NPT threaded fitting

[idesign73]

I have a 3/8" NPT threaded fitting that is basically used as a plug in the side of a water column. I am trying to do an analysis of this part for a CRN (Canadian Registration Number) report that I must submit. I am having difficulties determining the proper way to go about analyzing the fitting. It is threaded 3/8" NPT on the outside, the inside is bored out for a piece of Teflon to fit into (this is part of an electrode assembly). Because the Teflon seals off the internal portion of the fitting, I don't think I have to do a B31.1 hoop strength calc, but how do I assure that the part can withstand the pressure since the pressure would be external? Since it is part of a boiler system, I must stick with Section I and B31.1 Code, but B31.1 (104.1.3) points me to Section VIII Div. 1 for external pressure. I am thinking that I need to determine if the threads will withstand the force of the pressure acting axially on the electrode. Also I would assume that I need to determine that the fitting would not collapse. I have not been in this field too long, so any help you could give would be much appreciated.

 

[prex]

It is out of normality that you make the detailed analysis of such a component, for which detailed methods are not provided in B31.1 (though such an analysis may be required when you really don't know how to come out of it).
Look at para 106 of B31.1 (or the corresponding of ASME I, PG-42): you should demonstrate that your fitting complies by geometry and material with a standard fitting (a threaded plug) and that your pressure is within the rating.
Don't understand why the component is subject to external pressure: anyway if it is similar to a plug, it is so thick with respect to its diameter that it will not buckle and the resistance to external pressure equals that for internal pressure.
I suppose also that the rating of the corresponding standard fitting is much higher than your pressure (normally this is the case) so that you should have not too much troubles in getting it approved. prex

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[idesign73]

Thanks for the help, prex, it was much appreciated. The reason the fitting is exposed to external pressure is because the body of the fitting extends past the wall thickness of the water column. Once the fitting is screwed in all the way, a portion of the fitting (approximately 0.25&quot extends into the water column past the wall thickness.
Thanks again!

 

[idesign73]

I have reviewed what prex had said, and those suggestions would work, I could prove the dimensions comply with the requirements of 16.11, except that my material does not comply with that section of code. I am using SA-479 304/304L, which is not a forging. I can not find a section of code that will let me use my geometry with the given material. I need to analyze it as a hex-head plug but due to the material, I can not use anything I have found. Plus approximately 0.25" of the "plug" extends out past the wall thickness of the inside of the water column, so there will be external pressure. Anyone have any ideas on this?

 

[prex]

Concerning the material you should be able to determine the rating for a material other than those specified, either by comparing the allowable stresses for piping material in a B31 code ( likely B31.3) or by comparing the temperature-pressure ratings for flanged fittings per B16.5. The procedure should be outlined in B16.11 (have not it at hand to check): this is done for instance in B16.9 (buttwelding fittings) with reference to pipe material per B31.
Concerning the external pressure, if my understanding is correct, you have it because you have a hole in the fitting filled with a gasketing material on the inside of vessel. In my opinion you should not consider the presence of this material, and in that case the external pressure disappears, so you shouldn't account for it. On the contrary, if you want to account for its presence, then you should also consider it as able to avoid the buckling of the fitting. prex

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