Hammeringcontrol valves-diagnosis and solution of a stability problem

Plug-type control valves are often installed in the flow-to-close close-on-air-failure configuration. In this configuration, it is possible for the valve to become unstable even though the actuator can generate sufficient force, in the steady state, to overcome the forces opposing it. For a valve that is not fitted with a positioner, instability will occur if the curve of equilibrium valve actuator pressure against lift is not monotonically increasing. If a positioner is fitted, instability will occur if the curve of equilibrium mass of air in the valve actuator against lift is not monotonically increasing. This paper presents the stability analysis for both cases. It outlines areas where instability may occur and suggests a variety of solutions.