Centrifugal Pumps
Centrifugal pumps use a rotating impeller to impart kinetic energy to the fluid. This kinetic energy converts to pressure as fluid decelerates in the volute casing. Flow rate is determined by the balance between the pump curve (head vs flow characteristic) and the system resistance curve.
Key characteristics:
- Flow varies with system head — more resistance = less flow
- Efficiency peaks at best efficiency point (BEP) — running far from BEP causes heat, vibration, and seal damage
- Performance drops significantly with high-viscosity liquids (above ~500 cSt)
- Cannot self-prime (most designs) — requires flooded suction or priming
- Can run dry briefly before seal and impeller damage occurs
Positive Displacement Pumps
PD pumps mechanically trap a fixed volume of fluid per stroke or revolution and force it through the outlet. Types include gear pumps, lobe pumps, peristaltic (hose) pumps, piston pumps, and diaphragm pumps.
Key characteristics:
- Flow is nearly constant regardless of discharge pressure — flow is set by pump speed only
- Handles high-viscosity fluids well — efficiency often improves with viscosity (less internal slippage)
- Suitable for accurate dosing and metering
- Can self-prime and handle air in suction line
- Cannot be deadheaded — blocking the outlet builds pressure until something fails catastrophically. Always install a pressure relief valve.
Side-by-Side Comparison
| Parameter | Centrifugal | Positive Displacement |
|---|---|---|
| Flow vs pressure | Flow decreases as head increases | Constant flow regardless of pressure |
| Typical viscosity range | 0.5–500 cSt | 1 cSt to >1,000,000 cSt |
| Pulsation | None (continuous flow) | Pulsed (especially reciprocating types) |
| Shear-sensitive fluids | High shear — not suitable for polymers, blood, slurries | Gentle — suitable for polymers, biological fluids |
| Deadhead safety | Safe to deadhead (overheats slowly) | Dangerous — must have relief valve |
| Self-priming | Generally no | Generally yes |
| Maintenance | Lower (fewer wearing parts) | Higher (gears, rotors, diaphragms, seals) |
| Typical applications | Water supply, cooling, fire fighting, chemical transfer | Lube oil, food processing, polymer dosing, high-pressure hydraulics |
Viscosity and Pump Selection
Viscosity is often the deciding factor. Centrifugal pumps lose efficiency rapidly above 200–500 cSt as viscous drag increases. The Hydraulic Institute provides viscosity correction factors — at 1,000 cSt, a centrifugal pump may deliver only 60% of its water duty efficiency. PD pumps (especially gear and lobe types) are designed for high-viscosity service and maintain good efficiency up to millions of cSt.
Quick Selection Guide
| Application | Recommended type |
|---|---|
| Water supply, cooling towers, fire systems | Centrifugal |
| High-viscosity oil transfer (>200 cSt) | Gear pump (PD) |
| Chemical dosing / metering | Diaphragm or peristaltic (PD) |
| Slurry or abrasive service | Centrifugal (rubber-lined) or peristaltic |
| High-pressure hydraulic service | Piston pump (PD) |
| Food and beverage (hygienic) | Lobe pump (PD) |
| Shear-sensitive fluids | Peristaltic or lobe pump (PD) |