Pneumatic Solenoid Valve

Chapter 1: What are Pneumatic Solenoid Valves?

Pneumatic solenoid valves are electromechanical devices that control the flow of air or process gas. They are mostly used for controlling pneumatic actuators such as cylinders, turbines (pneumatic motors), diaphragms, and tubes. Pneumatic solenoid valves and actuators form auxiliary air circuits. These devices are used to control plant equipment.

Other pneumatic solenoid valves are used as an integral part of some pieces of equipment or processes. Examples are compressed air systems, vacuum systems, ventilation systems, and air-operated equipment.

Pneumatic solenoid valves are commonly seen in many industrial and manufacturing plants. One of their main benefits is being remote controlled by sending low-power electric signals over large distances. These electric signals are easily handled by the plant‘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.

Pneumatic solenoid valves in the fields of medicine, pharmaceuticals, food, and beverage are preferred because of their clean operation. They are much cleaner than hydraulic solenoid valves which mainly use oil. They can be sealed to prevent any product from being trapped within its internal cavities. This, in turn, lessens the risk of product contamination.

Robotics and automation systems also use pneumatic solenoid valves aside from electric motors and hydraulic actuators. Most robotic arms and end effectors rely on these valves for actuation. Pneumatically actuated types are commonly utilized in assembly, sorting, packaging, and material transfer units.

Chapter 2: Working Principle

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 its 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.

Chapter 3: Parts of a Pneumatic Solenoid Valve

The previous chapter generally describes the two main parts of a pneumatic solenoid valve, the solenoid, and the valve. To further understand its operation, it is useful to note its detailed components. Below are the parts of a pneumatic solenoid valve common to almost every design.

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 vary 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.

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 a current is applied.

Diaphragm:

The diaphragm is a flexible material that isolates the solenoid assembly. 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 durable materials such as PTFE or stainless steel.

Seat:

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.

Seals:

The seal, like the diaphragm, isolates the solenoid assembly and the external environment from the 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 valve 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 in the valve body. This is where air flows from the top of the diaphragm and into the downstream side of the valve.

Chapter 4: Types of Pneumatic Solenoid Valves

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 type of automatically actuated valves, 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:

Normally open solenoid valves are open at 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.

Normally Closed Solenoid Valves:

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.

Bistable Solenoid Valves:

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.

Direct Acting Solenoid Valves:

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:

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 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 due to the larger effective area on top of the diaphragm. When the valve is opened, the core opens the orifice which relieves pressure from the top of the diaphragm. The line pressure then opens the valve.

Externally Piloted Solenoid Valves:

This type of valve applies the same concept from 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 Solenoid Valves:

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 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.

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.

Two-way Solenoid Valves (2/2-way Valve):

These types of solenoid valves have one upstream and one downstream port. They are the most basic types which are used to block or allow airflow. Two-way solenoid valves are either configured as normally open and normally closed.

Three-way Solenoid Valves (3/2-way Valve):

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 (4/2-way Valves):

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 exhaust. There is no normally open or closed position, they mainly act as a directional control valve.

Five-way Solenoid Valves (5/2-way Valves):

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 types 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.

Five-way Solenoid Valves with Intermediate Position (5/3-way Valves):

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 types of 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.

Chapter 5: Other Factors to Consider

Aside from determining which type of pneumatic solenoid valve to choose, it is important to also specify other valve properties such as type of materials used, pressure, pipe or tubing size, voltage rating, and so on. These are enumerated below.

Valve Body Material:

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 thus 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.

Seat and Disc Materials:

These parts directly contact the process gas. They must have sufficient corrosion and erosion resistance. Air and other process gases 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.

Seal Material:

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 to have stable physical and chemical properties while being exposed to external factors. Common seal materials are EPDM, NBR, FKM, PEEK, and PTFE.

Line Size:

Line size is the nominal diameter of the solenoid valve ports to be fitted to the supply and actuator piping. Oftentimes, the connection type is also specified.

Maximum Operating Pressure Differential (MOPD):

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.

Minimum Operating Pressure Differential (Min. OPD):

Indirect, servo-assisted, or piloted solenoid valves require the Min. OPD to be specified. These types of solenoid valves have internal components which 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.

Valve Flow Coefficient (Cv):

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:

Working pressure, or safe working pressure, is the internal pressure range of the system‘s operating condition.

Proof Pressure:

This is the maximum pressure that the solenoid valve can sustain without suffering permanent damage. It is usually a few times larger than the working pressure of the system.

System Voltage:

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. For systems that use AC, the signal frequency is also specified which can either be 50 or 60Hz. AC voltages at 60Hz are 24, 120, 240, and 480 volts.

Response Time:

Response time is the period needed by the valve to go from one state (energized or de-energized) to another. It is influenced by both the mechanical and electrical parts of the solenoid valve. 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.

Operating Temperature Range:

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 that have high amounts of entrained water vapor, 0°C (32°F) is specified as minimum ambient temperature to prevent freezing issues.

Enclosure Protection Rating:

Enclosure protection rating describes the type of environment where the solenoid valve is safe to operate. Various industries expose solenoid valves to various 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.

Other Certifications:

In choosing a solenoid valve, not only are the design parameters considered, but also product certifications. This is a way of having an 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. Widely accepted certifications are Underwriter Laboratories (UL Listed or Recognized), CSA, FM Approval, CE, ATEX, and IEC.

Conclusion

• Pneumatic solenoid valves are electromechanical devices that control the flow of air or process gas. They are mostly used for controlling pneumatic actuators such as cylinders, turbines (pneumatic motors), diaphragms, and tubes.
• 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.
• Solenoid valves are classified according to their normal state, type of operation, and circuit function.
• Aside from determining which type of pneumatic solenoid valve to choose, it is important to also specify other valve properties such as type of materials used, pressure, pipe or tubing size, voltage rating, and so on.