Severe Cryogenic Cycling LNG Service 1

[tdhaberk]

I need some help. My company manufactures LNG fuel dispensers. The service is fairly severe as temperature fluctuates between ambient and -220°F. Think car pulls up, activates system, system goes cryo, car pulls away, system slowly returns to ambient. We have swing checks in the return line that have started to leak from the cap to body connection. Originally, they were supplied with a TFE sheet gasket and B8 class I bolting. After the problems, we discussed the issue with a gasket manufacturer and they advised us to use a Gylon material and upgrade the bolting to class II. We are uncertain if this will work. Does anyone out there have experience in severe cycling cryogenic service? Any help would be much appreciated.

 

[iainuts]

Hi tdhaberk. Welcome to the board... Bolting is generally not in good thermal contact with the parts they bolt together. If that's the case, I would agree that changing gaskets or bolts isn't the best solution. Generally, the bolts don't get as cold as quickly as the body/gasket so there is a loss of tension during cool down and the gasket leaks. If your gasket leaks during cool down but, if left alone, seals up after some short period of time, that's what's going on. The gasket has lost compression when the body cools down and shrinks but the bolts may 'catch up' as the entire valve comes to thermal equilibrium. It isn't uncommon to see the leak go away after a while as the bolts cool down. It could also be the body is made from a different material that shrinks more than the bolt material in which case, it might not stop leaking when it's cold.

Some pictures might help too...

One solution is to add belleville washers. They should be sized so that they just come flat when the proper bolt torque is applied.

A much better solution is to use cryogenic check valves that have no body seals. You'll find a few poppet style valves where the poppet is actually inserted into the body through one of the ports. That eliminates a body seal altogether. Pipe threaded ends are common and work well. Take a look at Generant and Check All Valve.

http://www.generant.com/

http://www.checkall.com/

 

[bcd]

Graphite will be more dimensionally stable at cyclic temperatures that PTFE based materials. Coefficient of thermal contraction of PTFE is several times greater than graphite and metal. If you don't change the valve type, you should at least use a graphite seal with adequate bolt loads.