Should I derate?

[jfoe]

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.

 

[metengr]

Is the reason for your question to evaluate corrosion pitting damage or some other wastage that resulted in the measured wall thickness of the propane tank falling below nominal wall thickness, which is min wall?

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.

 

[jfoe]

The reason I bring it up is because I am confused as to why the nominal stated on the U1 which is .271" is what I get when I do the t min calculation. The actual readings taken were between .270"-.276". It seems to me that there is very little wall loss yet the t-min calculates out to be the same as the original thickness. Maybe I am using the wrong calculation? I was using t=PR/SE-.6P I wanted to calulate the remaining life of the vessel based on the corrosion rate. Maybe you can help me determine based on the following info: 3'5" OD, E=1.0, MAWP=250psi@650F, Material=SA-455 and Nominal Thk.= .271

 

[codeeng]

Assuming no original corrosion allowance used then this is correct. Thats what MAWP means - its aleady been max'd out and registered this way. The 650F implies pre 1998 edition, again the temp was probably max'd out. If you want more corrosion allowance you'll need to rerate to a lower pressure.

 

[jte]

jfoe-

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

 

[jfoe]

The corrosion allowance on the U1 is O. It was built in 1991 to 1989 AD.
jte-I believe the allowable stress for this would be 18.8 @650F based on the thickness of .271"

 

[metengr]

According to my calculation, the minimum wall thickness of this vessel using the 2004 Edition of the ASME B&PV Code, Section VIII, Div 1, UG-27 is 0.294". This assumes your stated dimensions and an allowable stress value of 17.3 Ksi at 650 deg F for SA-455 plate material.

 

[jfoe]

Again I believe the stress should be 18.8 ksi@650F for the nominal thickness of .271" Let me know if I am wrong.

 

[metengr]

jfoe;
When was this propane tank fabricated?

 

[jfoe]

1991

 

[jte]

jfoe-

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