pressure derating for plastic/metal tapered thread connections

[ODtape]

It is my understanding that pressure derating for tapered thread (npt) pvc pipe/fitting connections is 50%. However, I know that FEMALE pvc threaded fittings are more susceptable to breakage (splitting). My question: would connecting a threaded sch 80 pvc nipple to a brass female fitting also involve a 50% derating? If so, I am assuming that this would be due to the reduction in wall thickness in the exposed threaded area(?).

 

[aybee]

Fist I would use UPVC which has a higher temperature /strength rating, I would check the info from suppliers (on the web) to get the strength reduction at the temperature you are considering to the minimum published which I think is 25C above that it drops off to virually 0 at 100C
the specific answer to your query re thread depth is "probably" but cant speak from knowledge of the subject

 

[jdarco]

Your assumptions are correct. The pressure derating comes from the cutting of the threads and the reduced wall thickness. The plastic pipe people always prefer to thread a male NPT plastic part into a female NPT metal part. It has more to do with expansion of plastic part vs. metal. The plastic male thread will expand "into" the metal female thread in a temperature rise, effectively providing a tighter seal. Check out the Spears website. They offer a female threaded fitting with a stainless reinforcing ring to provide additional pressure capabilities and prevent failure due to over tightening.

http://www.spearsmfg.com