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 consists of two primary components: the solenoid and the valve body. The solenoid is one type of actuator, with other types including manual, pneumatic, and hydraulic actuators, each suited to different applications. The specific parts of a solenoid can vary depending on the required action. Conversely, the valve body components are generally consistent across different solenoid valves, though they may vary in design and material. Below are the general components of both the solenoid and the valve body:
The coil is a crucial component of the solenoid, consisting of insulated copper wire wound tightly around a core tube. When an electric current is applied to the coil, it generates a magnetic field, which is essential for the solenoid's operation.
The core, also known as the armature or plunger, is the moving part of the solenoid. Made from soft magnetic metal, the core can be easily magnetized and demagnetized by low magnetic fields. When the coil is energized and generates a magnetic field, the core is attracted, causing it to move and thereby opening or closing the valve.
The core spring is responsible for returning the core to its original position once the magnetic field is removed. The design and configuration of the core spring can vary depending on the valve’s operation. For instance, in latching solenoid valves, a spring may not be used to return the core, as these valves rely on different mechanisms for maintaining position.
The core tube is where the coil is wound and serves as a soft magnetic core. It enhances the magnetic flux generated by the coil, improving the efficiency of the solenoid's operation.
This component is installed at the closed end of the core tube and also enhances the magnetic flux. It is made from a soft magnetic metal, which helps in improving the solenoid's efficiency.
The diaphragm is a flexible component that separates the solenoid assembly from the fluid, effectively containing the fluid's pressure.
The stem is a component of the valve that connects to the core or plunger. When the coil attracts the core, the stem moves in unison, thereby actuating the valve.
The disc prevents fluid flow when the valve is closed. In certain solenoid valve designs, diaphragms, bellows, or pinch mechanisms are used in place of a disc to block the flow. Depending on the application, the disc is typically constructed from materials resistant to corrosion and erosion, such as PTFE or stainless steel.
The seat is the orifice that presses against the disc to seal the valve when it is closed. Depending on the valve design, a seat may not be present. Like the disc, the seat is typically made from materials resistant to corrosion and erosion. If the seat or disc becomes damaged, the valve may leak and fail to stop the flow.
The seal, similar to the diaphragm, isolates the solenoid assembly and the external environment from the fluid. Various materials are available for seals, such as PTFE, FKM, NBR, and EPDM, depending on the application and the nature of the process fluid.
The valve bonnet sits at the top of the valve body. The core tube and stem extend through the bonnet and into the valve assembly.
The body is the central component of the valve that houses the diaphragm, disc, seat, as well as the inlet and outlet ports.
In indirect or semi-direct acting solenoid valves, a bleed orifice is typically installed on the diaphragm, though some designs use an equalizing hole instead. This bleed orifice allows the valve to utilize line pressure for opening or closing the valve.
In indirect acting solenoid valves, a pilot channel is incorporated into the valve body. This channel allows fluid to flow from above the diaphragm to the downstream side of the valve.
Solenoid valves are categorized based on their mode of actuation as well as the number and direction of flow paths. This chapter focuses on the modes of actuation: direct-acting, internally piloted, and externally piloted.
In this type of solenoid valve, static pressure forces increase with the orifice size. A larger orifice requires a stronger solenoid action and, consequently, a stronger magnetic field to handle the increased static pressure. Therefore, for a given fluid pressure, larger flow rates necessitate larger solenoids. As a result, fluid pressure and flow rate are directly proportional to the size of the solenoid. This type of solenoid valve is typically used for applications with small flow rates and operating pressures.
Internally piloted solenoid valves are used for high flow rate and high pressure applications. In these valves, pressure across the valve controls its opening and closing. An orifice or equalizing hole is typically installed for this purpose. In the usual design, the core blocks the flow through the orifice when the valve is closed, causing fluid to pass through the orifice and build up pressure on both sides of the diaphragm. This creates a shut-off force due to the larger effective area on top of the diaphragm. When the valve is opened, the core uncovers the orifice, relieving the pressure on top of the diaphragm. The line pressure then forces the valve open.
This type of valve operates on a similar principle to internally piloted valves, but it uses fluid from an external source to actuate the valve. It incorporates an additional port to integrate a separate fluid circuit for this purpose.
Both internal and external piloted solenoid valves are referred to as indirect or servo-assisted valves, where the primary actuating force is derived from the differential pressure between the upstream and downstream sides of the valve.
Semi-direct acting solenoid valves integrate the principles of both direct and indirect acting valves. In addition to the magnetic force from the solenoid, the pressure differential across the valve also aids in opening or closing the valve. When the plunger is actuated, it lifts the diaphragm to open the valve, simultaneously uncovering an orifice that relieves pressure on top of the diaphragm. When the plunger closes this orifice, pressure builds up on top of the diaphragm, causing the valve to close.
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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