Threaded Flanges: Where They Fit in Modern Piping Systems

Modern piping systems are engineered to safely and efficiently move liquids, gases, steam and other process media. Pipes are the primary flow conduits, but flanges are key connection points between pipes, valves, equipment and other elements. There are several types of flanges to choose from, with threaded flanges having their own unique use when welding might not be the preferred option or viable choice.

External threads are attached to pipes, and threaded flanges (also known as screwed flanges) are mated with the external threads. They are not usually welded into place, so they can be helpful in some pipe applications where welding would be difficult, undesirable, or limited.

What Is a Threaded Flange?

A pipe flange that has internal threads is known as a threaded flange. The threaded part is made to fit with the outside threads of the connecting pipe.

A threaded flange is not welded to the pipe, as with weld-neck or slip-on flanges. This feature can make installation easier in some cases.

These are some of the key features:

• Internal threaded bore
• Bolted flange connection
• Direct flange-to-pipe welding is not allowed.
• Relatively simple installation
• Designed for selected low- to moderate-temperature applications
• Various pressure and size options

Always verify the suitability of a threaded flange with the pertinent piping standard and engineering specification.

Where Threaded Flanges Are Commonly Used

Threaded flanges can be considered for piping systems where welding is not practical. They might be helpful in smaller diameter piping, utility systems and in some cases when the pipe material or installation environment is less conducive to welding.

Potential applications include:

• Utility piping
• Compressed-air systems
• Water lines
• Selected process piping
• Maintenance-friendly piping arrangements
• In some hazardous area applications where hot work is prohibited

The operating conditions are still a key factor. Various factors like temperature, pressure, vibration and thermal cycling can affect the applicability of a threaded connection.

Advantages of Threaded Flanges 

The advantage of threaded flanges is that they can solve the problem of direct welding between the flange and pipe.

This can provide practical advantages such as:

Easier Installation

Conventional tools can be used to assemble threaded connections without a welding operation at the flange connection.

Reduced Hot Work

When welding or other hot work is limited, a threaded connection might be an alternative where such a connection is allowed by the relevant engineering requirements.

Convenient Maintenance

Bolted flange connections can be taken apart for inspection, cleaning or component replacement.

Suitable for certain small bore systems

For specific smaller piping installations where their design and pressure specs are applicable, threaded flanges may be beneficial.

Limitations of Threaded Flanges 

While threaded flanges are beneficial, they come with their disadvantages. Applications requiring high temperature, high thermal cyclic, vibration or high pressure can be a factor.

When a structure or component is connected to others and the temperature changes, there may be thermal expansion differences. Likewise, if the system is not designed and secured properly, a vibration can also impact a threaded connection.

For this reason, engineers should assess:

• Operating temperature
• Pressure
• Thermal cycling
• Vibration
• Pipe material
• Fluid characteristics
• Corrosion conditions
• Codes and standards that apply to piping.

Hence, the choice of threaded flanges must be, not only for the ease of installation, but also for engineering requirements.

Threaded Flanges Compared With Other Flange Types 

There are different types of flange designs for various purposes. Weld neck flanges are typically used in high-stress applications, and slip-on flanges have a wide range of applications in general industrial systems. Blind flanges can be used to close a portion of piping and lap joint flanges can offer flexibility in assembling piping.

The threaded flange is a more specialized part. The difference between their is the threaded connection between the flange and pipe.

Project teams need to take the entire piping design into account when selecting flange types rather than the only functionality.

Material Selection for Threaded Flanges

Material selection also is important. The threaded flange can be made of carbon steel, stainless steel, alloy steel and other materials as required by the project.

For instance, if corrosion resistance is a concern, stainless steel threaded flanges can be chosen. In appropriate industrial and utility applications, carbon steel choices can be employed.

The material of the flange needs to be compatible with the pipe, fluid, temperature, pressure and surrounding environment.

Quality Control and Inspection 

It is particularly significant to the quality of threads as they are the main mechanical connection between pipe and flange. Damaged or improperly sized or improperly formed threads can cause difficulties in installation and impact connection reliability.

These are some of the possible quality checks that can be made:

• Dimensional inspection
• Thread inspection
• Visual examination
• Material verification
• If applicable, pressure-related testing is performed.
• Marking and traceability
• Final inspection

The specific inspection needs will depend on the document standard and project specification.

Conclusion

While not particularly common, threaded flanges still have their place in modern piping systems. Their "non-welding" capability allows them to be used in selected low- or moderate-service applications, in utility systems, for maintenance, and in areas where welding is impractical.

However, they should not be chosen just because of the ease with which they can be installed. All of this needs to be taken into account, including pressure, temperature, vibration, fluid properties, material compatibility and applicable standards.

Threaded flanges sourced using an experienced Flange Manufacturer in India can help contractors, engineers, architects and procurement teams ensure that the flanges are sourced according to the technical requirements of the project.