Cavitation in a river lift pump house that ran perfectly in the monsoon
A river lift scheme pumps 90 L/s against a static lift of 38 m through a 400 mm delivery main, 1.2 km long. The pump selected at the duty point runs at 1,450 rpm and has an NPSHR of 6.5 m. The pump axis has been fixed 4.2 m above the lowest river level, which occurs in April–May. Design conditions: water temperature 30 °C (vapour pressure 4.25 kPa), site elevation 600 m (atmospheric pressure about 10.1 m of water). The suction line is 250 mm diameter, 8 m long, with a foot valve with strainer and two bends; Hazen–Williams C = 130 and the sum of minor loss coefficients is K = 6.0. Commissioning took place in June with the river 2.5 m above its lowest level and the unit ran smoothly. In the following April the unit developed rattling noise, vibration and falling discharge, and the mechanical seal failed twice. The contractor states that the pump is defective and must be replaced under warranty.
1. Compute the NPSH available at the worst condition and decide whether the contractor's explanation is tenable.
Read the worked answer
Step 1 - suction velocity and velocity head A = π/4 × 0.25² = 0.0491 m²; V = 0.09/0.0491 = 1.83 m/s; V²/2g = 0.171 m.
Step 2 - friction loss (Hazen–Williams, in SI units) hf = 10.67 L Q^1.852/(C^1.852 D^4.87) Q^1.852 = 0.09^1.852 = 0.01157; C^1.852 = 130^1.852 = 8,218; D^4.87 = 0.25^4.87 = 0.001166 hf = 10.67 × 8 × 0.01157/(8,218 × 0.001166) = 0.988/9.58 ≈ 0.10 m over the 8 m length (≈ 12.9 m per 1,000 m).
Step 3 - minor losses: 6.0 × 0.171 = 1.03 m. Total suction loss = 1.13 m.
Step 4 - NPSHA NPSHA = Ha − Hvap − Hs − hf,suction = 10.1 − 0.43 − 4.2 − 1.13 = 4.34 m (NPSHR 6.5 m, so the deficit is 2.16 m, and about 2.7 m if a 0.5 m margin is added.)
Verdict: the pump is not defective. The system cannot supply the NPSH the pump needs at the lowest river level. In June the water was 2.5 m higher, giving NPSHA ≈ 6.8 m, which is why it ran quietly. The failure is a system/design error: the pump axis was set from a comfortable water level, not from the worst case. The seal failures, noise and falling discharge are classic cavitation symptoms, and they will recur until NPSHA is corrected - replacing the pump with an identical unit will not help.
2. Propose remedial options, quantifying each, and select one.
Read the worked answer
The required NPSHA is 6.5 m plus about 0.5 m margin = 7.0 m, so we must gain about 2.7 m.
Option 1 - enlarge the suction line to 350 mm V = 0.09/0.0962 = 0.94 m/s; V²/2g = 0.045 m. hf/m = 10.67 × 0.01157/(8,218 × 0.35^4.87) = 0.1235/49.5 = 0.0025 m/m → 0.02 m for 8 m; minor = 6 × 0.045 = 0.27 m. Total 0.29 m. NPSHA = 10.1 − 0.43 − 4.2 − 0.29 = 5.18 m - still short of 7.0 m. Useful, but not sufficient alone.
Option 2 - lower the pump axis Each metre lowered adds 1 m of NPSHA. With the existing suction line, we need 4.2 − 2.7 ≈ 1.5 m; with the 350 mm line, only about 4.2 − 1.8 = 1.8 m axis height (a drop of 1.8 m). This means a deeper dry well/pump house floor, with drainage and access, plus a check against flooding.
Option 3 - change the pump type/speed A vertical turbine or can-type pump with the first stage submerged below the lowest water level removes the suction-lift problem entirely; alternatively, NPSHR falls roughly as N² for the same duty, so a 20% speed reduction cuts NPSHR by about 36% - but the impeller must be resized and the duty point re-checked.
Option 4 - operate only above a minimum river level Cheap, but defeats the purpose of a lift scheme in a dry season; acceptable only as a temporary measure with an interlock and level switch.
Selection: for a river with a 2.5 m seasonal swing, the robust solution is a vertical turbine pump with the bowl assembly below the lowest water level, or a dry-well pump house with the floor lowered about 2 m and a 350 mm suction line. Either gives NPSHA ≥ 7.0 m with margin at 30 °C and the lowest river level. Add a low-level interlock and a vacuum/pressure gauge on the suction so the operator can see NPSHA in service.
3. What should have been specified and done differently in design and commissioning? Answer as if advising the department for the next scheme.
Read the worked answer
In the tender/design stage
- State the duty point at the worst case: lowest river level, highest water temperature, maximum flow - and require the manufacturer to certify NPSHR at that duty point, with a copy of the NPSH test curve, not just the best-efficiency point.
- Fix the pump axis from the lowest water level, not the working level; carry a design margin of 0.5–1.0 m.
- Do a proper suction-line design: velocity under 1.2 m/s, short and rising suction line, minimum bends, no unnecessary foot valve loss (a foot valve is often a large, avoidable loss where a priming system can be used).
- Consider the seasonal range explicitly in the civil layout; deep-set pumps or wet wells with adequate submergence should be part of the structural brief.
At commissioning
- Commission and test at the worst level, or at least record NPSHA at the level available and extrapolate; a monsoon commissioning test proves nothing about the dry season.
- Provide and calibrate a compound gauge on the suction and a flow meter; record suction lift, discharge pressure and power at several flows.
- Train operators on cavitation symptoms and set a minimum operating level interlock.
Contractually
- The vendor's guarantee should be against system NPSHA as well as pump performance, so that a dispute about 'defective pump' versus 'inadequate NPSHA' is settled by the design data. In this case the pump met its NPSHR, so the claim against the supplier would fail; the cost falls on the department's design. Also keep the operating records (April discharge and noise) as evidence for the corrective works estimate.