NPSF thread 3

[EngJW]

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.

 

[insult2injury]

little explanation here:

http://www.webster-inst.com/techinfo/pipeconn/pipefr/pipeconn.htm

I2I

 

[Cockroach]

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