Under pressure, the fluids fills onto the solenoid valves.
The coil winding is not energized, the core spring, because of fluid pressure, pushes the sealing gasket onto the solenoid valve body seat ( hole ) and keeps it closed
When the coil winding is energized, the plunger is attracted by the fixed core, the hole ( DN) is opened and fluid passes through.
The maximum pressure difference allowed (M.P.O.D.) decreases as the seat diameter increases