This article offers a detailed guide to pneumatic solenoid valves
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Pneumatic solenoid valves are electromechanical devices that control the flow of air or process gas and are used for controlling pneumatic actuators such as cylinders, turbines (pneumatic motors), diaphragms, and tubes. Using actuators, they form auxiliary air circuits to control plant equipment. The function of pneumatic solenoid valves is to control fluids by allowing flow or restricting it using a plunger located in the solenoid core.
When a pneumatic solenoid valve is activated, a wire coil, located on the outside of the solenoid, creates a magnetic field in the solenoid core that causes the plunger to move to close or open the flow of a system. There are several typical types od pneumatic solenoid valves, including direct-acting, pilot-operated, and two-way. In some cases where a standard pneumatic solenoid may not work, engineers develop customized ones to fit unique and unusual applications.
Machinery, equipment, and control systems are designed to make work easier and more efficient, which is the primary purpose of pneumatic solenoid valves. Modern industry relies on automation to complete production using a wide array of technological and computer-controlled systems. At the core of many of those systems are pneumatic solenoid valves that can be easily programmed to turn processes on and off without the need for human interference.
Pneumatic solenoid valves give engineers several options when deciding to control the processing of a system. Since they are easily installed and have a simple operation, they can be positioned in remote and inaccessible locations to be activated electronically. This aspect of pneumatic solenoid valves makes them a valuable and irreplaceable asset when designing complex systems for off-site operations.
The flexibility and adaptability of pneumatic solenoid valves make them an integral part of processing equipment and industrial operations. Compressed air systems, vacuum systems, ventilation systems, and air-operated equipment rely on pneumatic solenoids to safely and dependably control their operation.
One of the main benefits of pneumatic solenoid valves is their ability to be remotely controlled by sending low-frequency electrical signals over long distances. The nature of the electrical signals makes them easy to handle using a facility's control system. A control panel or unit distributes signals that manipulate the valve at unattended locations in the process area. Pneumatic solenoid valves are employed in a wide range of industries such as oil and gas, power generation, chemical, and plastics.
The fields of medicine, pharmaceuticals, and food and beverage production must dhere to a strict set of standards established by the Food and Drug Administration for the protection of the public. Any instrumentation or equipment used to produce medicines, drugs, or food must comply with FDA standards. Many of the processes used by these industries require strict control to ensure proper mixing and flow, which makes pneumatic solenoids an essential tool for quality purposes and production efficiency.
Unlike hydraulic solenoid valves, pneumatic solenoid valves operate without the use of fluids, which makes them much cleaner and contaminant free. This characteristic is why they are preferred for use in the production of medical instruments, pharmaceuticals, and foods and beverages. Pneumatic solenoid valves are sealed to prevent them from contaminating products that could seam into their core or inner mechanism. Their tight, impenetrable seal lessens the potentiality of product contamination.
Since pneumatic solenoid valves use pneumatic operation to control a pneumatic system and only require a small amount of electrical input, they are an ideal choice for the operation of robotic arms, automated assemblies, feeder mechanisms, and conveyors. The air pressure operation of pneumatic solenoids makes them a more efficient replacement for small DC motors and hydraulic actuators. Robotic arms, automated actuators, and end effectors depend on pneumatic solenoid valves for actuation. The wide assortment of applications that depend on pneumatic actuators includes assembly, sorting, packaging, material transfer, and safety control systems.
The heart of a pneumatic solenoid valve is the solenoid. A solenoid is an electromagnetic actuator that converts electrical energy into mechanical action. It consists of a coiled wire tightly wrapped around an iron core and a ferromagnetic plug or plunger. 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 their current axis. In the case of a current flowing through a looped coil, the circles combine, forming the magnetic field.
The magnetic field around the coil causes the ferromagnetic plunger to become attracted. The electromagnetic force generated can be increased using two ways. The first is by adding more loops or windings in the coil. This increases the number of magnetic field lines, or flux, emanating from the coil.
The second method is by increasing the amount of current flowing through the coil. This increases the supply voltage into the solenoid. Solenoids valves operate with either DC or AC voltages.
The other main part of a pneumatic solenoid valve is the valve. The valve is the part in contact with air or gas. It is made up of components designed to withstand the pressure of the system. It also resists corrosion and erosion brought by contaminants entrained into the pneumatic system.
Inside the valve is a disc that mates to a seat or opening. Plugging the seat with the disc stops the flow. The other side of the disc is directly attached to the ferromagnetic plunger through the stem. The disc can be lowered or raised depending on the action of the solenoid.
The previous chapter generally describes the two main parts of a pneumatic solenoid valve, the solenoid, and the valve. To further understand the valve's operation, it is useful to note its detailed components. Below are the parts of a pneumatic solenoid valve common to almost every design.
The core, also referred to as the armature or plunger, is the moving part of a solenoid. This is a soft (meaning it can easily be magnetized and demagnetized at low magnetic fields) magnetic metal. When the coil is energized and generates 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 vary depending on the valve operation. Some designs, such as the latching-type solenoid valves, do not use springs to create a return action.
The core tube is where the coil is wound. It also acts as a soft magnetic core that improves the coil's magnetic flux.
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 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 a current is applied.
The diaphragm is a flexible material that isolates the solenoid assembly. 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 to actuate 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-resistant and durable materials such as PTFE or stainless steel.
The seat is the orifice that presses against the disc when closing the valve. The seat and disc are usually made from the same material. Once the seat or disc is damaged, the valve will become passing and unable to stop the flow.
The seal, like the diaphragm, isolates the solenoid assembly and the external environment from the fluid. Several seal materials are 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 valve 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 in the valve body. This is where air flows from the top of the diaphragm and into the downstream side of the valve.
Solenoid valves are classified according to their normal state, type of operation, and circuit function. All these must be specified when selecting and incorporating a new solenoid valve into an existing system.
Like any other automatically actuated valve, solenoid valves are generally classified according to their normal (de-energized) state. This characteristic also signifies its fail-safe position. Springs inside the solenoid valve cause the plunger to revert to its normal position when power is cut off.
Normally open solenoid valves are open in their de-energized state. Activating the solenoid closes the valve. This is useful in applications where air or gas flow must be maintained in the system in the event of power failure.
In contrast to normally open solenoid valves, normally closed means blocked at its unpowered state. Sending power through the solenoid opens the valve. Normally closed solenoid valves are more common than normally open types. Most applications require the system lines to be closed or isolated during system upsets.
Normally open and normally closed solenoid valves are considered monostable valves. Bistable solenoid valves, on the other hand, have a second solenoid instead of a spring return mechanism. They do not have a normal position. When actuated, they remain in the same position even when the power is cut off.
Another classification of solenoid valves is the type of operation. They can activate through two main methods. The first is by direct action, which solely relies on the electromagnetic force generated by the solenoid. Next is through indirect methods, which use pressure supplied by pilot lines. These methods can also be combined to create a valve that activates through both electromagnetic force and line pressure.
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 certain amount of pneumatic pressure, larger flow rates require larger solenoids. The 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 solenoid valves a used for high flow rate and high-pressure applications. In this type of valve, pressure across the valve opens or closes it. 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 air passes through the orifice, and pressure builds up on both sides of the diaphragm. As long as the airflow is blocked, a shut-off force is created from the larger effective area on top of the diaphragm. When the valve is opened, the core opens the orifice, relieving pressure from the top of the diaphragm. The line pressure then opens the valve.
This type of valve applies the same concept as internally piloted valves, but the pressure used to actuate the valve comes from externally supplied air. A separate air circuit is integrated into the valve through an extra port.
Both the internally and externally piloted solenoid valves are called indirect or servo-assisted valves, where the main actuating force comes from the differential pressure between the upstream and downstream lines of the valve.
Semi-direct acting combines the principles of direct and indirect-acting valves. Aside from the magnetic force from the solenoid, the 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, relieving pressure on top of the diaphragm. Closing this orifice by the plunger creates more pressure on top of the diaphragm, closing the valve.
Lastly, solenoid valves are also categorized according to their circuit function. They can act as a simple isolation valve serving a single flow path. Other applications require multiple flow directions. An example is a pneumatic cylinder which requires pressurization and exhaust flow paths.
These types of solenoid valves have one upstream and one downstream port. They are the most basic types used to block or allow airflow. Two-way solenoid valves are either configured as normally open or normally closed.
Three-way solenoid valves have three ports: inlet (pressure port), exhaust, and outlet (actuator port). They have two states. The two states alternately apply and vent 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, air 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 have four ports: an inlet (pressure port), two outlet or actuator ports, and an exhaust port. The two states of this valve allow flow from the pressure port to one of the outlet ports while venting pressure from the other port to the exhaust. There is no normally open or closed position; they mainly act as a directional control valve.
Five-way solenoid valves are similar to four-way valves except for the addition of a second exhaust port. They also act as a directional control valve which allows flow on one line while venting the other. Each line has an independent exhaust port.
Five-way solenoid valves are better than four-way due to the different possible exhaust speeds of the two lines. When used in double-acting cylinders, the cylinder‘s retraction (or extension) speed can be made faster than the extension speed.
These types of solenoid valves are like ordinary 5/2-way valves but with the addition of a center position as its normal state. They have two solenoids and two spring return mechanisms to allow the actuator to return. Different 5/3-way valves are categorized according to the function of their normal state. Typically, the normal state is the valve "rest" state, which holds the position of the actuator.
Aside from determining which type of pneumatic solenoid valve to choose, it is important to specify other valve properties such as material used, pressure, pipe or tubing size, voltage rating, and so on. These are enumerated below.
The valve body material encloses the valve internals and provides structural support. This part must have high strength to withstand the pressure of the process line. Moreover, it is directly exposed to the environment and requires sufficient corrosion and weathering resistance. Common valve body materials are stainless steel, cast iron, bronze, brass, and engineering plastics such as PTFE and PP. Stainless steel and brass are mostly used for outdoor applications. PTFE and PP are used for food grade and ultra-clean applications.
These parts directly contact the process gas. They must have sufficient corrosion and erosion resistance. Air and other process gasses can entrain moisture, corrosive substances, dirt, and other particulate matter, which can gradually degrade the seat and disc. Typical seat and disc materials possess high corrosion resistance, abrasion resistance, and impact strength. Examples of these materials are stainless steel and PTFE.
Selection of seal materials is affected by the type of process gas or entrained chemicals in the air supply and the temperature of the system. They are designed to be flexible and have stable physical and chemical properties while exposed to external factors. Common seal materials are EPDM, NBR, FKM, PEEK, and PTFE.
Line size is the nominal diameter of the solenoid valve ports to be fitted to the supply and actuator piping. Often, the connection type is also specified.
MOPD means the difference in pressure between the inlet and outlet ports in which the solenoid valve is designed to operate. In most cases, the outlet port is connected to the atmosphere. In such cases, the MOPD is the same as the supply pressure of the system.
Indirect, servo-assisted, or piloted solenoid valves require the Min. OPD to be specified. These solenoid valves have internal components that exert force to close their pilot lines. The pilot lines are opened when the Min. OPD is achieved. Airflow through the pilot lines actuates the valve.
This defines the valve‘s flow capacity. It usually describes the correlation between flow and pressure drop across the valve orifice. It is particularly useful in comparing flow efficiencies between two similarly designed solenoid valves.
Working pressure, or safe working pressure, is the internal pressure range of the system‘s operating condition.
This is the maximum pressure solenoid valve can sustain without suffering permanent damage. It is usually a few times larger than the working pressure of the system.
This identifies the voltage of the solenoid‘s control circuit along with the current, frequency, and power specifications. The solenoid must be rated properly for the circuit. Otherwise, the device may not activate or may have a poor response.
Control circuits are usually in DC, but in some instances, AC is employed. Common DC voltages are 8, 12, 24, and 30 volts. The signal frequency for systems that use AC is also specified, which can either be 50 or 60Hz. AC voltages at 60Hz are 24, 120, 240, and 480 volts.
Response time is the period needed by the valve to go from one state (energized or de-energized) to another. It is influenced by the solenoid valve's mechanical and electrical parts. Response times can vary from 5 to 200 milliseconds. This characteristic is important in solenoid valves used in critical equipment that require an almost instantaneous activation.
This is the range of ambient temperatures in which the solenoid valve is designed to operate. Oftentimes, only the maximum ambient temperature is specified. For applications with high amounts of entrained water vapor, 32°F (0°C) is specified as the minimum ambient temperature to prevent freezing issues.
The enclosure protection rating describes the type of environment where the solenoid valve is safe to operate. Various industries expose solenoid valves to conditions such as rain, dust, snow, washdown, and explosive gases. Enclosure protection ratings are specified through NEMA and IP numbers.
A higher number indicates a better protection level. For general purpose indoor use, NEMA 1 to 2 or IP 10 to 11 rated enclosures are used. For outdoor applications, NEMA 3S to NEMA 4X or IP 54 to IP 64 are sufficient. These types protect against dust, rain, and snow. In cases of occasional washdown and immersion, NEMA 6 and IP 68 are used.
Aside from solid and liquid protection, enclosures are also rated according to their compatibility with an explosive environment. ATEX and IECEX markings are used to specify the hazardous applications of solenoid valves and other electronic devices. ATEX ratings are specified according to the type of hazardous area where the device will be used. It is important to specify this accurately since having higher protection ratings greatly increases the cost of the device. Also, having a higher rating does not mean a higher level of protection.
In choosing a solenoid valve, the product certifications are considered along with the design parameters. This is a way to ensure 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, etc. Widely accepted certifications are Underwriter Laboratories (UL Listed or Recognized), CSA, FM Approval, CE, ATEX, and IEC.
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