instrumentation questions and answers

8/12/2019 Instrumentation Questions and Answers

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INSTRUMENTATION QUESTIONS & ANSWERS

INTRODUCTION:

It is the branch of engineering which deals with the measurement, monitoring, display etc. of the severalof energy exchanges which take place during process operations. "In short Instrumentation is the studyof Instrument."

INSTRUMENT:

Instrument is a device, which is used to measure, monitor, display etc. of a process variable. 1. What is the process Variab e!The process Variable is: low. !ressure. Temperature. evel. #uality i.e. $ %&, '% &, p( etc.". De#i$e a the process Variab e a$% state their $it o# 'eas re'e$t. !( o): )efined as volume per unit of time at specified temperature and pressure

'onditions, is generally measured by positive*displacement or rate meters.U$its: kg + hr, litter + min, gallon + min, m + hr, -m + hr. gases/

*ress re: orce acting per unit 0rea. ! 1 +0U$its : bar, !ascal, kg + cm&, lb + in&.

+e,e : )ifferent between two heights.U$its: 2eters, mm, cm, percentage.

Te'perat re: It is the degree of hotness or coldness of a body.U$its : )egree 'entigrade, )egree ahrenheit, )egree 3elvin, )egree 4ankine.

Q a it-: It deals with analysis. !(, $ '%&, $ 5 &, 'onductivity, Viscosity/

. What are the pri'ar- e e'e$ts se% #or # o) 'eas re'e$t!The primary elements used for flow measurement are: %rifice !late. Venturi tube. !itot tube. 0nnubars. low -o66le. 7eir 8 lumes.

/. What are the %i##ere$t t-pes o# ori#ice p ates a$% state their ses!The different types of orifice plates are: 'oncentric. 9egmental. ccentric. #uadrant dge.

Co$ce$tric:The concentric orifice plate is used for ideal li;uid as well as gases and steam service. This orifice plate beta ratio falls between of 5.= for li;uids and 5.&5 to 5.>5 for gases and steam. ?est results!age 1 of/0


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@

occur between value of 5.A and 5.B, beta ratio means ratio of the orifice bore to the internal pipediameters.

A=@ beveled edges are often used to minimi6eriction resistance to flowing fluid/

flowd

@

fig


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Indication of an orifice plate in a line. The orifice diameter is marked on it. The material of the orifice plate. The tag no. of the orifice plate. The mark the inlet of an orifice.4. What is 5er$o i6s theore' a$% )here it is app icab e! !?ernoulliFs theorem states the "total energy of a li;uid flowing from one point to another remains

constant." It is applicable for non*compressible li;uids.7. 2o) %o -o i%e$ti#- the 2. *. si%e or i$ et o# a$ ori#ice p ate i$ i$e!The marking is always done (. !. side of the orifice tab, which gives an indication of the (. !. side.8. 2o) %o -o ca ibrate a D. *. tra$s'itter!The following steps are to be taken which calibrating:

)a- 'a$i#o % ,a ,e

( o) tra$s'itter

K *


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)a- 'a$i#o % ,a ,e =

*ri'ar- iso atio$ ,a ,e

( o) %irectio$

Ori#ice p ate

Stea' Ser,ice: O$ steam service the transmitter is mounted below the orifice plate with condensate pots. The pots should be at the same level.

Ori#ice p ate

( o) %irectio$


Co$%e$sate pot

)a- 'a$i#o % ,a ,e

? @ ( o) tra$s'itter

!age / of/0


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1". Dra) a$% e p ai$ a$- # o) co$tro oop!

Ori#ice p ate cB,*ositio$er


AIR SU**+ (RC

)a- 'a$i#o % ,a ,e

( o) tra$s'itter AIR SU**+

1 . A$ operator te s -o that # o) i$%icatio$ is 'ore! 2o) )o % -o start chec i$ ! irst flushing the transmitter. lush both the impulse lines. 0dGust the 6ero by e;uali6ing ifnecessary. If still the indication is more then.

'heck .!. side for choke. If that is clean then. 'heck the leaks on .!. side. If not. 'alibrate the transmitter.

1/. 2o) %o -o %o a ero chec s o$ a D.*. tra$s'itter!'lose one of the valve either (.!. or .!. open the e;uali6ing valve. The %+! should read 6ero.

10. 2o) )o % -o %o < -co #i i$ or #i sea i i%s i$ sea pots 4 Dra) a$% e p ai$. The procedure for glycol filling is : 'lose the primary isolation valves. %pen the vent on the seal pots. )rain the use glycol if present. 'onnect a hand pump on .!. side while filling the (.!. side with glycol. 3eep the e;uali6er valve open. 3eep the .!. side valve closed. 9tart pumping and fill glycol. 9ame repeat for .!. side by connecting pump to (.!. side, keeping e;uali6er open and (.!. side

isolation valve closed. 'lose the seal pot vent valves.

'lose e;uali6er valve. %pen both the primary isolation valves.

13. 2o) %o -o ca c ate $e) #actor #ro' $e) ra$ e si$ o % #actor a$% o % ra$ e!

# < L M!< # < 1 %ld flow factor/ F # & 1 -ew flow factor/ # & L M!& L M!< 1 %ld range L M!& F Ne) range

# & N M!< 1 #< N L M!&

!age 0 of/0


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# < G M!& # & F L M!<

%ld factor N new range -ew factor 1

L %ld range

14. 2o) )i -o ,e$t air i$ the D.*. ce ! What i# sea pots are se%! 0ir is vented by opening the vent plugs on a li;uid service transmitter. %n services where seal pots are used isolate the primary isolation valves and open the vent valves.

ill the line from the transmitter drain plug with a pump.

17. Wh- # o) is 'eas re% i$ s are root!low varies directly as the s;uare root of different pressure 1 3 s;uare root of 0!. 9ince this flow

varies as the s;uare root of differential pressure the pen does not directly indicate flow. The flow can bedetermined by taking the s;uare root of the pen. 9ay the pen reads =5$ of chart.

18. What is abso te press re!0bsolute pressure is the total pressure present in the system0bs. pressure 1 auge pressure K 0tm. pressure.

"9. What is abso te ero press re!0bsolute 6ero 1 >B5 mm (g Vacuum.

"1. What is the 'a i' ' Vac '!The maximum Vacuum 1 >B5 mm (g.

"". What is Vac '!0ny pressure below atmospheric pressure is vacuum.At' F 439 '' Hero Vac ' Hero


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2aterial Csed: phosphor bron6e, -i*spanc stainless steel.5e o)s :?ellows is a one * piece, collapsible, seamless metallic unit with deep folds

formed from very tin walled tubing. 2aterial used: ?rass, phosphor bron6e, and stainless steel. Csed for high

pressure.*re.spri$ :!ressure springs of helical or spiral shape used for measuring high pressures.

"/. 2o) )i -o ca ibrate a$ abso te press re tra$s'itter@ si$ ,ac ' 'a$o'eter Ra$ e 9@/99'' abs!The procedure for calibration is as follows: 'onnect air supply to the transmitter. 'onnect a test gauge of 5*


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+ E V E +

"8. 5rie# - e p ai$ the %i##ere$t 'etho%s o# e,e 'eas re'e$t!There are two ways of measuring level:


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'onnections are made at the vessel top and bottom, and to the two columns of the ).!. meter. The topconnection is made to the .!. column of the transmitter and the bottom to (.!. column of thetransmitter. The difference in pressure in the vessel is balanced out, since it is fed to both the column ofthe meter. The difference in pressure deducted by the meter will be due only to the changing, level ofthe li;uid.

d/ )isplacer type level measurement:The leveltrol is one of the most common instruments used measuring level in closed tanks. Thisinstrument works of 0rchimedes principle. The displacer in immersed in the li;uid due to which there isloss of weight depending on the specified gravity of the li;uid. This displacer hangs freely on a knifetransmitted to the pneumatic or electronic counterpart at the other end.

9. E p ai$ ho) -o )i 'eas re e,e )ith a %i##ere$t press re tra$s'itter.The bottom connection of the vessel is connected to high*pressure side of the transmitter.

)ifferent !ressure 1 ( H )

2%

2* +* K *

D B * TRANSMITTER

This difference pressure is applied to (.!. side of the transmitted and calibrated.

1. 2o) is D.*. tra$s'itter app ie% to a c ose ta$ !

In close tank the bottom of the tank is connected to the high*pressure side of the transmitter and top ofthe tank in connected to .!. side of the transmitter. In this way the vessel pressure is balanced.

". 2o) is D.*. tra$s'itter app ie% to a$ ope$ ta$ !%n an open tank level measurement the .!. side is vented to atmosphere. 7hatever pressure acts is onthe (.!. side, which is a measure of level.

9!0- 1 H/ 9p. rav/J 4% 9C!!4 99I%- 1 O/ 9p. rav/ J

2* +* ? @

!age 8 of/0


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. 2o) is D.* tra$s'itter app ie% to a c ose ta$ & ope$ ta$ )ith Dr- e !

9pan 1 H/ /( 7 at minimum level 1 J/ 9/ K O/ /( 7 at maximum level 1 J / 9 / K H K O / / 7here: 1 9pecific gravity of tank li;uid. 9 1 9pecific gravity of seal li;uid. ( 7 1 ;uivalent head of water. H, O 8 J are shown in fig


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%

xample:

H 1 55 inches O 1 =5 inches d 1 =55 inches 1 5.D 9 1 5.E9pan 1 55/ 5.D/ 1 &A5 inches( 7 minimum level 1 =5/ 5.D/ * =55/ 5.E/ 1 * A


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B. '%-T4% 9 TTI- C-IT: !rovides a motion of varying the set point.

3. E p ai$ the )or i$ a$ e ectro$ic e,e tro .

4K

4V)T )' 2%)%9'

K K * * )' 02! I I 4 9!0- S J 4%

The variation in buoyancy resulting from a change in li;uid level varies the net weight of the displacerincreasing or decreasing the load on the tor;ue arm. This change is directly proportional to the changein level and specific gravity of the li;uid. The resulting tor;ue tube movement varies the angular motionof the rotor in the 4V)T 4otary Variable )ifferential. Transformer/ providing a voltage change

proportional to the rotor displacement, which is converted and amplified to a ).'. current.4. 2o) )i -o re,erse a$ actio$ o# the e,e tro !

The reversing are serves as motion take off arm from the tor;ue tube. It is provided with a slot on eachside of the center so that link can be connected either for reverse or direct action.

7. What is i$ter#ace e,e ! 2o) %o -o ca c ate it!7hen a vessel is filled with two li;uids of two different specific gravities the level measurement refersto as interface level.)! 1 ( ) * d /

) -9ITO d

) -9ITO )

%n a level set the difference of two specific gravities.

!age 1" of/0

%!02!

(! !


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8. 2o) )i -o ca ibrate a e,e tro i$ the #ie %!

evel transmitter.

)isplacer chamber

Transparent !.V.' tube


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//. What is the se% o# S ppressio$ a$% e e,atio$!9uppression and elevation are used on evel applications where


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?oth type operate on the principle of thermal expansion. 7here the bulb is immersed in a heatedsubstance. The li;uid expands causing the pressure spring to unwind. The indicating, recording orcontrolling mechanisms are attached to pressure spring.'ompensated Thermometer 9ystem:'ompensations are provided in order to nullify the effect of changes in ambient temperature. Thecompensation in li;uid filled expansions thermal system consists of the second tubing and helicalelement, both li;uid filled. The two elements are so constructed that the measuring helical floats on amovable base the position of which is governed by the compensating helical. The two tubing and helicalare matched in volume so that variation in temperature at the instrument case and along the capillarytubing produce e;ual motion from both helical. 9uch motion nullity each other so that only motion produced by varying the bulb temperature actuates the recorder pen.


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X 1 'o. off of thermal expansion/.t 1 Temperature to be measured.These metals have a positive temperature co*efficient of expansion. Therefore resistance increases asthe temperature increases.

Types of material used:


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circ it si $a si $a

O*ERATION:The input to the instrument is a measurement of some in the processes using a sensing element such asthermocouple/ or a device to produce direct voltage, which is the voltage signal/. This voltage issubtracted from a voltage developed by a known constant voltage in a potentiometer measuring circuit.The subtraction occurs by connecting two voltages in series with the opposing polarity, difference between these two voltages produces signal, the voltage going to the amplifier. The errors will positiveor negative depending on which of the two voltages greater. 7hen amplified, the error signal will driveservo balancing motor in appropriate direction to adGust circuit actually drive the slide wire/ until thedifference between the feedback voltage and the input voltage is balance out. 0n error signal e;ual to6ero results null point/ the balancing servo motor is be longer driven/0 . What is the co$sta$t ,o ta e $it! 4 &1&E.A 4 2 4c

'r < 4 < K &k 'r A

' Meas ri$ e e'e$t Set poi$t Tra$s'itter

Co$tro ,a ,e Ori#ice p ateeed back control involves the detection of the controlled variable and counteracting of charges its it\s

value relative to set point, by adGustment of a manipulated variable. This mode of control necessitiesthat the disturbance variable must affect the controlled variable itself before correction can take place.(ence the term FfeedbackF can imply a correction FbackF in terms of time, a correction that should havetaken place earlier when the disturbance occurred.

(ee% #or)ar% co$tro : Ori#ice

O tp t (T

Co$tro er A%%iti,e re ator

eed forward control system is a system in which corrective action is based on measurement ofdisturbances inputs into the process. This mode of control responds to a disturbance such that isinstantly compensates for that error which the disturbance would have otherwise caused in thecontrolled variable letter in time.

eed forward control relies on a prediction. 0s can be seen from the figure of feed forward control a

necessary amount of input goes to the process. This measurement goes to the controller, which givesoutput to the control valve. The control valve regulates the flow.

!age / of/0

*ROCESS

*ROCESS


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(ee% bac co$tro :

In feed forward control no difference between the desired result and actual result need exist beforecorrective action is taken in feed back control a difference must exist. (ence, open loop or feed forwardcontrol is capable of perfect control, but feedback is not. )ue to economic impartibility of precision, predicting the amount of correction necessary to achieve satisfactory results with feed forward control,feed back control is most often used. In order to properly choose the type of feed back controller for a particular process application, two factors time and gain must be considered.

1"1. E p ai$ three e e'e$t #ee% )ater co$tro s-ste'!(ee% )ater co$tro Dr ' e,e co$tro Stea' co$tro

*" *1 */

S are root S are root e tractorE tractor *

Co'p ti$ e atio$:* F R =*" @ *1 @ > ? */ ? 9* F O tp t.*1; *" & */ F I$p t

1 F A%L stab e s ppressio$. 9 F A%L.5ias.

!age 0 of/0


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1"". E p ai$ A$ti@s r e co$tro ! Co'pressor

*DT

*i$ et *o t et

Ratio

(T ( (RC= ( o) co$,erter>

5-@*ass ,a ,e

This method of surge control uses the ratio of compressor pressure rise to inlet flow rate to set the flowin by*bass loop. 7hen the suction pressure drops and discharge shoots up, the compressor startssurging. The pdt senses this and gives the signal to the 4' which will open the by*pass valve.

or / @ "9 'a.etc. !A$s. : The transmitter output stance from what is known as "live 6ero". This system has specificadvantages :


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(ORCE 5A+ANCE *RINCI*+E:"0 controller which generates and output signal by opposing tor;ue\s".

The input force is applied on the input bellows which novas the beam. This crackles no66le backpressure. The no66le backpressure is sensed by the balancing bellows, which brings the beam to balance. The baffle movement is very less about 5.55&" for full*scale output.

MOTION 5A+ANCE *RINCI*+E:"0 controller, which generates an output signal by motion of its parts".The increase in input signal will cause the baffle to move towards the no66le. The no66le backpressurewill increase. This increase in back pressure acting on the balancing bellows, will expands the bellows,they\re by moving the no66le upward. The no66le will move until motion almost/ e;uals the input

baffle/ motion.A%,a$ta es o# #orce 5a a$ce:


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It is an inherent characteristic of the process, which aids in limiting the deviation of the controlledvariable.

CONTRO++ED VARIA5+E:The controlled variable is that ;uantity and condition which is measured and controlled.

CONTRO++ED MIDIUM:It is that process energy or material in which a variable is controlled. The controlled variable is acondition or characteristic of the controlled medium. or e.g. where temperature of water in a tank isautomatically controlled, the controlled variable is temperature and controlled medium is water.

MANI*U+ATED VARIA5+E:It is that ;uantity or condition which is varied by the automatic controller so as to affect the value ofthe controlled variable.

CONTRO+ A


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DRI(T:

0s undesired change in output over a period of time, which change is unrelated to input, operatingconditions, or load.

ERROR:

The difference between the indication and the true value of the measured signal.

S*AN ERROR:

It is the difference between the actual span and the specified span and is expressed as the percent ofspecified span.

HERO ERROR:

It is the error of device operating under the specified conditions of use when the input is at the lowerrange value.

STATIC


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RAN


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RATE ACTION:It is that in which there is a continuous linear relation between the rate of change of controlled variableand position of final control element. 4ate action produces value motion proportional to the rate ofchange of actual measurement.

*RO*ORTIONA+ 5AND:It is the range of values of the controlled variable, which correspond to the full operating range of thefinal control element.RESET RATE:It is the number of times+minute that the effect of proportional position action upon the final controlelement is repeated by proportional speed floating action.

There are two ways of expressing reset action:


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0 diode is the most basic solid state semi conductor/ device. The above figure shows a !.-. Gunction.The !. material has holes and the -. material has electrons.

(ORWARD 5IAS:

REVERSE 5IAS:

< / 7here the applied voltage overcomes the barrier potential the p side is more positive than the nside/ the current produce is large because maGority carriers cross the Gunction in large numbers. Thiscondition is called forward bias.

& / 7hen the applied voltage aids the barrier potential n side K ve than p side/ the current in small.This state is known as 4everse ?ias.

Q e. : What is a ha # )a,e; # )a,e a$% bri% e recti#ier !A$s. :2A+( WAVE RECTI(IER:

(U++ WAVE RECTI(IER:

5RID


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VO+TA


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1. OR @


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E c si,eOR

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