All you need to know about lap joint flange in this blog

Lap Joint Flange

All you need to know about lap joint flange in this blog

A lap joint flange is a pipe flange welded to the pipe at the lap joint. This allows easy access to the pipe’s interior for inspection and cleaning. Lap joint flanges are often used in low-pressure applications where welding is not required.

Lap joint flanges are less expensive than other types of pipe flanges, making them a good choice for budget-conscious projects. Lap joint flanges are available in a variety of sizes and materials, so in this blog, we shall share with you all the relevant information about lap joint flanges.

What is a lap joint flange?

A lap joint flange is a two-piece device that consists of a stub end that has a lap-joint ring flange placed over it. Normally, the stub end is welded to the pipe and the other component, the flange ring, is placed over it to be aligned with the mating flange.

Lap joint flanges are also sometimes called “loose” or “lapping” flanges because they can be “lapped,” or slipped over the end of a pipe, before welding. This provides easy alignment during welding and is particularly helpful when working with large-diameter pipes. Generally speaking, the lap joint flange and weld neck flange are similar in terms of applied size and pressure classes.

lap joint flange

Are lap joint flanges flat?

Lap joint flanges are actually not flat, but have a small radius at the bore of the flange. The radius is usually about 3/16″, but can vary depending on the size and schedule of the pipe. The purpose of this radius is to allow for a smooth transition from the flat surface of the pipe to the flange. This reduces stress on the weld and helps to prevent cracking.

Actually, The flat face of the lap joint flange has a smooth surface. But when the lap joint flange is combined with the stub end, the resulting sealing face is raised. The sealing face of a stub end may be plain, serrated, or grooved to allow for a ring-type joint.

Lap joint flanges are typically used in applications where welding is impossible or where the pipe needs to be removed frequently. Because they are not welded to the pipe, they can be easily disassembled and reassembled. However, because they are not as structurally sound as other types of flanges, they are not typically used in high-pressure applications.

What is the difference between slip on and lap joint flange?

There are several key differences between these two types of flanges that include: 1) how they attach to pipe, 2) their construction,3) their intended purpose, 4) their pressure, and 5) whether it is easy to install.

Actually, the main difference between a slip-on and a lap joint flange is that a lap joint flange is not welding onto the pipe. A lap joint flange is held in place by bolts and it can rotate around the stub end. This rotated position is important because it aligns the bolt holes in the flange to the bolt holes in the stub end. A stub end is a short piece of pipe with weld bevels on both ends that fits inside the Flange and is also welded to the pipe.

lap joint flange

Another difference between these two types of flanges is that a slip-on Flange has two conflicting faces. One face butts up against another pipe’s end while the other face accepts a backing ring or gasket. A lap joint Flange has only one weld bevel so it does not get in the way of another pipe’s end when trying to mate these two pieces together.

This single weld bevel allows for full access to clean and inspect the interiors of both mating surfaces, which helps prevent contamination during welding processes. For this reason, many high-purity (semiconductor, pharmaceutical, etc,) applications prefer lap joint flanges over slip-on flanges even though they are more expensive.

Besides, one more difference between these two types of flanges is their pressure ratings with Slip-On generally being lower than Lap Joint Flanges in most cases. All other factors being equal, a Slip-On will have a slightly lower pressure rating than an equivalent-sized and classed Lap Joint due to its design having only a weld bead as compared to 2 for the latter.

Lastly, the key difference between slip on and lap joint flanges is that the slip on flanges are easier to install than lap joint flanges, as they don’t require any welding. However, they’re not as strong as lap joint flanges and may leak if not installed correctly. Lap joint flanges need to be welded onto the pipe, but this gives them a much more secure connection. They’re also less likely to leak than slip on flanges.

However, lap joint flanges are more difficult to install, so you’ll need to factor this into your decision.

lap joint flange

What is the advantage of lap joint flanges?

In general, the lap joint flange has the following advantages:

1. Be applied when the piping system needs to be frequently assembled

A lap joint flange is designed to be used in applications where the piping system needs to be frequently assembled and disassembled. The biggest advantage of this type of flange is that it can “swivel” around the stub end and the pipes, which means that it can be used even if the two flanges’ bolt holes are misaligned.

This makes it ideal for use in applications where the piping system needs to be frequently accessed or repaired. Compared with the weld neck flange, for a Lap joint flange,  this will never be a problem as it is not necessary to align it during the assembly process.

But a weld neck flange needs to be aligned with the welding point before welding. If not, the uneven loading would lead to cracks or broken in the welding neck area.

Additionally, lap joint flanges are often easier to install than other flanges, making them a popular choice for many applications.

lap joint flange

2. Can be used in corrosive and explosive situations

Another advantage of using a Lap joint flange is that it can be used in corrosive environments where flammable and explosive conditions are not allowed. In this scenario, weld neck flanges cannot be used, and have to go for other options like the thread types or blind types.

Yet those types of connections would have drawbacks such as poor sealing or hard to install and disassemble. The lap joint flange could provide good sealing at a low price and is easier to handle. Also, because the backing flange and the stub end of the lap joint flange are separated, we can use two different materials for the two pieces, making them more flexible.

3. Be used in low-pressure applications

Finally, lap joint flanges usually have lower pressure ratings than weld neck or threaded types, even if they are built according to the same size and material grade. Therefore, when only a tiny bore and wall thickness is required for your piping system, you could consider using a lap joint flange which could offer both economy and safety conditions.

lap joint flange


In a nutshell, a lap joint flange is a type of pipe flange that is used in situations where disassembly and reassembly are required. It is also used if frequent inspection of the inside of the piping is necessary.

The lap joint flange consists of two pieces: the body flange and the stub end. This design allows the pipe to be disassembled quickly and easily for inspection or repairs. Lap joint flanges are most commonly used in low-pressure applications, such as HVAC systems. However, they can also be used in high-pressure applications, such as in-process piping.



Some other related articles:

What is a blind flange in plumbing?

All you want to know about threaded flanges

What is a weld neck flange used for?