PRT 140: Lesson 8 Introduction to Control Loops

Objectives

  • Describe Process Control
  • Explain the function of a control loop
  • Compare “Closed Loops” and “Open Loops”
  • Identify the components of a control loop
  • Describe signal transmission types

Reading

Terms to Know

  • Setpoint
  • Open Loop, Closed Loop, Feedback
  • Control, Measure, Manipulate
  • Sensor, Transmitter, Controller, Transducer, Final Control Element
  • Live Zero
  • Loop Error

What is Process Control?

The act of regulating one or more process variables so that a product of a desired quality can be produced”

How to control a process variable?

  1. sense/measure it
  2. compare to the desired value, ‘setpoint’
  3. calculate necessary change – the error
  4. make the change – correction

Variables

Controlled – sense this value to initiate signal

Measured – determine actual condition of variable

Manipulated – adjust a quantity or condition

Not always the same process variable – not always the same process stream.

OPEN LOOP

  • Instrument provides data
  • No connection to the change in the process – someone has to open/close the valve
    • No ‘feedback’
  • “Manual” mode
A diagram of an open control loop
An open control loop
[image 140-8-01]

CLOSED LOOP

  • Instrument provides data, and also determines the necessary corrections to make
  • Instruments control the valve position
  • ‘Feedback’ – as level changes, control loop will register the change, and valve position will change as needed
  • “Automatic” mode
A diagram of a closed control loop
A closed control loop
[image 140-8-03]
A diagram of a control loop block flow
Control Loop Block Flow
[image 140-8-5]

Control Loop – Components

Sensor Sensing
Transmitter Converting, Transmitting
Controller Compare, Calculate, Correct
Transducer Converting (signal type)
Final Control Element Manipulating
Also:
Indicator Displaying (values)
Computer Calculating, Converting

Sensor   (FE, TE, PE, LE, etc)

Flow Sensor in a Control Loop
[Image 140-8-06]

Transmitter (FT, TT, PT, LT, etc)

Flow Transmitter in a control loop
[image 140-8-7]

Controller (FC, TIC, PC, LIC, etc)

Flow Indicating Controller in a control loop
[Image 140-8-8]

Transducer (FY, TY, PY, LY, etc)

I/P Transducer in a control loop
[Image 140-8-9]

Final Control Element (FCV, etc…)

Final Control Element (pneumatic control valve) in a control loop
[Image 140-8-10]

Signal Types

  • Pneumatic – gas  std. 3-15 psig
  • Electronic – analog signal  std. 4-20 mA, 1-5 VDC
    • Often uses the same wires that provide power to instrument
  • Digital – binary – computerized – no std. range
  • Mechanical – physical linkage – no std. range

Signal Types on PID’s – Recap

Identify the analog electrical, digital, and pneumatic signals in this loop:

A diagram depicting various signal types in a loop
Various signal types in a loop
[image 140-8-11]

LIVE ZERO – Why isn’t 0 just 0?

  • 3-15 psig, 4-20 mA, 1-5 VDC – why not 0-12, 0-16, 0-4?
  • If 0 is 0, how do you tell the difference between a reading of 0 and a dead transmitter?
  • If 0 is 0, how do you handle any values <0?
  • How do you calibrate <0?
  • Remember that the range of an instrument is not necessarily 0-something – usually has a LRV and URV, so 0 doesn’t enter into it.

Control Loop Error

  • Each component in the loop has an error factor.
  • Cumulative error = Loop Error
  • Eloop = √[(E1)2 + (E2)2 + (E3)2 …(En)2]
  • Where E1, E2, …En = errors of all components in the loop.

Sample Problem, Loop Error

A control loop is composed of a transmitter (accuracy 0.5%); controller (accuracy 0.25%); I/P Transducer (accuracy 0.5%); and control valve (accuracy 1.5%).

  • Error = √[0.52 + 0.252 + 0.52 + 1.52]
  • Error = √[0.25 + .0625 + 0.25 + 2.25]
  • Error = √[2.8125]
  • Error = 1.68%

Accuracy calculation

[(measured value – true value)/(true value)] x 100%

“Accuracy” is usually expressed as “accurate +/- x%”.

It doesn’t matter if the value from the calculation is positive or negative…

Sample Problem, accuracy:

Pressure gauge true value is 100 psig, and it is reading 98 psig

  • [(98-100)/100] x 100%
  • [-.02] x 100%
  • -2%
  • Gauge is accurate +/- 2%

Loop Analysis procedure…

  1. List all instruments, full tag numbers
  2. Start at the sensing element, move through the loop to the final control element
  3. ‘Variable being controlled’ = variable being controlled OR manipulated
    • This variable changes as you move through a loop
    • Control Valves almost always manipulate FLOW

Flow Control Loop

Discuss components with class

 

Flow Control Loop
[image 140-8-12]
ComponentElement TypePV being controlledComponent Function (table 8-1)
FE-100Flow elementFlowSensing
FT-100Flow transmitterFlowConvert/Transmit
FC-100Flow controllerFlowCompare/Calc/Correct
FY-100Flow transducerFlowConvert signal
FCV-100Flow control valveFlowManipulating

Level Control Loop

  • Discuss components with class
Diagram for homework 11b
Level Control Loop
ComponentElement TypePV being controlledComponent Function (table 8-1)
LE-100Level elementLevelSense
LT-100Level transmitterLevelTransmit/convert
LC-100Level ControllerLevel/FlowCompare/calc/correct
LY-100Level transducerFlowConvert signal
LCV-100Level control valveFlowManipulate

Temperature Control Loop –

  • Discuss components with class
Temperature Control Loop
[Image 140-8-16]
ComponentElement TypePC being controlledComponent Function (table 8-1)
TWThermowellN/AProtecting
TTTemperature TransmitterTemperatureSensing/Convert/transmit
TICTemperature Indicating ControllerTemperature/flowCompare/calc/correct/display
TYTemperature transducerFlowConvert signal
TCVTemperature control valveFlowManipulate