Genuine Campbell Hausfeld replacement part, this item is sold individually.
When you use a compressor the air that is compressed inside the tank will condensate and leave water inside the tank.
If it is left in the tank too long it can start to rust out the inside of the tank.
The Drain Valve should be opened up and let the water drain out.
The water will build up if you have a tendency to leave air in your tank for a long period of time.
You should drain your tank after each time it is used to prevent this from happening.