PRT 140: Lesson 14 Symbology and Course Review

Objectives

  • Review Different Types of Drawings
  • Identify instrumentation on a P&ID, using the symbols from Chapter 12
  • Describe a control loop and its function, based on a P&ID layout

Reading

Process Flow

[image 140-14-02] Process Flow diagram

Piping and Instrumentation

[image 140-14-03] Piping and Instrumentation diagram

Symbology

  • Review Table 12-1 for how to interpret tag numbers (separate doc)
  • Demonstrate clouding to show modifications
    • Red-line/green-line
  • Look at Alpha Oil D-00-F1000
    • Legend, symbol key for set of PIDs

 

D-1010 Excerpt

[image 140-14-04]

D-1010 Questions

  • As level in Vessel rises, how should flowrate at FIC-122 (setpoint) change?
  • Will FV-122 be opening or closing as the signal from the FIC increases?
  • Is LIC-125 Reverse- or Direct-Acting?
  • Is FIC-122 Reverse- or Direct-Acting?

 

ComponentData to Fill In:0% Range (low end)Mid-range (setpoint)100% range (high end)
V-107 LevelVessel level - lowest setpoint, highestLowestSetpointHighest
Controller Input, LT-125 output signalSignal in mA, generally speaking4 mA12 mA20 mA
LIC-125 output signal (setpoint to FIC-122)Setpoint to FIC-122 -- Lowest, middle, highestLowestMiddleHighest
FIC-122 output signal to FV-122 "Lowest to Highest"Overall signal to FV - 122 -- Low, Middle, HighLowMiddleHigh
Overall FV-122 Valve position - "% open"Valve position - Closed, Middle, OpenClosedMiddleOpen

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 –

  • 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

 

PRT 140: Lesson 7 Analytics and Miscellaneous Measurement

Objectives

  • Discuss and identify major analytical instruments
  • Discuss miscellaneous/specialty instruments
  • Activity – PID/Instrument ID
  • Test review

Reading

Terms to Know and Discuss

  • Hand-Held, In-Line
  • Visual, photometric
  • pH, ORP, Conductivity
  • Opacity, Turbidity
  • Chromatograph, Spectrometer
  • CEMS, personnel monitors
  • Quantitative, Qualitative
  • Rectilinear speed, Rotational speed

Why Monitor Analytical Variables?

  • Environmental Monitoring/Reporting
  • Mechanical integrity of Fixed Equipment
  • Economics
  • Product Quality Assurance

Which reason do you think is most important?

CEMS

  • Continuous Environmental Monitoring Systems
  • Reports emissions for EPA guidelines
  • Sometimes on stacks, emissions from fired equipment
  • Other systems required by regulation
  • Mechanical Integrity
  • Monitor corrosion rates
  • Monitor corrosive atmospheres/liquids
  • Monitor vibration, other indicators for rotating equipment

Economics

  • Find problems real-time
  • Correct problems real-time
  • Save money

Product Quality

  • Test finished products
  • Test production streams
  • May be dozens of parameters on products like fuels, chemicals

Qualitative vs. Quantitative

Qualitative

  • To trigger response
  • To make conversation
  • Normal discussions

Quantitative

  • To analyze in detail
  • To make process corrections

Examples

  • Baby, It’s cold outside (qualitative)
  • It’s -47 °F outside (quantitative)
  • There is benzene present in the air in the lab (qualitative)
  • There is 0.5 ppb benzene present in the air in the lab (quantitative)

Sampling System

  • Obtain a representative sample from the process stream.
  • Transport the sample to the analyzer while maintaining its physical/chemical integrity.
  • Analyze the sample.
  • Return the sample to the process or discard it appropriately.

Handheld Instrument

 

 

An illustration of a handheld instrument
A personal H2S handheld gas detector

In-Line Instrument

  • In-Line instruments are permanently installed within the process unit.
  • The data from in-line instruments is used in product quality control, and/or environmental reporting.

Personal Monitor

An example of a personal dosimeter
A Personal Dosimeter
[image-140-7-3] By Elfabriciodelamancha (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Personal Dosimeter to Monitor VOC exposure

Employee wears dosimeter during regular work for required time period, then it is sent away for analysis –

Determines TWA, STEL readings –

QUANTITATIVE ANALYSIS

Lab Instrument

An example of a lab instrument
Lab Instrument
[image: 140-7-4] By Mirolka (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

pH

pH – acidity measurement – hydrogen ions specifically

  • Water systems and processes
  • Indications of corrosion, scaling issues
  • Scale = 0 to 14
    • <7 = Acid
    • >7 = Base (caustic)
    • 7 = neutral

ORP – Oxidation-Reduction Potential

  • Ratio of reducing agents to oxidizing agents in the sample
  • Free electron concentration
  • Similar to acid/base analysis

Chromatography

Packed columns for gas chromatography
[image 140-7-5] Image from LibrTexts.org
A chart showing a calibration curve
Chromatographic curve
[image 140-7-6]
  • Chromatography separates mixtures into components by forcing them through a ‘packed column’.
  • Gas flows through column, pushes material through – heavier molecules take longer to move through
  • As stream exits the column, different types of detectors used to read amount of material.
  • Data = time through column, size of peak on chart
  • Time through column = identify component
  • Size of peak on chart = amount of component
  • Use calibration standards to identify and quantify
  • Other methods of separation – ability to adsorb onto column, polarity, etc – same principles
pH   meter A Measures the amount of particulate matter in a gas stream by measuring the transmittance or absorption of light through the material
ORP meter B Measures the ratio of reducing agents to oxidizing agents
Conductivity meter C Separates the molecular components of a liquid or gas by forcing them through a packed column.
Chromatograph D Measures the hydrogen ion concentration
Turbidity analyzer E Measures the ability of a solution to conduct electricity
Opacity analyzer F Measures the amount of particulate matter in a liquid stream by measuring the amount of transmittance or absorption of light through the material

Vibration Monitors

  • Why is it important to monitor vibration on rotating equipment?
  • Excessive vibration is a sign that equipment is out of alignment
  • Excessive vibration is a sign that equipment is wearing out – could fail

Rotation/Speed monitors

  • Rectilinear = speed in a straight line – velocity
    • i.e. meters/second, feet/sec
  • Rotational = speed of revolution, for rotating equipment like pumps, motors
    • i.e rpm

Exercise

  • PID review
  • Instrument Functional description
  • Loop Analysis – how much can we do already?
  • Look Ahead…

 

A diagram for activity 1
Activity: Figure 1
[image-140-7-7]
Activity figure 2
[image 140-7-8]
 

An example instrument diagram
Activity 2
[image-140-7-9]
An example instrument diagram
Activity 3
[image-140-7-10]
An example instrument diagram
Activity 4
[image-140-7-11]

PRT 140: Lesson 6 Flow Measurement

Objectives

  • Define major terms associated with flow and flow measurement
  • Identify common types of flow sensing and measuring devices
  • Discuss and demonstrate the difference between total volume, flow rate, volumetric flow, mass flow
  • Net and Gross Flow (temperature corrections)
  • Review P&ID symbols for flow instrumentation
  • Demonstrate relationship between dP and flow rate

Reading

Chapters 6 and 7

  • Analytical Variables and Instruments
  • Miscellaneous Measuring Instruments

Terms to Know

  • Reynolds Number, Laminar, Turbulent
  • Volumetric Flow
  • Mass Flow
  • Net and Gross Flow – not in textbook, and important
  • Flow Instrumentation per lecture and notes

Flow – volumetric and mass

  • Movement of fluid
  • Flow rate = volume/time, or mass/time
  • gpm – gallons per minute
  • SCFH – standard cubic feet per hour – vapor
  • BPD – barrels per day – oil production
  • Lbs/hr – pounds per hour
  • m/s – meters per second – a velocity value

Reynolds Number

4 factors (Q: What haven’t we given you?)

  1. Velocity of fluid
  2. ID of pipe
  3. Density of fluid
  4. Absolute viscosity of fluid

LAMINAR flow – < 2,000

Streamlined flow; velocity varies over diameter of pipe

Laminar flow can increase risk of corrosion and scaling – why?

A diagram of laminar flow
Laminar Flow
[140-6-1a]

TURBULENT flow – >4,000

Fully turbulent at > 10,000

Fluid flow is consistent, well mixed, usually the desired condition

A diagram of turbulant flow
Turbulent Flow

[140-6-1b]

Mass Flow – LIQUID

  • Mass/Time (M/T)
  • Density = mass/volume = M/V
  • Volumetric flow = volume/time = V/T
  • Volumetric flow x Density = (V/T) x (M/V) = M/T
  • Density varies with temperature, so you need to know the exact density at the flowing temperature to calculate mass
  • Balance and convert all units as needed.

Net vs. Gross Flow – LIQUID

(also applies to total volume)

Gross Flow = volumetric flow rate at actual conditions (‘observed’) – what most flow instruments measure

Net Flow = volumetric flow rate converted to flow rate at standard conditions – usually 60 deg F

WHY NET? To keep consistent – measure flow at -20 deg F, then product heats up to 40 deg F – different observed amount of product. You wouldn’t want to keep changing the ‘amounts’ used in records, procedures, designs, sales, etc.

Volume Correction Factors (VCF)

  • Need T and VCF to calculate
  • VCF – different for each liquid
  • Discuss: Why don’t we need P?
  • Find VCF for observed T
  • Net Flow = Gross Flow x VCF
  • Net Volume = Gross Volume x VCF
  • Many different types of VCF data – charts, formulas, programs, internal programming in the instrumentation control system
Temperature
Deg F VCF
25 1.07
30 1.06
35 1.05
40 1.04
45 1.03
50 1.02
55 1.01
60 1.00
65 0.99
70 0.98
75 0.97
80 0.96
85 0.95
90 0.94
95 0.93
100 0.92
105 0.91
110 0.90

MASS FLOW, NET FLOW – GASES

  • Need both T and P to calculate – why?
  • Our old friend – Ideal Gas Law
  • Remember: P and T in ABSOLUTE units
    P1V1  =  P2V2
    _____       _____
    T1             T2
  • Instead of Volume, think volumetric flow: Ft3/hr

Net flow, gas = look at volumetric flow

Condition 1 = observed T, P

Condition 2 = standard T, P

  • 14.696 Pounds per Square Inch (psia)
  • 60 Degrees Fahrenheit (oF) (520oR)

Use this formula to calculate SCFH from CFH data

Flow Instrumentation

  • Direct vs. Indirect Measurement
  • Direct measurement – positive displacement
    • Sound familiar?
    • Very similar to P-D pumps – chamber within meter physically moves a set volume
    • Counter tallies the number of times the chamber fills/empties

Flow Elements

  • Most common indirect measurements use dP
  • Orifice Plate, Venturi, Flow Nozzles, Annubar, Pitot

NOTE: These are just the sensing elements – still need some kind of transmitter to create data from the change in dP (differential pressure).

A photo of an orifice plate
Orifice Plate is used for measurement.
[140-6-2a]
A diagram of a flow nozzle
Flow Nozzle Sections
[140-6-2b]

Example Problems

B Orifice Plate A Measures flow using a tube with several openings and then averaging all flow measures.
D Flow Nozzle B Measures flow using a metal disc containing a drilled opening
E Venturi Tube C Measures flow using an L-shaped tube and another tube that compares the impinging pressure with static pressure.
C Pitot Tube D Measures flow using a tapered inlet device inserted into a flange connection/spool piece
A Annubar ® E Measures flow using a cone-shaped device with inlet and outlet components

dP vs. Flow

  • Flow rate is proportional to the square root of the differential pressure
  • Consider the full range of flow and dP that we measure
  • Look at % of full range:
  • The % of full flow range will vary as the square root of the % of full dP range, or…
  • The % of full dP range will vary as the square of the % of full flow range
  • %F = √%dP, or
  • %dP = (%F)2
  • dP = differential pressure through an element
  • F = flow rate
  • We’ll look at a change in the dP, and a change in the Flow

Example Problems

  1. 60 % dP
    • 60% = 0.60, square root = 0.775
    • = 77.5% flow
  2. 50% dP
    • 50% = 0.50, square root = 0.707
    • = 70.7% flow
  3. 45% dP
    • 45% = 0.45, square root = 0.671
    • = 67.1% flow
  4. 36% dP
    • 36% = 0.36, square root = 0.60
    • = 60% flow

% Total Flow vs. % Total dP

%F = √%dP

A chart showing the resulting curve of %F = √%dP
%F = √%dP
[image 140-6-3]

% Total dP vs. % Total Flow

%dP = (%F)2

A chart showing the resulting curve of %dP = (%F)2
%dP = (%F)2
[image 140-6-4]

% Span Calculations

  • Material covered more thoroughly in Week 10, but we’ve been using it all along.
  • Range = the Lower (LRV) and Upper (URV) in the range of the signal, instrument, or process value.
    • Example: 4-20 mA signal, the Range is 4 mA (LRV) to 20 mA (URV)
  • Span = the difference between the URV and the LRV
    • 4-20 mA signal, Span is 16 mA

SPAN, Operating Range

  • SPAN = URV – LRV
  • Operating Range is ‘LRV to URV’
  • Temperature transmitter calibrated for operating range 100 deg F to 400 degF
    • Span = 300 deg F
  • Temperature transmitter calibrated for operating range 1500 deg F to 1800 degF
    • Span = ?????

Scaling – Determining Values for % range

  • Scale represents 0-100% of measured process variable
  • 4 mA = 0%
  • 20 mA = 100%
%Span equals Value minus LRV multiplied by 100%, divided by Span.
%Span Equation
[image 140-6-5]

Span, %Span

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

Scaling: What is %span for operating data?

  • Measure operating range – low end is LRV, high end is URV, difference is Span
  • Calculate % span through the range:
    • Operating valuex = (% desired x Spanx) + LRVx
  • Example: What is the 35% point in a temperature scale that reads between 55 and 172 deg F?
    • LRV = 55 deg F
    • URV = 172 deg F
    • Span = 117 deg F (172 F – 55 F)
  • (.35 x 117 F) + 55 F = 96 F

Homework problem – %span

  • Flowmeter calibrated from 45 gpm – 230 gpm
  • Analog signal = 4 mA – 20 mA
  • Testing flow rates at listed % span?
  • What is LRV, URV, Span of Flow data?
  • What is LRV, URV, Span of mA data?

VALUEB = [(% SPANA) x SPANB] + LRVB

Example:

  • Value = [(0.14) x 185 gpm] + 45 gpm
  • Value = 25.9 gpm + 45 gpm
  • Flow Value = 70.9 gpm
  • Value = [(0.14) x 16 mA] + 4 mA
  • Value = 2.24 mA + 4 mA
  • Signal Value = 6.24 mA

Shop Demo – dP vs Flow

  • DAC Pump demo unit
  • Globe valve to create 3 psi dP in upper spool
  • Take readings of flow, dP at the following settings
  • Flow at 15 gpm, 12 gpm, 9 gpm, 6 gpm
  • What is 100% dP range?
  • What is 100% flow range?
  • How closely does it follow the calc plan?

Flow Instruments

Rotameter – Fluid flows through the device, lifting a free-floating indicator called a float. The position of the float is referenced to calibrated marks to indicate the flowrate.

Magmeters – Produces a magnetic field that penetrates the flow tube; liquid is the conductor flowing at right angles to the magnetic field. This creates an electrical potential, sensed by electrodes. (voltage)

Flow Instruments – Turbine Meter

  • Free-spinning turbine (fan) in flowing liquid-
  • The rpm of spinning fan is proportional to flow rate
  • Rpm generates a pulse
  • Calibrated with the ‘K-factor’ to determine actual flow rate – pulses/gallon

Flow Instruments – Mass Flow

  • Coriolis Meter – does not need external compensation for temperature, etc.
  • Fluid flows through a vibrating coil – sensors measure the twisting, oscillation, and can calculate velocity, flow, mass, etc. from all this data.
  • Video illustrates this principle
  • Very powerful tools – lots of data from one instrument.

dP transmitters

  • We’ve seen dP transmitters used to calculate dP, level, density – now we can calculate flow
  • Uses the Bernoulli principle, and the relationship between dP and flow rate (already discussed)

Vortex Meter

  • Common process meter – minimal pressure drop
  • The vortex element extends into the process fluid, disrupts flow – creates eddies (vortices) around the ‘bluff body’ of the element
  • Sensors pick up the pressure fluctuations caused by these eddies – the even signal is proportional to flow rate (calibrated)

Flow Meters – Drawing Symbols

Flow meter drawing symbols
Flow meter drawing symbols
[140-6-6]
Flow meter drawing symbols
Flow meter drawing symbols
[140-6-7]

PID from homework

What’s happening with these instruments?

An instrument diagram
What is going on in this diagram?
  • Why is TE connected to the FY?
  • Is this a gas stream or a liquid stream? How do you know?
  • What does the C mean?
  • What type of flow meter is it?
  • What type of TE is it?
  • What type of signals are used?
TagFunctional
Description
Notes
FE-001Flow ElementNote that there is a separate FE/FT drawn. Not always the case. If you draw only one, use the FT
FT-001Flow Transmitter
FY-001Flow ComputerFlow Computer is calculating the mass flow rate using flow and temperature data. The "Y" can indicate different instruments - have to look at the function.
FI-001Flow Indicator
TW-002Thermowell
TE-002Temperature ElementNote that this must be a combined TE/TT since it sends a signal.
TI-002Temperature Indicator

PRT 140: Lesson 4 Level

Objectives

  • Define ‘level’ and related terms
  • Describe common types of Level instrumentation
  • Demonstrate level measurements – bubbler, sight glass
  • Perform level calculations and conversions
  • Review Level instrumentation symbols on PID’s
  • Perform pressure, temperature, level calculations – review

Reading

Terms to Know

  • Innage
  • Outage/ullage
  • Direct level measurement
  • Indirect level measurement
  • Interface
  • Meniscus
  • Hydrostatic Head Pressure
  • Level instruments, per presentation

Innage/Ullage

Question: Why might you use one or the other?

A diagram of fluid in a tank
Innage or Ullage
[image 140-4-01]

Direct/Indirect Level Measurement

Instrument directly reads the level

 

A diagram of a sight glass used for direct measurement
Direct Measurement
[image 140-4-02]

Instrument senses another property that is used to calculate level

A diagram of a pressure gauge to depict indirect measurement
Indirect Measurement
[image 140-4-03]

Direct/Indirect Examples

Direct Level Instruments

  • Sight glass
  • Float
  • Dipstick
  • Tape gauge

Indirect Level Instruments

  • Bubbler
  • dP cells
  • Displacer

Interface / Meniscus

Interface

a diagram showing interface
Interface
[image 140-4-04]
Meniscus

An image of a miniscus viewed through a pipette
An example of a meniscus
[image 140-4-05]

Level – Hydrostatic Head Pressure

Pressure exerted by the depth of the liquid column (we’ve looked at this before…)

P = (hydrostatic head pressure) = in. H2O

h = height of the liquid column (liquid level) = in.

SG = specific gravity of the liquid (water SG = 1)

P = (1 in wc) x (h in. ) x (SG)
                     in.

Or P = h x SG (if everything is in the right units)

UNITS of Level vs. Pressure

Units of level = a length, height measurement: ft, in, mm, etc.

Units of hydrostatic head pressure = length, height of a particular fluid:

  • in. H2O, mm Hg, in. w.c.,
  • Sometimes in ‘feet of head’, implying water (this is used for pump data, ref. PRT 130)

Example 1

An open tank contains a liquid with a specific gravity of 1.735. If the level of the liquid is 123 in., how much head pressure, in in. H2O, will it exert?

A diagram illustrating h and PI for the example problem.
The open tank which contains a liquid, specific gravity 1.735
[image 140-4-07]
 

Equation to use:

  • P = (1 in. H2O)(h)(SG)       or P = h x SG
    in.
  • SG = 1.735
  • h = 123 in.
  • P = (1 in. H2O)(123 in.)(1.735)
    in.
  • P = 213.4 in. H2O

Level

If you know the hydrostatic head pressure and the SG, you can calculate level:

  • P = h x SG, so
  • h = P/SG
  • h is in.
  • P is in. H2O

IMPORTANT: Data must be in the right units for this simple equation to work. Always verify units before using equations.

Example 2

An open top tank is filled with a liquid that has a specific gravity of 0.873. The liquid exerts a head pressure of 193 in. H2O. What is the liquid level in inches?

 

A diagram illustrating h and PI for the example problem.
The open tank which contains a liquid, specific gravity 1.735
[image 140-4-07]
  • P = (1 in. H2O)(h)(SG) /in.
    • P = 193 in. H2O
    • SG = 0.873
  • 193 in. H2O = (1 in. H2O)(h)(0.873)/in.
  • h = (193 in.) / 0.873
  • h = 221.1 in.

Be very careful with units – this is not water, so giving a level height as in. H2O makes no sense at all!

Level Instruments to Know

  • Sight Glasses
  • Float
  • Tape Gauge
  • dP cell
  • Bubbler
  • Displacer
  • Ultrasonic, Radar, Nuclear
  • Load Cells

Direct Measuring Instruments

  • Sight Glass – see examples
  • Float
  • Gauge Tape
Direct measurement example
An example of a direct measure instrument.
[image 140-4-06]

dP vs. PI

A diagram depicting how dP differs from PI
dP vs. PI
[image 140-5-06]

PI Measures All Pressures

  • PI measures PX (vessel P) + PH (hydrostatic)
  • P = PX + PH
  • PX = 5 psi
  • PH= h x SG = 67 in. x 1 = 67 in. H2O
    • 67 in. H2O = 2.42 psi – (how do we know this?)
  • P = 5 psi + 2.42 psi
  • P = 7.42 psi at the PI

dP Cell – Measures Level Regardless of Vessel Pressure

  • Low side of dP measures PX, vessel pressure
  • High side of dP measures PX + PH (hydrostatic)
  • Since it’s a differential, the final reading is
  • dP = (PX + PH) – PX
  • dP = PH
  • PH = h x SG
  • PH= h x SG = 67 in. x 1 = 67 in. H2O
    • 67 in. H2O = 2.42 psi
  • dP = 2.42 psi

Advantage of dP Cell?

  • dP cell measures only the hydrostatic head from the liquid level – the vessel/system pressures are canceled out.
  • To use a single PI, you have to KNOW the vessel pressure and include that in any calculations.
  • Consider how the reading on the PI will change with vessel pressure – how do you know the level is changing or not?

Level – Bubbler

A diagram of a bubbler with labeled components
A diagram of a bubbler
[image 140-4-08]
  • What is the pressure at the bottom of the dip tube?
  • What is the formula for the pressure at the bottom of the liquid level, h?
  • P = h x SG

Example Question

What is the water height in inches, h, if the pressure on the bubbler reads:

  • 1.5 psi
  • 13 psi
  • 72 psi

0.433 psi = 1 ft water     (WHERE DOES THIS COME FROM?)

0.433 psi = 12 in. water     (because 1 ft = 12 in.)

1 psi = 12 in. water/0.433

1 psi = 27.7 in. water

Bubbler       P = h x SG, so P/SG = h

FIRST – convert all pressures to in. water

  1. P = 1.5 psi = 41.6 in. H2O
  2. P = 13 psi = 360.1 in. H2O
  3. P = 72 psi = 1994.4 in. H2O
    • h = P/SG     (SG =1.3)
  4. h = 41.6/1.3 = 32 in.
  5. h = 360.1 / 1.3 = 277 in.
  6. h = 1994.4 / 1.3 = 1534.15 in.

(NOTE that the level units are different from P units)

How to Recognize Level Instruments in the Field

Many dP and P instruments are used to indirectly measure level. How do you identify them in the field?

  • Look at how they are connected/configured in the process.
  • Single PT – would have to be at the bottom of a liquid column, right? (not too commonly used this way)
  • dP is very common
  • First – make sure it’s a dP transmitter – H/L connections, possibly two visible leads to process
    • One at bottom of a liquid column, one to atmosphere
    • One at bottom of a vessel, one in the vapor space of a vessel
    • Two connections to a liquid-full container, at a specific height differential
    • One to a bubbler line, one to atmosphere
    • Probably many more

The bottom line is – look at how it’s configured, think what information you get from that data

Displacers

A diagram of a displacer
A displacer
[image 14-4-09]
A diagram illustrating buoyancy measured in pounds
Buoyancy reading
[image 140-4-10]

Other

Ultrasonic/Radar: Measure ullage distance, infers level. Used when physical sensors may not work (i.e. asphalt tankage)

Nuclear: uses gamma radiation to detect matter inside the vessel. Used for difficult measuring situations, extreme conditions

Load Cells: Weigh the contents, can calculate level

  • What other info do you need to make this calculation?

Level – PID Symbols

An example diagram with PIDs
PID symbols
[image 140-4-11]

Review

In a closed container,

P1V1/T1 = P2V2/T2

Pressure and Temperature must be in ABSOLUTE UNITS –

  • P = psia
  • T = K or R

Volume units just have to be consistent

Step 1 = get all units and variables identified

450 psig, 125 C, 437 ft3                      =                    235 psig, 564ft3, ???C

Variables Data, units
P1 450 psig
T1 125 C
V1 437 ft3
P2 235 psig
T2 ?
V2 564 ft3

Step 2 = get all units correct (absolute T, P)

Variables Data, units Data, in absolute units
P1 450 psig 464.7 psia
T1 125 C 398 K
V1 437 ft3 437 ft3
P2 235 psig 249.7 psia
T2 ? ?
V2 564 ft3  564 ft3

Step 3, set up and solve equations

(464.7 psia x 437 ft3) / 398 K = (249.7 psia x 564 ft3) / T2

T2 = (249.7 psia x 564 ft3 x 398 K) / (464.7 psia x 437 ft3)

T2 = 276 K

T2 = 3 C (answer should be in same units as original data, unless specified otherwise)

Scaling Calculations

Level instrument operating range is
5 ft – 85 ft. What is the 4-20 mA reading at different levels?

VALUEB = {[(VALUEA – LRVA)/SPANA] x SPANB} + LRVB

Level:  LRV = 5 ft  URV = 85 ft       Span = 80 ft

Signal:  LRV = 4 mA  URV = 20 mA   Span = 16 mA

So:

?? mA = {[(Level Reading – 5 ft)/80 ft] x 16 mA} + 4 mA

PRT 140: Lesson 3 Temperature

Objectives

  • Define ‘temperature’ and related terms
  • Identify common types of Temperature instrumentation
  • Perform temperature and pressure conversion calculations
  • Review Temperature instrumentation symbols on PID’s

Reading

Temperature – Terms to Know

  • Sensor – will respond to the process variable
  • Temperature
  • Heat Transfer, Conduction, Convection, Radiation
  • Phase Change, Heat of Vaporization
  • Latent Heat, Sensible Heat
  • BTU
  • Fahrenheit/Rankine; Celsius/Kelvin
  • Absolute Temperature

Heat Transfer Terms

Convection

  • Mixing two substances or direct contact with one substance (exhaust gases)
  • hot coffee and cold cream
  • Convection oven – blows hot air around food

Conduction

  • Contact through a solid – no mixing; indirect contact
  • Shell/tube heat exchangers
  • Pan on electric stove top – pan conducts heat from burner to food

Radiation

  • Electromagnetic waves – no contact
  • Sunshine, heat from woodstove, burner flames in furnace
  • Pan under a broiler – radiant heat from burner/flame onto food

Absolute Temp/Pressure

Absolute temperature – temperature scale where 0 = ‘absolute zerothe temperature where no more heat can be removed from a system. This corresponds to 0 K or -273.15°C.   Theory – no more molecular movement.

Absolute pressure – pressure measured from 0 = full vacuum. 0 psia = full vacuum

Gauge pressure – pressure measured from 0 = current atmospheric pressure. 0 psig = atmospheric pressure

Temperature Scales vs. Actual Temperature

Absolute Temperature:

(Kelvin) K = C + 273

(Rankin) R = F + 460

F = (C x 9/5) + 32

C = (F-32) x 5/9

K = R x 5/9

R = K x 9/5

A diagram showing comparisons of temperature scales
Temperature Scales vs. Actual Temperature
[image 140-3-01-01]

Latent Heat vs Sensible Heat

  • Sensible Heat – heat that can be ‘sensed’ by a thermometer – i.e. the temperature changes
  • Latent Heat – heat that cannot be ‘sensed’ by a thermometer – i.e. temperature doesn’t change when phase is changing.
  • Boiling water stays at 212 F until all of it is steam – even while we keep adding heat. This is called the latent heat of vaporization.
  • Same idea when water freezes – releases latent heat.

Temperature Instruments

  • Thermowell
  • Thermometer
  • BiMetallic Strip
  • RTD – Resistance Temperature Device
  • Thermocouple
  • Thermistor
  • Temperature Gauge

Thermowell

  • Not an instrument – it holds the instrument and protects it from the process, while allowing heat transfer
  • TW
A diagram of a thermowell
Thermowell
[image 140-3-02]

Thermometer

  • Glass Bulb – standard
  • Also IR – Infrared – non-contact
An infrared photo of a train in operation
Infrared Train, By Jagokogo (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
[image 140-3-03]

BiMetallic Strip

  • Two Dis-similar metals, bonded together
  • Expansion/Contraction with Temperature different for 2 metals
  • Movement of strip, moving temperature dial
  • Dial thermometer very common
A diagram of how bi-metalic strip reacts to templature.
Bi-metallic Strip
[image 140-3-04]

RTD – Resistance Temperature Device

  • Electrical resistance (ohms) will change with temperature – varying output signal
  • More accurate than Thermocouple
  • Smaller operating range (-200F to 900F)
  • Ohms resistance in the RTD correlates to temperature – see tables

100 (1000) Ohm Platinum RTD Resistance Chart
Generally RTDs are a 3- or 4- wire configuration – fine electrical wires

4-wire RTD
[Image 140-3-06]
A diagram showing typical RTD design
Typical RTD Design
[image 140-3-07]

Thermocouple

  • Most common, simplest
  • Two dissimilar metals – generate voltage at their junction when they are heated.
  • Measured junction – connects to process
  • Cold junction/reference – connects to transmitter
  • mV generated across Thermocouple correlates to temperature – see tables
Thermocouple – excerpt from voltage chart
Thermocouple – excerpt from voltage chart
[image 140-3-08]

Thermocouples

  • Generally 2-wire configuration
  • Wires are thicker
  • made of the dissimilar metals
An image of a diagram of an ANSI MC96.1 Color Coding table
ANSI MC96.1 Color Coding table
[140-3-9]

Thermistor

  • Ceramic resistor – same principle as RTD –
  • Electrical resistance through thermistor changes with temperature
  • Usually small bead/disk
  • Registers very small temperature differences

Temperature Gauge

No transmitter – gauge face (like PI)

Standard Bi-Metallic Thermometer
[Image 140-03-11]

Temperature transmitters – How to identify them in the field

  • Read the nameplate on the instrument
  • Should include calibration range
  • Will include model number – can look it up
  • May include facility tag number
  • Look at how it’s connected to the process
  • Temperature sensors almost always have THERMOWELLS to house the sensor
  • Temperature sensors usually extend into the process stream – so there will NOT be an isolation valve on the process connection.

Temperature Conversion Calculations

  • Absolute: (Kelvin) K = C + 273
  • Absolute: (Rankine) R = F + 460
  • F = (C x 9/5) + 32
  • C = (F-32) x 5/9

Temperature Conversion

32 C = ?? F

F = (C x 9/5) +32

F = (32 x 9/5) + 32

F= 89.6

32 C = 89.6 F

25 F = ?? C

C = (F-32) x 5/9

C= (25-32) x 5/9

C= -3.9

25 F = -3.9 C


25 C = ?? F

F = (C x 9/5) +32

F = (25 x 9/5) + 32

F = 77

25 C = 77 F

100 C = ?? K

K = C + 273

K = 100 + 273

K = 373

100 C = 373 K

Temperature ‘linear scaling’ – NEW

The conversions between F and C temperature scales are an example of how we use the linear scaling calculation.

We looked at this equation in week 1 – it seems very complicated:

VALUEB = {[(VALUEA – LRVA)/SPANA] x SPANB} + LRVB

But it’s not, and you’ve been doing it already:

LRV = lower range value = the lowest value in the operating range

URV = upper range value = the highest value in the operating range

Span = URV – LRV

We pick two equivalent operating ranges:

Temperature – range from freezing to boiling:

F:   LRV=32°F
URV=212°F
Span = 180°F
C:  LRV = 0°C
URV = 100°C
Span = 100°C

Convert any C reading to F
(Whenever we get data, that set of units becomes the “A” data)

F = {[(C reading – 0 C)/100 C] x 180 F} + 32 F

This equation is mathematically equivalent to:

F = (C X 9/5) + 32

Try it out! Convert 140°C to F

F = {[(C reading – 0 C)/100 C] x 180 F} + 32F

F = {[(140C – 0C)/100C] x 180F} + 32 F

F = {[140C/100C] x 180 F} + 32 F

F = {1.4 x 180 F} + 32 F

F = 252 F + 32 F

F = 284 F

140 C = 284 F

F = (C X 9/5) + 32

F = (140 x 9/5)+32

F = (252) + 32

F = 284

 

 

140 C = 284 F

Linear Scaling Calculation
VALUEB = {[(VALUEA – LRVA)/SPANA] x SPANB} + LRVB

The linear scaling calculation is used to relate all kinds of linearly related scales. For example:

  • Operating temperature range will be 200 F to 600 F
  • We want the 4-20 mA signal from the transmitter to reflect that range.

We can use the scaling calc to calculate the mA reading at any temperature in the range – try 440 F.

F:  LRV=200 F  URV= 600 F  Span = 400 F

mA:   LRV = 4 mA  URV = 20 mA  Span = 16 mA

mA = {[(440F – 200F)/400F] x 16 mA} + 4 mA

mA = {[240F/400F] x 16 mA} + 4 mA

mA = {0.6 x 16mA} + 4 mA

mA = 9.6 mA + 4 mA = 13.6 mA at 440 F

Process Variable Relationships

Pressure, Volume, Temperature in a closed container

\frac{P_{1}V_{1}}{T_{1}} = \frac{P_{2}V_{2}}{T_{2}}

Pressure/Temperature are in Absolute Units

T = K,R     P = psia

V1, V2 both in the same units

PRT 140: Lesson 2 Pressure

Objectives

  • Define pressure and formula P=F/A
  • Define terms associated with pressure and pressure instruments, per textbook
  • Identify common types of pressure-sensing/measuring instruments used in the process industry:
    • manometers
    • pressure gauges
    • differential pressure (d/p) cells
    • strain gauge transducers (‘piezoelectric effect’)
    • capacitance transducers
  • Describe the purpose and operation of pressure instruments
  • Discuss and perform pressure unit conversion calculations
  • Describe and identify P&ID symbols for Pressure instrumentation
  • Connect and read a pressure gauge, describe Bourdon tube operation

Reading

Pressure

P = F/A

Pressure = Force / Area

Measurements are in pounds/square inch

Parameters Affecting Force

  • FORCE = push/pull that causes change in direction
  • SPEED = temperature, how fast molecules move
  • MASS/Weight = amount of matter
    • Larger molecules weigh more – Hg vs. H2O
  • DENSITY = molecules/volume

Specific Gravity

Liquids

  • Specific gravity = density of x/density of water
  • Density water = 1.0 at 39 deg F

QUESTION – If SG <1, is material lighter or heavier than water?

Gases

  • Specific gravity = weight of x/weight of air
  • Air at standard conditions for reference

Pressure Instruments

  • Gauge (PI) or Transmitter (PE-PT or PT)
  • Local or Remote reading

Manometers

  • Essentially, open-ended tubes filled with liquid
  • Applying pressure to one end of the tube will cause the liquid to rise on the other side

Pressure Gauge – Bourdon tube

Principle of Operation: The Bourdon Tube flexes in response to pressure changes – mechanical response

The flexing tube is connected to the indicator needle with gears.

Pressure Gauge, showing the Bourdon Tube
[Image 140-02-2]
 

 

[/panel]

dP Gauge

Principle of Operation:

This example is also a mechanical device, similar to a Bourdon tube

A diagram of a dP gauge
dP Gauge
[image 140-2-05]

Strain Gauge Transducer

Principle of Operation:

Group of wires stretch when they are exposed to pressure. Current flows through the wires, and resistance changes as the wires are stressed. Emits an electrical signal.

A diagra of a strain gauge transducer
Strain Gauge Transducer
[image 140-2-06]

Capacitance Transducer

Principle of Operation:

Two metal capacitor plates are pushed closer together as they are exposed to pressure; distance between the plates changes the amount of electrical charge that the two plates can hold (electrical capacitance). Emits an electrical signal.

A diagram of a capacitance transducer
Capacitance Transducer
[image 140-2-07]
A diagram od a detail of a capacitance transducer
Capacitance Transducer Detail
[image 140-2-08]

Pressure transmitters – How to identify them in the field?

  • Read the nameplate tag on the instrument
    • Should show calibration range
    • Will give model number – can look it up
    • May include facility tag number
  • Look at how the instrument connects to the process
    • Sensors have an isolation valve at the process connection
    • If a dP cell, then there will be two sensing leads (one may be to atmosphere) – look for H/L stencil on the instrument body (High/Low)
    • If a P cell, then there will be one sensing lead
  • Be aware that many pressure transmitters are used as level or flow instruments

Differential Pressure

  • dP, DP
  • P2 – P1
  • Difference in pressure between 2 distinct points.
  • dP can be used to calculate other process variables, specifically FLOW or LEVEL (we’ll learn more later)
  • dP around equipment used for monitoring the fouling or plugging of the equipment (filters).

Absolute Pressure – Pressure scale where 0 = full vacuum

  • Pounds/square inch ABSOLUTE
  • Total Vacuum is 0 psia
  • Normal atmospheric P (sea level) = 14.7 psia
  • Most gauges read pressure above atmospheric pressure, called GAUGE pressure – psig
  • Normal atmospheric P (sea level) = 0 psig
  • Atmospheric pressure changes depending on elevation, conditions, but:
  • For practical purposes, psia = psig + 14.7

Pressure Units – many many

  • Need for different scales
  • Water, mercury, etc. can measure smaller variations in pressure more clearly
  • How is ‘in. H2O’ a pressure unit? Isn’t pressure force/area?
  • Look at columns of water, pressure

Pressure Units – In. W.C.

  • The unit “in. w.c.” or “in. H2O” means:
    • The pressure is equivalent to the pressure exerted by a column of water that high.
    • If you had a column of water that was 1 square inch in cross-sectional area, 27.7 inches high, the weight of that water would be 1 pound. 1 pound/sq. inch = 1 psi
    • The pressure exerted by a column of liquid is independent of the diameter of the column.
    • WHY is that?

Units – Mercury vs. Water – 1 psi

A diagram of a pressure gauge
Mercury vs. water – 1 psi
[image 140-2-09]

Conversions – Unit Equivalency

  • psia = psig + 14.7
  • psig = psia – 14.7
  • 1 psi = 2.04 in. Hg = 27.7 in. H2O = 0.069 bar
  • It goes on and on – how to convert between units?
    • Learn or look up conversion factors for every change – see Table 2.3….
    • Or learn the main ‘equivalencies’ and how to convert that way

Table 2-3, Pressure Unit Conversion Chart

 psibarmbarIn. HgIn. H2OmmHgmmH2O
psi114.5040.0145040.491180.0361270.0193370.0014223
bar0.06894610.0010.0338650.00249080.00133329.8068 x 10-5
mbar68.9461000133.8652.49081.33320.098068
In. Hg2.035929.5290.02952910.0735520.0393680.0028959
In. H2O27.68401.470.4014713.59610.535250.039372
mmHg51.714750.060.7500625.4011.868310.073558
mmH2O703.050.1019710.197345.3225.33913.5951
atm0.0680450.986920.000986920.0334220.00245830.00131589.6788 x 10-5

 

In this type of table, always best to confirm that you’re reading it right.

I know that 27.7 in. H2O = 1 psi, so I can figure out how to read this particular table:

1 of (Column Heading) = (table value) of (Row Heading)
Example: 1 mmHg = 0.53525 in. H2O

Conversion Table 2-3

You can use each column in the table as a string of ‘equivalent units’

Note: Not every table is the same – verify before you calculate. You know that 1 psi = 27.7 in H2O

 psibarmbarIn. HgIn. H2OmmHgmmH2O
psi114.5040.0145040.491180.0361270.0193370.0014223
bar0.06894610.0010.0338650.00249080.00133329.8068 x 10-5
mbar68.9461000133.8652.49081.33320.098068
In. Hg2.035929.5290.02952910.0735520.0393680.0028959
In. H2O27.68401.470.4014713.59610.535250.039372
mmHg51.714750.060.7500625.4011.868310.073558
mmH2O703.050.1019710.197345.3225.33913.5951
atm0.0680450.986920.000986920.0334220.00245830.00131589.6788 x 10-5

Equivalency Calcs

1 psi = 2.04 in. Hg = 27.7 in. H2O = 0.069 bar

 

New units     =            (known units)         x           (new equivalent units)

(known equivalent units)

Ex:  Convert 50 in. water to psi

New psi = (50 in H2O)/(27.7 in H2))   x (1 psi)

New psi = 1.8 psi

PID, PFD, Symbols Information

A chart showing PID and PFD symbols
PID, PFD, Symbols
[image 140-2-11]

P&ID Detail – Pressure Instruments

A diagram of a pressure instrument
P&ID Detail
[image 140-2-12]

Reading a P&ID – Instrumentation

  • To interpret instrumentation on a P&ID, you apply your own logic to the information given. Some rules of thumb:
    • The signal begins at the process and moves outward.
    • Signals move in one direction only, through each signal line shown.
    • Follow the signal path to describe what the instruments are doing.
  • Balloons used for instrumentation provide info on remote/local, mechanical/electrical, etc.