Section 3 Selective Control Flashcards
What is reset tieback and why is it used?
Reset tieback is when the output of the selector relay is routed back to each controller. This prevents reset windup of the deselected controller by letting controllers know what controller is selected and which is deselected.
What are the 3 steps to tuning an override control scheme?
1) Disconnect controllers from the selector relay
2) Tune controllers with whatever tuning method for a certain response
3) Connect the controllers back to the selector relay and observe their behavior in override control
What is auctioneering and when is it used?
Auctioneering is when the same process variable is measured in a number of positions in the process. Either the highest or the lowest measurement is chosen for control. This can be applied to a catalyst bed, the highest temperature across the bed controls the process cooling.
What is redundant instrumentation (standby) and when is it used?
Multiple pieces of equipment are operating but only one is used at a time as a fall back for safety if the equipment being used fails. This is common with composition analyzers to prevent a hostile environment should a failure occur.
What is hard constraints selective control and when is it used?
A control strategy that ensure the system follows the process variable until a certain limit. This is commonly used on furnaces to ensure they never become too rich with fuel.
