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Material Selection of Ethylene Storage Tank

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Jack Nicholson

Jack Nicholson

Chemical
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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?
 
 https://files.engineering.com/getfile.aspx?folder=fc6ddf2e-2080-4190-809c-979a04c85933&file=Ethylene_Storage_tank.png
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The designer is taking a punt on the definition of minimum allowable temperature (MAT) - see API 579-1, 1A.56. Noting something is not the same as demonstrating something. What is the owner's policy towards prevention of brittle fracture? Most operators in design phases adopt the minimum metal temperature approach, i.e. the lowest metal temperature is taken as the minimum design temperature regardless of the pressure existing simultaneously. Generally, it works until scenarios throw up expensive materials; that's when the fancy calculations start.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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Thanks Jones.
Owners policy: its obvious! No brittle fracture.

I couldn't fully get your point. LTCS cannot stand - 89 c temperature, so did the designer something Wrong???
 
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You may want to consider posting on the "Storage Tank Engineering" forum for a more specialized set of eyes on your problem ...

Not many new storage tank spheres are built these days .... It would be be very interesting to discuss your reasons for your choice of a sphere ...

Vacuum insulated liquid Ethylene Storage Tank are commonly available in different sizes and configurations for smaller volumes. The inner tank is stainless steel and the outer is low-temperature carbon steel with vacuum and perlite in annular space. Both vertical and horizontal cylindrical tanks are used

Large Liquid ethylene storage tanks are designed as a single wall flat bottom tank without a roof, The outer tank is a flat bottom tank, and Roof is domed steel. The roof supported on the outer tank wall with a suspended deck for the Insulation.


See page 10 of this document




MJCronin
Sr. Process Engineer
 
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@Jack
So the owner needs to be presented with the technical justification of why steel is considered acceptable at that temperature.

Steel can "stand" -89 deg C if the stresses it experiences at that temperature are sufficiently low. Which takes us back to the point made in the paragraph above. As stated previously, many operators would set the minimum design temperature as -89, equating it to the minimum metal temperature. The design codes would do the rest.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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Well the data sheet is 16 years old so maybe things were done differently then.

Also it does say "Equipment" to be capable, it doesn't say "tank"....

Design temp is clearly written as -40. If whoever wrote the data sheet wanted the design temp to be -89 then they had the chance to say so.

I suspect that the issue is one of temperature vs pressure, so at 0 barg, stresses are a lot lower goes the argument. At 5 bar they are probably somewhere in between.

Also LTCS is rather vague.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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So....Jack........ What was your final decision ???... Tell us about your final tank design and selection of materials

Or will you disrespect us by dropping this thread ?

We may not answer any more of your questions if we are disrespected ....

MJCronin
Sr. Process Engineer
 
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Dear Mr Jcronin, I'm not disrespecting You! I'm just evaluating and reading more references. I can not tell anymore, when I don't have something to Say!
 
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I think the simplest choice is 304 ss, with no pressure specified it apparently does not need to be thick so material cost should not be prohibitive when all fab costs are considered. Using a tough conventional steel will require testing of starting material and weld zones for each type weld.
 
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