Valve questions for natural gas / digester gas shutoff 3

[USAeng]

Hey guys,

Sorry multiple questions here...

Background: anaerobic digestion plant, natural gas 10PSI, digester gas 5PSI

We have some equipment at our plant that came with butterfly valves for digester gas shutoff. Seem to work great.

For field installed piping- all of the valves spec'd by the AE were plug valves. One option for small valves 2" and under was lubricated plug valve. The other option for anything 1/2"-12" was non lubricated resilient faced plug valve.

I have read that plug valves are "inherently fire safe".
Why is a plug valve fire safe?
Is this true for all plug valves or just some?
Is there a certification for fire safe plug valve?

The butterfly valves are very small size in comparison in the larger pipe size and are easier to operate. Is there any reason to stay away from butterfly valve for natural gas or digester gas shutoff?

Does anyone know of a rule for using a plug valve ONLY in gas line prior to boiler?

Thanks for any input!

 

[LittleInch]

This post gives you some idea of the mess surrounding what is isn't "fire safe"

http://www.eng-tips.com/viewthread.cfm?qid=299055

Plug valves are not much different from any other in terms of being better or worse in the event of a fire.

at your pressures, usage and size, I can't see anything wrong with butterfly valves. At those sorts of pressures you are probably better off with a valve type which provides some of the sealing itself and not relying on pressure to force the seat onto the plug or ball or the other way around. Wedge gate valves and butterfly valves essentially provide much of the sealing themselves.

I don't know of any "code" or regulation which specifies a type of valve to be used, but some would specify that you have an isolating valve and maybe that it should be actuated

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zdas04]

My rule is to never use plug valves for anything at all ever. My reason is that I work in raw field gas and even the so-called non-lube plug valves will fail to operate if not exercised frequently (and in Natural Gas Gathering we have valves that are only operated once a decade). If a plug valve sits in gas with water, condensible hydrocarbons, H2S, CO2, coal fines, frac sand, formation sand, blood, guts, and feathers for a few years, the chances of it either operating or sealing if you can make it move become very low. Non-maintained valves that are infrequently operated should not be plug valves. The idea that plug valves are "fire safe" comes from the metal on metal seals that the valves traditionally had. Resilient seats (like in a ball valve) tend to melt or deform in a fire and reduce the effectiveness of the seal. As plug valves have moved away from strict metal to metal sealing, they can no longer be called "fire safe".

Butterfly valves are inexpensive and do an OK job in low flow-rate service. Much above 10 ft/s in gas and I worry about the vortex-shedding characteristic of a bluff body in the flow stream breaking the flapper off. I've seen this happen 3 times and losing the isolation capability is small beans compared to the loose flapper lodging somewhere inconvenient. I just wouldn't risk it even if I didn't always consider that any gas flow line will eventually need to be pigged and the flapper makes a pig run problematic.

For in-stream isolation service I always use ball valves, and for normal operating pressures under 25 psig I tend towards trunnion mounted ball valves (so that I always have activated seals and am not relying on very low dP to shift the ball to the downstream seat). Ball valves tend to have prices very similar to plug valves which are much higher than butterfly valves.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[USAeng]

Thank you! much appreciated

 

[gerhardl]

I fully agree with both postings above. However, for information, an excentric plug- valve with (correct type) resilent sealing will give very good sealing, but I would not prefer that for the plant.


Full bore ball- or butterfly valves would be my choice for smaller valves; resilent seated BFL, preferably double eccentric and/or high quality, for higher dimensions.

Prevention of explosion by (forced) venting of surroundings, and explosion relief (bursting discs) for pipelines should have priority, as an explosion is more likely to occur than a fire.