1, you can judge the positive and negative valve discs by the working principle of the breathing valve. When the pressure of oil and gas in the tank is greater than the allowable pressure in the tank, the positive valve disk is topped open, the gas from the tank overflow with the same atmospheric pressure. When the tank pressure is less than the allowable pressure, the atmosphere outside the tank will open the negative valve disk into the tank with the same atmospheric pressure.
2, you can also look at the end of the valve shaft in that direction to judge, the shaft in the breathing valve is a negative valve, the shaft outside the breathing valve is a positive valve.