ASME VIII Div. 1 and 2 - Threaded Openings

[sgtpickles]

Hi Guys
I am curious if there is anything additional to note when following the ASME Section VIII Div. 1 and/or 2 rules for threaded openings. I have a high pressure vessel that I am designing (4000 psi and 400F) and wish to use threaded connections in the shell, c/w a seal weld. UG-36(3)(b) and UG-43(e) state that threaded connections can be used if they are less than NPS 2” and meet the minimum thread engagement of Table UG-43. Div. 2 part 4.5.3.1(c) also says the same thing referring to Table 4.5.1.

The vessel is made from 4.5” SA-479 round bad and has a wall thickness of 1.125”. I modeled the largest hole, ½” NPT, and performed a linear elastic FEA analysis. At these conditions, there is slight yielding on the inner surface of the nozzle which concerns me. I am even more concerned about a future vessel at 4700 psi which would have more yielding. However, the Section VIII code seems to be telling me not to worry about it. I know Div. 1 scope is technically up to 3000 psi, but Div. 2 still should apply. Thoughts?

 

[prex]

What you find seems to be a peak stress, especially if you modeled the threads (type of model unclear in your post).
That's in fact unrelevant to Div.1 and relevant to Div.2 only for fatigue calculations.
I assume that you are not in a high cycle condition, otherwise the choice of threaded couplings would be a bad choice. So my advice is not to worry about your FEM results, provided you meet code provisions.

prex

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[sgtpickles]

Thanks Prex for your response. For those interested, I used a static analysis with a fine mesh around the opening. I did not model the threads, just a 0.84" thru hole in the shell (conservative). On the hole inner surface I applied the equivalent horizontal thrust load (pressure X hole area) as well as the internal pressure.

The vessel will not be under heavy cyclic service. I guess I am concerned with when to follow the code without question and when to just take it as a "guide" and do supplementary analysis?

 

[TGS4]

If you are performing a "Design by Rules" design - Part 4 in Div. 2, then the rules stated are the minimum requirements for design.

I would recommend that you NOT start poking around with Design by Analysis work if you do not understand the full implications of what you are doing. It is apparent from your question that you are new to Design by Analysis, and I would gently steer you away from that line of work unless you have a mentor who can guide you.

That said, you are more than welcome to ask any design by analysis questions here, and we will be glad to help you, if we can.

 

[TGS4]

One other thing - you said that your vessel is not in "heavy cyclic service". I don't know what that means. Have you performed a fatigue screening evaluation as required in Part 5? Does it pass or fail the screening test?

 

[sgtpickles]

Thanks for you input TGS4. Based on 5.5.3, the fatigue will not be a concern.

I prefer to use "Design by Rules", however this does not mean that I should just turn my brain off and follow blindly. All I am suggesting is just because it is written in the code doesn't necessarily mean that it works for all cases, i.e. at this extreme pressure. Keep in mind there are changes made to the code all the time...

 

[TGS4]

Tell me about changes - you'll be pleasantly surprised to find many Code Committee members frequenting these fora.

I like your attitude about not "turning off your brain". Welcome to Eng-Tips. you'll find that we have had some good design by analysis discussions, so check with the archives/search to see what we've discussed.