Flange Bolt Torque Values: Complete Reference Guide for Every Pressure Class

Introduction

Proper flange bolt torque is critical for maintaining leak-free joints in industrial piping systems. Incorrect torque values can lead to gasket failure, bolt damage, flange warping, and dangerous leaks.

Why Bolt Torque Matters

Consequences of Improper Torque:

Under-Torquing:

  • Insufficient gasket compression
  • Joint leakage
  • Gasket blowout
  • Bolt loosening from vibration

Over-Torquing:

  • Bolt yielding or fracture
  • Flange face damage
  • Gasket crushing
  • Thread stripping

Bolt Torque Fundamentals

The Torque Formula:

T = K x D x P

Where: T = Torque, K = Nut factor, D = Nominal bolt diameter, P = Desired bolt preload

ASME Pressure Class Torque Tables

Class 150 Flange Bolt Torque Values

Bolt Size ASTM A193 B7 (ft-lbs) ASTM A193 B7 (Nm)
M12 45 61
M16 95 129
M20 185 251
M24 310 420
M30 600 814
M36 950 1288

Class 300 Flange Bolt Torque Values

Bolt Size ASTM A193 B7 (ft-lbs) ASTM A193 B7 (Nm)
M20 220 298
M24 370 502
M30 720 976
M36 1200 1627
M42 1850 2508

Bolt Material Specifications

ASTM A193 B7:

  • Chromium-molybdenum steel
  • Temperature range: -50F to 1000F
  • Most common for carbon steel flanges

ASTM A193 B8M:

  • 316 stainless steel
  • Excellent corrosion resistance
  • Temperature range: -425F to 1000F

Bolt Torquing Procedures

Step-by-Step Torquing Sequence:

  1. Preparation – Clean all threads and mating surfaces
  2. Hand Tightening – Install all bolts finger-tight
  3. Initial Torque (30%) – Use star/cross pattern
  4. Intermediate Torque (60%) – Follow same star pattern
  5. Final Torque (100%) – Complete star pattern
  6. Verification Pass – Recheck all bolts

Conclusion

Proper flange bolt torquing ensures safe, leak-free piping systems. Following established procedures and using calibrated equipment is essential.

Songhai Flanges provides technical support for flange assembly, bolt selection, and torque specifications.

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