The size of the DRIERITE Storage Tank Vent Drier required to protect a storage tank is determined by the amount of air and the rate at which the air enters the storage tank. Generally, the rate at which the air enters the tank is the pump-out rate; however, there are exceptions. A large tank that is nominally empty can have air entering the tank due to temperature and pressure changes faster than the pump-out rate. Fortunately, the amount of air entering the tank usually requires a drier large enough to handle the flow rate. DRIERITE Storage Tank Vent Driers are normally sized to last from one to six months per charge of desiccant. With shorter service cycles, the labor cost becomes prohibitive and, with longer cycles, the capital costs become higher. The size of the drier is determined by the amount of air that enters the tank to replace the liquid that has been removed from the tank, and the amount of air that enters the tank due to atmospheric pressure and temperature changes. A tank that averages half full for a month will breathe 60% of its volume per month with a 20° F temperature change from day to night. Each 10° F increase or decrease in the temperature differential adds or subtracts another 30% to the volume. As an example, a 10,000 gallon tank that is 3/4 full at the beginning of the month and 1/4 full at the end of the month would breathe approximately 6,000 gallons per month with a 20° temperature change per day. Add this to the 5,000 gallons of liquid removed from the tank, and the total breathing of the tank is approximately 11,000 gallons of air. Figure one shows that the DRIERITE Model 105 Storage Tank Vent Drier has a capacity of 30,000 gallons of air at 80° F and 80% RH. This drier will last about three months under these conditions. The Model 1100 will last twice as long. The instantaneous pump-out rate should not exceed the maximum pump out rate for the drier, nor should it exceed the design pump-out rate for more than thirty minutes. If it does, the next size larger drier should be used.