How to Install Weld Lap Joint Flanges

Lap joint flanges connect valves, heat exchangers, pipes, and more. Choosing the right type of flange and investing in the proper products and installation methods can make your piping systems more reliable and safe. Learn how to install weld lap joint flanges and common mistakes to avoid in this guide.

What Is a Weld Lap Joint Flange?

Weld Lap Joint Flange

A weld lap joint flange can connect two pipes or fittings together. Unlike other flange types, these are not welded directly onto the pipe. Instead, welders attach the stub-end fitting of the flange using bolts, with the flange itself sitting loosely on the pipe. This is why weld lap joint flanges can also be called loose flanges.

Numerous industries prefer weld lap joint flanges over alternative assemblies. The looser-fitting design makes it much easier to assemble and disassemble the pipe connections, allowing for faster maintenance cycles, modifications, and repairs over time.

Step-by-Step Guide

While weld flanges can make it easy to build and modify piping systems, it’s important to carefully install the joints to access the complete benefits they offer. Follow these steps to ensure a secure, leak-free connection that works properly.

Install the stub-end fitting. First, locate the stub-end fitting in the lap joint flange assembly and weld it directly onto the pipe. The model of the flange, the piping material, and other constraints will determine which welding method is best.
Install the main body of the weld lap joint flange. Next, slide the loose flange in place over the stub-end fitting. Place the flange so its empty bolt holes align with the bolt holes on the adjacent flange or pipe.
Secure the weld lap joint flange. Once you have the flange rotated with the bolt holes in the right position, insert the bolts through the holes using the specified torque to ensure a secure connection without overtightening.

Common Installation Mistakes

There are a few common installation mistakes that can lead to leaks and premature breakage. These include using incorrect bolt lengths, overtightening bolts, or failing to properly align the fitting and flange before securing it with bolts.

Misalignment. If the lap joint flange components don’t align, fluid will leak out. You’ll want to properly align the stub-end fitting and the backing flange with alignment tools before tightening the bolts in place.
Incorrect bolt length. Bolts that are too short or too long can negatively impact the joint. Every bolt should accommodate the flange assembly thickness so that the nuts fully engage with the threads, but the bolt shafts don’t extend past the nuts.
Using the wrong gasket. While all gaskets help prevent leaks, they need to be compatible with the fluids running through the pipe system and the temperature and pressure conditions. Compressed fiber, PTFE, and rubber are some popular material choices.
Skipping surface prep. Before installation begins, you’ll want to clean the flange faces to remove dirt and rust that can compromise the connection and proper seating position. You can scrub the debris away with a wire brush or emery cloth, or use a chemical cleaner. You’ll also want to visually assess the surface for damage before installing the flange.
Under- or over-tightening bolts. If installers overtighten the bolts, the flange may crack or deform. If it’s under-tightened, the flange will leak. It’s important to always follow the manufacturer’s instructions and torque specifications for best results.

Maintenance After Installation

Weld lap joint flanges require some maintenance. Immediately following the welding phase, installers should conduct visual inspections, radiographic testing, and/or dye penetrant testing to guarantee the weld is solid. You’ll want to routinely inspect the flange joints for visible leaks or corrosion. Replace the sealing gaskets as needed, and make repairs to flange joints when you detect damage to prevent performance issues down the road.

Weld Lap Joint Flanges at Coastal Flange

At Coastal Flange, we supply our customers with precision-engineered flanges, including standard and custom weld lap joint flanges. Organizations in the oil and gas, petrochemical, and waterworks industries trust our selection of flange products to perform in some of the harshest operating environments. Flanges range from 12 inches to 200 inches in diameter to meet your sizing requirements.