Dynamic behavior of direct spring loaded pressure relief valves in gas service: Model development, measurements and instability mechanisms

• A new mathematical model is derived of a direct-spring relief valve together with its inlet piping.
• Detailed laboratory tests are made for three different valves in gas service, with varying inlet pipe length.
• For sufficiently long pipes a violent oscillation is recorded in both valve displacement and pipe pressure.
• The model is able to show good agreement with the onset points and nature of the instability.
• The instability is due to a Hopf bifurcation where valve motion couples to a quarter-wave within the pipe.

A synthesis of previous literature is used to derive a model of an in-service direct-spring pressure relief valve. The model couples low-order rigid body mechanics for the valve to one-dimensional gas dynamics within the pipe. Detailed laboratory experiments are also presented for three different commercially available values, for varying mass flow rates and length of inlet pipe. In each case, violent oscillation is found to occur beyond a critical pipe length, which may be triggered either on valve opening or closing. The test results compare favorably to the simulations using the model. In particular, the model reveals that the mechanism of instability is a Hopf bifurcation (flutter instability) involving the fundamental, quarter-wave pipe mode. Furthermore, the concept of the effective area of the valve as a function of valve lift is shown to be useful in explaining sudden jumps observed in the test data. It is argued that these instabilities are not alleviated by the 3% inlet line loss criterion that has recently been proposed as an industry standard.