Air compressor water separators are usually installed in-line after the air has been compressed and left the air receiver tank on its way to your pneumatic device. If you have a regulator to reduce the pressure at a specific drop towards the end of your line. The water separator will come after the regulator as the regulator cannot be harmed by moisture. Another reason being due to the pressure drop across the regulator, more air will be cooled and thus the water separator will be able to capture a greater amount of water and provide your pneumatic tool with a greater quality of air!