Is it okay to mix threaded PP and PVC plastic pipe?

[davidd31415]

I recently ordered several fittings and ended up combining polypropylene and polyvinyl chloride. All the fittings were threaded and included elbows, couplings, nipples, and bushings.

I found that several of the fittings did not seal even though I used Teflon tape. The fittings felt loose too, as if the threads did not tighten together.

All fittings were NPT pipe (no tube fittings were used).

I've been suspecting that it is not okay to mix different types of plastic; is this correct?

Could mixing different schedules be the cause of my problem?

If not, does anyone have ideas on what other causes my problem might have?

If it is okay to link to distributor sites I'll post the part numbers/links.

Any help would be appreciated!

Thanks,

David

 

[chrishutt]

NPT refers to the US standard for pipe threads. Are you in the US? Otherwise why are you using US fittings?

 

[TicklyT]

If you do have any BSP in there anywhere, then it will be difficult to seal (different thread depths, flank angles etc.) even if the job seems mechanically sound. If the joints are all NPT / NPT, then all should be OK, unless someone is using non-standard thread sizes.

Mixing tapered and parallel threads can be problematic, as tapered threads are designed to seal on the thread, and parallel threads are designed as a fixing, usually with alternative arrangements for sealing, like the fibre washers in tap connectors or olives in compression fittings.

I usually test assemble threaded joints 'dry' (shows up any malformed threads etc) and check they can be made mechanically sound without 'bottoming out' at the end of the thread before applying plenty of PTFE tape, so the end of the pipe only just enters the fitting. A few turns just to cover the thread is nowhere near enough.

Wind the tape clockise, or it will unravel as the joint is tightened.

 

[davidd31415]

chrishutt- I am in the US, that's why I am using NPT.

TicklyT- I think the problem may be a mix of parallel and tapered threads. The distributor I ordered from has the fittings labeled as NPT only; no additional information on the thread... I did not realize there were two types of NPT.

You mentioned parallel threads being used as a fixing. I am unfamiliar with that term, what is a fixing?

Thanks for the tips.

David

 

[Space cat]

You mentioned parallel threads being used as a fixing. I am unfamiliar with that term, what is a fixing?

What TicklyT means is that parallel threads only make a good mechanical joint. The threads themselves are not watertight, still less gas tight. Something else is required to provide a fluid seal, the simplest example being a washer.

 

[chrishutt]

In the UK it is general practice for female threads to be parallel and male threads to be tapered. The result is that joints will slowly lock together and seal (provided a suitable sealant has been added).

 

[davidd31415]

Thanks.

Perhaps I should find a US forum . From what I've read it sounds like NPT threads are tapered by definition. I've contacted the distributor, perhaps they can shed some light on the situation.