P UM P A N D VA L VE T E C HN O L O G Y

Wellhead ch o ke va l ves, usually in sizes of up to 6inand in the API pre s s u re ra n ge 5000 to 15,000, havelong been considered an almost ex p e n d able item ofequipment. Ty p i c a l ly, the life of ch o ke va l ves can beve ry short. A matter of hours is not unheard of,although months is more common, but this is stillu n s at i s fa c t o ry.

The reason for this history of rapid ch o ke va l vefa i l u re is complex, and only re c e n t ly has it becomec o m p re h e n s ive ly unders t o o d. Essentially, it is a sys-tem pro blem — the interaction of many fa c t o rswh i ch constantly ch a n ge and produce a number ofl evels of ove rall instability and rapid wear out. Oneof these fa c t o rs is the composition of the fluid han-d l e d, wh i ch can ch a n ge fre q u e n t ly and unpre-d i c t ably. Another reason lies in the area of a diffe re n tand longe r- t e rm unpre d i c t ab i l i t y, design operat i n gconditions that are ap p ro p ri ate today can ch a n geover time and may, in a few ye a rs, extend beyond thec ap abilities of the ori ginal va l ve design. These con-t rolling fa c t o rs large ly ex clude operating pre s s u reand diffe rential pre s s u re as pro blem areas in ch o keva l ve design. To d ay, even at the high end of ch o keva l ve operating pre s s u res, the industry has a va s ts t o re of high-pre s s u re ap p l i c ation ex p e rience that ise q u a l ly ap p l i c able to ch o ke va l ve serv i c e.

Fluid composition. The fluid handled is a mixtureof crude oil, gas, wat e r, sand, and often other solidcomponents. Since the pro p o rtion of each compo-nent in the fluid mixture can ch a n ge constantly, it isd i fficult, if not impossibl e, to accurat e ly determ i n ethe actual effe c t ive fluid velocity because each of thecomponents travels at a diffe rent velocity in thebuild stream. Th e re fo re, velocity in the pure sensecannot be a useful and re l i able ch o ke va l ve designc ri t e rion. Howeve r, it is obvious that the sand, andp e r h aps other solids, is the bottom-line culprit ins h o rt ch o ke va l ve life. But this is far from the wh o l es t o ry.

C h a n ging operating conditions. While in time,ch o ke va l ve fl ows are re l at ive ly constant, we l l h e a dp re s s u res ch a n ge as the wells age. Usually, higho p e rating pre s s u res are associated with re l at ive lys t able dow n - well ge o l ogical fo rm ations wh e re lesssand is re l e a s e d. But as wellhead pre s s u res start to

fall, and these ge o l ogical fo rm ations start to col-l ap s e, the perc e n t age of the sand in the fluid fl owcan incre a s e. In some cases, this decreasing pre s-s u re / i n c reased sand re l ationship can be inve rt e d, fur-ther complicating the ch o ke va l ve design process. Ina ddition, decreasing operating pre s s u res with re l a-t ive ly constant fl ow can increase the re q u i red va l vec o e fficient (Cv) by as mu ch as a factor of 10, usuallywell beyond the ori ginal va l ve ’s design cap ab i l i t y.

While common single-stage ch o ke va l ves haveattempted to ach i eve accep t able perfo rmance lifet h rough improved mat e rials, this has not beenenough. As more and more large older wells arech a n ging from oil to gas production, ch o ke va l vei n t e rnal velocities are incre a s i n g, ex a c e r b ating thee ro s ive damage effect of the entrained sand. And, asva l ve trim erodes as a result, control is compro m i s e dand the fl ow needed re q u i res closer plug/seat opera-tion. This, in turn, accelerates plug/seat ero s i o n ,rega rdless of mat e rials. Another contri buting fa c t o ris increased vibration (and re l ated high noise leve l s )wh i ch the eroded va l ve trim and other re l ated oper-ating circumstances can cre at e.

So, it is this vicious circle that plagues operat i o n swith ex t re m e ly high maintenance costs and gro s s lyu n a c c ep t able short ch o ke va l ve life.

M u l t istage choke valve desi g nTo limit fluid velocity throughout the va l ve trim andto dra s t i c a l ly reduce the ero s ive effects of the sand,a mu l t i s t age va l ve design uses a stack of disks, eachof wh i ch incorp o rates a tortuous path fl ow pat t e rnby means of a series of right-angle turns in eachdisk. The velocity of all fluid components is therebycut by at least 75 per cent compared to single-stagep l u g / s e at and drilled hole-cage designs. By usingtungsten carbine trim throughout, the ero s ive effe c t sof the sand are minimised. In addition, long va l vestem stro ke increases ra n ge ability and enhances con-t ro l l ab i l i t y, so that fl ow set points can be maintainedand ch a n ges in operating conditions can be accom-m o d ated over time.

These va l ves are also ‘ch a ra c t e rised’. This meanst h at the tortuous paths in discrete groups of indiv i d-ual disks in the disk stack are diffe re n t ly designed;t h at is, fl ow per disk in one group is higher or lowe rthan in a disk in another group. The benefit in ch o keva l ve ch a ra c t e ri s ation lies in the ability to maintain a

3 3 I N T E R N A T I O N A L H Y D R O C A R B O N

P rolonging choke valve life The life of choke valves has typically been unacceptably short. Multistage valve technology hasbeen shown to prolong their life by as much as 10 times.

CCI Control Components, Inc

l a rger distance between the plug and the seat at lowfl ows; pre s s u re reduction is there fo re the same ast h at ach i eved within the tortuous path, mu l t i s t agedisks rather than by a throttling process through asmall plug/disk annular cl e a ra n c e.

The use of this mu l t i s t age va l ve tech n o l ogy andm o d e rn mat e rials of construction has borne fru i t .

Va l ve life has been demonstrated to be up to 10times gre ater than was ex p e rienced prev i o u s ly, espe-c i a l ly wh e re sand content was high. This gain inch o ke va l ve life has been shown to be true for newva l ves, replacement va l ves, and old va l ves that havebeen re t ro fitted with mu l t i s t age, ch a ra c t e ri s e d, long-s t ro ke, all tungsten-carbide tri m . ■

I N T E R N A T I ON A L H Y D R O C A R B O N 3 4

P U M P A N D VA L V E TE CH N O L O G Y

F i g u re 1. Retro f i t t e dtungsten-carbide multistagetrim with a longer plugt ravel for betterc o n t rollability and higherrangeability installed in anoriginal choke valve body.