Pressure does the work, the coil gives permission
The moving element is a diaphragm (or piston) resting on the main seat. A small bleed orifice through the diaphragm lets line pressure fill the control chamber above it. With the pilot closed, chamber pressure equals inlet pressure, but the chamber-side area of the diaphragm is larger than the seat-side area exposed to inlet pressure, so the net force holds the diaphragm firmly shut; line pressure is effectively clamping the valve closed with its own muscle. In the centre of the diaphragm sits a small pilot orifice, sealed by the solenoid plunger. Energise the coil, the plunger lifts, and the chamber vents through the pilot passage to the outlet faster than the bleed hole can refill it. Chamber pressure collapses, the inlet-side force wins, and the diaphragm lifts off the main seat, opening the full bore. De-energise, the pilot closes, the bleed refills the chamber, and the diaphragm is pressed shut again.
Why the minimum differential exists
The lifting force is inlet pressure minus outlet pressure acting on the diaphragm. If the two are nearly equal, venting the chamber changes little and nothing lifts; the pilot clicks, the main seat stays down. This is not a defect but the operating principle, and it is why the datasheet minimum differential, typically 0.2 to 0.5 bar, is a hard limit. Assisted-lift designs hang the diaphragm mechanically from the plunger, so the coil can drag it open at zero differential while pressure assistance still carries the load whenever it is available, a hybrid of both principles.
The two small holes rule the valve
Bleed and pilot orifices are sized as a pair: the pilot must outflow the bleed for the valve to open, and the bleed must refill the chamber promptly for it to close. Debris that blocks the bleed leaves the chamber unable to repressurise, so the valve hangs open; debris in the pilot prevents venting, so it stays shut with a perfectly good coil. Both failures are cured by strainers and clean commissioning, not by replacement coils. Orifice sizing also sets speed, and manufacturers exploit it in soft-closing variants that damp the diaphragm's return to tame water hammer on pump and booster lines.
Piston variants and steam
Replace the elastomer membrane with a metal piston running in a machined bore, seal it with PTFE rings, and the same pilot principle handles saturated steam and thermal oil, where any elastomer would harden and crack. Piston valves tolerate higher differentials and temperatures, at the price of more mass, slower action and closer attention to media cleanliness in the piston clearance.
Reading field symptoms
A valve that buzzes and warms its coil but never opens is usually below minimum differential or has a blocked pilot. One that dribbles when closed has debris or a worn seat on the main or pilot sealing face. One that hangs open after de-energising has a blocked bleed hole or a torn diaphragm, and a diaphragm tear often announces itself first as external weeping at the bonnet joint. Because the coil is rarely the culprit, a strainer and a spare diaphragm kit are better insurance than a spare coil in most plants.
