How Do Solenoid and Air Actuated Valves Differ?
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In industrial automation control systems, valves are key actuators, used to adjust, open or close the path of media flow. Among the many types of valves, solenoid valves (Solenoid Valve) and pneumatic valves (Air Actuated Valve) are widely used because of their convenient control. Although they all have automatic control functions, their working principles, applicable environments, control methods and maintenance requirements are completely different. This article will focus on the specific structure and operating principles of pneumatic valves, and reveal the advantages of the former through in-depth comparison with solenoid valves.
Pneumatic valves are more suitable for high-intensity operations
Due to its simple structure, small size and low cost, solenoid valves are widely used in automation control systems for small diameter, low pressure and low frequency opening and closing occasions. For example, laboratory piping, household water supply, internal cooling system of electrical cabinets, etc. This kind of environment has low requirements on driving strength, pressure resistance and working frequency. The solenoid valve can rely on the principle of rapid attraction of its electromagnetic coil to complete the opening and closing action with a millisecond-level response.
However, when the valve needs to be switched frequently for a long time or withstand high pressure and large flow, the driving method of the solenoid valve shows limitations. First of all, its driving process relies on the magnetic field attraction continuously generated by the electromagnetic coil to push the valve core to move. However, the output strength of the electromagnetic attraction is physically limited by the coil structure, current intensity and continuous energization time. Especially in the working rhythm of frequent on-off, the continuous heating of the coil will cause the accumulation of temperature rise, causing problems such as magnetic saturation of the iron core, insulation aging, electromagnetic response lag, etc., eventually leading to weakened suction, slow response, and even coil burnout.
In addition, solenoid valves require large driving force when opening and closing in high-pressure, large-diameter pipelines. If the opening and closing resistance exceeds the suction capacity of the electromagnetic coil, the action may become stuck or cannot be reset.
In comparison, pneumatic valves use compressed air as the driving source and use a piston or diaphragm as the actuating component. Its output force is proportional to the air source pressure, which can provide a driving force far exceeding that of a solenoid valve without increasing the volume of the actuator. This driving method can not only easily cope with high-pressure working conditions, but also achieve fast and stable action conversion, which is suitable for high-frequency opening and closing operations. Since the compressed air has good buffering properties, the actuator is not prone to fatigue or mechanical impact during repeated movements, significantly improving the stability and service life of the system.
Pneumatic valves have greater fault response capabilities
In industrial processes involving flammable, highly toxic, high-pressure or high-temperature media, valves are not only the control components for switching on and off the media, but also a key link in the safety protection chain of the entire system. Once control failure, power supply interruption, equipment abnormality or on-site accident occurs, the valve must respond automatically in a very short time and quickly enter the set "safe position" to cut off dangerous sources, prevent leakage, and protect the safety of personnel and equipment.
The driving mechanism of the solenoid valve determines that it cannot maintain any action when powered off. Since the solenoid valve generates magnetic force after the coil is energized instantaneously to pull the iron core to complete the opening and closing action, once the power supply is interrupted or the voltage is unstable, the magnetic force disappears instantly and the valve core cannot maintain the set position. This structural defect means that the solenoid valve cannot ensure reliable operation in scenarios such as emergency shutdown, fire, explosion or lightning strike.
In contrast, pneumatic actuators can be selected as "single-acting" types, that is, only one side is supplied with compressed air for opening or closing, and a strong spring provides reverse action on the other side. In this structure, once the air source is interrupted, the spring immediately drives the piston to move in the reverse direction, quickly returning the valve to the "preset safety position" (normally open or normally closed) without any electrical intervention. This safe reset action occurs quickly, making pneumatic valves ideal for applications such as gas shutoff, steam isolation or combustion control where extremely short response times are required.
Because solenoid valves essentially rely on electromagnetic attraction to achieve action, they can usually only switch between two fixed positions (i.e. fully open or fully closed), which is called "two-position control". It does not have the ability to stay stably in the middle position and cannot continuously adjust the medium flow rate. Therefore, solenoid valves are often suitable for systems that require fast response but simple control logic, such as cleaning, spraying, reagent switching, etc.
Pneumatic valves can be used in combination with "positioners". The positioner is an intermediate control device that can receive continuous analog signals (such as 420mA current signal or 010V voltage signal) from the upper system (such as PLC or DCS), convert the signal into the corresponding compressed air pressure, and then control the air volume entering the pneumatic actuator cavity. In this way, the distance or angle of the piston movement can be finely controlled, thereby driving the valve stem to accurately adjust the valve opening, achieving full range continuous adjustment from 0% to 100%.
Although the initial purchase cost of pneumatic valves is slightly higher than that of solenoid valves, the long-term operating costs are lower. First of all, from the perspective of structural design, pneumatic valves are composed of mechanical components such as cylinders, pistons, sealing rings, and valve stems. The core driving force comes from external compressed air rather than magnetic force excited by current. Therefore, the pneumatic valve does not have an electromagnetic coil that continuously generates heat during operation, and does not require complex electronic control circuits. This "low electrical dependence" significantly reduces common electrical fault hazards, such as coil burnout, insulation aging, electromagnetic interference and other issues. The core of the solenoid valve is the coil, which will inevitably produce thermal effects during long-term energization. Especially during high-frequency or long-term switching operations, excessive coil temperature will lead to a decrease in magnetism, insufficient suction, and then cause slow action or failure.
Secondly, in terms of maintenance difficulty, the operation and maintenance of pneumatic valves is easier to implement. Its maintenance focuses on three aspects: First, the quality of the air source, regularly check whether it contains water, oil, and particle blockage; second, the lubrication and wear conditions of sealing rings and sliding parts; third, the accuracy of action feedback signals. Most of these inspections can be completed through routine gas line inspections and operating status observations. Maintenance costs are controllable and the threshold for operator training is low. Solenoid valves require professionals to use instruments to test coil resistance, monitor temperature rise curves, and confirm whether there are hidden dangers such as local short circuits or demagnetization. These often require electrician intervention and shutdown, which increases maintenance interference and time costs.
