43 Excessive Power Consumption Excessive pump power is an indication that there

43 excessive power consumption excessive pump power

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4.3 Excessive Power Consumption Excessive pump power is an indication that there may be one of a number of different problems going on with the pump. One of the main problems that results in excessive power consumption, and possibly ultimately a motor trip, is the existence of small or high concentrations of particles in the fluid. Even though the fluid looks clean, it may have very small concentration of particles that can lead to long-term performance problems as running clearances gradually increase from the wearing down of parts. This can be found in any centrifugal pump application, regardless of how clear or clean the fluid looks like. This wearing away produces a loss of hydraulic performance, which will cause the control valve to gradually open wider without being noticed. The wider it becomes, the more power is required from the motor, which may result in a motor trip [26]. Other main causes of faults in a pump that could cause excessive power consumption are: the speed of the impeller is too high; the shaft packing is too tight; the liquid that is being pumped has a viscosity that is more than what the pump was designed for; there is a misalignment in the pump; the impeller is touching the case; the pumped liquid is denser than specified; the impeller is rotating in the wrong direction; the impeller is installed in the wrong direction [27]; the pump has trapped air inside; there is a serious leak in the delivery line to the pump; the pump is delivering more than its rated quantity; the neck rings on the impeller are worn excessively; the impeller is damaged; there is a mechanical tightness at the pump’s internal components; or the pipework is exerting a strain on the pump[9]. Corrective procedures that may be taken in order to eliminate the causes of the excessive power consumption include: periodic tracking of process control valve position versus flow rate; periodic recording of motor current and voltage if the pump is operating on
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11 the head/flow curve where the head flow curve is not steep; periodic measurement of pump shut-off head; renew defective gaskets; repair existing leaks; reduce the speed of the impeller; dismantle pump and restore clearances to original dimensions; stop pump, close delivery valve to relieve internal pressure on packing, slacken back the gland nuts and retighten to finger tightness; dismantle pump and renew impeller; dismantle pump, check internal clearances, and adjust as necessary; or stop pump and reprime [9, 26]. Due to the number of faults which can cause excessive power consumption, in order to have a correct diagnosis of the underlying fault a variety of condition based measurement data is needed, from process information to temperature and vibration measurements 4.4 Blockages From anecdotal data in logs obtained from companies in the water supply and sewerage industry, the authors conclude that blockages can also be a major fault mode of centrifugal pumps, however little evidence is able to be found in literature on this failure mode. The
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