8/13/2019 Valves in Piping Design

1/5

VALVES IN PIPING DESIGN

Valves are used for these purposes:

1) Process control during operation

2) Controlling services and utilities-steam, water, air, gas and oil

3) Isolating equipment or instruments, for maintenance

4) Discarging gas, vapor or liquid

!) Draining piping and equipment on sutdown

") #mergenc$ sutdown in te event of plant misap or fire

WHICH SIZE VALVE TO SE!

%earl$ all valves will &e line si'e ( one eception is control valves, wic are usuall$ one or twosi'es smaller tan line si'e* never larger+

t control stations and pumps it as &een almost traditional to use line-si'e isolating valves+owever, some companies are now using isolating valves at control stations te same si'e aste control valve, and at pumps are using .pump si'e/ isolating valves at suction and discarge+0e coice is usuall$ an economic one made &$ a proect engineer+

0e si'es of &$pass valves for control stations are given in "+1+4, under .Control valve)

stations/+

WHE"E TO PLACE VALVESee "+3+1 for valving pumps, under .Pump emplacement connections/+

Prefera&l$, place valves in lines from eaders on piperac5s) in ori'ontal rater tan

vertical runs, so tat lines can drain wen te valves are closed+ In cold climates, watereld in lines ma$ free'e and rupture te piping6 suc lines sould &e traced ( see "+7+2)

0o avoid spooling unnecessar$ lengts of pipe, mount valves directl$ onto flanged

equipment, if te flange is correctl$ pressure-rated+ ee "+!+1 under .%o''le loading/

relief valve tat discarges into a eader sould &e placed iger tan te eader in

order to drain into it+

8ocate eav$ valves near suita&le support points+ 9langes sould &e not closer tan 12

inces to te nearest support, so tat installation is not ampered

9or appearance, if practica&le, 5eep centerlines of valves at te same eigt a&ove

floor, and in-line on plan view

8/13/2019 Valves in Piping Design

2/5

8/13/2019 Valves in Piping Design

3/5

$A%E $AINTENANCE SI$PLE

Provide access for mo&ile lifting equipment to andle eav$ valves

Consider providing lifting davits for eav$ valves difficult to move &$ oter means ifaccess is restricted

If possi&le, arrange valves so tat supports will not &e on remova&le spools6

plug valve requiring lu&rication must &e easil$ accessi&le, even toug it ma$ not &e

frequentl$ operated+

$A%E $AINTENANCE SA&E

@se line-&lind valves, spectacle plates or te .dou&le &loc5 and &leed/ were positive

sutoff is required eiter for maintenance or process needs ( see 2+A

O"IENTATION O& VALVE STE$S

Do not point valve stems into wal5wa$s, truc5wa$s, ladder space, etc+

@nless necessar$, do not arrange gate and glo&e valves wit teir stems pointing

downward at an$ angle &elow te ori'ontal), as6-

1) ediment ma$ collect in te gland pac5ing and score te stem+2) proecting stem ma$ &e a a'ard to personnel+

If an inverted position is necessar$, consider emplo$ing a dripsield6

CLOSING DOWN LINES

Consider valve closing time in sutting down or trottling large lines+ Bapid closure of te valverequires rapid dissipation of te liquid/s 5inetic energ$, wit a ris5 of rupturing te line+ 8ongdistance pipelines present an eample of tis pro&lem+

liquid line fitted wit a fast-closing valve sould &e provided wit a stand pipe upstream andclose to te valve to a&sor& te 5inetic energ$ of te liquid+ standpipe is a closed vertical&ranc on a line6 air or oter gas is trapped in tis &ranc to form a pneumatic cusion+

I& THE"E IS NO P ' ID

Provide valves at eaders, pumps, equipment, etc+, to ensure tat te s$stem will &e

pressure tigt for $drostatic testing and to allow equipment to &e removed formaintenance witout sutting down te s$stem

Provide isolating valves in all small lines &rancing from eaders(for eample, see

figure "+12

8/13/2019 Valves in Piping Design

4/5

Provide isolating valves at all instrument pressure points for removal of instruments

under operating conditions

Provide valved drains on all tan5s, vessels, etc+, and oter equipment wic ma$ contain

or collect liquids

Protect sensitive equipment &$ using a fast-closing cec5 valve to stop &ac5flow &efore

it can gater momentum

Consider &utt-welding or ring-oint flanged valves for lines containing a'ardous or

.searcing/ fluids+ $drogen is especiall$ lia&le to lea5

Consider seal welding screwed valves if used in $drocar&on service see cart 2+3 inset

s5etc)

Provide sufficient valves to control flows

Consider providing a concrete pit usuall$ a&out 4 ft 4 ft) for a valve wic is to &e

located &elow grade

Consider use of temporar$ closures for positive sutoff-see 2+A

Provide a &$pass if necessar$ for equipment wic ma$ &e ta5en out of service

Provide a &$pass valve around control stations if continuous flow is required+ ee "+1+4

and figure "+"+ 0e &$pass sould &e at least as large as te control valve, and isusuall$ glo&e t$pe, unless " inc or larger, wen a gate valve is normall$ used see3+1+4, under .

8/13/2019 Valves in Piping Design

5/5

0e two isolating valves permit servicing of te control valve+ 0e emergenc$ &$pass valve isused for manual regulation if te control valve is out of action+

0e &$pass valve is usuall$ a glo&e valve of te same si'e and pressure rating as te controlvalve+ 9or manual regulation in lines " inc and larger, a gate valve is often te more economiccoice for te &$pass line-refer to 3+1+4, under .