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pressure derating for plastic/metal tapered thread connections

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ODtape

Agricultural
Jan 15, 2003
14
US
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(?).
 
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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

 
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.

 
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