I used to live near a garage that welded fuel tanks as one of their
services. The only method they employed for working on a tank that had
ever contained gasoline was to connect the tank to an exhaust line from
another vehicle which was running. The exhaust from the vehicle went
through the tank input and output the entire time work was being done. The
tank was never uncapped at either end until it was ready to be worked on.
I believe they also employed a fuel oil rinse before the exhaust
connection. There was another garage that had used steam to purge through
the tank........ it didn't work.
With our tank the fumes build in the same way and cannot be washed away
with water. Gas and water don't mix so the water cannot carry the residual
gasoline away, thus the fumes come back after the water is gone. If air
were purged through the tank at a high rate it would carry the fumes away
but also build a charge of static electricity on the tank walls which is an
ignition source. The trick is to displace the combustible fumes with
non-combustible fumes in a manner that won't cause static build up. A
slowly moving non combustible gas such as exhaust fumes (no O2) will
accomplish this. Of course you have to get the carbon monoxide out of the
shop so it doesn't kill you instead of the explosion.
When we specify chemical storage areas for flammable materials Class I
Division I everything has to be non sparking and gasketed so no fumes can
enter a device or exit the room. Chemical cabinets are grounded and
nitrogen purged, light switches are outside of the room. The wire passing
between in and out goes through a junction box filled with a non flammable
semi-liquid (goop) that also blocks fumes from passing down the conduit.
In short you can't get rid of the fumes and electro static discharge (ESD)
will definitely cause an explosion, be careful
Lee
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