Butterfly Valves : Single Eccentric vs Double Eccentric

[Valving]

Dear All,

I am trying to understand the preference on double eccentric over single eccentric.
Especially Awwa designs are single offset. Whereas Europen prefer
double eccentric. (what I see)

Options,
1.better sealing (any concrete number for proof?)
2.less wear on seal(any concrete number for proof?)
3.better control performance
4.Double one should need higher torque?(disadvantage)
5.Any other?

Thanks in advance

 

[hsbcn]

hi Valving:

there are many threads on the forum that deal with this, cf.

http://www.eng-tips.com/viewthread.cfm?qid=313416

and

http://www.eng-tips.com/viewthread.cfm?qid=126483,

and

http://www.eng-tips.com/viewthread.cfm?qid=145006,

some also dealing with triple offset butterfly valves.

good luck with the investigations!




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[Valving]

thanks but most of them are for double or triple.
some of them are not even aware of signle offset.
I read all of them coule of times.

 

[bao2ye]

Make a drawing and you will find your answer.

Good luck.

 

[thost]

A single offset valve will have the shaft placed behind the centerline of the sealing area. In addition on a double eccentric, the shaft is placed offset to one side of the valve centerline. Benefits of an double eccentric valve versus a single eccentric is that the seat and the disk will not have contact while traveling. The disc will cam away from the seat while opened, and contact is not established before the final shut off position.
Another advantage of the dbl offset is the difference of the pressure-area on the disc. Since the shaft is offset to one side the pressure area will be slightly bigger on one side of the shaft. If the valve is installed in the preffered direction the process pressure will help the valve making a better sealing.
I dont think a double offset valve needs higher torque than single offset valves. I have not checked any calculations, but I cant see why that should be necessary.

I can think about two disadvantages for a double offset vs a single offset. One is price and availability. The other disadvanteg is a slightly higher pressure drop on the double offset valve.

 

[Valving]

thanks thost.
So what you are saying is there is no wear between disc and seal by double eccentric design?
I dont understand why the 2nd offset (shaft offset position off the valve centerline) results this.

 

[bao2ye]

Double offset can reduce a little bit of wear between disc and liner compared with single offset but not significant.
That is the reason why we have to rely on triple offset design.

 

[Valving]

thanks bao2ye. Do you have any solid indication of this difference? Any parameteri figure?

 

[thost]

No Valving, I am not saying there is no wear between disc and sealing. Unlike a single eccentric, a double eccentric valve, will have no swiping contact between the disc and the sealing. This fact will reduce the wear, and prolong the lifetime of your valve. if your service is clean like firewater, freshwater etc. you will probably do fine with a soft seated dbl.ecc. If you have a more abrasive service,or you need a tight shut off. I will advise you to choose a metal seated trpl. ecc.