Mohamad Roostaei
PhD in Chemical Engineering
Mohamad Roostaei
@mhdroostaei
1
Keep the control system as simple as possible
• Everyone involved in the process, from the operators up to the plant manager,
should be able to understand the system, at least conceptually.
• Use as few pieces of control hardware as possible.
• Every additional gadget included in the system is one more item that can fail
or drift.
© Mohamad Roostaei
2
2
Use feed-forward control to compensate for large,
frequent, and measurable disturbances.
Wild stream: FA
×
FT
102
Ratio Control is a special type
of feed-forward control!
Desired
Ratio
FY
102
FC
101
FY
101
FT
101
I/P
FV
101
Manipulated stream FB
© Mohamad Roostaei
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3
Use override control to operate at or to avoid constraints
SP
TT
101
TIC
101
SP
FIC
101
FT
101
FIC-101 to be overridden by LIC-101
when low liquid level is detected.
LT
101
FY
101
LIC
101
SP
LV
101
© Mohamad Roostaei
FY
101
<
OVERRIDE
I/P
FC
4
4
Avoid large time lags and dead-times in feedback loops
• Control is improved by keeping the lags and deadtimes inside the loop as small as possible.
• This means that sensors should be located close to where the manipulated variable enters the process.
Stream 1
Stream 1
Stream 2
Stream 2
CC
101
© Mohamad Roostaei
AT
101
CC
101
AT
101
5
5
Use proportional-only level controls where the
absolute level is not important.
PIC
101
PT
101
PI-Controller
PI-Controller
LIC
101
LT
101
P-Controller
© Mohamad Roostaei
Feed Surge Drum
FIC
101
FT
101
6
6
Eliminate minor disturbances by using cascade control
systems where possible
Steam
FT
101
Slave Loop
FV
101
I/P
FY
101
FC
101
RSP
TIC
101
SP
TT
101
Process Fluid
Heat Exchanger
Master Loop
© Mohamad Roostaei
7
PC
7
Avoid control loop
interaction if possible, but if
not, make sure the
controllers are tuned to make
the entire system stable
LC
TC
Interacting Loops!
TC
LC
© Mohamad Roostaei
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8
Check the control system for potential dynamic problems
during abnormal conditions or at operating conditions that
are not the same as the design
• The ability of the control system to work well over a range of conditions is called flexibility.
• Startup and shutdown situations should also be studied.
• Operation at low throughputs can also be a problem.
• Process gains and time constants can change drastically at low flow rate, and controller
retuning may be required.
• Installation of dual control valves (one large and one small) may be required.
© Mohamad Roostaei
9
9
Avoid saturation of a manipulated variable
40 m3/h
LC
40 m3/h
LC
TC
TC
4 m3/h
Richardson's rule
© Mohamad Roostaei
4 m3/h
10
10
Avoid “nesting” control loops
Control loops are nested if the operation of the external loop depends on the operation
of the internal loop.
PC
LC
© Mohamad Roostaei
11
References
• Buckley, Page S. Techniques of process control. (1964).
• F. G. Shinskey. Process control systems: application, design, and
tuning. McGraw-Hill, 1996.
• Luyben, Michael L., and William L. Luyben. "Essentials of process
control." (1997).
© Mohamad Roostaei
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Mohamad Roostaei
@mhdroostaei
0
