Appendix 2 flanges on older vessels 1

[lcmechengr]

I am having to derate some older exchangers (manufactured 1957) due to component thinning. When performing the calculations for the channel and shell flanges using the allowable stresses from 1957 and the latest edition of code, I am finding that the origninal flange dimesnion do not pass calculations for either gasket seating or operating cases. Has the design requirements for the Appendix 2 flanges changed since these exchangers have been designed and manufactured? Is it appropriate to use the latest edition of code to derate these components?

 

[Christine74]

Some thoughts:

1) I'm not aware of any changes in the calculation methods of Appendix 2 since the time it first appeared in the ASME Code, but you'll have to review that to confirm this.

2) Are you using the bolting allowable stress from 1957 as well? Back then I believe the allowable stress for SA-193-B7 was 20 ksi, not 25 ksi. You generally cannot mix and match flange/bolting allowable stresses successfully.

3) Are you accounting for gasket seating in the pass partitions in your calculations? I don't think this was generally done at the time of manufacture.

4) It is not uncommon for flanges designed 50 years ago not to meet the required Appendix 2 calculations. To save time, manufacturers at that time often created there own standard flanges for 150#/300# ratings, but they did not pass Appendix 2 calculations for every pressure/temperature combination.

5) Use of the allowable stresses from the current Code might be possible. Check with your jurisdictional authority.

6) Changing the gasket type might help reduce the calculated stresses for the gasket seating case. If possible, reducing the gasket diameter/width might help reduce the stresses for the operating condition.

-Christine

 

[Christine74]

Forgot something--

Addenda to #5 above, you cannot use allowable stresses from the current Code to re-rate this exchanger since it was built prior 1968. See NBIC Interpretation 98-14.

-Christine