Flange Bolt Torque Chart: Proper Tightening Procedures

Proper bolt torque is critical for leak-free flange connections. Under-torquing causes leaks; over-torquing can damage gaskets or warp flanges.

Why Bolt Torque Matters

Flange bolts must create sufficient gasket compression to seal without exceeding material yield strength. The correct torque ensures:

• Uniform gasket compression
• Prevention of bolt relaxation
• Avoidance of gasket blowout
• Extended service life

Torque Calculation Formula

T = K × D × F

• T = Coppia (Nm o ft-lbs)
• K = Nut factor (friction coefficient, typically 0.2)
• D = diametro nominale del bullone (mm o pollici)
• F = Bolt load (N or lbs)

ASME B16.5 Bolt Torque Chart

Flange Size	Class 150	Class 300	Class 600
1/2″ (DN15)	15 ft-lbs	25 ft-lbs	40 ft-lbs
1″ (DN25)	45 ft-lbs	75 ft-lbs	120 ft-lbs
2″ (DN50)	95 ft-lbs	160 ft-lbs	250 ft-lbs
4″ (DN100)	185 ft-lbs	310 ft-lbs	490 ft-lbs
6″ (DN150)	280 ft-lbs	470 ft-lbs	740 ft-lbs
8″ (DN200)	380 ft-lbs	640 ft-lbs	1010 ft-lbs
12″ (DN300)	570 ft-lbs	960 ft-lbs	1520 ft-lbs

Proper Bolt Tightening Sequence

1. Cross-pattern tightening: Start at 12 o’clock, opposite at 6 o’clock
2. 30% torque: Snug all bolts in sequence
3. 60% torque: Second pass in same sequence
4. 100% torque: Final pass to specified torque
5. Check: Verify all bolts reach target torque

Common Torque Mistakes

• ❌ Using impact wrenches (causes uneven loading)
• ❌ Tightening bolts sequentially around circle (warps flange)
• ❌ Reusing damaged or stretched bolts
• ❌ Not lubricating threads (increases friction)
• ❌ Ignoring gasket type (different compression requirements)

Torque Tools Recommendation

• Calibrated torque wrench (click type or digital)
• Torque multiplier for large bolts
• Hydraulic torque wrench for critical applications
• Bolt tensioner for large diameter flanges