INTRODUCTION:
It is the branch of engineering whichdeals with the mesurement, monitoring, display etc of the various of energy exchanges which take place during process operations. "In shortIntrumentation is the study of Instrument. "
INSTRUMENT:
Instrument is a devics which is used to measure, moniter,display etc of a process variable.
1. What are the process Variable?
The processVariable are: 1) Flow 2) Pressure 3) Temperature 4) Level 5) Qualityi. e. % D2, C02, PH
etc.
2. Define all the process Variable and state their unit of measurement. ?
1) FLOW: Kg I hr, Litter I min, Gallon I min. M3 I NM3 I HR. (GASES)
2) PRESSURE: Force acting per unit Area. P = F/A Units: Bar I Pascals I Kg I CM I, Pounds
3) LEVEL: Different betweentwo heights. Units: Meters, M M, C M, %.
4) TEMPERATURE: It is the degree of hotness or coldness of a body. Units : Degree Centigrade, Degree Farenheit, DegreeKelvin, Degree Rankin.
5) QUALITY: It deals with analysisPH, % C02, % 02, Conductivity, Viscosity.
3. Whatare the primary elements usedfor flow measurement. ?
The primary elementsused for flow measurement are: 1) Orifice Plate. 2) Venturitube. 3) Pitot tube. 4) Annubars. 5) Flow Nozzle.6) Weir & Flumes.
4. What are the differnt types of orifice plates and state their uses?
The differenttypes of orificeplates are: 1) Concentric. 2) Segmental. 3) Eccentric.
CONCENTRIC: The concentric orifice plate is used for ideal liquidas well as gases and steam service. This orifice as a hole in concentricand hence known as concentric orifice.
Eccentric & Segmental: The accentricorifice plate has a hole eccentric. The use this is made in viscous and sherry flow measurement.
The segmental orificeplace has the hole in the form segment of a circle.This is used for colloidal and sherry flow measurement.
1. How do you identify an orificein the pipe line. ?
An orifice tab is welded on the orificeplate which extends our of the line giving an indication of the orifice plate.
2. Why is the orifice tab provided. ?
The orifice tab is provideddue to the following reasons. 1) Indication of an orificeplate in a line. 2) The orifice diameter is marked on it. 3) The materialof the orifice plate. 4) The tag no. of the orifice plate. 5) Themark the inlet of an orifice.
3. Whatis Bernoulli's theoram and where it isapplicable. ?
Bernoulli's theoram states the "total energy of a liquid flowing from one point to another remains constant." It is applicable for non compressible liquids.
4. How do you identify the H. P. side or inlet of an orifice plate in line. ?
The markingis always done H. P. side of the orifice tab which gives an indication of the H. P. side.
5. How do you calibrate a D. P. transmitter. ?
The following steps are to be taken which claribrating : 1) Adjust zero of the Xmtrs. 2) Static preasure test: Give equal pressure on bothsides of the transmitter. Zeroshould not shift. If it is shiftingcarry out static aligntment. 3) Vaccum test: Apply equal vaccum to both the sides. The zero should not shift. 4) Calibration Procedure: a) Give 20 psi air supplyto the transmitter. b) Vent the L.P. side to atmosphere. c) Connect output of the Instrument to a standardtest gauge. Adjustzero. d) Applyrequired pressure to highpressure side of the transmitter and adjust the span. e) Adjust zero again if necessary.
6. What is the seal liquid used for filling impulse lines on crude and viscousliquid?
Glycol.
7. How do you carry out pipingfor a Different pressure flow transmitter on liquids, Gas and steam services? Why?
Liquid lines: On liquid lines the transmitter is mounted below the orifice plate. Since liquids havea property of self draining.
Gas Service: On gas servicethe transmitter is mounted above the orificeplate because Gases have a property of self ventingand secondly condensate formation.
Steam Service: On steam service the transmitter is mounted belowthe orifice plate with condenlate pots. The potsshould be at the same level.
8. An operator tells you that flow indication is more? How would you start checking?
1) First flushing the transmitter. Flush both the impulse lines.Adjust the zeroby equalizing if necessary. If still the indication is more then. 2) Check L.P. side for choke. If that is clean then. 3) Check the leaks on L.P. side. If not. 4) Calibratethe transmitter.
9. How do you do a zero check on a D.P. transmitter?
Close one of the valve either H.P. or L.P. open the equalizingvalve. The O/P shouldread zero.
10. How would you do Glycol filling or fill sealliquids in seal pots 7Draw and explain.
The procedure for glycol filling is : 1) Close the primary isolationvalves. 2) Open the vent on the seal pots. 3) Drain the use glycol if present. 4) Connect a hand pump on L.P. side while filling the H.P. side withglycol. 5) Keep the equalizer valve open. 6) Keep the L.P. side valve closed.7) Start pumping and fill glycol. 8) Same reeat for L.P. side by connecting pump to H.P. side, keepingequalizer open and H.P. side isolation valve closed.9) Close the seal pot vent valves. 10) Close equalizervalve. 11) Open both theprimary isolation valves.
11. How do you calculatenew factor from new range using old factor andold range. ?
New Factor = _!New Range OldFactor = _IOld Range Flow = K_!Range
Q = Factor X Unit Flow
New Factor = Old Factor I _IOld Range X _/NewRange.
12. How will you vent air in the D.P. cell? What if seal pots are used?
1) Air is vented by opening the vent plugs on a liquid servicetransmitter. 2) On services where seal pots are used isolate the primary isolation valves and open thevent valves. Fill the line from the transmitter drain pluge with a pump.
13. Why isflow measured in square root?
Flow varies directlyas the square root of different pressure F = K square root of AP. Since this flow varies as the square root of differential pressure the pen does not directly inlicate flow. The flow can be determinded by taking the square root of the pen. Say the pen reads 50% of chart.
DEFINATION
1 ACCURACY: Anumber or quantitywhich defines the limit of error under reference conditions.
2 ATTENUA TION : A decreasein signal magnitudebetween two points,or between two frequencies.
3 DEAD TIME: The interval of time between initiation of an impectchange or stimulusand the start of the resulting response.
4 DRIFT : As undesired change in output over a pakiod of time, which change is unrelated to input, operating conditions, or load.
5 ERROR: The difference between the indication and the true value of the measuredsignal.
6 SPAN ERROR: Itis the difference between the actual span and the specified span and is expressed as the precent of specified span.
7 ZERO ERROR: Itis the error of device operating under the specifiedconditions of use when the input is at the lower range value.
8 STATIC GAIN: Itis the ratio of the output change to an input been change after the steadysttate has been reached.
9 HYSTERESIS : The maximum difference between the upscale and downscale indications of the measured signal during a full range traverse for the same input.
10 INTERFERENCE: Interference is any spurious voltage or current aristugfrom external sources and appeearing in the circuitsof a device.
11 COMMON MODE INTERFERENCE : It is the form of interference which appears between the measuring circuit terminalsand ground.
12 NORMAL MODE INTERFERENCE: It is the form of interference which appears betweenmeasuring circuit terminals.
13 LINEARITY: The closenessto which a curve approximate a straight line.
14 RANGE : The regionbetween the limitswithin which a quantity is measured received or transmitted, expressed by stating the lower and upper range values.
15 REPEATABILITY: The closeness of agreement among a number of consecutive measurements of the output for the same value of the measured signal under the same operatingconditions.
16 REPRODUCIBILITY: The closeness of agereement among repeated measurements of the output for the samevalue of the input made under the same operatingconditions.
17 RESPONSE: It is the general behaviour of the output of a device as a functionof input both with respect totime.
18 SIGNAL TO NOISE RA TIO : Ratio of signal amplitudeto noise.
19 TIME CONSTANT: The time required for the output to complete63.2 % of the total rise or decay.
20 SPAN: The algeabric difference betweenupper and lower range values.
21 ZERO SHIFT: Any parallel shift of the input output curve.
PRESSURE CONVERSIONS:
PRESSURE
lpsi 27.74" H2O 1 Kg/cm2 14.223 psi
1 Bar 14.504 psi 1 Kpa 0.145 psi
1Kg/cm2 10.000mm of H2O 1 Bar 1.0197 Kg/cm2
1 Kg/cm2 0.98 Bar 1 Torr 1 mm oflig
1 What is absolute pressure?
Absolute pressure is the total pressurepresent in the system
Abs.pressure = Gauge pressure+ Atm. pressure.
2 Whatis absolute zero pressure?
Absolute zero = 760 mm Hg Vaccum.
3 What is the maximum Vaccum?
The maximum Vaccum = 760 mm Hg.
4 What is Vaccum ?
Any pressure below atmospheric pressureis vaccum. Atm = 760 mm 0 Vaccum Zero Gauge
---
Maxm. Vaccum 760 mm HG
Zero Absolute.
S Whatare the primary elements for measuring pressure?
Theprimary elements used for measuringpressure are :
1. Bourdon tube.
2. Diaphragm.
3. Capsule.
4. Bellows.
5. Pessure Springs.
The above are known as elasticdefermation pressure elements. Type of Bourdon tubes.
1. c C 'type.
2. Spiral.
3.Helix.
Diaphragm: The diaphragm is best suited for low pressuremeassurement. Capsules: Two circulardiaphragms are welded together to form a pressure capsule. Material Used: phospher bronze,Ni-spanc stainless steel.
Bellows: Bellows is a one - piece, collapsible, seamlessmettalic unit with deep folds formed from very
tinwalled tubing.
Material used: Brass, phosphor bronze, stainless steel. Used for high pressure.
PRESSURE SPRINGS: Pressure springs of helical or spiral shape used for measuringhigh pressures.
6 Whatis the principle of a pressure gauge?
Pressure works on Hooks law.
Principle: "Measuring the stores in an elasticmedium"
7 Drawand explain a pressure gauge? Whatis the used of a Hair spring?
The parts of a pressuregauge are :
1. 'C' type Bourdn tube.
2. Connecting link.
3. Sector gear.
4. Pinion gear.
5.Hair Spring.
6. Pointer.
7. Dial.
Uses of Hair Spring: Hair spring serves two purposes name
1. Toeliminate any play into linkages.
2. It serves as a controlling torque.
S How will you calibratean absolute pressure transmitter vaccummanometer Range 0-400mm abs ?
Theprocedure for calibration is as follows :
1. Connect air supply to the transmitter.
2. Connect a testgauge of 0-1.4 Kg/cm2 to the output.
3. Connect Vaccum pump with tee off to the manometter.
4. Apply 760 mm Vaccum ( or Hearest) and adjust zero.
5.Apply 360 mm Vaccum adjust span. (760 - 360 = 400 mm abs. )
9 You are given a mercury manometer range 0 -760 mm ? A vaccum gauge reads60 mm vaccum. The test manometer reads 50 vaccum ? Which of the two in correct.
Thetransmitter is correct because 760 - 50 = 710 mm abs.
LEVEL
1 Explain the different methods of level measurement in brief?
There are two ways of measuring level : ( 1) Direct & ( 2) Indirect.
1) DIRECT LEVEL MEASUREMENT:
1. BOBAND TOP:
2.SIGHT GLASS: This consists of a graduated glass tube mountedon theside of the vessel.As the level of the liquid in the vessel change,so does the level of the liquidin the glass tube.
2) INDIRECT LIQUID LEVELMEASUREMENT :
1. PRESSURE GA UGE : Thisis the simplest method, for pressure gauge is locatedat the zero level of theliquid in the vessel. Any rise in level causesan invrease of pressure which can be mesured by a gauge.
2.PURGE SYSTEM: Inthis method a pipe is installed vertically with the open and at zero level.The other end of the pipe is connected to a regulatedai r supply and to a pressure gauge. To make a level measurement the air supply is adjustedso that pressure is slightlyhigher than the pressure due to height ofthe liquid. This is accimplished by regulating the air pressure until bubbles cab be seen slowly leaving the open end of the pipe.
The method above are suitablefor open tank applications. when a liquid is in a pressure vessel,the liquid column pressurecan't be used unless the vessel pressureis balanced out. This is done through the use of different pressure meters.
DIFF.PRESSURE METHOD: Connection are made at the vessel top and bottom, andto the two columns of the D.P. meter.The top connection is made to the L.P. column of the transmitter and the bottom to H.P. column of the transmitter. The difference in pressure in the vessel is balancedout, since it is fed to both the column of the meter. The differencein pressure deductedby the meter will be due only to the changing,level of the liquid.
DISPLACER TYPELEVEL MEASUREMENT: The level trol is one of the most common istruments used measuring level in closed tanks.This instrument works of Archimedes principle. The displacerin immensed in the liquid due to whichthere is loss of weightdepending on the specified gravity of the liquid. This displacer hangs freelyon a knif transmitted to the pneumaticor electronic counterpart at the other end.
2 Explain how you will measurelevel with a different pressure transmitter.
The bottom connection of the vessel is connectedto high pressure side of the transmitter. Different Pressure = H X D
This differencepressure is applied to H.P. side of the transmitted and calibrated.
3 How is D.P. transmitter applied to a close tank?
In close tank the botton of the tank is connected to the high pressure side of the transmitter and top of the tank in connected to L.P. side of the transmitter. In this way the vessel pressure is balanced.
4 Howis D.P. transmitter applied to an open tank?
On an open tank level measurement the L.P. side is ventedto atmosphere. Whateverpressure acts is on the H.P. side which is a measure oflevel.
5 Whatis purge levelsystem?
This method is also known as bubbler method oflevel measurement. A pipe is installed vertically with its open end at the zero level.The other end of the pipe is connected to a regulated air supplyand to a pressure gaugeor to /\P transmitter. To make a level measurement the air supplyis adjusted so that pressure is slightly higher than the pressure dite to the height of the liquid. Thisis accomplished by regulating the air pressure until bubbles can be seen slowly leavingthe open end of the pipe. The gage then measures the air pressureneeded to over come the pressure of the liquid.
AI> =H X D
USE: On for corrosiveliquids where the transmitter cannot be directlyconnected to processeg... Acids, Some organic liquids.
6 Explain the working of a level trol.
The level trol is used for measuring level 0 liquidsin a closed vessel.
PRINCIPLE: It works on archimedes principle "The loss in weight of a troly immersed in a liquid is equal to amount ofliquid displaced by the body". The level trol basically consistsof the following:
DISPLACER : It is consists of a cylinderical shape pipe sealed and filled inside with sandor some weight.The purpose of this is to convertchange in level to primarymotion. The variation in buyoancy resulting from a changein liquid level varies the netweight of the displacerincreasing or decreasing the load on the torque arm. This changeis directly propertional to change in level and specific gravity of the liquid.
RELA Y : Amplifies pressurevariations at the nozzles. REVERSING ARC: Itis used for the followingpurposes. l. Motion take of from Torque tube.
2. Means of rever control action.
3. Adjustment for specific gravity.
PROPERTIONAL UNIT: Converts primarymotion to a propertional outputair pressure.
CONTROL SETTING UNIT: Provides a motions of varying the set point.
7 Explain the working an electronic level troL
The variation in buyoancy resulting from a change in liquid level, varies the net weight of thedisplacer increasing or decreasingthe load on the torque arm. This change is directly proportional to the changein level and specificgravity of the liquid. The resulting torquetube movement varies the angularmotion of the rotor in the RVDT (Rotaryvariable diff. Transfermer) providing a voltagechange proportional to the rotor displacement, which is convertedand amplified to a D.C. current.
S How will you reversean action of the level trol.?
The reversing are serves as motion take off arm from the torque tube.It is provided with a slot on each side of the centre so that link can be connected eitherfor reverse or direct action.
9 What is interfacelevel? Howdo you calculate it?
When a vessel is filled with two liquids of two different specific gravities the level measurement regers to as interfacelevel.
DP = H (D - d)
On a level set the difference of two specificgravities.
10 How will you calibrate a level trol in the field?
l. First close both the primary isolation valves and drain the liquidinside the chamber.
2. Adjust the zero to get 0% output.
3. Connecta transparent PVC tube to the drain point as shown in hook up.
4. Fill it to the centre of the top flange.
5. Adjust the specificgravity or span adjustment (Electronic Level).
6. Fill it up to 50 %, check linearity.
11 How will you calibrate on interface level control.?
Two liquidsof two different specific gravities.
l. The level will be zero when it is full oflighter liquid. Zero % level = H X d.
Where, H = Displacerlength d = Specificgravity oflighter liquid.
2. The level will be 100 % when it is full of heaviour liquid.
100 % level = H X D.
D = Specific gravityofheaviour liquid. Calibration: (with water)
l. Fill H X d level with water adjust zero.
2. Fill H X D level with water adjust Sp. gravity or span.
3. Check linearity.
12 How will you apply wt. lest calibrationto a level troL
Wt. test calibration method:
1. Remove the displacerfrom the torque arm.
2. Applyequivalent weight on the torque arm that is equalto the wt. of the displcer. Adjust zero %
output.
3. For Span: V = TTr2h
Loss in weight = Wt.of float - wt. of the float immersed in liquid
Loss in weight = [wt. of float - Vol. x d] Span wt. = (wt. of float - Loss in wt.)
r = radius of the displacer. h = ht. of displacer.
4. Apply equivalentwt. equal to the (Wt. of float - Loss in weight).Adjust Span to get 100 % out put.
5. To check linerarity apply average of the two weights.
