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 5Enhanced
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
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 TCController OP CW (A) Steam (B)
0% Full open Full close
50% Full close Full close
100% Full close Full open
InsulatedBatch Reactor
Valve A
Cooling Water /Condensed Steam
Outlet
Valve B
CoolingWater
Steam
TC
SP
Light Ends Drum Pressure Control
Controller OP
Valve A Valve B
0% Full close Full close
50% Full open Full close
100% Full open Full open
PC
Feed
To Light EndsRecovery Units
KnockoutDrum
Valve A
Valve B
To flare
To CondensateRecovery Unit
SP
Heat Exchanger Temperature ControlController OP
Valve A Valve B
0% Full open Full close
50% 50% open 50% open
100% Full close Full open
Cooler
Valve A
Valve B
TCSP
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
T1
T2
T3
T4
HighSelector TC
SP = 400C
Hot oilinlet
ReactantInlet
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
FCLow Selector
LC
MainProduct
LC.SP= 25%
FC.SP= 16 KBD
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