NPT confusion

[emonje]

I just came up against this spec that gives different nominal OD of NPT threads than all other sources I've checked. For example, it says for 1/8 thread nominal OD is 0.391 while all others including Machinery Handbook says it is 0.405; for 3/4, it says OD is 1.022 but all others say it should be 1.05 & so on. Pitch is same for both in all cases.
Can anyone please tell me what am I missing?
Thanks.

 

[BigInch]

Which one is the source spec? Go to the source document. B1.20.1, B1.1, ISO 724?

More info here,

http://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[emonje]

Both says NPT, so it means both implies B1.20.1, right?

 

[BigInch]

American Standard is B1.20.1

I don't have the B1.20.1 spec, but my Piping Handbook shows 0.405 & 1.05 too.


"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[JLSeagull]

Several thread designations exist per 1.2 for NPS 1/8 including 27 NPSM, 27 NPSC AND 27 NPSL as well as left and right hand threads. The characters suggest the threads per inch and N for National Standard, P for Pipe, S for straight, M=mechanical, C=coupling, L=Lockout. Thread dimensions are pre B1.7.

I see nothing with a diameter listed as 0.391-inch. Table 2 shows the O.D. of the pipe as 0.405. The pitch diamter at beginning of external thread E0 is 0.36351 and E1 diameter (pitch diameter at gauging notch) is 0.37360. The diameter E3 wrench makeup length for internal trhreads is 0.35656 E5 nominal completer external threads diameter is 0.37537.

 

[unclesyd]

Could it be one the dryseal threads?

What is the media to be contained?

 

[emonje]

As far I can gather it's sort of company standard. So it's used for all sorts of things, pneumatic as well as hydraulic fluids. I think it's not meant for dryseal application.

 

[JLSeagull]

From API Manual of Petroleum Measurement Standards Chapter 14—Natural Gas Fluids Measurement
2.5.4.3 Pressure Tap Diameter
The diameter of the pressure tap holes at the inner surface of the meter tube and along the drilled length of the holes shall be 3?8 (0.375) inch ± 1?64 (0.016) inch, providing for a maximum diameter of 0.391 inch and a minimum of 0.359 inch for pipe with a nominal diameter of 2 or 3 inches; and shall be 1?2 (0.5) inch ± 1?64 (0.016) inch, providing for a maximum diameter of 0.516 inch and a minimum of 0.484 inch for pipe with a nominal diameter of 4 inches or more.

 

[rconner]

I think the outside diameter the original specifier perhaps was referring to is the approximate external thread diameter at the very "End of the External Thread" (that is of course smaller than the pipe diameter, that I think as others have noted is normally 0.405" for 1/8" dia. pipe).
In any case I think as others have said it is best practice to precisely identify what one is talking about with a standard specification and or drawing reference!