NPT versus FNPT 4

[Nellysdad]

I have a drawing which displays FNPT. I am familiar with NPT and its variants but not this one. What about FPT. Any info is appreciated.

 

[FredGarvin]

Most likely a shorthand for female NPT. Do they give the full thread callout?

 

[Nellysdad]

just "1/8 FNPT" (it is a female though) We also have 1/8 FPT on the same dwg. They are at right angles to each other. They do intersect, if that is relevant. I would expect them to be just NPT but as they are different....I question it. The designer is unavailable right now.

 

[Celebur]

I, too, would say FNPT = Female NPT. The company that I work for routinely uses it to mean this. The "correct" (i.e. standard) shorthand would be NPT(F).

 

[11echo]

As everyone has indicated FNPT is Female National Pipe thread Tapered, and conversely MNPT is Male National Pipe thread Tapered. I run across this all the time in the oil fields. You do know there is a straight pipe thread too? ...NPS Good Luck! ...Mark

 

[TBP]

I have a book on hydraulic systems ("IPT's Industrial Hydraulics Handbook&quot that refers to NPTF as:

"A variation of the NPT thread that is used for hydraulic purposes is the Dryseal ANSI Standard Taper Pipe Thread (NPTF). This thread has the same form and dimensions as the NPT thread, except that the truncation of the crests and roots is changed to ensure that the spiral clearance around the thread is eliminated.

THIS THREAD MAY ALSO BE REFERRED TO AS THE NATIONAL PIPE TAPERED THREAD FOR FUELS.

The interference at the crest and root of the mating parts of this thread eliminates the need for a sealant to seal any clearances. Be aware that assembling any thread system without lubrication can lead to galling, expecially with materials such as stainless steel or other nickel alloys. A thread sealant should be used with this thread for anti-galling purposes."

The information on the drawing you're looking at could mean either female national pipe thread, or dryseal. I'd make a phone call to whoever did the design, just to be sure what he meant.

The book goes on to say that special taps and dies are needed to cut this kind of thread, and unless specially marked, you can't really tell NPTF from NPT, just by looking at it.

 

[FredGarvin]

That's a good bit of info there TBP. Thanks for finding that.

 

[CADShark]

TBP...be careful. I also work with standard and pipe thread on a hourly basis. The ANSI standard is very clear to the thread designation that should be called out. If you are looking to call out a female pipe thread you should use FNPT, but if its a Dryseal thread it should be NPTF. Don't get the two mixed or you could get into a heap of trouble when it comes time to assemble and test your connections. Normally NPT's are fine under lower pressures, say 1500 PSI or lower. But over that I prefer to use the NPTF. We still use a teflon tape on the threads to ensure we don't gall the thread.

 

[TBP]

Abolutely, Sharkman - that's why I suggested checking with the designer. It wouldn't be the first time that somebody had used an abreviation that looks CLOSE to what they intend it to mean, but in fact means something else. "PRV" comes to mind - pressure reducing valve or pressure relief valve? To me, it means a pressure reducing valve, but I've noticed that this is not universal. Some people use "PRV" where they mean a relief valve. It's always better to clarify these things before material and components are acquired, and things start getting put together.

 

[FredGarvin]

BTW...what is the application?