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Threaded Flanges in Piping Systems

When choosing flanges, there are different shapes and sizes, ranging from the welding neck and long welding neck flanges to the slip-on, socket weld, lap joint, threaded, and blind flanges. So let us dive in and find out more about flanges a bit.

Flanges with Classifications -The Standard Types

The selection of a flange for a pipe installation largely depends on the needed robustness of the flanged joint. As opposed to a welded connection, a flanged joint can be disassembled rapidly and with ease, thus making maintenance easier.

It is easy to classify different flanges based on their individual shape. However, there are additional methods used. These include classification by material grade, type of face (raised, flat, etc.), finish (smooth, serrated, etc.), and dimensions (size and pressure rating).

Threaded Flanges – An overview 

Threaded flanges, called “screwed flanges,” are affixed to a pipe using a screw thread. The pipe’s end has a male thread, and the flange’s bore has a female thread. Subsequently, the male-threaded pipe is secured in the female-threaded flange.

The ASME B1.20.1-compliant threaded flange is especially useful in highly explosive environments where welding could be dangerous. An external taper pipe threaded pipe help connect the threaded flange to the pipe.

Generally, engineering organizations restrict their usage to services such as instrument air, plant air, nitrogen, potable water, and raw water of class 300 rating and sizes between 1/2′′ and 4′′ inches. 

Threaded Flanges are used in cast iron and galvanized piping. However, it is significant to note that the specifications for threaded joints outlined in the following paragraph apply to threaded flanges. Using threaded flanges in any service susceptible to extreme erosion, crevice corrosion, or cyclic loading is not recommended.

The tension of the piping system must be rigorously checked when implementing threaded flanges to safeguard against leakages. While threaded flanges are viable for a class 2500 pressure setting at room temperature, weld-neck slip-on or socket weld flanges have a higher quality.

Although seal welding of a threaded flange can hinder leakage through its threads, it is not expected to provide any extra strength.

The American Society of Mechanical Engineers (ASME) B31.3 specifies that threaded flanges are limited to Category D Fluid service when the pipe ends protrude from the flange and form a sealing surface.

Although they come in various diameters and pressure ratings, they are usually employed in small-diameter Piping Systems (under 4 inches) in non-toxic, low-pressure, and low-temperature systems. 12-inch to 2-inch threaded flanges are far more common than those larger than 2 inches, which feature flat and raised faces due to their application in low-pressure systems.

Threaded flanges do not necessitate welding for connecting pipes, making them desirable for use in hazardous environments such as fuel stations, hexane facilities, and places where flammable liquids are kept.

Furthermore, this form of union is also faster and more convenient to assemble and dismantle compared to welded flange joints. Unfortunately, they are unsuitable for applications as their geometrical shape tends to warp in high-temperature conditions, often causing leakage.

With thin wall thickness, cutting threads on pipes can be difficult. Hence, using threaded flanges on such pipes is usually not advisable. Instead, a seal weld can be utilized to increase the strength of the flange and reduce the chance of leakage,. However, this makes the union permanent and more intricate to install and dismantle.

Organizations and associations in engineering largely restrict threaded flanges to pressures no higher than 300 and sizes between 12 and 4 inches. Without a weld joint, threaded flanges are more likely to allow leakage than those with welded joints, and incorrect alignment can result in a damaged seal. For applications with severe consequences, threaded flanges should be avoided if there is a leak.

Flanges up to Class 400 are suitable for threaded assembly in compliance with ASME B16.5 para 6.9.1. The aim is for the pipe’s threaded end to align closely with the face of the flange when the two are joined together. In Classes 150, 300, and 400, the internal thread length must adhere to the ASME B1.20.1 standard stipulated in the appendix of ASME B16.5.

According to ASME B1.20.1, the thread length (L2) of a 24-inch pipe is 3.25 (83 mm). This measurement complies with the minimum thread length (T) of 83 mm for 24″ Flanges up to Class 400 as stipulated by ASME B16.5. For Flanges of Class 600 and above, Threaded Flanges should be assembled accordingly.

In order to meet ASME B1.20.1 requirements for threaded flanges in Class 600 or higher, the minimum thread length (T) must exceed the length of the effective external thread on the pipe end (L2). For instance, an ASME B16.5 24-inch Class 600 flange requires a minimum thread length (T) of 93 mm, but the available thread length (L2) for a 24-inch pipe is 83 mm.

To ensure compliance, a pipe wall thickness of schedule 80 or heavier must be used. The length of the effective external thread on the pipe end must be extended beyond ASME B1.20.1 specifications, bringing the end of the pipe closer to the mating surface of the flange.

ASME B165’s Table 1-1 further states that power equipment must be used to ensure proper interaction and installation of threaded flanges in Class 600 and higher.

Performance characteristics exhibited by Threaded Flanges

It is significant to consider different other factors that may affect the performance attributes of the flange. Below is a compilation of the features to be considered:

  • Durability

The durability of a pipe flange is determined by its strength and ability to resist stress or pressure. Therefore, the compatibility of the pipes to the flanges, the design, and the material’s strength all play an integral role.

  • Straightforward Assembling 

It is a qualitative measurement of the construction and deconstruction processes’ success. Achieving simple assembly and disassembly is necessary for situations where the flange is utilized as a permanent or interim link.

  • Weight

The size and density of the flange determine its weight. Therefore, the industrial buyer should ensure that the pipe and support system accommodate the extra mass for heavy-duty applications.


Threaded flanges are sized up to 4 inches and are made for various pressure ratings. Primarily, these are used for small piping with low-pressure, low-temperature utilities, such as water and air.

Moreover, threaded flanges are essential in environments such as gas stations and plants where welded connections may present a safety risk.


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9 months ago

Threaded flanges seem like a versatile solution, especially in environments with safety concerns around welding. It’s interesting how they’re limited to specific pressure ratings and sizes.

Eng Mike
Eng Mike
9 months ago

The classification of flanges based on shape, material grade, and other factors provides a comprehensive approach for selecting the right type for a given application.

9 months ago

What advantages do threaded flanges offer in terms of safety in hazardous environments?

Eng Mike
Eng Mike
9 months ago
Reply to  SteveM

Threaded flanges are safer to use in environments like gas stations and plants due to the absence of welding, which reduces the risk of fire or explosions caused by welding sparks. This makes them a suitable choice for locations where flammable substances are present.

S Ford
S Ford
9 months ago

Are there any limitations to using threaded flanges, especially in terms of pressure and size?

9 months ago
Reply to  S Ford

Yes, threaded flanges are typically limited to pressure ratings not exceeding 300 and sizes ranging from 12 to 4 inches. They are more suitable for smaller diameter piping systems with lower pressures. For higher pressure applications, other types of flanges like weld-neck or slip-on flanges are preferred.

9 months ago

The performance characteristics highlighted, such as durability and straightforward assembly, emphasize the importance of considering practical aspects beyond just technical specifications.