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.
Solenoid Magnetic Field
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. Solenoid 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. A 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 the 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.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.
Solenoid Magnetic Field
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. Solenoid 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. A 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 the 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.
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.
Coil
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.
Core
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.
Core Spring
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.
Core Tube
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.
Fixed core
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.
Diaphragm
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.
Stem
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.
Disc
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.
Seat
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.
Seal
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.
Bonnet
The valve bonnet seats at the top of the valve body. The core tube and stem extend through the bonnet and into the valve.
Body
The body is the main part of the valve which holds the diaphragm, disc, seat and the inlet and outlet ports.
Bleed Orifice
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.
Pilot Channel
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.
Types of Solenoid Valve Operations
Solenoid valves are described according to their mode of actuation, and the number and direction of flow paths. This chapter discusses the former, namely direct-acting, internally piloted, and externally piloted.
Direct-acting
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 become 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
Internally Piloted
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.
Externally Piloted
This type of valve applies the same concept as internally piloted valves, but the pressure used to actuate the valve comes from fluid from an external source. A separate fluid circuit is integrated into 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 from the differential pressure between upstream and downstream of the valve.
Semi-direct Acting
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.
4: Different Solenoid Valve Circuit Functions
Solenoid valves are also characterized by their 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.
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.
How a solenoid valve works:
Basic Structure: A solenoid valve consists of two main components: a solenoid and a valve body. The solenoid is an electromechanical coil of wire wound around a core. When an electrical current passes through the coil, it generates a magnetic field. The valve body contains a fluid passage with an inlet and an outlet, and it includes a sealing mechanism that opens and closes to control the flow of the fluid.
Electrical Control: The solenoid valve is controlled by an electrical signal. When an electric current is applied to the coil, it generates a magnetic field that attracts a movable plunger or armature located within the solenoid. This plunger is connected to the valve mechanism in the valve body.
Opening the Valve: When the electrical current flows through the coil, the plunger is pulled towards the core of the solenoid, creating a mechanical force. This force overcomes the spring or other mechanisms holding the valve closed, causing the valve to open. As a result, the fluid can flow through the valve from the inlet to the outlet.
Closing the Valve: When the electrical current is turned off, the magnetic field dissipates, and the spring or other mechanism returns the plunger to its original position. This closes the valve, preventing the flow of fluid.
The speed at whichh a solenoid valve opens and closes, as well as the precision of its control, can be adjusted by varying the current applied to the coil and the design of the valve itself. Solenoid valves can be categorized into two main types based on their operation:
Normally Closed (NC): In this configuration, the valve is closed when no electrical current is applied to the solenoid. Applying current opens the valve.
Normally Open (NO): In this configuration, the valve is open when no electrical current is applied. Applying current closes the valve.
Solenoid valves are valuable for their rapid response time and ability to be controlled remotely through electrical signals, making them essential components in various automated systems that require precise fluid control.
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