Pump Cavitation in a Hot Oil Transfer Loop
A centrifugal pump transfers a hot hydrocarbon (vapor pressure 180 kPa at 140 °C, density 720 kg/m³) from a surge drum at 1.2 bar absolute to a fired heater. The pump is rated at 45 m³/h and 60 m head. Operators report a rattling noise, fluctuating discharge pressure, and a slight loss in flow after the surge drum level was lowered to reduce residence time. The suction line is 4 m of 100 mm ID pipe with two elbows, a strainer, and a fully open gate valve. NPSH required by the pump is 3.5 m.
1. Calculate the available NPSH and determine whether cavitation is the cause.
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NPSH_A = (P_s − P_v)/(ρg) + z − h_f. P_s = 120 kPa abs, P_v = 180 kPa abs. That gives (120−180)×10³/(720×9.81) = −8.5 m. Even before friction, the available NPSH is negative - the liquid is already above its vapor pressure at the drum surface. Wait - check the drum pressure: if the drum is at 1.2 bar and the liquid is at 140 °C with vapor pressure 180 kPa, the liquid is subcooled only if the drum pressure exceeds 180 kPa. At 1.2 bar the liquid would boil in the drum. The realistic reading is that the drum pressure is 1.2 bar gauge (≈221 kPa abs), so NPSH_A = (221−180)×10³/(720×9.81) + z − h_f = 5.8 + z − h_f. With z ≈ 2 m and h_f ≈ 1.5 m (strainer, elbows, valve), NPSH_A ≈ 6.3 m, which exceeds NPSH_R = 3.5 m. So at the original level cavitation is marginal but not certain. Lowering the drum level reduces z, and if the level dropped by 2 m, NPSH_A falls to ~4.3 m - still above 3.5 m but with little margin. The noise and pressure fluctuation are consistent with cavitation at the impeller inlet, especially if the strainer is partially blocked, which adds h_f.
2. What is the immediate and long-term remedy, and what instrumentation confirms it?
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Immediate: raise the drum level back to restore static head, clean or bypass the strainer, and throttle the discharge (not suction) to move the operating point away from the cavitation region. Long-term: increase suction line diameter to reduce velocity and h_f, or lower the pump to increase z, or install a booster/inducer. Confirm with a suction pressure gauge at the pump inlet and a vibration/acoustic sensor: cavitation shows broadband high-frequency noise and a characteristic NPSH_A vs. flow curve crossing NPSH_R. Plot NPSH_A against flow - it falls with flow² - and ensure the margin is at least 1 m across the operating range. Also check the pump curve: if the system curve shifted, the pump may be running out on its curve, which lowers NPSH_A further.