Scaling and performance of a centrifugal pump with hydrodynamic disc seals running in water and mercury

Abstract

The performance of a small centrifugal pump, normally used to circulate mercury in an I.C.I. chemical plant, is examined in water and mercury over a range of rotational Reynolds Numbers (Reω). It is shown that variations in speed and viscosity are not necessarily equivalent for a given change in Reω. The pump design leads to an unusual relationship between leakage and hydraulic efficiency. A scaling technique, which incorporates the effects of leakage, is developed for predicting the pump performance in mercury from its characteristics in water. Two factors contributing to excessive wear of the pump neckrings are defined. A hydrodynamic seal (HDS) is designed to replace the upper neckring. The extra power requirement of this seal, which is partially offset by the lower leakage rate, is reduced by staging. HDS performance is shown to be similar whether it runs alone or as part of In mercury, the seal interface (with water) is found to be less the pump. stable than a water/air interface. entrainment of bubbles, which subsequently collect at the interstage region of a staged HDS, posing a potential problem in practical applications. At low seal inlet pressures in mercury, total interface breakdown can occur for a single or two-stage HDS, resulting in water throughflow.The peak pump efficiency may be accurately monitored by thermometric measurements in water. The same method is shown to be more suitable for fluids of low thermal capacity, such as mercury, if the experimental technique can be refined.

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