Internal Floating Roof Peripheral Vents

[Leinster]

Why is it necessary to have such large peripheral vents and a central vent on an Internal Floating Roof as per API 650. Surely if you satisfy the venting requirements as per API 2000 that should suffice?

 

[IFRs]

The vents are for circulation, to dilute the small amount of vapors that get past the floating roof until they are below the LEL.