Dear all, this is a sharing from a member of KMI (Indonesia Oil and Gas Community).
The case is,
There is a retubing process for power boiler. Then the client ask to vendor/contractor for cutting the old tube and replace with new one.
The problem is position of cutting for old tube will be same as the new welding line. So, the new welding line position will be same as previous welding joint line existing which got cutting.
Usually, we avoid new welding joint in existing welding line. Some people said that welding in an existing welding line will make structure is not good. Because of microstructure in HAZ will be change, and structure become hardness due to welding process more than one. But if we considering of basic normalishing, it doen’t matter as long as we keep the cooling rate, so that automatically the microstructure will not changed.
But, actually are there any code or rule for that condition? Or, what is the risk if we do that?
And, Yes, ASME allow it. We can refer to ASME VIII; UW-14 (for Pressure Vessel). For Power Boiler refer to ASME I; PW-14. And UW-38 for remove defect.
Beside that, practically it just a simple case. We can cut old tube, ground out old weld joint line and also clean out. Then we do a NDT test to see the crack or defect. If material okay, we can weld it with new tube.
Source : Summary of Indonesia Oil and Gas Mailing List.