PD Pump Engineering Center
Welcome to the Positive Displacement Pump Engineering Center — part engineering guide, part interactive tool suite. Follow the engineering workflow below, or jump straight to the tool you need.
Learn → Calculate → Select → Troubleshoot
| Stage | What you will find here |
|---|---|
| Learn | Working principle, pump types, performance curves |
| Calculate | Flow, power, displacement, torque, RPM, pressure tools |
| Select | PD vs centrifugal, type selection, sizing considerations |
| Troubleshoot | Common faults, causes and corrective actions |
What do you need to calculate?
| I want to calculate... | Use this tool | What it returns |
|---|---|---|
| Pump Flow (Q from power, head & efficiency) | Pump Flow Calculator | Flow rate in m³/h, L/s, GPM |
| Pump Power (shaft power from flow & head) | Pump Power Calculator | Required shaft power in kW |
| Displacement / Sizing (required pump size) | Pump Sizing Calculator | Design flow, head and motor power |
| Torque (from power & speed, T = 9550 × P/N) | Pump Power Calculator | Use with T = 9550 × P(kW) / N(rpm) |
| RPM (effect of speed change on flow & power) | Pump Affinity Law Calculator | New flow, head and power at changed speed |
| Pressure (unit conversion, bar/psi/MPa) | Pressure Unit Converter | Converted pressure in any unit |
How Positive Displacement Pumps Work
Positive displacement pumps move fluid by trapping a fixed volume in a cavity and mechanically forcing that volume into the discharge pipe, regardless of the downstream pressure. The theoretical flow rate is set by geometry and speed alone:
Where:
- Qtheoretical = theoretical flow rate (m³/s)
- Vd = displaced volume per revolution (m³/rev)
- n = rotational speed (rev/s)
In practice, actual flow is slightly lower because of internal leakage ("slip") past clearances:
Slip increases with discharge pressure and with lower fluid viscosity — a gear pump moving thin solvent at high pressure may lose 5-15% of its theoretical flow, while the same pump on a viscous oil may lose less than 1%. This is why PD pump flow is described as "relatively constant" rather than exactly constant: it is nearly independent of pressure compared with centrifugal pumps, but not perfectly so.
Volumetric efficiency captures the slip loss:
Flow Rate Unit Conversion
| Unit | Conversion from m³/h |
|---|---|
| m³/h | 1 |
| L/min | × 16.667 |
| L/s | × 0.2778 |
| US GPM | × 4.403 |
| Imperial GPM | × 3.666 |
| bbl/h (42 US gal) | × 6.290 |
For pump selection and power sizing, use the Pump Flow Calculator to convert between flow units, and the Pump Head Calculator to build the system head curve the pump must overcome.
PD Pump Power: Same Formula, Different Machine
The hydraulic and shaft power of a PD pump is calculated with the exact same equation as a centrifugal pump:
The difference is not the power math — it is the flow-head characteristic. Run any PD duty through the Pump Power Calculator to get shaft power in kW, then convert to motor horsepower (kW × 1.341). Because a PD pump delivers near-constant flow against rising pressure, its power rises with discharge pressure, unlike a centrifugal pump whose flow simply drops. Always size the PD driver for the relief-valve setting so the motor is not overloaded if the discharge is blocked.
| PD pump type | Typical efficiency | Power note |
|---|---|---|
| Plunger / piston | 80 - 95% | Highest efficiency, strong at high pressure |
| Gear | 70 - 90% | Clean, predictable duty |
| Screw | 70 - 85% | Stable across high viscosity |
| Progressive cavity | 60 - 75% | Slips more on low-viscosity fluids |
| Diaphragm (AODD) | 50 - 70% | Consumes compressed air — run cost is high |
For total driver operating cost, combine the shaft power with the Pump Energy Calculator. The comparison table below explains when a PD pump is the right family versus a centrifugal machine.
PD vs Centrifugal Pumps
Positive Displacement (PD) pumps move fluid by trapping a fixed volume and mechanically forcing it into the discharge pipe. Unlike centrifugal pumps, PD pumps produce flow directly proportional to speed and relatively independent of discharge pressure. Run your duty through the Pump Selection Calculator to get a quantitative PD-vs-centrifugal recommendation based on flow, head and viscosity.
| Characteristic | Centrifugal | Positive Displacement |
|---|---|---|
| Flow vs pressure | Flow drops as head increases | Flow constant regardless of pressure |
| Suitable viscosity | Low (<500 cP) | High (up to 1,000,000 cP) |
| Shear | High shear | Low shear (many types) |
| Self-priming | Poor | Excellent |
| Dry running | Will damage seals | Some types tolerate it |
| NPSH requirement | Increases with flow | Relatively constant |
| Max pressure | Lower (typically <40 bar) | Can be very high (to 700+ bar) |
Gear Pumps
Two meshing gears create voids that fill with liquid and carry it from suction to discharge around the casing.
- Types: External gear (two identical gears), internal gear (gerotor, gear within gear)
- Flow: Smooth, low-pulsation
- Pressure: Up to ~250 bar (external gear)
- Viscosity: 1 to 1,000,000 cSt
- Best for: Oils, polymers, fuels, hydraulic systems, high-pressure metering
- Not for: Solids/abrasives (close clearances wear quickly)
Progressive Cavity Pumps (Moyno)
A single helix rotor turns inside a double-helix elastomeric stator, forming cavities that progress from suction to discharge.
- Flow: Very smooth, non-pulsating
- Pressure: Up to 60+ bar (multiple stages)
- Viscosity: 1 to 1,000,000+ cSt
- Best for: Sludge, slurry, high solids, shear-sensitive polymers, food products
- Advantages: Gentle on product, handles large solids, self-priming
- Cannot run dry — stator elastomer burns out instantly without liquid lubrication
Screw Pumps
One, two, or three helical screws move fluid axially along the screw axis.
- Types: Three-screw (most common, for clean lube oil), twin-screw (handles some entrained gas), single-screw
- Flow: Very smooth, extremely quiet
- Pressure: Up to 300+ bar
- Best for: High-viscosity oils, fuel oil transfer, multi-phase (oil/gas/water)
- Advantages: Low noise, high reliability, handles gas entrainment
Lobe Pumps
Two (or three) lobe-shaped rotors rotate in opposite directions, trapping fluid between the rotor lobes and the casing and carrying it from suction to discharge. Lobe pumps look similar to gear pumps but use larger, rounded rotors with more clearance.
- Flow: Smooth, low-pulsation (tri-lobe designs smoother than bi-lobe)
- Pressure: Typically up to ~15 bar (some hygienic designs less)
- Viscosity: Low to very high, including shear-sensitive fluids
- Best for: Food and beverage, pharmaceuticals, cosmetics, shear-sensitive chemicals, fluids with soft solids
- Advantages: Hygienic CIP/SIP designs, gentle low-shear action, handles solids better than gear pumps (larger clearances), some designs run dry briefly
- Limitations: Lower pressure capability than gear or piston pumps; larger footprint for same flow
Because the rotors do not contact each other, lobe pumps are a common choice for sanitary and food-grade service where product integrity matters more than maximum pressure.
Diaphragm Pumps
Flexible diaphragm reciprocates to draw in and expel fluid; check valves control direction.
- Air-Operated Double Diaphragm (AODD): Compressed air drives two diaphragms; stall under pressure; intrinsically safe
- Mechanically actuated: For metering/dosing
| Attribute | Value |
|---|---|
| Flow | To 100+ m³/h |
| Pressure | To 20 bar (air limited) |
| Viscosity | To 50,000 cSt |
| Best for | Chemicals, slurries, abrasives, hazardous areas, shear-sensitive fluids |
| Advantages | Can run dry, self-priming, sealless, handles solids, air-operated = explosion-proof |
Peristaltic (Hose) Pumps
A rotating shoe/roller compresses a flexible tube, pushing fluid along. Liquid only touches the inside of the hose.
- Flow: Pulsating (multiple rollers reduce this)
- Pressure: Up to 15 bar
- Best for: Corrosive chemicals, food/pharmaceutical, abrasive slurry, metering
- Advantages: Complete fluid isolation (no seals), self-priming, reversible, can run dry
- Limitations: Hose life is the maintenance item; flow pulsation; not for high flow rates
Piston/Plunger Pumps
Reciprocating piston or plunger displaces fluid through check valves.
- Flow: Pulsating (triplex designs smooth this with three cylinders)
- Pressure: Up to 700+ bar (high-pressure water blasting, hydrotesting)
- Best for: High-pressure cleaning, water jetting, high-pressure injection, metering
- Advantages: Highest pressure capability, very high efficiency, accurate metering
- Limitations: Pulsation requires dampeners; check valves wear in dirty service
Metering/Dosing Pumps
Small, precise PD pumps for accurate chemical addition:
- Diaphragm metering: Most common; adjustable stroke length/frequency; to ~1000 L/h
- Plunger metering: Higher pressure; to 500+ bar
- Peristaltic metering: Simple, for low-pressure dosing
Typical applications: Chlorine injection, pH adjustment, corrosion inhibitors, polymer dosing.
PD Pump Selection Guide
PD Pump Types: Flow, Self-Priming & Dry-Run Comparison
A quick cross-type view on the operating traits that decide which PD pump survives your site conditions. Values are typical for standard commercial machines.
| Pump type | Flow character | Self-priming | Tolerates dry run | Flow control method | Pressure relief |
|---|---|---|---|---|---|
| Gear | Smooth, low pulsation | Fair | No - needs lubrication | Speed (VFD) | Yes - mandatory |
| Screw | Very smooth, quiet | Fair | No - needs lubrication | Speed (VFD) | Yes - mandatory |
| Lobe | Smooth, low pulsation | Fair | Some designs briefly | Speed (VFD) | Yes - mandatory |
| Diaphragm (AODD) | Pulsating | Excellent | Yes | Air regulation / stall | Built-in relief common |
| Progressive cavity | Very smooth | Excellent | No - burns stator | Speed (VFD) | Yes - mandatory |
| Peristaltic | Pulsating (rollers reduce it) | Excellent | Yes | Speed (VFD) | Not needed - hose self-seals |
| Piston / Plunger | Pulsating (triplex smooths) | Poor | No - needs lubrication | Speed / stroke | Yes - mandatory |
Use this with the selection table above to shortlist a pump family, then confirm with the Pump Selection Calculator.
| Need | Best PD Pump Type |
|---|---|
| High-viscosity oil | Gear or screw pump |
| Slurry with solids | Progressive cavity or diaphragm |
| Shear-sensitive fluids | Progressive cavity or peristaltic |
| Hazardous/explosive area | AODD (air-operated diaphragm) |
| High pressure (>100 bar) | Plunger or gear pump |
| Metering/dosing | Diaphragm metering |
| Completely leak-free/sealless | Diaphragm, peristaltic, mag-drive gear |
| Abrasive slurry | Peristaltic or heavy-duty diaphragm |
Seven PD Pump Types Compared: Working Principle, Pressure & Flow
Use this comparison to shortlist a pump family from your duty, then refine with the Pump Selection Calculator and the Pump Sizing Calculator. Flow figures are typical operating ranges for standard commercial machines, not absolute limits.
| Pump type | Working principle | Typical pressure | Typical flow | Best for | Avoid when |
|---|---|---|---|---|---|
| Gear | Two meshing gears create voids that carry fluid around the casing | Up to ~250 bar | Low to moderate (typical up to ~600 m³/h) | Clean oils, hydraulics, high-pressure metering | Solids or abrasives (close clearances wear fast) |
| Screw | Helical screws move fluid axially along the rotor | Up to 300+ bar | Moderate to high (typical up to ~1000+ m³/h) | High-viscosity oils, multi-phase fluids | Dry running, severe abrasives |
| Lobe | Lobe-shaped rotors carry fluid between the lobes and the casing | Up to ~15 bar | Low to moderate (typical up to ~500 m³/h) | Food, pharma, shear-sensitive fluids | High pressure duty |
| Piston / Plunger | Reciprocating piston displaces fluid through check valves | 100 to 700+ bar | Low to moderate, pulsating (triplex smooths it) | High pressure, water jetting, metering | Abrasive or very viscous fluids |
| Diaphragm | Flexible diaphragm reciprocates; check valves direct the flow | Up to ~20 bar (AODD) | Up to 100+ m³/h | Chemicals, hazardous areas, dry running | High flow rates, very high pressure |
| Progressive cavity | Helical rotor turns inside an elastomeric stator; cavities progress along the axis | Up to 60+ bar | Low to moderate (typical up to ~500 m³/h) | Slurries, sludge, shear-sensitive products | High speed, clean solvents |
| Peristaltic | Rotating rollers squeeze a hose, pushing fluid ahead of the roller | Up to ~15 bar | Low (typical up to ~80 m³/h) | Corrosive fluids, metering, dry running | High flow, continuous heavy duty |
Duty guidance on viscosity, shear sensitivity and solids content is also covered in the Pump Selection Guide. For low-viscosity clean fluids at high flow and low pressure, review the Centrifugal Pump Fundamentals before committing to a PD pump.
Key Sizing Considerations for PD Pumps
- Slip factor: Internal leakage (clearances) reduces net flow as pressure increases — more significant at low viscosity
- Viscosity correction: At very high viscosity, flow decreases and power increases; must derate speed
- NPSH: PD pumps generally have lower NPSHr than centrifugal, but suction line losses still matter
- Pulsation dampening: Most PD pumps produce flow pulsations that require dampeners on both suction and discharge
- Relief valve: Mandatory on discharge to protect against overpressure
- Speed reduction: For high viscosity, run slower (consult viscosity vs speed curves)
PD Pump Applications by Industry
| Industry | Typical Duty | Preferred PD Pump |
|---|---|---|
| Oil & gas / fuel transfer | Fuel oil, crude transfer, lubrication | Screw, gear, three-screw |
| Chemical processing | Dosing, corrosive chemicals, metering | Diaphragm, peristaltic, metering |
| Food & beverage | Sanitary transfer, CIP, shear-sensitive products | Lobe, peristaltic, progressive cavity |
| Water & wastewater | Sludge, slurry, polymer dosing | Progressive cavity, diaphragm |
| Pharmaceutical | Sterile filling, precise dosing | Peristaltic, diaphragm metering |
| Marine | Lube oil, fuel transfer, bilge | Gear, screw, piston |
| Hydraulics | High-pressure hydraulic power | Gear, piston (axial) |
| Pulp & paper | Coating, starch, high-solids slurries | Progressive cavity, lobe |
| Construction | Grouting, concrete pumping | Piston, progressive cavity |
| Mining | Abrasive slurry transfer | Peristaltic, heavy-duty diaphragm |
When the pumped fluid is viscous, shear-sensitive, solids-laden, or must be metered accurately, a PD pump is usually the right family. For clean low-viscosity fluids at high flow and low pressure, a centrifugal pump is often more economical. The full decision procedure is covered in the Pump Selection Guide, and the NPSH Calculator should be used to verify suction conditions for any PD installation.
Power Calculation for PD Pumps
To estimate the long-term operating cost of the driver, use the Pump Energy Calculator alongside the power figures below. Field verification of the installed driver is covered in the Pump Installation and Commissioning guide, and the required driver power is estimated with the Pump Power Calculator.
Hydraulic power is similar to centrifugal:
Brake power = hydraulic power / efficiency. PD pump efficiencies:
- Gear pumps: 70-90%
- Progressive cavity: 60-75%
- Screw pumps: 70-85%
- Diaphragm (AODD): 50-70% (and they consume compressed air!)
- Plunger/piston: 80-95%
PD Pump Performance Curves
PD pump performance curves look very different from centrifugal pump curves, and reading them correctly prevents misapplication.
- Flow vs speed: Flow is directly proportional to speed (Q = Vd × n). Doubling RPM roughly doubles flow — this is why variable-speed drives are the standard flow-control method for PD pumps. Explore this relationship with the Pump Affinity Law Calculator.
- Flow vs pressure: Flow is nearly flat against discharge pressure. Only internal slip reduces flow slightly as pressure rises, and slip is larger for low-viscosity fluids and worn clearances. Unlike a centrifugal pump curve, there is no drooping head-flow curve.
- Power vs pressure: Shaft power rises roughly linearly with discharge pressure because the pump is always displacing the same volume against system resistance. A pump selected for 10 bar can overload its motor if the relief valve is set at 15 bar — always size the driver for the relief valve setting.
- Viscosity effect: Higher viscosity increases internal resistance and shaft power, and normally requires derating speed. Most PD pump manufacturers publish viscosity-versus-speed correction curves.
- NPSH behavior: NPSH required is relatively constant across the operating range rather than rising steeply with flow, but suction line losses and fluid vapor pressure still govern the available NPSH.
Interactive visualisation of the flow-head-power operating point is provided in the Pump Flow Calculator and Pump Power Calculator performance curve modules.
PD Pump Troubleshooting
Work through the Pump Installation and Commissioning guide for site procedures. The table below covers the most common field faults for PD pumps.
| Problem | Possible causes | Recommended action |
|---|---|---|
| Flow too low | Low RPM; excessive internal leakage/slip; low volumetric efficiency; high fluid viscosity; suction restriction; worn pump components | Verify drive speed; check relief valve bypass; inspect clearances/rotor wear; confirm viscosity and speed derating; check suction strainer and line |
| Pump overheating | High discharge pressure; relief valve set too high; excessive speed; internal friction from high viscosity; insufficient cooling | Reset relief valve to duty; reduce speed for viscosity; check cooling flow; verify motor loading |
| Excessive noise | Cavitation (low NPSH); gas entrainment; worn bearings or gears; misalignment; pulsation resonance | Improve suction conditions; use NPSH Calculator to verify margin; bleed gas; inspect bearings and alignment; fit pulsation dampener |
| Excessive vibration | Cavitation; mechanical imbalance; worn internal parts; foundation/pipe strain; pulsation | Check NPSH margin; balance rotating parts; relieve pipe strain; install dampeners and flexible connections |
| Pressure too high | Blocked discharge; relief valve stuck closed; wrong relief valve setting | Open isolation valve; service relief valve; reset to system design pressure |
| Motor overload | Relief valve setting above driver rating; viscosity higher than design; mechanical seizure; low voltage | Set relief valve within motor rating; re-check viscosity derating; inspect for seizure; check supply voltage |
| Pump losing prime | Suction lift too high; air leak in suction line; clogged strainer; running dry | Check suction lift and NPSH; tighten suction joints; clean strainer; use foot valve; never run dry on lubrication-dependent types |
| Leakage | Worn mechanical seal or packing; corroded joints; overpressure | Replace seal; re-torque joints; verify relief valve setting |
| Pulsating flow | Reciprocating action (piston/plunger/diaphragm); air in system; worn check valves | Fit pulsation dampener; bleed air; inspect and replace check valves |
Frequently Asked Questions
What is a positive displacement pump? A positive displacement pump traps a fixed volume of fluid and mechanically forces it into the discharge pipe. Unlike centrifugal pumps, flow is produced by volume displacement rather than velocity, so flow stays roughly constant as discharge pressure changes.
Which is better, positive displacement or centrifugal pump? It depends on the duty. PD pumps are better for high viscosity, high pressure, shear-sensitive fluids, solids, self-priming, and accurate metering. Centrifugal pumps are better for high flow rates, clean low-viscosity fluids, and lower pressures where their smooth continuous flow and low cost per unit flow win.
How do you calculate positive displacement pump flow rate? Q = Vd × n − Qslip, where Vd is the displaced volume per revolution, n is speed, and Qslip is internal leakage. Flow is proportional to speed, so variable-speed drives are the standard way to vary PD pump flow.
Why must a relief valve be installed on a PD pump? A PD pump keeps displacing fluid even when the discharge is closed, so pressure rises until something fails. A pressure relief valve on the discharge (or an internal relief) protects the pump, pipework, and driver.
Can positive displacement pumps run dry? Only some types — air-operated diaphragm, peristaltic, and some lobe designs tolerate short dry running. Gear, screw, progressive cavity, and piston pumps require liquid lubrication and will be damaged quickly if run dry.
What viscosity range do PD pumps handle? PD pumps handle up to 1,000,000+ cSt (e.g., screw and progressive cavity pumps), versus roughly 500 cP practical limit for most centrifugal pumps. High viscosity normally requires derating pump speed.
Related Engineering Resources
- Pump Power Calculator — hydraulic power and motor sizing for PD pumps
- Pump Head Calculator — total dynamic head for viscous and high-pressure duty
- Pump Flow Calculator — volumetric flow and displacement rate
- Pump Efficiency Calculator — volumetric and mechanical efficiency
- Pump Sizing Calculator — complete pump selection workflow
- Pump Selection Calculator — quantitative PD vs centrifugal recommendation
- Pump Energy Calculator — operating cost of the driver
- NPSH Calculator — net positive suction head margin
- Pump Affinity Law Calculator — speed and impeller scaling
- Pressure Unit Converter — bar / psi / MPa conversion
- Pump Selection Guide — PD vs centrifugal selection criteria
- Pump Installation and Commissioning — site procedures and troubleshooting
- Centrifugal Pump Fundamentals — the complementary pump class
Summary
Positive displacement pumps excel at high viscosity, high pressure, solids handling, and metering where centrifugal pumps cannot operate. Gear pumps for clean oils, progressive cavity for slurries, AODD for versatility/safety, peristaltic for chemical isolation, plunger for ultra-high pressure. Always include a pressure relief valve on PD pump discharge — this is not optional.