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Threaded Connections in Piping: Threaded Pipe Fittings

What are Threaded Connections?

Threaded connections are frequently used when it comes to small bore pipework with nominal sizes of NPS 2 or less. This is the earliest and most well-liked way of linking pipes. For non-critical applications with lower temperatures and pressure services, threaded pipe fittings are employed. 

The plumbing industry uses threaded fittings the most frequently. The threads per inch, pitch diameter, and typical engagement lengths for all pipe sizes are provided by the American Standard, which also acts as the dimensional standard for taper pipe threads. 

Forging is used to create threaded steel fittings. They come in a variety of thread types, including NPT, BSPP, BSPT, PF, PT, and MPT. Even though threaded fittings are mostly used for connecting small bore pipework, they are nevertheless available in sizes up to 4″ and are occasionally employed.

Cast grey or malleable iron, cast brass or bronze, or forged alloy and carbon steel are the typical materials used to make threaded fittings.

Class 2000, 3000, and 6000 are the three pressure ratings that are offered.

Uses of Threaded Fittings

Threaded Fittings are not suitable for applications involving higher pressure, higher temperature, or cycles. As a result, they are employed in the following less crucial, low-pressure applications such as water distribution, potable water, plant air, cooling systems, and fire protection.

Tools Required to Identifying Thread

Two main tools are necessary for identifying the thread, they are:

  • Caliper- A good instrument for measuring the inside and outer diameter of a female and male thread is a caliper. The most accurate and exact measurements can be made with a caliper, but a straight steel ruler works just as well.
  • Pitch Gauge- Threads per inch are measured with a pitch gauge. This instrument counts the number of threads in metric threads.

How to Identify Different Types of Threads

It is necessary to examine and determine the type of thread to be used in order to ensure better application of the product. One can do so by ensuring certain dimensions such as:

Difference between male and female threads

In order to differentiate one may start by examining the locations of the threads. It’s a male thread if they’re on the outside of it. It’s a female thread if they’re on the inside of it. The functionality of the thread isn’t always impacted by if it is male or female. The difference merely acts as a means of separating the two connections.

Identifying Tapered and Parallel Threads

It is significant to note that parallel threads maintain their diameter while tapered threads get narrower as they move outward. A caliper can be a helpful tool if a visual inspection is not sufficient to determine this attribute. A person can measure the first, fourth, and last complete threads with the caliper to notice a difference between tapered and parallel threads. It is parallel if all of the measurements are the same. It is tapered if the measures get smaller.

A few types of tapered threads are NPT/NPTF, BSPT, and Metric Tapered. These threads wedge metal against metal or slightly distort the threads to establish a seal. To ensure a tight seal, parallel threads frequently need an o-ring or thread tape.

Pitch Size

The number of threads per inch or the space between threads for metric thread types determines a thread’s pitch size. A pitch gauge is a very advisable tool to be used because pitch sizes can be extremely similar, even though a ruler might be used to determine the pitch size. For obtaining the optimal fit, one may use a pitch measure to test a few different sizes.

Thread Size and Diameter

To determine the size, a person can start by measuring the outside diameter in case it is a male thread and the internal diameter in case of a female thread once again using the caliper. Small variances are unavoidable because various manufacturers produce the material slightly distinctly. 

It is important to determine whether the thread is a pipe thread (NPT/NPTF, BSPT, BSPP) or not (UN/UNF, Metric Parallel, Metric Tapered) before one begins with the applications. It is advisable to keep in mind that a tapered thread may or may not be a pipe thread. In order to distinguish, one may compare the thread size with a nominal size profile if the pipe has a thread. It can be done with the help of the caliper tool as it can determine the outside diameter of the tread for UN/UNF, Metric Parallel, and Metric Tapered non-pipe threads.

Finding the thread type

Establishing the standard is the last step in determining the thread type. Standards for thread types include NPT, PT, and G.

Types of Threads:

  • NPT/NPTF Thread Type (National Pipe Tapered Fuel)

North America is the region that uses this thread type the most. Its self-sealing, tapered outer and inner diameter will help the person identify it. The threads’ flanks compress against one another when they are torqued, creating a leak-tight seal. To get better performance and ensure a truly leak-tight seal, it is still advised to apply PTFE tape or another sealant material. 

NPTF is a partially compatible variation of NPT (National Pipe Taper Fuel). It ensures an even tighter seal with no leaks. However, it is crucial to remember that combining these types reduces their ability to remain leak-free. NPT threads need to be greased using lubricating paste or tape and clear of burrs. By doing this, the consumer can prevent thread corrosion, which might make future disassembly practically impossible.

  • BSP Thread Type (British Standard Pipe)

For the purpose of connecting and sealing pipe ends, this is a universally recognized standard thread type. It is employed throughout all of Europe. BSP threads come in two varieties: BSPP and BSPT. Parallel or straight threads are referred to as BSPP. BSPT, on the other hand, refers to tapered threads. BSPP threads and BSPT threads are occasionally referred to as G threads and R threads, respectively. 

It is important to note that BSPT thread and JIS tapered pipe thread (PT thread) can be used interchangeably

  • PT Thread

The PT and BSPT thread types are interchangeable and identical. However, the male PT thread cannot pair with the female BSPP swivel with a conical seat because it lacks a 30-degree flare. In order to guarantee a leak-free seal, it is also advisable to use a thread sealant with PT threads.

  • Straight Thread O Ring Boss

The 90-durometer Buna-N “O” Ring allows straight threads to seal, as specified by SAE, the Society of Automotive Engineers. This thread type is very dependable and reusable. The O-Ring on this thread type prevents the male and female threads from crushing together, which is required by some thread types to create a seal.

  • Parallel Thread Type

The most prevalent thread type in Europe is metric. It has a millimeter-accurate cylindrical inner and outer diameter. Metric tapered thread’s precise taper makes for the best possible force transmission. Metric threads are denoted in writing by a capital “M” and a mention of their nominal outer diameter (ex. M22 x 1.5). Last but not least, the consumer should make sure to use a metric pitch gauge when determining pitch size.

Different Fittings available for Threaded Pipe Systems:

  • Threaded 90 Degree Elbow- 

90 Degree Elbow can be used for threaded piping systems’ 90-degree piping turns. According to ASME B16.11, there is a size range of 1/8″ to 4″ that can be used for piping that needs to shift direction by 45 degrees.

  • Threaded 45 Degree Elbow- 

The 45 Degree Elbow can be used for piping that needs to shift direction by 45 degrees. According to ASME B16.11, sizes between 1/8″ and 4″ are available. 

  • Threaded Cap- 

It is typically used to seal the pipe’s end. Similar to others, according to ASME B16, sizes between 1/8″ and 4″ are available.

  • Threaded Straight/Equal and Reducing Tee- 

They are generally used to link 90-degree branches of the same or smaller size. According to ASME B16.11, sizes between 1/8″ and 4″ are available.

  • Threaded Cross-

The term “four-way fittings” also refers to threaded crosses or cross tees. It has three outlet connections and one intake. In pipe crosses, the flow is thereby dispersed in three directions. Two 90° branches from the main run pipe direction are created by threaded crosses. Crosses connect male piping components securely because they have female threads. They are also available in sizes between 1/8″ and 4″. 

  • Threaded Full Coupling- 

They are used to attach pipes to one another in a straight line.

  • Threaded Half Coupling- 

Commonly utilized to join large bore welded pipe to a small bore threaded branch. Threaded couplings come in a variety. The primary distinction between a full coupling and a half coupling is that a full coupling contains threads on both ends, allowing for the threading of numerous pipes at various possible spots to create a full coupling. Threaded complete couplings that can support 3000 pounds are offered. The only difference is that a half coupling threads only from one end. Many times, the half coupling does in fact have a length of the half, but they are termed half couplings because you can only couple one-half of the pipe to them. They are about half as long. Both full and half coupling are available in sizes between 1/8″ and 4″.

  • Reducing Coupling- 

These fixtures, as the name suggests, serve as reducers in networks of threaded piping. 

  • Hex Plug- 

They are used to seal off one end of a threaded fitting, like an elbow or a tee. The head has a hexagonal form.

  • Close Nipple- 

The outside diameter of the pipe is standard. The length can be determined according to the needs and demands of the project. It is generally constructed from a common pipe section.

  • Both End Threaded Nipple- 

It is made from common pipe pieces in accordance with project specifications. It is used to join two threaded fittings that are close to one another. The outside diameter of the pipe is standard.

  • POE TOE Nipple- 

Its function is to combine threaded small bore piping with small bore piping that has been socket welded.

  • Pipe to Tube Connector- 

The typical use comprises connecting instrumentation tubes to air pipes. When looking at other uses, usually utilized for analyzer tubing as well.

  • Hexagon Bushing- 

Hexagonal-headed threaded bushings are used to link threaded pipes of various sizes. Thus, the threaded bushing can help reduce the size.

  • Unions- 

Unions make it simple to install, remove, or replace the lengths of pipes, valves, or vessels in screwed piping systems. In order to remove a valve, it must have at least one adjacent union. Along with that, each outlet from a vessel with threaded connections needs to have a union between the valve and the vessel in order to remove the pipe from the vessel. Although different facings are available, ground-faced joints are favored within Union. 

The design of threaded unions is based on MSS-SP 83. Three connected pieces make up a threaded union, which is utilized for installation and maintenance. Male to female unions, female to female unions, lug nut unions, and Rockwood unions are all available. Both components were joined by the lug nut.

  • Reducing Insert- 

These fittings are utilized to join together two female threaded fittings of various diameters.

  • Square Head Plug- 

They are square in shape. They are used to seal one end of a female threaded fitting so that it can expand in the future. According to ASME B16.11, sizes between 1/8″ and 4″ are available.

  • Round Head Plug-

They have a circular shape to them. They are utilized in sealing vents and draining holes in valves, steam traps, and other devices. Further uses include sealing the female threaded pipe fittings’ ends.

  • Flush Bushing- 

They are utilized to join smaller-diameter pipes to female threaded fittings of larger diameter.

Materials Used for Threaded Fittings:

Following are the materials for construction:

A105– Standard Specification for Forgings of Carbon Steel for Piping Applications.

A18– Standard Specification for General-Purpose Piping Forgings Made of Carbon Steel.

A182– Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, Forged Valves, and Forged Parts for High-Temperature Service, Standard Specification.

A350– Standard Specification for Forgings of Carbon and Low-Alloy Steel, Requiring Notch Toughness Testing for Piping Components

Other materials that can be used are duplex stainless steel and cooper nickel. 

Standards for Threaded Connections:

Threaded pipe fittings are designed in accordance with the following standards:

  • According to the ASME B16.11 standards- forged fittings, socket welding, and threaded.
  • In accordance with MSS-SP-83- class 3000 Steel Pipe Unions, Threaded and Socket-Welded. 
  • As per ASME B16.3- Malleable iron threaded fittings. 
  • According to the ASME B16.4- Gray iron threaded fittings.
  • Malleable iron threaded pipe unions, as per ASME B16.39.
  • As per, ASME B16.34, Valves, Flanged, Threaded, and Welding End 
  • According to ASME B16.39, malleable iron threaded pipe unions, Class 150, 250, and 300. 

Advantages of Threaded Connections:

The primary benefits of threaded fittings are

  • Threaded Connections are rapid to install.
  • Their leakage integrity is reliable for applications requiring low temperature and they are highly suitable for low-pressure applications as well.
  • Installation is time-saving and doesn’t require any special skills.
  • The connecting process requires less force.
  • Their parts are detachable.
  • They are affordable and cheaper. Hence, they can be considered to be cost-effective. 

Disadvantages of Threaded Connections:

Along with advantages, they offer some disadvantages as well, they are:

  • They are not suitable for high-temperature-pressure applications.
  • Threads might not handle a corrosive environment well as there possess chances of corrosion.
  • Due to temperature changes, there may be a possibility of leakage problems.
  • Fatigue damage can occur for cyclic services.
  • Their strength is less compared to that of welding.
  • Due to the high stress amplification effects induced by the sharp notches at the base of the threads, vibration can cause fatigue failures of screwed pipe joints.

Further Suggestions:

In corrosive and erosive situations or where cyclic loads may take place, threaded joints should be avoided.

Threaded joint pipework should be laid correctly to minimize joint stress, paying particular attention to strains from thermal expansion and valve operation.

Under extremely cyclic situations, threaded components of a specialized character that are not subject to external moment loading, like thermometer wells, may be employed.

Difference between malleable iron and forged threaded fittings:

Plumbing and light industrial applications both use fittings made of malleable iron. These threaded fittings are composed of forged steel and have a high-pressure rating. One significant distinction is that you cannot weld a threaded fitting while you may with a fitting made of malleable iron. Again, this is a threaded fitting, but depending on your use, you could be better off using forged carbon steel if you need to weld it. Additionally, fittings made of malleable iron have 150- and 300-pound strength ratings. Much less pressure can be applied to a threaded fitting, which has a 3000 and 6000-pound pressure rating.

Difference between Threaded fittings and socket welded fittings:

When it comes to threaded fittings, screws are used to attach threaded fittings to pipes or pipe parts. They leak more easily and have less strength and dependability. They can be obtained in 2000#, 3000#, and 6000# pressure classes. 

On the other hand, Socket welded fittings are linked by fillet weldings. They are more trustworthy and give better results. They are stronger and more enduringly connected. Available in additional pressure class 9000#.


Pressure fitting is a wide range of fittings which includes threaded pipe fitting. The pressure ratings for threaded fittings range from 2000 pounds to 6000 pounds. 2000 pounds basically corresponds to schedule 80, whereas 3000 pounds roughly corresponds to schedule 160. The maximum pressure rating for the 3000-pound threaded fitting is 3000 pounds. The pressure rating is a function of temperature, therefore as the temperature rises, the pressure rating decreases, making this a bit more difficult. They come in forged carbon steel, A105, A105 N, stainless steel, and nickel alloys, among other materials.

They prove to be popularly used in the piping industry as they offer various advantages despite some drawbacks. They have high demand when the applications require delicacy in terms of the temperature along with the pressure put on them. 

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