There will be an air release valave on every air tank.
Air tanks come in different sizes, shapes and they are manufactured for different applications as well. Most air tanks must be emptied at regular intervals for maintenance or disposal. Some air tanks, like water pressure bladder tanks, will never have to be emptied unless they have lost their ability to retain air. Disposable air tanks, like propane air tanks, will only have a top bleeder valve that opens to dispense the air. In this case, once the air in the tank has been consumed, the tank must be recycled or disposed of. Does this Spark an idea?
1. Bleed the air from any reusable air tank by opening up the air release valve at or near the top of the tank. Air release valves come in several different designs; some will look much like a regular water valve that can be grasped and turned, while others may be a ball valve with a long horizontal handle. Smaller release valves may look like a standard tire valve in which the center valve stem must be pushed in to release the air inside.
2. Bleed the air pressure out of the air tank, then locate the drain bolt on the bottom of the air tank, if applicable.
3. Attach a wrench to the drain bolt and remove it to drain the air tank of water condensate. Place a bucket beneath the drain hole and catch any water that flows out; dispose of this down a drain. Air compressor tanks and air expansion tanks attached to boilers need regular maintenance in this manner to purge the water from the system. Once the water has been drained, replace the drain bolt.