What Is Hot Bolting? Online Flange Repair

Hot bolting is the sequential replacement of stud bolts on a flanged joint while the system remains pressurized and in operation. One bolt at a time is removed and replaced with a new bolt, which is then tightened to the required torque. The technique addresses bolt degradation (corrosion, stress relaxation, creep) without shutting down the plant. ASME PCC-2 (Repair of Pressure Equipment and Piping) provides the guidelines.

When Hot Bolting Is Used

Scenario	Purpose
Corroded bolts	Replace before they fail under load
Bolt relaxation (creep)	Restore bolt load on high-temperature joints
Leaking flange	Retighten or replace bolts to improve gasket compression
Post-startup retorque	Re-tension bolts after thermal cycling
Upgrade material	Replace with higher-grade bolts (e.g., B7 to B7M for NACE compliance)

Key Requirements (ASME PCC-2)

Requirement	Detail
Maximum bolt removal	One bolt at a time (some procedures allow two diametrically opposite)
Minimum bolts remaining	N-1 (or N-2 if diametrically opposite)
Pressure limit	Typically limited to 50-80% of design pressure during bolt removal
Temperature limit	Per bolt material limit and safety assessment
Risk assessment	Required before any hot bolting operation
Personnel	Trained and certified bolting technicians
PPE	Full face shield, fire-resistant clothing, gloves

Common Mistake

Hot bolting is NOT the same as hot tapping or hot work. It does not involve welding, cutting, or penetrating the pressure boundary. However, removing a bolt from a pressurized flange temporarily reduces the clamping load on the gasket. If the remaining bolts cannot maintain sufficient gasket stress, a leak will occur—potentially catastrophic in hydrocarbon service.

Hot Bolting Procedure

1. Perform engineering risk assessment (pressure, temperature, fluid, flange class)
2. Verify flange alignment and joint condition
3. Prepare replacement stud bolts and nuts (verified material, length, and thread)
4. Remove one bolt completely
5. Insert new bolt and hand-tighten nut
6. Torque new bolt to specified value per torque chart
7. Move to the next bolt (follow star/cross pattern)
8. After all bolts replaced, perform final pass torque on all bolts
9. Monitor for leaks during and after the operation

Pro Tip

Use hydraulic bolt tensioning for hot bolting on large-diameter, high-pressure flanges (Class 600+). Tensioners apply axial load without nut rotation, reducing the risk of gasket disturbance during the operation. Multiple bolts can be tensioned simultaneously, minimizing the time with reduced bolt count.

Limitations

Hot bolting is not permitted in all situations. Most operating companies restrict or prohibit it for:

• Lethal service (HF, H2S, chlorine)
• RTJ flanges (metal ring gaskets lose contact if bolt load drops)
• Flanges with fewer than 8 bolts (removing one bolt is >12% load reduction)
• Joints with known gasket degradation

Read the full guide to flanges