Should I derate?
Should I derate?
(OP)
In calculating the t-min for a small propane tank it seems that the t-min is equal to the nominal. The vessel is designed for 250 psi @650 F which it would never see. It is made from SA-455. Can I derate this just by lowering the Max temp to 450F and attaching a new plate with this info or what is my other option? Any help is needed here.





RE: Should I derate?
If the vessel is currently below min wall, you would need to derate in terms of design pressure not temperature. This would require changing the set pressure of the existing safety relief valve to a lower value if you derate. The 650 deg F temperature limitation only implies that the operation of the vessel is below time dependent effects on allowable stress (no creep). I would review API 579 just to be sure that you even need to derate this vessel. Also, review this information with your Inspector.
RE: Should I derate?
RE: Should I derate?
RE: Should I derate?
I guess the question to ask at this point is what is the stated corrosion allowance on the U1 form?
If this is a new (post 1999) vessel, then dropping the temperature would buy you some additional allowable stress (17.3 ksi at 650°F; 20 ksi at 300°F) which would translate into a bit of corrosion allowance.
Check with your jurisdiction / AI / insurance carrier as to how to handle the change. In my case, I'd consider it a rerate and work it through jurisdictional approval then proceed to have an alteration nameplate installed in addition to the original nameplate. I'd strongly suggest that you do not remove the original nameplate.
jt
RE: Should I derate?
jte-I believe the allowable stress for this would be 18.8 @650F based on the thickness of .271"
RE: Should I derate?
RE: Should I derate?
RE: Should I derate?
When was this propane tank fabricated?
RE: Should I derate?
RE: Should I derate?
Your numbers are correct. The min thk is 0.2712" according to my calcs based on Div. 1 A89 41" OD and 250 psi MAWP.
So... your choices remain the same:
* Talk to your AI and jurisdiction and insurance carrier.
* If the jurisdiction allows, you may rerate using post-'98 allowables which help you if you can lower the design temp. My jurisdiction "will not entertain requests" to approve rerates of pre-'98 vessels using post-'98 allowables.
* Trade some MAWP to buy more CA. This presumes that internal pressure governs, which is likely for such a small diameter vessel.
* If the MAWP is reduced, as metengr pointed out you'll have to re-set the pressure relief device accordingly.
jt