Differences Between Direct-Acting and Pilot-Operated Solenoid Valves

According to the Solenoid valve page on Wikipedia at the time this post was written, the invention of these devices dates back to 1910 by ASCO, now part of Emerson. Solenoid valves are electromechanical devices:

…in which the solenoid uses an electric current to generate a magnetic field and thereby operate a mechanism which regulates the opening of fluid flow in a valve.

Emerson's John Molloy

In a Valve magazine article, Solenoid Valves: Direct Acting vs. Pilot-Operated, Emerson’s John Molloy describes the differences in these two types of solenoid valves (SOVs). The question he most often hears as a training manager for ASCO solenoid valves is:

What’s the difference between direct-acting and pilot-operated SOVs and how do we make a choice?

Valve magazine: Solenoid Valves: Direct Acting vs. Pilot-OperatedJohn notes that first everyone should remember that the solenoid is the electrical actuator which moves the valve mechanical parts. A direct-acting SOV:

…is the type of SOV with the least amount of moving mechanical parts. Typically, a movable core (or plunger), spring and/or fulcrum lever are found inside the valve body. Once the SOV is energized, the magnetic field created in the solenoid coil attracts the movable core towards the coil’s center.

Pilot-operated solenoids use their coils:

…to lift a movable core, but these valves have a pilot and bleed orifice that enables them to use a fraction of the line pressure from the media source to assist in the lift of either a diaphragm or piston.

For direct-acting SOVs, the:

…main orifice size must remain smaller if a larger pipe size is allowed, the coil would need to be larger to create more power to pull the moving plunger through the greater volume of media.

Pilot-operated SOVs are:

…’borrowing’ from the media at the inlet, will also lift a diaphragm or move the piston enough to allow media to flow through a larger orifice, thus allowing pipe sizes to be greater than the direct acting.

Read the article for more on the differences and advantages/disadvantages of the two SOV technologies. You learn more about SOVs in the Solenoid Valves section on Emerson.com and connect with SOV experts in the Fluid Control & Pneumatics group in the Emerson Exchange 365 community.

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