National pipe thread

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Threaded pipe and elbow. The pipe has a male NPT thread, while the elbow is female.

National Pipe Thread Taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. In contrast to straight threads that are found on a bolt, a taper thread will pull tight and therefore make a fluid-tight seal. In 1864, William Sellers, being the president of Franklin Institute, presented a standard for nuts, bolts and screws, which became the National Pipe Tapered (NPT) thread.[1]

Usage[edit]

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 PTFE,[2] PVC, nylon, bronze, and cast iron.

The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft or 62.5 mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr-free and lubricated using a lubricating material like lubricating paste or tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.

Commonly used sizes are 18, 14, 38, 12, 34, 1, 1 14, 1 12, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Sizes smaller than 18 inch are occasionally used for compressed air, while sizes larger than 2 inches are uncommon, due to the use of alternative methods of joining that are used with these larger sizes.

Specifications[edit]

NPT is defined by ANSI/ASME standard B1.20.1.[3]

The taper rate for all NPT threads is 1 inch of diameter in 16 inches of length (34 inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance of thread. The angle between the taper and the center axis of the pipe is tan−1(132) = 1.7899° = 1° 47′ 24″.

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. Pipe of other schedules with a certain NPS has different wall thickness, but the same outside diameter and thread profile as Schedule 40, so the inside diameter of the pipe is therefore different from the NPS.

Thread form[edit]

NPTE[clarification needed] and NPS threads have a 60° included angle and have a Sellers thread form (flattened peaks and valleys).

National pipe taper fuel (NPTF)[edit]

A semi-compatible variant called National Pipe Taper Fuel (NPTF), also called Dryseal American National Standard Taper Pipe Thread, defined by ASME B1.20.3, is designed to provide a more leak-free seal without the use of PTFE tape (often referred to by the popular brand name "Teflon") or other sealant compound. NPTF threads have the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.

Informal subsets[edit]

Male/mechanical pipe tapered (MPT)
A subset of NPT for non-tapered male connections.
Female pipe tapered (FPT)
A subset of NPT for female connections.

Standard sizes[edit]

American Society of Mechanical Engineers National Pipe Thread Taper (NPT)[4]
Pipe size Thread
density
Thread pitch Hand-tight Engagement [5] Effective Thread [5] Overall
Length [5]
Pipe nominal
outside diameter, OD
Tap drill
Length Threads Diameter Length Threads Diameter
(inch) (inch−1) (inch) (mm) (inch) (threads) (inch) (inch) (threads) (inch) (inch) (inch) (mm) (inch) (mm)
116 27 0.03703704 0.9407 0.1600 4.32 0.28118 0.2611 7.05 0.2875 0.3896 0.313 7.950
18 27 0.03703704 0.9407 0.1615 4.36 0.37360 0.2639 7.13 0.38000 0.3924 0.405 10.287 0.339 8.6106
14 18 0.05555555 1.4111 0.2278 4.10 0.49163 0.4018 7.23 0.50250 0.5946 0.540 13.716 716 11.113
38 18 0.05555555 1.4111 0.2400 4.32 0.62701 0.4078 7.34 0.63750 0.6006 0.675 17.145 3764 14.684
12 14 0.07142857 1.8143 0.3200 4.48 0.77843 0.5337 7.47 0.79178 0.7815 0.840 21.3360 2332 18.2563
34 14 0.07142857 1.8143 0.3390 4.75 0.98887 0.5457 7.64 1.00178 0.7935 1.050 26.6700 5964 23.4156
1 11 12 0.08695652 2.2087 0.4000 4.60 1.23863 0.6828 7.85 1.25631 0.9845 1.315 33.4010 1 532 29.3688
1 14 11 12 0.08695652 2.2087 0.4200 4.83 1.58338 0.7068 8.13 1.60131 1.0085 1.660 42.1640 1 12 38.1000
1 12 11 12 0.08695652 2.2087 0.4200 4.83 1.82234 0.7235 8.32 1.84131 1.0252 1.900 48.2600 1 4764 44.0531
2 11 12 0.08695652 2.2087 0.4360 5.01 2.29627 0.7565 8.70 2.31630 1.0582 2.375 60.3250 2 732 56.3563
2 12 8 0.12500000 3.1750 0.6820 5.46 2.76216 1.1375 9.10 2.79063 1.5712 2.875 73.0250 2 58 66.6750
3 8 0.12500000 3.1750 0.7660 6.13 3.38850 1.2000 9.60 3.41563 1.6337 3.500 88.9000 3 14 82.5500
3 12 8 0.12500000 3.1750 0.8210 6.57 3.88881 1.2500 10.00 3.91563 1.6837 4.000 101.6000 3 34 95.2500
4 8 0.12500000 3.1750 0.8440 6.75 4.38713 1.3000 10.40 4.41563 1.7337 4.500 114.3000 4 14 107.9500
4 12 8 0.12500000 3.1750 5.000 127.0000 4 34 120.6500
5 8 0.12500000 3.1750 0.9370 7.50 5.44929 1.4063 11.25 5.47863 1.8400 5.563 141.3002 5 932 134.1438
6 8 0.12500000 3.1750 0.9580 7.66 6.50597 1.5125 12.10 6.54063 1.9462 6.625 168.2750 6 1132 161.1313
8 8 0.12500000 3.1750 1.0630 8.50 8.50003 1.7125 13.70 8.54063 2.1462 8.625 219.0750
10 8 0.12500000 3.1750 1.2100 9.68 10.62094 1.9250 15.40 10.66563 2.3587 10.750 273.0500
12 8 0.12500000 3.1750 1.3600 10.88 12.61781 2.1250 17.00 12.66563 2.5587 12.750 323.8500
14 8 0.12500000 3.1750 1.5620 12.50 13.87263 2.2500 18.00 13.91563 2.6837 14.000 355.6000
16 8 0.12500000 3.1750 1.8120 14.50 15.87575 2.4500 19.60 15.91563 2.8837 16.000 406.4000
18 8 0.12500000 3.1750 2.0000 16.00 17.87500 2.6500 21.20 17.91563 3.0837 18.000 457.2000
20 8 0.12500000 3.1750 2.1250 17.00 19.87031 2.8500 22.80 19.91563 3.2837 20.000 508.0000
24 8 0.12500000 3.1750 2.3750 19.00 23.86094 3.2500 26.00 23.91563 3.6837 24.000 609.6000
A NPT dimensions
Hand-tight and effective thread engagement lengths.

See also[edit]

References[edit]

Further reading[edit]

  • Oberg, Erik; Franklin D. Jones; Holbrook L. Horton; Henry H. Ryffel (2000). Christopher J. McCauley; Riccardo Heald; Muhammed Iqbal Hussain, eds. Machinery's Handbook (26th ed.). New York: Industrial Press Inc. ISBN 0-8311-2635-3. 

External links[edit]