Internal Pilot Operated Valves：
This type of valve is provided with a 2-Way pilot solenoid operator. A diaphragm or piston provides the seal for the main seat. These valves are used for switching pressures in conjunction with orifice sizes larger than those available with direct-acting valves. The floating diaphragm construction requires a pressure drop across the main valve seat to remain in the open position, or a coupled diaphragm or piston which is mechanically held open by the solenoid plunger (i.e. Chem-o-sol). The coupled type will operate with zero pressure drop across the main valve seat.
An Internal Pilot Operated 2-Way valve is When the solenoid is energized, the plunger opens the pilot orifice and relieves pressure from the top of the diaphragm to the outlet side of the valve, through the pilot channel in the main body. This results in an unbalanced condition which causes the main line pressure to lift the diaphragm off the main seat, thereby opening the valve. When the solenoid is de-energized, the pilot orifice is closed and full line pressure is applied to the top of the diaphragm, through the bleed orifice located in the diaphragm, from the inlet side of the valve, thereby providing a seating force for tight shut-off. As long as a pressure differential exists between the inlet and outlet ports, a residual shut-off force is available by virtue of the larger effective area on the top side of the diaphragm. In certain cases an internally piloted valve will require a minimum line pressure to insure sealing of the main orifice. This is due to the fact that the diaphragm is usually produced in a flat fashion and is made of some type of elastomeric material which will have a tendency to pull the diaphragm back to its normal position.
External Pilot Operated Valves：
With these types of valves, an independent external pilot source is used for controlling the pressure for actuation purposes. These valves are either piston or diaphragm types and may be actuated by either air or liquids.
A 3-Way solenoid pilot valve controls the independent external pilot media. In the case of externally piloted valves, the standard internal pilot valves are manipulated in a manner which separates the main valve into two sections. The upper section is the pilot pressure chamber and the lower section is for main media control. In the normal position, with pilot pressure, the main valve is closed preventing the flow of media. Pilot pressure is interrupted when the solenoid is energized. The plunger is pulled from its normal position to the raised position, closing off the pilot inlet orifice. At the same time the pressure is released from the pilot chamber of the valve by means of an exhaust port. When the solenoid is de-energized, the plunger returns to its normal position closing the exhaust port and the pilot pressure again builds in the pilot chamber, creating the diaphragm closing force. (Note - All externally piloted valves will fail to open if for some reason the pilot supply pressure is interrupted).
The Normally Open and closed functions of the main valve rely on the function of the pilot valve. For example, a Normally Closed external pilot valve requires a Normally Open pilot valve. Conversely, a Normally Open external pilot valve requires a Normally Closed pilot valve.