Analysis of Power Plant Deaerator Storage Tank Instabilities

Abstract

In some power plant deaerators storage tank steam is caused to flow over the surface of the water on its way to the condensing head. This steam flow, which may become very large under transient load operation, low pressure feed heater bypassing or isolation, may create water waves, a hydraulic gradient and possibly spray which carries over into the deaerator head via the steam transfer pipe to result in high differential pressure. This may lead to head flooding, surging discharge, tray stack damage, and also give rise to problems with associated components; for example vent condenser flooding. Steam transfer pipes may constitute a forcing system resulting in tank contents sloshing and severe vibration. These events in turn may precipitate an instability (the Kelvin-Helmholtz instability) whereby waves may grow to bridge the gap between water surface and tank crown to form a water-piston. A piston so formed is rapidly propelled along the tank to impact with a hammer-like blow causing serious internal and external damage and jeopardizing safety. The sequence of events finally precipitating the onset of the water piston is complex and renders an analytical approach which might establish design criteria difficult, for this reason simple modelling is suggested. This paper describes such studies using one-twelfth scale models of late generation 500-660 MW units, from which the parameters governing instability are evaluated.