Topic 5 Enhanced Regulatory Control Strategies

In The Last Lecture  Split-range control –What it is –When it is used –Problems associated with it  Selective Control –What it is –When it is used –Problems associated with it

What We Will Cover Topic 1 Introduction To Process Control Topic 2 Introduction To Process Dynamics Topic 3 Plant Testing And Data Analysis Topic 5 Enhanced Regulatory Control Strategies Topic 6 Process Control Hardware Systems Topic 4 Controller Actions And Tuning Topic 7 Control Valves Topic 8 Process Control Troubleshooting

In This Lecture  Ratio Control Scheme –Fixed ratio control –Variable ratio control

Ratio Control  Used to ensure that the values of two or more variables are controlled in a fixed ratio  Very commonly used, mostly with flow variables  Examples: Used in maintaining in proportion –The flow rates of two reactants into a reactor –The flow rates of air and fuel into a furnace –The flow rates of components into a mixer

Fixed Ratio Control Structure  Wild flow: The flow that is allowed to fluctuate  Controlled flow: The flow that is adjusted to maintain proportionality with the wild flow  Ratio controller is usually a multiplier block –FC2.SP = Ratio.SP × F1.PV –Ratio = Controlled flow / Wild flow  When Ratio.SP is constant, we call this Fixed Ratio Control

Variable Ratio Control Structure  Sometimes we need to vary Ratio.SP –Wild flow composition varies –If conversion is lower than SP, we may want to increase Ratio.SP  Cascade AC.OP to Ratio.SP  In many control systems, the SP in a multiplier block can be adjusted by the operator (AUTO) or via the OP of a primary controller (CAS)  When Ratio.SP is varied often, we call this Variable Ratio Control

Variable Ratio Control Structure  In some control systems, especially older ones, the SP in a multiplier block cannot be adjusted  Use PID controller instead of multiplier block  Need to calculate Ratio.PV –Ratio.PV = Controlled.PV / Wild.PV  Not desirable because it introduces lag time into the process and it also adds one extra PID loop to tune

Fixed Air-Fuel Ratio

Variable Air-Fuel Ratio

In This Lecture…  Ratio Control Scheme –Fixed ratio control –Variable ratio control