CHE 185 – PROCESS CONTROL AND

DYNAMICSSTANDARD CONTROL LOOPS

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.

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

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.

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:

COMPARISON BETWEEN PI AND PID FOR A LOW θp/τp RATIO

COMPARISON BETWEEN PI AND PID FOR A HIGH θp/τp RATIO

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

FLOW CONTROL

• CONTINUOUS CONTROL– THE STANDARD FLOW CONTROL LOOP SHOULD

INCLUDE THE FOLLOWING COMPONENTS– ALSO SEE FIG 7.10.1 IN TEXT

FC

FT

I PPI

S/PDISPLAY

FCV FE

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.

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.

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.

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

FLOW CONTROL

• HIGH/LOW FLOW SWITCHES CAN BE SETPOINTS ON THE FLOW CONTROLLER

• CONNECTED TO ALARMS• MEANT TO ALERT OPERATOR TO ACT

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

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

LEVEL CONTROL• CONTINUOUS LEVEL CONTROL LOOPS

CONTAIN THE FOLLOWING COMPONENTS

• ALSO SEE FIGURE 7.9.2

LC LTI PPI

S/PDISPLAY

LCV

LEPROCESS

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

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

LC

S/P

LCV

INCOMING FLOW

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.

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.

LEVEL CONTROL

TC

S/P

TCV

INCOMING STEAM FLOW

HEATED PROCESSFLOW

CONDENSINGHEATEXCHANGER

ENTERING PROCESSFLOW

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

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

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

PC

PT

I PPI

S/PDISPLAY

PCV

PE

PROCESS

VENT ORVACUUM

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.

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 HTTP://VACUUM.TUTHILL.COM/PRODUCTCATALOG/SCAN.ASP

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

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

SELF-CONTAINEDPRESSUREREGULATINGVALVE

SELF-CONTAINEDBACKPRESSUREREGULATINGVALVE

P P

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

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

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.

TEMPERATURE CONTROL• ANALYSIS OF PI CONTROLLER APPLIED TO

TYPICAL TEMPERATURE LOOP.

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.

TEMPERATURE CONTROL• CONTINUOUS TEMPERATURE CONTROL

HEAT EXCHANGERS

PROCESSFLOW IN

PROCESSFLOW OUT

UTILITYFLOW IN

UTILITYFLOW OUT

TC

HEATEXCHANGER

1ST PROCESSFLOW IN

1STPROCESSFLOW OUT

2NDPROCESSFLOW IN

2NDPROCESSFLOW OUT

TC

HEATEXCHANGER

PROCESS FLOW BYPASS

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

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

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

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

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.

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

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.

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.

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.

PID CONTROLLER SUMMARY• .

VARIABLE FREQUENCY DRIVES

• VARIABLE FREQUENCY DRIVES CAN ELIMINATE THE CONTROL VALVE IN A SYSTEM WITH A CONSTANT SPEED MOTOR

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VARIABLE FREQUENCY DRIVES

• VARIABLE FREQUENCY DRIVES CAN SIGNIFICANTLY REDUCE ENERGY CONSUMPTION

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VARIABLE FREQUENCY DRIVES

• BASIS OF ENERGY REDUCTION ARE AFFINITY LAWS

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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%

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VARIABLE FREQUENCY DRIVES

• PUMP AND DRIVE NEED TO BE SPECIFIED BASED ON SYSTEM CURVE

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VARIABLE FREQUENCY DRIVES• MOST PUMPS ARE NOT DESIGNED TO OPERATE AT MAXIMUM

FLOW• ENERGY SAVINGS USING A VFD ARE TYPICALLY SHOWN

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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.

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