In this article, you will learn more about the following.
A solenoid valve is a type of electromechanical device designed to regulate the flow of fluids. It comprises an electromagnetic actuator, known as a solenoid, coupled with a valve body. Acting as the valve's actuator, the solenoid-plunger assembly is tasked with opening and closing the valve. This system operates in a binary mode, meaning it is either completely open or entirely closed, without the capability for throttling flow. The valve body incorporates the pressure-retaining components that come into contact with the process fluid.
The solenoid mechanism transforms electrical energy into mechanical motion through a push and pull action. It features a tightly wound wire coil surrounding an iron core, accompanied by a ferromagnetic plug or plunger. Component specifics can vary based on design. When electrical current flows through the coil, it generates a magnetic field visualized as a sequence of concentric circles encircling the current path. As the current traverses the coiled wire, these circles converge, forming a unified magnetic field, as depicted below.
Increasing the number of loops in the coil amplifies the magnetic field lines or flux, enhancing the electromagnetic force of the solenoid. This boost offers more power for actuating the valve.
Another strategy to augment the attraction force is by increasing the current through the coil, achievable by elevating the solenoid's supply voltage. Solenoid valves function with either DC or AC power. Common DC voltages include 6, 12, 24, and 240 volts, whereas typical AC voltages at 60Hz are 24, 120, 240, and 480 volts.
Proportional solenoid valves form a distinct category that permits a smooth and continuous modulation of flow or pressure according to the electrical input. Unlike traditional solenoid valves that switch on and off, proportional solenoid valves serve as control valves. For proportional control, the plunger’s position is adjustable, managed by an external spring force. This spring adjusts by compressing or stretching to match the solenoid’s electromagnetic force. Variations in electrical current modify the force exerted on the spring, adjusting its compression or extension until a balance is achieved. This mechanism facilitates precise flow and pressure control across various applications.
Proportional solenoid valves may encounter frictional challenges that can hinder the balance between electromagnetic and spring forces. Specialized electronic controls are utilized to counteract this. A prevalent method for attaining proportional control in solenoid valves is pulse width modulation (PWM). PWM induces the solenoid to alternate quickly between energized and de-energized states, resulting in plunger oscillations that help it achieve a stable position. By fine-tuning the cycle of solenoid activation, known as the duty cycle, the plunger’s position can be accurately managed.
Diverging from standard on/off solenoid valves, proportional solenoid valves are applied in scenarios requiring automated flow regulation. These include the use of proportional pneumatic actuators, throttle valves, burner controls, and other systems where meticulous control over flow or pressure is vital.
A solenoid valve is a critical device used for precise fluid control in various industrial automation and process applications. It consists of two primary components: the solenoid actuator and the valve body. The solenoid serves as an electromechanical actuator, converting electrical energy into linear motion, while the valve body regulates the flow of gases or liquids within a piping or process system. Other actuator types include manual, pneumatic, and hydraulic actuators, each chosen based on automation requirements and system complexity. The specific assembly of a solenoid valve can be tailored to application needs, including variations in the coil’s voltage rating, response speed, and compatibility with corrosive or hazardous media. The valve body construction may also differ based on pressure requirements, operating temperature, and fluid characteristics. Below are the essential solenoid valve parts, with details relevant for selection, maintenance, and optimized system performance:
The coil is a crucial component of the solenoid, consisting of tightly wound, insulated copper wire around a core tube. When an electric current is applied to the coil, it generates an electromagnetic field, which actuates the core to open or close the valve. Coils are available in various voltage ratings (AC or DC), with options for explosion-proof, waterproof, or high-temperature environments. Maintaining proper coil selection is essential for ensuring energy efficiency and operational longevity in automated control systems.
The core, also known as the plunger or armature, is the moving ferromagnetic component inside the solenoid. Made from soft iron or other magnetic materials, the core is drawn toward the center of the coil’s magnetic field when energized, triggering the mechanical actuation of the solenoid valve to control fluid flow. The rapid and reliable motion of the core underpins solenoid valves’ exceptional response speed in irrigation, HVAC systems, industrial automation, and process control applications.
The core spring is responsible for returning the core to its original position once the electromagnetic field is de-energized. The spring’s material and tension are selected based on actuation force and operation mode (normally open or normally closed). In latching solenoid valves, a spring may not be required, as permanent magnets or other mechanisms hold the core in position, allowing for extremely low power consumption in energy-sensitive applications.
The core tube, sometimes referred to as the guide tube or stem tube, encloses the core and coil assembly, serving as a pathway for the moving core. Built from non-magnetic, corrosion-resistant metals such as stainless steel or brass, the core tube provides structural integrity and enhances the magnetic flux generated by the coil, improving the valve’s efficiency and actuation reliability.
This fixed component is installed at the closed end of the core tube and further amplifies the magnetic flux generated by the coil. Constructed from soft magnetic material, the fixed core works in concert with the moving core, ensuring precise magnetic circuit operation and lowering response times for high-performance solenoid valves in safety or emergency shutdown systems.
The diaphragm is a flexible membrane that separates the solenoid assembly from the process fluid and maintains system integrity by containing fluid pressure. In diaphragm-operated solenoid valves—commonly used for low-pressure, high-flow applications—the diaphragm’s material is carefully selected for chemical compatibility and resistance to wear. Typical materials include PTFE, EPDM, or NBR, depending on system media, ensuring performance in water treatment, food processing, and pharmaceutical industries.
The stem is the connecting link between the core (or plunger) and the valve’s closing element (such as the disc or plug). As the electromagnetic coil actuates the core, the stem transmits this force to move the disc or diaphragm, enabling rapid and tightly controlled opening or closing of the valve for precise flow regulation.
The disc is the main shut-off device in many solenoid valve designs, ensuring zero-leakage sealing when the valve is closed. Alternative shut-off mechanisms can include diaphragms or pinch elements in specialized valve types. Disc material—such as PTFE, stainless steel, or high-performance polymers—is selected based on process fluid characteristics like temperature, corrosiveness, or particulate content. Proper disc selection minimizes downtime by reducing the risk of erosion and provides operational reliability in demanding chemical, steam, or gas applications.
The seat is the precision-machined orifice against which the disc seals when the valve is closed, ensuring a leak-tight shutoff. Like the disc, the seat is crafted from corrosion-resistant and chemically inert materials. Accurate seat alignment and material compatibility are essential to prolong valve life, prevent valve leakage, and maintain consistent process control. Seat or disc damage is a primary cause of valve leakage and system inefficiency.
The seal, sometimes called an O-ring or gasket, isolates the solenoid assembly and external environment from the process fluid. Seal material choice—such as PTFE, FKM (Viton), NBR, or EPDM—depends on media compatibility, temperature range, and pressure requirements. High-quality seals ensure reliable, long-term performance in oil & gas, petrochemical, and water management systems, protecting sensitive electronics from fluid ingress.
The valve bonnet forms the protective upper cover of the valve body, with the core tube and stem extending through it into the valve assembly. Typically fabricated from robust metals for durability, the bonnet allows for maintenance access, facilitates coil replacement, and contributes to the overall mechanical strength and pressure retention of the solenoid valve assembly.
The body is the main mechanical housing of the solenoid valve, enclosing the internal elements like the diaphragm, disc, seat, and supporting the inlet and outlet ports. The valve body material—stainless steel, brass, or engineered plastics—should be selected based on chemical resistance, system pressure, and compliance with industry standards. A robust body design prevents leaks, withstands mechanical shocks, and extends service life in industrial fluid control systems.
In indirect acting and semi-direct acting solenoid valves, a bleed orifice—often integrated into the diaphragm—permits the controlled movement of fluid to balance pressure across the valve. This enables the main valve port to open or close using only low actuation force, making such solenoid valves highly efficient for applications requiring large flow rates with minimal power consumption, such as water supply lines and air handling systems.
Indirect acting solenoid valves utilize a pilot channel machined into the valve body. This pilot passage allows fluid communication from above the diaphragm to the downstream port, controlling main valve operation via a smaller pilot-operated mechanism. Pilot channels are essential for achieving rapid response, fine flow control, and energy efficiency in large industrial solenoid valves and automated fluid handling installations.
When choosing a solenoid valve for any fluid control application, several key technical specifications must be considered. These include the type of actuation (direct, indirect, or semi-direct acting), valve orifice size, flow coefficient (Cv value), port connection types (threaded, flanged, or push-fit), voltage and power requirements, and necessary certifications for hygienic or hazardous environments (such as ATEX or UL listing). Understanding the operational environment—such as pressure range, ambient temperature, and the type of process fluid—is critical for selecting solenoid valves that provide long-lasting, leak-free performance. Proper installation and preventative maintenance further ensure system reliability and safety in HVAC, industrial automation, irrigation, chemical processing, and food and beverage systems.
For specialized guidance, consult with leading solenoid valve manufacturers and fluid control solutions suppliers to assess technical datasheets and select products optimized for your application’s requirements. Modern solenoid valves offer enhanced energy efficiency, quick response times, and excellent compatibility with industrial automation protocols, meeting the stringent requirements of today’s smart manufacturing and process control systems.
Solenoid valves are essential components in fluid control systems and are categorized based on their mode of actuation as well as the number and direction of flow paths. Understanding the different types of solenoid valve operations is crucial for selecting the best valve for a given industrial automation, process control, or fluid handling application. This chapter focuses on the most common operational mechanisms: direct-acting, internally piloted, externally piloted, and semi-direct acting solenoid valves.
Direct-acting solenoid valves, also called direct-operated valves, operate entirely by the electromagnetic force generated by the solenoid coil. In this design, static pressure forces increase as the orifice size grows. A larger orifice requires a more powerful solenoid and a stronger magnetic field to manage higher static pressure and ensure reliable actuation. Therefore, for a given fluid media and system pressure, higher flow rates necessitate correspondingly larger solenoid coils. This relationship means that both the fluid pressure and flow rate are directly proportional to the physical size and power of the solenoid mechanism. Direct-acting valves are ideally suited for low flow rate and low-pressure applications, such as small-scale fluid dosing, laboratory automation, gas control, and point-of-use dispensing systems. These valves can be used with a wide range of fluids, including air, water, light oils, and inert gases.
Internally piloted solenoid valves, also known as pilot-operated solenoid valves, are specifically engineered for high flow rate and high-pressure operation in industrial fluid control systems. In these valves, fluid pressure itself assists in opening and closing the main valve mechanism. An orifice or equalizing hole is built into the diaphragm assembly to facilitate pressure communication. During normal operation, the solenoid core or plunger blocks the flow through this orifice, causing pressure to accumulate on both sides of the diaphragm. This pressure differential creates a shut-off force, ensuring a tight seal. When electrical power activates the solenoid, the core uncovers the orifice, allowing the pressure on the upper side of the diaphragm to relieve. As a result, the line or upstream process pressure lifts the diaphragm and opens the valve, permitting high volumes of liquid or gas to pass through. Internally piloted designs are widely used for industrial process automation, heating and cooling circuits, and water treatment systems, where large flow capacities or higher working pressures are required.
Externally piloted solenoid valves operate using fluid pressure from a separate, external source to actuate the valve mechanism. Unlike internally piloted valves, these models integrate an additional port to connect an independent fluid circuit. This allows for the precise control of large flow rates and higher line pressures, even when the process fluid is not suitable for pilot control. Externally piloted solenoid valves are ideal for applications such as pneumatic systems, safety shutoff mechanisms, and systems where the process medium is viscous, contaminated, or not compatible with internal components. The ability to use an external control medium increases reliability and versatility in specialized fluid automation scenarios.
Both internal and external piloted solenoid valves are known as indirect or servo-assisted valves. In these servo-assisted designs, the main actuating force is achieved through the differential pressure between the inlet (upstream) and outlet (downstream) sides of the valve, enabling efficient operation and energy savings in large-scale fluid management solutions.
Semi-direct acting solenoid valves—sometimes referred to as assisted lift solenoid valves—combine the functional principles of both direct and indirect operation. In this type of control valve, the solenoid coil’s magnetic force is supplemented by the system’s fluid pressure to actuate the diaphragm or piston. When the plunger is energized, it lifts the diaphragm to open the valve, simultaneously uncovering an orifice that relieves built-up pressure on top of the diaphragm, enabling a reliable and responsive opening action. Conversely, when the plunger seals this orifice, pressure accumulates above the diaphragm, securely closing the valve. This makes semi-direct acting valves particularly effective for applications with medium flow rates, fluctuating pressures, or low minimum pressure differentials. Common uses include food and beverage dispensing, irrigation systems, medical equipment, and automatic washing machinery where operational versatility is paramount.
Understanding the distinctions between direct-acting, internally piloted, externally piloted, and semi-direct acting solenoid valves helps engineers, facility operators, and procurement professionals select the optimal valve type for specific process requirements. Factors such as flow capacity, operating pressure, response time, media compatibility, and energy efficiency should all be considered when choosing a solenoid valve for any fluid automation, industrial control, or process management scenario. By selecting the right type of solenoid valve, businesses can improve process efficiency, ensure system safety, and reduce maintenance costs for critical applications.
Solenoid valves are also defined by their flow path or circuit function, which includes opening, closing, distributing, or mixing fluids. Understanding these circuit functions is easier when referring to standardized solenoid valve symbols.
ISO, the International Organization for Standardization, is a global federation of national standardization bodies. Together with the IEC (International Electrotechnical Commission), they developed ISO 1219, a standard for symbols and rules used in fluid power diagrams. This standard provides guidelines for creating symbols for components and circuit diagrams.
In the symbol above, the green boxes denote the number of valve positions, while the arrows indicate the direction of flow. A blue arrow represents pressurized flow, a red arrow signifies exhaust, and the purple T-lines indicate closed ports.
The following are the common types of solenoid valves categorized by their circuit function:
This type of solenoid valve features one upstream and one downstream port and is used to either block or permit fluid flow. It can be configured as normally open or normally closed, with the "normal" state referring to the de-energized position. A normally open valve remains open when de-energized and closes when energized, while a normally closed valve remains closed when de-energized and opens when energized.
Three-way solenoid valves feature three ports: an inlet (pressure port), an exhaust port, and an outlet (actuator port). They are used to alternately apply and exhaust pressure from an actuator or downstream equipment. These valves can be configured as normally open or normally closed, with an additional universal function. In a normally open three-way valve, when de-energized, fluid flows from the inlet port to the outlet port while the exhaust port remains closed. When energized, the inlet port is closed and the outlet port connects to the exhaust port. For normally closed valves, the configuration is reversed. The universal function allows for flexible control of flow direction between the ports.
Four-way solenoid valves have four ports: an inlet (pressure port), two outlet or actuator ports, and an exhaust port, with some designs featuring two exhaust ports. These valves do not operate in a normally open or closed mode. Instead, they direct flow either to the first outlet port or the second, depending on the valve's position.
Note that two-way, three-way, and four-way valves can be either direct or indirect acting.
When selecting the appropriate solenoid valve body and seal materials, several factors must be considered: the process fluid, the application environment, and cost. The process fluid influences the choice of materials for the valve trim or wetted parts, considering properties such as pH (acidic or basic), temperature, and pressure. The environment affects the material and construction of the entire valve body and the solenoid enclosure. This includes dealing with corrosive environments, such as chemical plants or seawater, as well as hazardous environments with flammable gases. Lastly, cost is a crucial factor. While specifying a valve made of stainless steel with high corrosion resistance may offer durability, it may not always be the most cost-effective choice.
Listed below are common materials used for valve bodies and wetted parts:
Stainless steel provides excellent corrosion resistance, strength, and durability, even at high operating temperatures. However, it can be susceptible to corrosion from chloride ions unless a higher grade of stainless steel is employed.
Bronze is an alloy of copper and tin. It is more resistant to cracking compared to cast iron and offers a degree of corrosion resistance.
Brass is an alloy of copper and zinc. It is more corrosion-resistant, durable, and malleable compared to bronze.
Cast iron provides significant strength but is prone to cracking due to its low ductility.
Polytetrafluoroethylene (PTFE) is a thermoplastic fluoropolymer that is chemically inert to most substances. It also has hydrophobic properties, making it ideal for ultra-clean and chemical applications.
Polyamide (PA) provides excellent mechanical properties comparable to those of PVC and is also resistant to salt water.
Polypropylene (PP) is a thermoplastic with mechanical properties comparable to steel and brass, and it offers superior resistance to acids, salts, and alkalis.
Polyphenylene Sulfide (PPS) performs well at high temperatures and exhibits strong resistance to acids and alkalis.
Polyvinyl Chloride (PVC) is suitable for use in seawater but has lower strength and temperature ratings compared to brass and copper.
Polyvinylidene Fluoride (PVDF) is easy to mold and weld. It offers good resistance to acids and alkalis, but it is not suitable for high-temperature applications.
The main advantage of solenoid valves is their compact profile and ease of control. They are widely used across various industries that require remote control.
Solenoid valves are employed to start or stop the flow of air or liquids (such as oil or refrigerant) within a circuit. Common applications include refrigeration, air compression, and lubrication systems. An example of a refrigerant circuit is illustrated below.
Solenoid valves are utilized whenever automatic control of fluid flow is required, such as in robotics or factory automation. These valves are managed by microcontrollers or programmable logic circuits using a predefined set of actions. This precise control of fluid flow enables robots to perform a defined sequence of actions with accuracy and consistency.
For fluids that are unsuitable for solenoid valves, pneumatic, hydraulic, and motor actuators are commonly used. However, because of their ease of control, solenoids are often integrated into pneumatic and hydraulic valve actuators. In industrial plants, control valves are typically actuated by a compressed air system that is managed by solenoid valves.
Solenoid valves are commonly used in medical equipment, including custom valves for dialysis machines, anesthesia machines, and ventilator systems. In pharmaceuticals and food processing, solenoid valves are employed to meet stringent hygienic requirements.
When selecting a solenoid valve, it is important to consider both the design parameters and product certifications. Certifications provide assurance that the product meets the safety standards set by national and international organizations. This is particularly crucial for solenoid valves used in applications that impact consumer health and safety, such as food manufacturing, fire protection, and flammable gas handling. Below are some of the certifying bodies that provide validation for solenoid valves.
Underwriters Laboratories (UL) is an independent organization based in the USA that inspects and certifies products for safety compliance.
The Canadian Standards Association (CSA) is the authoritative body for national standardization in Canada. Only products tested and certified according to relevant standards can bear the CSA mark.
FM Approvals is a global leader in third-party testing and certification, specializing in the evaluation of property loss prevention products and services for quality, technical integrity, and performance. FM Approvals conducts tests based on guidelines set by CSA, UL, IEC, and other certification bodies.
The CE marking is an administrative certification indicating that a product complies with the legal requirements of European Union directives concerning safety, health, and environmental protection.
ATEX (Atmosphères Explosibles) certification confirms that products meet European Union directives outlining minimum safety requirements for protecting health and safety in explosive environments. ATEX-certified products have different specifications based on the hazardous area classification and the method of protection employed.
The International Electrotechnical Commission (IEC) is a global organization that publishes consensus-based standards and assessment systems for electrical and electronic products, systems, and services.
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