Principles of Instrumentation & Control

8/18/2019 Principles of Instrumentation & Control

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Process Control Concepts

Process Control is to maintain certain processvariables (PV) of a process at a esire level (!etValues)

Instruments are use to obtain information aboutt"e process variables (PVs)

Controller ( a soft#are pro$ram resient in %C!)calculates t"e ifference bet#een !V an PV anaccorin$l sens a si$nal' calle ManipulateValue (MV) to final control elements (Valves'Vanes etc)


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Pressure Measurement

bsolute Pressure* Measure over totalvacuum./ero absolute

tmosp"eric.Barometric Pressure* Pressuree+erte b Eart"0s atmosp"ere

%ifferential Pressure* %ifference bet#een t#o pressures 1au$e Pressure* Pressure over t"e atmosp"eric

Pressure 2rostatic Pressure* Pressure e+erte b a li3ui

column


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4ine Pressure.5orkin$ Pressure.!tatic Pressure*

Pressure e+erte b a flo#in$ flui on t"e pipe

surfaces %namic Pressure* Pressure e+erte b a flo#in$

flui parallel to t"e flo# irection

Vacuum* Pressure belo# t"e atmosp"eric pressure

Compoun Pressure* Measure from a reference

#"ic" is neit"er atmosp"eric nor total vacuum


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Mission !tatement

clear statement of our compan0s lon$6

term mission ,r to use #ors t"at #ill

"elp irect t"e $ro#t" of our compan' but be as concise as possible


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%evices Pressure is a force applie.istribute over a

surface

Pressure measurement is use for pressure itself'flo#' level an even temperature

ll measurement sstems consists of *

7 Primar Element #"ic" inirectl. inirectl in contact

#it" flui an interacts #it" pressure c"an$es

7 !econar Element #"ic" translates t"is interaction

into values for use in inication' recorin$ an control


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Pressure Measurement %evices 7

!econar Elements

Mec"anical ,ransucers

7 Bello#s

7 Bouron ,ubes

7 %iap"rams

7 -orce Balanc


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Pressure Measurement %evices 7

!econar Elements

Electronic ,ransucers

7 Variable Capacitance

7 Variable Reluctance

7 Vibratin$ 5ire

7 Variable Resistance


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Mec"anical ,ransucers


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4evel Measurement

4evel measurement is ivie into t#o $roups*%irect 4evel an Pressure6operate

%irect 4evel Measurement* 7 2ook tpe

7 !i$"t 1lass

7 -loat 1au$in$

Pressure6Operate Measurement* 8se is mae of

t"e fact t"at pressure oes not epen on t"ecross6sectional area of a vessel' in stea it epenson li3ui "ei$"t an ensit


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%irect 4evel 6 4evel 1au$es


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4evel 1au$es

%irectin$ 4evel reain$ evice

,pes * ,ransparent an Reflu+

,ransparent tpe is use for*

7 Caustic' ciic' 2eav 2C services' 2i$" pressure

7 Coate #it" protective Mica

Reflu+ tpe is use for li$"t services*

7 9on6corrosive' non6viscous' Clear 2C 7 It "as special optical propert for clear emarcation


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4evel 1au$es

5"ile commissionin$' open t"e :er$usonvalve partiall till t"e $a$e is pressure up .

#arme up5"ile in service' ;er$uson valve s"all be

full in open conition


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Close Vessels 7 %ifferential

Pressure pproac"

" = P .>$ In close vessles' pressure above t"e li3ui affects

t"e pressure measure at t"e bottom ,o measure t"e true level' t"e vessel pressure must

be subtracteMakin$ a pressure tap at t"e top of t"e vessel an

connectin$ t"is to 4P sie accomplis"es t"is

%r 4e$* If $as is non6conensable5et le$* Conense 4i3ui introuces error

2ence calibrate #it" a reference li3ui


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4evel Measurement in Close

Vessels


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,emperature Measurement

,emperature is e$ree of "otness.colness

on a efinite scale

oC = ?.@ (o- 7 A)Most reliable met"os of measurement*

7 Bimetallic

7 ,"ermocouples

7 R,%


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ll temperature $a$es uses bimetallic principles

Bimetallic Principles*

7 Metals c"an$e in volume in response to temperature 7 an ifferent metals respon ifferentl to c"an$e in

temperature

Its accurac is not as $oo as a $lass t"ermometer


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Bimetallic 1a$e

Bimetallic Principles*

7 Metals c"an$e in volume in response to temperature

7 an ifferent metals respon ifferentl to c"an$e in temperature

ll temperature $a$es uses bimetallic principles

Most reliable met"os of measurement*

7 Bimetallic

7 ,"ermocouples

7 R,%


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,"ermocouples

!ummari/e

competition

Outline ourcompan0s

competitive avanta$e


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,"ermocouples

It is a t"ermoelectric temperature measurin$evice

It is forme b pressin$ t#o issimilar metals in

series5"en t"e ;unctions are at ifferent tempertures'

emf is $enerateMeasurin$ ;unction."ot ;unction is inserte into

t"e meium of #"ic" temperature is inserte Col :unction.Reference ;unction is inserte in t"e

measurin$ evice


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Resistance ,emperature

%etectors

8ses t"e c"an$e in resistance in response to

c"an$e in temperature

Resistance epen on metal si/e anresistivit of t"e metal


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-lo# Measurement

Orifice

7 Orifice ma be concentric or eccentric

7 Orifice plats constricts t"e flo# to prouce aifferential pressure

7 ,"e result is a "i$" pressure upstream an a

lo# pressure o#n stream

7 %p is proportional to t"e flo#

7 Orifice meters are ine+pensive


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-lo# Measurement

Venturi

7 4o# Pressure

7 2i$"l accurate

7 Bi$ !i/e Pipe


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nnubar

8ses Pitot ,ube

Principle

-lo# is proportionalto pressure rop

8ses multiple ports


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-lo# Measurement

rea Meters*

7 In "ea flo# meters' t"e restriction area kept

constant $eneratin$ a pressure ifferential 7 5"ere as in area meter' a variable area "ols

ifferential pressure constant

7 C"an$e in area is a measure of flo# rate

7 E+* Rotameter


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Rotameter6 !uitable for lo# flo#s


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Valves 7 Valve is a variable

Orifice

Valve ,pes*

7 Plu$6in6seat

7 Rotatable Plu$ 7 Rotatin$ Vane

7 Eccentric Rotation

7 Ca$e6$uie 7 %eformable -lo# Passa$e


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Ball Valve.Plu$ Valve

6 Rotatable Plu$

Plu$.ball rotate

,"rou$" a 3uarter

of a turn,"e ball "as a passa$e


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Butterfl valve. %ampers 7

Rotatin$ Vane


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Valve ccessories

ctuators* ,ranslates control si$nal intoaction' e$*Motors' %iap"rams' solenois'

cliners,ransucers an Positioners* Convert

electronic si$nals into pneumatic."raulic pressures t"at control actuators.valves

Positioner is use #"en more accurate positionin$ of valve steam is re3uire


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T%ere are t&o #asic "esi'ns o 'ate va!ves insi"e

scre& ste* an" otsi"e scre& ste*+Seatin' is perpen"ic!ar or at ri'%t an'!es to t%e !ine

o !o& , *eets it %ea" on+Repeate" *ove*ent o t%e &e"'e near t%e point o

c!osre n"er %i'% ve!ocit- !o& *a- create a "ra' ont%e seatin' sraces an" case 'a!!in' or scorin' on

t%e "o&nstrea* si"e+ A s!i'%t!- opene" &e"'e *a- a!so case tr#!ent

!o& &it% vi#ration an" c%atterin' o t%e &e"'e+


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Globe valve seating is parallel to the line offlow. A!! contact #et&een seat an" "isc en"s

&%en !o& #e'ins+ T%ese are a"vanta'es or

*ore eicient t%rott!in' o !o& &it% *ini**

&ire "ra&in' an" seat erosion+

.!o#e va!ves are 'enera!!- i"ea! or t%rott!in'

an" preera#!e or reent operation+


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Btter!- Va!ves are o t%e arter trn

a*i!- an" are so "esi'ne" #ecase a 01

"e'ree trn o t%e operator !!- opens or

c!oses t%e va!ve+

+ In so*e cases t%e- *a- #e se" ornon,critica! t%rott!in' app!ications+ T%e-

are !i'%ter in &ei'%t t%an conventiona!

va!ves+

Principles of Instrumentation & Control

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