What Are Industrial Valves?
A solenoid valve is an electrically controlled valve used to allow or prevent the flow of media through it. The basic principle of operation is a plunger that moves up and down based on the magnetic field generated from the electrical solenoid. This plunger either opens or closes an orifice that the media flows through. There are different circuit functions, designs, and construction material allowing them to be selected and designed for specific applications. They can be remotely and automatically controlled, making them ideal for a wide variety of industries from water treatment, automotive, and food processing applications.
Type Of Valves
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 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.
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 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
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.
Part Of Valves
A solenoid valve is composed of two major parts: the valve body and the solenoid. The solenoid is one of various actuator types, including: pneumatic, manual, and hydraulic. In contrast, the components of the valve body remain consistent across all valves, albeit with differing designs and materials. Figure 1 shows an illustration of the parts of the solenoid valve:
These parts are described below:
- Valve Body: The body forms the main structure of the valve, housing components such as: the diaphragm, disc, seat, and inlet and outlet ports. It provides a passage for the fluid to flow through and is designed to withstand the specific medium in the circuit.
- Coil Windings: The solenoid coil assembly consists of a series of wire turns wound around a ferromagnetic core, like steel. Inside the coil's hollow space, a plunger or piston is often present, and it is regulated by a spring. When an electric current flows through the coil, it generates a magnetic field that influences the movement of the plunger.
- Plunger/Piston/Core: When the solenoid coil is energized, the plunger generates a magnetic field that causes the plunger to move either upward or downward. The plunger's movement determines the valve's state, regulating the flow of the fluid through the valve. It often features a sealing element, commonly made of rubber, that helps close or open the valve ports.
- Inlet Port: The inlet port is where the fluid initially enters the solenoid valve. It acts as the opening for the fluid before it is directed to be controlled and either blocked or released through the valve's seal or disc. Solenoid valves can have one or more inlet ports depending on their intended function.
- Outlet Port: The fluid, once controlled by the solenoid valve, exits through the outlet port. This outlet is connected to the process or system that requires the controlled flow of fluid.
- Solenoid Coil: The solenoid coil, typically made of steel or iron, serves as an electromagnet assembly. When an electrical current flows through the solenoid, the coil's wire-wound structure creates a magnetic field. It is housed within the body of the solenoid valve, serving as an integral component of the assembly.
- Lead Wires: These external connections enable the passage of electrical current from a power supply to the solenoid valve when it's energized. When the power is turned off, the current flow to the valve ceases.
- Solenoid Spring: A spring is included to provide the necessary tension to hold the plunger in place. Otherwise, when the electrical current ceases, residual magnetism might cause the plunger to remain stuck either in an upward or downward position.
- Orifice: The orifice serves as a crucial element in connecting the inlet and outlet ports. It is managed by the movement of the plunger or piston, which is regulated by the spring and the electric current flowing through the valve. The sensor detects the process and controls the current flow to the valve accordingly, allowing the plunger or piston to open or close the orifice and, in turn, control the fluid flow through the valve.
- Diaphragm: A flexible material, the diaphragm isolates the solenoid from the fluid and is designed to withstand the fluid's pressure.
- Stem: The stem is connected to the valve and serves as the point of attachment for the core or plunger. When the coil attracts the core, it moves the stem, causing the valve to actuate.
- Disc: The disc prevents the fluid flow when the valve is in a closed position. Some valve designs make use of bellows, diaphragms, or pinch devices instead of a disc to obstruct fluid flow. Typically, the disc is made from corrosion-resistant materials, such as stainless steel or PTFE.
- Seat: The seat is the opening that the disc presses against during valve closure. Depending on the valve's design, the seat may or may not be present. It is also made of corrosion-resistant materials. Damage to the seat or disc can render the valve incapable of stopping fluid flow.
- Seal: Similar to the diaphragm, the seal acts as a barrier, separating the solenoid from the fluid. Various seal materials, such as FKM, PTFE, and EPDM, are available, depending on the application and the process fluid.
Valve Application
How do they work?
Solenoid valve is a control units which, when electrically energized or de-energized, either shut off or allow fluid flow. The actuator takes the form of an electromagnet. When energized, a magnetic field builds up which pulls a plunger or pivoted armature against the action of a spring. When de-energized, the plunger or pivoted armature is returned to its original position by the spring action.
A solenoid valve consists of two main components: a solenoid and a valve body (G). A solenoid has an electromagnetically inductive coil (A) around an iron core at the center called the plunger (E). AC coils have a shading ring (C), which prevents vibration and humming.
At rest, the valve can be normally open or normally closed. In the de-energized state, a normally closed valve is closed. When current flows through the solenoid, the coil is energized and creates a magnetic field. This creates a magnetic attraction with the plunger, moving it and overcoming the spring (D) force. The plunger lifts so that the seal (F) opens the orifice and allows the flow of the media through the valve. A normally open solenoid valve works in the opposite way.
Solenoid valves are used in a wide range of applications, with high or low pressures and small or large flow rates. These solenoid valves use different operating principles that are optimal for the application. The three most important ones are explained in this article: direct acting, indirect acting, and semi-direct acting operation.
Consideration When Buying
It is essential to understand your application before selecting a solenoid valve. Some important selection criteria are as follows:
- Housing material: Determine valve housing material based on the chemical properties and temperature of the media, but also the environment the valve is in. Common options are brass, stainless steel, PVC, aluminum, and cast iron. Refer to Select the right housing material for your solenoid valve for more information.
- Connection size: Ensure the solenoid valve's port sizes are the same as what the valve will connect to.
- Connector: A solenoid connector is a device that plugs into a solenoid valve to supply it with electricity, and it helps protect the connections from dirt and water.
- Voltage: Determine which voltage the power supply has and select a solenoid valve with a corresponding solenoid.
- Seal material: Seal material should be selected based on the chemical properties and temperature of the media. NBR, EPDM, FKM (Viton), and PTFE (Teflon) are common options. Refer to select the right seal material for your solenoid valve for quick reference on chemical resistance of seal materials.
- De-energized state: Determine if the application needs a normally open, normally closed, or bi-stable valve function.
- Circuit function: Determine whether the application requires a 2 or 3-way solenoid valve.
- Pressure: The valve must be able to withstand the maximum pressure required for the application. It is equally important to note the minimum pressure as a high-pressure differential can cause the valve to fail.
- Temperature: Ensure that the valve materials can withstand the minimum and maximum temperature requirements of the application. Temperature consideration is also essential to determine valve capacity as it affects the viscosity and flow of the fluid.
- Response time: The response time of a valve is the time needed for a valve to go from the open to the closed position or vice versa. Small direct-acting solenoid valves react much quicker than semi-direct or indirect-acting valves.
- Approvals: Ensure that the valve is appropriately certified depending upon the application.
- Degree of protection: Ensure the valve has the appropriate IP rating for protection from dust, liquid, moisture and contact.
