To prevent the possibility of insufficient air entering through the vent line during pump out operations causing the tank to implode, VETUS has developed a valve according to the ISO8099 standard. In case of significantly reduced pressure in the holding tanks, the valve will open automatically to let air into the tank. By using this valve fitting of a large diameter vent line is no longer necessary. The valve is made from synthetic materials and therefore absolutely corrosion-free. Hole size in the tank is 56 mm.