During idle operation, what keeps the valve shut?

• A. Water pressure below the diaphragm.
• B. The plunger locks the valve.
• C. Water pressure in the bonnet chamber above the diaphragm keeps it sealed.
• D. The diaphragm magnetizes to hold the seal.

Answer: (C)

In a diaphragm irrigation valve, the shut condition is created by pressure in the bonnet chamber above the diaphragm. That pressure clamps the diaphragm against the valve seat, preventing flow when there’s no signal. When the valve is energized, the solenoid releases that bonnet pressure (via a pilot valve), allowing the line pressure to move the diaphragm and open the valve. The other ideas—pressure below the diaphragm, a locking plunger, or magnetizing the diaphragm—don’t provide the sealing action used here.