Piping class building

[Ing. Aponte]

Good morning,

in this time I am working on piping class design after long time I done pure flexibility analysis. Design has to be done following the rules of B31.3. I started my career exactly doing piping class design so I know, but i don't remeber where is written, that ASME code requires to keep the same thickness on the piping net. That means that, for example, if an elbow need a thickness of 10mm for internal pressure and instead for pipe 8mm thickness would be enougt also pipe has to be chosen with 10mm. This rule is still followed and, if it is like this, where I can find this in code?

Second point are B16.9 fittings. ASME B31.3 is clear on this point: rating of that fitting is same of the connecting pipes so, if connecting piping is sized correctly nothing has to be done on fittings. Of course fitting will be thicker than pipe but it is nothing to evaluate here, ony specify the type B16.9 on fitting in the list.

In this moment my calculations appears as hybrid: if I follow the code, as indicated above, nothing has to be do on bends, reducers and tees. I sizing only pipe, specify B16.9 on fittings and it is finished. Instead the procedure here seams to evaluate also bends and reducers thicknesses: so in this moment I calculate the thickness for pipes, bends and reducers and, after that I implement the most higher value for all componens. For example if pipe need 8mm, bend 10mm and reducer still 8mm the final result will be 10mm for all. In my opinion it would be ok if Vendor fabricate the fittings following our thicknesses but, if we specify also B16.9 requirement on fitting we will finish to have double safety rate: one is our thickness propagation and the second one is the one included on B16.9 design. I am wrong? Your experience is design also in this way?

Last strange point to me is for B16.9 tees: here ASME B31.3 has not rules to evaluate the thickness (are only for direct branches or with reinforcing plate) so we spec them simply appling the rule of the rating of the conncting pipes, without look for the real needing thickness and, as indicated, propagate this thickness to pipes, reducers and bends. It is standard praxis work like this today?

Sincerely Aponte

 

[GBTorpenhow]

That means that, for example, if an elbow need a thickness of 10mm for internal pressure and instead for pipe 8mm thickness would be enougt also pipe has to be chosen with 10mm.

This isn't a thing as far as I'm aware. As you note weld fittings are rated based on the equivalent connecting straight pipe so no reason to upsize the pipe.

If the internal pressure needs 8mm straight pipe, elbows and bends are specified at 8mm to match. Areas of the ell/bend may be physically thicker than 8mm but that's the manufacturer's concern only, the ends will be 8mm and in terms of the piping spec its an 8mm elbow.

Reducers and tees are the only thing that gets tricky, if you have a STD run and XS branch you need an XS tee, but in this case the branch connection would just be back beveled locally to match the branch pipe wall thickness.

 

[LittleInch]

I agree.

Remember some fittings are made from pipe, not forged or cast.

Never seen a fitting specified at a higher wt than the pipe UNLESS it is a lower yield strength material than the pipe.

You say practice "here". Where? Your company or your country?

 

[Snickster]

B31.3 requires that you calculate the wall thickness of straight pipe and all fittings are specified as the same wall thickness as the matching pipe. I don't know what you are looking at that says to calculate wall thickness of pipe and fittings independently. It is not in B31.3. Attached is sample piping spec for 150# carbon steel.

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