Repairs on fuel oil storage tank

[mpeck1982]

Anybody know what are some good safe repair working practices on above ground double wall fuel oil storage tanks? We have some small corrosion damage on the top side of the outer shell that needs to be repaired. It will have a small gradual leak out of the storage tank when the outer shell is full. We found that out by a contractor filling the inside tank and the outer shell tank. What is the best repair to fix this tank?

 

[Duwe6]

Safe Repair requires either a visually verified Clean-to-Bare-Metal, or a purge of the oxygen from the area being welded. Reason for the visual verification is that 'varnish', polymers, etc. can still be on the back side of the area to be worked on, even after a lengthy steam-out. When the repair crew starts cutting & welding, those contaminants will volatilize and hopefully burn; but may cause an explosion if the oxygen-to-hydrocarbon mixture is just right [wrong?].

Not possible to verify the cleanliness of an annular space, so a purge is required to displace oxygen. Fill that space with something other than air. Water, nitrogen [10 volume changes] or argon [fill until O2 reading is 0.0%] all work very well. NOTE: corrosion of the steel surrounding the annular space MAY have already depleted the O2 down to essentially zero. But do NOT bet the lives of the repair crew on that being adequate, reinforce that natural 'purge' with some additional N2 and monitor the O2 content of the space within the vicinity of the Hot Work.