Modelling and Dynamic Analysis of Pneumatic Control Valve with Stiction

High performance of control loops is necessary to ensure high product quality and low cost in chemical plants. Often poor performance can be detected by the operator, but many times the problem may propagate to other loops, making it difficult to detect the root cause. One of the many reasons for degrading performance is stiction found in control valves. It hinders the proper movement of valve stem and consequently affects the control loop performance. It is therefore important for control engineers to understand stiction phenomena and to know how to deal with them. This paper focuses on the development of a model for control valve stiction, through simulation, which can be used for the dynamical analysis of valve stiction effects on process control loop performance. The model developed here use simple empirical relationships between controller output and the valve position to describe valve stiction with just a few parameters that can be determined from operating data. The proposed stiction model captures the stiction behaviour and can be used to investigate by analysis and simulation, the properties of stiction that are relevant to control design.