What is the Difference Between a Stub End Flange and a Flange Adapter?

The difference between a stub flange and a flange adapter lies in their use case: A stub end flange connects to another flanged pipe, typically of similar material and size, while a flange adapter joins pipes with different end types (e.g. flange to thread), materials, or sizes.

From the outside, especially when viewed within the larger piping system, stub flanges and flange adapters may appear quite similar in appearance and function. So, it’s not surprising why some may confuse one over the other, even though their applications differ significantly.

Read further as this blog will discuss in detail the key differences between the two. This will help you make informed purchasing decisions for your specific applications, ultimately avoiding any costly mistakes.

Appearance and Assembly

If you take a closer look into its anatomy, you will see that a stub flange is actually a two-part assembly ^[1]. It consists of a stub end and a backing flange, forming a complete lap joint flange.

The stub end is welded to the pipe, providing a short length that extends out, while the backing flange is the round disc with bolt holes, which allows for connection to another pipe or fitting.

Here’s a short video^[2]of a lap joint flange installation to give you a better sense of how a stub end looks and functions.

On the other hand, a flange adapter – also referred to as flange couplings or flange couplers – is a single-piece assembly composed of:

Flange end: One end of the adapter has a flange face, which looks like a flat or raised disc with bolt holes around its circumference. This end is designed to bolt onto a standard flange on a pipe or fitting.

Pipe connection end: The opposite end is adapted for a specific type of connection and can either be threaded, beveled (for welding) or socket-style. You may also come across other models like the flange to flange adapter and grooved flange adapter.

These adapters vary in length and diameter, but they’re usually compact to minimize space in the piping system, as you can see in this video ^[3].

Purpose & Features

A stub flange is primarily used to connect one flanged pipe to another, usually of the same material and size.

Its standout feature? The rotatable backing flange. This means you can easily adjust the backing flange so that the bolt holes match perfectly with the adjoining pipe’s flange.

This is particularly advantageous in cases where maintaining alignment can be challenging, such as in long pipe runs or large-diameter pipe applications.

In contrast, flange adapters are used in situations where two incompatible pipe types need to be joined. In this case, incompatible can mean:

Different end connections: For example, a pipe with a flanged end uses a flange adapter to connect to a threaded-end pipe
Different material types: Such as connecting steel to PVC or copper
Different sizes or diameters: Using a reducing flange adapter for different pipe sizes
Different industrial standards: Bridging standards like ANSI to DIN or JIS
Different pressure ratings: Matching pipes with varying pressure classes or ratings (e.g. ANSI Class 150 to Class 300)

Installation Requirements

In a stub end flange assembly, the stub end is welded directly onto the pipe. That said, this may require more time to install, and at times, the need for extra machines or equipment.

On the other hand, flange adapter installation is more straightforward. You’ll likely just align both adapter and pipe, and tighten any screws or clamps. As there’s typically no welding involved, no time is wasted waiting for welds to cool.

Pressure Resistance

Stub flanges are commonly used in relatively higher-pressure piping systems. This is because:

Stub flanges are typically welded to the pipe, creating a strong, leak-resistant joint capable of handling high pressures and temperatures.
Stub ends are often thicker and more sturdy, typically constructed from steel, stainless steel, or alloys designed to withstand higher pressure.

Flange adapters, while versatile, are generally better suited for lower-pressure applications. Key reasons include:

Most flange adapters don’t require welding. Instead, they use threaded, grooved, or push-fit connections, which are typically not as resistant to high pressure as a welded joint.
As mentioned earlier, flange adapters are often designed to bridge different materials which may include non-metallic materials that can’t withstand the same pressure levels as welded steel or other metal connections.
With threaded or grooved connections, there are more potential points of leakage under high pressure.
While flange adapters can be made of steel, they can also be made of materials like High-Density Polyethylene (HDPE) and Polyvinyl Chloride (PVC), which are generally not as strong as steel.

Industry Applications

Stub end flanges are widely used (and crucial) in more permanent installations within fixed locations, especially in large industrial facilities. These locations often include:

Industry/Setting	How Stub End Flanges Are Used
Chemical Processing Plants	Stub flanges provide reliable connections in pipelines that transport corrosive substances. The welds create leak-resistant joints capable of handling the stresses brought about by high pressure and temperature fluctuations.
Power Generation Facilities	Stub flanges are commonly found in steam and water lines that require durability and resistance to extreme conditions. The ability to withstand high temperatures and pressures makes them essential in these settings.
Oil & Gas Infrastructure	In refineries and upstream operations, stub flanges ensure strong pipe connections that can withstand the operational demands and potential risks that come with transporting volatile substances, like oil, gas, and other hydrocarbons.

In contrast, flange adapters are frequently used in temporary or modular setups where adaptability is key. For instance:

Industry/Setting	How Flange Adapters Are Used
Construction Sites	Flange adapters are often employed on construction sites where different pipe materials (e.g., PVC for drainage and metal for water supply) need to be connected. As construction progresses, pipe layouts can change, requiring flexible and adaptable connections. Flange adapters allow for quick modifications without extensive rework.
Water Treatment Facilities	These facilities often need to connect different types of pipes as they expand or modify their systems. Flange adapters allow for easy integration of various materials (like steel, PVC, and HDPE) as the facility’s requirements evolve.
HVAC Systems	In heating, ventilation, and air conditioning systems, flange adapters facilitate the connection between different pipe types and sizes, especially when retrofitting older systems with new components. The adaptability of flange adapters allows for easier installation and modification, reducing downtime and enhancing system efficiency.

References

[1] Lap Joint Flange

[2] 6-Lap Joint Flange

[3] Flange Adaptor for PE pipes assembling