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Topic 8

Topic 8. Process Control Troubleshooting. Topic 1 Introduction To Process Control. Topic 2 Introduction To Process Dynamics. Topic 3 Plant Testing And Data Analysis. Topic 6 Process Control Hardware Systems. Topic 7 Control Valves. What We Will Cover. Topic 4

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Topic 8

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  1. Topic 8 Process Control Troubleshooting

  2. Topic 1 Introduction To Process Control Topic 2 Introduction To Process Dynamics Topic 3 Plant Testing And Data Analysis Topic 6 Process Control Hardware Systems Topic 7 Control Valves What We Will Cover Topic 4 Controller Actions And Tuning Topic 5 Enhanced Regulatory Control Strategies Topic 8 Process Control Troubleshooting

  3. When a Plant is Suffering • There are two things that tell us when a plant is having a problem • Lab results • Off-spec results during normal operations • Process values • Out-of-ordinary values • Noisy values • Fluctuating values • Frozen values • Problems refer to • Plant upset • Off-spec product • Faulty instrumentation / Final control element

  4. Process Design Process Applications Process Control Considerations Process Operations Process Equipment Control Strategy Controller Tuning Field Devices Hierarchy of a Process

  5. Process Design • A result of errors in assumption, calculations, etc • Examples • Too few stages in a distillation column because a wrong property was used • Over-sized control valve due to use of wrong equation / assumptions • Long term problem

  6. Process Applications • Sizing is correct, but the wrong equipment is picked • Example • Cross-flow heat exchanger instead of shell-and-tube • CSTR instead of PFTR • Long term problem

  7. Process Operations • Sizing and application are correct, operations is wrong • Wrong procedure • Did not follow procedure • Operators misunderstand the use of the equipment / the process • Eg. Steam at the bottom of the FCC fractionator is to create turbulence, not for stripping

  8. Process Equipment • Equipment starts to malfunction or fail • Wear and tear (especially rotating equipment) • Fouling • Catalyst deactivation • Plugging of lines • Coking of furnace pipes

  9. Process Control Loop

  10. Primary Element • The element that is in physical contact with the process fluid • If something is wrong here, nothing is going to be right • *Primary element has a property that is used to INFER the process value

  11. Transmitters • Electronic devices – electronic board can fail • Subjected to “white noise” • Impulse lines plugged • Exp 4, LT always plugs • Can be calibrated wrongly

  12. I/P Converter • Converts 4-20 mA signals from DCS (controller OP) to a 3-15 psig pneumatic signal for control valve operation • Opposite of transmitters • Subjected to calibration problems • Can only work if the instrument air pressure is enough

  13. Control Valve • Control valve problems have been covered in Topic 8 • Deadband • Backlash • Hysteresis

  14. Controllers • Easiest to determine if problem is due to controller, but not so easy to solve the problem • To see if the problem is caused by the controller, take away its action and see if problem persists • For single-loops : put controller in MAN • For cascade loops : put secondary controller in MAN first; if no problem then put to AUTO • For feedforward loops : disconnect feedforward controller • For the loops above, when the problem disappears when the controller is in MAN, problem is with the controller

  15. Controllers • PID : Tuning? • Feedforward : Model? • FF gain, lead-lag, deadtime compensation • Note that some processes are inherently difficult to control • Long deadtimes (Try model based control instead) • Non-linear • Highly unstable

  16. Case Study 1

  17. Case Study 1

  18. Case Study 1

  19. Video Case Study 2:BP Texas City Refinery Explosion

  20. BP Texas City Refinery Explosion • Info from U.S. Chemical Safety and Hazard Investigation Board • Process Design • No level high cut out as part of safety interlock system • Process Operation • Operators deviated from procedure • Raffinate splitter tower level control supposed to be on AUTO but was placed in MAN. MV set at 0% (outlet valve fully closed) • SP should have been 50%, but level was allowed to exceed transmitter range • Happened in a number of occasions before this • Poor communication between shift teams • Operators worked 12hr shifts, 7 days a week for 29+ days • Insufficient supervisor and operator staffing • 1999, budget cuts caused reduced manpower • 2 operators reduced to 1 • Additional workload added • Insufficient operator training in abnormal situation management

  21. BP Texas City Refinery Explosion • Process Equipment Maintenance • Level transmitter miscalibrated so level reading erroneously showing declining level • Level gauge sight glass dirty so unreadable • Redundant level high alarm failed to sound • Process Control Strategy • Poor operator display design: Tower liquid holdup not shown • Tower inlet and outlet flows shown on separate displays

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