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.
| 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
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
| 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 |
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)
Power Calculation for PD Pumps
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%
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.