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Table 10-2. Typical Failure Modes of Positive Displacement Pump Assemblies FAILURE MODE FAILURE CAUSE FAILURE EFFECT Pump cavitation - Reduction in suction head - Pump noise and vibration - Eventual erosion of rotor, casing Component corrosion - Incorrect fluid - Excessive flow rate for fluid - Eventual catastrophic pump failure Low net positive suction head (NPSH) - Pump cavitation - Reduced pump efficiency Shaft unbalance - Torsional vibration - Shaft deflection and misalignment - Seal leakage - Bearing wear External Leakage - Seal failure - Worn mechanical seal - Scored shaft sleeve - Stuffing box improperly packed - Depends on type of fl uid and criticality as to time of failure Mechanical noise - Bent shaft - Worn/damaged bearing - Foundation not rigid - Cavitation - Eventual wear of piston or rotor, and other components Positive suction head too low - Clogged suction pipe - Valve on suction line only partially open - Suction cavitation - Noisy operation - Low discharge pressure - High output flow rate Pump discharge head too high - Clogged discharge pipe - Discharge line valve only partially open - Discharge cavitation - Noisy operation - Low output flow rate Suction line clogged - Contaminants - No pump output / reduced flow Pressure surges - Incorrect NPSH - Cavitation damage Increased fluid temperature - Incorrect fluid viscosity for pump - Misaligned pump driver 10.2.1 Cavitation Cavitation occurs when the pump suction is under a low pressure/high vacuum condition creating vapor bubbles at the inlet of the pump. Dynamic motion of the pump moves the bubbles to the discharge side of the pump where the discharge pressure Pumps 10-10 Revision C

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