Troubleshooting guide

Why Is a Pump Delivering Low Flow or Making Noise?

Cavitation, air in the line, a blocked suction, a worn impeller, wrong rotation and system changes: how to separate the causes from the symptoms.

Mechanical, HVAC & Fluids · Last reviewed

A centrifugal pump runs where its curve meets the system curve. Low flow means either the pump is not producing its curve, or the system has more resistance than expected. Gravel-like noise points toward cavitation.

Pumps involve rotating machinery, pressure and sometimes hot or hazardous fluids. Isolate, lock out and depressurize before opening a pump or its piping, and follow the site safety procedures.

SymptomLikely causeWhat to check
Crackling or gravel noise, falling flowCavitation: NPSH available is below NPSH required, so vapor bubbles form and collapseSuction pressure, fluid temperature, strainer, suction pipe size, and valve position
No flow, motor runsNot primed, air-bound, or wrong rotationPrime the pump and check the direction of rotation against the arrow
Low flow and low pressureWorn impeller, wear ring clearance, or a partly blocked impellerCompare the measured head and flow with the pump curve
Low flow and high pressureSystem resistance higher than designed: a partly closed valve, a clogged strainer or fouled coilValve positions, pressure drop across strainers and heat exchangers
Flow varies, pulsingAir entrainment, a vortex at the suction, or low liquid levelSuction pipe leaks, tank level, and any air in the line
High current and heatRunning far to the right of the curve (too much flow), or a bearing or alignment problemMotor current against nameplate, discharge valve position, alignment
VibrationMisalignment, cavitation, bent shaft, worn bearing or unbalanced impellerAlignment, base bolts, vibration spectrum

The cavitation check

Cavitation starts when NPSH available (NPSHa) falls below NPSH required (NPSHr), the value on the pump curve.

NPSHa = (P_suction_abs − P_vapor) ÷ (ρ g) + static head − friction loss

Example: open tank at 14.7 psia, water at 20 °C (vapor pressure about 0.34 psia), liquid level 2 m above the pump, friction loss 0.5 m. NPSHa = (101.3 − 2.3) kPa ÷ 9.81 kPa/m + 2 − 0.5 = 10.1 + 1.5 = 11.6 m. If the pump needs 4 m, there is a good margin. At 70 °C the vapor pressure is about 31 kPa and NPSHa falls to about 8.8 m. At 90 °C the vapor pressure is about 70 kPa and NPSHa falls to about 4.8 m, which is barely above the 4 m the pump needs.

Write a useful problem statement

  • Pump model, speed, impeller diameter and the design duty point.
  • Measured suction and discharge pressure, flow if known, motor current.
  • Fluid, temperature and tank level.
  • What changed: valves, strainers, fluid, piping, speed.

A second example: speed instead of a valve

The affinity laws describe how a centrifugal pump responds to a change in speed: flow varies with speed, head with the square of speed, and power with the cube.

  1. A pump is slowed by 10%, to 90% of its speed.
  2. Flow = 0.90 of the original. Head = 0.90² = 0.81 of the original. Power = 0.90³ = 0.73 of the original.
  3. Throttling a valve to get 90% of the flow does not cut power by the same amount, because the pump still runs at full speed against the added valve loss.

Preventive checks

  • Check the strainer pressure drop and clean it on a schedule.
  • Record suction and discharge pressure and motor current at a known operating point, to compare later with the pump curve.
  • Check alignment, seals and bearing temperature during routine rounds.
  • Keep the pump away from operating far to the left or the right of its best-efficiency flow.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

More troubleshooting

Questions

Why does a hot fluid cause cavitation?

Its vapor pressure is higher, so the margin above vapor pressure at the suction shrinks, and NPSHa falls.

Can I fix cavitation by throttling the suction?

No. Throttling the suction lowers the suction pressure and makes it worse. Throttle on the discharge side, or raise NPSHa.

What is the minimum continuous flow?

The lowest flow at which the pump can run continuously without damage from heating or recirculation. The pump data sheet gives it.

How do I know the impeller is worn?

Measured head at a given flow drops below the pump curve, and the motor current at the same flow is often lower than before. Compare against recorded baseline readings.

What is a vortex at the suction?

A swirling air-drawing funnel that forms when the liquid level above the suction is too low. It lets air into the pump and gives noise and unstable flow.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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