3 replies to this topic

[h.eghbali]

Dear all.

Hello.

I have a question.

In attached process data sheet, design temperature of ethylene storage tank is -40 C, and depressurizing temperature is -89 C. But the designer selected LTCS (with minimum temp.: -45C) and noted that this material should sustain depressurizing condition!!! How could is this possible?

Attached Files

•  Ethylene Storage tank.png   234.47KB   3 downloads

[Pilesar]

Minimum design metal temperature is a function of metal stress. When the stress is lowered below design when depressured, the metal design minimum temperature can be significantly reduced. You can view a graph here: http://alturl.com/3dh93

The concept makes sense when you consider that the vessel walls are thick for high pressure. If the vessel were designed for lower pressure, the walls could have been made thinner. So when the thick walled vessel is depressured, the walls get colder but there is 'extra' thickness available to handle the new conditions. Where the problem would arise is after the vessel is depressured and cooled. If the cold vessel were brought back to design pressure, then the low temperature metal combined with high stress would be much more dangerous. Note also that the mean metal temperature of the vessel wall will not get quite as cold as the contents of the vessel. 

You do not have to use LTCS for ethylene storage, but it is a cheaper alternative than stainless. When some companies choose to use LTCS for this service after thorough analysis, they also should establish safeguards so that operations cannot quickly repressure the vessel after depressuring. Low temperature embrittlement of ethylene storage resulting in loss of containment could be a huge problem. Continue your investigation until you are satisfied the operation would be safe enough. ASME has specific procedures to verify low temperature design and it is not a simple matter.

[Bobby Strain]

Ethylene is at less than -89 C at atmospheric pressure. The data sheet indicates ethane storage. You should talk to your insurance provider, too.

 

Bobby

[Pilesar]

Bobby, the extra-cold atmospheric pressure is not always considered. Some major companies only consider depressuring down to a specific fraction of design pressure. I was surprised when I ran across that spec since I could imagine a relief valve perhaps freezing open. I don't remember whether the client spec I used so many years ago was 50% of design or some other mid fraction, but I don't think it was below 30% of design pressure. It made allowing vessel material of construction to be LTCS in a similar service (liquid ethane or similar) I did not really care for the spec then, but client specs must have gone through some sort of risk analysis to be written into the design basis. The client spec was detailed -- something in the form of 'consider the scenario for controlled depressuring the vessel to 50% of design pressure in a twenty minute span for determining minimum design metal temperature.'