EngiMetric Arena / Diagnose the Failure
Diagnose the gravelly noise at the pump suction
Competency Fault diagnosis primary Quantitative reasoning supporting Constraints and trade-offs supporting
What you should take away: Compute available NPSH from the suction layout and distinguish cavitation from air ingress or mechanical wear.
The situation
What is happening
A pump draws 40 degC water from a sump 3.5 m below the impeller. Suction-pipe friction at the duty flow is 3.0 m and the pump requires 4.0 m NPSH. At high flow the pump emits a gravelly noise, discharge pressure fluctuates and vibration rises. Choose the diagnosis the numbers support.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Water temperature | 40 | degC | Sets the vapour pressure. |
| Suction lift | 3.5 | m | Sump level below the impeller. |
| Suction friction | 3.0 | m | At the duty flow. |
| NPSH required | 4.0 | m | From the pump curve at the duty flow. |
| Atmospheric pressure | 101.3 | kPa | Site atmospheric pressure. |
Assumptions this challenge makes
- The suction lift and friction are stated at the duty flow where the noise occurs.
- The fluid is water at 40 degC with a density of 1000 kg/m3.
- NPSH required is taken from the pump curve, not estimated.
Your call
Your answer
Hints
- Compute the pressure head available at the suction eye. Start with atmospheric head and subtract vapour head, lift and friction.
- Compare NPSH available with NPSH required. A negative margin is a cavitation risk, not a wear diagnosis.
- Match the symptom pattern: gravelly noise, fluctuating discharge and vibration at high flow all point to the suction side.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- At 40 degC the vapour pressure is 7.38 kPa, or 0.752 m of water. Atmospheric head is 10.33 m.
- NPSH available is 10.33 - 0.752 - 3.5 - 3.0 = 3.07 m. The pump requires 4.0 m, so the margin is -0.93 m.
- A negative margin means the pressure at the suction eye can fall to the vapour pressure, forming vapour cavities that collapse as they move into higher pressure. That produces the gravelly noise and vibration described.
- The diagnosis is therefore a suction-side NPSH deficit. Lowering the pump, reducing suction friction or cooling the water would raise the margin; replacing the impeller would not.
- The calculation does not prove that no other fault exists. It does show that this fault is present and must be corrected before the pump can run reliably at this duty.
What this challenge does not establish
- NPSH required is an input from the pump curve. A different curve at the actual speed would change the margin.
- The suction lift and friction are single values at the reported flow; a full system curve would show how the margin varies.
- The model does not assess dissolved-gas effects, entrained air or suction-vessel geometry.
- Cavitation damage and impeller wear require inspection; the calculation only establishes the hydraulic risk.