Water in a compressor

Q: When I drain the air out of my compressor at the end of the workday, I sometimes find that some brownish water is also discharged. Because the compressor tanks are sealed, is there any way to remove all of the water from the tanks? Is water remaining in the tanks a big problem? And is there any way to prevent the buildup of water in the tanks?

A: Drew Wiley of Berkeley, California, replies: When air is compressed, it is essentially concentrated, and any water vapor in the compressed air becomes concentrated as well. The higher the relative humidity of the air taken in by the compressor, the higher its water content, and the greater the amount of water vapor that can become trapped inside the air tank of the compressor. The concentrated water vapor condenses either when warm air from the pump meets the cooler walls of the storage tank or when air is left trapped in an inactive compressor and the entire system cools down.

Condensed water in a compressor tank is likely to contain corrosive agents due to industrial pollution (acid rain, for example) or, in coastal areas, harsh marine salts. Compressor tanks are typically made of galvanized steel and are vulnerable to these caustic substances. Stainless-steel compressor tanks are available, but they are very expensive.

The best way to minimize damage is to drain the compressor tank at the end of each workday. If the drain cock is not at the very bottom of the tank, the compressor may have to be tilted a little to facilitate drainage. If your compressor has two tanks, each should have a drain cock, and each should be drained. Having the tank fully pressurized before draining makes the job easier because pressure forces out nearly all of the water.

A small amount of corrosion in galvanized tanks is almost inevitable, and a little rust color in the discharge water is nothing to worry about. However, advanced corrosion is another matter. Rust scale can enter pressure switches or regulators, or even freeze up the pump, leading to costly repairs. In a worst-case scenario, corrosion may weaken the tank wall enough to become hazardous. In any event, compressors should always be equipped with line filters to prevent rust scale or any other particles from entering the air line and damaging tools as well.