Flange Bolt Selection & Step-by-Step Tightening Sequence for Pipeline Engineering
Flange connection is the core joint structure of industrial pipelines, thermal equipment and pumping units. The overall tightness and operation safety highly depend on standardized flange bolt selection and scientific tightening order. Random bolt tightening is the top cause of pipeline liquid and gas leakage, which brings huge maintenance cost and production shutdown loss for energy and chemical enterprises worldwide.
1. Four Flange Bolt Selection Principles
- Pipeline Working Pressure: Grade 10.9 and 12.9 high-strength bolts for high-pressure pipelines; grade 8.8 carbon steel bolts applicable for low-pressure auxiliary pipelines
- Operating Temperature: Hot-dip galvanized fasteners are forbidden above 250℃; heat-resistant alloy bolts are required for high-temperature thermal pipelines
- Corrosive Medium Resistance: A4 stainless steel bolts for chemical and salty medium pipelines to resist intergranular corrosion
- Standard Matching Rule: Avoid mixing DIN and national standard flange bolts, mismatched thread size will cause assembly dislocation
2. Standard Flange Bolt Tightening Sequence
The golden rule for flange installation is diagonal cross tightening, which balances compression force evenly on flange sealing surfaces. Clockwise sequential tightening is strictly prohibited, it will lead to flange deflection, gasket extrusion deformation and irreversible sealing failure. The whole installation process is divided into three stages: pre-tightening with low torque, intermediate locking, and final standardized torque calibration.
3. Applicable Bolt Types for Different Working Conditions
| Working Condition | Recommended Bolt Grade | Surface Treatment |
| Low-pressure normal temperature pipeline | Grade 8.8 | Hot-dip galvanizing |
| High-pressure heat supply pipeline | Grade 10.9 | Manganese phosphating |
| High-temperature industrial reaction equipment | Grade 12.9 | Heat-resistant anti-rust coating |
| Offshore subsea pipeline | A4 Stainless Steel | Surface passivation |
4. Critical Assembly Taboos
Never assemble mixed-strength or mixed-length bolts on a single flange joint. Discard aged deformed gaskets directly during overhaul, reused sealing parts will lose compression elasticity rapidly. We supply integrated flange fastener kits including matched bolts, nuts, washers and sealing gaskets, providing one-stop engineering matching service for global pipeline contractors.