Water Hammer Caused by Fast Closing Valves

Abstract

Operation of fast closing valves in piping systems can create an overpressure condition, resulting in permanent deformation, joint damage, leakage, or rupture. Fast closing valves are used in many piping systems to protect personnel, equipment and the environment from the danger of overpressure. When there is a sudden closure of a piping system valve, the change in the flow velocity produces a transient increase in pipe pressure. This increased pressure is commonly known as transient, fluid hammer waterhammer, or surge pressure. In a very simplistic system, the excess pressure created by this sudden closure of valves can be computed using a simple hand calculation using Joukowsky method. The method is applicable only for the initial pressure wave generated. In complex systems, where there are dead legs (e.g. closed by-pass valves) or branches, there is more chance of the pressure waves being reflected, transmitted and superimposed. The overpressure problem is even more severe if a liquid column separation and re-joining occurs during the transient conditions. The magnitude of the pressure in the system due to these effects may be higher than that estimated by Joukowsky method. Hence a transient analysis needs to be performed to estimate the overpressure in the system. In this case study, the transient conditions initiated due to closure of buckling pin valves (BPVs) are modeled using a proprietary software CE099. The objectives are to calculate the maximum surge pressures, dynamic loads, and to recommend mitigations to reduce transient pressures and loads. The results showed that pressures could be reduced by increasing the pipe size of few segments or adding expansion loops. The most sensitive parameter for transient pressure was pipe size and that for dynamic load was valve closure time. It is recommended that this kind of study be performed in the early phase of engineering design, so that any identified overpressures can be mitigated with simple, cost effective options such as increasing pipe size, altering valve closure times, and adding expansion loops.