A Review of Process Instrumentation, Control Valves, and the Control

LoopCM4120

Unit Operations LabJanuary 2011

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Outline

Process Instrumentation ReviewA look at Regulatory Control ValvesWhat is a Control Loop?PID Controllers and terminology

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Temperature Measurement

Thermocouples•low cost sensor

•needs transmitter/readout

•-440 to 5000ºF, typically 1 to 2ºF accuracy

•wide temperature range for various types

•rugged, but degrades over time

•many modern transmitters can handle T/C or RTD

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Temperature Measurement

RTD’s•-300 to 1150ºF, 0.1ºF accuracy or better

•more fragile, expensive than T/C

•very stable over time

•wide temperature range

•also needs readout/transmitter

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Pressure Measurement

Pressure Transmitters•available in gage pressure, absolute pressure and differential pressure

•typically 0.075% range accuracy

•50:1 turndown

•same transmitter and sensor body as in dP flow measurement and dP level

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Flow Measurement

Differential Pressure – Orifice Meter

•well-characterized and predictable

•causes permanent pressure (energy) loss in piping system, typically 8 ft. head loss (3 to 4 psi loss)

•5:1 rangeability

•requires straight run of 20 pipe diameters upstream, 5 downstream

•suitable for liquid, gas, and steam

•accuracy is 1 to 2% of upper range

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Flow Measurement

Turbine Flow Meter•accuracy is 0.25% of rate

•good for clean liquids, gases

•5 to 10 pipe diameters upstream/downstream

•10:1 turndown

•3 to 5 psig pressure drop

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Flow Measurement

Magnetic Flow Meter (Mag Meter)

•0.4 to 40 ft/s, bidirectional

•accurate to 0.5% of rate

•fluid must meet minimum electrical conductivity

•head losses are insignificant

•good for liquids and slurries

•upstream/downstream piping does not effect reading

•linear over a 10:1 turndown

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Flow Measurement

Vortex Flow Meter•suitable for liquids, steam, and gases

•must meet min. velocity spec

0.5 to 20 ft/sec range for liquid

5 to 250 ft/sec for gases

•non-clogging design

•not suitable if cavitation is a problem

•accuracy is ½% of rate

•up to 5 psig head loss

•linear over flow ranges of 20:1

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Flow Measurement

•used for steam, liquids, gases

•measure mass flow, density, temperature, volumetric flow

•expensive, but 0.2% of rate accuracy

•very stable over time

•100:1 turndown

•negligible to up to 15 psig head loss

Coriolis Effect Mass Flow Meter

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Level Measurement

Non-Contacting – Radar Level•suitable for liquids and solids

•foaming, turbulence, vessel walls and internals can effect signal if not installed correctly

•can use “stilling leg” if turbulence is extreme

•typically 0.1 inch accuracy

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Level Measurement

Contacting – dP Level•suitable for liquids only

•foaming and turbulence will effect signal

•can use “stilling leg” if turbulence is extreme

•typically 0.05% range accuracy

•100:1 turndown

•uses same dP transmitter as in dP flow measurement

Regulatory Control Valve

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Trim setdesirable to have flow linearly proportional to valve position for good control

Actuator(F.O. or F.C.?)

Valve Trim Inherent Characteristics

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Quick Openingsafety by-pass typeLarge 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

Flow Coefficient vs. Valve Position for each Valve Characteristic

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

Installed Characteristic

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Size the valve trim, then select valve characteristicw/ the most linear response:

…use Equal Percent Characteristic valve to achievea linear Installed Characteristic

0

50

100

150

200

0 20 40 60 80 100Stem Position (% Open)

Inst

alle

d Fl

ow R

ate

(GPM

)

Linear Valve

=% Valve

Typical Control Loop

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All elements of a loop have same loop number

“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

Controller TerminologyPID control

Dynamic equation that is used to match the controller’s response to a measured disturbance.Goal is to minimize disturbance and return to setpointEquation is “tuned” to match process response using up to 3 tuning constants

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Controller TerminologyTuning Constants:Proportional term – Adjusts output

proportional to the error multiplied by 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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Controller TerminologyController Modes of Operation:

Auto mode – Output of controller is set by PID algorithm based on features enabled and tuning parameters

Manual mode – Output is controlled by an operator (i.e. “not in control”)

Cascade mode – Output is controlled by PID but setpoint comes from the output of a “primary or master” controller

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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/, Micro

Motion, Inc., Oct. 2006.www.ametekusg.com/, Ametek, Inc. Oct. 2006.

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