National pipe thread

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Threaded pipe and elbow. The pipe has a male NPT thread, while the elbow's 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 America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864.[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]

Pipe threads are different from machine-screw and bolt threads. Those are designated NC (national coarse) and NF (national fine.) The biggest difference is the taper on pipe threads.

The taper rate for all NPT threads is 1 in 16 (34 inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance. 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 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 teflon tape or other sealant compound. NPTF threads are 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 thread (MPT)
A subset of NPT for non-tapered male connections.
Female pipe thread (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 Approx. thread length Approx. thread length,
hand and wrench
Pipe nominal
outside diameter, OD
Tap drill
(inch) (inch−1) (inch) (mm) (inch) (mm) (turns) (inch) (mm) (inch) (mm) (inch) (mm)
116 27 0.03703704 0.9407 0.313 7.950
18 27 0.03703704 0.9407 38 9.525 10 14 6.350 0.405 10.287 0.339 8.6106
14 18 0.05555555 1.4111 58 15.875 11 38 9.525 0.540 13.716 716 11.113
38 18 0.05555555 1.4111 58 15.875 11 38 9.525 0.675 17.145 3764 14.684
12 14 0.07142857 1.8143 34 19.0500 10 716 11.1125 0.840 21.3360 2332 18.2563
34 14 0.07142857 1.8143 34 19.0500 10 12 12.7000 1.050 26.6700 5964 23.4156
1 11 12 0.08695652 2.2087 78 22.2250 10 916 14.2875 1.315 33.4010 1 532 29.3688
1 14 11 12 0.08695652 2.2087 1 25.4000 11 916 14.2875 1.660 42.1640 1 12 38.1000
1 12 11 12 0.08695652 2.2087 1 25.4000 11 916 14.2875 1.900 48.2600 1 4764 44.0531
2 11 12 0.08695652 2.2087 1 25.4000 11 58 15.8750 2.375 60.3250 2 732 56.3563
2 12 8 0.125 3.1750 1 12 38.1000 12 78 22.2250 2.875 73.0250 2 58 66.6750
3 8 0.125 3.1750 1 12 38.1000 12 1 25.4000 3.500 88.9000 3 14 82.5500
3 12 8 0.125 3.1750 58 15.8750 13 1 116 26.9875 4.000 101.6000 3 34 95.2500
4 8 0.125 3.1750 1 58 41.2750 13 1 116 26.9875 4.500 114.3000 4 14 107.9500
4 12 8 0.125 3.1750 5.000 127.0000 4 34 120.6500
5 8 0.125 3.1750 1 34 44.4500 14 1 316 30.1625 5.563 141.3002 5 932 134.1438
6 8 0.125 3.1750 1 34 44.4500 14 1 316 30.1625 6.625 168.2750 6 1132 161.1313
8 8 0.125 3.1750 1 78 47.6250 15 1 516 33.3375 8.625 219.0750
10 8 0.125 3.1750 2 50.8000 16 1 12 38.1000 10.750 273.0500
12 8 0.125 3.1750 2 18 53.9750 17 1 58 41.2750 12.750 323.8500
14 8 0.125 3.1750 14.000 355.6000
16 8 0.125 3.1750 16.000 406.4000
18 8 0.125 3.1750 18.000 457.2000
20 8 0.125 3.1750 20.000 508.0000
24 8 0.125 3.1750 24.000 609.6000

See also[edit]

References[edit]

Further reading[edit]

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

External links[edit]