Effects of Compressibility on Flow Characteristics and Dynamics of Swing Check Valves—Part I

Abstract

In the design of natural gas compressor stations, a check valve is a critical element which is commonly placed on the discharge side of the compressor to prevent reverse flow that can cause serious damage to the compressor itself and other components such as seals and bearings. One of the selection criteria of the check valve for this particular application is the valve flow characteristics in steady flow, and its dynamic characteristics in unsteady flow operation. With regards to steady flow valve Characteristics, current models for the determination of the check valve open angle versus mean flow velocity are based on semi-empirical data obtained from water tests, which were found to deviate from measurements involving fluids of relatively higher compressibility. This paper presents results of steady flow testing of an NPS 4 swing type check valve in air. Mean flow velocities versus disk angles were measured together with several local pressure measurements at the back side of the valve disk. Comparison of these results with the EPRI model and Rahmeyer’s model revealed that these two models underestimate the mean flow velocity for a given disk angle in air. A model was thus developed based on further refinement of Rahmeyer’s model, but more suitable for fluids of relatively higher compressibility, and accounts for both torque contributions: (i) from jet velocity impingement (Kv), and (ii) from back pressure distribution (Kp). The work presented here points out the need for better design of the disk shape, particularly at the lower lip, and/or the valve body in order to create a lower disk back pressure to improve the disk lifting torque at lower mean flow velocity.