isolation valve linear movement

7/26/2019 Isolation Valve Linear Movement

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Isolation valves are a key component in any fluid system as they are used to stop the flow of fluid into a particulararea of the system. They are also sometimes used to manually control the flow of the fluid. The European standardEN 736-11!!" distin#uishes $etween isolatin#% re#ulatin# and control valves as follows

Isolating valve -& valve intended for use only in the closed or fully open position.

Regulating valve -& valve intended for use in any position $etween closed and fully open.

Control valve -& power-operated device which chan#es the fluid flowrate in a process control system.

Isolation valves are used in a wide variety of different applications where on'off type control is re(uired% these include

)ivertin# process media.

*low isolation to

- *acilitate maintenance- &llow the removal of e(uipment- &llow the shut down of plant

& multitude of different types and desi#ns of isolation valve have $een developed in order to meet this ran#e ofapplications and the diverse operatin# conditions in which they are used. +alves are commonly classified into two#roups ,see Ta$le 1.1.1% accordin# to the operatin# motion of the closure device ,or o$turator

Linear movement valves - The o$turator moves in a strai#ht line. Included in this cate#ory are #ate valves%

#lo$e valves% diaphra#m valves and pinch valves. These valves are covered in #reater depth within this

tutorial. Rotary movement valves - The o$turator rotates a$out an a/is at ri#ht an#les to the direction of flow. 0all

valves and $utterfly valves are the two most important rotary valves associated with steam applications andare covered in #reater depth in Tutorial 1.% Isolation +alves - otary 2ovement.

Table12.1.1 Obturator motion in the basic valve types

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inear movement valvesinear movement valves have $een developed from the early forms of sluice #ates used to control the flow of waterin irri#ation channels. 4ince then% a lar#e num$er of different desi#ns and types have $een developed for use inalmost every type of flow application. &lthou#h linear movement valves are characterised $y strai#ht-line o$turator
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movement% the flow of the fluid may $e at ri#ht an#les to this movement ,as in the case of #ate valves% or in thesame direction% as with #lo$e valves. The main feature of the linear movement valve is that ti#ht shut-off may $eachieved $y ti#htenin# down the o$turator on a threaded stem.

Gate valves5ate valves are pro$a$ly the most common valves in use today due to their widespread use in domestic watersystems% $ut it should $e noted that their popularity in industry has declined in recent years. owever% they are still

used where an uninterrupted flow is re(uired% $ecause the #ate fully retracts into the $onnet% creatin# a minimalpressure drop% when the valve is in an open position. 5ate valves are specifically intended for use in isolationapplications.

& #ate valve consists of four main components% the $ody% $onnet ,or cover% #ate and stem. & typical #ate valve isshown in *i#ure 1.1.1.

Fig. 12.1.1 Typical e!ge gate valve

The #ate% which slides $etween the seats% is lifted in a direction at ri#ht an#les to the flow until clear of the flow path.The fact that the #ate fully retracts into the $onnet ensures that the pressure drop across the valve is low.

5ate valves are divided into a num$er of different classes% dependin# on the desi#n of the #ate and its seatin# faces.

"oli! e!ge gate valveThe #ate is wed#e shaped and it seats on correspondin# faces in the valve $ody. The mechanical advanta#e of theactivatin# thread% to#ether with the wed#e an#le% ena$les ade(uate seatin# forces to $e applied a#ainst the fluidpressure without e/cessive handwheel effort. The seat can sometimes $e coated with T*E to assist a hi#h inte#rityshut-off. & typical solid wed#e #ate valve is shown in *i#ure 1.1.1.

Fle#ible e!ge gate valve&lthou#h there are several types of fle/i$le wed#e #ate valves% they all make use of a fle/i$le two-part disc% which isshaped like two wheels on a very short a/le. The fle/i$ility of the disc ensures ti#ht seatin# over a wide ran#e oftemperatures and pressures.

The most common type of fle/i$le wed#e #ate valve used in steam applications is the parallel slide valve. The twoplates that constitute the #ate are held a#ainst the seat $y a sprin#% encased $etween them. The fluid pressure


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moves the upstream disc off its seat% and the force is transferred onto the downstream disc% there$y ensurin# a ti#htshut-off. The hi#h de#ree of fle/i$ility in the #ate allows for e/pansion and contraction when su$8ected to temperaturevariations% makin# it suita$le for use in steam systems.

Globe valves5lo$e valves constitute a ma8or class of linear movement valves9 they have $ecome more popular than #ate valvesas there is a wide variety of confi#urations availa$le to suit most applications. The movement of fluid throu#h the

valve seat is lon#itudinal to the operatin# motion of the o$turator9 this means that for a valve in which the inlet andoutlet are hori:ontally opposed% the fluid must follow a chan#in# course. The main advanta#e of this arran#ement isthat a #lo$e valve opens more rapidly than a #ate valve as the disc only needs to move a small distance from its seatto allow full flow. This is an advanta#e when there is fre(uent operation of the valve. The disadvanta#e is that the fluidhas to chan#e course% increasin# the resistance to flow and #eneratin# tur$ulence. This results in a hi#her pressuredrop across a #lo$e valve than a #ate valve.

Fig. 12.1.2 $ conventionalglobe valve

5lo$e valves are less likely to leak than #ate valves% which means that they can $e used for hi#her pressure or hi#hervolume applications% for e/ample in steam systems% or where fluid loss can $e ha:ardous or costly. The increasedcost of #lo$e valves over #ate valves is therefore offset $y the additional safety they provide% and a reduced chanceof fluid loss.

The pressure of the fluid actin# over the area of the disc #enerates an a/ial load on the stem. This makes closin# thevalve difficult% so much so% that it limits the si:e of a standard #lo$e valve to )N";.


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Fig.12.1.% "chematic o& a typical balancing plug valve

'iston valves


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valve is fully opened% only the $ottom face of the piston is e/posed to the fluid as the rest of the $ody is protected $ythe upper sealin# rin#s. This means that the sealin# surfaces ,the sides of the piston are protected from erosion $ythe fluid flow.

Fig. 12.1.( $

piston valveIf the valve re(uires maintenance% all the internals can $e easily removed $y undoin# the cover nuts and withdrawin#the piston. The rin#s and the lantern $ush can then $e removed usin# an e/tractor tool. This operation is simple andcan $e undertaken without havin# to remove the valve from the pipeline. In #eneral% the piston should never have to$e replaced% $ut the sealin# rin#s may wear over a lon# period with fre(uent operation.

)iaphragm valves)iaphra#m valves constitute the third ma8or type of linear movement valves. The stem of the valve is used to pushdown a fle/i$le diaphra#m% which in turn $locks the path of the fluid. There are two different classifications ofdiaphra#m valve $ased on the #eometry of the valve $ody

*eir type -& weir is cast into the $ody% and when closed% the diaphra#m rests on the weir% restrictin# the

flow ,see *i#ure 1.1." ,a.

"traight-through type - The $ore runs laterally throu#h the $ody and a wed#e shaped diaphra#m is used to

make the closure ,see *i#ure 1.1." ,$.


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Fig. 12.1.+ The eir type ,a an!straight-through type ,b !iaphragm valves

The main advanta#e of a diaphra#m valve is the fact that the diaphra#m isolates the movin# parts of the valve fromthe process fluid. They are therefore suita$le for handlin# a##ressive fluids and for those containin# suspendedsolids. In addition% as the $onnet assem$ly is not e/posed to the fluid% it can $e made from ine/pensive materialssuch as cast iron% there$y reducin# the overall cost. The development of new diaphra#m materials ena$lesdiaphra#ms to $e used on most fluids. Their application is however limited $y the temperature that the diaphra#m canwithstand - typically less than 17">?. )iaphra#m valves are #enerally used on process fluid applications.

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inear movement valve stem optionsinear movement valves are availa$le with a num$er of different stem arran#ements

Risingnon-rising stems - If the stem is risin#% it will move vertically upwards when the

valve is opened% as opposed to only rotatin#% as with a non-risin# stem. The risin# stem indicates thede#reee of valve openin#% which in turn rou#hly reflects the amount of flow throu#h the valve. +alves with
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risin# stems do however re(uire more space a$ove the $onnet to accommodate the stem in the fully openposition. The use of non-risin# stems is recommended on #land packed valves% as they reduce the wear onthe packin#.

Fig. 12.1./ Rising ,aan! non-rising ,b stem valves

Insi!eoutsi!e stem scres -


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4tem sealin#In order to prevent leaka#e of the process media from around the stem of a valve% a $arrier must $e placed $etweenthe fluid and the environment. 4tem sealin# is usually achieved $y one of two methods% namely #land packin# and$ellows sealin#. 5land packin# consists of a polymeric material% typically T*E% packed ti#htly $etween the stem andthe $onnet of the valve% there$y preventin# any process media escapin#.

Fig. 12.1. ellos seale!valve

In $ellows sealed valves% a fle/i$le metallic $ellows is used. It is connected on one end to the stem and the other end

is connected to the $onnet% effectively producin# a $arrier $etween the fluid and the environment. This $ellowse/tends and contracts as the stem moves up and down. The $ellows is so effective% it produces a @:ero emissions@seal. *itted to the $ellows is an anti-torsion device% which prevents the $ellows from rotatin# with the stem. 4uch adevice is essential% otherwise the repeated twistin# of the $ellows would lead to the failure of the seal.

&lthou#h less costly than the $ellows sealed valves% the #land packed valve does not produce such a ti#ht seal as the$ellows. If a #land packed valve is not used for a si#nificant period% the #land packin# can stiffen% and leaka#e willoccur the ne/t time the valve is used. The $ellows sealed valve does not suffer from this pro$lem. *urthermore% #landpacked valves re(uire re#ular re-packin# of the #land% whereas a typical $ellows re(uires no maintenance for over 1;;;; cycles.

isolation valve linear movement

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