The amount of closing force applied determines the set point of the vent. The pressure in the tank's vapor space pushes against the pallet assembly, in opposition to the closing force. When the tank pressure reaches the vent set point, the pallet assembly lifts and vapors are allowed to escape from the tank through the vent. Larger size vents provide greater flow capability than smaller size vents. Methods of calculating these volumes for specific normal venting and emergency venting situations can be found in 29CFR - OSHA 1910.106 . 2.
Storage tanks are mechanical structures.
There are limits as to how much pressure and vacuum they can withstand before they are damaged. These limits are known as the tank's design pressure and vacuum. The relief vent will remain closed until its set pressure is reached. If there is a need to maintain some pressure in the tank during normal operations, the vent must be set so that it will not open and begin relieving below that pressure. Each size and style of vent will flow specific volumes of vapors at a given pressure.