FUNDAMENTAL PRINCIPLES OF PILOT-OPERATED REGULATORS

7/25/2019 FUNDAMENTAL PRINCIPLES OF PILOT-OPERATED REGULATORS

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F U N D A M E N T A L P R I N C IP L E S O F P I L O T O P E R A T E D R E G U L A T O R S

Class 6030

Steve. Berry

Fisher Contro ls In ternat ional , Inc.

Marsha l l town , Iowa 50158

A p ressure - reduc ing regu la to r must supp ly a quan ti ty

o f f low tha t m atches a downst ream, usage , de m an d.

I f the required f low rate is smal l , the regulator p lug or

d isk shou ld approach i ts sea t and res t ri c t the f low . I f

the demand increases, the p lug o r d isk shou ld move

away to crea te a la rger op en ing . Ideal ly , a regu la to r

shou ld p rov ide a constan t downst ream pressure

whi le de l iver ing the required f low. The refore,

regu la to rs au tomat ica l l y ad jus t the f low ra te to meet

th e d o wn s t r e a m d e m a n d .

For a l l pract ica l pu rpose s, regulators, used by the gas.

d istr ibut ion industry can. be. p laced, in e i ther o f tw o

categor ies:

I.. Sel f-O perate d or

I1. Pilot-Operated

This categor iz ing of a l l regulators (p lus a l l

const ruc t ion mod i f i ca t ions) tends to be an

overs imp l i f i ca t ion , , bu t excep t ions a re ra re . Le t 's

examine each o f them. c lose ly .

S e l f Op e ra t e d Re g u l a t o rs

An. example of a se l f-operated regulator is a spr ing-

opened va lve d i rec t ly opposed by a d iaphragm

assembly , see Figure 1. Ga s, . a t increasing

downst ream pressure , ac ts on the d iaphragm

assem bly to overco me the fo rce o f the . sp ring ,.

c losing, the va lve.. When downstream, pressure fa l ls,

spr ing force is. greater than the force of the gas

act ing on. the d iaphragm, and the va lve opens.

F

i l l q l

Figure 1

Self Operated Re gulato r Basic Design

Pressure regu la to r va lves have many va r ia t ions.

They can be. ba lanced or unbalanced, s ing le-por ted

or d ouble-po r ted.. But com mon, to. a ll . regulator va lves

is a contro l inaccurac y caused, by droop or o f fset . .

O f fse t e f fec t occurs as the demand fo r gas in the

downst ream system, increases. When the regu la to r

is requ i red to . pass more gas to the downst ream

system,, the regulator va lve, p lug m ust be. opened to

a l low more f low. . To a l low fo r th is open ing o f the

va lve , p lug , the dow nst ream pressure sensed by the

d iaphragm mus t decrease . By a l lowing the

downstream pressure, to . decrease, less force

opposes, the com pres sed spr ing an d the va lve, p lug

is pushed fa r the r open to meet the f low demand o f

the system. The ne t e f fec t i s a decrease o r o f fse t o f

downst ream con t ro l , p ressure f rom the des i red se t

p ressure .

The. service ty pe of reg ulator is a var ia t ion of the se l f-

opera ted regu la to r tha t deserves separa te ment ion ,

see Figure 2 . I t operates, the same as the simple

sel f-operated regulator except that a lever is used to

ga in m echan ica l adv an tage and . p rov ide a h igh

shutoff force.. In order to. ove rcom e droop effect,

boost ing, measures are employed, but these tend to .

ma ke the un i t unstable. .

F/gure 2

Self Operated Service Type Regulator

Mo st service, regulators a re a lso cursed wi th . an

unbalan ce prob lem. In fact, out le t pressure wi l l . vary

fo r every in le t p ressure . The on ly way to min imize

the. inherent drawbacks, o f service regulators is to

use. the sm a l les t p ract ica l o r i f i ce . One doesn ' t ge t

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more . fo r h i s mo ne y by spe c i f y in g a la rge or i f ice in . a

s e r v i c e r e g u l a t o r e x c e p t b y s a c r if i c in g s a f e t y ,

accuracy and s tab i l i t y . .

Sel f Operated Regulators Pressure Loaded

Ou r se arch leads , us to. a fo rm . o f the . se l f -op era te d

r e g u l a t o r t h a t is . p r e s s u r e - l o a d e d . A p r e s s u r e - l o a d e d

r e g u l a t o r , s e e

Figu re 3 .

opera tes , the same. as . the

spr ing loaded un i t except ra ther than. re l y ing upon.

the . fo rce , o f the . spr ing , to . pus h the . va lve ope n, an .

aux i l i a ry regu la to r i s used. to . rep lace , the mechan ica l .

spr ing w i th . an . a i r spr ing .

I

t

Figure 3

Se l f-Opera ted Press ure-Loade d Regu la to r w i th

A tmo sp h e r i c B le e d

T h i s a i r s p r i n g , t h e . o u t p u t p r e s s u r e o f t h e a u x i l i a ry

r e g u l a t o r , m a i n t a i n s t h e . s a m e u n i t fo rc e , o n t h e

d i a p h r a g m r e g a r d le s s , o f d i a p h r a g m p o s i t io n . ( T h e

u n i t f o r c e r e m a i n s c o n s t a n t . T o t al . d i a p h ra g m , f o r c e

c h a n g e s b e c a u s e a s. t h e . r e g u l a t o r s t r o k e s , t h e .

d i a p h r a g m , e f f e c t i v e a r e a c h a n g e s . ) . . C o n s e q u e n t l y , .

t h e. a i r s p r i n g a d d s s t a b i l it y a n d . p r o v i d e s a r e l a ti v e l y

con s tan t o u t le t p res su re . . In . fact ,, i f ou t le t p ressure .

ac cu rac y is . the. on ly cr i ter ia, , there, is no. nee d. to. look

for a super io r regu la to r . .

T h e . p r e s s u r e - l o a d e d r e g u l a t o r w i l l m a i n t a i n a

s t ra igh t - l i ne p ressure , and. can . use . the to ta l ava i lab le

va lve , t rave l fo r g re a ter capa c i t y . . . In. som e cases , , i t

u s e s t h e . i d e n t i c a l v a lv e , u s e d b y t h e . s p r i n g - l o a d e d

regulator . .

T h e . a d v a n t a g e s , o f a c c u r a c y a n d . i n c r e a s e d c a p a c i t y

d o n o t c o m e . e a s i l y - t h e y h a v e th e i r p r ic e . I n t h e

p r e s s u r e - l o a d e d r e g u l a t o r , l o a d i n g p r e s s u r e , m u s t

e i t h e r b e v e n t e d , t o a t m o s p h e r e , w i t h t h e p o s s i b i li ty o f

c r e a t in g a c o m b u s t i o n h a z a r d .

I f l o a d i n g g a s m u s t b e b l e d d o w n s t r e a m , a s m a l l

b leed, o r i f i ce i s requ i red , see .

Figure 4.. .

T h e l o c k - u p

o f t h e t o ta l, r e g u l a t o r t h e n b e c o m e s , t h e l o c k - u p o f t h e

l o a d in g r e g u l a t o r .

F / g u r e 4

Sel f -Opera ted Pressure Loade d Regu la to r w i th

D o w n s t r e a m B le e d

I f l oad ing gas . can be . b led to a tm osph ere , , the . un i t

c a n b e s p ri n g, o r p r e s s u r e o p e n e d . I f a d o w n s t re a m .

b leed, i s requ i red , then. a spr ing must be ins ta l l ed , .

wh ich , tend s to c lose , the . va lve . . . The. a tm osp he r i c

b l e e d , t y p e . o f a p r e s s u r e - l o a d e d r e g u l a t o r h a s .

e x c e l l e n t s h o c k c h a r a c t e r i s t i c s . . . . T h e . d o w n s tr e a m .

b l e e d t y p e o f a p r e s s u r e - l o a d e d r e g u l a t o r i s. s e v e r e l y

l im i ted in i t s ab i l i t y to . co pe w i th fas t c los in g load.

v a l v e s , i n i n d u s t r i a l a p p l i c a t i o n s . . T h i s l i m i t a t i o n

resu l t s f rom. the equa l i za t ion , tha t mus t f i na l l y occur

th rough, the . sm al l do wn st re am b lee d or i fi ce ..

Pilot Operated Regulators

T o o b t a i n t h e . a d v a n t a g e , o f b o th . th e . s p r i n g - l o a d e d

a n d t h e . p r e s s u r e - lo a d e d r e gu la t or s ,, o n e . m u s t

exp lo re , the use . o f a p i l o t -op era ted regu la to r . The .

p i lo t , a l so ca l led , a re lay , ampl i f i e r , mu l t i p l i e r , , e tc . ,

has the . ab i l i t y to . mu l t i p l y a smal l, cha ng e in

d o w n s t r e a m , p r e s s u r e i n t o a l a r g e c h a n g e , i n

pressure , app l ied to the . regu la to r d iap hragm . . . Due to .

the ga in fea ture o f the . p i lo t ,, thes e type s o f regu la to rs .

cont ro l , p ressure , as . accura te ly as . the pressure-

l o a d e d u n i t s , m e n t i o n e d a b o v e , , b u t w i t h o u t s o m e o f

the i r l im i ta t ions .

There . a re th ree bas ic t ypes , o f p i l o t -opera ted

r e g u l a t o r s . . T h e y a r e :.

1 . t h e . w o - p a t h c on t ro l. ( l o a d i n g s y s t e m )

2 . the un load ing , sys tem, , an d

3 . t h e i n s tr u m e n t s y s t e m

Al l. th ree , o f fe r des ign and. op era t ing personne l , som e

versa t i li t y .. Th ey a l l. em ploy a p i l o t and. a re . cap ab le o f

v e r y a c c u r a t e c o n t r o l . . A l m o s t a l l p i l o t - o p e r a t e d

regu la to rs , have. downst ream, cont ro l l i nes tha t

prov ide , versa t i l it y i n co n t ro l l i ng p res sure a t a . g i ven .

po int d ow nstre am , f rom. the. regu lator . . . A valve, in the.

dow nst ream , cont ro l l ine . can be . used. to c rea te a . l ag .

to. tune. the. stab i l i ty of th e un i t .

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Let 's . exam ine the three , modes, of p i lo t operat ion

more c losely .

The Tw o-Path Cont ro l. Sys tem, . see Figure 5 has. the.

advantage, of changes in forces, associated wi th.

chang es in . dow ns t ream pressure , ac t ing qu ick ly on

t wo. o f t h e i m p o r t a n t e l e m e n t s o f t h e r e g u l a t o r . Th e

quicke r act ion wo uld be. on. the . main ope rat ing

diaphragm,, tending to m ove the. ma in va lve, in . the.

desi red d i rection.. Th is is . a. h ighly inaccura te

mo vem ent and . wou ld n o t be . sa t i s fac to ry fo r f i na l

contro l . . I t is supple me nted by p i lo t act ion, that occurs.

as so on as. the p i lo t senses, the ch ang e in

down s t ream pressure . There fo re , . the . combina t ion .

o f the rap id ad jus tment f rom. the . ma in . d iaphragm

and the h igh ga in , , bu t s lower ad jus tm ent f rom. the

pi lot prov ides accurate, , fast and. s table contro l .

by p i lo t act ion. A p i lo t unloading system, is typ ical ly

in le t p ressure to open o r spr ing to open, wh i le the

t wo -p a t h s y s t e m i s s p ri n g , to . c l o s e . . A c o m m o n

un load ing sys tem des ign u t i li zes , an. e las tom er ic

diaphragm, as i ts pr imary contro l , e lement,, a f ixed.

restrict ion, an d the. unlo ad ing pi lot,, see.

Figure 6 .

Inlet

pressure, tends , to open . th is . dev ice by pushing, the.

e las tom eric d iaphragm, aw ay f rom the. in let and. out let

f low s lo ts . . Increasing loading, pressure. (pressure.

between, the. f ixed restr ic t ion and. p i lo t ) shuts , of f f low

by push ing , the . e las tomer ic d iaphragm aga ins t the

slots..

Figure 5

Pi lot-Operated Regulator

Two-Path Control System

The. p ilo t ge nera l ly consis ts , of a f ixed, and a var iable.

or i f ice, wi th the operat ing, o r loading pressure.

genera ted be tween them . . . Th is . l oad ing p ressure .

g ives the. p i lo t i ts gain , whic h permi ts , the. use. of the

total, avai lable, travel of the . m ain valve... W ith. a sm all .

change, in . dow nstre am pressure, , a h igh. gain. p i lo t

can com ple te ly pos i t ion , the . ma in . opera t ing va lve

any wh ere f rom. c losed, to fu l ly open. .

Tw o-pa th sy stem s are equipped, wi th. spr ings, c los ing

the. ma in valve . . Increasing. force of dow nstream, gas

a lso c loses the ma in va lve . Force o f the load ing gas

mus t , there fo re , overco me bo th the . fo rce , o f

downs t ream, gas and . the c los ing fo rce o f the . sp r ing .

The. h igher pressure, load ing gas. b leeds, dow nstream.

to c lose, the. valve . . (Wh en. the main. valve, locks up,.

the fo rce of dow nstream, ga s. a lso c loses, the p i lo t .) .

The p resence o f gas a t dow ns t ream, and . l oad ing

pressures, in . the spr ing case s add s an actua tor

st i f fness factor to th is type of uni t that ma kes, i t

excep t iona l l y s tab le .

The response, an d con t ro l o f the . ma in . regu la to r

e lemen t in a p i lo t un load ing sys tem is d i rec ted so le ly

Figure 6

Pi lot-Operated Regulator

Unloading System

The opera t ion o r th ro t t l i ng o f the ma in va lve i s

accom pl i shed by the . p i lo t change load ing p ressure .

The. p i lo t lowe rs pressure. (ma in va lve, opens), by

open ing and exhaus ting , gas . dow ns t ream fas te r than .

the f ixed restr ic t ion, sup pl ies, i t . . As the p i lo t c loses ,

the loading, pres sure increases (main. valve c loses), .

s ince, the f ixed restr ic t ion, suppl ies, mo re gas to

load ing , p ressure than the . p i lo t is. exh au s t ing

d o wn s t re a m .

The p i lo t un load ing sys tem does have . the . accurac y

assoc ia ted , w i th p i lo t ga in . . Resp onse t ime i s

somewhat less than two-pa th con t ro l , sys tems

because, of th e absence, of d i rect act ion..

Th e I n s tr u m e n t Co n t ro l . Sy s t e m , s e e Figure 7

al though, c om m only cal led, a. regulator , is . t ru ly a

diaphragm contro l , va lve and a pressure, contro l ler .

Th ese versati le , dev ices, are equipped, wi th. f ixed, and.

var iab le or if ices, and are ch aracter ized, by h igh. gain

operat ion. . Mo st are. equipped, wi th prop ort ional an d

reset adjustments, th at pro v ide, system, s tabi l ity an d

accuracy. . They have the d isadvantage, , however, , of

hav ing to ve nt gas. to a tmo sphe re ....

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SUPP Y

Figure 7

Pilot Operated Regulator

Instrument Control System

A l t h o u g h m a n y m a n u f a c t u re rs h a v e m a d e . a t t e m p t s

to . reduce the . am ount o f ven ted gas , the ins t rument

con t ro l sys tem, has the same prob lem as the

a tmospher i c b leed , p ressure- load ing sys tem.

The ins t rument con t ro l sys tem encompasses , a

range of valves and inst ruments, broad enough to. be.

the su b jec t o f a separa te d iscuss ion . . . There fo re , . we

wi ll . not explore them fur ther.

Re g u l a t o r T y p e An a l y s i s

Every co mm erc ia l l y ava i lab le regu la to r fa l ls i nto . one

of the. categories, prev ious ly m ent ioned . . . Th ey m ay

not loo k l ike. the ske tche s, but the. essent ia l , e leme nts

the restr ic t ing, , loading and measur ing, e lements. ,

a ll . ope rate as show n. Th e sketches, depic t pressure

redu ct ion uni ts on ly , not re l ie f va lves. .

Occasional ly , . one hears of the adopt ion, or the.

exc lus ion of a part icu lar type of regulator because, i t

is labeled fa i l open , or fa il . c lo se d . Su ch. labels are

easy to . ass ign i f one makes , the ba s ic assum pt ion .

We. can predic t the fa i lure, that w i l l occur. . That 's . a

ta l l o rder becaus e any th ing mad e. by m an can . fa il .

and. poss ib ly wi l l . , and at the w orst poss ib le t ime. .

I f, . then, , any part of the regulator can. fa i l , wi l l . the

regu la to r a lways , fa i l open o r c losed?

For ana lys is , purposes , some typ ica l regu la to r

failures, a re. liste d in.

Figure 8

( Ins t rument Cont ro l

Systems. are. om i t ted due. to. thei r m any var iat ions .)

Even. these. fa i lures can be. mod i f ied to. sho w tha t

the. in it ia l , tend enc y of the. regulator to o pen or c lose.

wi l l be. tem pora ry unt i l the regu lator contro ls a ne w

set poin t . . Th e lesson to. learn f rom . th is . chart is . that

in orde r to. predic t the fa i lure m ode , one. m ust def ine.

the. fa i l u r e . In fac t , under c lose , exam ina t ion , the

terms. fa i l. open , and. fa i l c lose d , requi re, so. muc h

qual if icat ion, they b eco me meaningless. .

C o n c l u s i o n

Every type o f regu la to r represen ts , a compromise

involv ing such factors as pr ice, capaci ty , accuracy, .

s tabi l i ty , , s impl ic i ty , , safety and speed of response. . A

carefu l analys is , of large volume gas contro l .

app l i ca t ions usua l l y revea ls tha t more o f these .

fac to rs a re . ava i lab le th rough the . use o f p i lo t -

operated, regulators .

R e g u l a to r T y p e s

Sel f -Opera ted Bas ic Des ign.

Se l f -Ope ra ted S erv ice Type .

Se l f -Opera ted .

P re s s u re L o a d e d. A t m o s p h e r i c B l e e d

Se l f -O p e ra t e d

Pressure Loaded Downs t ream. B leed .

P i lo t -Opera ted 2 -Path Cont ro l

P i lo t -Opera ted Un load ing

Main

Diaphragm

R u p t u r e s

Opens.

Opens.

Opens.

C l o s e s

Closes.

Closes.

T Y P ES O F F A IL U R E

Main

S p r i n g

Bre a k s

Closes.

Closes.

O p e n s

O p e n s

O p e n s

D o w n s t r e a m

Contro l L ine

Bre a k s

O p e n s

Opens.

O p e n s

O p e n s

O p e n s

Pilot

S u p p l y

Orifice

Plugs

Closes.

C l o s e s

Closes.

O p e n s

Pilot

E x h a u s t

Orifice

Plugs

O p e n s

Closes.

O p e n s

Closes.

Figure 8

Types of Regu lator Failures and Initial Effects O n

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FUNDAMENTAL PRINCIPLES OF PILOT-OPERATED REGULATORS