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Where: JiFD = Failure rate for the pump fluid driver, failures/million operating hours AFD.B = Base failure rate of pump fluid driver (See Table 10-4) CPF = Percent flow multiplying factor (See Section 10.8.1) Cps = Operating speed multiplying factor (See Section 10.8.2) CC = Contaminant multiplying factor (See Section 10.8.3) CSF = Service factor multiplying factor (See Table 10-5) 10.8.1 Percent Flow Multiplying Factor A centrifugal pump is designed to operate most reliably at one capacity for a given RPM and impeller diameter. This flow rate is called the Best Efficiency Point (BEP). At this flow, hydraulic loads imposed on the impeller are minimized and are steady. At flows greater than or less than the BEP the hydraulic loads increase in intensity and become unsteady due to turbulence in the casing and impeller. As a result, hydraulic loads, which are transmitted to the shaft and bearings, increase and become unsteady. Shaft deflection changes as a function of the fluid flow rate through the pump. As a pump's capacity increases or decreases, moving away from the point of maximum efficiency, fluid pressures around the impeller become unequal, tending to deflect it. Special casings, such as diffusers and double-volute and concentric casings can greatly reduce the radial thrust and, hence, the deflection. Also, the severity of these unsteady loads can cause failures of the mechanical seal. Operation at reduced flow rates that put the pump into its recirculation mode can also lead to cavitation damage in high suction energy pumps. The effect on reliability of operating a pump too far from its maximum efficiency point is shown in Figure 10.4. The Percent Flow multiplying factor (CPF) is dependent upon the casing type and pump capacity percentage. The pump capacity percentage is the actual operating flow divided by the maximum pump specification flow. Values for CPF are shown in Figure 10.4 and related equations are included in Table 10-3. 10.8.2 Operating Speed Multiplying Factor Operating speed affects the failure rate of a pump caused by the increased rotational or cyclic speed leading to accelerated rubbing wear of shaft and mechanical seal faces, increased bearing friction and lubricant degradation. Increased operating speed also increases the energy level of the pump which can lead to cavitation damage. The effects of wear on these components are almost linear as a function of speed. Pumps 10-10 Revision C

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