Process Instrumentation, part 2: Control Loops and the Control Valve CM4120 Unit Operations Lab January 2010
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Outline
What is a Control Loop? A look at Regulatory Control Valves PID Controllers and terminology Instrument Connections to a Distributed Control System
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Typical Control Loop
All elements of a loop have same loop number
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Elements of Control Loop Input side: TE → Element to measure temp RTD vs. T/C
TT → Transmitter sends signal Dashed line - signal transmission line
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Elements of Control Loop
Controller: TIC → Temperature Indicating Controller → Shared Display, Analog signal
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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 6
Regulatory Control Valve Actuator (F.O. or F.C.?)
Trim set desirable to have flow linearly proportional to valve position for good control
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Valve Trim Inherent Characteristics Quick Opening safety by-pass type Large flow response when valve starts opening is more important than linear response Equal Percentage ~ 80% of all control valves provides linear response to valve position Linear used when majority of system pressure drop is due to valve position 8
Valve Trim Sizing: Flow Coefficient vs. Valve Position
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f(x)
By definition: for Cv = 1, 1 gpm flow w/ 1 psi pressure drop across valve
QO 0.5
Linear =%
0 0
20 40 60 80 Stem Position (% Open)
100
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Valve Selection Example “Control flow of reflux to distillation column” Determine pressure drop: @ design flow @ expected min/max flow
C.W.
FT
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System Response based on Pump/ Piping System Design Range of flow is 100 to 200 gpm: 25 Pressure Drop (psi)
Increase in valve opening → less ΔP across valve, but w/ increased line losses and decreased total available head from pump
Pump Head
20 Valve Δ P
15 10
Line Losses
5 0 0
50
100 150 Flow Rate (GPM)
200
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Installed Characteristic
Installed Flow Rate (GPM)
Size the valve trim, then select valve characteristic w/ the most linear response: 200
Linear Valve
150 =% Valve
100 50 0 0
20 40 60 80 Stem Position (% Open)
100
…use Equal Percent Characteristic valve to achieve a linear Installed Characteristic 12
“PID” Controllers
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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
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Relating this to our Control Loop: Setpoint
Process Variable Output
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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 Equation is “tuned” to match process response using up to 3 tuning constants
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Controller Terminology Tuning Constants: 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
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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
Emerson’s DeltaV System – current state of the Technology PID control Discrete logic control Signal conversions Alarming Fuzzy control, etc. are continuously executed by the “MD” controller
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Wiring Systems Connect Transmitters to DCS – at the Instrument End:
Level transmitter
Wiring to field junction cabinet RTD or T/C head
Wiring from transmitter to temp measuring element Temperature transmitters 21
Wiring Systems Connect Transmitters to DCS – at a Marshalling Cabinet: Single pairs from field devices 8 pr. Cables to controller cabinet
8-pr. cables run from Field Junction Box (Marshalling Cabinet) to Distributed Control System 22
Wiring Systems Connect Transmitters to DCS – in 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
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Wiring Systems Connect DCS to Transducers – at Marshalling Cabinet: 8-pr. cable from controller cabinet
Current to pneumatic transducers Air lines to control valves
Wire prs. to transducers
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Regulatory Control Valve Air line from I/P transducer Actuator w/ positioner Control valve
Block valves Bypass valve 25
Output Signals from Control System to Control Solenoids Solenoids for 2-position air-actuated ball valves
8-pr. cable from controller cabinet
Air lines to ball valves Wire prs. to solenoids
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Installed Field Devices: Ball Valve w/ Actuator Air line from solenoid Ball valve body Actuator Process line
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DeltaV & Foundation Fieldbus (4) mass flows, (4) densities, and (4) RTD temps (3) 8-multiplexed RTD temps (2) temp-only transmitters (1) wire
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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. 29
