An ASME B16.5 Class 1500 LJ Flange does not actually seal the fluid itself. Instead, it is used in tandem with a stub end (a short segment of pipe with a flared shoulder). The flange acts as a loose backing ring that slips over the pipe. HT PIPE supply all different types and scales of flanges. Please feel free to contact us for further product information and pricing.
Why Choose a Lap Joint Flange in Class 1500?
Cost-Efficient in Exotic Alloys: If your process line requires high-grade stainless steel, titanium, or nickel alloys, you only need to buy a stub end made of that expensive material. The outer backing LJ flange can be made of cheaper carbon steel because it never comes into contact with the internal fluid.
Rapid Maintenance: High-pressure Class 1500 lines require routine testing and cleaning. Being able to quickly spin and slide the backing flange speeds up assembly and minimizes downtime.
Sealing Faces: RTJ vs. Raised Face (RF)
Ring-Type Joint (RTJ)
RTJ is the gold standard for high-pressure (＞Class 600) and high-temperature services. A deep, precision-machined groove is cut directly into the face of the stub end. A solid metal ring gasket (either octagonal or oval) is compressed into the groove by the backing flange, forming a metal-to-metal seal.
Raised Face (RF)
RF stub ends feature a 0.25-inch raised portion (for Class 1500) machined with a concentric or spiral serrated finish (usually 125 to 250 micro-inches roughness). This surface bites into a semi-metallic gasket, like a spiral wound gasket with an inner ring.
Critical Pitfalls to Avoid in Class 1500 Piping Design
Because Class 1500 systems operate under incredibly unforgiving pressures, minor dimensional mistakes can lead to major structural failures or leaks. Watch out for these three technical nuances:
1. The Lap Joint Fillet Radius
Look closely at the Radius column in the dimensional table. The inside corner of the lap joint flange has a specific machined curve designed to clear the weld fillet on the stub end.
Warning: If you buy a generic flange or use a Slip-On (SO) flange in place of a dedicated Lap Joint flange, the square inside edge of the SO flange will bind on the stub end fillet. This prevents the sealing faces from making flat contact, causing catastrophic joint failure when bolted down.
2. Tolerance Stack-Up at High Pressures
A Class 1500 system handles extreme mechanical loads. Because the flange is floating, its Bore is machined slightly wider than the outside diameter of the pipe. Ensure your piping contractor uses stub ends that strictly match the schedule (wall thickness) of your pipe run so the ID (Inner Diameter) remains completely flush.
3. Bolt Circle Torqueing Sequence
With bolt hole sizes starting at 7/8" for small piping and reaching up to 3-5/8" for 24-inch systems, these flanges require immense torque. When working with Class 1500, always utilize a cross-pattern torqueing sequence in multiple passes (typically 30%, 60%, and then 100% target torque) to avoid warping the backing flange.
ASME B16.5 Class 1500 LJ Flange Dimensions
