tank roof welds

Date 01-15-2007 13:29

Good morning every one! My question of the day is, where does it say in API 650 that fixed roofs do not have to be welded on the under side? It seems that I was told that anything above the overflow didn't need backwelded unless the engineer called for it. We don't usually weld the underside of roofs but we have a batch of old tanks (304 SS) that the shell to roof welds are cracking terribly from overpressurization. The engineers want to fix the cracks, but leave this the weak spot. Should these be backwelded? And should we backweld all new tank roofs? Thanks in advance, Henry

By dbigkahunna (****)

Date 01-15-2007 14:02 Edited 01-15-2007 14:35

API 650 Section 3.10.2.5 (Note the word "shall"). The roof is designed to fail vs. the shell failing. ie-the roof blows off vs. the tank blows out. Was the tank constructed based on API 650 Appendix F? Even that does not allow backewlding the roof plates to the compression bar or top angle. See API 650 Appendix F.3,Figure F-2.
If you back-weld the roof to the compression bar or top angle, you have defeated a safety mechanism for the tank. The compression bar or top angle is where the tank gets its strength. The structure of the roof is where it gets it rigidly. The plate over the structure is pretty much to keep dirt and bird poop from getting in. This is on 650 tanks.
Why are 650 tanks being pressurized?
API 620 tanks are designed to hold pressure. Only tank designed to 650 Appendix F are for pressurized service
If you are getting cracks in the welds along the top angle and the tank the tank was not designed to Appendixf F, there may be a settlement problem, an anchorage issue, or it may be a result of the material the tank is holding. If the tank is on a concrete ring-wall and you can safely say it is not a settlement issue (see API 653 Appendix B)it could be one of the following:
the tank is on a concrete ring-wall but not anchored and the tank can meet the requirements of Appendix F, it could be an anchorage issue,
the tank can meet the requirements of API 650 Appendix Fand is properly anchored the owner may be looking at a corrosion cracking issuedue to the tanks contents.
Before I would turn a welder loose on the cracks, I would determine what caused them. Do a thorough API 653 inspection and show the inspector what you have seen.
BABRT's

By Ariel D C (**)

Date 01-16-2007 06:08

Hi Henry, You can refer to 3.10.2.5 (addendum March 2000) which states " roof plates shall be attached to the top angle of the tank with a continuous fillet weld on the top side only" You may want to review the tank design if this connection ask for frangible joint (not exceeding 5mm continuous weld).