I looked at the copy on our library, 6th edition dated November 2009.

In the scope it states "This International Standard does not apply to external floating-roof tanks".  If you have internal floating roof tanks, you could use API 2000 in  my opinion.

Two inbreathing and outbreathing rates are given, one for low vapor pressure product (basically 1 volume of air for 1 volume of liquid) and for high vapor pressure product which for outbreathing, has a higher allowance because of the vapors being displaced. With an internal floating roof tank, as it rises or falls you simply need to outbreathe or inbreathe the corresponding volume.

Annex E gives a good discussion of the theory behind the standard.  In general, I don't see why the same approach wouldn't apply to a fixed roof tank with an internal floating roof.  Since a floating roof prevent product loss into the vapor space, I would likely pick the venting requirements for a low vapor pressure product.

Ultimately, you need to move air in and out of the tank to prevent overpressure or vacuum from damaging the tank.  I haven't looked at the openings set by API 650 and backcalculated what pressure/vacuum they correspond to.  My guess is the pressures are very, very low even relative to atmospheric tank design pressures.  

Thanks TD2K.

The version of API 2000 I had looked at was the 5th edition, 1998, which mentions both external floating roof, and free venting internal floating roof tanks. I just looked at the 2009 version and like you say, it excludes only external floating roof tanks, there is no mention of internal floating roof.

API 650 (10th edition) does go on to say if gas blanketing is used in place of circulation vents, then API 2000 should be used for the calculations. The 11th edition makes no mention of this, however I assume it will remain same.

Anyways, the client wants free venting system, which excludes blanketing. I will look further into the Annex E of API 2000 and see what it says.  

This would be an excellent discussion if posted in the "Storage Tank Forum"....

   

Good point.

Is there a way to just move the thread and its contents directly, or will I have to start the topic all over?

Regards

Start a new thread in that forum and put a link to this one. Maybe even paraphrase it to from a FAQ.

Wazobia - the 11th edition does mention that if circulation vents are not needed you must inert the space above the floating roof.  Here it is in full - H.5.2.2.3 If circulation vents (per H.5.2.2.1 and H.5.2.2.2) are not installed, gas blanketing or another acceptable method to
prevent the development of a combustible gas mixture within the tank is required. Additionally, the tank shall be protected by pressure-vacuum vents in accordance with 5.8.5, based on information provided by the Purchaser.