2007 DIV2 Limit Load, Tables 5.4 and 5.5 2

[Ishmael]

Hi,

I have a simple but fundamental question about the Limit Load Analysis according to 2007 Div.2 part 5, and on the concepts of LFRD approach.

Let's suppose I have to design a vessel with the following data:
Design Pressure: 70psi
Operating Pressure 50psi

According to the assessment procedure (5.2.3.5e) convergence shall be achieved for the prescribed load combinations.
For example, the first global criteria is:
1.5*(P+Ps+D)

Here comes my question: according to the code the first term is the "max allowable working pressure".
Shall I assume the design value (70psi) or the operating one 50psi)?
If use the operating value, then the design pressure becomes "useless", on the other hand it seems weird to me to apply a further magnification factor to the design value, which already includes a safety coefficient.

I found out in 4.1.5.2 two separate definitions, one for Design Pressure, and the other for MAWP, so I'm assuming that the 50psi value shall be used, but I would rather like to have your opinion on the matter.

Thanks

 

[Jonezaz]

I share the confusion over the definitions, but here is how I understand it.

Design Pressure - maximum setting for safety pressure relief devices required by the ASME code. These devices are required to be 100% open at this setting, but may begin to open before the pressure vessel reaches design pressure. In the case where a burst disk is used as the safety device, it may burst before the design pressure.

MAWP - The design pressure is not suitable for routine use of the pressure vessel, especially where the vessel holds expensive and/or toxic materials. The MAWP is chosen suitably below the design pressure so safety devices can be selected that will not relieve during normal operation.

 

[vesselfab]

in the past, not sure about 2007 edition, in Div. 2, the vessel was always stamped the DESIGN pressure as listed ber the USER spec.

could be a typo or the newest code may be using MAWP now

 

[TGS4]

The answer is crystal clear - refer to 4.1.5.2(b).

Maximum Allowable Working Pressure – The maximum gage pressure permissible at the top of a
completed vessel in its normal operating position at the designated coincident temperature for that
pressure. This pressure is the least of the values for the internal or external pressure to be determined
by the rules of this Division for any of the pressure boundary parts, considering static head thereon,
using nominal thicknesses exclusive of allowances for corrosion and considering the effects of any
combination of loadings specified in the User’s Design Specification at the designated coincident
temperature. It is the basis for the pressure setting of the pressure relieving devices protecting the
vessel. The design pressure may be used in all cases in which calculations are not made to determine
the value of the maximum allowable working pressure.

I think that it would be appropriate to actually read ALL of the definitions in 4.1.5.2 BEFORE plowing in to Part 5. Remember, the various Parts of the Code are not stand-alone documents, they are all in the cohesive context of one Division of the ASME Code. It ALL goes together.

 

[Ishmael]

TGS4,
thanks for your reply. As I said in the opening post, I read all the 4.1.5.2 definitions, before posting. As you correctly pointed out, a good part "n-1" knowledge is requested before going to part "n".
Nevertheless, still was not sure about the correct answer. Please do not take my word as offending, simply I'm not english mothertongue, therefore I may be missing something that is obvious to you.
So I'm correct in assuming that, in my case, I shall assume the MAWP=PDES because in my case "calculations are not made to determine the value of the maximum allowable working pressure"?
So, in the formula
1.5*(P+Ps+D)
I shall assume P=70psi?

Thank you
Ishmael

 

[jte]

Ishmael-

Yes. Set P=70 psi.

Now, no offense, but please, please also read the Foreword to the code on page xix, in particular

The Code is not a handbook and cannot replace education, experience, and the use of engineering judgment. The phrase engineering judgment refers to technical judgments made by knowledgeable designers experienced in the application of the Code.

I do not doubt that you are a competent engineer. What concerns me is that the question seems to indicate that you do not have previous experience in dealing with the design of ASME vessels nor do you have a mentor who is helping you understand the ASME philosophy. Although I recognize that some large, relatively low pressure vessels can find economic benefit from using Div. 2, it is not commonly used for a vessel at 70 psi. Another indicator of potential lack of experience with ASME pressure vessel design.

So again, no offense, but get help. Paid help, not the free kind like this forum where you get what you pay for, though sometimes more.

jt

 

[Ishmael]

jte,

I understand your concern. Just some explanations.

a) 70 and 50 psi are two sample values, used just for the sample, although I actually deal with vessel in that pressure range.
b) I cannot disclose further details, but the vessel has some shape discontinuities that cannot be handled by Div 1 rules.
c) In addition, the use of Div 2 is not up to my decision, is always in the Customer requisiton for this kind of applications.
d) I'm a Mechanical Engineer in structural mechanics, I have 9 yrs experience in FEM and vessel design, but I usually do not use limit load analysis. Typically I adopted the past Div.2 Appendix 4 stress categories, because with the 2004 and previous editions of the Code the use of non linear FEM in div 2 was much less direct.
I'm now practicing (late, I know) with the 2007 new part 5, because in the last two years I used different european codes, like the EN13445-3, which addressed much more clearly the use of FEA in the DBA route.
I'm not saying I'm the best expert in the world, but my calculations have been revised and approved by Customer engineering contractors, independent Notified Bodies (PED), third part consultant in Europe, USA, Canada,and Japan. I'm not a kid playing with engineering toys, I just had a (may be silly) doubt in a for me new application I wanted to share.
I think that there are no silly questions, just silly answers. I want to point out that this IS NOT THE CASE OF YOUR ANSWER, I'm sincerely grateful to your and TGS4 replies and warning. I do understand your concern. I asked several times my managers for a Div1/Div2 2007 training course, I hope I could attend it in a near future although no such courses are available in Europe, and an oversea course is kind of expensive.
e) I have seen much more concerning questions asked in the eng-tips forums

Sincerely,
Ishmael

 

[jte]

Ishmael-

Ok. Glad to see you have a background in vessels. In you previous FEA work, when someone came to you with a specific part (say for a Div. 1 vessel) which needed FEA approach (linear or nonlinear), did you use the DP or MAWP as you pressure load? The new Div. 2 defines MAWP pretty much the same way as Div. 1 does, so there should be no confusion. Operating pressure does not play a role here until you start looking at issues like fatigue.


jonezaz-

Sorry, can't let this one by since someone may read it and think its true:

MAWP - The design pressure is not suitable for routine use of the pressure vessel, especially where the vessel holds expensive and/or toxic materials. The MAWP is chosen suitably below the design pressure so safety devices can be selected that will not relieve during normal operation.

First, I'll note that DP is not mentioned in the Manufactuer's Data Report - take a look at Div. 2 Table 2.D.3.

Secondly,

Specified design pressure. The specified design pressure is the design pressure, see paragraph 4.1.5.2.a, required at the top of the vessel in its operating position. It shall include suitable margins required above the maximum anticipated operating pressure to ensure proper operation of the pressure relief devices. The MAWP of the vessel may be set equal to this specified design pressure. If the actual MAWP of the vessel is calculated, it shall not be less than the specified design pressure.

Note that MAWP shall not be less than the specified design pressure. So to state that the MAWP is chosen at some value below the DP is flat out wrong. The same holds for Div. 1. See UG-98 and also UG-99(b) footnote 34 which indicates that you don't actually have to determine the MAWP - you may use the DP as the MAWP. In other words, using the DP as the MAWP is conservative. In other words, MAWP is always higher than DP.

jt