ASME Section I vs ASME Section VIII Div.1

[CFDFEAGURU]

I recently rejected a code calculation because the flat head thickness on a 6" NPS Sch. 80 header was thought to be too thin. This calculation was for an unreinforced flat head per Section 1, PG-34 sketch g-1. The design has tubes that are within the distance of 2*sqrt(d*ts) and the value of m was not 1. I thought this was incorrect and the value of m had to be 1. It was pointed out to me by an A.I. (Authorized Inspector) that there was a code case from 1988 that allowed for a fully reinforced hole to be place within that distance and a value of m less than 1 code be used.

I was curious to see what Section VIII Div. 1 had to say about this. In that section this is not permitted unless rigorous stress analysis is performed or you built one to scale and test it until it fails under pressure.

The two sections are in total disagreement on this issue. Can anyone explain why the codes differ?

Thanks
Matt

 

[Astronuc]

How is each section applied? In other words, to what systems, and at what temperature and pressure ranges, and what materials?

 

[CFDFEAGURU]

The situations are identical. The temperature only affects the allowable stress. The allowable stresses for both sections are the same. The pressures are the same.

Per PG-34 in Section I or UG-34 in Section VIII, Div.1 the calculation for the flat unstayed circular head of geometry in sketch g-1 there is a constant C that is equal to 0.33m. If m is less than one and a hole is within the distance 2*sqrt(d*ts) according to section VIII, Div.1 rigorous stress analysis (FEA) or a full size test of the design has to be tested until it fails. However, in Section I, you are allowed to place a hole within that distance and no additional stress analysis or testing is required.

In both situations it is assumed that the hole is properly reinforced.

Thanks
Matt

 

[Astronuc]

Materials?

 

[CFDFEAGURU]

The materials are the same.

I will draw up a sketch for this problem tomorrow.

Thanks
Matt

 

[Q_Goest]

Hi Matt,
I'm not familiar with this paricular part of the code, and I'm sure you use the code much more than I do, but I'll toss this out for consideration.

The allowable stresses for both sections are the same.

That may be true for your particular material, but my understanding is that, in general, the allowables are different. Div 1 uses 3.5 to ultimate, Div 2 uses 3.0. However, often times, it's the yield strength that governs the allowable stress and I believe they both use 1.5 to to yield. Here's a decent summary of the differences between the codes:
http://www.absa.ca/faq/SectionVIIIcomparison.PDF

In general, Div 2 requires more analysis but allows higher stress which has the potential for vessels to use less material and thus they can be less expensive to manufacture, but to achieve this, there is more analysis required for Div 2. I'd assume the difference between the hole placement is due to the potential for higher stresses in Div 2.

 

[CFDFEAGURU]

Yes, the allowable stress values are different between Div.1 and Div.2 of Section VIII. However, like you pointed out, the analysis is much more indepth. In many cases FEA is almost always needed when designing to Div. 2.

This thread is about the differences between Section I and Section VIII Div.1

Basically, all that is happening is that Section I is allowing reinforced holes to be placed within a certain distance of a flat head with the value of m being less than 1 without additional stress analysis and that Section VIII Div.1 does not allow this to happen without additional stress analysis.

I think it comes down to differences between the code comittees and chairmen. Section I is very mature compared to Section VIII.

Thanks
Matt

 

[CFDFEAGURU]

For clarification in post #3 PG-34 should be PG-31.

See attachment for the sketch.

Here is an example calculation.

m = ration of tr/ts (m cannot be less than 0.20)

where tr = thickness required for pressure of seamless shell, pipe, or header

ts = minimum specified thickness of shell, pipe, or head.

Now if a reinforced hole is placed within a distance of 2*(d*ts)^1/2 the value of m cannot be less than 1 (without rigorous stress analysis) per Section VIII, Div.1.

So the thickness for a flat head for a 6" NPS Sch. 80 pipe is calculated from this equation.

t = d*((C*P)/S)^1/2

with an allowable stress of 14400 psi, a pressure of 650 psig, a diameter of 5.761" and a C value of 0.33 the head will have to be

t = 0.703" (plus a corrosion allowance if one is specified)

Now the same calculation per PG-31 in Section I with m less than 1 and C = 0.2

t = 0.547" (plus a corrosion allowance if one is specified)

So Section I allows the flat head to be 22.14% thinner then Section VIII for the same materials, geometry, and conditions.