NPT threaded fitting
NPT threaded fitting
(OP)
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.





RE: NPT threaded fitting
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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RE: NPT threaded fitting
Thanks again!
RE: NPT threaded fitting
RE: NPT threaded fitting
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
http://www.xcalcs.com
Online tools for structural design