Butterfly valve design: conc, ecc, doubl ecc or triple eccentric? 5

[heymans]

Hello,

Is there somebody how really knows the different (dis)advantages (when and where to use wich type?) of the different butterfly valves designs, offset.
Specially the differnece between single and double eccentric is a bit of a mystery for me. Has it something to do with selingforces (moments?) or gives double eccentric less friction, ....?


Thanks

greetings, Pieter

 

[JimCasey]

The non-offset valves are the low end of the spectrum. They are generally "rubber" lined. Typically capable of sealing to 150 psi, temp-limited by the elastomer of the seats, and the shaft seals by interference fit between the shaft and liner. These can work on dry granular solids, or with a polished vane and pedigreed liner they are used on food-or pharmaceutical applications. As the valve opens and closes, some part of the vane is always wiping the seat, causing friction and wear. These valves are position-seated, When the vane is at 90-degrees to the pipe centerline, they are as closed as they get.

I don't think there IS a single-offset valve, but if I am wrong someone will tell me soon.

Double-offset valves, also known as high-performance butterfly valves, have the shaft mounted slightly offset from the pipe centerline, and the shaft is mounted behind the sealing band of the seat. The seat is a narrow resilient band is clamped in the body and must be resilient. PTFE is resilient enough to seal, and the disc only wipes the seat for a few degrees of travel at seating. So there is a lot less friction, the seats can last longer, and the seats can be made from materials that are capable of dealing with higher temperatures than the rubber seats of the lined valve. Metal seats are available for HP Butterfly valves allowing operation to around 1200F. SInce the disc is offset from the shaft, there is a lot of metal in the flow stream in the wide-open position, and double-offset valves have somewhat less maximum capacity than flat-disc, non-offset valves. These valves are also position-seated. Unlike the lined valves, these valves can actually damage the seats if they are overstroked so they almost all have a travel-stop pin in the body.

Triple offset valves are set up much like double-offset valves, except that the profile of the outside diameter of the vane is machined as a conic section. The cone would be projected upstream, tilted along the edge of the pipe wall on the leading edge of the disc. The cone is not centered in the pipe-hence the third "offset". What this does is to allow seating without rubbing, and the seats hardly have to be resilient at all. SO once the valve is comanded to move, there is no breakaway torque, the running torque is essentially just packing friction, and even the dynamic torque produced by the flowing media over the vane is low. I said the seats are hardly resilient: Ususally the seats are something like a stack of thin stainless sheets bonded with graphite, so they can deform slightly and conform to the seating surfaces. Triple offset valves are not positon-seated-they are torque-seated. When the disc hits the seat it is metal-to-metal contact and they cannot be overstroked. But to get tight shutoff, it is necessary for the actuator to lean on them mightily to squeeze the seating surfaces together. SO a triple offset can give you tight shutoff at elevated temperatures, there are no soft elastomers/polymers to be damaged by particles or velocity effects, and they work pretty well for control.

If you paid attention in geometry class you will remember that an angled slice through a cone is not a circle but an ellipse. That's right-the seats and vane are NOT ROUND. As long as there is a good SPC/ISO manufacturing plan the pieces are interchangeable, so repair parts will still fit.

Search the web for Bray or Keystone for the rubber-lined valves, search for the Flowserve BX2000 or the Keystone K-Lok for the double-offset valve, or the Tyco Vanessa or Tri-Centric for the triple offset valve.

 

[Ashereng]

If you google TRICENTRIC and WEIR, they have a great little paper that explains all of this, and great illustrations too.

I can't insert the diagram here, but here is the description of the 3 offsets.

Tricentric® Sealing System
TRIPLE OFFSET

Offset #1 The shaft is positioned forward of the valve
seat providing an uninterrupted seating surface.

Offset #2 Centerline of disc rotation is located away
from the centerline of the seat allowing the seal to lift
off and away from the seat on opening.

Offset #3 The cone axis is offset from the centerline
of the seal to provide a conical sealing surface that
allows the seal to rotate in and out of the seat without
interference, sliding or jamming.

 

[Ashereng]

Jim,

You can buy a single offset disc valve. Many are going to double and triple offset due to better sealing, as you mentioned above. Many others, like me, are going to other valve types as costs are coming down.

Great descriptions by the way!

 

[khan101]

Asereng,
could you please post the link for that paper here. I tried to google it but "it was not found"

Thanks

 

[Ashereng]

http://www.weirvalve.com/weir/valves/home.nsf/Page/DownloadsProduct_literatureButterfly_Valves

The PDF is titled "Tricentric Butterfly Valve".

 

[Ashereng]

khan101,

You are buying the beers the next time I see you in the field.

 

[GeoSmith]

Jim,

Your explaination is so detailed. I learned much from you on valves. In fact, I work as distributor, one of my clients complain the triple offset valve need to have a larger torque to have a bubble-tight, compared with double-offset. Do you have any idea how to reduce the torque? Thanks a lot

 

[JimCasey]

Geosmith: The torque is needed to cause the seating surfaces of the Triple Offset Butterfly (TOB) to deform enough to seal. Using a soft seat would require less torque, if you can give up some margin on the pressure/temperature limits.

Most of the time, the triple offset is used for isolation in lieu of a gate valve. THe Tyco Vanessa valve is actually marketed as a Quarter-Turn Gate (QTG). Gate valves need gorilla seating loads, don't really shut off all that tight, and are a LOT taller than an equivalent TOB valve. The TOB valve might cost a little more but it saves you a lot of room in a pipe chase, shuts off tight, and has substantially less lifetime environmental leakage, too. The only real offsetting features are that with the vane and shaft in the flowstream, the capacity of the TOB is somewhat less that the gate valve, and the gate valve is piggable.

 

[Ashereng]

GeoSmith

Another alternative to the TOB is the slab gate valve. The gate sort of looks like a spectable blind - the orifice moves up and down.

Here is are a couple of links with a good picture:

http://www.fmctechnologies.com/SurfaceWellhead/Valves/Series100Valves.aspx

http://www.coopercameron.com/cgi-bi...m?OID=1072354&pageid=cambrands&thismenu=menu1

Instead of a traditional knife or wedge type gate valve, which may be prone to gunk in the channel, causing sealing problems, this type has a full plate that moves an orifice/solid blind in and out of the flow path. Because the orifice slides completely out of the flow path (versus a knife coming into a seat), there is better sealing as the seats are parallel to the movement of the sliding plate.

This type of valve gives you:
- lower seating loads (don't need "gorilla" torgque)
- very good shutoff classifications (API 6A)
- high working pressures
- bi directional sealing

 

[NGiLuzzu]

GeoSmith,
where is the seating element of the tripple-offset valves you are talking about? Is it a flexible ring bolted to the disc (against an hardened seat surface on the body) or is it fixed to the body?
This may affect the torque requirements of the valve... (in any case, usually the most "critical" are the opening ones, i.e. the torque needed to unseat the valve).



JimCasey,
I think that the "Quarter-Turn Gate (QTG)" denomination once used by Vanessa refers to face-to-face dimension only (and not to torque requirements), in order to distinguish the valves with an "extended" length in the pipe axis direction, intended to replace gate valves in existing plants.
(See, for example:

http://www.vanessavalves.it/series3-1.htm#ppp,

5^th paragraph after the table...).



To All:
there are a lot of discussions already written about this issue, maybe you'll find useful to take also a look at
- thread408-101237,
- thread408-141966,
- thread408-126483
- thread408-72775,
- thread408-129793 and
- thread408-64692
within this Forum...!



Hope this helps, 'NGL

 

[sterl]

If you really need to get to some high seating forces without huge torque and breakaway, see Trunion valve...Has some shortcomings compared to pivoting disk valves, but has some distinct advantages too.

 

[JimCasey]

Anegri,
The QTG DOES, In fact, have a body with the same face-to-face as a gate valve. My point was, if a user is distressed by the torque required by triple-offset a butterfly valve, he will not be relieved by going to alternative technology gate valves. We could do paragraphs about solid wedge, flex wedge, double-disc-parallel seat types of gate valves, but in every case you have to drop them into the seated position then lean on them enthusiatically to get them to seal well.
Ashereng's slab gate is a valve to consider. It would be a position-seated valve. It is less environmentally friendly because of the long stroke of the stem thru the packing, makes for a tall assembly, and usually slab gates (conduit gates) have soft seats with the inherent benefits and disadvantages thereof. But they are truly obstructionless in the Wide-open position.

If you have not yet engraved your granite slab, please grab one and chisel in the following thought:
There is a best valve for every application.

 

[GeoSmith]

Thanks for all suggestions for the high torque required for triple offset valve. On the other hand, my client found out and complain to me that the screws in the ring (seat retainer) are not strong enough to hold the seal. What can I do? Any formula to calculate the sizes or number of cap screws are strong enough to hold the pressures? I think we need to change the manufacturers. So bad!!!

 

[JimCasey]

The screws in the seat ring retainer are only there to maintain assembly before the valve is installed. After you squash it between two flanges and torque a large number of large bolts down on it, it ain't going anywhere. Exception: Lugged valve for end-of-line service. Solution: Bolt it in place with a flange anyway. Here where I work, the retainer for our HPBVs is just snapped in place with a Viton o-ring. This gives an uninterrupted gasket surface, and it's easy to pop out the retainer to replace the seal. The o-ring is NOT for sealing, and the valve is not rated for unsupported end-of-line service.

MOST triple offset valves have a fixed seat in the body and the conformal seal in the disc, held in place with a lot of smallish screws and a relatively beefy ring.

 

[NGiLuzzu]

GeoSmith,
simply contact the triple offset valve Manufacturer: they should have, of course, design calculations good to demonstrate that the retainer flange bolts will withstand the service conditions.

Otherwise (if you can't get replies about this issue)... switch to a more serious Manufacturer!! ;-)
(See for example the ones mentioned in thread408-126483...)

Hope this helps, 'NGL