National pipe thread
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National pipe thread (NPT) is a U.S. standard for tapered threads, originally used to join steel or brass pipe and fittings. The 1/8 inch, 1/4 in, 3/8 in, 1/2 in, 3/4 in, 1 in, 1 1/4 in, 1 1/2 in, and 2 in sizes are commonly used, appearing on pipe and fittings by most U.S. suppliers. Smaller sizes than those listed are occasionally used for compressed air. Larger sizes are used less frequently because other methods of joining are more practical at 3 inch and above in most applications.
Nominal Pipe Size (NPS) is loosely related to the inside diameter of schedule 40 pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Other schedules of pipe have different wall thickness but the OD (outer diameter) and thread profile remain the same, so the inside diameter of the pipe is therefore different from the nominal diameter.
Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. These threads are now used in materials other than steel and brass, including PVC, nylon, bronze and cast iron.
The taper on NPT threads allows them to self-tap slightly when torqued and form a seal as the threads compress against each other, as opposed to straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. This means that NPT fittings are easiest to make leak free with the aid of Teflon tape or a similar thread sealant compound. (The use of Teflon tape or pipe dope will also help to limit corrosion on the threads, which can make future disassembly nearly impossible.) It also means that NPT fittings should only be assembled and disassembled a limited number of times, because the deformation of the threads will result in a poorer quality seal over time.
There is also a semi-compatible variant called NPTF or Dryseal, designed to provide a more leak-free seal without the use of teflon tape or other sealant compound.
Sometimes NPT threads are referred to as MPT (Male Pipe Thread) or FPT (Female Pipe Thread), for male (external) or female (internal) threads, respectively. An equivalent designation is MIP (Male iron pipe) and FIP (Female iron pipe).
Pipe thread sizes
| Nominal NPT Size | Outer Diameter | Threads per inch |
|---|---|---|
| 1/8 " | 0.405" | 27 |
| 1/4 " | 0.540" | 18 |
| 3/8 " | 0.675" | 18 |
| 1/2 " | 0.840" | 14 |
| 3/4 " | 1.050" | 14 |
| 1 " | 1.315" | 11½ |
| 1¼ " | 1.660" | 11½ |
| 1½ " | 1.900" | 11½ |
| 2 " | 2.375" | 11½ |
| 2½ " | 2.875" | 8 |
| 3 " | 3.500" | 8 |
References
See also
External links
- [Georgefischer Co.] Shows the Whitworth form BSP or ISO pipe thread.
- [RIDGID], the Ridge Tool Company, manufacturer of a full line of internal and external threaders for professional plumbing use. Site includes many downloadable instructional videos.
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