12.1 Pumps: Types, Curves & Cavitation
Key Takeaways
- Centrifugal pumps add velocity that is converted to pressure and deliver variable flow against varying head, while positive displacement pumps deliver a fixed volume per stroke regardless of pressure.
- Total dynamic head is the sum of static head, friction head, and velocity head, and the operating point is where the pump curve intersects the system curve.
- Cavitation occurs when suction pressure falls below the vapor pressure of the liquid, forming and collapsing vapor bubbles that pit the impeller.
- Net positive suction head available must exceed net positive suction head required, with a safety margin, or the pump will cavitate.
- A positive displacement pump discharge must never be valved closed, so a relief valve is mandatory on the discharge side.
12.1 Pumps: Types, Curves & Cavitation
The WPI Need-to-Know Criteria weight Equipment Operation and Maintenance at 20 to 26 questions on the water treatment exam and 28 to 39 questions on the wastewater exam, where it is essentially tied with process control for the largest area. Pumps are the largest single slice of that. This is not optional material.
1. The Two Families
| Centrifugal (kinetic) | Positive displacement | |
|---|---|---|
| Principle | An impeller imparts velocity, which the volute or diffuser converts to pressure | A fixed volume is captured and displaced each stroke or revolution |
| Flow vs. head | Flow varies with system head — a rising head reduces flow | Flow is essentially constant regardless of head |
| Discharge valve closed | Survives briefly (churn), but overheats | Catastrophic — pressure builds until something bursts |
| Self-priming | Generally no — must be primed | Yes, generally |
| Best for | High flow, low to moderate head, clean or solids-bearing liquid | Metering, viscous fluids, sludge, high pressure |
| Examples | End suction, split case, vertical turbine, submersible, non-clog | Progressive cavity, plunger, diaphragm, peristaltic, rotary lobe, piston |
Centrifugal types you must recognize
| Type | Application |
|---|---|
| End suction | General service, small to medium |
| Horizontal split case | High-flow raw and finished water, high-service pumping |
| Vertical turbine | Deep wells and wet pit applications; multiple stages stack head |
| Submersible | Lift stations, wells; motor is submerged and cooled by the liquid |
| Non-clog / solids-handling | Raw wastewater and sludge; large, unobstructed passages |
| Recessed impeller (vortex) | Grit and stringy solids; impeller sits out of the flow path |
Positive displacement types you must recognize
| Type | Application |
|---|---|
| Progressive cavity | Thick sludge, polymer; gentle, steady flow. Never run dry — the stator burns up in seconds |
| Diaphragm / metering | Chemical feed; precise, adjustable stroke length and frequency |
| Peristaltic (hose) | Chemical and polymer; liquid touches only the tube |
| Plunger / piston | High pressure, lime slurry |
| Rotary lobe | Sludge transfer, blowers |
2. Head — The Language of Pumping
| Term | Definition |
|---|---|
| Static suction head/lift | Vertical distance from the supply liquid surface to the pump centerline. Head if the supply is above the pump; lift if below |
| Static discharge head | Vertical distance from pump centerline to the discharge liquid surface |
| Total static head | Discharge surface elevation minus supply surface elevation |
| Friction head | Head lost to pipe wall friction and fittings — rises with about the square of flow |
| Velocity head | Energy in the moving water, $v^2/2g$; usually small |
| Total dynamic head (TDH) | Total static head + friction head + velocity head — what the pump must actually produce |
Worked example. A pump lifts water 12 ft from a wet well up to the pump, then discharges to a tank whose surface is 96 ft above the pump. Friction losses total 18 ft.
3. Pump and System Curves
- The pump curve plots head the pump can produce against flow. For a centrifugal pump it slopes downward to the right — more flow, less head.
- The system curve plots head the system requires against flow. It starts at the total static head at zero flow and rises steeply as friction grows with the square of flow.
- The operating point is where the two curves intersect. The pump will run there and nowhere else.
What moves the operating point
| Change | Effect |
|---|---|
| Throttling the discharge valve | Steepens the system curve; operating point moves left — less flow, more head |
| Tank level rises | Raises static head; system curve shifts up; flow decreases |
| Pipe tuberculates | Increases friction; flow decreases at the same pump speed |
| Slowing the pump (VFD) | Lowers the pump curve; flow decreases with far less energy waste than throttling |
Best efficiency point (BEP)
Every centrifugal pump has a best efficiency point on its curve. Operating far from BEP causes radial thrust, shaft deflection, vibration, and premature bearing and seal failure. Two specific errors:
- Running far left of BEP (excessive throttling or a closed system) — recirculation, heating, vibration, and eventually the pump churns and boils.
- "Running off the end of the curve" — flow far right of BEP, typically when discharge head is much lower than design. The motor overloads and trips because power demand rises with flow.
4. Cavitation — the classic exam topic
Cavitation occurs when pressure at the pump suction (specifically at the impeller eye) drops below the vapor pressure of the liquid. Water flashes to vapor, forming bubbles that are carried into the higher-pressure region of the impeller where they collapse violently.
Symptoms
- A sound like gravel or marbles rattling inside the pump
- Vibration, fluctuating discharge pressure, erratic flow
- Pitting erosion of the impeller vanes and volute — the metal is literally hammered away
- Loss of capacity and efficiency; eventual bearing and seal failure
Net Positive Suction Head
NPSH available (NPSH_A) is what the system delivers at the suction. NPSH required (NPSH_R) is what the pump needs, published by the manufacturer and rising with flow. NPSH_A must exceed NPSH_R with a safety margin, commonly 2–5 ft.
What lowers NPSH available — and therefore causes cavitation
| Cause | Mechanism |
|---|---|
| Clogged suction screen or strainer | Adds suction-side friction loss. The single most common field cause |
| Partially closed suction valve | Same. Never throttle on the suction side |
| Excessive static suction lift | Wet well level too low |
| Long, undersized, or convoluted suction piping | Friction loss |
| High liquid temperature | Raises vapor pressure, so cavitation begins at a higher absolute pressure |
| High altitude | Lower atmospheric pressure |
| Running far right of BEP | NPSH_R rises with flow |
The operating rule that follows: throttle on the discharge, never on the suction. Discharge throttling raises head and is merely inefficient; suction throttling destroys NPSH and cavitates the pump.
5. Priming and Air Binding
A centrifugal pump cannot pump air. If the casing is not full of liquid, it loses prime and moves nothing.
| Condition | Cause | Fix |
|---|---|---|
| Loss of prime | Air leak in suction piping or packing, failed foot valve, wet well level below the suction bell, vortexing | Repair the leak, verify foot valve, raise level, install anti-vortex baffle |
| Air binding | Air trapped at a high point in the casing or suction line | Vent the casing; eliminate suction-line high points; slope suction piping upward toward the pump with no high points |
| Vortexing | Insufficient submergence over the suction bell | Increase submergence, add a vortex breaker |
Self-priming pumps and pumps with vacuum priming systems handle this automatically; conventional pumps require a flooded suction or a foot valve.
6. Pump Troubleshooting Table
| Symptom | Likely causes |
|---|---|
| No flow, pump running | Lost prime, air bound, suction valve closed, impeller clogged, wrong rotation, discharge valve closed |
| Reduced flow | Clogged impeller or suction, worn wear rings, higher system head, partially closed valve, low speed, air entrainment |
| Rattling / gravel sound | Cavitation (see above) |
| Motor overload trips | Running off the end of the curve, higher specific gravity or viscosity than design, mechanical binding, bearing failure, low voltage or single phasing |
| Excessive vibration | Misalignment, unbalanced or clogged impeller, worn bearings, cavitation, operating far from BEP, loose baseplate or resonance |
| Overheated bearings | Over- or under-lubrication, misalignment, excessive belt tension, contaminated grease |
| Packing leaking excessively | Worn packing, scored shaft sleeve, over-tightened gland (which burns packing and scores the sleeve) |
| Mechanical seal failure | Ran dry, abrasives, misalignment, thermal shock, wrong seal for the service |
Packing note: stuffing box packing is supposed to drip — typically 20 to 60 drops per minute — because that leakage lubricates and cools the packing and shaft sleeve. An operator who tightens the gland to stop the drip destroys the packing and scores the sleeve. Mechanical seals, by contrast, should not visibly leak.
A centrifugal pump produces a sound like gravel rattling inside the casing, with vibration and fluctuating discharge pressure. What is occurring and what is the most common field cause?
A pump lifts water 10 ft from a wet well, discharges to a tank 85 ft above the pump centerline, and friction losses total 22 ft. What is the total dynamic head?
Why must a positive displacement pump never be operated against a closed discharge valve?
Stuffing box packing on a raw water pump is dripping about 40 drops per minute. What should the operator do?
A centrifugal pump's motor trips on overload after the discharge head drops well below design. What is the explanation?