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

American National Standard Pipe Thread standards, often called national pipe thread standards for short, are United States national technical standards for screw threads used on threaded pipes and pipe fittings. They include both tapered and straight thread series for various purposes, including rigidity, pressure-tight sealing, or both.[1] The types are named with a full name and an abbreviation, such as NPT, NPS, NPTF, or NPSC.

MIP is an abbreviation for male iron pipe, and FIP is an abbreviation for female iron pipe.[2]

Outside North America, some US pipe thread sizes are widely used, as well as many British Standard Pipe threads and ISO 7–1, 7–2, 228–1, and 228-2 threads.

Types

The various types are each named with an abbreviation and a full name, as follows:[3]

Abbreviation[3] Shorthand expansion Full name[3] Comment[3]
NPTNational pipe taperAmerican National Standard Taper Pipe ThreadTapered and needs thread sealant; for connections in nearly every type of service
NPSNational pipe straightAmerican National Standard Straight Pipe ThreadFor rigidity; sealable with o-ring or gasket; sometimes male straight is mated with female tapered for low-pressure sealing (need to verify)
NPSCNational pipe straight–couplingAmerican National Standard Straight Pipe Thread for CouplingsFor general couplings
NPSFNational pipe straight–fuel and oilDryseal USA (American) Standard Fuel Internal Straight Pipe ThreadInternal only, straight
NPSHNational pipe straight–hoseAmerican National Standard Straight Pipe Thread for Hose CouplingsFor hose couplings
NPSINational pipe straight–intermediateDryseal USA (American) Standard Intermediate Internal Straight Pipe ThreadSimilar to NPSF but slightly larger; internal only, straight
NPSLNational pipe straight–locknutAmerican National Standard Straight Pipe Thread for Loose-fitting Mechanical Joints with LocknutsFor use with locknuts
NPSMNational pipe straight–mechanicalAmerican National Standard Straight Pipe Thread for Free-fitting Mechanical JointsFor various rigid mechanical uses
NPTFNational pipe taper–fuel and oilDryseal USA (American) Standard Taper Pipe ThreadFor dryseal connections in nearly every type of service, especially fuel connections
NPTRNational pipe taper–railingAmerican National Standard Taper Pipe Thread for Railing JointsFor railings
PTF-SAE SHORTPipe taper, fuel, SAE, shortDryseal SAE Short Taper Pipe ThreadNamed after SAE International; similar to NPTF but one turn shorter

Uses

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, 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 a seal. The various types of pipe threads are designed for use both with or without thread sealants,[1] as particular applications demand. The sealants, where used, are typically thread seal tape thread sealant or a thread seal paste pipe dope (sometimes similar in composition to plumber's putty).

Because 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 lubricant like lubricating paste or thread seal 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 1⁄8, 1⁄4, 3⁄8, 1⁄2, 3⁄4, 1, 1+1⁄4, 1+1⁄2, 2, 2+1⁄2, 3, 4, 5, and 6 inch, as pipe and fittings by most US suppliers. Sizes smaller than 1⁄8 inch are occasionally used for compressed air, while sizes larger than 6 inches are often joined by other methods.

Specifications

NPT is defined by ANSI/ASME standard B1.20.1.[4]

The taper rate for all NPT threads is 1 inch of diameter in 16 inches of length (3⁄4 inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance of thread. The half-angle (between the taper surface and the axis of the pipe) is arctan(1⁄32) ≈ 1.7899° ≈ 1° 47′.

The standard for Nominal Pipe Size (often abbreviated NPS, which should not be confused with the abbreviation NPS for the straight thread form standard) is loosely related to the inside diameter of Schedule 40 series of sizes. Because of the pipe wall thickness of Schedule pipe, the actual diameter of the NPT threads is larger than the Nominal Pipe Size diameter, and considerably so for small sizes. Pipe of a given size in a different Schedule than Schedule 40 provides a different wall thickness while maintaining the same outside diameter and thread profile as Schedule 40. Thus the inside diameter of a given size of Schedule pipe differs from the Nominal Pipe Size, while outside diameters for a given nominal Schedule size are the same between Schedules.

Thread form

National Pipe Thread series have the same 60° included angle as Sellers thread. On the other hand, for sealing capability, tapered (pressure-tight) threads have crest and root shapes that differ from Sellers. For example, the NPT has a crest width of 0.038 to 0.062 pitch, which is sharper than the 0.125 pitch of Sellers. [4]

National pipe taper fuel (NPTF)

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 another 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.

Standard sizes

American Society of Mechanical Engineers National Pipe Thread Taper (NPT)[5]
Nominal
pipe size		 Thread
density		 Thread pitch P Hand-tight engagement[6] Effective thread[6] Overall
length L4[6]		 Actual outside
diameter D		 Tap drill
Length L1 Turns Diameter E1 Length L2 Turns Diameter E2
inch inch−1 inchmm inch inch inch inch inch inchmm inchmm
1⁄16 27 0.03703704 0.9407 0.16004.320.28118 0.26117.050.2875 0.3896 0.313 7.950
1⁄827 0.03703704 0.9407 0.16154.360.37360 0.26397.130.38000 0.3924 0.405 10.287 0.339 8.6106
1⁄4 18 0.05555555 1.4111 0.22784.100.49163 0.40187.230.50250 0.5946 0.540 13.716 7⁄16 11.113
3⁄8 18 0.05555555 1.4111 0.24004.320.62701 0.40787.340.63750 0.6006 0.675 17.145 37⁄64 14.684
1⁄2 14 0.07142857 1.8143 0.32004.480.77843 0.53377.470.79178 0.7815 0.840 21.3360 23⁄32 18.2563
3⁄4 14 0.07142857 1.8143 0.33904.750.98887 0.54577.641.00178 0.7935 1.050 26.6700 59⁄64 23.4156
1 11+1⁄2 0.08695652 2.2087 0.40004.601.23863 0.68287.851.25631 0.9845 1.315 33.4010 1+5⁄32 29.3688
1+1⁄4 11+1⁄2 0.08695652 2.2087 0.42004.831.58338 0.70688.131.60131 1.0085 1.660 42.1640 1+1⁄2 38.1000
1+1⁄2 11+1⁄2 0.08695652 2.2087 0.42004.831.82234 0.72358.321.84131 1.0252 1.900 48.2600 1+47⁄64 44.0531
2 11+1⁄2 0.08695652 2.2087 0.43605.012.29627 0.75658.702.31630 1.0582 2.375 60.3250 2+7⁄32 56.3563
2+1⁄2 8 0.12500000 3.1750 0.68205.462.76216 1.13759.102.79063 1.5712 2.875 73.0250 2+5⁄8 66.6750
3 8 0.12500000 3.1750 0.76606.133.38850 1.20009.603.41563 1.6337 3.500 88.9000 3+1⁄4 82.5500
3+1⁄2 8 0.12500000 3.1750 0.82106.573.88881 1.250010.003.91563 1.6837 4.000 101.6000 3+3⁄4 95.2500
4 8 0.12500000 3.1750 0.84406.754.38713 1.300010.404.41563 1.7337 4.500 114.3000 4+1⁄4 107.9500
4+1⁄2 8 0.12500000 3.1750 5.000 127.0000 4+3⁄4 120.6500
5 8 0.12500000 3.1750 0.93707.505.44929 1.406311.255.47863 1.8400 5.563 141.3002 5+9⁄32 134.1438
6 8 0.12500000 3.1750 0.95807.666.50597 1.512512.106.54063 1.9462 6.625 168.2750 6+11⁄32 161.1313
8 8 0.12500000 3.1750 1.06308.508.50003 1.712513.708.54063 2.1462 8.625 219.0750
10 8 0.12500000 3.1750 1.21009.6810.62094 1.925015.4010.66563 2.3587 10.750 273.0500
12 8 0.12500000 3.1750 1.360010.8812.61781 2.125017.0012.66563 2.5587 12.750 323.8500
14 8 0.12500000 3.1750 1.562012.5013.87263 2.250018.0013.91563 2.6837 14.000 355.6000
16 8 0.12500000 3.1750 1.812014.5015.87575 2.450019.6015.91563 2.8837 16.000 406.4000
18 8 0.12500000 3.1750 2.000016.0017.87500 2.650021.2017.91563 3.0837 18.000 457.2000
20 8 0.12500000 3.1750 2.125017.0019.87031 2.850022.8019.91563 3.2837 20.000 508.0000
24 8 0.12500000 3.1750 2.375019.0023.86094 3.250026.0023.91563 3.6837 24.000 609.6000
A NPT dimensions
Hand-tight and effective thread engagement lengths

History

In 1864, William Sellers, then president of Franklin Institute, presented a standard for nuts, bolts, and screws. Out of this effort came the first United States Standard threads, including pipe threads.[7]

See also

References

  1. 1 2 Machinery's Handbook (1996), pp. 1772–1781.
  2. ↑ Cauldwell, Rex (2006). Plumbing. Newtown, CT: Taunton Press. ISBN 1561588172.
  3. 1 2 3 4 Machinery's Handbook (1996), pp. 1776–1778.
  4. 1 2 "ANSI Thread Series Designations". Archived from the original on 2008-05-14. Retrieved 2008-04-27.
  5. ↑ Engineering toolbox
  6. 1 2 3 "National Pipe Thread".
  7. ↑ "BSP and NPT threads - Detailed Explanation, Differences". Archived from the original on 2016-04-26. Retrieved 2016-04-17.

Bibliography

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