Safety Relief Valve Connectors w/ flexible joint 2

[UtilityLouie]

I've just run across some of these in a power plant - but have never specified or installed one. I think I understand what they are used for - the flex connector isolates the relief valve discharge piping from the valve itself both from thermal movement and movement due to force.

Is there a general guideline that should be used in applying these items? My personal feeling is that the relief piping on at valve set at 15# with a low flow wouldn't need one, but there must be some number where it makes engineering sense to use them.

 

[NozzleTwister]

UtilityLouie,

What kind of flexible joint? An expansion joint or flexhose?

Intalling a weak link like this in a relief sysytem doesn't sound like a good idea to me. The higher the pressure, the more I would advise against it.

If you do do something like this, you piping on both sides of the flexible connection must be very well restrained.

My 2 cents,

NozzleTwister
Houston, Texas

 

[UtilityLouie]

The items I was trying to ask about are on the attached pages. They are some type flex joint, but they appear to be beefed up and restrained.

http://railmex.com/proinf/bej/svc/svc-w.html

OR

http://www.hyspan.com/relifeConnectors.html

I've never seen them before - installed or sold to me.

 

[NozzleTwister]

UtilityLouie,

Thanks for you quick reply.

I've never seen these either. As a pipe stress engineer, I know I would have a hard time getting the project to accept using something like this except as the last resort in a low pressure application. Additional drain piping/tubing would also be requird to drain pocketed liquid.

NozzleTwister
Houston, Texas

 

[EGT01]

The way the Hyspan picture looks, seems like this is just a more elaborate version of the safety relief installation found in ASME B31.1 (power piping code) with a drip pan elbow.

http://www.conbraco.com/products/safety/techtemplate_1.asp?pid=Drippan

I suppose the bellows provides an additional measure of containment to address the potential for blowback where the smaller pipe is inserted inside the larger.

 

[JimCasey]

It is necessary to consider the reaction force when a Safety valve opens. Many critical valves in a power plant have dual outlets to cancel the reaction force, or at least if multiple valves are on a header the outlets are alternating on opposite sides of the header. There have been cases where entire pipes were torn loos by reaction force, with tragically fatal consequences.

With the bellows connector shown, it appears that blowback would be avoided. The stack would need to be supported much like a drip pan stack, but free to follow the valve outlet as the header jumps around. Also, if would be highly prudent to provide a positive drain so the outlet of the valve could not fill with condensate/rainwater.

 

[StressGuy]

I must confess, I'm having trouble figuring out what that is supposed to get you. With the bellows as shown, the valve inlet connection is going to see the full bending moment due to the pressure thrust of the valve outlet. The bellows is going to stretch some amount as the inlet connection rotates under the applied moment. You are going to get some serious stresses on the inlet pipe if you have much pressure thrust.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[UtilityLouie]

My sentiments exactly StressGuy.

I look and look at these and can't figure out what exact problem they solve. I'll have a flash of an idea, but when I put the logic to it - it doesn't pan out.

What I've found is that the plant engineers where these are installed - think they are required on all PRV's - because they are installed on their existing PRV's. I'm trying to work through with them why they are not needed - and that we just need to support the line properly.