The PCV valves are composed of coil springs, which keep the covers closed when the pre-set pressure limits are respected. In case of overpressure inside the silos, the outer ring-shaped cover opens for the vent, because the pressure is higher than the force generated by the springs; while, in the case of depression, the compression of the springs causes the disc-shaped lid to open and the air to enter from the external environment, to rebalance the pressure and close the disc.