A bit depending on service and pressure class, but some points.
a) A double or triple offset (eccentric) valve, especially in this size, will have some improved qualities compared to normal centric valves, and will be selected because of better properties for the service, or for longer lifetime/lower necessary maintenance (often case for water service for instance).
b) A flanged valve will be easier to mount and center, and mechanically a stronger construction than wafer construction. For this size, in building length compared to wafer building length, a U-shaped flange will probably also be cheaper than screwed lugging.
c) If wafer construction is selected, at least 'stearing lugs' for hanging up and centering when mounting, should be selected. The only reason to select wafer is price.
d) If water service or comparable simple service you should also check and compare price solidity/quality of construction against complete cast body form with flanges (Type EN F4, normal short valve building length or other similar standards) and only double offset (disc and stem offset, not sealing offset).
e) Final choice to be made on quality (price over required lifetime) consideration.
Avoid wafer valves in hydrocarbon or toxic service. Fire poses problems with wafer valves as the expanding through-bolts will feed the fire. Some lugged body designs accommodate bolting a few cap screws into the body. Bolting the body to one flange can help to hold the valve in place for piping spool removal.
Thanks All.
Gerhardl, can you pls explain me more on the U shaped flange?What way is it different from a normal flange.
With regard to the dead end service capability:
My requirement though is not dead end service, customer wanted us have the end connection selected along with appropriate reasons.
The U-shaped flange is the form used where the building length of the valve is the same as short building length used for wafer centric valves.
I am not sure this building length will actually exist for double or triple eccentric valves in this size. It does exist for centric valves, but anyway this flange type will fit to be installed against normal counter-flanges in the given pressure class, have the same number and shape of bolt holes, but because of the short building length of the valve you must take into consideration the limited space in how to mount (put in) the bolts and mount and screw the nuts.
Wafer valves are inexpensive. There is less metal and fewer machining operations. the disadvantage is that they require additional work to install. The pipes must be hung and aligned. Then the lower bolts mst be installed. the valve is then dropped between the flanges to net on the installed bolts, then the last bolts are added.
JLseagull mentioned that the exposed bolts can expand more than the valve and pipe in a fire, thus loosening the connection. COnversely, the somewhat long bolts get some pre-stretch when they are torqued in place, so most of the time the gasket loading is more consistent through thermal transients.
Lug type valves have the same face-to-face as wafer bodies. Heavier and more maching required so you pay a higher price. You get that back in ease of installation because the contractor hangs a pipe, bolts a valve on the end, and hangs another pipe. All the fussy prealignment is not required. The bolts are shielded from fire. Bolt length is critical because it's necessary to have as much thread engagement as possible, but you have to make sure the bolts don't interfere in the lugs, preventing gasket compression.
Flanged bodies are usually longer than lugged bodies, frequently (but not always) meeting a standard such as B16.10.- same as gate valves and many ball valves. At least one manufacturer offers a flanged butterfly valve series promoted as a "Quarter-Turn Gate" valve to be interchangeable with B16.10 gate valves. Even more expensive than lugged bodies, but as easy to install, and the bolt length is not critical.
Clearly, wafer lugged valves will be less expensive than flanged valves, particularly when considering the reduction in bolting. Are you proposing threaded lugs or clearance lugs? If threaded, it is sometimes a pain to screw lengthy studbolts through the lugs, particularly if the lug threads have not been chased out properly and bind. Also, tapped lugged valves cannot be simply dropped out like a flanged unit. At least one adjoining spool must be removed to take out the valve.
If weight is a serious consideration, e.g offshore, then the lugged valces are significantly lighter.
If there is a lugged valve out there that does not have the lugs threaded, I haven't seen it. It is customary for the 2 or 4 locating lugs in a wafer-valve to be clear-through. I can understand how a lug valve would be installed with studs, and there would be advantages in that the body would only be exposed to pure compression, but it would be a true B*tch to service. Lugged valves in my experience have threaded lugs and are installed with capscrews.
Caveat: no matter what-somebody does it the other way, too.
A. 'Lugs' is also a question of definition and semantics, and I have seen different wordings used. To avoid misunderstanding and try to be precise we have generally followed following definitions in my company:
i) Threaded lugs - > always used theese two words together as description of this type. (Wafer type valve with threaded lugs)
ii) Steering lugs - > generally used for all types of unthreaded lugs and open 'ears'.
iii) 'Wafer type valve' or 'valve with lugs' we do not consider a full description. Found in inquiries it will
generally be taken as the cheapest type of valve, eg. no lugs at all necessary to offer, or unthreaded lugs to be offered.
iiii) 'Valve with 'U-formed flange' -> usually meaning a centric type butterfly valve with short building length, building length equal to 'wafer type' standards, flanges with undrilled holes, boring to normal flange standards (Note! Short bolts each side or throughgoing bolts as ppractical possible), elastomere inside sealing stretched onto flange side.
B. Price over lifetime (periode before necessary maintenance, dependable expected standtime) have not been taken into consideration in the many interesting postings above.
A double eccentric, double flanged, BFL valve will for larger sizes and higher pressure classes, at least for water service, (drinking water or hydroelectric plants), come far better out in this respect than a centric butterfly valve.
The reason for this is a number of points, partly based on the serial production cost for each type and different material and strength in construction (double eccentric generally stronger stem and bearing construction than standard types and standard stem for cheaper type centric), but mainly because of far longer standing time of seat sealing on double eccentric valves. (Closing and opening geometry and adjustable profile sealing mounted on disc for double eccentric).
We have seen double eccentric, double flanged BFL valves in dimension 800mm and larger in pressure classes up to PN25 (bar) being left in open or closed position in more than 35 years still fully functional and closing dropthight.
Far to many projects are in my personal opinion selecting valve types based on buying price instead of based on lifetime cost and dependability in a longer time aspect.
