API 620 Temperature Limits

[b8]

Why does API 620 only go to 250 deg F? For a low pressure tank (5 psig) designed to 320 deg F, API 620 still seems like the best fit (this doesn't fall perfectly into any code). Since the allowable stress for 320 deg F is not significantly lower than that for 250 deg F can the API 620 equations be used for tank design? Do I need to use API 620 with API 650 appendix M?

 

[JStephen]

ASME Section VIII could be used.

 

[b8]

Thanks for the reply, but wouldn't ASME VIII be overdesigning the tank? Why couldn't API 620 design formulas be used as long as the allowable stresses don't exceed those listed in ASME VIII for the same material at the design temperature?

Why does API 620 limit itself to 250°F?

 

[JStephen]

My best guess on the limitation would be that the applications for which the standard is used generally don't go that high. The main products for elevated temperature storage are either asphalts or sulfur, both stored near atmospheric pressure.

It is not uncommon to have situations where an item doesn't fall completely under any one standard. If you don't have a customer requirement or building code requirement to comply with a particular standard, it would be at the designer's discretion as to how to handle the situation. Common examples are welded steel silos and bins, which are built to tank standards "to the extent applicable."

 

[JStephen]

I'm not sure why it would be used as a limitation, but the 250 degrees is also the pressure of saturated steam at 30 psi absolute, ie, the maximum pressure allowed under API-620.

 

[Duwe6]

I would still take the material strength derate from 650 Appx M. It is negligable, so it should not change your material type or thickness, and using it shows "due dilligance". You always want to be able to show that you considered all the design conditions and engineered the job based on those conditions. Especially when you are using a temperature higher than the 'generic' maximum one given in API-620.

 

[MJCronin]

The real question that seems to be avoided in the above discussion is:

"At what temperature does the designer no longer want to select a flat bottomed tank design ?"

Flat bottomed metallic tanks simply cannot expand in a uniform, predictable manner as compared to ther designs

API has, by fiat, determined that 250F is the maximum.

Aside from random management or contract insanity, there is nothing to prevent the tank designer from going to an ASME VIII Horizontal tank design......on two saddles.

Another alternative might be a more detailed engineering FEA approach to the design

Perhaps there is something in an early Jawad and Farr text...

 

[JStephen]

Actually, API-650, which is ONLY for flat-bottom tanks, allows temperatures up to 500 degrees F. But API-620, which is applicable to a variety of different tanks other than flat-bottomed, is limited to 250 degrees. So I don't think that's the issue at all.

 

[IFRs]

Look for the next 620 publication to have raised the temperature limits.

 

[b8]

IFRs,

Are you speaking from inside knowledge or just guessing.