What could go wrong? Well, the tank blowing up, for one thing.
Actually, there are procedures for hot-tapping and similar operations, discussed to some extent in API-653. If the welding is far enough below the liquid level that it can't ignite vapor, and is minor enough that it's not weakening the structure, then it can be done.
It would be best to have someone more local that was familiar with this type of operation and the tank in question to go through it to determine a suitable and safe procedure, if feasible, or if not, to justify draining and cleaning the tank.
Hire any reputable Tank Erection firm.
Use a tape & float through the sample hatch to confirm the accuracy of your level gauging.
Mark where you want the supports welded on.
Stand back and watch the professionals rapidly and safely perform the work.
Draining a diesel tank makes it MORE dangerous, not safer. Tanks are NEVER empty until they have been rinsed, then steamed for DAYS, and then the air tested in several areas, at several elevations. Basically, someone has to enter that tank and verify complete cleaning. Otherwise, there may be a really big boom when the welding starts.
An 'empty' tank tends to develop an explosive atmosphere due to a massive intake of air while emptying it. WAY safer to verify the liquid level, and weld 3+ feet below the liquid level. And get your work done by a pro. This is nothing for an amateur. And if you haven't done at least 5, you are an amateur.
I am aware of both approaches to the task; I've heard of Keeping the tank filled to maximum/fill trip level and then weld only below this level. I know of Draining degassify and clean the tank then do the welding.
I have welded on active bunker c tanks and pipelines before but I am more comfortable to degassify & weld a diesel but there is always constraints in removing a tank from service as it must be a planned event.
I have only heard that welding below the fuel level(well below vapour space) can be done, but that is one's experience, so outside of persons experience the question is: Is there a code or standard, procedure or guideline to do an external weld an active diesel tank shell
API-653 covers welding on in-service, filled tanks, just as JStephen stated. The basic constraints are given there. But a detailed procedure is not given. It is expected by API that the firm doing the work will have developed a detailed procedure prior to performing the work. Thus my recommendation to 'farm out' this job to an experienced firm that has developed just that procedure.
And they will expect you to give them the exact level of product, not just the tape-gauge reading. A 'maximum level' of fill generally will not be required by the firm, just that they will be working their specified distance below the liquid level, and that the tank will remain 'still' [no inflow, or pumping out].
Thanks all
Your contribution has been helpful! found it though - "Safe Hot Tapping Practices in the Petroleum & Petrochemical Industries" API Recommended Practice 2201, 5th edition Section 9
