Looking for assistance with vacuum insulation of a cryogenic storage vessel and VJ piping

[jtylerb72]

I work with ASME Section 8 cryogenic storage tanks. I am interested in finding some folks to converse with about vacuum insulation between the inner and outer vessel of a cryogenic storage tank. These tanks would be insulated with either MLI and vacuum or perlite and vacuum. I am interested in learning more about perlite and identifying moisture issues as well as drying out moist perlite. I work with vacuum jacketed piping as well and I am working to learn better ways to design and install VJ piping as well as establishing a high vacuum level on said VJ piping. Thank you in advance.

 

[csk62]

Mli installation is a learned trade in itself, and most cryo fabrication facilities are protective of proprietary techniques. I've seen spiral wrapping on inner vessels, but again techniques at transition geometries are crucial. If layers are not interleaved with sufficient accounting for metal dimensional shrinkage during cooldown, then heat transfer may not be ideal. High temperature bakeout with multiple vacuum pump and purges to remove moisture is may be necessary to achieve sufficient vacuum pressures.

Perlite is easy by comparison and still used today in large bulk tank installations.

 

[IRstuff]

MLI takes a LONG time to bakeout, particularly if it's been sitting out for a while before the vacuum space was pumped down. Lots of nooks and crannies for "stuff" to get adsorbed.

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[jtylerb72]

I completely agree with the MLI and perlite comments. We keep the MLI in a heated storage fridge. We use a line heater and a vacuum pump to dry out the MLI upon installation. We get things dry and ready to ship in about five days now. We have made some progress here. Most of the bulk tanks we deal with are indeed insulated by perlite and vacuum. I don't know where to start to try and find some technical experts from the cryogenic vacuum world. I would guess there are only about 50 people like this in the whole country. We have been toying with the idea of slow purging the annular space of these tanks with dry NF grade nitrogen. We often break back the annular vacuum to almost atmosphere and then throttle the vacuum pump to prevent any moisture from freezing out before we can pump it out. I need to run some theories by someone with regards to moisture in perlite, getter options, etc. Any help is appreciated. Thank you for your support.

 

[georgeverghese]

In the air separations plant I used to work at, the entire free space in the cold box was filled with perlite and continously purged with dry N2. The cold box is kept at a slight positive pressure - we have never had issues with moisture in perlite. Every few years, perlite level is topped up to account for settlement. The HP and LP columns in the cold box were operating at -160degC and -190degC. There are some design details on calculations for heat loss in perlite filled vessels and vac insulated vessels in Perry Chem Engg Handbook 6th edition.

 

[EdStainless]

We had to do major repair on a liq Ar tank once. The field service guy basically said 'Watch this' and he cut a few days off of the vac purge time. He brought a trailer in to provide N2 to work with and for 24 hours purged the perlite with heated pure N. He set it up with inlet at the top and outlet at the bottom, but that was just so that he would have good distribution when he started pumping from the bottom.
Then be cut the N flow and kicked on the vac pump. He didn't try to pull a very deep vac just to sweep the space. After about 2 days of that (monitoring DP of the gas to the pumps) he shut off the N, turned on the other pump also and went to deep vac.
When he was done our boil off rate was a fraction of what it had been when the tank was new.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[jtylerb72]

georgeverghese and Ed, I am familiar with cold boxes and their nitrogen purge. The major difference between that application and a bulk tank is that the inner vessel is designed to withstand outward pressure, not inward pressure. So to purge the perlite, or the annular space in general, you would need to pressurize the inner vessel and also ensure that you don't pressurize the annular and cause the inner vessel to collapse. Evidently this has happened before so we avoid exceeding about 20 PSI in the annular. We obviously then need to take precautions and provide safety devices, flow restriction, etc. Often times tanks come in with very wet perlite because the outer vessel was cracked in the field, with the tank in service. With the inner vessel operating at about -250F to -300F and the outer cracked the cold inner acts like a cold trap and a lot of outside moisture gets ingested by the annular space and freezes to the perlite and anything else in the vacuum space. We are often left to deal with this sort of issue. I am looking for creative ways to reduce the time required to deal with these occurrences.

 

[georgeverghese]

From memory, we had maybe 1inch WG nitrogen pressure or so on the annular free space, nowhere near your 20psi. We would often call in a "maintenance painting" crew to paint the cold box skin to avoid corrosion. Once you've lost pressure containment on this outer skin, all bets are off. Why should you accept vessels with ruptured outer skins? It could take ages to dry out moisture held up in perlite deep inside the annular space.

 

[EdStainless]

The fastest way to remove moisture is with heated (any temp helps) dry nitrogen. I don't think that it really matters if this is done at a slight positive pressure or a slight vacuum. Watch the DP of the vented gas and as it drys out keep lowering the pressure.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[IRstuff]

Pressure helps in the sense of reducing the mean free path of the purge atmosphere, which makes it more likely nooks and crannies get cleared out. Usually, the issue is thermal limits on some of the materials involved.

TTFN (ta ta for now)
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[csk62]

Regarding industry standard numbers for vacuum pressures in the VJ space of cryogenic liquid bulk tanks in the warm condition (obviously in the cold condition these pressures should be lower), I have seen specified:

10 mTorr for MLI insulation
100 mTorr for Perlite insulation

This is absolutely crucial to reduce residual gas conduction heat leak into the liquid product. Especially for MLI, if these numbers are not met, your insulation effectiveness will be significantly reduced. Also your NER (normal evaporation rate) will be higher than your tank manufacturer competitors.

I have seen perlite baked out in 2 different ways, the first way with warm N2 gas (via trim heater) attached to the pump-out port on the bottom of the vessel and flowed up and out through the parallel-plate VJ relief orifice on the top of the vessel. Sounds similar to what EdStainless was describing. The second way was to cycle pumping and purging. The flow method usually works better.