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NPSF thread
3

NPSF thread

NPSF thread

(OP)
Some of our old drawings of cast housing show npsf threads. I looked them up to see what they are, but what would lead one to use these threads instead of nptf? Thanks.

RE: NPSF thread

I typically replace any NPT thread form with a Unified National alternative with equivalent pitch diameter and a male o-ring gland.  The pitch on the UN thread form can be computed directly from axial load on threaded joint, a classical boundary value problem closed form solution set at best.

You can check the wall stresses under the male gland using tri-axial paradigm similar to Von Mises-Hencky, Thick Wall Pressure Vessel (Infinite Pipeline Model) or classical analysis in fracture mechanics.  Obviously the wall below the male gland and bore of the pipeline is the worst case scenario, I like to carry factors of safety near two (2).

Hope this helps.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

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