Advanced Regulatory Control Techniques for Improved Averaging Level Control Performance

Averaging level control is used in liquid processing plants to optimally use the available volume in surge drums and storage tanks to stabilize the feed to downstream equipment. This paper describes three averaging level control techniques, namely, integral gap control (IGC), ramp horizon control (RHC), and a proposed combination of the two techniques called ramp horizon integral gap control (RHIGC). RHIGC is a novel advanced regulatory control (ARC) technique and can be implemented by using standard distributed control system (DCS) functions. The advantage of RHC is that it will keep levels from going over limits when disturbances larger than typical process disturbances occur. However, it does not move the level away from the limit; therefore, the level will violate the limit if a subsequent disturbance occurs. Based on tuning, IGC is able to move the level away from limits very quickly, but it will let the level violate the set limits if a larger than expected disturbance occurs. RHIGC aims to exploit the strengths of both techniques while compensating for individual weaknesses. The techniques are compared in simulations, followed by the implementation of RHIGC on an industrial process plant. RHIGC shows good results in preventing the level from violating the set high and low limits while simultaneously decreasing the variability of the manipulated variable. In process plants, keeping the level between set limits may avoid alarm or trip limits being exceeded, while the decreased variability of the manipulated variable leads to an increased stability in downstream process equipment.