Lap Joint Flanges Explained
A quick primer on how lap joint flanges function, why you might choose them, and common alternatives available.
As covered in our guide to flange basics, flanges both connect pipe systems to various equipment and components while also providing added flexibility when it comes to assembly, disassembly, and maintenance.
Lap joint flanges are one such flange type popular in both piping and duct system design.
But as with all flanges and piping system components, understanding both how the component functions and how various design differences will impact performance are essential to safe, reliable operation and optimal cost-effectiveness.
In this guide, we’re going to take an in-depth look at how lap joint flanges work, the various options available when buying a lap joint flange, and key considerations to keep in mind when purchasing them.
What is a Lap Joint flange?
Lap Joint flanges go by a range of names, including:
Back Up flanges
Loose Ring flanges
Loose Hubbed flanges
Van Stone flanges
They’re a two-part flange consisting of a Lap Joint Stub End which is butt welded to the pipe and a backing lap joint flange which is slid onto the pipe before welding the Stub End.
Lap Joint flanges are very similar to Slip On flanges.
However, Lap Joint flanges include a radius at the intersection of the flange face and the bore to accommodate the flanged portion of the Lap Joint Stub End making it only compatible to Lap Joint Stub Ends
Materials for Lap Joint flanges and Lap Joint Stub Ends vary and may even differ in a single implementation to allow for greater cost flexibility.
Materials typically used for Lap Joint flanges include a range of stainless steel, carbon steel and other alloy materials.
These flanges are available in common dimensions like other flange types making it easy to integrate them into existing piping systems or use alongside other piping components.
Stub Ends also match both pipe material and wall schedule to maintain both inside and outside diameters of your piping process. This style is typically used in engineered piping systems with critical wall thickness requirements.
Stub Ends are also available in two different lengths -- MSS or Short lengths and ASA or Long lengths -- as well as different face finishes depending on your needs.
For example, serrated or phonograph finishes can help to increase gasket adherence.
How Do Lap Joint Flanges Differ from Rolled Angle Rings?
Rolled Angle Rings -- also known as Vanstone Rings or Vanstone Rolled Angle Rings -- offer comparable benefits to Lap Joint Stub Ends with similar designs and installation requirements.
However, there are subtle differences that will often make one preferable over the other in most cases. It is important to note that Rolled Angle Rings are not registered fittings.
Like Lap Joint flanges, the two-part design means the rolled angle ring and backing flange can be comprised of different materials for added flexibility and cost optimization potential. An example of this would be using a 316/L SS rolled angle ring with a Carbon Steel, Galvanized backing flange.
The backing flange is also free to rotate on the pipe, allowing for easier alignment.
Unlike Lap Joint flanges, rolled angle rings are available in two different installation configurations:
Butt-Weld
Slip-On
Butt-Weld Rolled Angle Rings install similarly to Lap Joint Stub Ends, requiring welding to the end of the pipe and matching of the inner diameter of the pipe or fitting in use. Slip-On Rolled Angle Rings slide over the pipe, attaching to the outside of the pipe wall instead of the end.
Rolled Angle Rings are also not manufactured to match the schedule of the pipe or fitting involved.
In most cases, they’re made from 11 gauge Angle Iron though larger pipe sizes might use 7 gauge Angle Iron.
Some smaller rings -- often called Pressed Angle Rings or I.D. Pressed Vanstone Rings -- are hydraulically pressed from a piece of plate cut to specific inner and outer diameters.
This provides a seamless ring design that is easy to produce and replicate.
However, achieving the exact 90-degree angles of Rolled Angle Rings is difficult using hydraulic presses.
Why Choose Lap Joint Flanges or Rolled Angle Rings?
With an understanding of how these popular flange types work, we can talk about why you’d want to use them in your piping systems.
The biggest limitation to lap joint flange usage is pressure ratings.
While many Lap Joint flanges will accommodate greater pressure levels than Slip-On flanges, they’re still not suited for high-pressure applications. If you are unsure, always consult with an engineer before purchasing flanges for use with your piping systems.
With the limitations out of the way, both designs offer three major benefits depending on the industry in which you’re involved.
The first is the ability to use different materials for the Backing flange than the Stub End or Angle Ring.
This means you can match the piping materials as required where components touch piped materials while using more affordable -- or otherwise desirable -- materials in the outer components which do not interact with piped materials.
The second is the ability to realign and rotate the flange freely to ensure proper connection and speed up maintenance processes in systems that require frequent maintenance.
The ability to utilize flanges that do not require filet welds on the plates can also decrease installation times and provide further upfront cost savings.
Finally, in high-corrosion or high-erosion processes, Lap Joint flanges allow you to salvage the flange for re-use while replacing Stub Ends or Angle Rings as required for safe, cost-effective operation.
As a leading provider of stainless steel piping and components across Canada and North America, Unified Alloys has more than 4 decades of experience serving industries of all sizes. Our expert sales analysts are available today to help you choose the ideal lap joint flanges or rolled angle rings for your needs. Contact us today to learn more about our selection of high-quality stainless steel products.
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Nothing in this publication shall create or imply any warranty whether expressed or implied.