Pumps, problems cavitating-type

As the mechanical integrity of the pump system changes, the amplitude of vibration levels change. In some cases, in order to identify the source of vibration, pump speed may have to be varied, as these problems are frequency- or resonance-dependent. Pump impeller imbalance and cavitation are related to this category. Table 10-11 classifies different types of pump-related problems, their possible causes and corrective actions. 

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

This type of damage is dealt with comprehensively in Section 8.8. It can be particularly severe in seawater giving rise to cavitation corrosion or cavitation erosion mechanisms, and hence can be a considerable problem in marine and offshore engineering. Components that may suffer in this way include the suction faces of propellers, the suction areas of pump impellers and casings, diffusers, shaft brackets, rudders and diesel-engine cylinder liners. There is also evidence that cavitation conditions can develop in seawater, drilling mud and produced oil/gas waterlines with turbulent high rates of flow. 

A true vortex breaker is normally inserted just ahead of the vessel s liquid exit nozzle as shown in Fig. 13.1.1. This is a very important feature in the geometry at hand since the angular momentum of the incoming gas-liquid mixture will produce bulk rotation of the liquid pool. If a vortex is allowed to form, some of the incoming gas may exit out the underflow and create pump cavitation or other problems downstream. The vortex will also act as a type of fluidic choke and restrict the flow rate out the bottom liquid exit nozzle. 

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