Simplified Approach for the Optimal Sizing of Throttled Air Chambers

Abstract

Among water hammer damping devices, air chambers are often used in pumping plants to reduce pressure surges to acceptable values. The chamber is more effective if a throttling device is used, resulting in a reduction of the required volume. Design charts for a simple and fast sizing of air volume and orifice diameter are available in the literature using the rigid column theory (incompressible flow) and the De Sparre rule. Nevertheless, in many cases the pressure pattern is far from being constant during the first quarter of the transient period and lower pressures can be attained; furthermore, the rigid column model is not able to reproduce elastic phenomena arising from throttling, and so water hammer equations should be used instead. Although orifice induced pressure waves are evident only in the first part of the transient, differences between air chamber pressure and pipe pressure can be quite significant. Negative pressure surge should be limited because column separation and cavitation could occur as a consequence of low pressures. Because the maximum down surge inferred from the design charts does not ensure safe design, a simplified approach was proposed in this paper to design throttled air chambers under the elastic hypothesis. The analysis showed small deviations between the minimum pressure and the design pressure, unlike the inelastic approach, which exhibits even very large differences with water hammer equations.