there are couple reasons as on the pneumatic circuit, the on/off valve(in this case, a solenoid) is always placed before the flow restrict/control valve.
1. once the solenoid valve open, there is "rush" of pressured air to fill the other side of the solenoid, instant air pressure increase may cause trouble. If the solenoid is before the needle valve, the needle valve can act as flow restrictor to prevent this "rush" to the rest of the air hose space.
To place a needle valve before the solenoid is fine only if the cavity volume between the needle valve and the solenoid is small, so with only limited amount of pressurized air in this cavity. once the solenoid open, it doesn't "push" too much to the rest of the air hose space or create trouble.
2. water issue, if check valve fail, the water always gets into the needle valve first, but hard to go through needle valve(then reach the solenoid) because the orifice is too small. If solenoid after the needle valve, the water gets in the solenoid first, that is what we don't want to see.
Last edited by Bettatail; 12-10-2012 at 04:23 AM.