Picking the right solenoid valve for operating a shutoff valve
Electro-pneumatic solenoid valves come with many options such as direct-mounted, remote-mounted, 3-way, 4-way, 5-way, ac, dc, normally-open, and normally closed? There are so many choices that the user might end up asking why this matters if the purpose is to just open and close a valve.
What is an electro-pneumatic solenoid valve?
An electro-pneumatic solenoid valve takes an electrical signal (ac or dc Voltage) and opens or closes the air supply going to your pneumatic actuator. Very basic solenoids consist of an electromagnetic coil, a plunger or piston, a piston return spring, and a valve body. When you apply electrical power to the coil, it attracts the piston and causes it to move inside the valve body. This, in turn, opens or closes ports inside the valve body allowing your air supply to operate the pneumatic actuator. When the electric supply is removed, the piston spring returns the piston to its original position and shuts off the air supply to the pneumatic actuator.
To choose the correct solenoid for an actuator, the user needs to know what is really happening inside the solenoid as well as what is happening inside the actuator.
How to select the correct solenoid
Most pneumatic valve actuators fall into one of two categories: Double-acting (fail in place) or spring return (fail-open or fail-closed). Many pneumatic actuators consist of either one or two pistons inside a cylinder that connects to a rod or pinion which operates your valve.
There are also diaphragm actuators that utilize a rubber diaphragm with a spring attached to a rod, which operates the valve. Spring return actuators have springs on the ends of the pistons that return the pistons to a specific position when the air is removed from the actuator. When you add air to one side of the piston(s) or diaphragm, the user must vent air from the opposite side in order for the piston or diaphragm to move and operate the valve. If you do not vent the actuator, it will not completely stroke the valve. You must also vent the pressure side of the piston(s) or diaphragm as the actuator is returning to its normal position.
Modern rack and pinion actuators have a universal NAMUR supply port configurations that allow the use of special direct mount solenoids to eliminate the need for tubing or fittings. Remote mount solenoids can be mounted on or near the actuators and require the air to be piped to the actuator. Remote mount solenoids are useful if the actuator is in a harsh or hot environment that might affect the operation of the solenoid’s electronics.
The terms "3-way," "4-way," and "5-way" refer to the port configurations inside the solenoid valve body. These ports direct your air supply as well as the actuator’s exhaust when the electric coil is energized or de-energized. For example, a 3-way solenoid has one pressure inlet port, one pressure outlet port that goes to operate the pneumatic actuator (coil energized), and it has an exhaust port to vent pressure from the actuator’s pressure side when the coil is de-energized. Three-way solenoids are used primarily on spring return actuators. 3-way solenoids only pressurize and vent one side of the actuator’s pistons. The spring return actuator usually has a built-in vent for the non-pressure side of the piston(s) or diaphragm.
Four-way solenoids have one pressure inlet port, two pressure outlet ports and share an exhaust port for both sides of the actuator piston. A 5-way solenoid is similar to the 4-way except they have one inlet pressure port, two outlet pressure ports, and two independent exhaust ports. 4-way and 5-way solenoids are primarily used on double acting actuators. They allow you to pressurize and vent each side of the actuator’s pistons. Four-way and 5-way solenoids may have either one or two electromagnetic coils. Single coil versions will cause your actuator to be fail closed or fail open on loss of power. Dual coil solenoids will cause your actuator to remain in place if there is a loss of power. If you do not have a 4-way or 5-way solenoid readily available, the user can use two 3-way solenoids to control each side of a double acting actuator independently. This configuration will duplicate the operation of a dual coil solenoid.
Generally, you will need normally-closed solenoids for automating your actuators. Normally-closed or normally-open refers to the position of the solenoid when the coil is de-energized. A normally closed solenoid will block the air supply from the actuator when the coil is de-energized. Normally open solenoids allow an open path for the air supply to reach the actuator when the coil is de-energized.
Solenoid questions to answer: When picking a solenoid valve, be sure to ask these 11 questions:
- What type actuator do I have (fail open or fail closed)?
- What do I want my actuator to do if I lose power to my coil?
- Do I want a direct mount or remote solenoid?
- Do I need a normally closed solenoid?
- Do I need 3-way, 4-way, or 5-way?
- What is my signal voltage?
- What port size do I need in my solenoid body?
- What materials of construction do I need the solenoid made from?
- Does the solenoid need to be explosion proof or just watertight?
- What is the ambient temperature the solenoid will see?
- What is the supply pressure of my air?
Having the answers will make it much easier for the solenoid supplier to provide the correct type for a particular application. Most suppliers have access to sizing tools to calculate the speed in which the solenoid will stroke an actuator, but this will require answers to many more questions.
Ronnie Moore is Cross Company‘s resident valve expert working in inside sales and support. A Knoxville native, Ronnie spent time in the Air Force as crew chief during the first Gulf War. This article originally appeared on Cross Company’s Instrumentation blog. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, firstname.lastname@example.org.