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I. TYPE OF CONTROL 1. BOILER – A. ANALOG CONTROL INTENSIVE AND COMPLICATED (FOR ALL TYPES) B. DIGITAL CONTROLS GAS OIL FIRED SOLID FUEL FIRED FLUIDIZED COMB. VERY CRITICAL CRITICAL CRITICAL C. MONITERING I. SWAS II. STACK GAS MONITORING
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2. TURBINE – A. ANALOG CONTROLS – LESS COMPLICATED & FEW IN NOS B. DIGITAL CONTROLS – VERY CRITICAL & FOR TRIP & DRIVE CONTROL C. TSI & GOVERNOR CONTROLS – VERY CRITICAL WITH RESPECT TO CONTROL AND SPEED OF RESPONSE
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II. FAIL SAFE PHILOSOPHY OF CONTROL (FOR BOILER + TURBINE) FAIL SAFE: <ul><li>INTERLOCKS / SHUTDOWN </li></ul><ul><li>TRANSMITTERS / SENSORS </li></ul><ul><li>FINAL CONTROL ELEMENTS </li></ul>TYPE OF FAILSAFE POSITIONS <ul><li>FAIL OPEN </li></ul><ul><li>FAIL CLOSE </li></ul><ul><li>STAYPUT </li></ul>FAIL OPEN / FAIL CLOSE IN CONTROL VALVES <ul><li>ON SIGNAL FAILURE </li></ul><ul><li>ON POWER AIR FAILURE </li></ul>STAYPUT: ON POWER AIR FAILURE
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REDUNDANCY REQUIREMENTS MOST OF THE EMERGENCY CONTROL SYSTEMS ARE STATIC . i.e. THEY JUST REMAIN WITHOUT OPERATING FOR QUITE SAME TIME. HOW ARE WE SURE THAT THE SYSTEM OPERATES WHEN IT IS REQUIRED TO OPERATE?
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I. IN NORMAL LIFE FOLLOWING ARE FEW EXAMPLES <ul><li>UPS / diesel engine operation on failure of EB supply. </li></ul><ul><li>2. A fuse to blow on high current or short circuit. </li></ul><ul><li>3. An umbrella to unfold on sudden rain. </li></ul><ul><li>4. Kerosene stove to light on when gas cylinder getting </li></ul><ul><li>emptied at home. </li></ul><ul><li>5. Torchlight to light while power is off. </li></ul><ul><li>6. Fire fighting co2 cylinder to open while fire catches </li></ul><ul><li>up in a public place like cinema theatre. </li></ul><ul><li>7. Alarm clock to work for a critical early morning wakeup. </li></ul><ul><li>8. A relay to energize and trip the equipment on faulty </li></ul><ul><li>condition. </li></ul>
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II. TO IMPROVE ON THE DEPENDABILITY OF THE CONTROL SYSTEM IT IS NORMALLY HELD IN ENERGIZED CONDITION AND DE- ENERGIZED DURING A FALLTY CONDITION.
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A. SAFE FAILURES These are called safe failures OR FALSE TRIPS OR NUISANCE TRIPS Since they stop the process unnecessarily These faults REDUCE THE AVAILABILITY AND CAUSE PRODUCTION LOSS A wire opens Output De-energizes Process shutdown I. Fault in system Output De-energizes Process shutdown II.
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B. DANGEROUS FAILURES Faults that cause the outputs to remain Energized are generally not detected and are Considered “ DANGEROUS FAILURES” Because the system cannot tell the difference Between ‘Normal Operation’ and ‘Failure’ And therefore cannot take the process to a Safe state when required to do so. FAULT TOLERENCE “ Ability to tolerate a single failure and Continue to operate”. Can be implemented using various redundancy Schemes.
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CHARACTERISTICS OF SYSTEM ARCHTECTURES FOR SAFETY SHUTDOWN A 1 00 1 1 Out Of 1 S Process WHEN A FAILS, PLANT TRIPS: SAFETY - LOW AVAILABILITY - LOW
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B 1 Out Of 2 S Process WHEN ‘A’ OR ‘B’ FAILS, PLANT TRIPS: SAFETY - V.HIGH AVAILABILITY - LOW A 1 00 2
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B 2 Out Of 2 S Process WHEN A & B FAIL THE PLANT TRIPS: SAFETY - LOW AVAILABILITY – V.HIGH A 2 00 2
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Process A S B C WHEN 2 (OR 3) OUT OF THE THREE (A,B,C) FAIL THEN ONLY PLANT TRIPS: SAFETY - HIGH AVAILABILITY - HIGH 2 Out Of 3 2 00 3
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REDUNDANCY AT SENSOR LEVEL FOR ANALOG SIGNALS HIGH HIGH 2 OUT OF 3 TRANSMITTER 2 00 3 HIGH AVERAGE 2 OUT OF 1 TRANSMITTER 1 00 2 LOW LOW SINGLE TRANSMITTER 1 00 1 AVAILABILITY SAFETY
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2 00 3 FT 1 FT 2 MID VALUE SELECTION OUTPUT FT 3
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REDUNDANCY IN CONTROL SYSTEM 1. REDUNDANCY IN DCS / PLC a) AT PROCESSER LEVEL b) AT POWER SUPPLY LEVEL c) AT COMMUNICATION LEVEL d) AT I/O LEVEL e) AT OPERATOR STATION LEVEL
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CONTROLLER AND DATA ACQUISITION SYSTEM I/O MODULES OPERATING & ENGINEERING STATION (BOP) OPERATING STATION (BOILER) CONTROLLER AND DATA ACQUISITION SYSTEM LOG PRINTER ALARM &EVENT PRINTER OPERATING STATION (TURBINE)
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In order to achieve proper automatic control of a process, it is necessary to know the characteristics of the process material itself, as well all the devices used for process control. A given process might require the control of temperature, pressure, density and level,etc,. Also regulating one variable can affect other variables.
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For example, regulating temperature in a process might affect the pressure and the density. Regulating the flow rate of material to a process may also affect the level of material in storage vessel. It is important, therefore, to select a controlling device which most accurately affects the variable to be controlled.
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ON – OFF CONTROL In this control, when the value of the measured or controlled variable is at or above the SP, the final control element is closed. When the range is below the SP, the final control element is open. A common example of ON – OFF control action is a thermostat. Consider the thermostat is set for 68 . F. If the temperature drops below 68 . F, the heating unit is actuated.
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0% 100% FE CLOSED FE OPEN RANGE OF MEASUREMENT SP OFF ON ON-OFF CONTROL
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PROPORTIONAL ACTION This action provides the control valve with variant positions between ON and OFF. The position of the FCE is not simply open or closed, but varies, depending on how much the value of the measured variable is above or below the SP. The amount of energy to the process varies accordingly.
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Its actuating output is proportional to the error signal. c(t) = K c e(t) + C s where, c(t) - controller output K c – proportional gain of the controller e(t) - error signal C s – controller’s bias signal (i.e, its actuating signal when e(t)=0)
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A proportional controller is described by the value of its proportional gain K c or equivalent by its proportional band PB, where PB=100/K c . The proportional band characterizes the range over which the error must change in order to drive the actuating signal of the controller over its full range . usually, 1 < PB < 500
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0% RANGE OF MEASUREMENT 100% FINAL ELEMENT FULLY CLOSED FINAL ELEMENT FULLY OPEN INTERMEDIATE POSITIONING OF FCE SP PROPORTIONAL CONTROL
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PROPORTIONAL ACTION WITH RESET This action enables the FCE to assume intermediate position. In addition, it can shift the relationship between the FCE and the value of the measured or controlled variable. The controller continues the corrective positioning of the FCE until the measured variable returns to the desired valve.
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Its actuating signal is related to the error by the equation c(t) = k c e(t) + e(t)dt + C s where, T I – Integral time constant or reset time, in min. The reset time is an adjustable parameter and is sometimes referred to as minutes per repeat. Usually it varies in the range 0 < T I < 50 min 0 t k c T I
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Some manufacturers do not calibrate their controllers in terms of T I but in terms of its reciprocal, 1/T I (repeats/min), which is known as reset rate. Consider the error changes by a step of magnitude e.Initially the controller output is K c e (the contribution of the integral term is zero). After a period of T I min the contribution of the integral term is k c T I e(t)dt 0 T I k c T I = eT I = k c e
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It is clear from the above equation that the integral control has repeated the response of the proportional action. This repetition takes place every T I min and has lent its name to the reset time.
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PROPORTIONAL-PLUS-RESET ACTION C s C s + k c e C s + 2k c e C s + 3k c e 0 T I 2T I Time c(t)
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PI CONTROLLER CONTROLLER DP CELL FLOW SP e FLOW ELEMENT
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PROPORTIONAL ACTION WITH RESET AND DERIVATIVE Derivative, or rate action, adds even more flexibility to the movement of the FCE. Response is slow in same processes because a long period of time is needed to detect and correct changes in the measured variable. Derivative action provides correct positioning of the FCE related to the rate at which the value of the manipulated variable is changing.
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The output of this controller is given by where, T D – Derivative time constant,in min. With the presence of derivative term(de/dt), the PID controller anticipates what the error will be in the immediate future and applies a control action which is proportional to the current rate of change of error. c(t) = k c e(t) + e(t)dt + k c T D + C s k c T I 0 t de dt
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Due to this property, the derivative control action is sometimes referred to as anticipatory control. DRAW BACKS OF DERIVATIVE CONTROL ACTION: For a response with constant non zero error it gives no control action since de/dt=0. For a noisy response with almost zero error it can compute control action, although it is not needed.
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PID CONTROLLER TO PROCESS STEAM HDR SPRAY WATER TE TT PT DSH PRV STEAM FROM BOILER
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<ul><li>FEATURES OF CONTROLLERS </li></ul><ul><li>PROPORTIONAL CONTROLLER </li></ul><ul><li>Accelerates the response of a controlled process. </li></ul><ul><li>Produces an offset (i.e, non zero steady state error) as a result of sustained load change. </li></ul>
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<ul><li>Eliminates any offset. </li></ul><ul><li>Produces sluggish, long oscillating responses. </li></ul><ul><li>If we increase the gain K c to produce faster response, the system becomes more oscillatory and may lead to instability. </li></ul>INTEGRAL CONTROLLER
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<ul><li>DERIVATIVE CONTROLLER </li></ul><ul><li>Anticipates future errors and introduces appropriate action. </li></ul><ul><li>Introduces a stabilizing effect on the closed loop response of a process. </li></ul>
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SELECTING THE TYPE OF CONTROLLER Following rules can be adopted in selecting the most appropriate controller for a process. 1. If possible, use proportional controller. Proportional controller can be used if we can achieve acceptable offset with moderate values of K c P Controller is recommended for liquid level control where there is not sustained load change.
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2. If a simple P controller is unacceptable, use PI controller. A PI controller should be used when proportional control alone cannot provide sufficiently small steady state errors (offsets). Therefore, PI will be generally used in liquid level systems where there is sustained load change and also for flow control.
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The response of a flow control system is rather fast. Consequently, the speed of the closed loop system remains satisfactory despite the slowdown caused by the integral control mode.
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3. Use a PID controller to increase the speed of the closed loop response. The PI eliminates the offset but reduces the speed of the closed-loop response. For a multicapacity process whose response is very sluggish, the addition of a PI controller makes it even more sluggish.
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In such cases the addition of the derivative control action with its stabilizing effect allows the use of higher gains which produce faster responses without excessive oscillations. Derivative action is recommended for temperature and composition control.
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Consider 2 cars ‘A’ and ‘B’ running on a main road. OBJECTIVE : Driver of car A aims at running his car along with car B ( i.e., car B’s position is the Set Point) say, speed of B is 50 km/h. A B 50 km/h 50 km/h
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PROPORTIONAL ACTION Consider driver A is a proportional controller.Therefore, whenever speed of B changes A’s speed will change according to the distance between A and B. Assume B’s speed changes to 60 km/h(Load change) which means in 1 hr B will be ahead of A by 10 km. A’s speed changes according to the distance between A and B say 1 km/h increase per 1 km of distance.
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So A’s speed goes on increasing and B goes more and more away from A. At one point(when distance between A and B is 10 km) A’s speed will be equal to B’s speed and A and B will have a constant distance between them. But as per our objective A cannot be along with B.This constant distance between them is offset.
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INTEGRAL ACTION Now consider driver of A is a integral controller.Here, after each km the driver of car A senses the distance between A and B and increases its speed accordingly.As it approaches B, A slows down its speed and when along with B, A drives in a constant speed.
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Consider A is a derivative controller.Assume A and B are together.When B accelerates his speed A senses the rate of change of B’s speed by sensing the rate of change of distance between A and B.A accelerates its speed in proportion to the rate of change of distance between them. Thus, combination of proportional + integral + derivative action shall keep cars A and B together. DERIVATIVE ACTION
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GAIN (P) KC X + _ INPUT SET PT e (t) T I (A/B) B X INTEGRATOR SCALE 100 COUNTS / SEC INPUT – 100 – 0 - +100 DEAD TIME 1 SEC + _ X to K C X DERIVATIVE TIME TD IN SECS. CONSTANT d (e) dt I + OUTPUT CD (CONTROLLER’S B, AS SIGNAL D INTEGRAL TIME CONST IN SECS A K C . e(t) PROP. OUTPUT (P)
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