SIZING STORAGE TANK VENT DRIERS
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 pumpout 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 pumpout 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 pumpout rate should not exceed the maximum pump out rate for
the drier, nor should it exceed the design pumpout rate for more than thirty minutes. If it
does, the next size larger drier should be used.
