The electro-pilot of the pilot-operated solenoid valves is not directly responsible for opening the solenoid valve.
The solenoid valve presents three sequential chambers (see drawing below):
- inlet chamber Ci (upstream the diaphragm)
- compensating chamber Cc (downstream the diaphragm and upstream the electro-pilot)
- outlet chamber Cu (downstream the electro-pilot).
In the 2/2-way normally closed (N.C.) solenoid valve, Ci and Cc are communicating by means of a compensating hole when the coil is de-energized. Therefore, the diaphragm is in a condition of  balanced pressure level and DN tightness is ensured by the retaining spring load of the same diaphragm.
When the coil is energized by acting on the electro-pilot, there is an immediate communication between Cc and Cu: the sudden increase in volume of the pressurized fluid over the diaphragm (Cc +  Cu > Ci) causes, according to the Boyle-Mariotte’s law, a pressure drop.
The diaphragm is no longer in a condition of balanced pressure level (pressure under it is higher than pressure over it) and swells up in the higher pressure direction raising up and opening the DN to  let the fluid flow.
In the 2/2-way normally open (N.A.) solenoid valve, Cc and Cu are communicating when the coil is de-energized and pressure under the diaphragm is higher than pressure over it and as a result, the diaphragm is raised to let the fluid flow.
When instead the coil is energized, the communicating passage between Cc and Cu is closed. The volume available to the pressurized fluid above the diaphragm is suddenly reduced and, always  because of the Boyle-Mariotte’s law, this results in a pressure increase above the diaphragm. The lack of balance so produced makes the diaphragm close on the seal.

18000 SERIES	18500 SERIES