PRT 140: Lesson 13 Control Loops, Control Valves, and Regulators

Objectives

  • Identify components of control valves and regulators
  • Describe various operating scenarios
  • Discuss valve actuators and positioners
  • Explain the ways to reverse controller output signals, and need for such changes
  • Describe types of pressure regulator devices

Reading

Terms to Know

  • Actuator, Valve Plug, Valve Assembly
  • Valve positioner
  • Sliding Stem valve
  • Butterfly, Ball, Globe, Three-Way valves
  • Reverse action – valves and actuators
  • Direct action – valves and actuators
  • Failure Position – Actuators and Valve Assemblies
  • Regulators

Control Valve Components

Pneumatic Control Valve, annotated
[Image 140-13-01]
 

ACTUATOR Action/Failure Positions

Schematic of a reverse-acting control valve actuator
[Image 140-13-02]
 

Air to Open – Fail Down (Closed)

Reverse-Acting Actuator

  • Air enters under the diaphragm
  • Spring is pushing down
  • Air fails = spring pushes all the way down
  • Determines position of the valve stem

Schematic of a direct-acting control valve actuator
[Image 140-13-03]
 

Air to Close – Fail Up (Open)

Direct-Acting Actuator

  • Air enters over the diaphragm
  • Spring is pushing up
  • Air Fails – spring pushes all the way up
  • Determines position of the valve stem

 


ACTUATOR Action/Failure Conditions

A diagram of an actuator (fail last)
An actuator (fail last)
mage [140-13-04]
 

Fail Last

  • Electric motor actuators
  • Piston actuators
  • Double-acting positioners

Action Failure Position – How Do You Know?

  • Location of air inlet to diaphragm actuator
  • Labeled on the valve in the field
  • Labeled on PIDs
  • Equipment Data on specific valve
  • Overall Valve Failure Position is dependent on the design/operation of both the actuator and the valve (‘valve assembly’)

Actuator /Valve Types

  • Actuators and valves – both can be Direct Acting or Reverse Acting
  • Combination of Actuator/Valve Actions possible:
    • Direct/Direct
    • Direct/Reverse
    • Reverse/Direct
    • Reverse/Reverse

Actuators: Action Recap

  • Direct Acting:
    • Air enters on top of diaphragm
    • Increased pressure = extend stem = push down
    • ATC on Direct-Acting Valve
    • Fail Up (Fail Open on Direct-Acting valve)
  • Reverse Acting:
    • Air enters underneath diaphragm
    • Increased pressure = retract stem = pull up
    • ATO on Direct-Acting Valve
    • Fail Down (Fail Closed on Direct-Acting Valve)

Actuators – look at detail

Direct Action – Air Extends (pushes down) the Stem – (ATC, Fail Up)

Reverse Action – Air Retracts (pulls up) the Stem – (ATO, Fail Down)

A diagram showing the mechanical difference between direct and revers actuator action.
Direct action vs. Reverse action
Images [140-13-05+06]

VALVES: Plug Action – New Level of Detail

  • Direct Acting =   by far the most common
    • Push plug down to close
    • Closes as stem extends (as stem pushes down)
    • ATC with Direct-Acting Actuator
    • ATO with Reverse-Acting Actuator
  • Reverse Acting =
    • Pull plug up to close
    • Closes as stem retracts (as stem pulls up)
    • ATO with Direct-Acting Actuator
    • ATC with Reverse-Acting Actuator

Valve – Direct vs. Reverse Acting

Direct Acting – Stem Down to Close

Air Extends (pushes down) the Stem – (ATC, Fail Up)

Reverse Acting – Stem Up to Close

Air Extends (pushes down) the Stem – (ATC, Fail Up)

Direct action vs. Reverse action
Images [140-13-07+08]

Questions

Why are there different actions for –

  • Actuators?
    • Failure positions needed
    • Control response needed
    • Better control of actuator pressure (vs process pressure)
  • Control Valve Plugs?
    • System pressure
    • System fouling
    • Push-down-to-close (Direct) is most common

 

Valve Action Discussion

  • Most important?
    • Overall Valve Failure Position – FO or FC
    • Air to Close (ATC) or Air to Open (ATO)
  • Next Importance
    • Actuator Direct (Air on Top) – “Fail Open”
    • Actuator Reverse (Air on Bottom) – “Fail Closed”
  • Next Importance
    • Valve Plug Direct (push down to close) – almost all valve plugs are direct. Actuator Failure = same as Overall Valve Failure. If Actuator failure doesn’t match Overall Valve Failure – then look at valve plug action
    • Valve Plug Reverse (pull up to close)
Overall Effect of Actuator Action / Valve Action

[Image 140-13-11]
 

Activity

A direct acting controller sends a standard pneumatic signal to a control valve. The control valve has a direct-acting actuator, and the valve itself is reverse-acting. Fill in the information in this table:

Controller output, % of spanAction dir/rev (if known)0%25%50%75%100%
Controller output, % in psig
Stem Position - "all the way up" to " all the way down" (depending on the actuator action)
Valve % open, or "fully open" to "fully closed" - (depending on the plug action)

Check your answers on this table.

Pneumatic Actuators

Pneumatic Valve Actuator

[image 140-13-12]
 

Piston Actuators

note that they can be hydraulic or pneumatic

Piston actuator diagram [image 140-13-13]

Valve Positioners

  • Positioners – make the valve position match the controller output signal
    • Position the valve
    • Reverse the action
    • Mimic a valve trim type –
      • not discussed much here
    • Provide split range control –
      • if valve must respond to only part of the signal
  • How to Reverse the Controller Output signal
    • Can be changed at the I/P transducer
    • Can be changed at the Valve Positioner
    • Physically reconfigured to respond in reverse
    • NOTE: Actuator will still fail in position dictated by spring/air configuration
    • Why would you do these reversals?

Valve positioner diagram [image 140-13-14]

Activity

The tag on a pneumatically actuated control valve identifies it as air-to-open. If the positioner has been configured to reverse the signal, then the valve will fail _____________ on loss of instrument air supply.

A. Open

B. Closed

C. In its last position prior to the loss of air

D. None of the above

Types of Control Valves

  • Globe
  • Three-Way
  • Butterfly
  • Ball/Segmented Ball

Three Way Control Valve

  • Three ports
  • Mixing or diverting
  • As plug moves, one inlet closes while the other opens

[image 140-13-15]

Butterfly Control Valve

  • Higher flow capacity
  • Rotary stem/motor
  • All types of fluids

[image 140-13-16]

Ball, Segmented Ball Control Valve

  • Rotary valve
  • Spherical plug
  • Segmented has shaped plug – flow characteristics
  • Used in slurries

[image 140-13-17]

Air Regulators

Air Regulator at a control valve station

[Image 140-13-18]
 

 

Symbols for Mechanical Regulators

  • Note: small regulators on actuators, control valves are not normally shown on PIDs
  • In-line mechanical pressure regulators are shown:
  • Note direction of the angled line –
    • Indicates whether pressure is controlled upstream or downstream of valve…
P&ID Symbol for mechanical backpressure regulator

[Image 140-13-21]

 

PID, PFD, Symbol Information

  • PIDs, PFDs will indicate 3-way valves
  • PIDs will normally indicate FO, FC of overall valve
    • Will not be specific about actuator vs valve action
    • If you need to know that, where do you look?
  • PFD will probably not show FO, FC

 

PRT 140: Lesson 12 Control Loops, Control Elements

Objectives

  • Define basic controller terms – overview
  • Outline the common controller settings
  • Identify physical configurations for controllers
  • Describe final control elements
    • Identify components
    • Identify applications

Terms to Know

  • Control valves: ATO, ATC
  • Control Valve failure modes – FC, FO, Fail-Last
  • Controller Action: Direct vs Reverse
  • Local/Remote Controllers (location)
  • Local/Remote Control/Setpoint (where SP comes from)
  • Streams: Manipulated vs. Controlled vs. Measured
  • Bump – Bumpless Transfer
  • Controller Modes: Integral/Derivative – definitions
  • Proportional Band/Proportional Gain – calculations
  • Controller/Control Scheme configurations:
    • Cascade/Split-Range/Ratio

Analog Controllers

Analog controller
Analog Controller
[140-12-01a]
A diagram of the Siemens PAC 353 display
Analog Controller, Siemens PAC 353
Image [140-12-01b]

Controllers – Terms to Know

  • Direct Action
    • Increased Input = increased output
  • Reverse Action
    • Increased Input = decreased output
  • If Output signal = valve opener – think of scenarios for direct/reverse action
    • Level control
    • Flow Control

Auto/Manual Bumps

  • Auto/Manual – Bumpless Transfers
  • Bump – controller output changes dramatically because of a switch from auto/manual, manual/auto – can cause a Process Upset
  • Bumpless transfer from manual to automatic
    • Adjust SetPoint to match current Process Value
    • Switch manual to auto
    • Monitor for drift
    • Reset the SetPoint if it’s not where you want it

Controller Modes

(You’ll work with this more in the future… definitions only)

  • How the output signal responds to input signals, how smoothly the control functions
    • Integral Action
      • Figures time into the equation – how long has PV been different from SP
      • Control is working to ‘reset’ to the process setpoint
    • Derivative Action – RATE
      • Figures rate of change into the equation – how fast is PV moving away from SP

(Know calculations here)

  • Proportional Band and Gain
    • How much a change in input affects a change in output – how big a response is needed for a change in PV
    • Example: How does steam output respond to fuel input?

Process Gain, Proportional Band Calcs

  • Process Gain = Process\; Gain = \frac{\Delta\; output / output\; transmitter span}{\Delta\; input / input\; transmitter span}
    • Dimensionless, can be positive or negative
    • Δ = final – initial
  • Proportional Band – (1/GAIN) x 100%
  • PB = \left [ \frac{\Delta \; input / input\; transmitter\; span}{\Delta \; output / output\; transmitter\; span } \right ] \times 100%
    • A percentage, use absolute value

Questions for Consideration

  • What is Δ output / output transmitter span ?
  • How can you change the gain or PB on a control loop?
  • Why?

Calculate Process Gain

  • Fuel flow to a boiler is adjusted in order to control the volume of steam produced.
  • Fuel flow transmitter is calibrated from 45 gpm – 375 gpm.
  • Steam flow transmitter is calibrated from 3,000 lbs/hr – 10,000 lb/hr
  • If the fuel flow is changed from 75 gpm to 100 gpm, the steam production rate changes from 4,000 lb/hr to 4,300 lb/hr.
  • Input = ??  Output = ??
  • Variables needed:
    Change in input (new – old) or (final – initial)
    Change in output (new – old) or (final – initial)
    Input transmitter span (URV-LRV)
    Output transmitter span (URV-LRV)

Input Fuel; Output Steam

  • Change in output = 4300 lb/hr – 4000 lb/hr = 300 lb/hr
  • Change in  input = 100 gpm – 75 gpm = 25 gpm
  • Output span = 10,000 lb/hr – 3,000 lb/hr = 7,000 lb/hr
  • Input span = 375 gpm – 45 gpm = 330 gpm
  • GAIN = \frac{300\: lb/hr \; /\; 7000\; lb/hr}{25\; gpm / 330\; gpm}
  • What are the units?

Types of Controllers to Know

  • Physical location
    • Local controller
    • Remote controller
  • Setpoint origin – remote vs local setpoint
  • Type of control scheme – Both the programming and the physical setup
    • Split Range
    • Cascade
    • Ratio

Controlled Stream vs. Manipulated Stream

  • Discussion:  Is the ‘controlled’ stream the same as the ‘manipulated’ stream?
[Image 140-12-2]
A diagram of a manipulated stream
Manipulated Stream
Image [140-12-02b]
 

Split Range Control

One controller, two control elements

A diagram of split range control
Split Range Control
Image [140-12-03]
 

  • Split range requires two final control elements
  • Each FCE responds to a portion of the controller output signal
  • Typical application: Tank blanketing system – needs to vent OR pressurize, depending on current pressure in tank

Cascade Control

Two controllers, on control element

Cascade Control Loop
[image 140-12-04]
 

  • Output of one controller = remote setpoint for another controller
  • TIC- Temperature Indicating Controller – Primary Controller
  • Secondary (flow) controller receives setpoint from the Primary Controller
  • This scenario – control temperature on Stream B outlet by changing flow setpoint on Stream A in.
  • Option for secondary Controller to operate on a local setpoint
  • Secondary controller affects the value of the primary variable

Why not just do this?

An exampe diagram.
Proposed diagram
Image [140-12-05]
 

  • Better control
  • Reduced lag times
  • Need to control the flow rate separately under certain conditions, etc.

Ratio Control

2 controllers, 1 or 2 control elements

A diagram of a ratio control system
Ratio Controller
Image [140-12-06]
 

  • Proportion one flow based on another
  • Which is the primary transmitter?
  • Note which flow is controlled vs. which flow is measured vs. which flow is manipulated..
  • Note that the secondary controller does not affect the value of the primary variable – it responds to it

Final Control Elements

  • Valves – most common, used in this class
  • Louvers, Dampers
    • Ex.:Change position to manipulate air flow
  • Motors
    • Can have variable speed drives, which change the output of the associated pump/compressor, etc.

Valve Operation – Overall

  • ATO – Air To Open – Fail Closed
    • As 3-15psig signal increases, valve opens
    • Loss of air = valve slams closed
  • ATC – Air To Close – Fail Open
    • As 3-15 psig signal increases, valve closes
    • Loss of air = valve slams open
  • Fail Last, Fail-In-Place
    • Operates differently
    • Loss of air = valve stays where it was

Activity

If the level gets too high (goes over the setpoint, sending an increasing signal to the controller), the control valve starts to open, to drop the level in the tank. If the control vale is ‘air-to-open’, does it take an increasing or decreasing signal to open it? Given that, is the LIC a reverse-acting or direct-acting controller?

Level Control Loop
[Image 140-12-07]
 

Surprise! I can solve this with a table:

Set up Instrument Loop Analysis Chart

  • Include the actual controlled process value on the beginning, final manipulated process value at the end.
  • Instruments in order, tracing through the signal path.

Identify any ‘actions’ or ‘valve fail-safe’ configurations known (i.e. reverse/direct, ATO/ATC)

  • If action is not discussed, or we haven’t discussed reverse/direct for that type of instrument, leave blank
  • Note that valve fail-safe positions (marked on PID) lead to the ATO/ATC designation

Set up columns for well below setpoint (minimum process value), process value at setpoint, and process value well above setpoint (maximum process value).

  • You may want intermediate spots, as well, to illustrate more complicated schemes.

Fill in the data you know, based on a description of how the loop functions.

  • At each point, consider how the system needs to RESPOND to head towards the setpoint.

Step through from instrument to instrument, filling in blanks that make sense.

  • You may be working from both ends of the loop – just move through it very orderly

When complete, read through the whole chart to see if it makes sense, based on a description of what’s happening.

Fill In What Is Known

Component/StepACTION - dir/rev/NA or ATO/ATC/otherPosition or Signal at lowest PVPosition or Signal at PV SPPosition at highest PV
Process value - Level in tank 5
LT-100
LC-100
LY-100
LCV-100
Process value - flow of steam out

 

Solution

Component/StepACTION - dir/rev/NA or ATO/ATC/otherPosition or Signal at lowest PVPosition or Signal at PV SPPosition or Signal at highest PV SP
Process value - Level in tank 5NALevel = too LOWLevel = Just rightLevel = too HIGH
LT-100Direct4 mAMiddle20 mA
LC-100Direct - Figured from looking at signal in from LT to signal out to LY4 mAMiddle20 mA
LY-100NA (yet)3 psigMiddle15 psig
LCV-100ATOClosedMiddleOpen
Process value - flow of stream outFlow = LowFlow = just rightFlow = High

PRT 140: Lesson 10 Control Loops, Sensors, and Transmitters

Objectives

  • Describe the relationship between sensors, transducers, and transmitters in process control loops
  • Compare and contrast the transmitter/transducer input and output signals
  • Calculate:
    • % span
    • Scaling: Input to Output (linear)
  • Review control loop function based on a process control scheme diagram

Reading

Terms to Know

  • Discrete Sensing Element
  • Integrally Mounted Sensing Element
  • Linear Scaling
  • LRV, URV
  • Span
  • Operating Range
  • Standard Signals

Sensors

  • Pressure, Temperature, Level, Flow
  • Discrete Sensors or Elements– wired or connected to the transmitter
    • Thermocouples, RTDs
    • Should be shown on PID as TE and TT (and TW)
    • Flow orifices – The orifice is the Flow Element, often discrete from the transmitter, even though the ‘pressure sensor’ is integral to the sensor
  • Integrally Mounted Sensors – physically part of the transmitter
  • d/p cell, TT, PT
  • Note the need to connect to the Process – external to the sensor in a d/p
    • PID: The process connections are not normally shown for the d/P connection points
  • Can be shown on PID as PE/PT or PT or PE

Sensor Signals

What are the standard signals?

  • Electronic  ???
  • Pneumatic  ???
  • Digital  ???

Sensor outputs are most likely non-standard

  • Ex. Thermocouple in mV
  • RTD – resistance – ohms
  • Pressure – actual process pressure

Controllers need standard input signals

Transducers

  • Convert non-standard input signals to standard output signals
  • I/P  Current to Pneumatic – very common
  • P/I  Pneumatic to Current
  • I/E  Current to Voltage
  • E/I  Voltage to Current
  • E/P  Voltage to Pneumatic
  • Etc.

Sensor output to Transmitter

A diagram of sensor output to a transmitter
Sensor output to transmitter
[image 140-9-1]

SPAN, Operating Range

  • SPAN = URV – LRV
  • Operating Range is ‘LRV to URV’
  • Temperature transmitter calibrated for operating range 100 deg F to 400 deg F
    • Span = 300 deg F
  • Temperature transmitter calibrated for operating range 1500 deg F to 1800 deg F
    • Span = ?????
  • Transmitter output signal calibrated for operating range 4mA to 20 mA

Transmitter Scaling

  • Output of Transmitter represents 0-100% of measured process variable
  • 4 mA = 0%
  • 20 mA = 100%

\frac{Value - LRV}{Span} x 100 = Span

Span, %Span

Percent of ScaleInputOutput
0%500ºF4 mA
25%625ºF8 mA
50%750ºF12 mA
75%875ºF16 mA
100%1000ºF20 mA

Scaled Sensor Input – Transmitter Output

A table showing scaled sensor input - transmitter output
Scaled sensor input – Transmitter output
[image 140-9-2]

Transmitters: Input to Output

Transmitter Input vs. Output

(linear)

VALUE_{B} = \frac{VALUE_{A} - LRV_{A}}{SPAN_{A}} \times SPAN_{B} + LRV_{B}

Where:

A = Original Scale (input)

B = New Scale (output)

LRV = Lower Range Value

URV = Upper Range Value

SPAN = URV – LRV

Sample Scaling Problem: In a standard I/P transducer, an 8-mA input corresponds to what output signal?

Input = electrical signal

Output = pneumatic signal

Data Equations
VALUEA 8 mA VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}
LRVA 4 mA
URVA 20 mA
SPANA 16 mA Value B = \frac{(8 mA - 4 mA)}{16 mA} \times 12 psig + 3 psig
LRVB 3 psig
URVB 15 psig
SPANB 12 psig ValueB = 6 psig

 

Scaling Problem : A temperature transmitter uses a thermocouple sensor and is calibrated to 100 deg F – 300 deg F as a 4-20 mA output signal. If the fluid temperature is 200 deg F, what is the output signal in mA?

 

Data Equations
VALUEA 200ºF VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}
LRVA 100ºF
URVA 300ºF
SPANA 200ºF Value B = \frac{(200ºF - 100ºF)}{200ºF} \times 16 mA + 4 mA
LRVB 4 mA
URVB 20 mA
SPANB 16 mA Value B = 12 mA

 

Scaling Problem: A pressure transmitter is calibrated at 0-300 psig, with an operating setpoint of 175 psig. What is the percent span of the setpoint?

 

Data Equations
VALUEA 175 psig Insert Equation
LRVA 0 psig
URVA 300 psig
SPANA 300 psig Insert equation
LRVB
URVB
SPANB % Span = 58.3%

Scaling Problem: A thermocouple has an operating range of 150 deg F – 700 deg F. Current reading is 220 deg F. What is the scaled output from a standard electronic transmitter at this reading?

 

Data Equations
VALUEA 220ºF VALUE_{B} = \frac{(220ºF - 150ºF)}{550ºF} x 16mA + 4mA
LRVA 150ºF
URVA 700ºF
SPANA 550ºF VALUEB = (70/550) x 16mA + 4mA
VALUEB  6.04 mA
LRVB  4 mA
URVB  20 mA

VALUE – 2.04 mA + 4 mA

6.04 mA output signal

SPANB 16 mA
VALUE_{B} = \frac{(VALUE_{A} - LRV_{A})}{SPAN_{A}} \times SPAN_{B} + LRV_{B}

Example: Pressure transmitter is calibrated to measure from 0-80 psig, and it is measuring 20 psig. What is the output of its standard 4-20 mA transmitter?

VALUEA ?

LRVA?

URVA?

SPANA?

LRVB?

URVB?

SPANB?

Why is I/P one of the most common transducers?

A diagram of an I/P Transducer [140-10-01]
An I/P Transducer
[140-10-01]

REVIEW/DISCUSSION

Is this control loop open or closed?

A diagram of a control loop
Flow control loop – open or closed?
[140-10-02]
ComponentElement TypePV being controlled or manipulatedComponent Function
TW-002Thermowelln/aHousing the sensor
TE-002Temperature elementTemperatureSensing the temperature
TI-002Temperature indicatorTemperatureIndicating and transmitting the temperature
FE-001Flow elementFlowSensing the flow
FT-001Flow transmitterFlowTransmitting the flow of data
FY-001Flow transducer or flow computerFlow/TemperatureCalculation - temperature and flow to calculate net of mass flow
FI-001Flow indicator (net)FlowIndicates the final flow rate

 

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 –

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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