Air Cooled Heat Exchanger Plug Torque Calculation

Here's how I've approached the problem so far. Using the gasket dimensions for the plug header I calculated the gasket surface area. Since the gaskets are soft iron flat metal gaskets I used a target bolt stress of 18,000 psi. I multiplied the target stress by the surface area of the gasket to determine what the target plug preload should be. I then used T=KFd to calculate the torque required.

As a check I calculated the plug tensile area and used the calculated preload to check the tensile stress in the bolt. Since it was suffeciently low I doubled the torque to account for any unknowns I had missed and rechecked the stress using the new preload.

Bolt stress was about 10 ksi so I assumed this would be good.

Any holes in my method?

Gasket OD: 2.1875
Gasket ID: 1.890
Stud Size: 1.875 (n=12tpi)
K Factor: 0.16
Target Seating Stress: 18,000 psi (Lamons Handbook)
Calculated Target Preload: 13306 lbs. (Gasket Area*Target Stress)
Calculated Target Torque: 330 ftlbs. (T=kFd)
Bolt Tensile Area: 2.527 in^2 (pi/4 * (d-0.9743/n)^2)
Bolt Stress: 5265 psi (Bolt Stress=Preload/Tensile Area)

Doubled torque value as a safety factor yielding a stud preload of 26611 lb requiring a torque of 660 ftlbs.
This would produce a bolt stress of about 10.5 ksi.

Thanks in advance for your help.

-Blue
Mechanical Integrity Engineer
5 Years Experience (Refinery)
2 Years Experience (Chemical Plant)

BlueEngineer, not running the numbers, but I'd say your approach is about right. Identify a target gasket stress, calculate a load, check stresses, calculate a torque. Not commenting on the uncertainties in converting fastener load to torque

Perhaps you could check against your company standards, see if you (they) are in the range.

Regards,

Mike