weld neck flanges in between 2 butterfly valves 3

[Journeyman1972]

Is there anything wrong in having weldneck flanges welded back to back and placed in between 2 butterfly valves? the space in between the flanges ranges in between 3 to 5 inches.

 

[Zapster]

Most butterfly valves are not bubble tight. If this is for liquid service then it is up to you to ensure that there is no possible way for bubble tight valves to be installed unless you provide thermal pressure relief.

 

[Journeyman1972]

thanks zapster. But i'm not sure I understand. I got the butterfly valve part but what's being bubble tight anything to do with the flanges?

 

[PatBethea]

Journeyman,

I've done this with low- and high-pressure applications, with both liquids and gas and never had a problem.

 

[RossABQ]

If the valves were bubble tight, and both were closed, there would be trapped liquid or gas. Any change in temperature upwards would cause the pressure in the trapped gas/liquid to build, possibly above the ratings of the pipe/flanges/valves. Not very likely but a proper design consideration.

 

[zdas04]

I don't even want to ask why you are putting two butterfly valves back to back, I'm sure it makes sense on some level.

I'm pretty sure that there is enough offset within a RFWN flange that the flappers wouldn't contact each other so that isn't a problem.

Welding two flanges back to back is not illegal, immoral, or counter to any code that I know of, but if I was doing it, I'd put a pup between the flanges to have a place for a vent/drain/sensing point, but that is just because you can't ever have too many vents/drains/sensing points.

David

 

[JLSeagull]

Consider the potential for the butterfly to extend through the flanges and interfere.

 

[legs]

Unlike in former days of many years ago, Butterfly valves are now bubble tight and are used by most industries. As long as the flanges do not interfer with the flapper, there is no prob using flg to flg and it is frequently done.

tks!
legs

 

[stanier]

Have you considered using lap joints with backing flanges.(MSS SP43), This would enable field fit up and a connection for a tapping ppint if considered necessary.

I presume that you have two butterfly valves as one is for control and one maintenance isolation or for dual isolation for safety reasons. More detail would help.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[Gator]

Physical operation - can the discs interfere with each other when both are fully open?

If it's process-related and Geoff is right with his guess about isolation, you might need a vent/drain between the two depending on the fluid/temperature/fluid (or any combination). Does the control valve (if it exists) fail closed?

Paul

 

[STYMIEDPIPER]

Gator
Good point about the vent and drain.
I just realized that I recently comnpleted a project which resulted in the very scenerio that you have mentioned which required a vent and drain between the valves