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