Industrial Instrumentation: Instrumentation is Measurement and control of process variables.
Process: Sequence of operations to produce useful product from raw material.
For e.g. Crude Oil (raw material)——(processed)–à(Product)Gasoline, Naphtha, Kerosene, Bitumen, Sulphur etc
Crude oil also known as fossil fuel is natural resource of energy, naturally formed in millions of years after decomposition of fossils (buried dead organisms) underground. Crude oil in its original form is not useful, it is Processed, refined to make useful products like Gasoline etc.
Crude oil is sent to Distillation unit where its Heated, during heating different components of like, diesel, Kerosene , Naphtha which have different boiling point forms vapour, go up get cooled & settle on trays above where its taken out through pipes.
Process are controlled automatically with slight operation intervention using Automatic Controllers.
Controllers are industrial CPUs with programmed logic to read the input signals from the field sent by transmitters, transducers, limit & proximity switches etc then to control the plant process as per the programmed logic using final control elements like Control valves, motors etc. Eg. Of controllers are DCS(Distributed Control System) & PLCs( Programmable Logical Controllers).
CCR Central Control Room
Pic shows Control room to control the plant operation.
Operators can see the process variables (i.e. temperature, flow, level, pressure etc) values on Screen & give set point to control the process.
Switch Vs Transmitter
Switches indicate either 0 or 1 or On or Off state for eg a Flow switch will send signal to demonstrate whether there is flow or No flow in pipe.
Eg. Pressure Switch, Level Switch, Temperature Switch, Flow Switch etc.
Pressure Switch
Temperature Switch
displacer type level switch
magnetic Level switch
Ultrasonic Flow Switch
Vibrating fork level switch
ultrasonic level switch
Flow Switch
Transmitter
Transmitters are electronic devices which generates a continuous signal representing the current value of the desired process parameter e.g. 0 to 100oC gives 4-20mA.
Electromagnetic Flow Transmitter
Coriolis Flow Transmitter
Orifice Differential Pressure Flow Transmitter
Vortex Flow Transmitter
Ultrasonic Flow Transmitter
Flow Transmitters
Transmitters transmit process values(i.e. Pressure, Temperature, Flow, Level etc) to controllers like DCS or PLC.
2 Wire / 4 Wire Terminology
Transmitters with 2 wires doesn’t need additional power to operate, it can work with loop power. 4 wire transmitters need power supply, thus 2 wires for power & 2 for signal.
2 wire instrument draws it’s power from the instrument loop i.e. the instrument can tap off whatever power it needs from the 4-20mA loop (without impacting the 4-20mA).
Temperature Measurement :
RTD Resistance Temperature Detector (PT100)
Principle Of Operation: The PT100 (Platinum Thermometer) is an RTD (Resistance Temperature Detector). The PT100 has an electrical resistance of 100 ohms at 0oC and 138.5ohms at 100oC. Its resistance increases with the increase in temperature.
RTD is used as 4th resistance of Wheatstone Bridge, change in temperature causes change in resistance thus unbalancing the Wheatstone bridge & resulting in current, which represent equivalent temperature at the measuring field.
rtd
Wheatstone Bridge
3 wire RTD connected to Wheatstone Bridge in balance condition
R1+R3+A+C=R2+B+C+RTD
R1+R3+A=R2+B+RTD
If R1=R2,
R1+R3+A=R1+B+RTD
R3=RTD+B-A,
Since lead resistance of 2 wires of cables are similar B-A≈ 0
So
R3=RTD hence using 3 wire RTD, cancels lead wire resistance & gives accurate measurement of rtd resistance, so we get more accurate reading of temperature.
Thermocouple
Thermocouple is Bimetallic junction which works on Seebeck Effect. If junction of 2 dissimilar metals are kept at different temperatures, it generates mV.
Thermocouple working principle
Thermocouple Cutaway
T1 is at hot temperature & T2 is at low temperature, this difference in temperature induces miliVolts, which is equivalent to temperature.
Application:
Temperature Transmitter TT(pics shown below) converts Resistance value of RTD or mV of thermocouple in to 4-20 mA/FF signal & transmits it to controller, controller with programmed logic controls control valve by sending respective 4-20mA/FF signal to Control Valve.
Credit Emerson 3144P TT
Pressure Measurement
Pressure is Force applied on per unit area. P=F/A where P is pressure, F-Force, A- Area
Atmospheric pressure is natural pressure in environment approx. 1.01325 bar.
Gauge Pessure: Gauge pressure is only the pressure inside closed equipment/pipeline.
Absolute Pressure: is sum of Gauge pressure & atmospheric pressure.
Pressure Gauges: Pressure gauges are pressure measuring devices, which uses following “pressure elements for eg. Bordon tube, Bellows, Diaphragm etc to measure pressure”
As the inlet pressure changes pressure elements expand or contract causing movement of pointer. This movement is calibrated to display pressure reading as per international standards.
C type Bourdon tube Pressure gauge
Bellow type PG
Bellow type pressure gauge
Pressure Measurement –Transmitter
Principle Of Operation:
1. Metal Sensors: Normally sensors are oil filled. The pressure acts on a diaphragm which acts on the oil fill which acts upon the plate of a capacitor. The transducer converts the change in capacitance into corresponding pressure.
2.Ceramic Sensors: Pressure acts directly on a ceramic disk. The transducer converts the change in properties of the disk resistance (strain gauge) or capacitance into corresponding to pressure. Ceramic sensors are more strong & less prone to damage. Metal sensors are more hygienic.
inline pressure measurement
On Vessel Installation
Differential Pressure Transmitter
Differential Pressure Transmitters can also be used to measure pressure if Low pressure side of the transmitter is not connected to process.
Differential pressure transmitter measures Difference in Pressure in the system(pipeline/vessel). Differential pressure transmitter is used normally with Orifice plate, Venturi tube, flow nozzle, pitot tube etc which acts as obstacle & reduce the pressure, so a pressure difference at the inlet & outlet of these elements is generated which is measured by Differential Pressure Transmitter. As it can be seen Capacitance value will be changed based on input pressure from HP & LP side, this change in capacitance value will be proportional to differential pressure in the system.
Q&As
Q1. Is flow through a control valve- Turbulant or laminar?
एक नियंत्रण वाल्व के माध्यम से प्रवाह होता है- अशांत या शांत?
A1: Flow through control valve is always turbulant. Laminar flow takes place with liquid operating at low Reynold no. This occurs with liquids that are viscous, working at low velocities.
नियंत्रण वाल्व के माध्यम से प्रवाह हमेशा अशांत होता है। कम रेनॉल्ड नं पर तरल संचालन के साथ लामिना का प्रवाह होता है। यह उन तरल पदार्थों के साथ होता है जो चिपचिपे होते हैं, कम वेग पर काम करते हैं।
Q2. What is vapour pressure?
वाष्प दाब क्या है?
A2: Vapor pressure is natural equilibrium pressure that exists inside a closed vessel containing the liquid.
वेपर प्रेशर (वाष्प दबाव) प्राकृतिक संतुलन दबाव है जो तरल युक्त एक बंद बर्तन के अंदर मौजूद होता है।
Q3. Specific Gravity is the ratio of density of liquid to water, what is specific gravity of gas?
विशिष्ट गुरुत्व द्रव के घनत्व का पानी से अनुपात है, गैस का विशिष्ट गुरुत्व क्या है?
A3: Specific gravity of gas is the ratio of the density of gas to density of air at standard operating atmospheric pressure 1.01325 bar & 15°C
गैस का विशिष्ट गुरुत्व मानक ऑपरेटिंग वायुमंडलीय दबाव 1.01325 बार और 15 डिग्री सेल्सियस पर हवा के घनत्व के लिए गैस के घनत्व का अनुपात है
Q4: Difference between Safety Valve, Regulator, On off valve & Control valve.
A4: Safety valve is pressure relief valve, which has spring of required pressure, when pressure in the system goes above it, relief valve releases pressure & protect pressurizing of pipeline or vessel.
Pressure Regulator are devices to regulate the input pressure to control valve.
On Off Valve: On off valve is valve which operates in only 2 positions On or Off.
Control Valve: Control valves uses positioner to control the opening /closing of the valve from 0-100%.
Q5. What process data is required to size a control valve?
A5: Medium- which fluid is passing through it. if it is special liquid need specific gravity at operating temperature, critical pressure, vapour pressure, viscosity.
valve Cv can be described as the number of gallons per minute (GPM) at 60oF (15.5oC) that will pass through a valve with an associated pressure drop of 1 psi.
Q9. What are Open Loop & Closed Loop?
A9: If process variable measured(i.e. pressure, level, temp, flow etc) are just transmitted to see the valve. its called open loop.
If Process variable measured are compared with setpoint & action is taken to correct the deviation from setpoint its called closed loop. for eg if in a loop final control element like control valve exists, its called closed loop.
Q10. Types of orifice plates?
A10. Concentric- for liquid, steam & gas services,
Eccentric- viscous & slurry flow,
segmental- slurry & colloildal flow
Orifice plates
Q.11 What is I/P?
A11: I/P is current to pneumatic converter. It releases equivalent air when 4-20 mA is applied to it.
Q12. Whats is alarm value of NH3?
A12. 25ppm High, 35 ppm Hi Hi
Q13. What is alarm value of H2S?
A13. 10 ppm Hi, 20 ppm Hi Hi
Q13. Whats the purpose of shield?
A.13: Shield protects instrument cable from electrical noise. It protects capacitance through air between electrical signal & instrument conductor.
Twisted pairs cancels induced currents due to magnetic field in each twists.
Q14. How would you differentiate between two wire RTD & TC?
A14. Two wire of RTD are usually Red & white. while TC wire color depends on its type.
Testing with multimeter will show mV in case of TC & 100+ ohms resistance if its RTD.
Q.15 What are skind thermocouple?
A15. Skin thermocouple are directly connected to pipe skin without inserting it using thermowell. Used in Fired Heaters to measure process pipe temperature inside heater.
Q16. How to zero check dp transmitter?
A16. Close LP side valve, Open Equalize valve, so the pressure at both side of sensor will be same & Difference in pressure will be 0. If transmitter shows 0 its calibration is good. but if it doesnt adjust with Hart communicator.
Q17. What is trim in control valve?
A.17. Trim consist of parts of the valves that affect the flow i.e. Plug, Seat.
Q.18. What is Critical Temperature & Critical pressure?
A18: Critical temperature is that temp of gas above which fluid can not be liquefied by pressure alone.
Critical pressure is pressure value at critical temperature.
Q.19 What is TMR?
A.19 TMR is tripple Modular Redundancy technique. TMR is patented technology of Triconex ESD/IPS system, it uses 3 main processors to decide tripping of system. 3 transmitters are installed to measure same process variable. For eg. 2oo3 logic is used in ESD, system will trip only if 2 out of 3 transmitters have alarms. Single Alarm/Fault will not trip the system. its often called Fault Tolerant system.
Q.20 What is SIL rating?
A.20 SIL(Safety Integrity Level) ratings are ratings provided by manufacturer of the instrument indicating its probability to fail on demand
Q.21 What are the parts of Control valve?
Control valve consists of Valve Body, Actuator, Positioner, Pressure Regulator.
Q.22 What is Hold Off Distance?
Q.23 How to calibrate Displacer Level Transmitter
A.23 Dry Calibration: Zero setting-> hang displacer on torque tube arm rod of displacer transmitter & set 0(full weight)/ 4 mA.
Span Hang weight=displacer weight-volume x specific gravity of liquid being measured
set span value. Volume of displacer = πr2h r- radius of displacer, h-height of displacer
Q.24 What is Pressure & Temperature compensation?
A.24: Gases/steam volume depends on its pressure & temperature, if compressed more gas will pass through same volume, if heated less mass will go through same volume as gas molecules will expand. So to measure actual flow Pressure & Temperature compensation is done on flow reading.
Q.25 What are pressure sensors?
A.25 Pressure sensors are instruments that measure pressure. i.e. Bourdon tube, diaphram, capsule, bellows, pressure spring.
Q. What is absolute 0 pressure & vacuum?
A.26 Absolute 0 pressure = 760mmHg pressure below 0 pressure is called vacuum.
Q.27 find the miliamps required to move valve to 25%
A.27 Formula mA=((16 * %value/100)+4), puting % value in this formula we get mA req=8 mA
Q.28 What is Under range & over range values?
A.28 3.8 mA is Under range value, 20.8 mA is over range value, so transmitter will send 3.8 mA to indicate under range value, & 20.8 mA to indicate over range value.
Q.29 What is direct acting actuator?
A.29 Fail Open/ Air to Close type actuator is Direct acting actuator.
Q.30 What is DD file, what is full form of .cff?
A.30 DD(Device Description) file of any FF device is like driver of printer. To configure FF device DD files are must. DD files can be identified by .sym & .ffo file extensions.
.cff is common file format, it shows capability & values.
Q.31 Why do we fill glycerin in tubing?
A.31 If gas above liquid is condensable, LP side tube will slowly fill up with liquid. & Cause Error Measurement, to eleminate this error tube is filled with glycerin. Another reason to fill the seal fluid is to prevent hazardous gases to leak through transmitter.
Q.32 Explain Zones
A.32 Zone 0- Flammable gases always present e.g. inside tank.
Zone 1- Flammable gases regularly present at random interval during normal operation (between 1-1000 hrs) e.g. Pressure relief valve, sample point etc.
Zone 2- Flammable gases rarely present more than 1000 hrs
Class 1- Flammable gases present
Class 2- Combustible dust present
Class 3- ignitable fiber present
Div 1- Flammable/combustible atmosphere exist in normal operation
Div 2- low chance of flammable/ combustible atmosphere.
So
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