July 18, 2025
Flanges play an essential role in piping systems, enabling secure and durable connections between various pipes, valves, and other components. Among the various flange types used in industrial applications, the most commonly employed are those defined by ASME B16.5 standards: Welding Neck, Slip On, Socket Weld, Lap Joint, Threaded, and Blind flanges. Each of these flanges serves specific functions and is chosen based on the requirements of the piping system in question. In this article, we explore each of these flange types in detail, including their construction, advantages, and best-use scenarios.
The Welding Neck Flange is widely recognized by its long, tapered hub that gradually transitions to the thickness of the pipe or fitting. This design serves an important purpose: providing reinforcement for high-pressure, sub-zero, and elevated temperature applications. The taper ensures a smooth transition between the flange and pipe, which significantly reduces the potential for turbulence and erosion at the joint. This is especially beneficial in systems that experience frequent bending or line expansion, which could otherwise lead to joint failure.
Welding Neck flanges are bored to match the inside diameter of the mating pipe, ensuring there is no restriction of product flow. This also reduces any potential turbulence at the joint. Furthermore, these flanges distribute stress uniformly through the tapered hub, which enhances their strength and durability. As a result, Welding Neck flanges are ideal for systems that require high reliability, such as oil and gas pipelines and power plants. They are welded to the pipe or fitting using a full penetration V-weld, ensuring a strong and secure connection.
The Slip On Flange is another commonly used flange type, known for its simplicity and cost-effectiveness. While it offers lower strength than the Welding Neck flange, it is suitable for systems that do not require the same level of pressure resistance. The strength of a Slip On flange under internal pressure is approximately two-thirds that of the Welding Neck flange, and its fatigue life is about one-third that of the latter. The flange is connected to the pipe using two fillet welds—one on the outside and one on the inside of the flange.
The connection process requires a gap (denoted as "X" in the diagram), which is typically the wall thickness of the pipe plus 3mm. This gap prevents the flange face from being damaged during welding. One of the primary drawbacks of the Slip On flange is that it requires the pipe to be welded first, followed by the fitting. Additionally, because of its design, it cannot be used with elbow or tee fittings, which lack the straight ends necessary to slide into the Slip On flange.
Originally designed for use in smaller high-pressure piping systems, Socket Weld flanges are similar to Slip On flanges but with increased strength. Their static strength is comparable to that of Slip On flanges, but their fatigue strength is 50% greater, making them suitable for applications that experience fluctuating pressures or cyclic loading. The connection between the flange and the pipe is made using a single fillet weld on the outside of the flange. However, before welding, it is necessary to leave a small gap (approximately 1/16" or 1.6mm) between the flange and pipe.
This gap, known as bottoming clearance, helps reduce residual stresses that could lead to cracking during the welding process. While the Socket Weld flange offers superior fatigue resistance compared to the Slip On flange, it is prone to corrosion, particularly in stainless steel pipe systems, due to the gap that forms between the pipe and the flange. This issue should be carefully considered when choosing this flange type.
Lap Joint flanges are similar to Slip On flanges in terms of their dimensions, but they differ in the absence of a raised face. Instead, they are used in conjunction with a "Lap Joint Stub End," which fits into the flange. This type of flange is designed for applications where the ability to rotate the flange around the pipe is important. This swiveling feature makes it easier to align opposing bolt holes. While the Lap Joint flange is convenient and allows for easy installation, its pressure-holding capacity is not significantly better than that of a Slip On flange.
In fact, its fatigue life is only one-tenth that of the Welding Neck flange. However, it does offer specific advantages, such as the ability to use inexpensive carbon steel flanges with corrosion-resistant pipes. Additionally, since the flange does not make direct contact with the fluid inside the pipe, it is ideal for systems that are prone to erosion or corrosion, as the flanges can be salvaged and reused.
Threaded flanges are designed to be screwed onto a pipe, eliminating the need for welding. This makes them ideal for applications where welding is not feasible or desirable. Threaded flanges are generally used in smaller pipe sizes and are commonly found in systems where ease of installation is critical.
However, threaded flanges are not suitable for pipes with thin wall thicknesses, as cutting threads into a thin-walled pipe could compromise its integrity. To ensure a secure connection, thicker-walled pipes (at least Schedule 80) are required. Threaded flanges are typically used in systems with lower pressure and temperature requirements, such as in water or gas pipelines. They can also be used with seal welds for added security in high-pressure applications.
Blind flanges are solid flanges with no bore, designed to seal the ends of piping systems, valves, and pressure vessels. They are used in a variety of industries, including oil and gas, power generation, and chemical processing. Blind flanges are particularly useful in situations where a temporary closure is needed, or when future expansion is anticipated. These flanges are highly stressed, particularly in larger sizes, due to the internal pressure and bolt loading they must withstand. The primary stresses acting on Blind flanges are bending stresses near the center.
Despite these stresses, Blind flanges are suitable for high-pressure and high-temperature applications. Since Blind flanges can be custom-made to meet the specific needs of the system, they offer a flexible and reliable solution for various piping systems.
Each flange type plays a unique role in the piping system, and the selection of the appropriate flange is crucial to ensuring a safe and efficient installation. Whether you're dealing with high-pressure applications, systems that require frequent disassembly, or situations where welding is not an option, understanding the characteristics of each flange type is key to making the right choice. From the robust Welding Neck flange to the versatile Blind flange, each type offers distinct advantages tailored to different needs in industrial piping systems.
