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How Does a Solenoid Valve Work: Normally Closed & Normally Open
Solenoid valves play a key role in controlling fluid and gas flow, but how do they actually work? If you’ve ever wondered how does a water solenoid valve work or how does an electric solenoid valve work, this article provides a clear explanation. From direct-acting and pilot-operated designs to their applications in HVAC, water systems, and automation, understanding the working principle of solenoid valves is essential for efficient system operation. Read on to explore the different types and their functions.
How Does a Solenoid Valve Operate?
Solenoid valves come in various types, each designed for specific functions and configurations. These valves are essential in pneumatic and hydraulic systems. Let’s take a closer look at how solenoid valves work in these applications.
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Pneumatics
Pneumatic solenoid valves control the flow of compressed air in a system, which is used to actuate cylinders, drive machinery, or regulate vacuum applications.
3/2-Way Valves: These valves have three ports—one for the inlet, one for the outlet, and one for the exhaust. They are commonly used in single-acting pneumatic cylinders and vacuum applications. The valve works by directing compressed air into the cylinder, extending it, and then venting the air to allow the cylinder to return to its original position.
5/2-Way Valves: These valves feature five ports—two inlets, two outlets, and one exhaust. Their primary function is to control double-acting cylinders, where air needs to be alternated between two ports to extend and retract the actuator. The exhaust port allows for smooth operation by venting used air. These valves are widely used in automation, robotics, and packaging machinery, where precise motion control is required.
5/3-Way Valves: Similar to 5/2-way valves, these also have five ports but include three positions, offering greater control over the actuator’s movement. These valves are useful for applications that require intermediate stops or controlled holding positions in motion control systems. The center position can be configured in different ways:
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All ports blocked: Holds the actuator in position.
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All ports open: Allows free movement of the actuator.
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Pressure applied to both outlets: Ensures equalized pressure on both sides, keeping the actuator balanced.
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Hydraulic
Hydraulic solenoid valves regulate the flow of pressurized oil or fluid in hydraulic systems, which operate under high pressure to drive actuators and motors.
4/2-Way Valves: These valves have four ports—two inlets and two outlets—allowing control of the flow direction in hydraulic systems. They are used for double-acting hydraulic cylinders, where shifting the valve redirects fluid to extend or retract the actuator. When the solenoid is energized, it changes the valve’s position, altering the fluid path and controlling movement.
4/3-Way Valves: Like 4/2-way valves, these also have four ports, but they include three positions to provide additional control. The neutral (center) position allows for different configurations:
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Closed-center position: All ports are blocked, stopping fluid movement and maintaining pressure in the system.
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Float-center position: The actuator is left free to move, allowing it to return to its original state.
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Pressure-relieved position: The system pressure is reduced to prevent overloading.
These valves are useful in systems that require intermediate holding positions, precise fluid control, or pressure relief functions.
For a 2/2-way valve, the way it works depends on its internal design. Let’s break down how the three main types—direct-acting, semi-direct-acting, and pilot-operated—function in detail.
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Direct-acting
In a direct-acting 2/2-way valve, the plunger (core) directly blocks or opens the valve orifice. When no power is applied, a spring holds the plunger down, sealing the orifice and stopping flow. When the coil is energized, the magnetic field pulls the plunger upward, overcoming the spring and lifting the seal, allowing fluid to pass through. There's no need for a pressure differential—this valve type operates independently of system pressure.
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Semi-direct-acting
A semi-direct-acting valve combines both direct and pilot principles. It has a diaphragm or piston with a small pilot orifice and a plunger connected to the diaphragm. When energized, the plunger lifts and opens both the pilot hole and assists the diaphragm to rise. This allows the line pressure to help open the valve fully, enabling high flow rates even at zero pressure differential.
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Pilot-operated
Pilot-operated solenoid valves are designed for high-pressure and high-flow systems, but they require a minimum pressure differential to function. When de-energized, the pressure above the diaphragm keeps the valve closed. Energizing the coil opens a small pilot hole, releasing pressure above the diaphragm to the outlet side. This pressure drop allows higher inlet pressure to push the diaphragm up, opening the valve. Once de-energized, pressure equalizes again, and the spring forces the diaphragm closed.
Another key distinction is whether a valve is normally closed or normally open, determining its default state when not powered. Here are the working principles of these solenoid valves:
A normally closed (NC) solenoid valve stays closed when de-energized, preventing any fluid or gas from passing through. When an electrical current is applied to the coil, it generates a magnetic field that pulls the plunger upward, overcoming the spring force. This action lifts the seal and opens the orifice, allowing media to flow through the valve. When power is cut off, the spring pushes the plunger back down, sealing the orifice and stopping the flow.
NC solenoid valves are commonly used in safety systems, fuel control, and industrial automation, where the valve needs to remain closed for extended periods, ensuring energy efficiency and secure operation.
A normally open (NO) solenoid valve remains open when de-energized, allowing uninterrupted flow. When electrical current is supplied, the coil produces a magnetic field that forces the plunger downward, overcoming the spring force. This movement closes the orifice and prevents media from passing through. Once the power is turned off, the spring returns the plunger to its open position, restoring flow.
NO solenoid valves are ideal for applications that require continuous flow, such as cooling systems, ventilation, and pressure relief systems. They enhance energy efficiency and safety by preventing overpressure or overheating in critical processes.
Components of Solenoid Valves
A solenoid valve consists of two main parts: the solenoid (actuator) and the valve body.
The solenoid includes an electromagnetic coil, core (plunger), and core spring, which work together to open or close the valve. The valve body contains key components such as the stem, disc, seat, and seal, which control fluid flow. In indirect-acting valves, a diaphragm and pilot channel help regulate pressure.
Seals and diaphragms are made from materials like PTFE, FKM, NBR, and EPDM, ensuring compatibility with different fluids. The valve body is typically made from brass, stainless steel, or bronze, providing durability and corrosion resistance.
Where to Buy Solenoid Valves? Choose KUHNWAY for Reliable Solutions!
After understanding how solenoid valves work, it's clear why they’re widely used in water distribution, HVAC, industrial automation, and chemical processing. These valves provide efficiency, reliability, and precision, even in the most demanding environments. At KUHNWAY, we offer top-quality solenoid valves to meet your specific needs.
Our MD Series Direct-Acting Diaphragm Solenoid Valves provide fast response times and zero-pressure differential operation, ideal for air, water, and light oil applications. For high-temperature and high-pressure systems, the MT Series Pilot-Operated Piston Solenoid Valves ensure durability and precise control, handling steam and heavy oil at temperatures up to 200°C.
If your system requires large flow control, our BMF Series Large-Diameter Pilot-Operated Solenoid Valves offer manual override and flexible flange connections from 2" to 16", making them ideal for industrial and high-volume applications.
At KUHNWAY, we provide high-performance valve solutions tailored to your needs. Contact us today to find the right solenoid valve for your system!
How Does a Solenoid Valve Work: Normally Closed & Normally Open
How do solenoid valves work? Learn about the workings of solenoid valves, including both pneumatic and hydraulic systems for efficient fluid control.