NPT Threads in Plastic 4

[lilkitty]

I am designing a plastic drain pan with a molded-in 3/4" female NPT (tapered) pipe fitting. Does anyone know if design guides exist for designing the plastic surrounding the threaded hole? Tapered threads can produce extremely high hoop stresses and I don't want the HVAC installer to crack the threads (and pan) causing a leak. I'd appreciate some place to look for more information.

 

[insideman]

You are right in suspecting trouble with plastic female pipe thread because of high hoop stress leading to leaks because of relaxation or possibly cracking. A steel ring around the joint MAY help.

 

[donf]

Lilkitty,

You have a valid concern. The fitting could crack at the time of installation - or (even worse) after it's been in service for a while. (I've seen both)

Here are a three options to consider: (note - I only "linked" Spears and Banjo - there are other mfrs of similar fittings)

Use an outer metal "collar" (as insideman suggested)

http://www.spearsmfg.com/prod_brochures/SR-2-0801_0801.pdf

Use a metal insert:

http://www.spearsmfg.com/prod_brochures/SR-2-0801_0801.pdf

Use a bulkhead fitting:

http://www.banjocorp.com/products/prod.asp?PMPN=TF075++++++++++

Good luck.

Donf

 

[StoneCold]

lilkitty
Why not change the npt to a socket fitting. Similar to PVC fittings.

 

[PUMPDESIGNER]

lilkitty,
You are so very correct to be concerned.
In my view, you are asking for trouble to provide female threads for the 800 pound gorillas to screw something into.

I have seen manufacturers try everything to protect themselves from this, including thicker (does not help much), metal rings (still a problem), warning decals, etc.

If you can find some way to get away from the thread you will save yourself tons of warrantee claims, returns, phone calls and faxes, and money.

My company has outlawed all plastic threads.

We use grooved couplings, quick connects like grooved couplings. Adapt to a metal thread somehow, anything to get away from plastic.

PUMPDESIGNER