Air valves and liquid length variation in hydraulic transients
Enrique Cabrera Marcet, Jorge García-Serra García, Iglesias P., Izquierdo J., Abreu J.
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The use of air valves for avoiding cavitation and/or preventing high pressures caused by entrapped air pockets is widely recommended and well known and established in technical books and papers dealing with this subject. The peculiar behaviour of large-flow, low-head sewage outfalls gives rise to very important underpressures when the pump trips. The easiest way of avoiding its effects consists in using air valves. The liquid column length variation can be important because of the system inertia. Taking it into account appears to be necessary for the problem to be suitably modeled. In this paper, both the elastic and inelastic models, taking into account liquid column variation, are applied to a submarine sewage outfall. Using the first model amounts to adopting a moving boundary while, for the second one, and in order to fit the air valve into the model, the main contribution consists in dividing the transient into three perfectly defined stages. Both models are compared for the first cycle of the oscillation and the results they give, although different from a formal outlook, are similar from a practical point of view. It is worth noting here that in certain applications the rigid model exhibits clear inefficiency to completely analyze the transient. The definite influence of the air valve characteristics, with regard to the system response, is finally considered to elaborate some criteria for a suitable selection of this protecting device.