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Process Instrumentation,

part 2:Control Loops and the ControlValve

CM4120Unit Operations Lab

January 2010

1

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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

3

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

5

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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

7

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

8

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

9

By definition:for Cv = 1,

1 gpm flow w/

1 psi pressure dropacross valve

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Valve Selection Example

C.W.

FT

10

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

11

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

12

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

20

PID control

Discrete logic control

Signal conversions

AlarmingFuzzy control, etc.

are continuouslyexecutedby the MD controller

http://www.easydeltav.com/index.asp

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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:

23

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

http://www.easydeltav.com/index.asp

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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

25

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

http://plantweb.emersonprocess.com/Home/index.asphttp://www.easydeltav.com/index.asp

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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.

29

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/