Namur solenoid valve is a solenoid valve designed specifically for superior operation of pneumatic actuators. All spring return actuators that use a 3-way solenoid valve require air to enter and leave the spring chambers. As compressed air is introduced to the actuator to push the pistons apart, air from the spring chambers must be allowed to vent. During the fail or spring stroke, the compressed air between the pistons must exhaust to permit the springs to drive the pistons together. This piston movement creates a vacuum in the spring chambers which is usually filled by ambient air that may contain dirt, abrasives, corrosives and moisture of which are bad for the actuator - reducing its performance and life.
A wrong solution for this problem is the use of a 4 way solenoid valve (sometimes referred to as "air assist"). It does prevent the ambient air from entering the actuator, but it will also pressurise the spring chambers. The actuator now acts like an air to open, air to close unit. In the unlikely event of a spring breakage or if the valve torque increases, due to line media build up within the valve, the actuator would still appear to operate normally until its fail action was needed in an emergency. At this most critical time, with supply air not available, the actuator could not perform as intended and it's function as a safety device would be defeated.
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