Valve Flange Gasket Leak
Flange joint leakage at a valve is one of the most common maintenance issues in industrial plants. The remedy appears simple (tighten the bolts) but incorrect technique can worsen the leak or damage the flanges.
Symptoms
Root Causes
Gasket compression set
All gaskets lose seating stress over time through creep, thermal cycling, and normal relaxation. Spiral-wound gaskets have less creep than compressed fibre gaskets.
Incorrect bolt torque
Incorrect initial bolt-up torque (under-torqued = insufficient gasket seating stress; over-torqued = gasket crushed and extruded).
Gasket damage on assembly
A damaged, off-centre, or eccentrically positioned gasket on assembly will leak from initial startup.
Flange face damage
Radial scratches on the raised face, corrosion pitting, or weld spatter on the seating face prevent gasket seating.
Misaligned flanges
Pipe strain causing angular or parallel misalignment of the flanges requires the gasket to bridge the gap non-uniformly.
Safety Precautions
- Hot bolting: PTW required for most sites; not permitted on hazardous services at many facilities
- Full LOTO for joint replacement
- Confirm zero pressure and fluid drainage before breaking the joint
Tools Required
- Calibrated torque wrench (correct range for stud size)
- Feeler gauge
- Flange alignment bar
- Gasket scraper
- Wire brush
- Depth gauge (for RTJ grooves)
Supplies Needed
- Replacement gasket (correct specification - spiral wound, ring joint, or fibre as applicable to the class and service)
- New studs and nuts (if corroded)
- Stud lubricant (SAE 30 or nickel-based as specified in ASME PCC-1)
Step-by-Step Repair Guide
- 1
Identify the leak location within the joint
Carefully observe where the leak originates: along a single bolt position (localised low torque), all around the gasket OD (uniform low seating stress), or at one quadrant (flange misalignment). For hot services, use thermal imaging to identify cold (leaking) spots versus hot (sealed) spots on the flange face.
- 2
Hot bolt tightening (if process conditions permit)
For non-hazardous services at low pressure and temperature: with the line in service, carefully tighten each bolt by a quarter turn in a cross-star sequence (diagonally opposite bolts, never consecutive). Apply torque at 50% of the target value, traverse all bolts, then apply at 75%, then 100%. Do not over-torque - obtain the correct torque value from the flange specification (ASME PCC-1 Table 1 or the plant's bolting torque register).
Hot bolting is not permitted on flammable, toxic, or high-pressure services without a specific permit and may be prohibited entirely at many sites. Check site procedures.
- 3
Full joint replacement if hot bolting is not permitted or fails
Isolate and depressurise. Ensure the pipeline is free to move (undo any pipe clamps that might impose strain on the joint). Remove all studs. Clean both flange faces with a wire brush and inspect for radial scratches or corrosion pits. Remove old gasket completely and clean residual gasket material from faces with a gasket scraper and solvent. Install a new gasket (correct specification: class, size, and material). Make up the joint and torque per the applicable torque sequence and values in ASME PCC-1.
For API RTJ (ring joint) flanges: the groove depth must be checked with a depth gauge before installing a new ring. A worn groove will not provide adequate gasket seating even with correct torque.
- 4
Verify flange alignment before final bolt-up
Before final torquing, verify parallelism of the two flanges with a feeler gauge at 4 points around the circumference. ASME PCC-1 limits acceptable angular misalignment to 1 mm per 300 mm flange diameter. If misalignment exceeds this, the root cause (pipe stress, support issue) must be corrected, not compensated by bolt torque.
When to Replace Instead of Repair
Replace the valve and flanges if: flange facing is corroded or damaged beyond grinding repair, the raised face is not flat within 0.025 mm radial TIR, or repeated joint leakage is caused by pipe strain that cannot be relieved.
Applicable Standards
Related Products
Key Terms Explained
Unfamiliar with any terms used in this guide? Each links to a full engineering definition.
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