How To Choose Flange: Key Considerations for Optimal Pipeline Performance

Last time, we mentioned 15 different flanges; with so many flanges, how do we choose the correct flange to connect your pipes? This article will detail the five critical factors for selecting a flange.

1. Application Environment and Pressure Rating

When selecting flanges, consideration of the application environment and pressure rating is crucial. Here's a detailed description:

Application Environment:

• Temperature: Firstly, it's essential to consider the temperature range of the pipeline system. High-temperature environments may require materials like alloy steel flanges to ensure stability and corrosion resistance.
• Corrosiveness: Understanding the corrosiveness of the medium to the flanges is essential. Corrosive mediums require materials with good corrosion resistance, such as stainless steel or special alloy steel.
• Environmental Conditions: Consider the environmental conditions the pipeline system is exposed to, such as outdoor exposure or humid environments. Choose materials and coatings with good weather resistance to extend the flange's lifespan.

Pressure Rating: 

• Operating Pressure: Understanding the required operating pressure level of the pipeline system is critical. Select flanges with appropriate pressure ratings to withstand the system's operating pressure without leakage or rupture.
• Design Pressure: Additionally, consider the design pressure of the pipeline system to ensure the selected flanges' pressure ratings not only meet the current operating pressure but also accommodate potential future pressure increases.
• Emergency Situations: To account for potential emergencies, choose flanges with slightly higher pressure ratings than the operating pressure to provide a safety margin and prevent failure in unexpected situations.

2. Flange Material Selection:

The choice of material for the flange is crucial as it directly impacts its corrosion resistance, wear resistance, and temperature range. Common flange materials include carbon steel, stainless steel, and alloy steel. Select the appropriate material based on the properties of the medium, corrosion resistance, and temperature requirements.

• Carbon Steel: Carbon steel flanges are among the most common choices due to their excellent mechanical properties, machinability, and cost-effectiveness. They are suitable for general industrial applications and low-temperature environments but may have limitations in high-temperature or corrosive environments.
• Stainless Steel: Stainless steel flanges offer excellent corrosion resistance and mechanical properties, making them suitable for applications sensitive to corrosive media, such as chemical processing, food processing, etc. The choice of stainless steel flanges typically depends on the caustic nature of the medium and environmental conditions.
• Alloy Steel: Alloy steel flanges have excellent high-temperature and corrosion resistance, making them suitable for high-temperature, high-pressure, and corrosive media pipeline systems. Alloy steel flanges are commonly used in refining, chemical, and energy industries but come with higher costs.
• Cast Iron: Cast iron flanges are widely used in low-pressure, low-temperature, and general industrial applications due to their relatively low cost. However, cast iron flanges have poor corrosion resistance and mechanical properties and are unsuitable for some demanding environments.
• Plastic: Plastic flanges can be an option for special applications, such as chemical transportation or corrosive media. Plastic flanges are typically made of polyethylene and polypropylene and offer good corrosion resistance. However, they may have limitations in high-temperature or high-pressure environments.

3. Size and Specifications:

Ensure that the selected flange size and specifications match the requirements of the pipeline system. This includes outer diameter, inner diameter, flange thickness, number, and diameter of bolt holes. Inappropriate sizes and specifications may lead to installation difficulties or poor performance

how to measure flange size？

• Nominal Diameter: First, measure the nominal diameter of the pipe, which is typically the outer diameter of the pipe that the flange connects to.
• Flange Outer Diameter: Use a tape measure to measure the outer edge of the flange, the distance around the very perimeter of the flange.
• Bolt Hole Center-to-Center Distance: Measure the distance between the centers of bolt holes that are directly opposite each other. This measurement is crucial for confirming the model and standard of the flange.
• Bolt Hole Diameter: Measure the diameter of the bolt holes to determine the size of the bolts needed.
• Number of Bolt Holes: Count the number of bolt holes on the flange.
• Flange Thickness: Measure the flange from one side to the other, that is, the thickness from one face of the flange to the opposite side.

4. Connection Method

When choosing a connection method, you need to consider the specific requirements of the pipeline system, including sealing performance, load-bearing capacity, ease of installation and maintenance, and other factors. Depending on the needs, choosing the appropriate connection method can ensure the safe and reliable operation of the pipeline system.

Weld Neck Flange:

It has a long tapered neck that enhances strength and supports heavier pipelines. It is also designed to withstand heavy stress conditions. The excellent stress distribution and smooth transition area make it ideal for high-temperature and pressure applications.

Application: Widely used in petroleum and natural gas pipelines, chemical plants, and power stations' high-pressure and high/low-temperature systems.

Slip-on Flange:

Slip-on flanges are easy to install and less costly. Limited by their ability to withstand pressure, they are unsuitable for high-pressure applications. The slip-on flange is connected to the pipe internally and externally by welding.

Application: Suitable for medium-pressure systems such as water supply and fire fighting systems and applications with low temperature and non-destructive fluids.

Blind Flange:

Blind flanges have no center hole and seal the ends of piping systems or pressure vessel openings. They are commonly used for industrial pressure testing or when the flow line needs to be blocked.

Application: For pipeline maintenance or where closing valves are insufficient to isolate the system.

Threaded Flange:

This flange has internal threads and can be directly screwed onto pipe ends with external threads. This type of connection can avoid welding and is suitable for situations where welding is inconvenient due to flammable, explosive, or high temperatures.

Application: Used in low-pressure and non-cyclic temperature changes, such as small-diameter cold and hot water systems and gas transmission.

Socket Weld Flange:

It is quicker to install than weld neck flanges and less costly. The socket weld flange is suitable for small-diameter pipes, and its internal welding design requires welding skills.

Application: Usually used in small-size high-pressure pipeline systems, commonly seen in the chemical industry and shipbuilding.

Lap Joint Flange:

Used in conjunction with stub ends, it facilitates pipe alignment and installation. The "stub end" must be installed on the pipe to accommodate the lap joint flange. This design allows the flange to rotate freely without moving the pipeline, which is suitable for frequent disassembly.

Application: Mainly used in systems requiring frequent inspection disassembly, such as low-pressure corrosive fluid processing.

Ring Type Joint Flange (RTJ):

RTJ flanges have a circular groove in which a metal ring gasket is placed to maintain a robust seal. This design is meant to endure extremely high pressure and temperatures.

Application: Mainly used in oil exploration, refining, maritime, and high-pressure environments like deepwater offshore oil wells.

5. Flange Sealing

Sealing performance is a crucial consideration when selecting flanges, as it directly affects the safe operation of pipeline systems and the risk of media leakage. Here's a discussion on sealing performance:

1. Sealing Surface: The sealing surface of flanges is a critical area for maintaining the seal between pipes. A well-designed sealing surface ensures tight contact between flanges, preventing media leakage. Therefore, the flatness and smoothness of the sealing surface are essential for sealing performance.
2. Gasket Material: Gaskets are typically placed between flanges to fill the gaps between sealing surfaces and improve sealing performance. Choosing a suitable gasket material is crucial to ensure a good seal. Common gasket materials include rubber, iron, and polytetrafluoroethylene (PTFE), each with specific applications and advantages.
3. Bolt Preload: Proper bolt preload ensures a tight connection between flanges, improving sealing performance. Insufficient bolt preload may result in gaps between flanges, affecting sealing performance, while excessive preload may cause gasket deformation or damage, reducing sealing performance.
4. Media Properties: The properties of different media also affect sealing performance. Some corrosive media may affect the performance of gasket materials, leading to leakage. In contrast, high-temperature or high-pressure media may exert additional pressure and challenges on gaskets and sealing surfaces, affecting sealing performance.

Summary

When selecting flanges, it is necessary to consider multiple factors comprehensively to ensure the safe operation and stability of the pipeline system.

Firstly, it is vital to understand the pipeline system's application environment and pressure rating and choose flanges with suitable materials, sizes, and pressure ratings.

Secondly, sealing performance is crucial, and appropriate sealing surfaces, gasket materials, and bolt preload should be selected to prevent media leakage.

Additionally, the connection method is also an important consideration, and the appropriate one should be chosen according to the requirements of the pipeline system to ensure firm connections and good sealing.