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Basic process control

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Basic process control

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Basic process control

  1. 1. Basic Process Control
  2. 2. Basic Control Principles Level (Controlled Variable) Overflow Pump Inflow Supply l ll l ll
  3. 3. Open vs Closed Loop ! Closed loop ! Automatic control ! Open Loop ! Manual control ! Person takes the place of the controller
  4. 4. Feedback Control Comparator Amplifier Process (Tank) Error (SP - M) Manipulated Output (Controlled Variable - Level) Disturbances Variable Process Sensor Signal Measurement Set Point Controller
  5. 5. Feedback vs. Feedforward ! Feedback ! Control action after an error exists ! Feedforward ! Reacting to the disturbance before the error occurs
  6. 6. Typical ON/OFF Control System P1 L1 L2 V Qin SP Qout 1 P1 closes @ L1 P1 opens @ L2 Power Supply Solenoid Solenoid valve Electrically Operated Inflow l ll l ll
  7. 7. Typical ON/OFF Response L2 Periodic Time L1 SP time
  8. 8. Proportional Control
  9. 9. Level Control of Open Tank Qout LT SP Qin V1 20 - 100 kPa A/C l ll l ll
  10. 10. Simple Proportional System Pivot Outflow Float Valve Inflow lll ll l
  11. 11. Open Tank Control Qin Qout SP LT 20 - 100 kPa A/O LIC
  12. 12. Controller Action Qin Qout SP LT 20 - 100 kPa A/C LICl ll l ll Qin Qout SP LT 20 - 100 kPa A/O LICl ll l ll Direct Reverse
  13. 13. Proportional Control + - k•e Setpoint Measurement Error Bias + + Output m= k(SP-M) + bias
  14. 14. Proportional Band and Gain Proportional Band– the input change required to change the output 100% PB gain %100 = input output gain ∆ ∆ =
  15. 15. Narrow, Wide, High & Low Wide PB – Low Gain Narrow PB – High Gain OutIn
  16. 16. Proportional Control Response Curve Offset Outflow Loss in Volume New mass balance occurs here New level below setpoint timet 1 1t0t 0t time at setpoint Level originally Input/Output Level t Inflow
  17. 17. Proportional Response with Narrower PB Level originally Offset tt 0 time 1Level at setpoint below setpoint New level New mass balance occurs hereLoss in Volume Inflow Input/Output 0 t 1 t t Outflow time
  18. 18. Response Versus PB, Proportional Control Only time Step Disturbance LoadChange Offset Offset time SystemResponse SP SP SP "Wide" PB "Moderate" PB "Narrow" PB
  19. 19. ¼ Decay Response Curve A A/4 A/16 SystemResponse time
  20. 20. For You To Do ! Read pp. 89-105 ! Answer Questions pp. 121-122, #1-19
  21. 21. Reset or Integral
  22. 22. Response Curve: Proportional Control Only SystemResponse SP Step Disturbance time Offset
  23. 23. Additional Control Signal Restores Process to Setpoint Setpoint Initial mass balance Inflow Outflow Final mass balance Reset Action time Offset Removed
  24. 24. Integral Action biasedt TR kem +      += ∫ 1 + - k•e Setpoint Measurement Error Bias + + Output ∫edt TR 1
  25. 25. Units ! Minutes per repeat ! MPR ! The length of time that it will take the integrator to add an amount equal to the proportional response ! Repeats per minute ! RPM ! The number of times the proportional response is repeated in one minute
  26. 26. Proportional Plus Reset, Open Loop Response time e Fast Reset Normal Reset Slow Reset error ke Proportional Response} ControlSignal
  27. 27. A problem Output initially 50%, Gain = 2, reset = 2 minutes per repeat A direct acting controller control is subjected to a sustained error of 5% What is the output after 4 minutes? Proportional Response = ke = 2x5=10% Integral Action- in 4 minutes the control will go through 2 repeats. Integral action = 2 x 10 = 20% Total output change is proportional + integral = 30%
  28. 28. A Couple More Things ! Reset Windup ! Instability because of lag
  29. 29. For You to Do ! Read over text pp. 89 –110 ! Answer questions pp. 121-122, 1-24
  30. 30. Rate or Derivative
  31. 31. Proportional and Derivative – Open Loop Pressure time InputOutput Derivative Derivative ceases as error stops changingProportional Action
  32. 32. Derivative Control + - k•e Setpoint Measurement Error Bias + + Output ∫edt TR 1 dt de TD + + + bias dt de kTedt T kem D R ++      += ∫ 1
  33. 33. Simple Flow Control System lll FT FC A/C √ _
  34. 34. The open Loop Response of Proportional Plus Derivative (PD) Action to Rapidly Changing Error Signals 0 t 1 t 2 t 0 t 1 t 2 t time time A B C Rate action due to cessation of increase in e Rate action B -C Rate action A - B Proportional action A - B Proportional action B - C Rate action due to cessation of increase in e Control signal at end of excursion Control Signal % Process
  35. 35. Large System Under Proportional and Proportional Plus Derivative Control time Control Signal Setpoint Level Load Disturbance Applied Prop. + Derivative Prop. Only
  36. 36. Multiple Control Modes ! Virtually all controls have a proportional response ! Integral and derivative are added to improve performance ! Majority have proportional and integral ! Some, typically heat exchangers have derivative added
  37. 37. Open Tank Level Control With Valve In Inflow Qi A/O l ll l llQo SP LIC LT
  38. 38. Typical Flow Control Loop ll FT FIC A/O √ _ SP
  39. 39. Pressure Control – Constant Bleed PIC PT A/C Feed SP Pressure Vessel Pressure Bleed
  40. 40. Split Ranged Feed and Bleed Pressure Control PIC PT A/C Feed SP Pressure Vessel Bleed A/O
  41. 41. Representative Hot Bleed/Cold Service Water Heat Exchanger TT TC A/C SP Cooled Bleed Hot Bleed Cold
  42. 42. For You To Do ! Read pp. 106-120 ! Answer Questions pp. 122-123, #20-38

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