal clearances. At the low end of the typical operating range — say 100 RPM on a hand-cranked or belt-driven heritage installation — slip flow is a large fraction of total flow because the pressure differential has time to push fluid back through the clearances. At the high end, around 600 RPM on a modern electric-driven unit, slip becomes negligible but rotor cavitation starts to bite if suction lift exceeds 5 m. The sweet spot for most builds is 200 to 400 RPM, where volumetric efficiency holds above 88% and the rotors run cool.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Actual volumetric flow rate delivered at the discharge | m³/s | gal/min |
| Vr | Swept volume per revolution (geometric displacement of both rotors combined) | m³/rev | in³/rev |
| N | Shaft rotational speed | RPM | RPM |
| ηv | Volumetric efficiency accounting for slip flow through tip and mesh clearances | dimensionless (0 to 1) | dimensionless (0 to 1) |
Worked Example: Heppel Rotary Pump in a craft cidery wort transfer pump rebuild
You are rebuilding a 1908 Heppel-pattern bronze rotary pump for sweet apple juice transfer duty at a craft cidery in Herefordshire, lifting fresh-pressed juice from a 2000 L collection sump up 3.5 m to a settling tank ahead of fermentation. The rotors are twin-lobe with a swept volume of 0.18 L per revolution, the pump is belt-driven from a 0.75 kW geared motor, and you need to know what flow rate you will see across the practical operating speed range so you can size the discharge line and the settling tank fill cycle.
Given
- Vr = 0.18 L/rev
- Nnom = 300 RPM
- ηv = 0.90 dimensionless
- Suction lift = 3.5 m
Solution
Step 1 — at the nominal 300 RPM operating speed, calculate theoretical displacement before slip:
Step 2 — apply the volumetric efficiency to get actual delivered flow at nominal speed:
That is roughly 0.81 L/s — fast enough to move a full 2000 L sump in about 41 minutes, which matches the cidery's pressing cycle nicely. The pump runs cool at this speed and the rotors stay well below cavitation onset on a 3.5 m lift.
Step 3 — at the low end of the typical range, 150 RPM (belt slowed for delicate handling of the first juice run, which carries pulp solids):
Note the volumetric efficiency drops to about 85% at low speed because slip flow has more time to leak back through the tip clearances at any given discharge pressure. You can hear it — the discharge stream pulses slightly and the pump sounds like it is working harder per unit of output.
Step 4 — at the high end, 500 RPM, pushing the rebuild for a fast tank turnover:
In theory you get nearly 83 L/min, but in practice on a 3.5 m suction lift the pump starts to cavitate above about 450 RPM with cool juice — you will hear a gravelly rattle at the suction flange and the discharge flow becomes erratic. Drop the lift to under 2 m or accept 400 RPM as a safe operating ceiling.
Result
At the nominal 300 RPM operating point the pump delivers 48. 6 L/min of cider juice, enough to clear the 2000 L sump in about 41 minutes. The 150 RPM low-end run gives roughly 23 L/min — useful when you want gentle handling of pulpy first-press juice — while the 500 RPM theoretical 83 L/min is unreachable in practice because suction-side cavitation kicks in above 450 RPM on a 3.5 m lift. If you measure flow well below the predicted 48.6 L/min at 300 RPM, the three usual suspects are: (1) tip clearances opened beyond 0.2 mm from rotor wear, which dumps volumetric efficiency into the 70s, (2) air ingress at the suction-side gland packing, which you will spot as a milky discharge stream and a hissing sound at the gland, and (3) a partially blocked suction strainer raising effective lift past the cavitation threshold.
Choosing the Heppel Rotary Pump: Pros and Cons
The Heppel Rotary Pump occupies the middle ground between piston pumps and centrifugals. It is worth comparing it directly against the two alternatives a practitioner is most likely to consider for the same duty — a reciprocating piston pump for the high-pressure end and a centrifugal pump for the high-flow end.
| Property | Heppel Rotary Pump | Reciprocating Piston Pump | Centrifugal Pump |
|---|---|---|---|
| Typical operating speed | 100 to 600 RPM | 30 to 200 RPM | 1500 to 3500 RPM |
| Maximum discharge pressure | 8 to 12 bar | 200+ bar | 3 to 10 bar (single stage) |
| Flow pulsation | Low — 4 to 6 small pulses per rev | High — single pulse per stroke | None — smooth flow |
| Self-priming capability | Yes, up to 6 m lift | Yes, up to 7 m lift | No — needs flooded suction or priming aid |
| Shear on pumped fluid | Low — gentle on yeast, pulp, emulsions | Low to moderate | High — destroys delicate biology |
| Tolerance to suspended solids | Moderate — soft solids OK, hard grit wears rotors | Poor — destroys valve seats | Good with open impellers |
| Capital cost (relative) | Medium | High | Low |
| Service interval (clearance check) | 2000 to 4000 hours | 500 to 1000 hours (valves) | 8000+ hours |
| Volumetric efficiency | 85 to 92% when fresh | 95%+ | Variable with head |
Frequently Asked Questions About Heppel Rotary Pump
This is almost always a static-leak path through the rotor mesh, not the shaft seals. When the pump is running, the rotors carry fluid past the mesh point fast enough that the small clearance leakage doesn't matter. Stopped, the trapped column of fluid in the suction line slowly drains back through the mesh clearance under gravity, and the pump loses prime.
Two fixes worth trying before you re-machine the rotors: fit a swing check valve in the suction line within 1 m of the pump inlet, or wet the rotors with a more viscous fluid before shutdown so the surface tension at the mesh point holds longer. If neither works, your mesh clearance has likely opened past 0.25 mm and the rotors need regrinding or replacing.
Progressing cavity wins on pure viscosity handling — it will move honey at 10,000 cP without breathing hard, where a Heppel starts to struggle above about 5,000 cP because slip flow rises sharply with viscosity-driven pressure differential. But progressing cavity pumps cost roughly 2 to 3 times more, the stator is a wear part you replace every 3000 to 5000 hours, and they don't tolerate dry running at all.
For 25 °C honey, which sits around 10,000 cP, go progressing cavity. For 40 °C honey at 2,000 cP, the Heppel handles it fine and costs less to own. The crossover viscosity in my experience sits around 5,000 cP — below that, Heppel; above that, progressing cavity.
Pressure-dependent knocking points to timing gear backlash. As discharge pressure rises, the torque transmitted between the two rotors through the timing gears rises with it, and any backlash in those gears lets one rotor lag the other by a fraction of a degree before the gear teeth take up the load. That sudden take-up is the knock you hear, once per rotor revolution per lobe.
Pull the timing gear cover and check backlash with a dial indicator on the rotor tip — anything over about 0.1° measured at the tip means worn gears or loose keyways. Left alone, it ends with the rotor lobes contacting at the mesh and galling within a few hundred operating hours.
More sensitive than most rebuilders expect. Bronze rotors in a bronze casing give the cleanest result because both parts have the same thermal expansion coefficient — clearances stay constant from cold start to operating temperature. Bronze rotors in a cast iron casing tighten up by about 0.04 mm as the pump warms from 15 °C to 60 °C, which can take a marginal clearance into rubbing contact.
If you are rebuilding with mixed materials, set cold tip clearance at the high end of the spec — 0.15 mm rather than 0.10 mm — to leave room for differential expansion. The original 1860s Heppel works specified all-bronze construction for exactly this reason.
Yes, the geometry is fully symmetric — reverse the drive direction and the inlet becomes the outlet. But two practical problems bite. First, the shaft seal arrangement on most heritage Heppels is a stuffing box that relies on a slight pressure differential to seat the packing; reversed, you may pull air in through the gland and lose prime. Second, if the timing gears use single-direction helical cuts, reversing puts axial thrust on a thrust face that wasn't designed to take it.
For occasional reverse-drain duty, fine. For regular bidirectional service, fit double mechanical seals and confirm the timing gears are spur-cut or double-helical.
The hydraulic power figure (flow × pressure) only accounts for useful work done on the fluid. Real Heppel pumps lose another 25 to 40% of input shaft power to mechanical friction at the gland packing, viscous drag of the rotors through the fluid, and timing gear losses. A 40% overage on a properly assembled pump is right on the edge of normal — anything above 50% suggests over-tightened gland packing.
Quick check: loosen the gland nuts until you get a steady drip of 5 to 10 drops per minute at the packing, run the pump for 20 minutes to let the packing settle, then measure power again. If draw drops by 15% or more, your packing was strangling the shaft.
References & Further Reading
- Wikipedia contributors. Rotary vane pump. Wikipedia
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