Solenoid Valve Basics: Normally Closed, Direct-Acting and Pilot-Operated

Based on Wikipedia's solenoid valve article. The Parker "Solenoid Valves 101" page could not be read (it needs JavaScript).

How it works

Current through a coil creates magnetism that pulls a plunger, opening or closing the fluid passage. A normally closed (N.C.) valve is closed when unpowered; a normally open (N.O.) valve is open when unpowered.

Two main designs

Direct-acting: the solenoid moves the main valve itself. Response is typically 5 to 10 ms. Simple, but needs more power, especially at high pressure or large orifices.
Pilot-operated: the solenoid opens a small pilot passage and line pressure moves the main diaphragm. Response is typically 15 to 150 ms and it uses less energy. It needs enough pressure difference between inlet and outlet and does not work without it. It is also more prone to clogging from solid impurities.

Practical points

Automatic sprinkler systems use solenoid valves driven by a controller. The article's example: a 3/4-inch, 150 psi, 24 VAC residential valve drawing 7.2 VA inrush and 4.6 VA holding.
If outlet pressure exceeds inlet pressure, the valve can open regardless of the solenoid.
Iron-based core parts can corrode, so material choice matters.

Not verified: ratings for any specific product. Check the manufacturer's data sheet.

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General information only. Not plumbing, legal or code advice. Confirm with a licensed professional and your local authority.