Solenoid valves convert electrical energy to mechanical energy, which causes a magnetic response. The solenoid activates when an electrical current comes through the wire coil. When activated, solenoids in hydraulic valves and pneumatic valves control the flow of fluid when a fluid power system is in motion. There are many different types of solenoid valves available, though direct-acting and pilot-operated valves are the two main categories. When selecting a solenoid valve, it is important answer questions about the application and ensure that you are choosing a suitable valve.
Types of solenoid valves
Direct-acting valves
Solenoids in direct-acting valves open the valve magnetically through direct action, hence the name. The coil pushes and pulls a shaft and causes the seat of the valve to shift. When there’s no electrical current, the spring coil returns to the default position. Direct-acting valves need full power to operate the valve. Whether that position is normally open or normally closed depends on the application.
In an energized normally closed valve, the inlet pressure is released when the solenoid coil retracts, thereby allowing the fluid to pass through the outlet. The solenoid coil is typically larger in a direct acting valve, compared to a pilot operated valve.
Pilot-operated valves
Pilot operated valves use the pressure of the fluid in the line to open the valve. One of the benefits is the ability to use a smaller coil and a lower cost. Although pilot-operated valves uses less electrical power, you must have the available pressure to operate the valve.
Choosing a Solenoid Valve
These are questions that you need to answer to select the types of solenoid valves that will be a right fit for your application.
TYPE – 2-way, 3-way, or 4-way
OPERATION – Is it a Normally Open (N/O), Normally Closed (N/C) or Universal (U) operation?
MEDIA
What is the going through the valve? Air, gas, water, something else?
What is the source of the fluid?
What is the temperature of the fluid?
Is the fluid clean?
Is the fluid being heated, cooled, or treated before it reaches the valve?
SIZE
What is the port size at the inlet and outlet of the valve?
What is the flow capacity?
PRESSURE – The pressure of the line helps the valve open
What is the maximum pressure?
What is the differential pressure? Inlet pressure compared to the outlet pressure. Upstream pressure, pressure of the fluid between its source and the valve, compared to its downstream pressure, after it leaves the valve. The answer
ATMOSPHERE
Where will be valve be installed? When the valve goes into service, you want to make sure it won’t be affected the environment like moisture, heat, freezing temperatures, etc.
What is the range of the ambient temperatures?
What type of NEMA enclosure does the valve require?
VOLTAGE – Ask the customer what the voltage will be for the valve.
EXTRAS – Are there any special options needed by the customer? The sealing material may be one extra specification
Example:
TYPE – 2-way
OPERATION – Normally Closed (N/C)
MEDIA – Water
SIZE – ½” NPT
PRESSURE – 65 PSI, 10 PSI minimum
ATMOSPHERE – 70 F
VOLTAGE – 120 VAC
EXTRAS – None
Type of Valve
Solenoid valves are available as 2-Way, 3-Way, and 4-Way. The application will dictate the types of solenoid valves you chose.
2-Way Valves
A 2-way valve is used in On/Off applications that requires a quick response and higher flow. Two-way valves can replace a lever to automate a process. In venting applications, the valve is actuated to release excess pressure on the fluid.
3-Way Valves
A 3-way valve has three ports. This allows for it to be in 1 of 2 states, or circuits. A 3-way valve can open, close, convey, or distribute, or combine media.
4-Way Valves
In a 4-way valve, all ports are in the off position by default. They create two states, or actions, and have two exhaust ports. You control the speed of each action by attaching a flow control valve to each exhaust port.
Operation: Normally Closed vs. Normally Open
Normally Closed
The valve remains in the “Closed” position when de-energized and is the most common method of operation. Generally, you will find Normally Closed valves being used in On/Off or venting applications here you want the process to stop when there is a loss of power. In the event of power loss, the valve closes and the fluid will not exhaust.
Normally Open
The valve remains in the “Open” position when de-energized. The most common uses of Normally Open valves are safety applications where the process is exhausted when there is loss of power.
Universal Valves
The valve can be either Normally Closed or Normally Open based on the way the valve is piped up. You would typically see this in 3- and 4-way valves where you are able to put pressure on any port in the valve. For example, a 3-way valve would have a supply port, exhaust port and a delivery port. It provides flexibility in the application and allows you to plumb it how you see fit.
Solenoid Valve Pressure
Maximum Pressure vs. Differential Pressure
The differential pressure is the difference between the inlet pressure (fluid as it enters the valve) and the outlet pressure (fluid as it leaves the valve). It is important to determine differential pressure so you know whether to select a pilot operated solenoid valve or a direct acting solenoid valve.
For example, an inlet pressure (P1) of 90 PSI and an outlet pressure (P2) of 80 PSI is a differential pressure of 10 PSI.
In the other example below, the inlet pressure is 90 PSI, and the outlet pressure is 0 PSI because it exhausts to the atmosphere. In this instance, the differential pressure is 90.
A valve with a maximum pressure of 100 PSI will work for the application with a differential pressure of 10. However, the same valve will struggle in the application with a differential pressure of 90. A valve with increased capabilities would be a much better choice.