Dewar Conversion, LN2 to LNG 3

Tango:

If you are trying to store LNG at a saturated pressure of 50 psig in a conventional, Liquid Nitrogen (LIN) Dewar you are probably going to run into problems. All the Dewar containers that I'm familiar with were designed to operate with saturated cryogen at atmospheric pressure. Therefore, no attempt is made to handle or contain the subsequently, normally-generated saturated vapors once the cryogen is introduced into the Dewar. This is (was) not considered a hazard with LIN, LAR (liquid Argon), or even LOX (liquid Oxygen). However, any emission of saturated LNG vapors out of a vessel would be considered a very hazardous scenario. I wouldn't trust the operation unless I knew specifically that the Dewar was designed to withstand the MAWP of the LNG at the saturated conditions you propose.

The Dewar is a portable vessel. Even if you put a relief device on it, what are the safety hazards surrounding the relieving of the vessel while it is in transit or storage? I would treat this application with a lot of questions and even more creative thought before passing on it.

Perhaps you mean something else when you say "Dewar pressures are around 50 psig", but I have to go with what I read in English. Perhaps you could elucidate.

Thanks, Art. I was hoping I'd get your attention. The dewars we use here are DOT compliant, Code stamped, or Code equivalent for pressures up to 270 psig, complete with pressure regulation, relief and vent systems, and vacuum jacket insulated. They are transported on dedicated truck trailers. Our commodities are LN2, LOX, LAR, and Liquid Hydrogen. We need to convert some to liquid methane (essentially LNG) service for research. We're hesitating to convert LH2 dewars because of differences in weight of fluids, but LN2 should be an easy switch once we recalculate relief capacity and add vent/purge tubing.

I was wondering if there might be any material compatibility issues. Do the mercaptans in the methane odorant provide enough sulfur to attack stainless steel? The inner vessel of all the dewars is stainless.

Larry

Larry:

Thanks for the rest of the story. I’m a stickler for knowing all the basic data and scope and this one bears out my concerns. You are to be commended for the engineering scrutiny and detailed investigation you’ve done on this application. I’m hoping a lot of the veterans on this Forum jump in and contribute what I expect to be some valuable experience.

Yours is a classic example of how to start treating what would normally be considered a benign and un-interesting project. To a non-experienced engineer, the proposal makes all the sense in the world: saturated temperatures for the cryogens at atmospheric pressure are LNG = -260 oF, LIN = -320 oF, LOX = -297 oF, LAR = -303 oF , and LH2 = -423 oF. As you’ve indicated, the specific gravity of the LIN is above that for LNG and, the temperature is ‘warmer’, then the application of substituting LIN with LNG within the Dewar should be OK. HOWEVER, there is another constraint besides the temperature design and the mechanical hydraulic design. This additional constraint is, as you’ve noted, the chemical composition of the cryogen and its potential attack on the metallurgy of the Dewar – resulting in a weakened and possibly dangerous vessel. This, in my opinion, is a valid and important question to bring up and not proceed forward until it is resolved with full approval.

I would expect the type of Stainless employed in the Dewars is OK for handling the mercaptans and other sulfides that may be present in the LNG and I would also expect any chemical attack at cryogenic temperatures to be retarded – BUT it must be checked out and confirmed. I would not just assume that it would be OK. Any mechanical failure of the Dewar at -260 oF is simply unacceptable – which I know is your position as well. Any other metallurgy attached to the Dewar – like your tubing and relief devices should be checked as well. I believe you’re on the right track in challenging the proposal and asking the right questions.

Above and beyond the material compatibility issues, Art is right in questioning the capacity of the dewar to seal effectively. As you know, the relief valves (in your application set at 50 psig) need to vent. In a stationary application you might get away with running a line into your stack or outside (assuming you do the flammability calculations). Once you start jostling the dewar around during transport, however, it might want to let off some pressure, and that is where you are going to run into other problems. LN2, LAr, and LOx don't pose too many problems with this unless you're in a confined space. Just try to keep in mind that the dewar may be a little "leaky."

Hope that helps.

Worked once making Dewars. Not many tricks for LNG. Probably the greatest issue is the peripherials. Valves, pumps etc. Brass, bronze, aluminum confinements just is not good enough.

The single most important calculations you are working on. It can be confusing to calculate the capacity with density going up and the pressure goes down. And that I think can reverse for different fluids.

Once you get past your material compatibility issues LNG dewars are really straight forward to design and work well.

Stainless steels and mercapitans. To the best of my recollection these dewars had mercapitans added afterwards. The mercapitans, being a little heavier, will I think collect on the bottom of the tank. But my memory is starting to be a little fuzzy so do not go to the bank with that one.

Would suggest to get someone who has been through it all before to help. There are a number of issues to work through.

CNG is made from pipeline gas I think the mercapitans are simply compressed and do not settle out. This is an advantage to this technology.