Help with valves for a dewar

[paulraphael]

Hi everyone,

I'm going to be working with liquid nitrogen for the first time and have some questions about valves. We would have prefered to work with a non-pressurized dewar, for the sake of simplicity and safety, but meager budgets led us to a used Alloy Products 304 stainless pressurized one.

It appears to be in good condition, but as a matter of principal I don't trust the valves on the thing, especially the relief valve.

Can anyone recommend a good source for these parts? I'd prefer a place where a knowledgeable sales person could talk to me on the phone, patiently, as if I'm a 2 year old.

We would like to get an appropriate pressure relief valve, some kind of fail-safe relief (a blow off valve or equivalent), and a gate valve or something we can use to dispense LN2 into a smaller dewar.

As an unrelated question, something strikes me as odd about this dewar: it's not insulated. Is this normal? What's the idea behind a single-wall stainless LN2 dewar? Its got about 18L capacity and is rated to 140psi. How long might the LN2 actually stick around in something like this? And wouldn't it get encased in ice from condensation?

Thanks in advance for any thoughts,
Paul

 

[iainuts]

Hi Paul,
Just to clear up any misunderstanding, the term "dewar" is used to refer to a vacuum insulated container which is often pressurized. James Dewar invented it back in 1892.
Dewars today can be found with maximum pressures of 0 psig to about 500 psig.

http://en.wikipedia.org/wiki/Dewar_flask

So you may want to verify you have a "dewar" and not simply an uninsulated, stainless steel vessel of some sort. Here's a decent web page that goes over the basics and provides a few pictures:

http://www.industrialgasplants.com/storing-cryogenic-gas.html

If your container isn't insulated, I wouldn't use it except for extremely brief periods of time and only outdoors in a well ventilated area, otherwise you'll be asphixiated by the gas boiling off. The nitrogen gas will kill you very quickly.

Regarding valves for these things, the main manufacturer for valves is Rego:

http://www.regoproducts.com/cryoflow/

They also make relief valves. However, I'd recommend also Generant for relief valves.

http://www.generant.com/home.htm

In my opinion, the Generant reliefs are better. Both Rego and Generant have very inexpensive products.

 

[paulraphael]

Thanks iainuts.

If it's helpful, here's a link with pictures and a brief description.

http://www.inxs-inc.com/cgi-bin/menu.cgi?a=view_prod&id=4631

The one I have is a larger size, and doesn't have the ports plugged up with bolts, but is otherwise identical.

If this is not actually suitable for LN2 I'll be unhappy.

 

[iainuts]

Don't worry about it. This is a typical, if not a bit small, vacuum insulated dewar. You should be happy with this one.

 

[paulraphael]

I guess I wasn't clear ... it's just like the one I have and it's not insulated. Just a single wall of stainless steel. It surprised me when it showed up ... I was expecting it to be insulated.

 

[iainuts]

If the vessel you have is small like this one and it isn't vacuum insulated, boil off will be huge. You could put it inside a crate packed with insulation, but even then the heat leak will be at least an order of magnitude higher than the vacuum insulation. If you're planning on keeping liquid in it overnight, it probably won't work even with lots of fiberglass or other insulation wrapped around it.

 

[paulraphael]

I'm wondering if the thing is mislabeled, and isn't supposed to be an LN2 dewar. I've seen used ones in a few places online, and they're all called LN2 dewars. Maybe it's for very short term storage? I'd still think handling it would be an issue.

It's supposed to pressurize to 140 psi (close to 10 atmospheres). I'm assuming that won't raise the boiling point enough to make a huge difference ... is this correct?

 

[dcasto]

When you fill the container, the temperature will be -320 F. Once it is sealed and heat starts entering the liquid N2, the temperature and pressure will increase. At 140 psig, the N2 will be -270F.

The tank looks like it has vapor and liquid draw off lines.

 

[danthemeterman]

Paul, I use the following company for Rego LPG equipment. They are a verygood distributor. If they can't help you, they'll tell you where you can go. I hope this isn't out of ET's rules of use.

http://www.lpgandnh3.com/contactinformation.htm

 

[paulraphael]

Thanks Dan.

At this point I'm strongly considering getting rid of the dewar. I'm afraid that being uninsulated, it's not going to be useful (and may eveen be dangerous, as ianuts suggests). I need something that wouldn't lose more than 25% over the course of several days.

It also seems to me this thing would get encased in ice from condensation, and be difficult to handle even if it didn't.

Unless someone thinks I'm wrong about all this, I'm going to look for a new owner for it ... someone who has use for a pressurized, uninsulated LN2 tank.

 

[EdStainless]

What makes you think that this is uninsulated? It should be all SS and double walled with a vacuum drawn on the space between them. You need to check to see if there is a good vacuum on this one. You will need a thermocouple vacuum gage to check.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[someguy79]

The link you sent shows something that looks to be a vacuum insulated dewar with an ASME code stamp. If you're quite sure it is not insulated, it's no good to you or anyone else as an LN2 container.

If you're not sure, get a hold of the manufacturer or use Edstainless's suggestion.

As far as valves go, you have to be much more specific. There are many manufacturers who can supply a decent valve for use with LN2, but the size, type and pupose must be known to be of much help.

For relief valves, I've had good luck with Farris and Anderson-Greenwood. However, we did all the sizing and selection in-house and just ordered the parts. For an 18L container with good insulation, most relief valves are going to be bigger than you really need. Look into API codes 520, and 521 for guidance. Some suppliers may be willing to help too.

 

[paulraphael]

Well, the walls seem in every way to be a single layer. And when I put hot water into it, the outside immediately got hot.

In other words, more like a beer can than a coffee thermos.

 

[unclesyd]

As stated above chunk this piece of _________ and contact your LN2 supplier and rent one. They are very reasonable and will do the job. If they don't have one small enough get their smallest as they hold for long time.

Heed warning about N2 in a closed space. All our Dewars in the labs and shop are vented to outside.
Take a look at thread725-217421 about Argon gas in a confined space. Though this one is about Argon, Nitrogen will do the number a lot quicker.

Personal addenda:

I've help recover two bodies from tank cars where N2, due to O2 depletion was the killer. I damn near had to fight to prevent two other people from rescuing their coworkers. It was real bad as one of the victims was his brother.

More personal I got a coworker, our group, out of vessel just in time as the N2 was building up very rapidly. He was unaware anything wrong and a little giddy as he was sent to medical. The doctor later told me that he had about 1 minute left on his clock when removed. This was on may way to lunch. Lunch was very enjoyable that day.