Pumps & Valves

DEMM Engineering & Manufacturing - - NEWS -

In­tro­duc­tion

Valves are es­sen­tial com­po­nents of a pip­ing sys­tem in any ma­chin­ery pack­age, pack­aged equip­ment or fa­cil­i­ties; valves al­low the work­ing fluid to be con­trolled and di­rected on its jour­ney through the pack­age. They are ex­pen­sive en­gi­neered items, and it is im­por­tant that the cor­rect valve is spec­i­fied for the func­tion and that it is con­structed of the cor­rect ma­te­rial for the work­ing fluid. There are two meth­ods of op­er­at­ing a valve: man­u­ally, with a hand-wheel, lever, wrench, or ac­tu­a­tor; or through au­to­mat­i­cally con­trolled valves. This ar­ti­cle fo­cuses on valves; it dis­cusses the prac­ti­cal note and im­por­tant guide­lines for se­lec­tion, op­er­a­tion and re­li­a­bil­ity of valves in pack­ages.

Com­po­nents of valves

Com­po­nents of a valve are usu­ally cat­e­gorised by the re­quire­ments of the valve’s task; the pres­sure con­tain­ing en­ve­lope is that vol­ume ex­posed to the full op­er­at­ing con­di­tions of the fluid tem­per­a­ture and pres­sure. Wet­ted de­scribes a com­po­nent di­rectly ex­posed to the process fluid, ei­ther fully or par­tially; pres­sure-con­tain­ing com­po­nents and com­po­nents with ar­eas in con­tact with the han­dled fluid( such as the body and bon­net) need care­ful ma­te­rial se­lec­tion. They are usu­ally fab­ri­cated from metal­lic ma­te­ri­als. All com­po­nents in this group should have both the me­chan­i­cal strength to cope with the de­sign con­di­tions and the cor­rect ma­te­rial chem­i­cal com­po­si­tion to han­dle the cor­ro­sion char­ac­ter­is­tics of the han­dled fluid. If the com­po­nent falls into the non-pres­sure-con­tain­ing group, then pres­sure con­tain­ment is not an is­sue, but the ma­te­rial cho­sen should have the me­chan­i­cal strength for its cho­sen func­tion. For ex­am­ple, a stem ma­te­rial should be able to sup­port the torque ap­plied to open and close the valve with­out fail­ure. Also, as a wet­ted com­po­nent( in con­tact with the fluid ), the stem should have cor­ro­sion re­sis­tance char­ac­ter­is­tics for the fluid. There have been many bolts used in dif­fer­ent valves; bolts should be of suf­fi­cient strength to seat the gas­ket when bolt loads are ap­plied and cre­ate an ef­fec­tive seal.

Hand-wheels should be con­structed of a ro­bust ma­te­rial to en­sure that they do not crack and fail when be­ing op­er­ated. En­vi­ron­men­tal con­di­tions should also be con­sid­ered, and some valve com­po­nents may re­quire an ad­di­tional coat­ing, as is the case of valves in cor­ro­sive or harsh en­vi­ron­ment of some plant’s lo­ca­tions, which may re­quire a spe­cial coat­ing and paint­ing. Ex­am­ples of non-metal­lic ma­te­ri­als in valves:

• Pri­mary seals – pres­sure con­tain­ing and wet­ted.

• Sec­ondary seals – pres­sure re­tain­ing and par­tially wet­ted.

• Soft seats – pres­sure con­tain­ing and wet­ted.

• Gas­kets – pres­sure con­tain­ing and par­tially wet­ted. All non-metal­lic com­po­nents form some sort of seal, ei­ther a pri­mary seal (the first seal, and di­rectly in con­tact with the process fluid and ex­posed to full de­sign con­di­tions, pres­sure, and tem­per­a­ture) or a sec­ondary seal (any seal af­ter the pri­mary seal and not in di­rect con­tact with the process fluid and full de­sign con­di­tions, pres­sure, and tem­per­a­ture).

There are dif­fer­ent valve de­sign stan­dards, such as ASME, BS, API, and in each ref­er­ences the nu­mer­ous com­po­nents in­cluded in the var­i­ous types of valves. It is es­sen­tial that all the valve com­po­nents are suit­able for the process fluid and the de­sign con­di­tions. A chain is as strong as its weak­est link, so it is point­less to se­lect suit­able ma­te­rial for all but one com­po­nent, be­cause this in­fe­rior part may lead to the to­tal fail­ure of the valve and costly main­te­nance.

Valve tech­ni­cal de­tails

The ASME/ANSI stan­dard B16.34 is a valve stan­dard used in con­junc­tion with the ASME codes on boil­ers and power pip­ing in ASME de­sign in­stal­la­tions and, sep­a­rately, as a valve stan­dard in it­self. The stan­dard is rel­e­vant to flanged, weld-neck, and threaded end-types for all ap­pli­ca­tions. The­o­ret­i­cally it cov­ers all valves down to the very small­est sizes but, in prac­tice, its main ap­pli­ca­tion is for those with a nom­i­nal bore of above ap­prox­i­mately 60 mm. The parts of ANSI B16.34 rel­e­vant to de­sign, man­u­fac­ture, and in­spec­tion are spread through­out var­i­ous sec­tions of the doc­u­ment. The key ones are:

• Dif­fer­ent valve classes.

• Ma­te­ri­als and di­men­sions.

• NDT scope.

• NDT tech­niques and ac­cep­tance cri­te­ria.

• De­fect re­pair.

Valve classes di­vides valves into two main ‘classes’: “”stan­dard classes” and “spe­cial class” (ASME/ANSI B16.34). There is a third one called ‘lim­ited’ class, but it is not used very of­ten. There are a se­ries of pres­sure–tem­per­a­ture rat­ings for each type, des­ig­nated as 150, 300, 400, 600, 900, 1500, 2500, and 4500#; th­ese are re­lated pre­dom­i­nantly to the valve in­side di­am­e­ter and its min­i­mum wall thick­ness. The higher the num­ber, the larger the wall thick­ness and the max­i­mum de­sign pres­sure.

Ball valve de­signs

Ball valves have been spec­i­fied in dif­fer­ent de­signs for ex­am­ple, in forms of split-body ball valves and float­ing-ball ball valves. Re­gard­less of the ma­te­ri­als for con­struc­tion of a split body, float­ing ball valve, all such valves have (more or less) the same prin­ci­pal com­po­nents. The body in a split body de­sign can be made in two pieces or three pieces. Both de­signs al­low the ball valve to be re­moved from the line and re­paired lo­cally or, ideally, in a work­shop. The three-piece ver­sion is more ex­pen­sive but eas­ier to main­tain, be­cause you can work on both side soft he ball. The float­ing ball de­sign means that the ball is sus­pended from the stem and rests on the soft seats. It is used for smaller sizes and lower- and medium-pres­sure classes. As the line size in­creases, the mass of the ball in­creases and reaches a weight at which it should be sup­ported from below with a trun­nion. The valve is usu­ally avail­able with a re­duced port (usu­ally one size down from the line size, e.g., 6 × 4 in.) or a full port (the port and line size are the same, e.g., 6 × 6 in.) An an­ti­static de­vice is also in­cluded to pre­vent a static charge as the metal ball trav­els over the soft seats, which could be made of PTFE. De­pend­ing

on the process con­di­tions, some of the ma­te­ri­als could change; oth­ers re­main the same. This par­tic­u­lar valve is of­ten de­signed to a com­bi­na­tion of API-6D and BS-5351 spec­i­fi­ca­tions. The flanged ends are de­signed and drilled to the spec­i­fi­ca­tions of ASME B16.5. The an­ti­static de­vice can be ac­cord­ing to BS-5361. The face-to-face di­men­sions are from API-6D and ASME B16.10. It is fire safe to an un­de­fined code.

An­other im­por­tant type of ball valve is split-body, trun­nion mounted type; trun­nion-mounted valves are spec­i­fied when the mass of the ball is such that it re­quires ad­di­tional sup­port at its base or for ser­vice at higher pres­sure rat­ings, when it is es­sen­tial that the con­struc­tion of the valve be more ro­bust and the ball main­tained in a fixed po­si­tion when the valve is fully closed and not forced up hard against the soft seats, which risks squeez­ing them out of their re­tain­ing seat ring. There are many other types of ball valves which are not men­tioned in de­tails here, they are usu­ally used in spe­cific ap­pli­ca­tions.

Con­trol valves

Con­trol valves are valves used to con­trol con­di­tions such as flow, pres­sure, tem­per­a­ture, and liq­uid level by fully or par­tially open­ing or clos­ing in re­sponse to sig­nals re­ceived from con­trollers that com­pare a “set-point” to a “process vari­able” whose value is pro­vided by sen­sors that mon­i­tor changes in such con­di­tions. The open­ing or clos­ing of con­trol valves is usu­ally done au­to­mat­i­cally by elec­tri­cal, hy­draulic or pneu­matic ac­tu­a­tors. Po­si­tion­ers are used to con­trol the open­ing or clos­ing of the ac­tu­a­tor based on elec­tric, or pneu­matic sig­nals. A con­trol valve con­sists of three main parts in which each part ex­ist in sev­eral types and de­signs. Be­cause of its de­sign, the globe valve pat­tern is the most suit­able valve to con­trol flu­ids for a wide range of pres­sures and tem­per­a­tures and the most com­monly spec­i­fied. Al­though globe valves are avail­able in sizes (say above 14 inch), for com­mer­cial rea­sons, at the larger sizes a but­ter­fly valve is of­ten spec­i­fied, for the sav­ing on space and weight.

An ex­am­ple of a con­trol valve ap­pli­ca­tion is the pump con­trol valve; this type of valve is used on pumped sys­tems to con­trol or elim­i­nate surges caused by pump start and stop. It of­ten op­er­ates by us­ing a springloaded clo­sure mem­ber that opens or closes slowly to re­strict the ini­tial flow of wa­ter when a pump starts and stops.

Safety re­lief valves

All con­ven­tional pres­sure re­lief valves op­er­ate on the prin­ci­ple of sys­tem pres­sure overcoming a spring load, al­low­ing the valve to re­lieve at a de­fined set-point. When the re­lief valve is closed dur­ing nor­mal op­er­a­tion, pres­sure act­ing against the seat­ing sur­faces is re­sisted by the spring force. Ideally with the sys­tem pres­sure below set pres­sure by more than one or two per­cent, the re­lief valve should be com­pletely leak- free. As sys­tem pres­sure is ap­plied to the in­let of the valve, force is ex­erted on the base of the disc as­sem­bly. The force pro­duced by the com­pres­sion of the spring coun­ters this up­ward force. When the op­er­at­ing sys­tem is below set pres­sure, the spring hous­ing (or body) of the valve and the out­let are at at­mo­spheric pres­sure; as op­er­at­ing pres­sure be­gins to ap­proach set pres­sure of the re­lief valve, the disc will be­gin to lift. This will ideally oc­cur within one to two per­cent of set point value and an au­di­ble sound will be pro­duced, termed the ‘sim­mer’ of the valve. As the disc lifts the gas is trans­ferred from the seat area to the ad­di­tional area, hence sub­stan­tially in­creas­ing the area be­ing acted on. The re­sult is that the amount of force be­ing ap­plied against the spring com­pres­sion is dra­mat­i­cally in­creased. This causes the disc as­sem­bly to rapidly ac­cel­er­ate to the lifted po­si­tion or ‘open’ con­di­tion, re­sult­ing in a ‘pop­ping’ sound.

The disc will not stay in the full open po­si­tion and will be­gin to drop un­til an ad­di­tional pres­sure build-up oc­curs. This over-pres­sure con­di­tion will main­tain the re­lief valve in the full open po­si­tion and al­low it to dis­charge at max­i­mum rated ca­pac­ity. As the sys­tem pres­sure be­gins to drop and the spring force over­comes the force cre­ated by the disc, the valve will be­gin to close. The sys­tem pres­sure should drop below the set pres­sure be­fore the re­lief valve will close. This process is termed the ‘blow­down’ of the valve.

Con­sid­er­a­tions re­gard­ing noz­zle open­ing are im­por­tant for re­lief valve op­er­a­tions. Noz­zle open­ings in equip­ment or ma­chiner­ies above a cer­tain di­am­e­ter (spec­i­fied in in­di­vid­ual ves­sel codes for ves­sels) need the ad­di­tion of a re­in­forc­ing pad to re­store shell strength. When strength com­pen­sa­tion is needed, noz­zles may be ‘set-in’ or ‘set-through’ the ves­sel shell. An al­ter­na­tive type of noz­zle used for some ap­pli­ca­tions (typ­i­cally on thick-walled head­ers) is the ‘wel­do­let’. Th­ese are heav­ily cham­fered where they fit into the ves­sel or header shell and re­quire a large multi-lay­ered weld. In all noz­zle fit­tings, the spec­i­fi­ca­tion of the weld leg length and throat thick­ness is crit­i­cal to the joint strength; in some cases it plays a part in the com­pen­sa­tion for loss of strength from the noz­zle open­ing.

Check valves

Check valves au­to­mat­i­cally check or pre­vent the re­ver­sal off low. Ba­sic types are the swing check, lift check, ball check, and wafer check de­signs. An­other des­ig­na­tion used for some ap­pli­ca­tions (such as some waste sys­tems) is a back­wa­ter valve. The swing check has a hinged disk( some­times called a flap per) that swing son a hinge pin. When flow re­v­erses, the pres­sure pushes the disk against a seat. The flap per may have a com­po­si­tion disk, rubber or Te­flon, rather than metal when tight clo­sure is re­quired. Swing checks of­fer lit­tle re­sis­tance to flow. The lift check has a guided disk that is raised from the seat by up­ward flow pres­sure. Re­ver­sal of flow pushes the disks down against the seat, stop­ping back flow. Lift checks have con­sid­er­able re­sis­tance to flow, sim­i­lar to that of a globe valve. They are well suited for high-pres­sure ser­vice.

An­other com­mon check is a wafer de­sign which fits be­tween flanges in the same fash­ion as a but­ter­fly valve. Wafer checks come in two types: a dual flap per that is hinged on a cen­tre post and a sin­gle flap­per that is sim­i­lar to the stan­dard swing check; they are gen­er­ally used in larger size pip­ing (4 in and larger) be­cause they are much lighter and less ex­pen­sive than tra­di­tional flanged end swing check valves.

A de­mand check value is of two-piece con­struc­tion, with one piece hav­ing a springloaded clo­sure sim­i­lar to the air val­ues found on au­to­mo­bile tires. The se­cond piece, when in­serted into the first, opens the valve, al­low­ing free pas­sage of air. The de­mand check valve is used for con­nect­ing gauges, al­low­ing re­moval with­out per­mit­ting air to es­cape from the pipe.

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