Pressure Test on Cryogenic Vessels

Removing all of the water would be difficult. As it is the tanks have to be purged with hot air (200 to 300°f) to evaporate any contaminates that condense out of the liquids stored within.

The military uses purge tables computed for the size of each tank. You have to observe the temp of the air at the outlet and depending on how hot you can get it is how long you have to purge the tank for. Higher temp shorter purge.

As I understand, the reason is water would be difficult to remove because the tank is not designed to hold water.

Drew

Drew is correct. water and cryogenic systems do not go together and it is very costly and time consuming to de-rhyme these vessels after hydro-testing.

Just a word of caution and you are probably aware anyway, pneumatic testing is very dangerous and must be very well controlled. The stored energy can be immense and any rupture potentially lethal.

Thank you for your answers!

But what would be the actual damage cause by having water in cryogenic vessels (presuming you use extrmemly low chloride content water for the hydro test, so chances of SCC in austenitic stainless steel vessels become very small)? If there are water in the cryogenic vessels, ice will form when the vessel operates, but how does that actually causes damage to the vessel by having a thin layer of ice inside the vessel? Because you are operating the vessel at such a low temperature, the effect of ice on your material brittleness is probably negligible.

I can only think of contamination of the product, but is this really enough of a reason to justify the usage of a more dangerous pneumatic testing instead of hydrostatic testing?

Any water would be a problem as the ice formed would block the cryo valves and you then cannot get the product out. If the Dew point is very low (if I remember correctly below-80C) then you should be OK but using water to pressure test is a no-no because it is extremely difficult to get to such low dew points. Further some of the gas applications require very low dew points e.g. inert gases for heat treatment applications that need dew points below -60C at least. Hope this helps.

Like David O says, even if you are not worried about contamination at cryogenic temp water ice is very hard and will damage valves strainers and such. All of my experience with cryogenic products was liquid oxygen (aviator breathing ox) and liquid nitrogen (various USAF uses). Contamination of either was a big deal so water was not an option at all. We did have a bad batch of nitrogen come in once, it had small ice crystals that clogged up my filters and ruptured a burst disk on the tank when pressure got too high faster than I could vent it.

What is your cryogenic tank for?

Drew

Unless a tank is used for water, pressure testing with air or an inert gas such as nitrogen is best. As mentioned this keeps any moisture out of the tank. If moisture gets in you have to pull a vacuum on the tank to remove all moisture. Heating a tank that has moisture already inside will not assure removal of the moisture. Also when you pressurize a tank the air inside heats up. Depending on the type of testing being used, such as pressure loss over time, moisture can cause false readings with the test equipment. There is a method to the madness of testing tanks and vessels that hold or contain liquids or solids. When testing tanks moisture is bad, unless you are hydro-statically testing them

Heating a tank will not ensure all water removal. When I purged my liquid oxygen tank, I used heated air blowing through the tank. Once I achieved a good outlet temp I would open and close the pressure build-up valves forcing the air to circulate throughout the piping of the tank ensureing all moisture/contaminent removal.

We were required to do periodic purges for purity of liquid oxygen. We received our oxygen at 99% purity, but because some impuritites have a lower boiling point than LOX they would accumulate as we dispensed our product. The only way to be sure we removed all contaminates was to purge the tank. Our purge unit discharged air at about 300 to 400°f and on a 5000 gallon LOX tank I could get an outlet temp of almost 200°f. After a predetermined time (depending on max sustainable temp) we would close off the tank leaving a positive pressure in it to make sure it didn't suck in any moist air when opened to fill. A tank could be left in storage like that until needed when the seals would be broke and the tank cooled as product was slowly introduced.

I might have pictures somewhere if anyone wants to see.

Drew

NEVER USE WATER TO PROOF TEST A CRYOGENIC VESSEL!!!

1) The vessel IS NOT designed for the weight of water

2) Ice WILL damage your valves or pumps

3) Moisture is extremely difficult (nearly impossible) to remove even using missile grade air or nitrogen

4) Now you must purge/pressure vent the vessel numerous times to remove the air or GN as BOTH will freeze in a Liquid Hydrogen or Oxygen vessel. Air (the O2) component will freeze in a Liquid Nitrogen storage vessel

5) Using gas to pneumostatic test the vessel can be done safely using relief valves of the proper size and setting.

Thanks for the reply all!

The info provided was very helpful!