In this article, you will learn more about the following.
- Solenoid valve principles
- Components of solenoid valves
- Operation of solenoid valves
- Circuit functions of a solenoid valve
- Materials and construction
- Common uses of solenoid valves
- Solenoid valve approvals and certifications
- And much more…
Chapter 1: What are Solenoid Valves?
A solenoid valve is an on/off electromechanically operated valve which consists of an electromagnetic actuator (solenoid) and a valve body. The solenoid-plunger assembly is the valve actuator responsible for opening and closing the valve. This actuator can be arranged in such a way that the plunger action can either open or close only. There is no intermediate, or in-between position, so there is no way for a solenoid to throttle flow. The valve body consists of the pressure containing parts in-contact with the process fluid.
The solenoid converts electrical energy into a mechanical pull/push action. This consists of a coil of wire tightly wrapped around an iron core, and a ferromagnetic plug or plunger. Components vary depending on the design. As an electrical current passes through the coil, a magnetic field is generated. The magnetic field lines can be imagined as a series of circles with the direction of its current axis. In the case of a flowing current along a looped coil, the circles combine forming the magnetic field shown below.
Adding more loops will increase the amount of magnetic field lines or flux. This increases the electromagnetic force of the solenoid, which also means more force for actuating the valve.
Another way to increase the force of attraction is to increase the amount of current flowing through the coil. This is done by increasing the supply voltage into the solenoid. Solenoids valves can operate with either DC or AC. Common DC voltages are 6, 12, 24 and 240 volts; while AC at 60Hz are 24, 120, 240 and 480 volts.
Proportional solenoid valves
Proportional solenoid valves are a special type of solenoid valve that provides a smooth and continuous variation in flow or pressure in response to the electrical input. This type can be classified as a control valve. For a solenoid valve to become a proportional valve, the plunger position must be controlled. It is achieved by balancing the plunger through an external force usually done by a spring. The spring will compress until the external force is equal to the electromagnetic force of the solenoid. If the position of the plunger must be controlled, the current must be changed resulting in an imbalanced force on the spring. The spring will compress or stretch until force balance is established.
One problem with this type is the effects of friction. Friction disrupts the smooth balancing of the electromagnetic and spring forces. To negate this effect, special electronic control is used. Common method used to solenoid valves proportional control characteristics is pulse width modulation or PWM. Applying PWM signal as the control input causes the solenoid to energize and de-energize successively at a very fast rate. This puts the plunger in oscillation which nets into a stable position. In order to change the position of the plunger. The on and off states of the solenoid, also termed duty cycle, are controlled.
Unlike ordinary on/off solenoid valves, proportional solenoid valves are used in applications where automated flow control is required such as proportional pneumatic actuators, throttle valves, burner controls, and so forth.
Chapter 2: Solenoid Valve Components
A solenoid valve, as mentioned earlier, can be separated into two major parts: the solenoid and the valve body. The solenoid is just one type in a variety of actuators such as manual, pneumatic, hydraulic and so forth. The solenoids have varying parts depending on the type of action required. The valve body components, on the other hand, are the same for every valve but with different designs and materials. Listed below are the general solenoid and valve body components.
The coil is one of the main parts of the solenoid which consists of an insulated copper wire wound tightly around a core tube. As described earlier, a magnetic field is generated when current is applied.
The core, also referred to as the armature or plunger, is the moving part of a solenoid. This is a soft magnetic metal; soft, meaning a ferromagnetic metal that can easily be magnetized and demagnetized at low magnetic fields. When the coil is energized generating a magnetic field, the core is attracted which opens or closes the valve.
The core spring returns the core to its original position when the magnetic field is removed. The core spring design and configuration in the solenoid assembly varies depending on the valve operation. In some designs, such as the latching type solenoid valves, it does not use springs to create a return action.
The core tube is where the coil is wound. This also acts as a soft magnetic core which improves the magnetic flux generated by the coil.
This is installed at the closed end of the core tube which also improves the magnetic flux. The material is also a soft magnetic metal.
The diaphragm is a flexible material that isolates the solenoid assembly from the fluid. The diaphragm is designed to contain the pressure of the fluid.
The stem is part of the valve where the core or plunger is attached. As the core is attracted by the coil, the stem moves along with it actuating the valve.
The disc blocks the flow of fluid when the valve is closed. In some solenoid valve designs, diaphragms, bellows or pinch devices are used instead of a disc to block fluid flow. Depending on the application, the disc is usually made of corrosion and erosion resistant materials such as PTFE or stainless steel.
The seat is the orifice that presses against the disc when closing the valve. Like the disc, the seat may not be present depending on the valve design. The seat is also made of corrosion and erosion resistant material. Once the seat or disc is damaged, the valve will become passing and unable to stop flow.
The seal, like the diaphragm, isolates the solenoid assembly and the external environment from the fluid. Depending on the application and the process fluid, there is a variety of seal materials available such as PTFE, FKM, NBR and EPDM.
The valve bonnet seats at the top of the valve body. The core tube and stem extend through the bonnet and into the valve.
The body is the main part of the valve which holds the diaphragm, disc, seat and the inlet and outlet ports.
For indirect or semi-direct acting solenoid valves, a bleed orifice is installed on the diaphragm. Some valve designs use an equalizing hole. The bleed orifice enables the valve to use the line pressure to open or close the valve.
For indirect acting solenoid valves, a pilot channel is included into the valve body. This is where fluid flows from the top of the diaphragm and into the downstream side of the valve.
Chapter 3: Types of Solenoid Valve Operations
Solenoid valves are described according to their mode of actuation, and number and direction of flow paths. This chapter discusses the former, namely direct-acting, internally piloted and externally piloted.
With this type of solenoid valve, the static pressure forces increase as the orifice size increases. The increase in static pressure requires a stronger solenoid action; thus, a stronger magnetic field. This means for a given fluid pressure, larger flow rates require larger solenoids. The fluid pressure and flow rate then becomes directly proportional to the required size of the solenoid. This type of solenoid valve is usually used for applications with small flow rates and operating pressures.
For high flow rate and high pressure applications, internally piloted solenoid valves are used. In this type of valve, pressure across the valve opens or closes the valve. To achieve this, an orifice or an equalizing hole is installed. The usual design involves the core blocking flow on the orifice. When the valve is closed, the fluid passes through the orifice and pressure builds up on both sides of the diaphragm. As long as fluid flow is blocked, a shut-off force is created due to the larger effective area on top of the diaphragm. When the valve is opened, the core opens the orifice and pressure is relieved from the top of the diaphragm. The line pressure then opens the valve.
This type of valve applies the same concept from internally piloted valves, but the pressure used to actuate the valve comes from a fluid from an external source. A separate fluid circuit is integrated to the valve through an extra port.Both the internal and external piloted solenoid valves are called indirect or servo-assisted valves where the main actuating force comes for the differential pressure between upstream and downstream of the valve.
Semi-direct acting combines the principles of direct and indirect acting valves. Aside from the magnetic force from the solenoid, pressure differential across the valve assists in opening or closing the valve. When the plunger is actuated, the diaphragm is lifted to open the valve. At the same time, an orifice is opened causing pressure to be relieved on top of the diaphragm. Closing this orifice by the plunger creates a larger pressure on top of the diaphragm closing the valve.
Chapter 4: Different Solenoid Valve Circuit Functions
Solenoid valves are also characterized by its flow path or circuit function. Solenoid valves can open, close, distribute or mix fluids as expressed by this category. To better understand solenoid circuit functions, one must first take a look at the standardized solenoid valve symbols.
ISO, or the International Organization for Standardization, is a worldwide federation of standardization bodies. In collaboration with the IEC (International Electrotechnical Commission), they created a standard of symbols and rules for devising fluid power symbols for use on components and in circuit diagrams. This standard is ISO 1219.
In the symbol above, the green boxes represent the number of positions of the valve. The arrows, on the other hand, indicates the direction of flow. The blue arrow means pressurized flow, while the red means exhaust. The purple T-lines are the closed ports.
The following are the common types of solenoid valves according to circuit function.
Two-way Solenoid Valves
This type of solenoid valve has one upstream and one downstream port and is used to block or allow fluid flow. The solenoid valve can be configured as normally open and normally closed; normal state means the de-energized state. A normally open valve opens when de-energized and closes when energized. The opposite is true for normally closed valves.
Three-way Solenoid Valves
Three-way solenoid valves have three ports: inlet (pressure port), exhaust and outlet (actuator port). These are used to alternately apply and exhaust pressure from an actuator or downstream equipment. Three-way solenoid valves can also be configured as normally open and normally closed, with the addition of a universal function. For a normally open three-way valve, when the valve is de-energized, fluid flows from the inlet port to the outlet port, while the exhaust port is closed. When energized, the inlet port is closed, and the outlet port connects to the exhaust port. The opposite is true for normally closed valves. The universal function, on the other hand, is used for selecting the direction of flow from one port to another.
Four-way Solenoid Valves
Four-way solenoid valves have four ports: inlet (pressure port), two outlet or actuator ports, and an exhaust port. Some designs have two exhaust ports. In this type of configuration, there is no normally open or close. Flow either goes to the first outlet port or to the second.
Note that these types of valves, either two-, three- or four-way valves, may either be direct or indirect acting.
Chapter 5: Materials and Construction
Several factors are needed to be considered in selecting the right solenoid valve body and seal materials. These are the process fluid, the environment of application, and cost. The process fluid determines the material of the valve trim or the wetted parts of the valve. Fluid properties of concern are pH (basic or acidic), temperature and pressure. The environment, on the other hand, will determine the material and construction of the whole valve body, and the enclosure of the solenoid. Like the process fluid, the environment may be corrosive such as in chemical plants and sea water environments. Also, the environment may be hazardous where flammable gases are present. Last will be the cost. Anyone can specify a valve to be stainless steel with the highest resistance to corrosion and durability, but it will not become cost efficient in most circumstances.
Listed below are the common valve body and wetted part materials.
Stainless steel offers high corrosion resistance, strength and durability even on high operating temperatures. However, stainless steel can be corroded by chloride ions, unless a higher grade of stainless steel is used.
Bronze is an alloy of copper and tin. Bronze is more resistant to cracking than cast iron and possesses some corrosion resistance.
Brass is an alloy of copper and zinc. Brass is more corrosion resistant, durable and malleable than bronze.
Cast Iron offers great strength but is prone to cracking due to low ductility.
Polytetrafluoroethylene (PTFE) is a thermoplastic fluoropolymer which is chemically inert to most substances. This material also offers hydrophobic properties making it suitable for ultra clean and chemical applications.
Polyamide (PA) offers good mechanical properties comparable to PVC and resistant to salt water.
Polypropylene (PP) is a thermoplastic that has comparable mechanical properties to steel and brass with better resistance to acids, salts and alkalis.
Polyphenylene Sulphide (PPS)
Polyphenylene Sulphide (PPS) offers good performance at high temperatures with strong resistance to acids and alkalis
Polyvinyl Chloride (PVC)
Polyvinyl Chloride (PVC) suitable for use in seawater but has lower strength and low temperature ratings than brass and copper.
Polyvinylidene Fluoride (PVDF)
Polyvinylidene Fluoride (PVDF) is easy to mould and weld. This material also offers good resistance to acids and alkalis but is not suitable for high temperature applications.
Chapter 6: Common Applications
The main advantage of using solenoid valves is its compact profile and ease of control. Almost all industries that require remote control use solenoid valves.
Pneumatic and Hydraulic Circuits
Solenoid valves are used to stop or allow supply of air or liquid (usually oil or refrigerant) along a circuit. Common applications under this category are refrigeration, air compression and lubrication systems. An example of a refrigerant circuit can be seen below.
Solenoid valves are used whenever fluid flow must be controlled automatically, such as in robotics or factory automation. These solenoid valves are controlled by microcontrollers or programmable logic circuits with a coded set of actions. This control of fluid flow allows a defined sequence of actions to be executed precisely and accurately by a robot.
Industrial and Manufacturing Plant Valve Actuators
For fluids that cannot be handled by solenoid valves, pneumatic, hydraulic and motor actuators are commonly used. However, due to the ease of control using solenoids, solenoid valves are integrated into pneumatic and hydraulic valves actuators. Control valves in industrial plants are usually actuated by a compressed air system controlled by solenoid valves.
Medical, Pharmaceutical and Food Manufacturing
Common uses of solenoid valves in medical equipment are custom valves for dialysis machines, anesthesia machines and ventilator systems. Pharmaceuticals and food processing use solenoid valves complying with hygienic requirements.
Chapter 7: Approvals and Certifications
In choosing a solenoid valve, not only the design parameters must be considered, but also the product certifications. This is a way of having assurance that the product conforms with the safety standards mandated by national and international organizations. This is especially significant for solenoid valves used in applications that directly affect consumer health and safety such as food manufacturing, fire protection, flammable gas handling, and so forth. Listed below are some of the certifying bodies that cover solenoid valves.
UL (Listed or Recognized)
Underwriter Laboratories is an independent organization with headquarters in the USA that inspects and certifies products with respect to their safety.
- UL (Listed) – certified complete devices ready to be purchased and installed
- UL (Recognized) – certified specific component used to create a device
The Canadian Standard Organization is the authoritative body for national standardization in Canada. Only products that are tested and certified according to the relevant standard can obtain the CSA mark.
FM Approvals is an international leader in third-party testing and certification that tests and verifies property loss prevention products and services for quality, technical integrity and performance. FM tests products according to the guidelines set by the CSA, UL, IEC, and other certification bodies.
The CE marking is an administrative marking that proves products complying with the legal requirements of the European Union directives in terms of safety, health and environmental protection requirements.
ATEX (Atmospheres Explosible) certification verifies the products’ compliance to directives set by the European Union describing the minimum safety requirements for improving health and safety protection at explosive environments. ATEX certified products have varying specifications depending on the hazardous area classification and method of protection used.
The International Electrotechnical Commission is a global organization that publishes consensus-based standards and assessment systems for electric and electronic products, systems and services.
- A solenoid valve is an electromechanically operated valve which consists of an electromagnetic actuator (solenoid) and a valve body. Ordinary solenoid valves are two-position valves. There is no intermediate, or in-between position, so there is no way for the solenoid to throttle flow.
- Proportional solenoid valves are a special type of solenoid valve that can throttle flow or pressure in response to the electrical input. For a solenoid valve to become a proportional valve, the plunger position is controlled by balancing it using an external force usually done by a spring.
- A solenoid valve has two main parts: the solenoid which is responsible for the opening and closing of the valve; and the valve body which contains the fluid and its pressure.
- Solenoid valves can be categorized according to its operation and flow paths or circuit function. Solenoid valve operation can be direct-acting, internally piloted, externally piloted or semi-direct.
- Circuit functions of solenoid valves are two-way which is the normal open and close valves; three-way which features an exhaust port for relieving pressure; and four-way which is used in flow diversion.
- The valve body can be constructed by a variety of materials. Factors such as the fluid properties, environment characteristics and cost affect the material specifications of the valve.
- Specifying the required approvals and certifications is one of the ways to ensure purchasing a satisfactory solenoid valve. Compliance to standards set by international organizations guarantees product safety and quality.