Dear,

can someone explain under which circunstances is needed to have storage tanks containing flammable liquids under nitrogen atmosphere?
Any tip or guideline is welcome.
Thanks.

If there is a high possibility of an explosive mixture forming by inbreathing of air, you should consider inert gas blanketing. Annex F of API Standard 2000 is helpful.

Jeremy, thanks a lot for your input.
API Std2000 also says (in 4.5.3. Inert gas blanketed tanks): "the use of inert gas systems instead of a vacuum relief device is beyond the scope of this Internation Standard". It recommends using inert gas to reduce potencially explosive atmosphere, but in no case says under which conditions it's a must.
Going into a real case, I know that could be a explosive atmosphere in a tank, containing solvent, since the concentration of solvent in gas can be between LEL and UEL in a particular case. It has been designed a system where the relief valve outbreaths to a recovery system before going to the atmosphere.
But if an ignition source would be into this atmosphere in the tank, an explosion could happen.
Anyway, I still wonder if there is a rule / standard for designing this systems in a sistematic way.

Francisco,
Here is a quote from section 5.4.3 (g) of AS 1940 2004 "The storage and handling of flammable and combustible liquids"

"Where several tanks are interconnected by a common venting system and the vapours
in the vapour are within the explosive range, measures shall be taken to prevent the
possibility of flashback or flame propagation through the system from one tank to
another, e.g. by the use of flashback arresters, barometric dampers, nitrogen inerting
or ensuring that the vapour concentration is always above the explosive range.
NOTE: Additional requirements may be necessary where vapour recovery is adopted."

In reality the only time I have used it was in relation to a very toxic conbustible fluid ie we didn't want to put the fumes through our vacuum system but still needed to have a positive pressure in the storage tank.

Hope this helps.

Thanks a lot (all of you) for your helpful information.
It seems that regulation is not clear enough regarding inertization systems. Anyhow, I would like going further looking at what it is done/real practices in the oil&gas industry.
As an example, is it used nitrogen for inerting gasoline storage tanks? Gasoline has a flash point (if I'm not wrong) of -43°C. Is any example known where it is a common practice inerting API storage tanks containing flammable liquids?
The solvent I'm dealing with has a flash point of -21°C.

Remember that nitrogen adds the hazard of asphyxiation to your process. The risk assessment should take this into account. In addition, it is not necessary to totally replace air with nitrogen (which saves the expense of nitrogen in extensive flushing). Depending on the material, reducing oxygen content to 10 or 5% is usually adequate. I would recommend an oxygen monitor to warn if there is any failing of the nitrogen system.

Large scale gasoline tanks in oil refineries use floating roofs. This prevents the build up of a flammable vapour space and removes the need for inerting. It would be too costly to inert as the tank sizes are too large,

i agree with Martin. You can use floating roofs with double seals for large scale gasoline or naphtha tanks. Diesel or VGO tanks require nitrogen blanketing for keeping oxygen away from the HC to prevent gum formation and fouling in the downstream units.

You should determine the flammabiltity range of the mixture of the nitrogen and the liquid(this should be done stochiometrically). Then keep the mixture in an atmosphere that has a flammability outside of the range for the mixture.(that is outside of the a/f for the mixture,the f taken to stand for the mixture)

Thanks for your information. It seems that if the atmosphere inside the tank can be a risk in terms of flammability, the industrial solutions are floating roofs for large scale tanks or nitrogen blanketing for keeping the flammable compound below the LEL concentration.
Any other preventive method?
New comments and inputs are welcome!