Many thanks for the reply.
There is no plan to re-sold my client's real big cryogenic tank. It will just undergo its annual preventive maintenance but they don't have the proper procedure to decommission it. I know that handling cryogenic tank is a different ball game where extra care has to be followed slowly to prevent the possibility of internal collapse or any damage?
it depends on what's in them. But in any case, just let them warm up naturally without adding heat is the best. If they need to be taken out of service quickly, injecting 130F nitrogen in is acceptable too.
Do not get water or other fluids that have freeze points below their normal temps anywhere near them. We had an operations group once that injected methanol into our LNG system, they thought the metahnol would inhibit some water that had entered the system that was open to the atmosphere. Methanol turns to a solid around -160F.
That's the other warn, keep humid air out or be prepared for a recommissioning problem.
Many thanks again, dcasto. Actually, it's an ethylene and a propylene storage tanks that will be decommission for preventive maintenance with some repairs to be done. I understand your advice but it's too generalized. Appreciate if you have a more detailed procedure from your Operations group? For example, what are the pre-requisites before starting to remove the tank's contents?
My primary concern is safety. There might be some unforseen activity that could inflict severe damage to the tanks although warming them up naturally is the safest option?
Are these carbon steel pressurized storage (bullet) tanks for LPG? Cryogenic tanks gave the impression that you might have refrigerated storage.
For pressurized storage tanks, pump out all the liquid first. Keep HC vapor pressure on the tank until until all the residual liquid is vaporized to avoid brittle facture. Blowing (shutdown) or holding pressure (start-up) with plant N2 is a common but serious error since N2 partial pressure does not suppress the vaporization (cold temperatures). I created an FAQ sometime back on this subject: faq124-1702. If you see ice forming, you have some liquid heel and are going way too fast.
With propylene there isn't near as much danger of brittle fracture since your tank is probably good to -50F.
