Cavitation and Overcurrent in a Lift-Irrigation Pump
At a lift irrigation scheme, a horizontal split-case pump is rated for 180 L/s at 45 m total head and is driven by a 110 kW, 415 V, three-phase, 50 Hz induction motor through a flexible coupling. During trial running, the pump delivers only 150 L/s, makes a crackling noise near the suction flange, and shows a vibration level of 7.5 mm/s. The suction pipe is 6.5 m long with a 1.2 m vertical lift, one foot valve, and two 90° bends; the suction strainer is partly choked. The motor draws 195 A against a rated 185 A; line voltages are 395, 402 and 428 V. The pump efficiency is 80% and the motor efficiency is 90% at this load. The operator wants to throttle the discharge valve to reduce current and continue running until the canal rotation ends.
1. Calculate the approximate hydraulic power and motor input power at the measured duty, and comment on whether the motor is genuinely overloaded.
Read the worked answer
Hydraulic power = ρgQH = 1000 × 9.81 × 0.150 × 45 ≈ 66.2 kW. Shaft power required = 66.2 / 0.80 ≈ 82.8 kW. Motor input power = 82.8 / 0.90 ≈ 92 kW.
At 415 V and a plausible power factor of 0.86, 195 A corresponds to about √3 × 415 × 195 × 0.86 ≈ 120 kW input - well above the 92 kW needed for the measured duty. Even allowing for a lower power factor, the current is disproportionate. Part of the explanation is voltage unbalance: 428 V against 395 V is roughly 8%, which causes current unbalance and motor derating. The motor is not productively overloaded; the high current indicates extra losses, low power factor, voltage imbalance, mechanical drag or a measurement error. Throttling to hide the current is not a diagnosis and may worsen cavitation by changing the operating point.
2. Distinguish the likely causes of the noise: cavitation versus air ingress or mechanical fault. What immediate action is justified?
Read the worked answer
Crackling near the suction flange, with a choked strainer, a 6.5 m suction line and a 1.2 m lift, strongly suggests cavitation due to excessive suction vacuum and insufficient NPSH margin. Air ingress through a loose flange, worn seal or faulty foot valve can produce a similar noise, but usually with air bubbles in the discharge and an unstable vacuum reading. A mechanical fault such as bearing damage or coupling misalignment normally produces noise and vibration at the pump bearing housing, often at 1× or 2× running speed, not at the suction flange.
Immediate action: stop the pump and investigate. Clean or replace the suction strainer and foot valve, check suction joints for air leaks, measure suction vacuum, check per-phase voltage and current, and take a vibration reading. Do not continue running throttled. Running with cavitation damages the impeller and casing, and the voltage imbalance will overheat one phase even if the average current looks acceptable.
3. Recommend a corrective plan before restart.
Read the worked answer
First remove the suction restriction: clean the strainer, verify the foot valve, and check that the suction pipe is not fouled. Reduce suction losses by using a larger suction pipe, fewer bends and a lower vertical lift if feasible. Calculate NPSH available against the pump NPSH required at the actual flow and temperature, and correct the installation if the margin is inadequate.
Second, correct the electrical supply: the 8% voltage unbalance is unacceptable for continuous operation. Check transformer tap settings, loose neutral or contactor terminals, and whether a large single-phase load is causing the imbalance. If the supply cannot be balanced, derate the motor or use a suitable starter/drive. Third, check coupling alignment, bearing condition and impeller wear. Restart only after the strainer is clean, suction vacuum is within limit, per-phase currents are balanced, and vibration is acceptable. Record a new performance test at the duty point.