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principle, each revolution delivering a definite volume of liquid to the system. Rotary pumps include gear, lobe, piston, screw and vane. 10.2 FAILURE MODES Due to the large number of pump types and applications, some failure modes are more prevalent than others for a specific category of pump. For example, with displacement pumps there is a much greater chance for cyclic fatigue to have an effect on the system than with centrifugal pumps. This is due to the inherent difference in designs. Displacement pumps have pressure transients which cause temporary unbalanced forces to be applied to the pump and its system. The displacement pump and driver shafting can experience much higher stresses during operation due to the uneven torque loading caused by this natural imbalance. A positive displacement pump does not have a shut-off head as does a centrifugal pump and cannot be operated against a closed valve on the discharge side of the pump. If a positive displacement pump is allowed to operate against a closed discharge valve or other obstruction, it will continue to produce flow, increasing the pressure in the discharge line until either the line bursts or the pump is severely damaged or both. Thus a relief valve incorporated within the pump assembly or incorporated on the discharge side of a positive displacement pump is an absolute necessity. Centrifugal pumps tend to be more balanced than positive displacement pumps and aren't as susceptible to large stress variations. There are three main factors that affect the reliability of a centrifugal pump including operating speed, impeller diameter and flow rate. Operating speed affects the wear in the rubbing contact surfaces of shaft seals, bearing life and the heat generated by the bearings and lubricants. Impeller diameter affects reliability through the loads it imposes on the shaft and bearings. And flow rate affects reliability of a centrifugal pump when it is different than that established by the Best Efficiency Point (BEF) causing increasing and turbulent loads on the impeller. BEF operation is discussed in more detail in Section 10.8.1. A major effect on performance of a positive displacement pump is a loss of flow due to slippage. The expanding cavity on the inlet side of a positive displacement pump creates a low pressure area that needs to be filled with fluid. This cavity can be filled with fluid from the inlet line in normal performance. However, if slippage occurs, the cavity will also be partly filled with fluid flowing back through the pump clearances from the outlet side. When a positive displacement pump is operating under a slippage condition, the pump loses the ability to deliver the volume of fluid it is theoretically capable of pumping. For a given pump and fluid, the slippage is proportional to the pressure differential from outlet to inlet. If the pump had no slippage the volume pumped would be directly proportional to the rotational speed of the pump. Typical failure modes of pumps are shown in Table 10-1 and Table 10-2. Pumps 10-10 Revision C

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