Heat Exchanger Tubes Thickness under external pressure & ASME Section VIII 1968 versus latest ed

[AdeelKhan]

Dear All,


Back-ground:
Heat Exchanger with SB-111 UNS 71500 HR50(3/4", 9 BWG - 3.7 mm) designed on ASME Section VIII 1968 Edition and installed in 1977. Now we plan to replace with the same design conditions, material and design code ASME Section VIII Div. 1. The manufacturers have told that the tubes thickness IS NOT ADEQAUTE UNDER EXTERNAL PRESSURE as per latest ASME code and proposed 4.7 mm thick tubes. Old calculations from original manufacturer are not available.

I am not sure why it is failing. Change in thickness will impact thermal performance and other installation issues. I need your expert advise on the following issue:
1) What are the changes in ASME Code calculations from 1968 to latest edition?
2) What are the changes in Code allowable stress values?
3) When checked the tubes thickness on ASME Section VIII Div 1 & Code Case 2286. Tubes thickness is okay but there is a limitation for using code case i.e., it is limited to ~ 65 Deg C for copper and copper alloys. Our design temperature for shell and tube side is 210 Deg C and 50 Deg C respectively. Can we use Code Case considering Mean metal temperature of the tubes which is less than 65 Deg C instead of shell side design temperature?

Requesting urgent replies.

Regards,

 

[MJCronin]

Can you post pictures of the tube failures ?

Do the failures show the characteristic "external crushing" mechanism type of external pressure failure ?

What was the corroded remaining wall thickness of the tubes when you tested them ??

MJCronin
Sr. Process Engineer

 

[AdeelKhan]

There have been no failures due to external crushing / pressure. Failures were due to long service of around 40+ years & some tubes are plugged only.