Vent H 5.2.2.2

API 650 doesn't say "if they are not needed", it says "if they are not installed".. Big difference in meanings there.

It also states the minimum number of circulation vents as being 4, not 2 as you propose, and it gives you the positions and required open area of that ventilation with regard to the circumference of the tank.

The free vent is simply to protect the tank from pressure fluctuations as the internal roof rises and falls with volume. It is there to prevent physical rupture of the structure.

The rise and fall of the internal roof and the free vent alone will not create sufficient air circulation in the void to prevent the accumulation of volatile gases, you need constant air movement to achieve this.

You should read the standard again. Here is a link to API 650 that tells you all you need to know - see page 297.

Personally, I would be wary of the bit about "(unless otherwise specified by the Purchaser)" as that sort of statement has a habit of coming back to bite you if things go awry...Better to do it to the standards than to the budget.

I'm also more than a little concerned that you appear to be looking to take short cuts on a project that absolutely cannot have any...Try employing a qualified engineer to do this right for you