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.