Flanges are used throughout industrial piping systems to connect pipes, valves, pumps, and equipment. The connection is bolted, which makes it easier to assemble and take apart compared to welded joints. In most projects, flange selection ends up influencing more than just connectivity. It affects sealing performance, pressure containment, and even how easily the system can be maintained later. A flange that is technically compatible may still not perform well if operating conditions are not fully considered. This guide covers the types of flanges, along with where they tend to be used and what makes them suitable in each case.
- 1. What is a Flange?
- 2. Weld Neck Flange
- 3. Slip-On Flange
- 4. Blind Flange
- 5. Socket Weld Flange
- 6. Threaded Flange
- 7. Lap Joint Flange
- 8. Orifice Flange
- 9. Flange Pressure Classes
- 10. Flange Face Types
- 11. Flange Materials for Corrosive Environments
- 12. How to Select the Right Flange Type
- 13. Frequently Asked Questions
- 14. Conclusion
What is a Flange?
A flange is a ring or collar attached to a pipe or fitting that provides a surface for bolting another component to it. A gasket is placed between the two faces to create a seal. One practical advantage of flanged joints is that they can be opened without cutting the pipe. That matters in systems where inspection or modification is expected over time. In large plants, this is less of a convenience and more of a requirement.
Most flanges are manufactured to ASME standards. ASME B16.5 covers sizes up to NPS 24, while ASME B16.47 applies to larger diameters. These standards define dimensions, tolerances, and pressure-temperature ratings.
Weld Neck Flange
Weld neck flanges are commonly used in high-pressure or high-temperature service. The long tapered hub is not just a design feature, it helps reduce stress concentration at the weld.
Because the bore matches the pipe ID, flow remains smooth through the joint. That becomes important in systems where turbulence or erosion is a concern. The flange is attached using a butt weld, which provides a strong and reliable connection. In critical systems, this type is often specified by default.
You will typically see weld neck flanges in refineries, power plants, and offshore installations, where operating conditions are more demanding than average. These flanges are available in all pressure classes from 150 to 2500.
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Slip-On Flange
A slip on flange is easier to work with during installation. The pipe is inserted into the flange and fillet-welded from both sides, which allows some flexibility during alignment.
That flexibility is one of the reasons they are used so widely. In many general service lines, the reduced cost and easier fit-up make them a practical choice. They are, however, not as strong as weld neck flanges. The difference is not always critical, but in high-stress conditions, it becomes relevant. This is why they are usually limited to moderate pressure and temperature service. They work well where the demands are predictable and not too severe.
Blind Flange
A blind flange is used to close the end of a piping system. There is no opening in the centre, so it completely stops the flow. Even though it looks simple, it carries full system pressure. Because there is no pipe supporting it from the inside, the stress distribution is different compared to other flange types. Blind flanges are used for isolation, testing, and future tie-ins. In some cases, they are installed temporarily and removed later when the system is extended.
Socket Weld Flange
A socket weld flange is generally used on smaller pipe sizes. The pipe is inserted into a recessed area and welded around the outside.
This creates a fairly clean internal bore, which helps maintain flow characteristics. It also provides better resistance to fatigue compared to slip-on flanges in certain applications. They are commonly used in high-pressure lines with smaller diameters, such as steam or hydraulic systems.
A slight gap is usually left between the pipe end and the internal shoulder. This allows for thermal expansion and helps prevent cracking during operation.
Threaded Flange
A threaded flange is connected to the pipe using threads, so no welding is required. This makes them useful in situations where welding is not feasible.
Installation is relatively straightforward, and removal is just as simple. However, their application is limited. They are generally used in low-pressure, low-temperature systems. Under cyclic loading or higher temperatures, threaded connections can become unreliable over time.
You will often see them in utility services like water or compressed air lines, and occasionally in hazardous areas where welding is restricted.
Lap Joint Flange
A lap joint flange is slightly different because they are used with a stub end. The stub end is welded to the pipe, while the flange itself remains loose. This allows the flange to rotate, which makes bolt alignment much easier. In large piping systems, that alone can save a significant amount of installation time.
Another advantage is material flexibility. Only the stub end needs to match the process fluid, so the backing flange can be made from a less expensive material. This is often used in systems where corrosion-resistant alloys are required, but the cost needs to be controlled.
Orifice Flange
An orifice flange is designed for flow measurement. It is installed in pairs and holds an orifice plate between them. Pressure taps are provided on either side so that differential pressure can be measured. That measurement is then used to calculate the flow rate. They are commonly found in process industries where accurate flow monitoring is required. The design itself is specialised, so they are not used outside of measurement applications.
Flange Pressure Classes
A flange pressure class defines how much pressure a flange can handle at a given temperature. These ratings are specified in ASME B16.5. Higher class numbers indicate higher pressure capability, but the actual rating depends on both material and temperature. Refer to the flange rating chart below for pressure values.
| Class | Approximate Max Pressure at Ambient Temp (psig) |
|---|---|
| 150 | 285 |
| 300 | 740 |
| 400 | 990 |
| 600 | 1480 |
| 900 | 2220 |
| 1500 | 3705 |
| 2500 | 6170 |
Flange Face Types
Flange faces determine how the gasket sits and seals the joint. Common flange face types are explained below.
- A raised face flange is the most common and is used in a wide range of applications.
- Flat face flanges are typically used with cast iron or non-metallic components, where full-face contact is required.
- A ring type joint flange is used in high-pressure systems and relies on metal ring gaskets.
- Tongue and Groove: One flange has a raised ring (tongue) that fits into a groove on the mating flange. Provides positive gasket alignment.
- Male and Female: Similar to tongue and groove, but with a wider contact area.
Flange Materials for Corrosive Environments
Standard carbon steel flanges (ASTM A105) are suitable for many applications, but corrosive environments require speciality alloys
| Alloy | Key Benefit | Application |
|---|---|---|
| Hastelloy C276 | Extreme acid resistance | Chemical processing |
| Hastelloy C22 | Versatile corrosion resistance | Pharma, chemical |
| Inconel 600 | High-temperature resistance | Furnaces, nuclear |
| Inconel 625 | Seawater and acid resistance | Marine, offshore |
| Monel 400 | HF acid resistance | Acid alkylation |
| Duplex S31803 | Chloride SCC resistance | Offshore, desalination |
| Super Duplex S32750 | High strength + corrosion | Subsea, oil & gas |
How to Select the Right Flange Type
Choosing a flange involves several considerations.
- Pressure and temperature requirements are usually the starting point. From there, pipe size, maintenance needs, and installation constraints come into play.
- Cost is also a factor, especially in large projects. In some cases, a slightly lower-grade option is acceptable if the service conditions allow it.
- Design codes such as ASME B31.3 or B31.1 may also affect the final selection, especially in controlled environments.
Frequently Asked Questions
Which flange type is strongest?
Weld neck flanges are generally considered the most reliable due to their design and method of attachment.
What is the difference between ASME B16.5 and B16.47?
B16.5 covers smaller sizes up to NPS 24, while B16.47 applies to larger flanges.
Can different flange types be connected?
Different flange types can be connected, provided the size, pressure class, and flange face type are compatible.
What gasket is used with raised face flanges?
Non-metallic or spiral wound gaskets are commonly used with raised face flanges. This depends on the pressure requirements.
How do I choose between weld neck and slip-on flanges?
Weld neck flanges are used for more demanding service. Slip-on flanges are used where conditions are less severe and cost matters
Conclusion
Flanges are an important part of any piping system. Choosing the right flange type affects both how well the system works and its durability. Different flange types are used for different reasons. Some are built for strength, others are easier to install, and some are made for special uses like measuring flow. The suitable choice depends on the operating conditions, the materials used, and the applicable standards. Balancing all these factors is what leads to reliable, long-term system performance.
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