1. CHE 185 – PROCESS CONTROL AND DYNAMICS
STANDARD CONTROL LOOPS

2. BASIS FOR PID SELECTION
PROPORTIONAL CONTROL
SATISFACTORY FOR PROCESSES WHERE RESPONSES ARE QUICK
OFFSET IS NOT A PROBLEM.
INVENTORY AND PUMP TANKS
For integrating processes, P-only control provides offset-free operation. In fact, if as integral action is added to such a case, the control performance degrades.
Therefore, for integrating processes, P-only control is all that is usually required.

3. BASIS FOR PID SELECTION
PROPORTIONAL-INTEGRAL CONTROL
PROCESSES WHERE OFFSET IS A PROBLEM
PROCESSES THAT ARE NOT SLUGGISH, SINCE INTEGRAL SLOWS DOWN THE RESPONSE
MOST LOOPS - CSTR LEVEL CONTROL, HEAT EXCHANGERS, PRESSURE CONTROL
Over 90% of control loops use PI controller

4. BASIS FOR PID SELECTION
PROPORTIONAL-INTEGRAL-DERIVATIVE
PROCESSES WHERE OFFSET IS A PROBLEM
PROCESSES THAT NEED TO RESPOND FASTER THAN P-I CONTROL
PROCESSES THAT DO NOT HAVE NOISY SIGNALS
CRITICAL TEMPERATURE OR PRESSURE CONTROLS WHERE VARIATIONS ARE FREQUENT.

5. BASIS FOR PID SELECTION
PID: use for sluggish processes (i.e., a process with large deadtime to time constant ratios) or processes that exhibit severe ringing for PI controllers. PID controllers are applied to certain temperature and composition control loops. Use derivative action when:

6. Comparison between PI and PID for a Low θp/τp Ratio

7. Comparison between PI and PID for a HIGH θp/τp Ratio

8. SELECTION FOR Several Commonly Encountered Control Loops
THE EXAMPLES ARE FOR CONTINUOUS SYSTEMS, BUT CAN APPLY TO BATCH OPERATIONS.
Flow control loops
Level control loops
Pressure control loops
Temperature control loops
Composition control loops
DO control loop
Biomass controller
USE OF VARIABLE FREQUENCY DRIVES

9. FLOW CONTROL CONTINUOUS CONTROL
THE STANDARD FLOW CONTROL LOOP SHOULD INCLUDE THE FOLLOWING COMPONENTS
ALSO SEE FIG IN TEXT

10. FLOW CONTROL
THIS LOOP HAS QUICK RESPONSE WITH THE SLOWEST COMPONENT BEING THE CONTROL VALVE
P-I CONTROL WITH SHORT RESET TIMES
A VALVE POSITIONER MIGHT BE ADDED TO SOME VALVES WHERE THE CONTROL VALVE LAG IS CRITICAL
Almost always use PI controller.

11. FLOW CONTROL Deadband of industrial valves is between ±10%-±25%.
As a result, small changes in the air pressure applied to the valve do not change the flow rate.

12. FLOW CONTROL
A control valve (deadband of ±10-25%) in a flow control loop or with a positioner typically has a deadband for the average flow rate of less than ±0.5% due to the high frequency opening and closing of the valve around the specified flow rate.

13. FLOW CONTROL EMERGENCY CONTROL HIGH-HIGH/LOW-LOW FLOW SWITCHES
ADDED IN THE LINE AND NORMALLY CONNECTED THROUGH A PLC
THESE ARE INDEPENDENT OF THE CONTROLLER AND CONNECTED TO ALARMS AND INTERLOCKS

14. FLOW CONTROL
HIGH/LOW FLOW SWITCHES CAN BE SETPOINTS ON THE FLOW CONTROLLER
CONNECTED TO ALARMS
MEANT TO ALERT OPERATOR TO ACT

15. FLOW CONTROL FLOW PREVENTERS
THESE ARE TYPICALLY SELF-CONTAINED DEVICES THAT MAY OR MAY NOT HAVE AN INDICATOR FOR CURRENT POSITION
BLOW-OUT PREVENTERS ARE PART OF THE SAFETY EQUIPMENT IN OIL DRILLING.
SEAL OFF THE WELL IN THE EVENT OF ANY TYPE OF PRESSURE SURGE
NORMALLY SPRING OR HYDRAULICALLY LOADED

16. FLOW CONTROL BACK-FLOW PREVENTERS
INSTALLED TO PREVENT A FLOW REVERSAL EITHER DUE TO HIGH PRESSURE DOWNSTREAM OR LOW PRESSURE UPSTREAM
USED TO PROTECT STANDBY PUMPS

17. LEVEL CONTROL
CONTINUOUS LEVEL CONTROL LOOPS CONTAIN THE FOLLOWING COMPONENTS
ALSO SEE FIGURE 7.9.2

18. LEVEL CONTROL
THESE CAN ALSO BE SET UP ON A CASCADE BASIS TO HAVE THE OUTPUT FROM THE LEVEL CONTROLLER SENT AS THE SETPOINT TO A SEPARATE FLOW CONTROL LOOP.
CONTROLLING LAG TIME IS BASED ON THE SYSTEM TIME DELAY, WHICH DEPENDS ON VOLUME AND FLOW RATE
IF ACTUAL LEVEL CAN VARY, THEN P-ONLY CONTROL MAY BE ADEQUATE
IF ACTUAL LEVEL IS IMPORTANT, THEN PI CONTROL IS APPROPRIATE

19. LEVEL CONTROL LEVEL CONTROL FOR PUMP TANKS
PUMP TANK LEVELS TYPICALLY CAN VARY ABOVE A MINIMUM - FOR CONSTANT DISCHARGE FLOWS
MAINTAINING A CONSTANT PUMP TANK LEVEL WILL RESULT IN INTEGRATED DOWNSTREAM FLOWS THAT EQUAL THE AVERAGE OF INCOMING FLOWS

20. LEVEL CONTROL LEVEL CONTROL FOR PUMP TANKS
THE PUMP TANK IS A FIRST ORDER PROCESS WITH RESPECT TO FLOW
GAIN AND TIME DELAY ARE A FUNCTION OF
THE VOLUME OF INCOMING FLOW
THE TANK VOLUME
AND THE TANK DIAMETER.

21. LEVEL CONTROL LEVEL CONTROL FOR HEAT TRANSFER
LEVEL CONTROL CAN BE USED FOR CONTROL OF HEAT TRANSFER WITH SYSTEMS THAT INCLUDE PHASE CHANGE
PHASE CHANGE RESULTS IN HIGH HEAT TRANSFER COEFFICIENTS, SO ONE WAY TO CONTROL THE TOTAL HEAT TRANSFER IS TO CONTROL THE AREA OF THE EXCHANGER ACCESSIBLE TO PHASE CHANGE.

22. LEVEL CONTROL

23. LEVEL CONTROL
THE AVAILABLE AREA IS REDUCED BY INCREASING THE LEVEL OF CONDENSATE IN THE SHELL OF THE EXCHANGER
THIS IS A REVERSE ACTING SYSTEM SINCE AND INCREASE IN LEVEL DECREASES THE PROCESS FLOW EXIT TEMPERATURE

24. LEVEL CONTROL
HIGH LEVEL TYPICALLY REQUIRES SHUTDOWN, SINCE ENTERING PROCESS LINES DO NOT HAVE A LARGE INVENTORY VOLUME
LOW LEVELS CAN BE USED TO SHUT DOWN PUMPS AND/OR CLOSE VALVES

25. PRESSURE CONTROL
THE STANDARD CONTINUOUS PRESSURE CONTROL SYSTEM SHOULD INCLUDE THE FOLLOWING COMPONENTS
THIS LOOP HAS SLOW RESPONSE WITH THE SLOWEST COMPONENT BEING THE PROCESS TIME DELAY - WHICH DEPENDS ON THE VOLUME OF THE SYSTEM

26. PRESSURE CONTROL
The sensor is generally faster than the actuator, which is faster than the process.
Use P-only controller if it is an integrating process otherwise use a PI controller.

27. PRESSURE CONTROL
THE PROCESS PRESSURE IS NORMALLY CONTROLLED BY REMOVING NON-CONDENSIBLES FROM THE SYSTEM
A CONDENSER IS USED TO CONTAIN THE PROCESS MATERIALS
THE CONTROL VALVE MAY BE ON A STEAM EJECTOR OR THE MOTOR ON A VACUUM SYSTEM. VAC. PUMP FROM

28. PRESSURE CONTROL
THERE MAY BE A SEPARATE SYSTEM FOR INITIALLY EVACUATING/PRESSURIZING THE SYSTEM
THIS SYSTEM SHOULD INCLUDE A SEPARATE CONTROL MODE TO ALLOW FOR HIGHER FLOWS
THE TRANSITION MAY BE AUTOMATED OR MANUAL

29. PRESSURE CONTROL SELF-CONTAINED PRESSURE CONTROL SYSTEMS
PRESSURE RELIEF VALVES AND BACKPRESSURE RELIEF VALVES ARE USED TO CONTROL PRESSURES AT POINT ADJACENT TO THE VALVE
THESE UNITS HAVE A VERY QUICK CONTROLLER RESPONSE TIME

30. PRESSURE CONTROL PRESSURE RELIEF SYSTEMS
SELF-CONTAINED VALVES THAT ACT LIKE BACKPRESSURE REGULATING VALVES
THESE ARE DESIGNED FOR HIGH FLOW RATES AFTER A SPECIFIED PRESSURE HAS BEEN ACHIEVED
MAY OR MAY NOT RESEAT AFTER ACTIVATION
MAY HAVE ACTUATORS FOR LARGE SYSTEMS

31. TEMPERATURE CONTROL CONTINUOUS TEMPERATURE CONTROL HEAT EXCHANGERS
STANDARD HEAT EXCHANGER CONTROL CONSISTS OF PROCESS FLUID TEMPERATURE MEASUREMENT AND SOME TYPE OF CONTROL ON THE HEAT TRANSFER FROM THE UTILITY
BYPASS IS AN OPTION WHEN HEAT IS BEING RECOVERED FROM A SECOND FLUID, BUT THERE IS A MINIMUM FOR EXIT TEMPERATURE OF THAT FLUID

32. TEMPERATURE CONTROL
The dynamics of the process and sensor are usually slower than the actuator.
Use a PI controller unless the process is sufficiently sluggish to warrant a PID controller.

33. TEMPERATURE CONTROL
Analysis of PI Controller Applied to Typical Temperature Loop.

34. TEMPERATURE CONTROL
Analysis of PI Controller Applied to Typical Temperature Loop.
Note that as the controller gain is increased, i.e., KcKp increase, the closed loop time constant becomes smaller.
Also, note that as the controller gain is increased, the value of z decreases.

35. TEMPERATURE CONTROL
CONTINUOUS TEMPERATURE CONTROL
HEAT EXCHANGERS

36. TEMPERATURE CONTROL PHASE CHANGE TEMPERATURE CONTROL
BOILERS/CONDENSERS CAN BE ALSO BE CONTROLLED BY ADJUSTING THE SYSTEM PRESSURE. THE LOWER THE PRESSURE, THE LOWER THE CONDENSING OR BOILING TEMPERATURE
DIRECT-FIRED HEATERS
FIRED HEATERS USE CONTROLS ON THE FUEL SUPPLY TO THE BURNER THAT ARE CONNECTED TO RATIO CONTROL FLOWS FOR OXIDANTS
SOME BURNERS USE STAGED COMBUSTION TO MINIMIZE MAXIMUM TEMPERATURE AND THEREBY REDUCE NOx FORMATION

37. TEMPERATURE CONTROL HIGH AND LOW TEMPERATURE ALARM CONDITIONS
LOW TEMPERATURE IS NORMALLY A HAZARD WITH
LOSS OF FLAME FOR A COMBUSTION UNIT
FALLING BELOW VAPORIZATION TEMPERATURE
FALLING BELOW CRYSTALLIZATION TEMPERATURES
IN EACH CASE ABOVE, THE NORMAL RESPONSE IS TO STOP FLOW OF THE COOLANT TO THE SYSTEM AND RESTART AFTER ANALYSIS

38. TEMPERATURE CONTROL HIGH AND LOW TEMPERATURE ALARM CONDITIONS
HIGH TEMPERATURE IS NORMALLY A HAZARD WHEN
SYSTEM COOLING CAPACITY IS EXCEEDED BY HEAT GENERATION
EXCESSIVE TEMPERATURE CAUSES PRODUCT DEGRADATION
HIGH TEMPERATURES AND LEAD TO MATERIAL FAILURES
IN EACH CASE ABOVE, THE NORMAL RESPONSE IS TO STOP THE HEAT GENERATION PROCESS, MAXIMIZE THE FLOW OF THE COOLANT TO THE SYSTEM, AND RESTART AFTER ANALYSIS

39. COMPOSITION CONTROL CONTINUOUS CONTROL ON COLUMNS
COMPOSITION CONTROL ON COLUMNS CAN BE BASED ON ON-LINE SAMPLES OR SAMPLES ANALYZED OFF-LINE
ON-LINE SAMPLING USES SPECTROPHOTOMETRIC METHODS OR AN INDIRECT VARIABLE, SUCH AS CONDUCTIVITY OR VISCOSITY
OFF-LINE SAMPLING USES CHROMATOGRAPHY OR OTHER CHEMICAL METHODS
THE DEAD-TIME FOR SAMPLING IS TYPICALLY LESS THAN DEAD-TIME FOR PROCESS CHANGES
ACTUAL ADJUSTMENTS MAY BE MANUAL SETPOINT CHANGES

40. COMPOSITION CONTROL
The process is usually the slowest element followed by the sensor with the actuator being the fastest.
Use a PI controller unless the process is sufficiently sluggish to warrant a PID controller.

41. COMPOSITION CONTROL CONTROL IN REACTORS/BLENDING OPERATIONS
THE SAME METHODS AS DISCUSSED FOR DISTILLATION MAY BE APPLIED FOR REACTORS AND BLENDING
OTHER TECHNIQUES INCLUDE CONTROLLED FLOWS WHEN THE COMPOSITIONS ARE CONSISTENT
BATCH OPERATIONS ALSO CAN BE APPLIED

42. COMPOSITION CONTROL START-UP/SHUTDOWN OPERATIONS
CONTROL FOR STARTUP IS GENERALLY BASED ON ESTABLISHING STEADY STATE CONDITIONS BY USING MATERIAL FROM THE PREVIOUS STEADY-STATE OPERATION
SHUTDOWN CONTROLS DEPEND ON WHETHER THE MATERIALS CAN BE INVENTORIED WITHOUT DEGRADATION.

43. DO Control Loop
The process and the sensor have approximately the same dynamic response.
This is a fast responding process for which offset-free operation is desired. Therefore, PI controller should be used.

44. Biomass Controller The process for this system is the slowest element.
Because the process is a high-order sluggish process, a PID controller is required.

45. Pid controller summary.

46. Variable frequency drives
Variable frequency drives can eliminate the control valve In a system with a constant speed motor

47. Variable frequency drives
Variable frequency drives can significantly reduce energy consumption

48. Variable frequency drives
Basis of energy reduction are affinity laws

49. Variable frequency drives
affinity laws
Flow or volume varies linearly with speed. If speed decreases by 50%, flow decreases by 50%
Pressure or head varies as a square of the speed. If speed decreases by 50%, the pressure decreases to 25%
Power or energy consumption varies as a cube of the speed. If speed decreases by 50%, power consumption decreases to 12.5%

50. Variable frequency drives
PUMP AND DRIVE NEED TO BE SPECIFIED BASED ON SYSTEM CURVE

51. Variable frequency drives
MOST PUMPS ARE NOT DESIGNED TO OPERATE AT MAXIMUM FLOW
ENERGY SAVINGS USING A VFD ARE TYPICALLY SHOWN

52. Variable frequency drives
EXAMPLE FOR A WATER PUMPING SYSTEM
PAYBACK PERIODS RUN IN MONTHS, DEPENDING ON THE SIZE OF THE UNIT AND THE NORMAL OPERATING CONDITIONS.