Butterfly vlv.s to Knife Gates? 1

[11echo]

Need some help here please ...doing a project where the client wants to trade out butterfly valves for knife gate valves (S.Stl.). This is suppose to happen on a gas line, from sizes 6" to 16"-150# service. My problem is I'm not all that familiar with knife gates. The only time I've seen one used was on a surrey line, but with the exposed gate it doesn't seem like a good valve to use in this particular situation (gas service). The ONLY reason I have been told that we are using these valves is that they are a direct replacement for the butterfly valve (...i.e. they only have to bolt them up with no changes to existing piping). Anybody have any experience with this type of valve? ...Any opinion on this application? THX! ...Mark

 

[NGiLuzzu]

Mark,
sorry, I don't have any answer, at the moment... but just another question ;-)

Why must the butterfly valves be replaced? Is it a matter of weight and/or costs?
And... what kind of butterfly valves (rubber lined or metallic, high performance or tripple-offset, ...) are they?


Many thanks, 'NGL

 

[11echo]

anegri ...The client wants to replace the butterfly valves because they say the valves are leaking. There are approximately 15 valves (in varying sizes) that they want to replace. At this point I don't know exactly what type of butterfly valves they are or who the manufacture is. I have just been introduced to this "project" and I just feel that for sake of convenience the client is making a bigger mistake. Problem there is I really don't have the experience with the knife gates to make that call, hence my question. So any help you can provide would greatly be appreciated! ...Mark

 

[NGiLuzzu]

11echo,
I agree with your impression. I'm not axpert about this type of valves either, but as far I can see they're usually employed for "dirty" services: pulp and paper, slurries, etc...

Just take a look to a brief list of Manufacturers: may be it helps getting an idea of the animal ;-)

http://www.redvalve.com/Knife_Gates.html

http://www.velan.com/products/catalogs/kg.htm

http://www.thomasregister.com/olc/73231680/knife2.htm

http://www.pulpandpaper-technology.com/contractors/valves/orbinox/

I think the valves that leak are simple or "high performance" butterfly valves... if it is so, how about tripple-offset metal seated valves?

Bye, 'NGL

 

[jet1749]

I have only used knife gate valves on wood pulp slurry service ie for paper making and viscose fibre production, in which they proved ideal. I wouldn't think that on a clean gas duty they would be the valve of choice. I do however stand to be corrected.

 

[magicfrog]

Hi,

There is absolutely no reason not to use butterfly valves for a gas application, whereas knife gate valves are NOT a good choice. As indicated by jet1749 before, knife gate valves are good at "cutting" through media, and do not have a very low leakage rate as you might need for gasses.
The question of the selection of type of butterfly valve and materials is what kind of gas it is, at what pressure, what temperature and if it is used for control or just on/off.
In general rubber lined BFV's are used up to 16 bar and 100°C - any higher and you'll get in the "high performance" type BFV ranges, usually with reinforced PTFE or metal seated valves. Please note that also these valves can have a "zero" leakage rate (eg. the tripple eccentric valve Anegri talkes about) for valves up to 1500# and 28". Now we're talking about some big valves...
Resuming: I strongly suggest against knifegates in gas applications. Use BFV's following the process conditions for this specific application.
Good luck, Magicfrog

 

[jeffvalve]

11echo,

Whereas I agree with the prevoius contributors that the knife gate valve is primarily used in the paper & waste water industries, there are specialist manufacturers who make 'wafer pattern' bonneted knife gate valves. These have a conventional rising stem/gland packing arrangement. Also, a resilient body seat to give bubble tight shut off. We have used them in the past on Hexane vapour duty and they performed well. The valve are not inherently firesafe and are not manufactured to any particular dimensional or design standard ( dont think there is one for knife gates anyway)...

Jeff

 

[NGiLuzzu]

Another reference about knife gate valves:

http://www.knifegate.com

They seem specialized in slurries, too.

Bye, 'NGL

 

[JimCasey]

I'll throw in my 2 cents worth: As explored earlier, knifegate valves are good for streams with entrained solids. The seats MAY be provided with a resilent seal ring and you MAY get good shutoff initally. HOWEVER, the packing arrangement of a knifegate provides a VASTLY greater possibility of leakage to the environment than a butterfly valve. Butterfly valves rotate a relatively small stem 90 degrees through the stem seal, but knife-gates drag the entire gate through the packing by a distance equal to the pipe diameter. They leak. Most people really DON"T want a lot of free natural gas wafting around in their plants. It's dangerous on several levels.

Use a good double-offset (High-performance) butterfly valve. They have good shutoff, good seat life, and almost zero environmental leakage. Two models are Durco BX, Tyco Keyston K-lok, and there are many others.