Topic 5 Enhanced Regulatory Control Strategies

In the last lecture  Feedforward Control –Measured Vs Unmeasured Loads –Purpose of feedforward control –Feedforward gain –Deadtime compensation –Lead-lag compensation –Testing feedforward loops –When feedforward control cannot be used

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 7 Process Control Hardware Systems Topic 4 Controller Actions And Tuning Topic 8 Control Valves Topic 9 Process Control Troubleshooting

In This 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

Split-range Control

 One controller sending one OP to 2 final control elements  Used when a single final control element is unable to control a CV across the entire range of operations  Insulated batch reactor example in notes –Temperature controller outputs to a cooling water flow valve and a steam flow valve –In this case, at any one time, only one valve can be opened –When temperature is above SP, steam valve will close. When steam valve is fully closed, CW valve will open –When temperature is below SP, CW valve will close. When CW valve is fully closed, steam valve will open

Insulated Batch Reactor TC Controller OPCW (A)Steam (B) 0%Full openFull close 50%Full close 100%Full closeFull open

Light Ends Drum Pressure Control Controller OP Valve AValve B 0%Full close 50%Full openFull close 100%Full open

Heat Exchanger Temperature Control Controller OP Valve AValve B 0%Full openFull close 50%50% open 100%Full closeFull open

Problems With Split-Range Control  The two final control elements usually does not have the same process dynamics, tuning is a problem –Get dynamics for both MVs Do plant test for OP between 0% & 50%; and Do plant test for OP between 50% & 100% –Calculate tuning parameters for both –Use the more conservative parameters in your controller to avoid instability  Calibration (OP split between valve A & B) has to be very accurate

Selective Control (Override Control)

Selective Control  Also known as Override Control or Constraint Control  Used when there are more CVs than MVs –Use MV to control the most important CV. Importance may change depending on process conditions  Generally uses a “selector” –The selector takes in a number of input signals and selects one as an output signal –The criteria for selection generally falls under one of the following: Highest value Lowest value Average of all the input values Middle of 3  Inputs can be the –PVs from a number of transmitters; or –OPs from a number of controllers

Optimizing a packed bed reactor  Consider a packed bed reactor containing an exothermic reaction  The reactor has a number of thermocouples that measure the temperature at different points in the reactor  Control objective: Keep the average temperature in the reactor as high as possible (for max reaction rate) so long as none of the local temperatures exceed 400C

Optimizing a packed bed reactor

Problems with Selective Control  Since the inputs come from different transmitters, we can expect different process dynamics with the same MV depending on which CV is selected –For the packed bed reactor, this is largely due to the thermocouple location (e.g. higher up the reactor we can expect longer deadtime and/or lag time)  Each CV will have different “ideal” tuning parameters  Do the plant test for each CV separately  Calculate PID parameters for each CV  Use the most conservative tuning parameters

Controlling a tower sidestream flow  Consider a distillation column with a sidestream product flow  The sidestream flow rate affects the liquid level at the bottom –More sidestream flow reduces the level  Control objective: Control the sidestream flow rate to SP provided a minimum tower liquid level of 25% is maintained

Controlling a tower sidestream flow

In This 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

In The Next Lecture…  Ratio Control –Fixed ratio –Variable ratio