Control Valves and the Control Loop
Transcript of Control Valves and the Control Loop
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Process Instrumentation,
part 2:Control Loops and the ControlValve
CM4120Unit 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
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All elements of a loop have same loop number
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Elements of Control Loop
Input side:
TE Element to measuretempRTD 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 LoopOutput side:
TV Valve to regulatesteam flow
TY Transducer converts
electric signal topneumatic
Solid line w/ dashes ispneumatic signal line
F.C. is Fail position
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Regulatory Control Valve
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Trim setdesirable to have
flow linearly
proportional to
valve position forgood control
Actuator
(F.O. or F.C.?)
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Valve Trim I nherent Characteristics
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Quick Openingsafety by-pass type
Large flow response when valve starts opening ismore important than linear response
Equal Percentage~ 80% of all control valvesprovides linear response to valve position
Linearused when majority of system pressure drop isdue to valve position
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Valve Trim Sizing: Flow
Coefficient vs. Valve Position
0
0.5
1
0 20 40 60 80 100Stem Position (% Open)
f(x)
=%
QOLinear
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By definition:for Cv = 1,
1 gpm flow w/
1 psi pressure dropacross valve
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Valve Selection Example
C.W.
FT
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Control flow of reflux to distillation column
Determine pressure drop:
@ design flow
@ expected min/max flow
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System Response based on
Pump/ Piping System
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Design Range of flow is 100 to 200 gpm:
Increase in valveopening lessP
across valve, but w/
increased line lossesand decreased total
available head from
pump 0
5
10
15
20
25
0 50 100 150 200Flow Rate (GPM)
PressureD
rop(psi)
Line Losses
Pump Head
Valve P
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Installed Characteristic
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Size the valve trim, then select valve characteristic
w/ the most linear response:
use Equal Percent Characteristic valve to achieve
a linearInstalled Characteristic
0
50
100
150
200
0 20 40 60 80 100Stem Position (% Open)
InstalledFlowRate
(GPM
)
Linear Valve
=% Valve
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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:
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Process Variable
Output
Setpoint
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Controller TerminologyPI D control Dynamic equation that is used to match the
controllers response to a measureddisturbance.
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 TerminologyTuning Constants:Proportional term Adjusts outputproportional to the error, Gain
I ntegral 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 Controllers1930s Pneumatic Controllers air pressure w/ flappers, bellows, and valves
adjust valve position based on measured
process variable for P, PI, later PID control1950s Electronic Controllers transistors, resistors, and capacitors for P, PI,
PID control capable of remote installation1960s Mainframe Computer Control Refineries were typical users
Alarming capability and supervisory control Single point of failure, no user-friendly
graphical interface
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Evolution of Controllers
Late 1970s Distributed Control Systems(DCS)
Networked computers distributed thru plant Pre-configured controllers Data archival capabilities Included an operator console Hardware is proprietaryLate 1990s DSCs built on commodity
hardware platforms Better scalability Affordable Interactive graphical interface
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Emersons DeltaV System current state of the Technology
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PID control
Discrete logic control
Signal conversions
AlarmingFuzzy control, etc.
are continuouslyexecutedby the MD controller
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Wiring Systems ConnectTransmitters
to DCS at the Instrument End:
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Wiring to fieldjunction cabinet
RTD or T/C head
Temperature
transmitters
Wiring from
transmitter to temp
measuring element
Level transmitter
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Wiring Systems ConnectTransmitters
to DCS at a Marshalling Cabinet:
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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
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Wiring Systems ConnectTransmitters
to DCS in the Controller Cabinet:
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DeltaV MD controller
8 pr. cables from
field junction cabinet
Power-limiting
Zener barriers
I/O cards
2nd I/O chassis w/4-20
mA Output cards
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Wiring Systems Connect DCS to
Transducers at Marshalling Cabinet:
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Wire prs. to transducers
Current to pneumatic
transducers
Air lines to
control valves
8-pr. cable from
controller cabinet
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Regulatory Control Valve
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Air line from I/P transducer
Actuator w/ positioner
Control valve
Block valves
Bypass valve
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Output Signals from Control System
to Control Solenoids
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8-pr. cable from
controller cabinetSolenoids for 2-position
air-actuated ball valves
Air lines to
ball valves
Wire prs. to solenoids
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Installed Field Devices:
Ball Valve w/ Actuator
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Air line from
solenoid
Actuator
Ball valve body
Process line
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DeltaV &Foundation Fieldbus
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(4) mass flows, (4) densities,
and (4) RTD temps
(3) 8-multiplexed RTD temps(2) temp-only transmitters
(1) wire
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ReferencesMiller, 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/, MicroMotion, Inc., Oct. 2006.
www.ametekusg.com/, Ametek, Inc. Oct. 2006.
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http://www.emersonprocess.com/rosemount/http://www.emersonprocess.com/micromotion/http://www.ametekusg.com/http://www.ametekusg.com/http://www.emersonprocess.com/micromotion/http://www.emersonprocess.com/rosemount/