American National Standard Pipe Thread standards, often callednational pipe thread standards for short, are United States nationaltechnical standards forscrew threads used onthreaded pipes andpipe fittings. They include bothtapered 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 asNPT,NPS,NPTF, orNPSC.

MIP is an abbreviation for male iron pipe, andFIP is an abbreviation for female iron pipe.^[2]

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

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

Threaded pipes can provide an effective seal for pipes transportingliquids,gases,steam, andhydraulic fluid. These threads are now used in materials other than steel and brass, includingPTFE,PVC,nylon,bronze, andcast 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 orcompression 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 typicallythread seal tape thread sealant or a thread seal pastepipe dope (sometimes similar in composition toplumber'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 beburr-free and lubricated using alubricant 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 are1⁄8,1⁄4,3⁄8,1⁄2,3⁄4, 1,1+1⁄4,1+1⁄2, 2,2+1⁄2, 3,3+1⁄2, 4, 5, and 6 inch, aspipe and fittings by most US suppliers. Sizes smaller than1⁄8 inch are occasionally used for compressed air, while sizes larger than 6 inches are often joined by other methods.

NPT is defined byANSI/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) isarctan(1⁄32) ≈ 1.7899° ≈ 1° 47′.

The standard forNominal 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 ofSchedule 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.

National Pipe Thread series have the same 60° included angle asSellers 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, also calledDryseal American National Standard Taper Pipe Thread, defined by ASME B1.20.3) is designed to provide a more leak-free seal without the use ofPTFE 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.

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

• Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H. (1996), Green, Robert E.; McCauley, Christopher J. (eds.),Machinery's Handbook (25th ed.), New York:Industrial Press,ISBN 978-0-8311-2575-2,OCLC 473691581.

• "NPT Thread Dimensions".AmesWeb. Archived fromthe original on 1 June 2018.
• ASME B1.20.-1983 Pipe Threads, General Purpose, Inch
• NPT Taper Pipe Threads

• Thread standards
• Piping
• Plumbing
• Standards of the United States

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