Pressure vessel weight reduction 4

[MotorMan12]

For a pressure vessel, my supplier has performed calculations according to ASME VIII- section 1. There are concerns regarding the weight of the unit at the location where it will be used. We have therefore taken his drawings, made a 3-d model and run it in ANSYS. It is no surprise to find that stress levels suggest that there are large margins. By heat treating the material in question, the yield stress is increased by 60%. This will allow for a significant reduction in casing thickness.

Calculations on the shell are rather straigh-forward, but when it comes to the flanges, however, I struggle a bit due to lack of experience: Does ASME VIII dictate how a given flange size should look (we are well outide regular flange sizes, ID=25"), or is it, in layman terms, just a matter of designing a flange and showing that you do not get separation of the flanges and that the stresses are not too high?

My idea is to play with the model until I get results that look decent and where I have a better utilization of the material. Based on this, the supplier could perform a new ASME VIII – section 1 calculation, as the current calculations also reveal margins. The ideal would of course be to perform a full ANSYS calculation according to section 2, but my supplier is not quite there yet.

 

[TGS4]

If you are designing a vessel to ASME Section VIII, Division 1, then you are required to follow the rules, where there are rules presented. You are NOT permitted to take credit for material properties in excess of those listed in ASME Section II, Part D. Only in the even that there exist no rules in Division 1, may you apply article U-2(g).

If you want a "lighter" vessel, then you may design the vessel in accordance with ASME Section VIII, Division 2. In that case, you may perform a Design-By-Analysis per Part 5. Note that the design-by-analysis rules in Part 5 are extremely stringent, and must be followed precisely. Merely running an "ANSYS 3-d model" is insufficient.

Are you aware of the multiple failure modes that must be protected against in order to properly design a pressure vessel? Your second paragraph about flanges implies that you do not. (Note that flanges need to be designed not only for protection against over-stress, but also uniformity of gasket contact pressure, deformation during bolt-up and operation (including differential thermal expansion effects, including those due to transient temperature events) to ensure that an adequate gasket contact stress (and contact width of the gasket) is maintained, among many others.

 

[MJCronin]

TGS4,

An excellent and complete response !!!

 

[vpl]

I would note that using Section VIII, Division 1 does result in a thicker vessel. If you want the benefits of the FEA, you're going to need to pay your supplier to do that analysis.

Patricia Lougheed

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[boilerone]

I want to mention that it is a 'NO, NO' to perform the heat-treatments and try to increase the strengths of the pressure vessels after they are manufactured per the codes and ready for installations. Performing such heats at sites can very possibly mess up shop-manufactured conditions including shop heat-treatments (but these are NOT for the purpose of getting higher strengths) and sometimes be dangerous because cracks can form due to un-even heatings or sudden coolings to cause thermal or residual stress in the vessels.

Also explained is that the pressure vessel materials selected by the codes are intentionally more elastic (given by the required material elongation properties) which means having much lower yield strengths compared with their tensile strengths because of safety considerations to avoid sudden crackings or eruptions without signs or predictions. Therefore you can notice that the pressure vessel materials used are typically low on carbons and not super-strong. Performing strength-increasing heat treatment methods such as quenchings can totally defeat this purpose and is typically not allowed by the installation codes. Plus, the higher strengths obtained from such heat-treatments can very possibly be lost during operation if the vessels are designed for or could possible experience high-temperature situations.

I guess the pressure vessel experts here can say a lot more about this topic.

 

[racookpe1978]

Absolute minimum weight in high pressure pressure vessels at high flows is one reason there were so many rocket motor and piping/control failures in the space program. And why as many as 1/4 of military ejection seats ever built have been used - because military jets are built with very, very low safety limits.

Please don't "experiment" or "play with the model" to find out why the Codes are explicitly careful about NOT letting people stretch the design envelope.

 

[SnTMan]

titanium