Valve Characteristics and Their Effect on Transient Surge Pressures in Delivery Terminals

Abstract

A hydraulic transient occurs when there is a sudden change in the steady state condition of a fluid in a pipeline. The rapid change in velocity of a fluid causes a pressure wave to travel along the pipeline, potentially causing damage to the equipment and piping. Usually, different scenarios are studied depending on whether the fluid is being injected or delivered from the pipeline into a tank. The most commonly studied causes of sudden fluid velocity change in a pipeline are: closing of a fast acting valve (ESD or control valves which close within seconds) and stopping or starting of pumps. When a pipeline is delivering fluid into a tank, the closure of a valve can completely block the flow and create transient surge pressures that exceed acceptable pressure limits, and require mitigation. Although the closure of a fast acting valve is a commonly analyzed scenario, the closure of a motor operated valve (MOV), which is less commonly analyzed, can also create surge pressures which can put the pipeline at risk. There are three characteristics of an MOV that can significantly impact the surge pressures it creates when it is closed. These valve characteristics are: flow coefficient, valve curve and stroke time. Hydraulic simulations were performed to study the effect of these three valve characteristics on transient responses when delivering from pipelines into tanks. Simulation results show that a faster stroke time leads to higher pressure surges, as well as a valve with a quick closing or linear curve. However, the flow coefficient of the valves will have varying effects on transients depending on the piping system being analyzed. The purpose of this paper is to not only highlight the importance of valve characteristics when modeling transient surge events but also to provide key learnings that can be used to design safer delivery terminals.