National pipe thread

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Threaded pipe and elbow. The pipe has a male NPT thread, while the elbow is female. Malefemalepipe.jpg
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 (NPT) standards for short, are U.S. 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 various types are each named with a symbol and a full name; examples of the symbols include NPT, NPS, NPTF, NPSC, and others.

Contents

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

Outside North America, some US pipe threads are found, 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 a symbol and a full name, as follows: [3]

Abbreviation [3] Shorthand expansionFull name [3] Comment [3]
NPTNational pipe taperAmerican National Standard Taper Pipe ThreadTapered for sealing, often without any thread sealant; for connections in nearly every type of service
NPSNational pipe straightAmerican National Standard Straight Pipe ThreadFor rigidity; sealable only with sealant; sometimes male straight enclosed by female tapered for low-pressure sealing
NPSCNational pipe straight–couplingAmerican National Standard Straight Pipe Thread for CouplingsFor general couplings
NPSFNational pipe straight–fuelDryseal 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–fuelDryseal 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

Use

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, [4] 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. 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). 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 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 18, 14, 38, 12, 34, 1, 1 14, 1 12, 2, 2 12, 3, 4, 5, and 6 inch, as pipe and fittings by most U.S. suppliers. Sizes smaller than 18 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. [5]

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″.

The standard for Nominal Pipe Size (often abbreviated NPS, which should not be confused with the symbol 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 outside 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

NPT and NPS threads have a 60° included angle and have a Sellers thread form (flattened peaks and valleys). The trough between the trapezoidal threads is 18 of the pitch in width. On external threads the flat crest is also 18 of the pitch wide (making the thread height 0.64952 × pitch), but on internal threads it is 14 of the pitch wide (making the thread height 0.54126 × pitch). [6]

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) [7]
Nominal
pipe size
Thread
density
Thread pitch Hand-tight engagement [8] Effective thread [8] Overall
length [8]
Actual outside
diameter, OD
Tap drill
LengthTurnsDiameterLengthTurnsDiameter
inchinch−1inchmminchinchinchinchinchinchmminchmm
116270.037037040.94070.16004.320.281180.26117.050.28750.38960.3137.950
18270.037037040.94070.16154.360.373600.26397.130.380000.39240.40510.2870.3398.6106
14180.055555551.41110.22784.100.491630.40187.230.502500.59460.54013.71671611.113
38180.055555551.41110.24004.320.627010.40787.340.637500.60060.67517.145376414.684
12140.071428571.81430.32004.480.778430.53377.470.791780.78150.84021.3360233218.2563
34140.071428571.81430.33904.750.988870.54577.641.001780.79351.05026.6700596423.4156
111 120.086956522.20870.40004.601.238630.68287.851.256310.98451.31533.40101 53229.3688
1 1411 120.086956522.20870.42004.831.583380.70688.131.601311.00851.66042.16401 1238.1000
1 1211 120.086956522.20870.42004.831.822340.72358.321.841311.02521.90048.26001 476444.0531
211 120.086956522.20870.43605.012.296270.75658.702.316301.05822.37560.32502 73256.3563
2 1280.125000003.17500.68205.462.762161.13759.102.790631.57122.87573.02502 5866.6750
380.125000003.17500.76606.133.388501.20009.603.415631.63373.50088.90003 1482.5500
3 1280.125000003.17500.82106.573.888811.250010.003.915631.68374.000101.60003 3495.2500
480.125000003.17500.84406.754.387131.300010.404.415631.73374.500114.30004 14107.9500
4 1280.125000003.17505.000127.00004 34120.6500
580.125000003.17500.93707.505.449291.406311.255.478631.84005.563141.30025 932134.1438
680.125000003.17500.95807.666.505971.512512.106.540631.94626.625168.27506 1132161.1313
880.125000003.17501.06308.508.500031.712513.708.540632.14628.625219.0750
1080.125000003.17501.21009.6810.620941.925015.4010.665632.358710.750273.0500
1280.125000003.17501.360010.8812.617812.125017.0012.665632.558712.750323.8500
1480.125000003.17501.562012.5013.872632.250018.0013.915632.683714.000355.6000
1680.125000003.17501.812014.5015.875752.450019.6015.915632.883716.000406.4000
1880.125000003.17502.000016.0017.875002.650021.2017.915633.083718.000457.2000
2080.125000003.17502.125017.0019.870312.850022.8019.915633.283720.000508.0000
2480.125000003.17502.375019.0023.860943.250026.0023.915633.683724.000609.6000
Hand-tight and effective thread engagement lengths NPT Dimensions.svg
Hand-tight and effective thread engagement lengths

History

In 1864, William Sellers, the 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. [9] (See also Screw thread § History of standardization .)

See also

Related Research Articles

Plumbing Systems for conveying fluids

Plumbing is any system that conveys fluids for a wide range of applications. Plumbing uses pipes, valves, plumbing fixtures, tanks, and other apparatuses to convey fluids. Heating and cooling (HVAC), waste removal, and potable water delivery are among the most common uses for plumbing, but it is not limited to these applications. The word derives from the Latin for lead, plumbum, as the first effective pipes used in the Roman era were lead pipes.

Flange External or internal ridge, or rim which provides strength

A flange is a protruded ridge, lip or rim, either external or internal, that serves to increase strength ; for easy attachment/transfer of contact force with another object ; or for stabilizing and guiding the movements of a machine or its parts. The term "flange" is also used for a kind of tool used to form flanges.

Gasket Type of mechanical seal

A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression.

Pressure vessel A container designed to hold gases or liquids at a pressure substantially different from the ambient pressure

A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.

The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

Compression fitting

A compression fitting is a fitting used in plumbing and electrical conduit systems to join two tubes or thin-walled pipes together. In instances where two pipes made of dissimilar materials are to be joined, the fittings will be made of one or more compatible materials appropriate for the connection. Compression fittings for attaching tubing (piping) commonly have ferrules in them.

Screw thread A helical structure used to convert between rotational and linear movement or force

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.

Garden hose

A garden hose, hosepipe, or simply hose is a flexible tube used to convey water. There are a number of common attachments available for the end of the hose, such as sprayers and sprinklers. Hoses are usually attached to a hose spigot or tap.

Pipe (fluid conveyance)

A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number. Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule. NPS is often incorrectly called National Pipe Size, due to confusion with the American standard for pipe threads, "national pipe straight", which also abbreviates as "NPS". The European and international designation equivalent to NPS is DN, in which sizes are measured in millimetres, see ISO 6708. The term NB is also frequently used interchangeably with DN.

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Threaded pipe

A threaded pipe is a pipe with screw-threaded ends for assembly.

Thread seal tape

Thread seal tape is a polytetrafluoroethylene (PTFE) film tape commonly used in plumbing for sealing pipe threads. The tape is sold cut to specific widths and wound on a spool, making it easy to wind around pipe threads. Thread seal tape lubricates allowing for a deeper seating of the threads, and it helps prevent the threads from seizing when being unscrewed. The tape also works as a deformable filler and thread lubricant, helping to seal the joint without hardening or making it more difficult to tighten, and instead making it easier to tighten.

Coupling (piping)

A coupling is a very short length of pipe or tube, with a socket at one or both ends that allows two pipes or tubes to be joined, welded (steel), brazed or soldered together.

The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.

Piping and plumbing fitting

A fitting or adapter is used in pipe systems to connect straight sections of pipe or tube, adapt to different sizes or shapes, and for other purposes such as regulating fluid flow. These fittings are used in plumbing to manipulate the conveyance of water, gas, or liquid waste in domestic or commercial environments, within a system of pipes or tubes.

Copper tubing

Copper tubing is most often used for heating systems, and as a refrigerant line in HVAC systems. Copper tubing is slowly being replaced by PEX tubing in hot and cold water applications. There are two basic types of copper tubing, soft copper and rigid copper. Copper tubing is joined using flare connection, compression connection, pressed connection, or solder. Copper offers a high level of corrosion resistance but is becoming very costly.

ASME is a non-profit organization that continues to develop and maintains nearly 600 codes and standards in a wide range of disciplines. Some of which includes the Boiler and Pressure Vessel Code (BPVC), Elevators and Escalators, Piping and Pipelines, Bioprocessing Equipment (BPE), Nuclear Facility Applications (NQA), Process Performance Test Codes (PTC), and Valves, Flanges, Fittings and Gaskets (B16).

Despite fire hose and hydrant coupler standardization efforts that are at least 144 years old, there remain significant areas in Canada, the United States, and Mexico that use fire hose and hydrant threads and other couplings that are incompatible with those used by neighboring fire departments. This is notable because the first fire hydrant was invented by Manhattan fire fighter George Smith in 1817, making these devices 200 years old.

Preferred metric sizes are a set of international standards and de facto standards that are designed to make using the metric system easier and simpler, especially in engineering and construction practices. One of the methods used to arrive at these preferred sizes is the use of preferred numbers and convenient numbers such as the Renard series, the 1-2-5 series to limit the number of different sizes of components needed.

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. http://www.micromold.com/threaded_piping.html PTFE threaded pipe and fittings manufacturer
  5. "ANSI Thread Series Designations". Archived from the original on 2008-05-14. Retrieved 2008-04-27.
  6. http://www.rjsales.com/products/pipenipples/tab6.gif
  7. Engineering toolbox
  8. 1 2 3 http://www.piping-designer.com/index.php/disciplines/mechanical/84-stationary-equipment/fastener/1156-national-pipe-thread?title=National_Pipe_Thread
  9. "Archived copy". Archived from the original on 2016-04-26. Retrieved 2016-04-17.CS1 maint: archived copy as title (link)

Bibliography