Eagleview
Veteran Member
Just shows how dangerous welding on tanks can be and using steam to inert the tank can make it worse . http://www.hss.state.ak.us/dph/ipems/occupation_injury/reports/docs/00AK006.pdf
Welding tanks and steam myth .
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Wis Bang
Gold Member
I worked in the chemical tank transport industry for close to 30 years.
Steam is not sufficient to remove potential volatile substances from a tank before hot work. It is part of the cleaning process but hydrocarbon products require soap or caustic solutions to break down the hydrocarbons and a hot water rinse to flush them away.
Reading the findings I know steam opens the pores of the metal and would free up the material allowing it to accumulate in the tank atmosphere. There should have been additional steps taken to break down and rinse away the hydrocarbons.
The report stated this facility had a confined space entry training which was ignored. Welding on the exterior of this vessel with out using a combustible gas monitor to check the tank atmosphere should have been part of the Confined Space program.
Most likely the welder was left to alone and had reduced the flow of the steam so he could see where he was welding. Had the full flow of the steam hose been utilized the atmosphere could have pushed the material beyone the explosive range but created less than optimal working conditions for the welder.
What a shame. DOT regulations prevent tank transport companies from welding on the transport units, repairs have to be completed by a facility w/ an ASME R or U stamp; i.e. a pressure vessel certified manufacturer or repair facility.
They could have used Nitrogen to inert the tank. Chances are they felt that the waste water had already diluted the residual in the tank ans tha the steam was 'good enough'.
Steam is not sufficient to remove potential volatile substances from a tank before hot work. It is part of the cleaning process but hydrocarbon products require soap or caustic solutions to break down the hydrocarbons and a hot water rinse to flush them away.
Reading the findings I know steam opens the pores of the metal and would free up the material allowing it to accumulate in the tank atmosphere. There should have been additional steps taken to break down and rinse away the hydrocarbons.
The report stated this facility had a confined space entry training which was ignored. Welding on the exterior of this vessel with out using a combustible gas monitor to check the tank atmosphere should have been part of the Confined Space program.
Most likely the welder was left to alone and had reduced the flow of the steam so he could see where he was welding. Had the full flow of the steam hose been utilized the atmosphere could have pushed the material beyone the explosive range but created less than optimal working conditions for the welder.
What a shame. DOT regulations prevent tank transport companies from welding on the transport units, repairs have to be completed by a facility w/ an ASME R or U stamp; i.e. a pressure vessel certified manufacturer or repair facility.
They could have used Nitrogen to inert the tank. Chances are they felt that the waste water had already diluted the residual in the tank ans tha the steam was 'good enough'.
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Egon
Epic Contributor
It was not the steam but the failure to imliment proper procedures before allowing hot work to proceed.
Wis Bang
Gold Member
It was not the steam but the failure to imliment proper procedures before allowing hot work to proceed.
You are correct about following procedures. If they had tested the O2 and combustible gas content, and kept the monitoring equipment in the area this welder might still be here.
Steam alone might not be enough. When we cleaned ethanols w/ out oily denaturants and very volatile solvents like Toluene the standard wash was a hot water rinse, steam, and dry. The blower has a steam coil on a timer so the drying air started out very hot & then cooled off to chill the tank down to try and prevent condensation.
The same tank going out to a repair shop would still have a caustic solution wash as an additional step. Same thing for the annual internal inspections and ultrasonic thickness tests we performed in-house.
We had our inspector in a harness connected to a retrieval device thru a pulley mounted on an hoist frame rolled over the manway and an attendant always present with a Confined Space Entry permit and these were not considered 'hot' entries.
Even with strict procedures, repair shops still have unfortunate incidents. I know of a double fatality where a guy decided to check a Liquid Molten Sulpher tank [almost impossible to clean] and was overcome as was the guy who jumped in to 'save' him...and this was a shop with highly trained technicians doing ASME repairs. He did not follow procedure.
Wis Bang
Gold Member
Enough would have kept the flammable vapors outside the 'explosive range' but the billowing cloud escaping the tank would have made it impossible to see enough to weld and may have prevented striking an arc...
This guy said 'screw it it's just waste water'...
That's why his employer should have followed procedures that were already written down in the safety plan. I wouldn't want to have their new insurnance invoice...or the OSHA fines!
polo1665
Veteran Member
This may be an ignorant question, but I'll ask it none the less.
In this application (as I read it) the steam was used to displace oxygen in the tanks, was it not? The reason it failed seems to be that the welder (victim) reduced the steam pressure enough to allow oxygen into the tank, the oxygen, coupled with the volatile chemicals and a spark from the welder then caused the explosion. Is this correct? If it is then I will have some follow up questions, if it isn't please let me know and I'll shut up and read.
I'm interested as I have some experience with purging pressure vessels with inert gasses.
Mark
Wis Bang
Gold Member
My take was that the steam was supposed to inert the atmosphere inside the vessel containing residue of hydrocarbon contaminated waste water while he welded on top of the tank. Reducing the steam [it was still flowing] allowed the vaporized residue inside the tank to fall between the LEL and UEL as it continued to escape the vessel; exactly the opposite of what was intended.
Egon
Epic Contributor
The steam is to heat up the vessel to evaporate combustibles and carry them out of the vessel. Then the vessel is purged with air, checked for combustibles, oxygen and whatever other toxins may have been in the vessel. After that it may be entered wearing the proper gear for further checking and perhaps the cleaning of any remaining substances that are in the vessel. At no time would anyone be allowed inside the vessel if an explosive atmosphere was present or materials that could make an atmosphere explosive be present.
At times nitrogen may be used to flood the vessels as some chemical byproducts in vessels will react with oxygen.
Many companies will have set standard procedures on how to do this on a general basis but for the actual situation each and every vessel must have it's own procedure.
Note: my experience only applies to a very limited range of hydrocarbon facilities operations and as my memory fails a little every year I'm almost to the beginning of the learning cycle.
At times nitrogen may be used to flood the vessels as some chemical byproducts in vessels will react with oxygen.
Many companies will have set standard procedures on how to do this on a general basis but for the actual situation each and every vessel must have it's own procedure.
Note: my experience only applies to a very limited range of hydrocarbon facilities operations and as my memory fails a little every year I'm almost to the beginning of the learning cycle.
Redneck in training
Elite Member
I think that the steam could actually draw the air in. If the steam flow was reduced it started to condense dropping its volume and pulling air in. Possibly there was other factor: water breaks down to Hydrogen and Oxygen above about 550 C or 1020F when iron is present as a catalyst water vapor can dissociate at even lower temperature. If enough O2 and H2 would accumulate it could contribute to explosion. Perhaps some chemist can tell us if that is possible.
