A Comparison: Full-Face vs Lap Joint FRP Flanges
There are two main types of FRP flanges commonly used in the industry, full-face (FF) and lap joint (LJ).
A full-face flange is characterized by its solid, circular design, which covers the entire face of the pipe end.
A lap joint flange is used to connect two piping components and allows for easy alignment, disassembly, and maintenance. The LJ flange contact face only covers the area inside the bolt holes and not the entire face of the flange like FF flanges. It consists of two main components: (i) Stub End: This is a short, cylindrical pipe section with a larger OD than the pipe at the end. It is typically attached to one of the pipes being joined. (ii) Backing Flange (also known as the backing ring): The backing flange is a separate, flat plate with bolt holes. It is usually made of a material such as carbon steel, stainless steel, or FRP. The backing flange is not laminated to the pipe and is free to rotate and move.
The table below summarizes the difference between FRP FF flanges and FRP LJ flanges:
| Performance Aspect | FF | LJ | Comments |
| Design Pressure | ✓ | ✓ | For the common pressure ratings for FRP flanges, there is no difference between FF and LJ and both can be designed to meet pressure specifications. |
| Design Temperature | ✓ | ✓ | There is no difference between FF and LJ regarding the design temperature |
| Installation | ✓ | With a loose backing ring, it is easier to align and install LJ flanges | |
| Backing ring | ✓ | Since the backing ring is separate from the flange, a different material can be selected for the backing ring to take advantage of higher strength material, since there is no contact with service fluid | |
| Sealing | ✓ | Since the contact area for LJ flanges is smaller than FF flanges, a certain amount of torque will lead to higher seating stress in the gasket providing an easier seal. |
To review, a flange is a mechanical component that serves as a connection point in a system of machinery or piping. Flanges are used to facilitate the assembly and disassembly, alignment, and secure attachment of various components, such as pipes, valves, or other mechanical elements. They have bolt holes that allow for the connection of multiple parts, forming a sealed mechanical connection.
For an additional discussion of FRP flanges, see Pressure Design of FRP Piping Components – Flanges in our Technical Center.