Instrument Location Selection Standards

8/11/2019 Instrument Location Selection Standards

1/9

Need And Location Of

Measuring InstrumentsPrepared By FAIZA AHMED

Octagon Management & Engineering o!utions "OMEOL#

$$$%omeso!%com


2/9

F "F!o$ ontro!#

'uare root ca!cu!ation is re'uired to con(ert t)e measured (a!ue of differentia!

pressure across t)e orifice p!ate into f!o$ rate* +ecause t)e differentia! pressure at

orifice p!ate is proportiona! to t)e s'uare of f!o$ rate%

F, "F!o$ ontro! ,a!(e# is a!most a!$ays !ocated ad-acent at t)e do$nstream side of

orifice measurement point* +ecause an orifice p!ate for f!o$ measurement re'uires at

!east ./ D straig)t pipe !engt) in t)e upstream side to esta+!is) a sta+!e f!o$ pattern%

Hence* as a (a!(e $ou!d generate f!o$ tur+u!ence in t)e out!et side* a F, s)ou!d +e

!ocated at t)e do$nstream side of t)e orifice p!ate%

0)e contro! response of F is fast and noisy* and P1I contro! is usua!!y emp!oyed for

f!o$ contro! $it)out D1action%


3/9

F "F!o$ ontro!#

0$o F2s s)ou!d not +e p!aced a!ong t)e same stream !ine* +ecause t)ese 0$o

Fs conf!ict eac) ot)er and one of t$o F, $i!! +ecome fu!!y open%

In t)e same conte3t* at -unction points of t)ree or more stream !ines* a!!

stream !ines s)ou!d not +e F contro!!ed +ut one stream )as to +e !eft free

from F contro!% 0)e f!o$ rate of t)is stream s)ou!d +e esta+!is)ed +y f!o$

contro!!ers of ot)er f!o$ contro!!ers% ontro! !oop configurations must satisfy

t)e degree of freedom of process contro!%


4/9

L "Le(e! ontro!#

L is a 4ind of )o!d up contro!* and t)ere are t$o cases of L app!ication5 one

case is t)at L, "Le(e! ontro! ,a!(e# is !ocated at t)e upstream of a drum*

and anot)er case is t)at L, is !ocated at t)e do$n stream of a drum%

0)e se!ection of up1stream L, or do$n1stream L, depends on $)et)er

incoming f!o$ rate is set or outgoing f!o$ rate is set% 6)en in1coming f!o$

rate is set* L, )as to +e !ocated at t)e do$n1stream side* and con(erse!y

$)en outgoing f!o$ rate is set* L, )as to +e !ocated at up1stream side as

s)o$n in Fig


5/9

L "Le(e! ontro!#


6/9

L "Le(e! ontro!#

If up1stream f!o$ rate is set and L, is a!so !ocated at up1stream side* t)is

contro! configuration is un$or4a+!e +ecause F and L conf!ict eac) ot)er ass)o$n in Fig 7%.%8+ +e!o$ and +ot) F, and L, cannot +e p!aced a!ong t)e

same stream !ine% If t)is contro! configuration is emp!oyed* F, or L,

reac)es at a fu!! open position due to t)e (io!ation of t)e degree of contro!

freedom%


7/9

P "Pressure ontro!#

P is used for 4eeping t)e process condition at t)e re'uired pressure so t)at

product specifications +e met for reaction condition or separation condition%

P is a!so considered as a )o!d up contro! for gas p)ase simi!ar to L for

!i'uid% 0)ere are t$o cases of P app!ication5 one case is t)at P, "Pressure

ontro! ,a!(e# is !ocated at t)e upstream of t)e drum* or P, is !ocated at

t)e do$n stream of t)e drum% 0)e se!ection of up1stream P, or do$n1

stream P, depends on $)et)er incoming gas f!o$ rate is set or outgoing gas

f!o$ rate is set% 6)en t)e in1coming gas f!o$ rate is set* P, )as to +e

!ocated at do$n1 stream side* and con(erse!y $)en t)e outgoing gas f!o$ rate

is set* P, )as to +e !ocated at up1stream side% 0)is is t)e same argument as

descri+ed for !i'uid !e(e! contro!%


8/9

P "Pressure ontro!#

0)e pressure in a region enc!osed +y contro! (a!(es* pumps and compressors

is essentia!!y t)e same $it) some pressure drops a!ong t)e pipe !engt)%

0)erefore* t$o or more pressure contro!!ers s)ou!d not +e p!aced in t)e same

region* +ecause t)ese redundant pressure contro!!ers conf!ict eac) ot)er%


9/9

0 "0emperature ontro!#

0emperatures are important operation conditions for reaction and separation

processes% 9sua!!y dynamic response of temperature c)ange is s!o$ due to

!arge )eat capacity and : or s!o$ )eat transfer rate* and in order to ma4e t)e

c!osed response faster* P;I;D contro!!er are emp!oyed for temperaturecontro!%

Ho$e(er* temperature contro! for !ine mi3ing is rat)er fast* and p!ease note

t)at temperature sensor for mi3ing needs to +e insta!!ed at ./ D or more

distance from t)e mi3ing point to ensure comp!ete mi3ing%

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Instrument Location Selection Standards