What is npt threading and Why Do We Use Them?

Threaded fittings are an essential component in many fluid control systems, including solenoid valves and related products. One type of thread you might come across is the National Pipe Thread Taper (NPT) thread. In this blog post, we'll explain what NPT threads are, their benefits, and how they are used in solenoid valves and related products.

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NPT threads are a type of tapered thread commonly used in piping systems throughout North America. The NPT thread has a unique design that helps ensure a tight seal and prevents leaks, making it a popular choice in many industries, such as plumbing, HVAC, and pharmaceutical manufacturing.

In the following sections, we'll take a closer look at NPT threads, how they are used in products like electric solenoid valves and related products, and the advantages they provide.

What Are NPT Thread Fittings?

NPT threads are a specific type of tapered thread commonly used in piping systems throughout North America. The term "National Pipe Taper" (NPT) refers to a specific type of tapered thread that connects pipes and fittings. They can be made of metal, like cast iron, steel, bronze, brass, or plastic, like nylon or PVC.

The NPT thread is a standard thread type and is defined by the American National Standard for Tapered Pipe Threads (ANSI/ASME B1.20.1). This standard specifies the size, angle, and depth of the thread and ensures that NPT threads are consistent in size and form, which helps to ensure reliable and leak-free connections.

Why are NPT and NPTF pipe threads tapered?

The tapered design of NPT and NPTF (National Pipe Thread Fuel) threads creates a tighter seal as the fitting is threaded deeper into the pipe. This is an important feature when dealing with liquid and gas flowing systems since it helps to prevent leaks. This design is particularly important in fluid transfer parts of systems where leaks can be dangerous or costly.

The taper on NPT and NPTF threads creates a mechanical interlocking effect between the fitting and the pipe. As the fittings and pipe are screwed together, the threads compress slightly as they are threaded deeper into the pipe, creating a tighter seal.

NPT Male and Female Fittings

The fitting is considered male if the threads are on the outside. Male fitting threads may be called MPT, MNPT, or NPT(M). Threads on the inside indicate it is female. Female fitting threads may be called FPT, FNPT, or NPT(F). Both NPT (male) and NPTF (female) threads have the same pitch, 60-degree angle, and flat peaks and valleys shape.

If you're wondering about the difference of MNPT vs NPT, MNPT is the specific NPT male thread. While in the case of NPT vs NPTF, NPTF is the specific female NPT fitting thread.

With tapered connections, the male fitting has a thread that locks into the female fitting and then expands until the amount of force between the two can withstand pressure from materials traveling through the pipes. Sealant is required on threads before assembly due to the small clearance between the crest and root of the connecting NPT threads.

The NPTF thread does not require sealant. Instead, it contacts and deforms the mating thread crest. Although lubricant is often used to help with sealing, the NPTF provides a dry thread seal by design.

NPT Thread Sizing and Tolerance

NPT threads are measured and sized according to a specific and standardized system (ANSI/ASME B1.20.1). Pitch diameter, major diameter, and minor diameter are used in the sizing of NPT threads. The tolerance requirements for NPT threads are critical for achieving optimal performance and reliability of threaded connections. Tolerance affects the tightness and stability of the connection, and precise tolerances are necessary to prevent leaks and other issues in fluid transfer systems.

Two Categories of ANSI/ASME Threading Standards

Within the commonly used classification of thread fittings, there are two specific types that some people may occasionally mix up: National Pipe Taper (NPT) and National Pipe Straight (NPS).

As mentioned above, NPT threading has a tapered design that helps form a connection that prevents leaks in fluid/gas systems. NPS threads (or sometimes called parallel) fittings, on the other hand, are straight threads that are typically used for quick connections that do not require a tight seal.

NPS threads are level and are mainly used for electrical and mechanical needs. However, proper sealing for NPS fittings also requires additional pieces, such as gaskets or o-rings.

You can determine whether your fitting is a straight or tapered thread by simply measuringthe first, fourth/fifth, and last threads. It is a tapered thread style if the diameter increases or decreases from one end to the other. If the diameter remains the same across, it is a straight thread.

The sharply angled tapered threads of an NPT fitting compress as the pipe screws into a joint, which creates a strong seal. Using a thread sealant with NPT style fittings is best to achieve a fully effective, leak-free seal. (The exception is NPTF fittings, which we'll discuss further down in this article.) In contrast, straight threads only hold the pieces together. NPS connections allow a nut to move freely up and down the connection, whereas NPT connections lock tightly once in place.

NPT and NPS thread fittings are not interchangeable as they will not seal together properly. Inherently, neither system is better than the other. Depending on the purpose, each thread works effectively.

Both threading type specifications are set and maintained by American National Standards Institute (ANSI)

NPT vs. BSP

The other most common type of tapered threaded fittings used in fluid transfer systems is known as BSP (British Standard Pipe) threads. The United States and Canada use the NPT classification system, while the rest of the world mainly uses BSP.

Both standards specify the types of screw threads that fit and seal pipe fittings that are used for different purposes. Within their own classification system, some varieties of threads are compatible. Still, you cannot mix and match between the two classification systems due to the different pitches and thread forms.

One of the main differences between NPT and BSP threads is the pitch of the thread. NPT threads have a finer pitch, which means that the threads are closer together and there are more threads per inch.

Another characteristic to note is that the peaks and valleys differ. Peaks and valleys are flat with an NPT thread, while BSP peaks and valleys are rounded. In addition, NPT fittings will have threads with a 60-degree spread, and BSP threads will have a 55-degree spread. These differences make them incompatible. Therefore, it is necessary to correctly identify your connection to create a safe and leak-resistant seal.

If you do not know which type of fitting you have, you can use a pitch gauge to measure the spread of the threads. The video below shows how to use a screw pitch gauge for something like this.

Finding the Standard Trade Size Match

You will want to know what size thread fittings you are working with. To find the standard trade size, you must first know whether the thread is straight or tapered. Then you can measure the diameter of any thread on an NPS fitting, or if it is an NPT fitting, you will need to measure the fourth or fifth thread.

This measurement is then referenced in a thread table to match its trade size. Be aware that the trade size does not directly represent what is measured, i.e., ½ inch NPT does not equal ½ inch measured.

NPT is the most widely known and used fitting. The most common US supplier sizes are 1'8, 1'4, 3'8, 1'2, 3'4, 1, 11'4, 11'2, 2, 21'2, 3, 4, 5, and 6 inches. Fittings less than ' inch are sometimes used with compressed air, while the larger sizes (over 2 inches) are less common, mainly because other joining methods are needed.

Advantages and Disadvantages of Using NPT Fittings

What Are the Advantages of Using NPT Fittings?

NPT threads are commonly used in industries such as oil and gas, chemical processing, water treatment, and many others where leak-free connections are of the utmost importance. The tight seal and stability provided by NPT threads make them well-suited for use in challenging environments, such as high-temperature, corrosive, and abrasive conditions. Additional benefits includes:

• An effective lubricated seal.
• Readily available with vendors.
• Easy to find in multiple sizes.
• Various materials are available (cast iron, bronze, steel, brass, PTFE, nylon, PVC).
• Easier to assemble without the use of sleeves, seals, or o-rings.

What Are the Disadvantages To Using NPT Fittings?

Though NPT threading is optimal for most fluid and gas systems, there are some negatives:

• Requires lubricant (except NPTF), which can cause system contamination.
• Not suitable for the highest pressures.
• Best used for single assembly as sealing performance can decrease with reassembly.
• Larger sizes are more prone to leaking due to leak path size.
• Prone to damage from over-tightening.
• Harder to orient shaped connectors.

Other Common American Thread Types

NPT and NPS are not the only types of American threading types. There are several others, including NPSF, NPSC, NPTR, NPTF, NPSH, NPSM, NPSI, and NPSL. Each of these threads has its own unique characteristics and is used in specific applications.

• NPSF: National Pipe Straight Fuel - for dry seal applications with soft materials.
• NPSC: American Standard Straight Coupling Pipe Thread - for general couplings.
• NPTR: American Standard Taper Railing Pipe Thread - for railings.
• NPTF: American Standard Pipe Thread Tapered (Dryseal) - for dry seal connections in nearly every type of service, especially fuel connections. A lubricant is often used to ease assembly.
• NPSH: National Pipe Straight Hose - for water suction pipes and discharge hose couplings.
• NPSM: American Standard Straight Mechanical Pipe Thread - for rigid mechanical uses.
• NPSI: National Pipe Straight Intermediate - for free-fitting mechanical joints for fixtures.
• NPSL: American Standard Straight Locknut Pipe Thread - for use with locknuts.

Other Common British Thread Types

BSP (British Standard Pipe) is the standard used in the commonwealth countries and Australia. BSP is based on trade size rather than actual diameter.

• BSPP: British Standard Pipe Thread Parallel - requires a bonded seal ring.
• BSPT: British Standard Pipe Thread Tapered - metal wedging creates the seal.

Both fittings in a BSPT connection do not have to be tapered, unlike NPT connections. For instance, you can use a female fitting that is tapered with a straight male fitting. However, all BSPT connections must use a thread sealant.

NPT/NPS style threads are similar to BSP, but as we mentioned above, there are differences in thread dimensions, pitch, and angle, which make them incompatible. Another important difference between BSP threads and NPT threads is that BSP threads are measured in metric units, while NPT threads are measured in imperial units.

In Conclusion

It is vital to know whether you are working with NPT or BSP standard thread fittings. With that information, you will want to consider your pipes' purpose to determine which specific thread type you need for your valves and other fittings. You can then move forward with finding the correct thread fittings: straight, tapered, male, or female.

Being clear on your NPT thread fitting will help to ensure you have the proper materials to achieve the results you're looking for.

Thank you for taking the time to learn about NPT threads and their variants. If you're in need of solenoid valves or automated ball valves with NPT threaded fittings for your fluid transfer system, electricsolenoidvalves.com has a wide selection to choose from, available with the shortest lead times. Our knowledgeable team is here to answer any questions you may have and help you find the best threaded fitting for your specific needs. Don't hesitate to reach out for assistance ' we're here to help!

U.S. national technical standards for threads on pipes and pipe fittings

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. 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

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

Abbreviation Shorthand expansion Full name Comment NPT National pipe taper American National Standard Taper Pipe Thread Tapered and needs thread sealant; for connections in nearly every type of service NPS National pipe straight American National Standard Straight Pipe Thread For rigidity; sealable with o-ring or gasket; sometimes male straight is mated with female tapered for low-pressure sealing (need to verify) NPSC National pipe straight'coupling American National Standard Straight Pipe Thread for Couplings For general couplings NPSF National pipe straight'fuel and oil Dryseal USA (American) Standard Fuel Internal Straight Pipe Thread Internal only, straight NPSH National pipe straight'hose American National Standard Straight Pipe Thread for Hose Couplings For hose couplings NPSI National pipe straight'intermediate Dryseal USA (American) Standard Intermediate Internal Straight Pipe Thread Similar to NPSF but slightly larger; internal only, straight NPSL National pipe straight'locknut American National Standard Straight Pipe Thread for Loose-fitting Mechanical Joints with Locknuts For use with locknuts NPSM National pipe straight'mechanical American National Standard Straight Pipe Thread for Free-fitting Mechanical Joints For various rigid mechanical uses NPTF National pipe taper'fuel and oil Dryseal USA (American) Standard Taper Pipe Thread For dryseal connections in nearly every type of service, especially fuel connections NPTR National pipe taper'railing American National Standard Taper Pipe Thread for Railing Joints For railings PTF-SAE SHORT Pipe taper, fuel, SAE, short Dryseal SAE Short Taper Pipe Thread Named after SAE International; similar to NPTF but one turn shorter

Uses

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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, 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, 3+1'2, 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

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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.° ' 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

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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)

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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

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American Society of Mechanical Engineers National Pipe Thread Taper (NPT)

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Nominal
pipe size Thread
density Thread pitch P Hand-tight engagement

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Effective thread

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Overall
length L4

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6

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Actual outside
diameter D Tap drill Length L1 Turns Diameter E1 Length L2 Turns Diameter E2 inch inch'1 inch mm inch inch inch inch inch inch mm inch mm

1

'

16

27 0. 0. 0. 4.32 0. 0. 7.05 0. 0. 0.313 7.950

1

'

8

27 0. 0. 0. 4.36 0. 0. 7.13 0. 0. 0.405 10.287 0.339 8.

1

'

4

18 0. 1. 0. 4.10 0. 0. 7.23 0. 0. 0.540 13.716

7

'

16

11.113

3

'

8

18 0. 1. 0. 4.32 0. 0. 7.34 0. 0. 0.675 17.145

37

'

64

14.684

1

'

2

14 0. 1. 0. 4.48 0. 0. 7.47 0. 0. 0.840 21.

23

'

32

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18.

3

'

4

14 0. 1. 0. 4.75 0. 0. 7.64 1. 0. 1.050 26.

59

'

64

23. 1

11

+

1

'

2

0. 2. 0. 4.60 1. 0. 7.85 1. 0. 1.315 33.

1

+

5

'

32

29.

1

+

1

'

4

11

+

1

'

2

0. 2. 0. 4.83 1. 0. 8.13 1. 1. 1.660 42.

1

+

1

'

2

38.

1

+

1

'

2

11

+

1

'

2

0. 2. 0. 4.83 1. 0. 8.32 1. 1. 1.900 48.

1

+

47

'

64

44. 2

11

+

1

'

2

0. 2. 0. 5.01 2. 0. 8.70 2. 1. 2.375 60.

2

+

7

'

32

56.

2

+

1

'

2

8 0. 3. 0. 5.46 2. 1. 9.10 2. 1. 2.875 73.

2

+

5

'

8

66. 3 8 0. 3. 0. 6.13 3. 1. 9.60 3. 1. 3.500 88.

3

+

1

'

4

82.

3

+

1

'

2

8 0. 3. 0. 6.57 3. 1. 10.00 3. 1. 4.000 101.

3

+

3

'

4

95. 4 8 0. 3. 0. 6.75 4. 1. 10.40 4. 1. 4.500 114.

4

+

1

'

4

107.

4

+

1

'

2

8 0. 3. 5.000 127.

4

+

3

'

4

120. 5 8 0. 3. 0. 7.50 5. 1. 11.25 5. 1. 5.563 141.

5

+

9

'

32

134. 6 8 0. 3. 0. 7.66 6. 1. 12.10 6. 1. 6.625 168.

6

+

11

'

32

161. 8 8 0. 3. 1. 8.50 8. 1. 13.70 8. 2. 8.625 219. 10 8 0. 3. 1. 9.68 10. 1. 15.40 10. 2. 10.750 273. 12 8 0. 3. 1. 10.88 12. 2. 17.00 12. 2. 12.750 323. 14 8 0. 3. 1. 12.50 13. 2. 18.00 13. 2. 14.000 355. 16 8 0. 3. 1. 14.50 15. 2. 19.60 15. 2. 16.000 406. 18 8 0. 3. 2. 16.00 17. 2. 21.20 17. 3. 18.000 457. 20 8 0. 3. 2. 17.00 19. 2. 22.80 19. 3. 20.000 508. 24 8 0. 3. 2. 19.00 23. 3. 26.00 23. 3. 24.000 609. Hand-tight and effective thread engagement lengths

History

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In , 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

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References

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Bibliography

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