Vapor control devices are rated by throughput of gas at specified conditions with a specified concentration of vapors. At higher throughput rates the pressure differential across the device will rise. If the pressure rise is great enough one or more tank relief vents may relieve. The vapor control device must be designed to handle the maximum expected gas flow rate at the highest temperature and at a specific pressure. The vent system must then be designed to deliver the gas at that pressure.
Pressure is very important.
The higher the pressure at which the system can operate, the more efficiently the system will operate. I have seen systems operate at nearly one psig, about two feet of water pressure. The difficulty with this is that this is the upper limit of allowable internal pressure for most atmospheric tanks and it is very difficult to find relief vents rated at this pressure. Most of the closed vent systems that are currently installed are constructed of normal weight pipe, i.e. rated for 150 psig. This is not necessary however, pneumatic steel tubing rated for full vacuum and 15 psig will be more than sufficient.