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Posted By James Perry
Colleagues,
We have a need to perform welding maintenance on our hot oil system pipework, which we have drained down in preparation.
We intend to flood the pipework within the affected area with nitrogen to prevent combustion.
Our welders are aware of maximum Oxygen levels to create an adequate purge for cleaning, however they are unaware of what level no longer supports combustion.
As we have a limited supply of nitrogen we will need to test to ensure O2 levels remain low enough while the contractor completes his work. 0% is not practicable.
Does anyone know of the minimum % level v/v for combustion?
Would anyone have an RA for welding pipework with flammables in, that I could adapt? (The Oil has a flash point of 200 degrees)
Any relevant info would be appreciated.
Regards
Jim Perry
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Posted By John Murgatroyd
All instructions for welding on containers that have contained flammable materials or liquids says to thoroughly clean the container before attempting to weld...you're going to subject a liquid hydrocarbon to heat in excess of a thousand degrees C...the oil will break down anyway and release hydrogen carbon and oxygen, in varying amounts and mixtures.
I consider that an explosion is highly likely.
I wouldn't weld it.
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Posted By Merv Newman
Different materials will require different levels of oxygen to burn. Highly combustible materials will ignite at much lower levels of oxygen than will weakly combustible materials. Suggest you check with msds or manufacturer.
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Posted By James Perry
John
Thanks for the warning, the system has to be welded, its leaking. We are now in discussion with the manufacturer of Oil as to a safe remedy.
Any other advice would be appreciated.
Regards
Jim
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Posted By Jane Blunt
There is an HSE publication CS 15, 'Cleaning and gas freeing of tanks.....' which is aimed at dealing with exactly this sort of problem.
It recommends cleaning, but also describes inerting techniques.
Jane
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Posted By John Webster
For the benefit of the doubters, it is perfectly normal for pipes in oil and gas service to be welded. It is virtually impossible to guarantee complete removal of product in this situation - it can be safer to weld a petrol pipe than a heavy oil pipe simply because petrol can be evaporated to dryness at ambient temperatures - so inerting is the accepted control method. Sorry, I don't have any data on safe 02 content now I am out of the oil & gas sector, but I have known cooled gas turbine exhaust used for this purpose. Somebody in the oilfield maintenance business should be able to help.
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Posted By Robert K Lewis
Jim
Have you considered using foam fill as it is more stable and less prone to leakage during the work. Nitrogen purging does rely on the ability to keep the gas from leaking out whilst foam can stay put and also prevent any local flame spread.
When I was involved in this back in my petrochem days we used to reckon on 2% oxygen as the break point, checking for explosive gases actually requires a 1.5 - 3% O2 concentration for the thyristor to work. There are a number of companies involved in foam filling - Look for those involved in Tank pulls from petroleum forecourts. This is the standard method of making tanks safe for transport.
Bob
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