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10.4 FAILURE RATE MODEL FOR PUMP SEALS, ASe The pump assembly will contain several types of seals such as O-rings on the fluid connectors, gaskets and mechanical seals. Failure rate equations for O-rings and gaskets are contained in Chapter 3. Mechanical seals of the pump require a specific evaluation of reliability. Mechanical seals for rotating shaft applications move the point of the seal away from the shaft to specially designed sealing faces that gradually wear down. A balanced mechanical seal is designed so that the effective contact pressure is always less than the fluid pressure, reducing friction at the seal faces. The result is less rubbing wear, less heat generated and higher system fluid pressure capability. In an unbalanced seal, fluid pressure is not relieved by the face geometry; the seal faces withstand full system fluid pressure plus surge pressures and spring pressure. Thus, the face contact pressure is greater than or equal to system fluid pressure. The balanced seal design is more expensive than the unbalanced design but provides higher reliability and longer life. Chapter 3 contains specific failure rate equations and failure modes for mechanical seals. 10.5 FAILURE RATE MODEL FOR PUMP SHAFT, ASff A typical pump shaft assembly is shown in Figure 10.3. The reliability of the pump shaft itself is generally very high when compared to other components of the pump. Torque limits of the shaft must be compatible with requirements of the pump application. High viscosity/low speed applications produce high torque requirements. Studies have shown (Reference 26) that the average failure rate for the shaft itself is about eight times less than mechanical seals and about three times less than that of the ball bearings. The possibility that the shaft itself will fracture, or become inoperable is very unlikely when compared to the more common pump failure modes. Usually the seals or bearings will cause problems first. The effect of the shaft on reliability of other components is of greater importance than the reliability of the shaft itself. For centrifugal pumps, there is a large difference in deflection among the types of pump casing design. In a single volute casing, there are varying amounts of fluid pressure distributed about the casing causing unequal distributions of forces on the pump shaft. This imbalance causes shaft deflection and greater seal and bearing wear. The amount of radial thrust will vary depending on the casing design and fluid flow. The thrust load will increase from normal operation for any type of casing design when the pump is not run at its optimum flow rate speed. When the pump is not operating at its optimum rate, then the type of casing design will have a significant effect on the radial load. Pumps 10-10 Revision C

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