A bolted flange connection is a complex mechanical system whose components must be selected and assembled properly to provide reliable sealing over a wide range of operating conditions.
All of the various components of the assembled bolted flange connection are important to the proper operation of the joint.
The components consist of the piping, or vessels, the flange(s), the gasket(s) and bolts.
In addition to the components themselves, the joint design and assembly are critical to the long-term operation of the joint.
To achieve a successful seal, the gasket must be resilient enough to conform to any irregularities in the mating surfaces. The gasket must also be sufficiently tough (rugged) enough to resist extrusion, creep and blowout under the operating conditions and unexpected pressure/temperature excursions. The seal is created by the clamping forces acting upon the gasket surface, compressing the gasket and causing the gasket to conform to flange imperfections. The conformance of the gasket material to the flange surface under the compressive load (contact pressure) fills any leak paths and prevents the escape of the contained fluid from the bolted flange connection while maintaining a specified leakage rate.
- Flange-to-flange alignment of mating flanges shall be checked to verify that tolerances are within those given in Figure 1.
- Misalignment outside the tolerances of Figure should be brought to the Site Supervisor. The Site Supervisor will consult the Asset Integrity Specialist to determine the path forward.
- Excessive force shall not be used to align flange faces.
- Flange centerline should align such that no binding exists when the studs are inserted. Studs should be parallel to the pipe centerline.
- Flange faces should be checked for proper gasket insertion gap.
- The gap between flanges should be sufficient to allow for gasket insertion.
- If additional space is necessary to allow gasket insertion, a flange spreader shall be used.
- No wedges are to be used.
- Care shall be taken to avoid scratching or otherwise damaging the gasket contact surfaces.
- A gap which is more than 1/8″ (1/64″ for machinery) larger than the gasket should be corrected before proceeding further with joint assembly.
- A larger gap is acceptable provided closing the gap with bolts using hand tools does not induce detrimental strain.
- In addition to being aligned initially, the joint shall also remain in alignment during assembly and initial tightening. Misalignment (indicated by uneven gasket compression) is indicative of gasket damage, slippage, or undesirable contact between flanges. .
- Ensure that the joint is still aligned within the tolerances indicated in Figure 1 above.
- Insert all remaining studs and nuts so that they are hand tight.
- Ensure all bolts are installed such that there is complete thread engagement in both nuts.
- Begin lightly tightening the bolts with hand wrenches (approx. 15-20 ft. lbs.) using the sequential pattern shown in Figure 2. Verify that the gap between the flanges stays uniform by measuring the gap between the flanges at 4 equally spaced locations for NPS ≤ 8 (or 8 equally spaced locations for NPS > 8).
- Additional torque may be applied to the two bolts that straddle the point of maximum gap to achieve uniformity, but the torque load should not exceed 40% of the final load. If the gap is not pulled back to the tolerance shown in Figure using this procedure, the joint should be rejected and disassembled to locate the source of the problem.
- The entire bolt pattern should be tightened at least three times around the flange at 30%, 70% and 100% of the torque value using the pattern in Figure 2.
- At least one circular pattern (Figure 2) shall then be applied at 100% of torque. For soft gaskets, where the movement between the flange faces during torqueing is relatively large e.g. Spiral wound, PTFE, etc.) more than one circular pass is recommended until no nuts turn.
- The final torque the joint shall not be less than the values listed in the tables provided by supplier of the gaskets.
- All torqued flanges shall be marked with torque values, date and initials using a paint marker. Marking higher alloy flanges (example stainless steel, duplex, etc.) shall be made with a suitable marker/paint that is low in chlorides and sulphur. If flanges are required to be taped for gas testing purposes, then the tape is to be removed after the test. If tape is observed during the monthly WAITs or anytime while the flange is in service, it will be removed to prevent the possible creation of a corrosive environment under the tape.