ASME Section VIII Div I U-1(d) Exception

[MechE2010]

Hi,

I have read some threads in here about the minimum pressure requirement on Section VIII Division 1. However, I am still confused as to when it is acceptable to design a vessel per div 1 for a pressure higher than 3,000 psig. Many of you have said that you have seen vessels stamped with div1 for higher than 3000 psig but that an experience engineer should be consulted...well I did...but I am still confused.

We have designed a vessel per Div 1 and the criteria is as follow...12"OD Schedule 160 Helium vessel with a MAWP of 3,992 psig and ambient operating temperature. The Authorized Inspector wants us to justify the use of Div 1 at this pressure.

I spoke to a senior engineer and he basically told me that since we have no other loading (no occasional, no nozzle load, no thermal gradient, etc) other than gravity (which should relieve the circumferential stress) and internal pressure that it would be OK to use div 1. However, doesn't div1 allow you to take into account other loadings? I think understand the analysis differences between div1 and div2 but can't wrap my head around the div1 limitation when pressure is higher. And...what max pressure can you get away with using div 1?

In advance, thanks for your help and response.

 

[jte]

Never heard of a 12" OD pipe...

Gravity relieves the circ stress? Huh?

Yes, Div. 1 not only allows other loadings, it mandates that they be considered.

I'd start by looking up thick shell theory and equations. If you have a copy of Roark's Formulas 6th edition see 12.6, 7th edition see 13.6. In a nutshell, with thin wall theory the thru thickness radial stress is neglected. Thick wall theory includes that stress. The "nuisance factor" of -0.6P in the Div. 1 cylindrical shell formula is allegedly there to account for this.

Compare the Div. 1 results with those derived using thick wall equations. Let us know how it works out...

jt

 

[MechE2010]

Thanks for your response jt.

I meant 12" nominal diameter.

 

[jamesl]

There is no problem to use Div.1 code on this vessel. I have seen much larger Div 1 vessels close to 5000 psi design pressure.

Before the Div 2 code 2007 edition, the typical way to design high pressure Div. 1 vessels was using Div.1 allowable and applying all material testing and NDE requirements in Div.2.

In your case, I recommend checking your design against latest Div.2 code and make sure it complies, then present your findings to AI.

 

[rneill]

You may need to consider the effect of long PWHT times on the thick walls resulting from the high pressure. Depending on the plate material used, the properties can degrade from the required PWHT times so some would specify that material properties have to be confirmed in a simulated PWHT condition.