Skip to main content
Back to module
Mechanical Systems & Maintenance · Fluid Power

Pumps and compressors

Reading46mLesson 3 of 3 in this module

Compare centrifugal and positive displacement machines and diagnose common pump problems such as cavitation.

Why it matters

Pumps move water, chemicals, coolant and product; compressors make the plant's air. Together they are some of the most maintained machines in industry. Many pump "failures" are really system problems: a clogged strainer, a closed valve, a tank run low. A good mechanic diagnoses the system before tearing the pump apart.

Two families of pumps

Centrifugal pumps spin an impeller that flings liquid outward into a casing (the volute), converting speed into pressure. They are the most common type. Their flow depends on the system: as the discharge is throttled, flow falls and pressure (head) rises along the pump curve. They should not run for long against a closed discharge valve, because the trapped liquid heats up.

Positive displacement (PD) pumps trap a fixed volume and push it out: gear, lobe, vane, diaphragm, piston and progressive cavity pumps. Flow stays nearly constant whatever the pressure, so a PD pump against a closed valve keeps building pressure until something breaks. That is why PD pumps need a relief valve.

Compressors split the same way: dynamic (centrifugal) and positive displacement (reciprocating piston, rotary screw, scroll).

Key terms

  • Head: pressure expressed as the height of liquid the pump could lift, in feet or meters. Pump curves use head because it does not depend on the liquid's density.
  • Best efficiency point (BEP): the flow where the pump runs most efficiently. Running far from BEP raises vibration, heat and bearing and seal wear.
  • NPSH (net positive suction head): the margin of pressure at the pump inlet above the liquid's vapor pressure. The system provides NPSHa (available); the pump needs NPSHr (required). If NPSHa falls below NPSHr, the pump cavitates.
  • Affinity laws for centrifugal pumps: flow is proportional to speed, head to speed squared, and power to speed cubed. Slowing a pump by 20 percent cuts power to about 0.8³ ≈ 51 percent, which is why variable speed drives save so much energy.

Cavitation

When inlet pressure drops too low, vapor bubbles form in the liquid and then collapse violently as they reach higher pressure inside the impeller. It sounds like the pump is pumping gravel. Over time it pits and erodes the impeller. Common causes:

  • Clogged suction strainer or filter.
  • Partly closed suction valve (suction valves should be fully open; throttle on the discharge).
  • Low level in the supply tank, or the tank too far below the pump.
  • Hot liquid (higher vapor pressure).
  • Running far to the right of the pump curve (too much flow).

Seals

Shaft seals keep liquid in where the shaft leaves the casing. Packing is meant to drip slightly to cool and lubricate itself. Mechanical seals should not visibly leak. Running a mechanical seal dry, even for seconds, can destroy it, and misalignment and vibration shorten seal life.

Worked example: diagnosing a pump

A coolant pump's flow has dropped and it sounds rough. Work from the outside in:

  1. Check the system first. Is the tank level normal? Are suction and discharge valves in the right positions? Is the suction strainer clean?
  2. Read the gauges. Low suction pressure points to an inlet restriction. Normal suction with low discharge pressure points to a worn impeller, wrong rotation or air in the pump.
  3. Check rotation. A centrifugal pump running backward (after motor wiring work) still pumps, but poorly. Compare with the arrow on the casing.
  4. Listen and feel safely. Gravel noise suggests cavitation; a steady rumble suggests bearing trouble.
  5. Look for leaks at the seal and for heat at bearings.
  6. Compare with records. The CMMS history may show the same fault six months ago.

Only after the system checks out do you lock out and open the pump.

Common symptom table

Symptom                     Likely causes
No flow                     Not primed, closed valve, wrong rotation, blocked suction
Low flow / low pressure     Worn impeller or wear rings, air leak at suction, speed low
Gravel noise                Cavitation: starved suction, hot liquid, strainer blocked
High vibration              Misalignment, cavitation, bent shaft, worn bearings
Seal leaking                Seal run dry, misalignment, worn seal faces
Motor overloads             Too much flow (running off the curve), thick liquid, binding

Common mistakes

  • Throttling the suction valve to reduce flow, which causes cavitation.
  • Running a pump dry to "check rotation".
  • Starting a PD pump against a closed discharge without a working relief.
  • Replacing the impeller without fixing the low tank level that caused the cavitation.

Safety

Pumps can hold hot, pressurized or hazardous liquids even when stopped. Lockout includes isolating and draining the pump and following the chemical's safety data sheet. Compressors restart automatically and store energy in receivers. Your employer's training and permits are required before you work on either.

Recap

  • Centrifugal pumps follow a curve; PD pumps give nearly constant flow and need relief protection.
  • Cavitation happens when NPSH available drops below NPSH required.
  • Diagnose the system (levels, valves, strainers, gauges, rotation) before the pump.

Try it

Open the Do tab to work through a pump diagnosis on paper.

Also in this lesson: