INSTRUMENTATION INSTRUMENTATION QUESTION & ANSWERS
INTRODUCTION :
It is the branch of engineering which deals with the measurement, monitoring, display etc. of the various of energy exchanges which take place during process operations. "In short Instrumentation is the study of Instrument." INSTRUMENT:
Instrument is a device which is used to measure, monitor, display etc. of a process variable. 1. What are the process “varia!es"#
The process proc ess “variables” “variables” are !low. ressure. Temperature. #evel. $uality i. e. % &', (&', p) etc. • • • • •
$. De%ie a!! the process 'aria!e a( state their )it o% *eas)re*et. # +!o,: *efined as volume per unit of time at specified temperature and pressure
(onditions, is generally measured by positive+displacement or rate meters. Uits: kg hr, litter min, gallon min, m - hr, m- hr. / gases 0
-ress)re: !orce acting per unit 1rea. 2 !1 Uits: bar, ascal, kg cm', lb in'. eve!: *ifferent between two heights. Uits: 3eters, mm, cm, percentage. Te*perat)re: It is the degree of hotness or coldness of a body. Uits: *egree (elsius, *egree !ahrenheit, *egree 4elvin, *egree Q)a!it/: It
5ankin.
deals with analy6ers. analy6ers. /p), %(&', %&', (onductivity (onduc tivity,, 7iscosity0 7iscosity0
0. What are the pri*ar/ e!e*ets )se( %or %!o, *eas)re*et#
The primary elements used for flow measurement are &rifice late. 7enture tube. itot tube. 1nnubars. !low o66le.
• • • • •
age ; of -;
'89:89:8;.doc
•
=eir > !lumes.
. What are the (i%%eret t/pes o% ori%ice p!ates a( state their )ses#
The different types of orifice plates are (oncentric. ?egmental. @ccentric. $uadrant @dge.
• • • •
Cocetric:
The concentric orifice plate is used for ideal liAuid as well as gases and steam service. This orifice plate beta ratio fall between of B.;C to B.DC for liAuids liAuids and B.'B to B.DB for gases, and steam. Eest results occur between value of B.9 and B.8, beta ratio means ratio of the orifice bore to the internal pipe diameters. <
/9C< beveled edges are often used to minimi6e friction, resistance to flowing fluid0
< d
<
flow
/fig. ;0 Eccetric:
The eccentric orifice plate has a hole eccentric. Fse full for measuring containing solids, oil containing water and wet steam. @ccentric plates can use either flange or vena contracta taps, but the tap must be at ;:B< or GB< to the eccentric opening. <
@ccentric orifices have have the bore bor e offset from center to 3inimi6e problems in services of solids+containing materials.
/fig. '0 Se2*eta!:
The segmental orifice place has the hole in the form segment of a circle. This is used for colloidal and slurry flow measurement. !or best accuracy, the tap location should be ;:B< from the center of tangency. age ' of -;
'89:89:8;.doc
<
?egmental orifices orifices provide another version of plates useful for solids containing materials.
/ fig - 0 Q)a(rat E(2e:
It common use in @urope and are particularly useful for pipe si6es less than ' inches. <
$uadrant edge orifices orifices produce a relatively constant coefficient of discharge for services with low 5eynolds 5eynolds numbers numbers in the range from ;BB,BBB down to C,BBB. / fig fig 9 0 3. 4o, (o /o) i(eti%/ a ori%ice i the pipe !ie#
1n orifice tab is welded on the orifice plate which extends our of the line giving an indication of the orifice plate. 5. Wh/ is the ori%ice ta provi(e(#
The orifice tab is provided due to the following following reasons. 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.
• • • • •
6. What is 7ero)!!i8s theore* a( ,here it is app!ica!e#
EernoulliHs theorem states the "total energy of a liAuid flowing from one point to another remains constant." It is applicable for non compressible liAuids.
9. 4o, (o /o) i(eti%/ the 4. -. si(e or i!et o% a ori%ice p!ate i !ie#
The marking is always done ). . side of the orifice tab which gives an indication of the ). . side. . 4o, (o /o) ca!irate a D. -. tras*itter#
The following steps are to be taken which calibrating age - of -;
'89:89:8;.doc
;. 1dust 1dust 6ero of the Jmtrs. '. ?tatic ?tatic pressure pressure test Kive eAual eAual pressure pressure on both sides of the transmi transmitter tter.. Lero should should not shift. If it is shifting carry out static alignment. -. 7acuum test 1pply eAual vacuum to both the sides. The 6ero should not shift. 9. (alibration rocedure Kive 'B psi air supply to the transmitter. 7ent the #.. side to atmosphere. (onnect output of the Instrument to a standard test gauge. 1dust 6ero. C. 1pply reAuired pressure to high pressure side of the transmitter and adust the span. 8. 1dust 6ero again if necessary. • • •
1;. What is the sea! !i<)i( )se( %or %i!!i2 i*p)!se !ies o cr)(e a( visco)s !i<)i( #
Klycol.
11. 4o, (o /o) carr/ o)t pipi2 %or a (i%%eretia! press)re %!o, tras*itter o !i<)i(s= 2as a( stea* services# Wh/# i<)i( !ies: &n liAuid lines the transmitter is mounted below the orifice plate because
liAuids have the property of self draining.
Ori%ice p!ate +!o, (irectio
-ri*ar/ iso!atio va!ve
>IQUID SER'ICE? 0 ,a/ *ai%o!( va!ve
M
+
+!o, tras*itter
&n gas service the transmitter is mounted above the orifice plate because gases have the property pr operty of self venting venting and secondly condensate formation. @as Service: Service:
M
+
age 9 of -;
+!o, tras*itter
'89:89:8;.doc
0 ,a/ *ai%o!( va!ve > @AS SER'ICES ?
-ri*ar/ iso!atio va!ve
+!o, (irectio Ori%ice p!ate
&n steam service the transmitter is mounted below the orifice plate with condensate pots. The pots should be at the same level. level. Stea* Service:
Ori%ice p!ate +!o, (irectio
-ri*ar/ iso!atio va!ve
Co(esate pot
0 ,a/ *ai%o!( va!ve B +!o, tras*itter 1$. Dra, a( ep!ai a/ %!o, cotro! !oop
cv positioer Ori%ice p!ate
-ri*ar/ iso!atio va!ve AIR SU--
+RC
0 ,a/ *ai%o!( va!ve
age C of -;
'89:89:8;.doc
+!o, tras*itter AIR SU--
10. A operator te!!s /o) that %!o, i(icatio is *ore# 4o, ,o)!( /o) start checFi2# •
• • •
!irst flushing the transmitter. !lush both the impulse lines. 1dust the 6ero by eAuali6ing if necessary. 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. 4o, (o /o) (o a Gero checFs o a D.-. tras*itter#
(lose one of the valve either ).. or #.. open the eAuali6ing valve. The & should read 6ero. 13. 4o, ,o)!( /o) (o @!/co! %i!!i2 or %i!! sea! !i<)i(s i sea! pots 6 Dra, a( ep!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. 4eep the eAuali6er valve open. 4eep the #.. side valve closed. ?tart pumping and fill glycol. ?ame repeat for #.. side by connecting pump to ).. side, keeping eAuali6er open and ).. side isolation valve closed. (lose the seal pot vent valves. (lose eAuali6er valve. &pen both the primary isolation valves. • • • • • • • •
• • •
15. 4o, (o /o) ca!c)!ate e, %actor %ro* e, ra2e )si2 o!( %actor a( o!( ra2e#
$; $'
N O;
H
N O'
$; 2 &ld flow/factor0 $' 2 ew flow /factor0 N O; 2 &ld range N O' H ew range
$' × N O; 2 $; × N O' $' H
$; × N O' N O; &ld factor × N ew range age 8 of -;
'89:89:8;.doc
ew factor 2
N &ld range
16. 4o, ,i!! /o) vet air i the D.-. ce!!# What i% sea! pots are )se(# • •
1ir is vented by opening the vent plugs on a liAuid 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.
19. Wh/ %!o, is *eas)re( i s<)are root #
!low varies directly as the sAuare root of different pressure ! 2 4 sAuare root of 1. ?ince this flow varies as the sAuare root of differential pressure the pen does not directly indicate flow. The flow can be determined by taking the sAuare root of the pen. ?ay the pen reads CB% of chart. 1. What is aso!)te press)re #
1bsolute pressure is the total pressure present in the system 1bs. pressure 2 Kauge pressure M 1tm. pressure. $;. What is aso!)te Gero press)re#
1bsolute 6ero 2 D8B mm )g 7acuum. $1. What is the *ai*)* 'ac))*#
The maximum 7acuum 2 D8B mm )g. $$. What is 'ac))*#
1ny pressure below atmospheric pressure is vacuum.
At* H 65; **
ero 'ac))* ero @a)2e
ero aso!)te
Ma.vacc)* H 65;**42
$0. What are the pri*ar/ e!e*ets %or *eas)ri2 press)re#
The primary elements used for measuring pressure are Eourdon tube. *iaphragm. (apsule. Eellows. • • • •
age D of -;
'89:89:8;.doc
ressure ?prings. The above are known as elastic deformation pressure elements. •
T/pe o% 7o)r(o t)es: • • •
H(H type. ?piral. )elix.
Diaphra2*: The diaphragm is best suited for low pressure Caps)!es:
7e!!o,s:
measurement.
Two circular diaphragms are welded together to form a pressure capsule. 3aterial Fsed phosphor bron6e, i+spanc stainless steel.
Eellows is a one + piece, collapsible, seamless metallic unit with deep folds formed from very tin walled tubing. 3aterial used Erass, phosphor bron6e, stainless steel. Fsed for high pressure.
-re.spri2 :
ressure springs of helical or spiral shape used for measuring high pressures.
$. 4o, ,i!! /o) ca!irate a aso!)te press)re tras*itter )si2 vac))* *ao*eter Ra2e ;B;;** as.#
The procedure for calibration is as follows (onnect air supply to the transmitter. (onnect a test gauge of B+;.9 4gcm' to the output. (onnect 7acuum pump with tee off to the manometer. 1pply D8B mm 7acuum /or nearest0 and adust 6ero. 1pply -8B mm 7acuum adust span /D8B + -8B 2 9BB mm abs.0 • • • • •
$3. o) are 2ive a *erc)r/ *ao*eter ra2e ; B65; **. A vac))* 2a)2e rea(s 5; ** vac))*. The test *ao*eter rea(s 3; vac))*. Which oe o% the t,o is correct#
The transmitter is correct because D8B + CB 2 D;B mm abs. $5. Wh/ is a ic!ie( *ao*eter )se(#
It is used to extend the scale of the instrument, because the manometer is at an angle to the vertical. $6. What is the pricip!e o% a press)re 2a)2e#
ressure works on )ooks law. rinciple "3easuring the stress in an elastic medium" $9. Dra, a( ep!ai a press)re 2a)2e# What is the )se( o% a 4air spri2#
The parts of a pressure gauge are ;. H(H type Eourdon tube. '. (onnecting link. -. ?ector gear.
age : of -;
'89:89:8;.doc
9. C. 8. D.
inion gear. )air ?pring. ointer. *ial. )air spring serves two purpose To eliminate any play into linkages. It serves as a controlling torAue.
Uses o% 4air Spri2: • •
E'E $. 7rie%!/ ep!ai the (i%%eret *etho(s o% !eve! *eas)re*et#
There are two ways of measuring level ;. *irect '. Indirect.
T1@ 3@1?F5@
;. *irect level measurement /a0 Eob and tape
TANJ
)eighest point reached by liAuid
1 bob weight and measuring tape provide the most simple and direct method of measuring liAuid level.
*istance to be measured IQUID
after tape is taken out of Tank. Eob /weight0
age G of -;
'89:89:8;.doc
/b0 ?ight glass This consists of a graduated glass tube mounted on the side of the vessel. 1s the level of the liAuid in the vessel change, so does the level of the liAuid in the glass tube. Indirect level measurement /a0 ressure gauge This is the simplest method, for pressure gauge is located at the 6ero level of the liAuid in the vessel. 1ny rise in level causes an increase of pressure which can be measured by a gauge. /b0 urge system In this method a pipe is installed vertically with the open and at 6ero level. The other end of the pipe is connected to a regulated air r supply and to a pressure gauge. To make a level measurement the air supply is adusted so that pressure is slightly higher than the pressure due to height of the liAuid. This is accomplished by regulating the air pressure until bubbles cab be seen slowly leaving the open end of the pipe.
-ress)re 2a)2e As !eve! 2a)2e
Air re2)!ator N $ p)r2e rota*eter
Air s)pp!/
The air pressure to the bubbler pipe is minutely in excess of the liAuid pressure in the vessel, so that air pressure indicated is a measure of the level in the tank.
The method above are suitable for open tank applications. when a liAuid is in a pressure vessel, the liAuid column pressure canHt be used unless the vessel pressure is balanced out. This is done through the use of different pressure meters.
age ;B of -;
'89:89:8;.doc
/c0 *ifferential pressure meter (onnections are made at the vessel top and bottom, and to the two columns of the *.. meter. The top connection is made to the #.. column of the transmitter and the bottom to ).. column of the transmitter. The difference in pressure in the vessel is balanced out, since it is fed to both the column of the meter. The difference in pressure deducted by the meter will be due only to the changing, level of the liAuid. /d0 *isplacer type level measurement The leveltrol is one of the most common instruments used measuring level in closed tanks. This instrument works of 1rchimedes principle. The displacer in immersed in the liAuid due to which there is loss of weight depending on the specified gravity of the liAuid. This displacer hangs freely on a knife transmitted to the pneumatic or electronic counterpart at the other end.
0;. Ep!ai ho, /o) ,i!! *eas)re !eve! ,ith a (i%%eretia! press)re tras*itter.
The bottom connection of the vessel is connected to high pressure side of the transmitter. *ifferential ressure 2 ) x *
4 ( 4- -
M +
D - TRANSMITTER
This difference pressure is applied to ).. side of the transmitted and calibrated. 01. 4o, is D.-. tras*itter app!ie( to a c!ose taF#
In close tank the bottom of the tank is connected to the high pressure side of the transmitter and top of the tank in connected to #.. side of the transmitter. In this way the vessel pressure is balanced. 0$. 4o, is D.-. tras*itter app!ie( to a ope taF #
age ;; of -;
'89:89:8;.doc
&n an open tank level measurement the #.. side is vented to atmosphere. =hatever pressure acts is on the ).. side which is a measure of level. ?1 2 /J0 /?p. Krav0 L@5& ?F5@??I& 2 /P0 /?p. Krav?
K
4- B
00. 4o, is D.- tras*itter app!ie( to a c!ose taF & ope taF ,ith Dr/ !e2#
?pan 2 /J0 /K#0 )= at minimum level 2 /L0 /K ?0 M /P0 /K#0 )= at maximum level 2 /L0 /K?0 M /J M P0 /K#0 =here K# 2 ?pecific gravity of tank liAuid. K? 2 ?pecific gravity of seal liAuid. )=2 eAuivalent head of water. J, P > L are shown in fig /;.;0 Ope taF
C!ose taF ,ith (r/ !e2
31J.#@7@# 31J #@7@# K
K
3I.#@7@#
3I #@7@#
@xample &pen tank with J 2 -BB inches P 2 CB inches L 2 ;B inches K# 2 B.: K? 2 B.G ?pan 2 /-BB0 /B.:0 2 '9B inches )= at minimum level 2 /;B0 /B.G0 M /CB0 /B.:0 2 9G inches )= at maximum level 2 /;B0 /B.G0 M /CB M -BB0 /B.:0 2 ':G inches (alibrated range 2 9G to ':G inches head of water age ;' of -;
'89:89:8;.doc
C!ose taF ,ith ,et !e2:
?pan 2 / J 0 / K# 0 )= at minimum level 2 / P 0 / K# 0 Q / d 0/ K? 0 )= at maximum level 2 / J M P 0 / K# 0 Q / d 0 / K? 0 =here K# 2 ?pecific gravity of tank liAuid K? 2 ?pecific gravity of tank liAuid )= 2 @Auivalent head of water J, P and L are shown in fig. 31J #@7@#
K
(
3I #@7@#
@xample
J 2 -BB inches P 2 CB inches d 2 CBB inches K# 2 B.: K? 2 B.G ?pan 2 / -BB 0 / B.: 0 2 '9B inches )= minimum level 2 / CB 0 / B.: 0 + / CBB 0 / B.G 0 2 + 9;B inches )= maximum level 2 / -BB M CB 0 / B.: 0 Q / CBB 0 / B.G 0 2 + ;DB inches (alibrated range 2 + 9;B to Q;DB inches head of water. / minus sings indicate that the higher pressure is applied to the low pressure side of the transmitter 0 0. What is p)r2e !eve! s/ste* #
This method is also known as bubbler method of level measurement. 1 pipe is installed vertically with its open end at the 6ero level. The other end of the pipe is connected to a regulated air supply and to a pressure gauge or to R transmitter. To make a level measurement the air supply is adusted so that pressure is slightly higher than the pressure due to the height of the liAuid. This is accomplished by regulating the air pressure until
age ;- of -;
'89:89:8;.doc
bubbles can be seen slowly leaving the open end of the pipe. The gage then measures the air pressure needed to over come the pressure of the liAuid. S 2 ) J * USE : &n for corrosive liAuids where the transmitter cannot be directly connected to process eg... 1cids, ?ome organic liAuids. 03. Ep!ai the ,orFi2 o% a !eve! tro!.
The leveltrol is used for measuring level of liAuids in a closed vessel. ;. 5I(I#@. It works on 1rchimedes principle "The loss in weight of a body immersed in a liAuid is eAual to amount of liAuid displaced by the body". The leveltrol basically consists of the following '. *I?#1(@5 It is consists of a cylindrical shape pipe sealed and filled inside with sand or some weight. The purpose of this is to convert change in level to primary motion. The variation in buoyancy resulting from a change in liAuid level varies the net weight of the displacer increasing or decreasing the load on the torAue arm. This change is directly proportional to change in level and specific gravity of the liAuid. -. 5@#1P 1mplifies pressure variations at the no66les. 9. 5@7@5?IK 15( It is used for the following purposes. 3otion take of from TorAue tube. 3eans of reverse control action. 1dustment for specific gravity. C. 5&@5TI&1# FIT. (onverts primary motion to a proportional output air pressure. 8. (&T5 ?@TTIK FIT rovides a motions of varying the set point. • • •
05. Ep!ai the ,orFi2 a e!ectroic !eve!tro!.
M
5@K &?(
57*T
*( 3&*
M +
& 1 3
M + *( 13#I!I@5 ?1
L@5&
The variation in buoyancy resulting from a change in liAuid level, varies the net weight of the displacer increasing or decreasing the load on the torAue arm. This change is directly proportional to the change in level and specific gravity of the liAuid. The resulting torAue tube movement varies the angular motion of the rotor in the 57*T /5otary 7ariable *ifferential. Transformer0 providing a voltage change proportional to the rotor displacement, which is converted and amplified to a *.(. current. 06. 4o, ,i!! /o) reverse a actio o% the !eve!tro!.#
age ;9 of -;
'89:89:8;.doc
The reversing are serves as motion take off arm from the torAue tube. It is provided with a slot on each side of the center so that link can be connected either for reverse or direct action. 09. What is iter%ace !eve! # 4o, (o /o) ca!c)!ate it #
=hen a vessel is filled with two liAuids of two different specific gravities the level measurement refers to as interface level. * 2 ) / * + d 0
*@?ITP d
*@?ITP * ) #
&n a level set the difference of two specific gravities. 0. 4o, ,i!! /o) ca!irate a !eve!tro! i the %ie!( #
*isplacer chamber #evel transmitter.
Transparent .7.( tube
;. !irst close both the primary isolation valves and drain the liAuid inside the chamber. '. 1dust the 6ero to get B% output. -. (onnect a transparent 7( tube to the drain point as shown in hook up. 9. !ill it to the center of the top flange. C. 1dust the specific gravity or span adustment / @lectronic #evel 0. 8. !ill it up to CB %, check linearity. ;. 4o, ,i!! /o) ca!irate o iter%ace !eve! cotro!. #
&n an interface leveltrol there are two liAuid of two different specific gravities. ;. The level will be 6ero when it is full of lighter liAuid. age ;C of -;
'89:89:8;.doc
Lero % level 2 ) J d. ) 2 *isplacer length d 2 ?pecific gravity of lighter liAuid. ' The level will be ;BB % when it is full of heavier liAuid. ;BB % level 2 ) J *. * 2 ?pecific gravity of heavier liAuid. (alibration with water ;. !ill ) J d level with water adust 6ero. '. !ill ) J * level with water adust ?p. gravity or span. -. (heck linearity. 1. 4o, ,i!! /o) app!/ ,t. !est ca!iratio to a !eve!tro!.
=t. test calibration method ;. 5emove the displacer from the torAue arm. '. 1pply eAuivalent weight on the torAue arm that is eAual to the wt. of the displacer. 1dust 6ero % output. -. !or ?pan 7 2 πr 'h #oss in weight 2 =t. of float + wt. of the float immersed in liAuid #oss in weight 2 U wt. of float + 7ol. x d V ?pan wt. 2 /wt. of float + #oss in wt.0 r 2 radius of the displacer. h 2 ht. of displacer. 9. 1pply eAuivalent wt. eAual to the /=t. of float + #oss in weight0. 1dust ?pan to get ;BB % out put. C. To check linearity apply average of the two weights. $. What ,i!! happe i% the (isp!acer has %a!!e (o, ,hi!e i !ie #
The output will be maximum.
0. What ,i!! happe i% the (isp!acer has a ho!e i it ,hi!e i !ie #
The output will be minimum.
. What is the )se( o% S)ppressio a( e!evatio #
?uppression and elevation are used on #evel applications where /;0 transmitters are not mounted on some level /'0 =et leg. i.e. condensable vapors are present. 3. What are the !i*itatios o% !eve!tro! #
The limitations of a level control that it cannot be used for lengths more than D' inches. 5. 4o, ,i!! /o) co**issio D.-. tras*itter i %ie!( i press)riGe( vesse!.
;. '. -. 9. C. 8.
(lose both the isolation valves, 7ent the ).. side. !ill it with the sealing liAuid. &pen the #.. side vent valve. 1dust 6ero with suppression spring. (lose the #.. side vent valve. &pen both the isolation valves.
age ;8 of -;
'89:89:8;.doc
6. 4o, ,i!! /o) checF Gero o% a !eve! D.-. tras*itter ,hi!e is !ie #
;. (lose both the isolation valves. '. &pen the vent valve on #.. leg and ).. leg drain. -. (heck and adust 6ero if necessary. 9.Ep!ai the ,orFi2 o% a Era% !eve! 2a)2e#
The @nraf precise level gauge are based on servo powered null+balance techniAue. 1 displacer serves as a continuous level sensing element. rinciple 1 displacer with a relative density higher than that of the product to be measured, is suspended from a stainless steel wire E, that is attached to a measuring drum. 1 two phase servo meter controlled by a capacitive balance system winds or unwinds the measuring wire until the tension in the weighing springs is in balance with the weight of the displacer partly immersed in the liAuid. The sensing system in principle measures the two capacitance formed by the moving center sensing rod @ provided with two capacitor plates and the side plates. In balance position the capacitances are of eAual value. 1 level variation will a difference in buoyancy of the displacer. The center sensing rod will move in the direction of one of the side capacitor plates. This causes a difference in value of these capacitances. Ey an electronic circuit this change is detected and integrated. *uring the rotation of the servo motor the cam driven transmitter continuously change the voltage pattern to a remote indicator of which the receiver motor drives a counter indicating level variation. TEM-RETURE . What are the (i%%eret *etho(s o% te*perat)re *eas)re*et # Ep!ai.
The different methods of temperature measurement are ;. 3echanical '. @lectrical. Mechaica! *etho(s: 1. Merc)r/ i 2!ass ther*o*eters :
This consists of a glass tube of very fine bore oined to a reservoir at the bottom and sealed at the top. 1 measured Auantity of mercury is the enclosed. =hen the thermometer is heated the mercury expands much more than the glass and is therefore forced to rise up in the tubing 1 scale is fixed at the side. $. 7i*eta!!ic Ther*o*eter : Two metals whose coefficient of linear expansion is different are welded and rolled together to the desire thickness. The actual movement of a bimetal is its flexivity with one end fixed, a straight bimetal strip (e%!ects i proportion to its temperature, to the sAuare of its length and inversely with its thickens. 0.-ress)re Spri2 Ther*o*eters : There are four classes of pressure spring thermometers. ;. #iAuid filled 2 class ; '. 7apor pressure 2 class ' -. Kas filled 2 class 9. mercury filled 2 class 9 i<)i( %i!!e( & Merc)r/ %i!!e( :
age ;D of -;
'89:89:8;.doc
Eoth type, operate on the principle of thermal expansion. =here the bulb is immersed in a heated substance. The liAuid expands causing the pressure spring to unwind. The indicating, recording or controlling mechanisms are attached to pressure spring. (ompensated Thermometer ?ystem (ompensations are provided in order to nullify the effect of changes in ambient temperature. The compensation in liAuid filled expansions thermal system consists of the second tubing and helical element, both liAuid filled. The two elements are so constructed that the measuring helical floats on a movable base the position of which is governed by the compensating helical. The two tubing and helicals are matched in volume so that variation in temperature at the instrument case and along the capillary tubing produce eAual motion from both helicals. ?uch motion nullity each other so that only motion produced by varying the bulb temperature actuates the recorder pen. @as %i!!e( Ther*o*eters :
This type depends upon the increase in pressure of a confirm gas /constant volume0 due to temp. increase. The relate between temp. and pressure in this kind of system follow (harles law and may be expressed. ; T; ' T' The system is filled under high pressure. The increase pressure for each degree of temperature rise is therefore greater than if the filling pressure were low. itrogen the gas most after used for such systems, because it chemically insert and possesses a favorable coefficient thermal expansion. 'apor B -ress)re Ther*o*eters :
7apor pressure thermometers depend upon vapor pressure of liAuid which only partially fills the system. 1t low temperatures the vapor pressure increase for each unit temperature charge is small, at higher temperature the vapor pressure change is much greater . E!ectrica! *etho( o% te*perat)re *eas)re*et:
;. Thermocouples It is a simple device consisting of a dissimilar metal wires oined at their ends. when an of each wire is connected to a measuring instrument thermocouples becomes an accurate and sensitive temperature measuring device.
cold or reference unction
)ot or measuring unction
Thermocouples Types and 5ange Type T( /W0 /40 /@0
ositive wire > color Iron > =hite (hromel > Pellow (hromel > urple
egative wire > color (onstantan > 5ed 1lumel > 5ed (onstantan > 5ed age ;: of -;
5ange °! +-BB to;9BB +-BB to'-BB +-BB to;8BB '89:89:8;.doc
/T0 /50 /?0
(opper > Elue latinum and ;B%5hodium > Elack latinum and ;-%5hodium > Elack
(onstantan > 5ed latinum > 5ed latinum > 5ed
+-BB to 8CB -' to 'DBB -' to 'DBB
5esistance Q Temperature *etectors /5T*0 5T*Hs are generally used for precise temperature measurement. It consists of a five wire wrapped around an insulator and enclosed in a metal. The most sheath of a resistance thermometer resembles that of bimetallic thermometer bulb. 5I(I#@ "5esistance increases as temperature increase" Rt. H Ro >1 L t ?
5t. 2 5esistance of Temperature to measured. 5o. 2 5esistance of 6ero temperature. X 2 (o. off of thermal /expansion0. t 2 Temperature to be measured. These metals have a positive temperature co+efficient of expansion. Therefore resistance increases as the temperature increases. Types of material used /;0 latinum /'0 ickel These metals have a positive temperature co+efficient of expansion. Therefore resistance increases as the temp. increases. (alculation of 5esistance or t;BB. 5o. 2 ;BB x for platinum 2 B.BB-:C c. To calculate 5esistance at ;BBHc. 5;BB 2 ;BB U ;M / -:.C x ;B 9 x ;BB 0 V 2 ;BB M /;BB x B.-:C0 5;BB 2 ;-:.C Resistace at 1;;8c H 109.3 3;. What is -t 1;; *ea. #
t;BB means ;BB &)3? at BH( for a platinum resistance bulb. 31. What is t,o ,ire a( three ,ire R.T.D. s/ste* #
Two wire 5 .T .*. system Two wire 5T* system use for short distance like a compressor field local panel. Three wire ?ystem Three wire system use for long distance coke a field to control 5un. The third wire is used for compensation of lead wire resistance. T,oB,ire R.T.D
ThreeB,ire R.T.D
B R 1
K al
B R $
R 1 R $
age ;G of -;
K al '89:89:8;.doc
R 0
R 0
R.T.D R.T.D
3$. Dra, a potetio*eter te*p. *eas)ri2 circ)its a( ep!ai its# Ther*o co)p!e T,o (i%%eret si2a!
AM-
U Fo, si2a! Servo a!aci2 *otor Meas. circ)it
Costat vo!ta2e si2a!
Jo, si2a!
O-ERATION :
The input to the instrument is a measurement of some in the processes using a sensing element / such as thermocouple 0 or a device to produce direct voltage, which is the voltage /signal0. This voltage is subtracted 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 error will positive or negative depending on which of the two voltages greater. =hen amplified, the error signal will drive servo balancing motor in appropriate direction to adust circuit / actually drive the slide wire 0 until the difference between the feedback voltage and the input voltage is balance out. 1n error signal eAual to 6ero results / null point 0 the balancing / servo motor is be longer driven 0 30. What is the costat vo!ta2e )it #
(r ; (; D89Y
5 '2'G.9 5 ; 'k
5 5esistance lead of
age 'B of -;
5 3
(r9 (r -
5c
M + (5 T 2 -9-.-'89:89:8;.doc
The constant voltage circuit consists of a rectifier, (5, a filter capacitor (;, followed by two stages of 6ener regulation. 1bridge configuration is provided to ;amp line voltage regulation 6ener (5-, 5; and 5' combine provide relatively constant current to 6ener (59, Thus variations. 5esisters 5' and 5- form a bridge that any remoment line voltage effects. 3. Ep!ai the ,orFi2 o% a a!aci2 *otor.
?ignal in control winding appears as and capacitor of amp. board.
due tank circuit formed by winding 5@*
(ontrol signal winding from amplifier
K5@@
Mve signal It lags GBH from due to line phase capacitor amp. board. +ve signal #eads GBH from line due to line phase capacitor of amp. board. The servo / balancing 0 motor is an induction motor that functions by creating a rotating magnetic field in the stator. The rotor / armature 0 turns by following this field. The field is developed by the use of two windings in the stator. It has got two windings, one of which is continuously energi6ed by the line voltage. The other winding is energi6ed by the power amplifier, with a current whose phase with respect to line current determines the direction of rotation of motor.
33. What is )ro)t %eat)re # Ep!ai.
Eurnout provides the warning feature of driving indicator the end of scale if the input circuit should open. 1 burnout resistor is provided which develops a voltage drop between the measuring circuit and the amplifier. The polarity of the signal determines the direction of the servo drive upon an open circuit in the input. Fpscale burnout 5 value ;B 3 *ownscale burnout 5 value '.' 3 35. Ep!ai the !ocF (ia2ra* o% a a*p!i%ier i a te*p. recor(er. 36. Wh/ is a coverter )se( i a te*p. recor(er #
The converter is designed to convert *. (. input voltage into an 1. (. input voltage proportional in amplitude to the input. age '; of -;
'89:89:8;.doc
39. Wh/ are Ther*o,e!!s )se( #
In numerous application it is neither desirable nor practical to expose a temperature sensor directly to a process material. =ells are therefore used to protect against damage corrosion, erosion, abrasion and high pressure processes. 1 thermowell is also useful in protecting a sensor from physical damage during handling and normal operation. ?electing a thermowell The significant properties considered in selecting a material for the well are as follows ;. 5esistance to corrosion and oxidation. '. 5esistance to mechanical and thermal shock. -. #ow permeability / 5esistance to gas leakage 0. 9. 3echanical strength. C. Thermal conductivity. 3aterial for =ells ;. ?tainless steal. '. Inconel. -. 3onel. 9. 1lloy steal. C. )astelloy H(H. 3. 4o, ,i!! /o) ca!irate a te*p. recor(er )si2 a potetio*eter #
(onnect the potentiometer output to the input of temp. recorder. ;. (onnect the / Mve 0 to the / Mve 0 and / +ve 0 to the / +ve 0. '. If ambient compensation is provided in potentiometer set it to the correct ambient temp. -. If no ambient compensation is provided take a thermometer and measure the correct ambient temp.. !ind out the corresponding m7 s for that temp. for the given input type of thermocouple. 9. =hile feeding subtract the ambient temp. m v s from the corresponding temp. every time. C. 1dust the necessary adustments. 3easuring Temperature =ith a otentiometer ;.(onnect the input of the potentiometer to the thermocouple. '.If no ambient compensation is provided find out the corresponding millvolts for that ambient temp. for the type of thermocouple used. -.1dd the ambient temp. millvolts to the corr. input millvolts measured. !ind out from the chart the corresponding temperature. 5;. What t/pe o% sesi2 e!e*et ,o)!( /o) )se to *eas)re ver/ !o, te*perat)re #
The sensing element used for measuring very low temperature is 5. T. *. / 5esistance Temperature *etector 0 51. What are sFi te*perat)re ther*oco)p!es #
?kin thermocouples are those which are directly connected to the process without any thermowell. Fsed for measuring the skin temperature of heaters furnaces, flue gas etc. age '' of -;
'89:89:8;.doc
5$. What is the specia!t/ o% ther*oco)p!es !ea( ,ires #
They should be of the same material as the thermocouple.
50. What is the (i%%erece the a Wheatstoe ri(2e a( a potetio*eter #
The difference between a potentiometer and a =heatstone bridge measuring instrument is that potentiometer is a voltage measuring instrument and =heatstone bridge is a current measuring instrument. 5. Ep!ai the coti)o)s a!ace potetio*eter s/ste* )si2 R. T. D.8s.
(onverter
7oltage ower amplifier amplifier
@nergi6ing coil ?lide wire Ealancing motor R.T.D
In a balance =heatstone bridge resistance thermometer a resistance bulb is connected into one branch of a d.c. bridge circuitZ in another branch is a variable resistance in the form of a calibrated slidewire. 7ariations in temp. of the measured medium cause a change in resistance of the bulb and a conseAuent unbalance of the bridge circuit. 1 self balancing =heatstone bridge recogni6es the condition of unbalance, determines its direction and magnitude and position the slidewire contractor to rebalance the bridge and indicate the temp. on the scale. The *. (. potential appearing at 11 is converted by the converting stage to an 1. (. voltage appearing at EE and is multiplied by the voltage amplifier to a large value at cc. It is then used to control the power amplifier output ** which drives the balancing motor in the proper direction to balance the bridge. The polarity of the signal at 11 determines the phase of the alternating voltage at EE which in turn determines the direction of rotation of the balancing motor. 53. 4o, is a)to*atic Re%erece )ctio co*pesatio carrie( o)t i te*p. recor(ers # Rheostat
M +
age '- of -;
'89:89:8;.doc
scale
'aria!e Resistor
@
M +
Re%erece )ctio
4ot )ctio -
!or automatic reference unction compensation a variable nickel resister is used. 1s the temperature changes, so does its resistance. This reference unction compensatory is located, so that it will be at the temperature of the reference unction. The reference unction is at the position where the dissimilar wire of the thermocouple is reoined, which invariably is at the terminal strip of the instrument.
CONTRO SSTEMS 55. Ep!ai the app!icatio o% proportioa! ite2ra! a( (erivative actio# -roportioa! cotro! o!/ :
roportional control only attempts to return a measurement to the set point after a load upset has occurred. )ow ever it is impossible for a proportional controller to return the measurement exactly to the set point. Use It is normally used for level controls. It reduces the effect of a load change but it can not eliminate it. -roportioa! p!)s reset cotro!:
5eset action is introduced to eliminate offset. It will integrate any difference between measurement and setpoint and cause the controllerHs output to change until the difference between the measurement and set point is 6ero. 5eset will act as long as the error exists. Use: roportional M 5eset controllers are by far the common types used in industrial process control and where predominate dead times occur. -roportioa! p!)s reset p!)s (erivative:
*erivative or rate action helps the controller overcome system inertia and result in faster, more precise control. *erivative action occurs whenever the measurement signal changes. Fnder study conditions the rate action does not act. *erivative allows the controller to inect more corrective action. Use : &n temperature controls.
age '9 of -;
'89:89:8;.doc
56. What is (i%%erece 2ap cotro! #
*ifferential gap control is similar to on off control except that a band or gap exists around the control point. Use: In industry differential gap control is often found in non critical level control applications where it is desirable only to prevent a tank from flooding or drying. Whe a measured variable exceeds the upper gap the control valve will open fully or be closed fully. ?imilarly when it exceeds the lower gap it will open or close fully. 59. Where is o o%% cotro! )se( #
&n off control is used when ;. recise control is not needed. '. rocesses that have sufficient capacity to allow the final operator to keep up with the measurement cycle. -. It is mainly used in refrigeration and are conditioning systems. 5. What is resetB,i( )p#
=hen reset action is applied in controllers =here the measurement is away from the set point for long periods the rest may drive the output to its maximum resulting in rest wind up. =hen the process starts again the output will no come off its maximum until the measurement crosses the so point causing large overshoots. This problem can be avoid by including anti+reset wind up circuit which eliminates the problem of output saturation. 6;. Wh/ is reset ca!!e( ite2ra! a( Rate (erivative #
5eset is called integral because of the mathematical relationship to the output. 5ate is called derivative because t2i
&i 2 f ∫ e / dt 0 M &B
&i 2 r / de dt 0 M &B
t2o
&i 2 &B 2 e 2 t 2 f 2
is the &utput at any given time is the out put at time 6ero or 6ero error. is the error signal is time. is the reset rate in respects per minute.
r 2 is the rate time
61. Ep!ai t)i2 o% cotro!!ers.
Tuning basically involves adustment of proportional. Integral and derivative parameters to achieve good control. The gain, time constants, and dead times around the loop will dictate the settings of various parameters of the controller . T)i2 *etho(s are roa(!/ c!assi%ie( ito t,o :
;. (losed #oop 3ethod e.g. Fltimate Kain 3ethod. '. &pen #oop 3ethod e.g. process 5eaction curve. U!ti*ate 2ai *etho(:
The term ultimate gain was attached to this method because its use reAuire the determination of the ultimate gain /sensitivity0 and ultimate period. The ultimate sensitivity 4u is the maximum allowable value of gain /for a controller with only roportional mode0
age 'C of -;
'89:89:8;.doc
for which the system is stable. The ultimate period is the period of the response with the gain set at its ultimate value. -rocess reactio c)rve :
To deter mine the process reaction curve, the following steps are recommended. ;. #et the system come to steady state at the normal load level. '. lace the controller on manual. -. 3anually set the output of the controller at the value at which it was operating in the automatic mode. 9. 1llow the system to reach the steady state. C. =ith controller on manual, impose a step changes in the output of controller, which is an signal to value. 8. 5ecord the response of controlled variable. D. 5eturn the controller output to its previous value and return the controller to auto operation. 6$. Ep!ai the ,orFi2 o% a e!ectroic -.I.D. cotro!!er.
Input from the measurement transmitter is compared with the set point voltage to produce a deviation signal. The deviation signal is combined with a characteri6ed feed back signal to provide the input for the function generator amplifier. This amplifiers output is delivered to the feed back network, and to the final output which is a ;B+CBm.a. do signal for actuation of final operators. ?et 3anual 3easurement
*eviation
1mplifier
3anual 1utomatic
Kain
To final operator
It is a obtained by adusting the magnitude of feed back signal. 1n increase in negative feed back means less effective gain and thus a broader proportional band. Reset actios: It is obtained by charging the reset capacitor at a rate determined by the value of reset resister. The reset resister is variable, and constitutes reset adustment. Derivative actio: The connection of a derivative capacitor across the feedback circuit delays feedback until the capacitor is charged to a value approaching amplifier output. This delay is controlled by value of derivative resister. This resister is variable and constitutes derivative adustment. -roportioa! actio:
60. What is a aa!o2)e ite2rator a( a aa!o2)e (i%%eretiator # Aa!o2 ite2rator:
age '8 of -;
'89:89:8;.doc
R
C
'i
A'
B1
B
'; H
'o)t
'i(t RC
Aa!o2 (i%%eretiator: R C ( '; H B RC
'i
'i
A'
(t
B
'o)t
6. What is ati reset ,i( )p #
If the limit acts in the feed back section of the control amplifiers integral circuit, the controller output will immediately begin to drive in the opposite direction as soon as the process signal crosses the set point. This approach is referred to as antireset wind up. 63. What are DeBsat)rators #
=hen, in some processes, e.g. batch process, long transient responses are expected during which a sustained deviation is present the controller integral action continuously drives the output to a minimum or maximum value. This phenomenon is called "integral saturation of the control unit". =hen this condition. 65.What is the e%%ect o% ,eep ho!e o ca!c)!atio o% ori%ice ore# >(*? $
( H (* 1 ;.33
,here: (* H Meas)re( (ia*eter o% ori%ice.
P
(h H Drai ho!e (ia*eter. ( H Correcte( (ia*eter ori%ice siGe.
(h 66. Ep!ai the ,orFi2 o% Rota*eter#
OUT ET
Tapered glass tube
?cale
age 'D of -;
The flow rate varies directly as the float rises and falls in the tapered tube.
'89:89:8;.doc
IN ET
7ariable area meters are special form of head meters. =here in the area of flow restrictor is varied. ?o as to hold the differential pressure constant. The 5ota meters consists of a vertical tapered tube through which the metered fluid flows in upward direction. 1 "float" either spherical or cone shaped, actually more dense than the fluid being measured, creates an annular passage between its maximum circumference and the weight of the tapered tube. 1s the flow varies the "float" rises or falls to vary the area of the passage so that the differential across it ust balances the gravitational force on the "float" i.e. the differential pressure is maintained constant. The position of the "float" is the measured of the rate of flow. 69. Ep!ai the ,orFi2 o% a *a2etic *eter.
1n electric potential is developed when a conductor is moved across the magnetic field. In most electrical machinery the conductor is a "wire"Z the principle is eAually applicable to a moving, electrically conductive liAuid. The primary device of commercial magnetic meters consists of a straight cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube wall and located at opposite ends of a tube diameter. 1 uniform a.c. magnetic field is provided at right angles to electrode diameter and to the axis of the tube. The a.c. voltage developed at the electrodes is proportional to the volume flow rate of fluid, and to a magnetic field strength. This device is limited to electrically conducting liAuids. The magnetic meter is particularly suited to measurement of slurries and dirty fluids, since there are no location for solids to collect except the walls of the tube itself. 6. Ep!ai the ,orFi2 o% a t)rie *eter.
Turbine meters consist of a straight flow tube within which a turbine or fan is free to rotate, about its axis which is fixed along the center line of the tube. ?traightening vanes upstream of the turbine minimi6es possible rotational components of fluid flow. In most units a magnetic pick+up system senses the rotation of the rotor through the tube wall. The turbine meter is a flow rate device, since the rotor speed is directly proportional to flow rate. The output is usually in the form of electrical pulses from the magnetic pick+up with a freAuency proportional to flow rate. Turbine meter are primarily applied to measurement of clean and non+corrosive hydrocarbons. 9;. Ep!ai the ,orFi2 o% a -itot t)e.
The pitot tube measures the velocity at point in the conduct. If Auantity rate measurement is desired, it must be calculated from the ratio of average velocity to the velocity at the point of measurement. -ricip!e : If a tube is placed with its open and facing into a stream of fluid, then the fluid impinging on the open end will be brought to rest, and the kinetic energy converted to pressure energy. This the pressure built up in the tube will be greater than that in the free stream by the impact pressure or pressure produced by loss of kinetic energy. The increase in pressure will depend upon the sAuare of the velocity of the stream. The difference is measured between the pressure in the tube and static pressure of the stream. The static pressure is measured by a tapping in the wall of the main or by a tapping incorporated in the pitot static tube itself. The difference between the pressure in the tube and static pressure will be a measure of the impact pressure and therefore of the velocity of the stream oil. age ': of -;
'89:89:8;.doc
91. Where is the ite2ra! ori%ice )se( #
Integral orifice is used to measure small flow rates. It is mounted directly on the secondary device. The integral orifice diameter varies between B.B'B inch and B.'CB inch diameter. The integral orifice finds considerable use in laboratory and pitot plants. (alculation of flow rate Q +c H Js C,i +a +*
@p @e =
h,
9$. Ep!ai the ,orFi2 o% a tar2et *eter.
The target meter combines in a single unit both a primary element and a force balance flow rate transmitter. 1 circular disc /or target0 supported concentrically in the pipe carrying the flowing fluid results in an annular orifice configuration. ressure difference developed by the fluid flow through this annular orifice produces a force on target proportional to the sAuare of the flow rate. This force is carried out of the pipe through a rod passing through a diaphragm seal, and is measured by a pneumatic or electronic force balance system identical with the mechanism of the force balance *.. cell. The advantages of the target meter lies primarily in its single unit construction the primary device and responsive mechanism in a single structure. This eliminates the diff. pressure fluid connections in most heads meters. This is particularly used for sticky and dirty material which may plug up differential connections and for liAuids which reAuire elevated temperatures to avoid solidification, this elimination of liAuid connection is useful. =m ' !2 (st !a !m !c rf 90. Where is a <)a(rat ori%ice )se( #
If the fluid is viscous and the operating 5eynolds number is low Auadrant orifice is preferred 9. What are t/pes o% taps )se( %or ori%ices # 1. +!a2e taps:
This are most commonly used on pipe si6es of ' inches or larger. They are located in the orifice flange ' inch from upstream and ; inch downstream from the faces B orifice plate. $. Corer taps:
&n pipe si6es less than ' inches corner taps located directly at the face of the orifice plate. 0. 'ea cotracta a( ra(i)s taps:
7ena contracta taps located at ; pipe diameter upstream and at point of minimum pressure downstream. There are mostly widely used for measurement of steam. 5adius taps are located ; pipe diameter upstream and [ pipe diameter downstream for the inlet face of the orifice are a close approximation to vena contracta taps up to B.D' d *. . +)!! %!o, taps:
!ace flow taps are located at '[ pipe diameter upstream and E pipe diameter downstream. !ull flow taps at '[ and E pipe diameter have the same advantage as vena contracta or radius taps. 93. What is Re/o!(s )*er #
age 'G of -;
'89:89:8;.doc
*ynamic similarity implies a correspondence of fluid forces in two systems. In general situation there are many classes of forces that influence the behavior of fluids. ?ome of these are inertial viscous, gravitational, compressibility, pressure and elastic forces. (ertain dimensionless ratio are developed based on fluid properties. 7elocities and dimension, which are essentially force ratio. The more important of these are 5eynolds number vD RH
' H ve!ocit/ D H isi(e (ia*eter o% pipe H %!)i( (esit/ H viscosit/
!or most applications in practical flow measurement the 5eynolds number is taken to be sufficient criterion of dynamic similarly. The magnitude of 5eynolds number not only indicates whether the flow is laminar or turbulent but also furnishes the probable shape of velocity profile. *ue to the strong role it plays as an indicator of varying flow characteristics, many of the deviation from the theoretical eAuations are called 5eynaldo number effects. 95. 4o, ,o)!( /o) choose (i%%eretia! ra2e #
The most common diff. range for liAuid measurement is B+;BB" ) '&. This range is high enough to minimi6e the errors caused by uneAual heads in the seal chambers, differences in temps. of load lines etc. The ;BB" range permits an increase in capacity up to 9BB" and a decrease down up to 'B" by merely changing range tubes or range adustments. 96. What is positive Disp!ace*et *eters # pricip!e: The principle of measurement is that
as the liAuid flows through the meter it moves a measuring element which seals off the measuring chamber into a series of measuring compartments each holding a definite volume. 1s the measuring element *oves= these compartments are successively filled and emptied. Thus for each complete of the measuring element a fixed Auantity of liAuid is permitted to pass from the inlet to the outlet of the meter. The seal between measuring element and the measuring chamber is provided by a film of measured liAuid. The number of cycle of the measuring element is indicated by means of a pointer moving over the dial, a digital totali6er or some other form of register, driven from the measuring element through an adustable gearing. The *ost co**o %or*s o% positive (isp!ace*et *eters are :
;. '. -. 9. C. 8. D.
5eciprocating iston type. 5otating or &scillating iston type. utating *isc type. !luted ?piral 5otor type. ?liding vane type. 5otating vane type. &val Kear type.
99. Wh/ are t,o p!)2s provi(e( o a D.- tras*itter#
;.The top plug is a vent plug for venting the air entrapped inside the cell. '.The bottom plug is a drain plug for draining the liAuid accumulated inside the cell.
age -B of -;
'89:89:8;.doc
CONTRO 'A'ES 9. What is a cotro! va!ves #
1 control valve is the final control element, which directly changes the valve of the manipulated variable by changing the rate of flow of control agent. 1 control valve consists of an operator and valve body. The operator provides the power to vary the position of the valve plug inside the body. The plug is connected to the operator by a stem, which slides through a stuffing box. The air signal from the controller is applied above the diaphragm. The increasing air signal from the controller is applied above the diaphragm. 1n increasing air signal will push the operator stem downwards against the force exerted by the spring on the diaphragm plate. The valve is adusted in such a way that the plug starts moving when - psi is applied to the diaphragm and touches the seat when ;C psi is applied to the diaphragm. Thus an increase in air pressure will close the valve. )ence the home "1ir to (lose". 1nother type is "1ir to open", such that - psi on the diaphragm the value is closed and ;C psi air signal it in fully open. ;. What are the (i%%eret t/pes o% cotro! va!ves #
The commonly used control valves can be divided as follows. ;. *epending on 1ction. '. *epending on the Eody. 1. Depe(i2 o actio:
*epending on action there are two types of control valves, /;0 1ir to close, /'0 1ir to open. $. Depe(i2 o o(/:
;. Klobe valves single or double seated. '. 1ngle valves. -. Eutterfly valves. 9. Three way valves. 1. What is the )se o% si2!e seate( va!ve #
The single seated valve is used on smaller si6es, and in valve of larger si6es, where an absolute shut off is reAuired. The use of single seated valve is limited by pressure drop across the valve in the closed or almost closed position. $. What is the )se o% (o)!e seate( va!ve #
In double seated valves the upward and downward forces on the plug due to reduction of fluid pressure are nearly eAuali6ed. It is generally used on bigger si6e valves and high pressure systems. 1ctuator forces reAuired are less i.e. 1 small si6e actuator. 0. What is Cv o% a va!ve #
(v is the capacity of a valve and is defined as "o of gallons per minute of water which passes through a fully open valve at a pressure drop of ; psi. (7 2 A / \ K 0 =here (v 2 7alve co+efficient A 2 7olumetric flow rate / gallons minute 0 age -; of -;
'89:89:8;.doc
\ 2 ressure drop across the valve in psi. K 2 ?pecific gravity of flowing fluid. The valve coefficient (v is proportional to the area H1H between the plug and valve seat measured perpendicularly to the direction of flow. . What are the (i%%eret t/pes o% act)ators #
The different types of actuators are ;. *iaphragm &perated. '. iston &perated.
3. What t/pes o% oets ,o)!( /o) )se o% hi2h te*p. a( ver/ !o, te*p. #
age -' of -;
'89:89:8;.doc
4i2h te*perat)re:
getting damaged.
Eonnets are provided with radiation fins to prevent glad packing from
O ver/ !o, te*perat)re:
getting free6ed.
@xtended bonnets are used to prevent gland packing from
5. 4o, ,i!! /o) ,orF o a cotro! va!ve ,hi!e it is !ie #
=hile the control valve is in line or in service, it has to be by passed and secondly the line to be depressuri6ed and drained. 6. What is the )se o% a va!ve positioer #
The valve positioner is used for following reasons ;. $uick 1ction control valve. '. 7alve hysteresis. -. 7alves used on viscous liAuids. 9. ?plit 5ange. C. #ine pressure changes on valve. 8. 7alve Eench set not standard. D. 5eversing valve operation. 9. Whe ca a / pass e ot )se( o a positioer #
1 by pass on a positioner cannot be used when ;. ?plit 5ange operation. '. 5everse 1cting ositioner. -. 7alve bench set not standard. . What is the )se o% )tter%!/ va!ves #
age -- of -;
'89:89:8;.doc
Eutterfly valves are used only in systems where a small pressure drop across the valve is allowed. The butterfly is fully open when the disc rotates by GB. 1 (ra,acF o% this valve is that even a very small angular displacement produces a big change in flow. 1;;. What is the )se o% three ,a/ va!ves #
Three way control valves are only used on special systems, where a dividing or mixture of flows according to a controlled ratio is reAuired. 1;1. What are the (i%%eret t/pes o% p!)2s #
The different types of plugs are generally used are / ; 0 7. port plug / ' 0 (ontoured plug 'Bport p!)2:
orted plug are generally used on double seated valves. This is because ported plugs, have a more constant off balance area. Coto)re( p!)2:
(ontoured plugs are generally used on single seated valve with small trim si6es. 1;$. What is a ca2e va!ve #
1 cage valve uses a piston with piston ring seal attached to the single seated valve "plug". )ere the hydrostatic forces acting on the top or the piston or below the valve plug tend to cancel out. The seat ring is clamped in by a cage. (age valves are generally used for noise reduction. 1;0. What are the a(vata2es o% Ca*%!e va!ves #
(amflex valves are intermediates between globe valve and butterfly valve. The plug rotates 8BH for full opening. A(vata2es:
;. 1ctuator forces reAuired are very less. '.@xtended bonnet and hence can be used on any service i.e. on high temp. and very low Temperature. -. 7ariations in flow. 9. #ight weight. 1;. What is the )se o% !iF coecte( to the va!ve positioer #
The link serves as the feed back to the value. 1nt valve movement is sensed by this link. ?ometimes due to line pressure changes on ).. service the valve position may be changed, the link in turn senses this change and the positioner will produce an output which will operate the valve to the original position. 1;3. What is the )se o% ooster re!a/s #
Eooster relays are essentially air load, self contained pressure regulators. They are classified into three broad groups 1. 'o!)*e 7oosters : These are used to multiply the available volume of air signal. $. Ratio Re!a/s : Fse to multiply or divide the pressure of an input signal. 0. Reversi2 Re!a/s : This produces a decreasing output signal for an increasing input signal. 1;5. What is the )se o% A2!e va!ves #
age -9 of -;
'89:89:8;.doc
1ngle valves are used where very high pressure drops are reAuired and under very severe conditions. =here the conventional type of valve would be damaged by erosion. 1;6. What are the (i%%eret va!ve characteristic #
The different types of valve characteristic are ;. #inear '. @Aual ercentage
-. $uick &pening.
1.iear: The valve opening to flow rate is a linear curve $.E<)a! perceta2e: !or eAual increments of valve opening it will give eAual increment in
flow rate range. 1t small opening the flow will also be small.
0.Q)icF opei2: 1t small opening the increments in flow rate is more. 1t higher opening the flow
rate becomes steady.
1;9. What is a so!eoi( va!ve # Where it is )se( #
age -C of -;
'89:89:8;.doc
1 solenoid is electrically operated valve. It consist of a solenoid / coil 0 in which a magnetic plunger moves which is connected to the plug and tends to open or close the value. There are two types of solenoid valves ;. ormally open '. ormally closed USE It is used for safety purpose. 1;. 4o, ,i!! /o) cha2e the va!ve characteristics ,ith positioer #
The positioner contains different types of came in it. selection of the proper cams in it.Ey selection of the proper cam the valve opening characteristics can be changed. 11;.4o, ,i!! /o) cha2e the actio o% a cotro! va!ve #
;. If the control valve is without bottom cap. The actual needs to be changed. '. If bottom cap is provided. a0 *isconnect the stem from the actuator stem. b0 ?eparate the body from the bonnet. c0 5emove the bottom cap and the plug from body. d0 *etach the plug from the stem by removing the pin. e0 !ix the stem at the other end of the plug and fix the pin back. f0 Turn the body upside down. (onnect it to the bonnet after inserting the plug and stem. g0 (onnect back the stem to the actuator stem. h0 !ix back the bottom cap. i0 (alibrate the valve. 111. 4o, ,i!! /o) se!ect the cotro! va!ve characteristics #
The graphic display of flow various lift shows then the *esired or inherent characteristic is changed by variations pressure drop. This occurs as the process changes from condition where most of pressure drop takes place at the control valve is a condition where most of the pressure drop is generally distributed through rest of the system. +!o, : This variation in where most of the total drop take place is one of the most important aspects is choosing the proper valve characteristics for give process. +!o, cotro! : ormally @Aual percentage valve is used. -ress)re Cotro! : ormally linear valve is used to maintain a constant pressure drop. Te*p. Cotro! : ormally eAual percentage valve is used. i<)i( eve! Cotro! : ormally linear valve is used. Easically in selecting a valve characteristic two important point have to be taken into account. a0 There should be a linear relationship between the position of the plug and the flow through the valve in a wide range of change in the pressure drop across the valve. b0 The pressure drop across a valve should be as low as possible.
Cotro! va!ve siGi2 11$. What is the e%%ect o% pipe re()cers o va!ve capacit/#
age -8 of -;
'89:89:8;.doc
=hen control valves are mounted between pipe reducers, there is a decrease in the actual valve capacity. The reducers create an additional pressure drop in the system by acting as contractions of enlargements in series with the valve. 3etric formula for inlet and outlet reduces. $ R H
1 1.3
($
Cv$
D$
;.;($
1B
!or outlet reducer only or inlet reducer with entrance angle less than 9B inches. $ R $
H
1 1.3
($
Cv$
D$
;.;( $
1B
( H va!ve siGe **. D H !ie siGe **. Cv H re<)ire( va!ve coBe%%iciet.
To compensate for reducer losses at sub+critical flow, divide (v calculated by 5. 110. A operator te!!s /o) that a cotro! va!ve i a st)cF # 4o, ,i!! /o) start checFi2 #
;. '. -. 9.
!irst of all get the control valve is passed from operation. (heck the lingual to the diaphragm of the control valve. *isconnect it possible the actuator stem from the control valve stem. ?troke the actuator and see whether the actuator operates or not. It not then the diaphragm may be punctured. C. If the actuator operates connect it back to the plug stem stroke the control valve. If it does not operate loosen the gland nuts a bit and see if it operates. If it does not then the control valve has to be removed from the line to wshop. 11. Where is a Air to c!ose a( Air to ope cotro! va!ves )se( # Air to c!ose:
;. 5eflux lines. '. (ooling water lines. -. ?afety 5elief services. Air to ope:
;. !eed lines. '. ?team ?ervice. 113. Wh/ (oes cotro! va!ve operate at IS psi #
&n higher pressure the actuator si6es becomes bigger in area. The actual force produced by the actuator. !orce 2 ressure x 1rea. 2 ;C psi x 1rea, If 1rea 2 ;C" !orce produced 2 ;C psi x 'C in' 2 -DC pounds. 1ctual force acting on a control valve 2 -DC pounds.
age -D of -;
'89:89:8;.doc
@ENERA QUESTIONS 115. Ep!ai Casca(e Cotro! s/ste* ,ith a (ia2ra*. What ,o)!( happier i% a si2!e cotro!!er ,ere )se( #
TT
TIC Master cotro!!er
S!ave or seco(ar/ Cotro!!er
Stea*
-T +ee( ,ater +)e! 2as -'
(ascade means two controllers is series. &ne of them is the 3aster or rimary and the second is the secondary of slave controller. The output of the secondary controller operates the final control element, that is the valve. oop ep!aatio:
The output of the temp. transmitter goes as measurement signal to the TI( which is the master controller. ?imilarly the output of pressure transmitter goes as measurement signal to the I( which is the secondary controller. The output of TI( comes at set point to I( which is turn operates the valve. The reAd. temp. is set on the TI(. Use o% casca(e s/ste*:
(ascade loops are invariably installed to prevent outside disturbances from entering the process. The conventional single controller as shown in the diagram cannot responds to a change in the fuel gas pressure until its effect is felt by the process temp. sensor. In other words an error in the detected temperature has to develop before corrective action can be taken. The cascade loop in contrast responds immediately correcting for the effect of pressure change, before it could influence the process temperature. The improvement in control Auality due to cascading is a function of relative speeds and time lags. 1 slow primary /3aster0 variable and a secondary /?lave0 variable which responds Auickly to disturbances represent a desirable combination for this type of control. If the slave can respond Auickly to fast disturbances then these will not be allowed to enter the process and thereby will not upset the control of primary /master0 variable. It can be said that use of cascade control on heat transfer eAuipment contributes to fast recovery from load changes or other disturbances. 116. Ep!ai ratio cotro! s/ste*.
age -: of -;
'89:89:8;.doc
U cotro!!e( %!o,
/ 10
+T
Ratio cotro!!er +1 +$ Seco(ar/ cotro!!er
/ E 0
+T
Cotro!!e( %!o,
+'
1 ratio control system is characteri6ed by the fact that variations in the secondary variable do not reflect back on the primary variable. In the above diagram B a ratio control system the secondary flow is hold in some proportion to a primary uncontrollable flow. If we assume that the output of primary transmitter is 1, and the output of the secondary transmitter is E, 1nd that multiplication factor of the ratio relay is 4, then for eAuilibrium conditions which means set valve is eAual to measured valve, we find the following relation 41 + E 2 B or E1 2 4, where H4H is the ratio setting of the relay. 119. Ep!ai %)e! to air ratio cotro! o% %)races. Air +RC
Master stea* +RC
+T
+C
+RC RS-
pri*ar/
seco(ar/ +'
RR
+T
Ratio cotro!!er +)e! 2as 11. What is +)race Dra%t cotro! #
Ealanced draft boilers are generally used negative furnace pressure. =hen both forced draft and induced draft are used together, at some point in the system the pressure will be the same as that of atmosphere. Therefore the furnace pressure must be negative to prevent hot gas leakage. @xcessive vacuum in the furnace however produces heat losses through air infiltration. The most desirable condition is that the one have is a very slight /about B.;" )'B 0 negative pressure of the top of furnace. 1$;. What is %ee( acF cotro! # What is %ee( %or,ar( cotro! # Disc)ss its app!icatio # +ee( acF cotro!:
age -G of -;
'89:89:8;.doc
Cotro!!er>+IC? Meas)ri2 e!e*et Tras*itter
Set poit
-ROCESS
Cotro! va!ve
Ori%ice p!ate
!eed back control involves the detection of the controlled variable and counteracting of charges its it]s value relative to set point, by adustment of a manipulated variable. This mode of control necessities that the disturbance variable must affect the controlled variable itself before correction can take place. )ence the term HfeedbackH can imply a correction HbackH in terms of time, a correction that should have taken place earlier when the disturbance occurred. +ee( %or,ar( cotro! : Ori%ice -ROCESS
O)tp)t +T
Cotro!!er
A((itive re2)!ator
!eed forward control system is a system in which corrective action is based on measurement of disturbances inputs into the process. This mode of control responds to a disturbance such that is instantly compensates for that error which the disturbance would have otherwise caused in the controlled variable letter in time. !eed forward control relies on a prediction. 1s 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 gives output to the control valve. The control valve regulates the flow.
+ee( acF cotro! :
age 9B of -;
'89:89:8;.doc
In feed forward control no difference between the desired result and actual result need exist before corrective action is taken in feed back control a difference must exist. )ence, open loop or feed forward control is capable of perfect control, but feed back 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. Ep!ai three e!e*et %ee( ,ater cotro! s/ste*# +ee( ,ater cotro! Dr)* !eve! cotro! Stea* cotro! -$ -1 S<)are root Etractor
- S<)are root etractor -0
Co*p)ti2 e<)atio: -0 H R > -$ B -1 B J ? - J ; -0 H O)tp)t. -1= -$ & - H Ip)t J 1 H A()sta!e s)ppressio. J ; H A(.7ias. 1$$. Ep!ai AtiBs)r2e cotro!# Co*pressor 3
-DT
age 9; of -;
'89:89:8;.doc
-i!et
- o)t!et
Ratio +T
+ +RC > +!o, coverter? 7/B-ass va!ve
This method of surge control uses the ratio of compressor pressure rise to inlet flow rate to set the flow in by+bass loop. =hen the suction pressure drops and discharge shoots up, the compressor starts surging. The pdt senses this and gives the signal to the !5( which will open the by+pass valve.
@ENERA QUESTION B II Q)e. : Dra, a e!ectroic t,o ,ire s/ste* cotro! !oop. As. : Q)e. : What are Itrisica!!/ sa%e s/ste* # As. : Intrinsic safety is a techniAue for designing
electrical eAuipment for safe use in locations made ha6ardous by the presence of flammable gas or vapors in air. "*efn. " Intrinsically safe circuit is one in which any spark or thermal effect produce either normally or under specified fault conditions is incapable of causing ignition of a specified gas or vapor in air mixture at the most easily ignited concentration. 4AARDOUS AREAS :
The specification of products or systems sold as intrinsically safe must state in what ha6ardous areas they are infect intrinsically safe. Fniversal cooling of ha6ardous areas has age 9' of -;
'89:89:8;.doc
not, unfortunately, been adopted in all countries. )owever two sets of codes in common use are. Q)e. : What (oes a tras*itter o)tp)t start %ro* 0B13 psi or >;.$ B 1 J2C*$? or B $; *a. etc. # As. : The transmitter output stance from what is known as "live 6ero". This system has
specific advantages ;. The systems automatically alarms when the signal system becomes inoperative. '. The output areas is linear / 5atio of ; C 0. DEAD ERO SI@NA :
The advantage is that it does not have to be biased to true 6ero. 1 "#ive 6ero" gives the computer additional information, so that it can takes appropriate alarm action in case of a measurement failure, because it can discriminate between a transmitter operating, but transmitting a 6ero measurement and a failure, in the signal system. Q)e. : What is %orce a!ace a( *otios a!ace pricip!e # As. : +ORCE 7AANCE -RINCI-E :
"1 controller which generates and output signal by opposing torAue]s". The input force is applied on the input bellows which novas the beam. This crackles no66le back pressure. The no66le back pressure is sensed by the balancing bellows which brings the beam to balance. The baffle movement is very less about B.BB'" for full scale output.
MOTION 7AANCE -RINCI-E :
"1 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 back pressure will increase. This increase in back pressure acting on the balancing bellows, will expands the bellows, there by moving the no66le upward. The no66le will move untill motion /almost0 eAuals the input /baffle0 motion. A(vata2es o% %orce 7a!ace :
;. 3oving parts are fever. '. Eaffle movement is negligible. -. !rictional losses are less.
AUTOMATIC CONTROER :
It is a device which measured the value of variable Auantity or condition and operates to correct or lie it deviation of this measured value from a selected reference. AUTOMATIC CONTRO SSTEM :
It is any operable arrangement of one or more automatic controllers in closed loops with one or more processes. SE+ O-ERATED CONTROER :
age 9- of -;
'89:89:8;.doc
It is one in which all the energy needed to operate the final control element is derived from the controlled medium through the primary element. REA O-ERATED CONTROER :
It is one in which the energy transmitted through the primary element is either supplemented or amplified for operating the final control element by employing energy from another sources. -ROCESS :
1 process comprises the collective function performed in and by the eAuipment in which a variable is to be controlled. SE+ RE@UATION :
It is an inherent characteristic of the process which aids in limiting the deviation of the controlled variable. CONTROED 'ARIA7E :
The controlled variable is that Auantity and condition which is measured and controlled. CONTROED MIDIUM :
It is that process energy or material in which a variable is controlled. The controlled variable is a condition or characteristic of the controlled medium. !or e.g. where temperature of water in a tank is automatically controlled, the controlled variable is temperature and controlled medium is water. MANI-UATED 'ARIA7E :
It is that Auantity or condition which is varied by the automatic controller so as to affect the value of the controlled variable. CONTRO A@ENT :
It is that process energy or material of which the manipulated variation is a condition or characteristic. The manipulated variable is a condition or characteristic of the control agent. !or e.g. when a final control element changes the fuel gas flow to burner the manipulated variable is flow the control agent is fuel gas. ACTUATIN@ SI@NA :
The actuating signal is the difference at anytime between the reference input and a signal related to the controlled variable. This basically known as error signal. DE'IATION :It
is the difference between the actual value of the controlled variable and the value of the controlled variable corresponding with set point. O++SET :
It is the steady state difference between the control point and the value of the controlled variable corresponding with set point CORRECTI'E ACTION :
age 99 of -;
'89:89:8;.doc
It is the variation of the manipulated variable produced by the controlling means. The controlling means operates the final control element / control value 0 which in turn varies the manipulated variable. RE+ERENCE IN-UT :
It is the reference signal in an automatic controller. SET -OINT :
It is the position to which the control point setting mechanism is set. CONTRO -OINT :
It is the value of the controlled variable which under any fixed set of conditions the automatic controller operates to maintain. D E + I N A T I O N. ACCURAC :
1 number or Auantity which defines the limit of error under reference conditions.
ATTENUATION :
1 decrease in signal magnitude between two points, or between two freAuencies.
DEAD TIME :
The interval of time between initiation of an impact change or stimulus and the start of the resulting response. DRI+T :
1s undesired change in output over a period of time, which change is unrelated to input, operating conditions, 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 of specified span.
age 9C of -;
'89:89:8;.doc
ERO ERROR :
It is the error of device operating under the specified conditions of use when the input is at the lower range value. STATIC @AIN :
It is the ratio of the output change to an input been change after the steady state has been reached. 4STERESIS :
The maximum difference between the upscale and downscale indications of the measured signal during a full range traverse for the same input. INTER+ERENCE :
Interference is any spurious voltage or current arising from external sources and appearing in the circuits of a device. COMMON MODE INTER+ERENCE :
It is the form of interference which appears between the measuring circuit terminals and ground. NORMA MODE INTER+ERENCE :
It is the form of interference which appears between measuring circuit terminals. INEARIT :
The closeness to which a curve approximate a straight line. RAN@E :
The region between the limits within which a Auantity is measured received or transmitted, expressed by stating the lower and upper range values. RE-EATA7IIT :
The closeness of agreement among a number of consecutive measurements of the output for the same value of the measured signal under the same operating conditions. RE-RODUCI7IIT :
The closeness of agreement among repeated measurements of the output for the same value of the input made under the same operating conditions.
age 98 of -;
'89:89:8;.doc
RES-ONSE :
It is the general behavior of the output of a device as a function of input both with respect to time. SI@NA TO NOISE RATIO :
5atio of signal amplitude to noise. TIME CONSTANT :
The time reAuired for the output to complete 8-.' % of the total rise or decay. S-AN :
The algebraic difference between upper and lower range values. ERO S4I+T :
1ny parallel shift of the input output curve. - R E S S U R E. -RESSURE CON'ERSIONS : 1psi H 1 J2c*$ H 1 7ar H 1 Jpa H 1 J2c*$ H 1 7ar H 1 J2c*$ H 1 Torr H
$6.6 V 4$O 1.$$0 psi
1.3; psi ;.13 psi 1;.;;;** o% 4$; 1.;16 J2c*$ ;.9 7ar
1 ** o% 42.
Q)e. : Ep!ai the ,orFi2 o% a Era% !eve! 2a)2e # As. : The @nraf level precise level gauges are based
on servo powered null balance techniAue. 1 displacer serves as continuos level sensing element. age 9D of -;
'89:89:8;.doc
-ricip!e :
1 displacer 1 with a relative density higher than that of a product to be measured, is suspended from a stainless steal wire E tat is attached to a measuring drum. 1 two phase servo motor controlled by a capacitive balance system winds unwinds the measuring wire until the tension on the weight springs is in balance with the wt. of the displace part immersed in the liAuid. The sensing system in principle measures the two capacitance formed by the moving central sensing rod @ provided with two capacitor plates and the si plates. In balance position the capacitance are of eAuip value. 1 level variation will cause a difference in buoyancy of the displacer. The center sensing rod will move in to direction of one of the side capacitor plates. This causes difference in value of this capacitance. Ey an electrolyte rotation of the servo motors the can driven transmitter continuously change the voltage pattern to remote indicate of which the receiver motor drives a counter indicating low variation.
-RIMAR +EED7ACJ :
It is the signal which is related to the bWtrW)^_-`d^j)
age 9: of -;
'89:89:8;.doc
3ith the reference input to obtain the actuating signal. ?imply stated primary feedback is the actual measurement of the controlled variable which when compared with the desired measurement of the controlled variable produces the actuating signal. -OSITIONIN@ ACTION :
It is that in which there is a predetermined relation between the value of the controlled variable and the position of the final control element. -RO-ORTIONA ACTION :
It is that in which there is a continuous linear relationship between the value of the actual measurement of the controlled variable and the value position. +OATIN@ ACTION :
It is that in which there is a predetermined relation between the deviation and speed of final control element. DERI'ATI'E ACTION :
It is that in which there is a predetermined relation between a time derivative of the controlled variable and position of final control element. REST ACTION :
It is the value movement at a speed proportional to the magnitude of deviation. RATE ACTION :
It is that in which there is a continuos linear relation between the rate of change of controlled variable and position of final control element. 5ate action produces value motion proportional to the rate of change of actual measurement. -RO-ORTIONA 7AND :
It is the range of values of the controlled variable which correspond to the full operating range of the final control element. RESET RATE :
It is the number of timesminute that the effect of proportional position action upon the final control element is repeated by proportional speed floating action. There are two ways of expressing reset action ;. 5eset time and '. 5eset 5ate It is commonly expressed as a number of "repeats" per minute. It is determined by dividing. a0 Travel of final control element / 7alue stroke 0 in one minute as a result of the effect of proportional speed floating action. b0 The travel as a result of the effect of proportional position action with the same deviation in both cases. 1. Reset Rate :
It is the time interval by which the rate is commonly expressed in minutes. It is determined by subtracting. $. Reset Ti*e :
age 9G of -;
'89:89:8;.doc
a0 The time reAuired for a selected motion of the final control element resulting from combined effect of the proportional position plus rate action. b0 The time reAuired for the same motion as a result of the effect of proportional position action alone with the same rate of change of controlled variable in both cases or expressed in another way. It is the time lead in terms of air pressure on the control value produced by rate action compared with proportional position action for the same rate of change of actual measurement in both cases.
EECTRONICS Q)e. : What is a (io(e# As. : 1 diode consists of two
electrodes /;0 1node /'0 (athode. The current flow is only
in one direction. 1 diode is the most basic solid state /semi conductor0 device. The above figure shows a .. unction. The . material has holes and the . material has electrons. +ORWARD 7IAS : RE'ERSE 7IAS :
/ ; 0 =here the applied voltage overcomes the barrier potential /the p side is more positive than the n side0 the current produce is large because maority carriers cross the unction in large numbers. This condition is called forward bias. / ' 0 =hen the applied voltage aids the barrier potential /n side M ve than p side0 the current in small. This state is known as 5everse Eias. Q)e. : What is a ha!% ,ave= %)!! ,ave a( ri(2e recti%ier # As. : 4A+ WA'E RECTI+IER : +U WA'E RECTI+IER : 7RID@E RECTI+IER : -EAJ IN'ERSE 'OTA@E :
cycle.
3aximum 5everse voltage across the diode during the
Q)e. : What is a %i!ter )se( %or # As. : The half wave and full wave signals are pulsating * . ( . voltages. The use of such
voltages is limited to charging batteries, running *. (. motors, and a few other applications. =hat we really have is a *.(. voltage that is constant in valve, similar to the voltage from a battery. To get a constant voltage from this, we can use a capacitor input filter.
age CB of -;
'89:89:8;.doc
Q)e. : What is eer Dio(e # What is a vo!ta2e Re2)!ator # As. : The breakdown region of a p n diode can be made very
sharp and almost vertical *iodes with almost vertical breakdown region are known as Lever *iodes. 1 Lener diodes operating in the breakdown region is eAuivalent to a battery. Eecause of this current through Lener diode can change but the voltage remains constant. It is this constant voltage that has made the 6ever diode an important device in voltage regulation. 'OTA@E RE@UATOR :
The output remains constant despite changes in input voltage due to Lever effect. Q)e. : What is trasistor # What are the (i%%eret t/pes #
1ns. 1 transistor is a three lagged semi conductor device. Easically a transistor means /transfer + resister0. =hether the transistor is pnp or npn it resembles two diodes /back to back0. The one of the left is called emitter diodes, and the one on the right is the collector diode. ?ince two types of charges are involved transistor are classified as bipolar devices. 7iasi2 the trasistor :
@mitter+ Ease + !orward Eias. (ollector + Ease + 5everse Eias. Q)e. : What is C7= CE a( CC co%i2)ratio #
1ns. Euffer 1 device or a circuit used to isolate two ptHne circuits or stages. The emitter follows is a example of buffer. 1lpha 2 I( I@ Eeta 2 I( IE Q)e. : 4o, ,i!! /o) test a trasistor ,ith a *)!ti*eter. #
1ns. ;. @mitter Mve of meter and Ease +ve output 2 #ow resistance '. @mitter +ve of meter and base Mve output 2 )igh resistance. -. (ollector Mve and Ease +ve output 2 #ow. 9. (ollector +ve and base Mve output 2 )igh. @mitter (ollector 2 )igh 5esistance. -N- : &pposite 5esults. Q)e. : What is a th/ristor # What are its )ses #
1ns. 1 thyristor is a special kind of semi conductor device that uses internal feedback to produce latching action. Use : Fsed for controlling large amounts of load power in motors, heaters, lighting systems etc. Ep!aatio : Eecause of the unusual connection we have a Mve feedback also called regeneration. 1 change in current at any point in the loop is amplified and returned to the starting point with the same phase. !or instance if the B' base current increases, the B' collector current increases. This force base current through B;. In turn this produces a large B; collector current which drives the B' base harder. This build up in currents will age C; of -;
'89:89:8;.doc
continue until both transistors are driven in saturation. In this case the latch acts like a closed switch. &n the other hand , if something causes the B' base current to decrease, the B' collector current will decrease. This reduces the B; base current. In turn, there is less B; collector current, which reduces the B; base current even more. This regeneration continues until both transistors are driven into cut off. 1t this time the latches like a open switch. This latch will always stay in open or close position. Q)e. : What are !o2ic 2ates # Ep!ai ,ith tr)th ta!e.
1ns. K1T@ 1 gate is a logic circuit with one output and one or more inputs . 1n output signal occurs only for control combination of input signals.
1. OR B @ATE : SM7O : TRUT4 1 TA7E :
B B ; ;
E
P
B ; B ;
B B B ;
De%ie : 1n &5 Kate has one output if any or all of its input are ;Hs. $. AND @ATE : De%ie : 1n 1* Kate has output when all inputs are present. SM7O : TRUT4 1 TA7E :
B B ; ;
E
P
B ; B ;
B B B ;
0. NOT @ATE :
1 not gate is also known as an inverter. This circuit has one input and one out put . 1ll it does is invert the input signalZ if the input is high, the output is low and vice versa. TRUT4 TA7E :
Inpu t B
&utpu t ; age C' of -;
'89:89:8;.doc