8.1 Pumps and Drivers: Centrifugal and Positive Displacement
Key Takeaways
- A centrifugal pump develops head rather than pressure, so the same pump produces the same head in feet regardless of the fluid's specific gravity.
- The affinity laws state that flow varies directly with speed, head varies with the square of speed, and power varies with the cube of speed.
- A positive displacement pump must never be operated against a closed discharge, which is why a relief valve is required in its discharge line.
- Cavitation occurs when the available net positive suction head falls below the pump's required NPSH and the liquid flashes to vapor at the impeller eye.
- Running a centrifugal pump far off its best efficiency point drives up radial load on the shaft and shortens bearing and seal life.
Two ways to move a liquid
Domain 7, Pumps, Drives, and Plates, contributes 11 of 115 items and references 32108 Pumps and Drivers, 32305 Setting Baseplates and Prealignment, and 32307 Installing Belt and Chain Drives.
Every pump falls into one of two families:
- A centrifugal (kinetic) pump accelerates liquid outward with a spinning impeller and then converts that velocity into pressure in a volute or diffuser. Flow depends on the system it is pumping into: raise the discharge resistance and the flow falls.
- A positive displacement (PD) pump traps a fixed volume and forces it out on every cycle. Flow is nearly constant regardless of discharge pressure: raise the resistance and the pressure rises until something gives.
That difference drives the single most important operating rule in this module.
| Property | Centrifugal | Positive displacement |
|---|---|---|
| Flow versus pressure | Flow falls as head rises | Flow essentially constant |
| Closed discharge | Recirculates and overheats; survivable briefly | Pressure rises until a component fails — a relief valve is mandatory |
| Self-priming | Generally no | Generally yes |
| Viscous liquids | Performance drops sharply | Handles viscosity well |
| Flow character | Smooth | Often pulsating |
| Throttling | Discharge throttling is acceptable | Never throttle the discharge |
Centrifugal pump construction
| Part | Function |
|---|---|
| Impeller | Adds velocity energy; open, semi-open, or enclosed |
| Volute or diffuser | Converts velocity to pressure |
| Casing | Contains pressure; may be split axially or radially |
| Wear rings | Sacrificial close-clearance rings that limit recirculation from discharge back to suction |
| Shaft and sleeve | Transmits torque; the sleeve protects the shaft in the seal area |
| Stuffing box or seal chamber | Houses packing or the mechanical seal |
| Bearing housing | Carries the radial and thrust bearings |
| Wear plate / back plate | Sets impeller clearance on some designs |
Head, not pressure
A centrifugal pump imparts a certain velocity to the liquid, and the resulting head in feet depends only on impeller diameter and speed — not on the fluid's density. The same pump raises water and gasoline to the same height. What changes is the pressure produced:
where pressure is in psi, head is in feet, and SG is specific gravity. A pump producing 200 feet of head makes about 87 psi on water (SG 1.0) and about 62 psi on gasoline (SG 0.72).
Affinity laws
For a centrifugal pump, changing the speed changes three things at different rates:
| Quantity | Relationship |
|---|---|
| Flow (Q) | Varies directly with speed |
| Head (H) | Varies with the square of speed |
| Power (P) | Varies with the cube of speed |
So raising the speed 10 percent raises flow 10 percent, head about 21 percent, and required power about 33 percent. That cube relationship is why a small speed increase can overload a motor, and why speed reduction saves so much energy.
NPSH and cavitation
Net positive suction head available (NPSHa) is the margin, in feet of liquid, by which the pressure at the pump suction exceeds the vapor pressure of the liquid. NPSH required (NPSHr) is what the pump needs to fill its impeller eye without flashing. The rule:
NPSHa must exceed NPSHr, with a margin.
When it does not, liquid flashes to vapor at the low-pressure side of the impeller vane, the bubbles are carried into the higher-pressure region, and they collapse violently against the impeller. The result is the classic symptoms:
- A sound "like pumping gravel."
- Erratic discharge pressure and falling flow.
- Pitting damage on the leading face of the impeller vanes.
- High vibration and rapid seal and bearing failure.
Causes to check in order: suction strainer or filter plugged, suction valve partly closed, suction level too low, liquid too hot (vapor pressure too high), suction line too small or too long, or air leaking into the suction.
Cavitation is not the same as air entrainment. Cavitation is vapor formed from the pumped liquid; entrainment is air drawn in through a leak or vortex. Both sound rough, but the fixes differ.
Positive displacement types
| Family | Examples | Character |
|---|---|---|
| Rotary | Gear, vane, lobe, screw, progressive cavity, peristaltic | Continuous, relatively smooth flow; good with viscous fluids |
| Reciprocating | Piston, plunger, diaphragm | High pressure; pulsating flow needing a pulsation dampener |
A progressive cavity pump handles shear-sensitive and solids-laden fluids but must never run dry — the stator burns out in seconds. A gear pump is the standard for lube oil service. A diaphragm pump isolates the drive from the fluid entirely, so it suits corrosives and slurries.
Drivers
| Driver | Notes |
|---|---|
| Electric motor | The default; constant speed unless driven by an adjustable speed drive |
| Steam turbine | Common on critical pumps so they continue running on a power failure |
| Internal combustion engine | Remote sites and emergency fire pumps |
| Gear or belt drive between driver and pump | Where the pump speed must differ from the driver speed |
Whatever the driver, the mechanic's responsibilities are the same: correct rotation direction before the first start, correct coupling and guard, correct alignment, and correct lubrication.
Why must a positive displacement pump have a relief valve in its discharge line?
A centrifugal pump running at 1,750 rpm draws 20 hp. If the speed is increased to 2,100 rpm, approximately what power will it require?
A centrifugal pump sounds like it is pumping gravel, discharge pressure fluctuates, and the impeller vanes show pitting on their leading faces. What is happening?