Many facilities believe putting a flange together is far easier than paying for a “certified weld” joinment. They see this as a low-risk, cost-saving solution, believing any “maintenance mechanic” can put a flange together. Hence, most LOPC events I have had in my time were due to a flange that was NOT assembled per ASME B31.3. This article is for safety professionals seeking to understand the fundamentals of B31.3 as they apply to flanges. Keep in mind, this goes FAR BEYOND PSM/RMP requirements.
Under ASME B31.3 (Process Piping), flange joint integrity is not just about tightening bolts; it is a critical “engineered system” where the flange, gasket, and bolting must work in harmony to maintain a seal under thermal and pressure loads.
The code categorizes flange joints as “Mechanical Joints,” and their assembly is governed primarily by ASME B31.3, Section 335.2.
A flange joint works through Gasket Stress. You must apply enough force through the bolts to “seal” the gasket into the microscopic imperfections of the flange faces.
- Seating Force: The initial load required to deform the gasket into the flange serrations.
- Operating Load: The force required to keep the gasket compressed even when internal pressure is trying to push the flanges apart (hydrostatic end force).
ASME B31.3 references ASME B16.5 for flange dimensions and finishes.
- Serrations: Most standard flanges have a “phonographic” or concentric serrated finish.
- Alignment (335.1.1): This is a critical compliance point. The code requires that flange faces be parallel within 1/16 in./ft (0.5%) of the diameter, and that bolt holes be aligned within a maximum offset of 1/8 in.
The gasket must be chemically compatible with the fluid and rated for the pressure/temperature. Common types include:
- Spiral Wound (Flexitallic): Often required for high-pressure/temperature service.
- Ring Type Joint (RTJ): Used in very high-pressure applications where a metal ring sits in a machined groove.
Bolting materials (typically ASTM A193 B7 studs) must provide the “spring” that holds the joint together.
- Bolt Length: B31.3 requires the bolt to extend through the nut by at least three (3) threads (referred to as “full thread engagement”). NOTE: Newer code language allows the bolt to be flush with the outside edge of the nut.
- Lubrication: Friction accounts for nearly 90% of torque resistance. Consistent lubrication on threads and nut faces is mandatory for accurate tensioning.
To ensure the gasket is compressed evenly, B31.3 emphasizes proper assembly procedures (often pointing to ASME PCC-1, Guidelines for Pressure Boundary Bolted Flange Joint Assembly).
- Stepwise Torque: Tightening is sometimes done in increments (e.g., 30%, 60%, 100% of the final torque).
- The Star Pattern: Bolts are tightened in a cross-over sequence to prevent the flange from “cocking,” which would pinch the gasket on one side and leave a leak path on the other.
Under B31.3, the inspector (or “The Owner’s Inspector”) looks for:
- Verification of Materials: Ensuring the heat numbers on the flanges and the grade of the studs match the piping specification.
- Visual Examination: Checking for “bottoming out” of gaskets or damage to the flange serrations (scratches or dings that create leak paths).
- Leak Testing: Once assembled, the joint is typically verified via a Hydrostatic (1.5x design pressure) or Pneumatic (1.1x design pressure) test.
Summary Table: B31.3 Flange Compliance
| Feature | Requirement / Reference |
| Parallelism | Max 1/16″ per foot of diameter |
| Bolt Hole Offset | Max 1/8″ |
| Bolt Engagement | At least 1 thread beyond the nut |
| Finish | Per ASME B16.5 (typically 125–250 Ra) |
| Procedure | Standardized via ASME PCC-1 |