Process Instrumentation, Part 2: Control Loops and the Final Control Element

1. Process Instrumentation, Part 2:Control Loops and the Final Control Element CM4110 Unit Operations Lab October 2008

2. Outline • What is a Control Loop? • A look at Regulatory Control Valves • Types of Controllers • Instrument Connections to Control Systems

3. Typical Control Loop

4. Elements of Control Loop Input side: TE → Element to measure temp • RTD vs. T/C TT → Transmitter sends signal Dashed line - signal transmission line

5. Elements of Control Loop Controller: • TIC → Temperature Indicating Controller • → Shared Display, Analog signal All elements of a loop have same loop number

6. Elements of Control Loop Output side: • TV → Valve to regulate steam flow • TY → Transducer converts electric signal to pneumatic • Solid line w/ dashes is pneumatic signal line • F.C. is Fail position

7. Regulatory Control Valve Actuator Trim set desirable to have flow linearly proportional to valve position for good control

8. Valve Trim Inherent Characteristics Quick Opening safety by-pass type, quick opening is more important than linear response Equal Percentage ~ 80% of control valves – provides linear response to valve position Linear used when majority of system pressure drop is due to valve position

9. Flow Coefficient vs. Valve Position By definition: for Cv = 1, 1 gpm flow w/ 1 psi pressure drop across valve

10. Valve Selection Example Typical flow control problem: “Control flow of reflux to distillation column” Determine pressure drop: @ design flow @ expected min/max flow

11. System Response based on Design Conditions Range of flow is 100 to 200 gpm: Increase in valve opening → less ΔP across valve, but w/ increased line losses and decreased total available head from pump

12. Installed Characteristic Size the valve, then select valve characteristic w/ the most linear response: Use Equal Percent Inherent Characteristic valve to achieve a linear Installed Characteristic

13. Types of Controllers

14. Review of Controller Terminology Process Variable (PV) = Measured variable of interest, in EU Setpoint (SP) = Desired value of the PV, in EU Output (OP) = Controller output, 0-100% Error = Difference between Setpoint and PV

15. Typical Control Loop Setpoint Process Variable Output

16. Controller Terminology PID control • Dynamic equation that is used to match the controller’s response to a measured disturbance. • Goal is to minimize disturbance and return to setpoint Proportional term – Adjusts output proportional to the error, Gain Integral term – Added to output based on error existing over time, Reset Derivative term – Additional adjustment to output based on rate of change of error, Rate

17. Evolution of Controllers 1930’s – Pneumatic Controllers • air pressure w/ flappers, bellows, and valves adjust valve position based on measured process variable for P, PI, later PID control 1950’s – Electronic Controllers • transistors, resistors, and capacitors for P, PI, PID control • capable of remote installation 1960’s – Mainframe Computer Control • Refineries were typical users • Alarming capability and supervisory control • Single point of failure, no user-friendly graphical interface

18. Evolution of Controllers Late 1970’s – Distributed Control Systems (DCS) • Networked computers distributed thru plant • Pre-configured controllers • Data archival capabilities • Included an operator console • Hardware is proprietary Late 1990’s – DSC’s built on commodity hardware platforms • Better scalability • Affordable • Interactive graphical interface

19. DeltaV & MD Controller – Today’s DCS PID control Discrete logic control Signal conversions Alarming Fuzzy control, etc. are continuously executed by the MD controller

20. Wiring Systems Connect Transmitters to DCS – at the Instrument End: Wiring to field junction cabinet Level transmitter RTD or T/C head Wiring from transmitter to temp measuring element Temperature transmitter

21. 8 pr. Cables to controller cabinet Wiring Systems Connect Transmitters to DCS – at the Marshalling Cabinet: Single pairs from field devices 8-pr. cables run from Field Junction Box (Marshalling Cabinet) to Distributed Control System

22. Wiring Systems Connect Transmitters to DCS – at the Controller Cabinet: DeltaV MD controller I/O cards Power-limiting Zener barriers 8 pr. cables from field junction cabinet 2nd I/O chassis w/4-20 mA Output cards

23. Wiring Systems Connect DCS to Transducers – at the Marshalling Cabinet: 8-pr. cable from field junction cabinet Current to pneumatic transducers Wire prs. to transducers Air lines to control valves

24. Regulatory Control Valve Air line from I/P transducer Actuator w/ positioner Control valve Block valves Bypass valve

25. Output Signals from Control Systemto Control Solenoids 8-pr. cable from field termination cabinet Solenoids for 2-position air-actuated ball valves Air lines to ball valves Wire prs. to solenoids

26. Installed Field DevicesBall Valve w/ Actuator Air line from solenoid Ball valve body Actuator Process line

27. DeltaV & Foundation Fieldbus (4) mass flows, (4) densities, (4) RTD temps (3) 8-multiplexed RTD temps (2) temp-only transmitters

28. References Miller, Richard W., Flow Measurement Engineering Handbook, 3rd Ed., McGraw-Hill, New York, 1996. Riggs, James B., Chemical Process Control, 2nd Ed., Ferret Publishing, Lubbock, TX, 2001. Taylor Instrument Division, The Measurement of Process Variables, no date. www.emersonprocess.com/rosemount/, Rosemount, Inc., Oct. 2006. www.emersonprocess.com/micromotion/, Micro Motion, Inc., Oct. 2006. www.ametekusg.com/, Ametek, Inc. Oct. 2006.