Liquid nitrogen transfer into nitrogen purged enclosure

[abedmcnulty]

I am trying to build a system to transfer liquid nitrogen (LN) into a roughly 6'x4'x2' sealed nitrogen gas-purged enclosure. The LN is in a pitcher-style dewar like this http://static.coleparmer.com/large_images/03773_51.jpg but with an open top, and the container I'd like to transfer it into inside the enclosure will also be open-topped. The enclosure has several threaded holes (1-1/4 NPT) at the top I can use for my transfer apparatus, has a relief valve for overpressure, and is under a continuous flow of nitrogen gas on the order of 30 SCFH.

Ideally I'd like to do this as simply as possible, something like pouring the LN slowly into a funnel leading into a wide tube, which will allow the excess nitrogen gas generated by the LN boiling off to flow up the tube and into the room. I know I need some kind of valve at the wall of the enclosure, preferably something that will just open under the weight of the LN, without allowing oxygen into the enclosure, which should be maintained at ppm levels of O2. Any thoughts on what kind of valve I can use to do this, or if this plan is even feasible? Thanks!

 

[Baluncore]

I would consider a flap valve, hinged on a single pin, with a counterweight to keep it closed.
Construction from stainless steel sheet.
LN2 poured down the tube would pass this valve, then it would swing back up to close the pipe.
The fit does not need to be good since venting N2 will flush the tube.

Take a look at the flap valve at the top of truck's diesel engine exhaust stack.
You need an inverted one of those. Come to think of it, they are available in stainless.
Re-purpose one by inverting it and increasing the counter weight.

 

[etudiant]

Do you even need a valve given that the N2 will be boiling off at a fair clip and the pipe is pretty narrow?
This is a pretty sizeable volume (almost 50 cubic feet) of N2 and at room temperature, even a pretty decent dewar will have steady venting.