Centrifugal vs Positive Displacement Pumps: Head vs Flow Curves, Viscosity & Efficiency
Centrifugal pumps transfer energy dynamically via rotating impellers converting velocity to pressure head, causing flow rate to drop steeply as discharge pressure increases. Positive displacement pumps trap fixed fluid cavities and discharge them mechanically, maintaining constant flow regardless of system head pressure.
Technical Comparison & Conversion Matrix
| Operational Parameter | Centrifugal Impeller Pumps | Positive Displacement Pumps (Gear/Piston/Screw) |
|---|---|---|
| Operating Principle | Dynamic kinetic acceleration converted to static head in volute | Mechanical entrapment and positive boundary displacement of fluid |
| Head vs Flow Relationship | Flow decreases significantly as system head pressure rises | Flow remains virtually constant across wide discharge pressure fluctuations |
| Viscosity Performance | Efficiency plunges rapidly above 100 cSt (viscous drag on impellers) | Efficiency actually increases with higher fluid viscosity (lower slip) |
| Discharge Shut-Off Behavior | Pressure deadheads at maximum shut-off head without rupture | Pressure escalates toward mechanical destruction (requires safety relief valve) |
| Self-Priming Capability | Non-self-priming (requires flooded suction or foot check valve) | Naturally self-priming (can evacuate air and lift suction dry) |
| Output Flow Characteristics | Smooth, continuous, pulse-free discharge stream | Pulsating flow requiring pulsation dampeners on reciprocating models |
| Primary Industrial Applications | Municipal water supply, HVAC hydronics, chemical transfer, fire protection | Hydraulic power, crude oil, metering pumps, high-viscosity sludge transfer |
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Frequently Asked Questions
What happens if a centrifugal pump discharge valve is closed while running?
The pump operates at shut-off head. Flow drops to zero, and power consumption drops to its minimum. However, fluid churning inside the casing generates friction heat that can boil the liquid and destroy mechanical shaft seals if sustained.
Why must every positive displacement pump have an external pressure relief valve?
Unlike centrifugal pumps, a positive displacement pump continues pushing fluid into the discharge pipe regardless of resistance. Closing a discharge valve causes instantaneous pressure spikes that will burst pipework, crack pump casings, or stall the drive motor.