Torque Tightening

The answer to this depends on whether you simply want to bring the flanges together or, if you want to ensure a leak-free, safe joint.

If the former, refer to any of the myriad bolt-to-torque tables that you can easily find on the 'net (or by following the link in the previous post). Apply the torque as directed. Then, pressurize the system, slay a goat for the gods and pray that the joint won't leak either immediately or shortly thereafter.

On the other hand, if you think that it might be better not to be faced with a leak, you'll need to:

• determine the minumum bolt stress required for proper seating of the particular gasket that you're using
  
• see what the maximum stress is for the flange and bolt material that you're working with
  
• back-off this stress value if it might result in gasket extrusion
• using the optimized stress figure,
  
• calculate the resultant bolt elongation
• calculate the "target torque"
  
• apply the target torque in the proper manner
• measure the bolt elongation
• "tune" the fasteners by applying more or less torque so that the desired elongation has been acheived (which also means "so that the necessary bolt stress has been acheived")

Too much work, huh? Well, here's a link that might make it easier for you to understand why this should be done : The Dangerous Misconception of "Torque"