How Do Fail-Safe Features Work in Air Actuated Valves?
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In the field of modern industrial automation, fluid control is a crucial link. Air actuated valves have been widely used in many industries such as petrochemical, electric power, water treatment, food and medicine, etc. due to their simple structure, fast response, and high safety. However, in industrial environments, unexpected interruptions to control systems or energy supplies often occur. In order to ensure that the production process can be in a preset safe state under such unexpected circumstances and avoid equipment damage, environmental pollution and even casualties, air actuated valves are usually designed with a "Fail-Safe" function. This article will delve into the basic concepts and working principles of the fail-safe function of air actuated valves and its importance in ensuring industrial safety.
What is fail-safe?
The so-called "fail-safe" means that when the system encounters the failure of key components (such as control signals and power sources), it can automatically switch to a preset safe state that can minimize risks without external intervention. For air actuated valves, this "safe state" usually refers to the valve being fully open (Fail-Open, FO) or fully closed (Fail-Close, FC). Which state is selected depends on the safety requirements of the valve in the specific process.
For example, the fail-safe state of a valve used to emergency cut off fuel supply should be "Fail-Close" to prevent fuel from continuing to flow in the event of loss of control; while the fail-safe state of a valve used for cooling water supply may need to be set to "Fail-Open" to ensure that critical equipment can continue to be cooled when control fails to avoid overheating damage. The purpose of fail-safe design is to rely on the internal mechanism to force the valve to reach this preset safe position when normal control capabilities are lost.
Fail-safe mechanism
The most common way for air actuated valves to achieve fail-safe functions is to use single-acting air actuated valves with spring return functions (Single-Acting Spring-Return Actuator). This actuator contains an air chamber and a return spring. Its working principle can be divided into two stages: During normal operation, the control system passes compressed air into the air chamber of the actuator, and the gas pressure acts on the piston or diaphragm. The generated thrust overcomes the preload force of the spring and the friction and medium force of the valve operation, driving the valve stem to move, so that the valve reaches the position required for work (for example, if it is a "fail-closed" type, ventilation causes the valve to open during normal operation).
During this process, the spring is compressed or stretched, storing potential energy. Once a failure occurs, such as an interruption in the compressed air supply (such as a rupture of the air source pipeline, shutdown of the air compressor) or a loss of power in the solenoid valve controlling the air circuit, causing the air circuit to be cut off, the pressure entering the actuator air chamber will disappear rapidly. At this point, the stored potential energy of the previously compressed or stretched spring is released, pushing the piston or diaphragm in reverse motion, driving the valve stem to drive the valve to a preset fail-safe position (for example, a "fail-close" valve will automatically close). The entire process relies entirely on the mechanical force of the spring and does not rely on external energy or signals, thus ensuring operational reliability in the event of a failure.
Fail-safe trigger
In modern industrial automation systems, the "fail-safe" function of air actuated valves is usually triggered by two main reasons: interruption of the actual supply of compressed air, and failure or interruption of the control signal.
First, from an air source perspective, loss of compressed air supply is the most common trigger. During operation, if the air compressor stops working, the gas tank pressure drops to the set lower limit, there is an obvious leak in the main pipeline, or the gas supply pipeline ruptures due to corrosion, vibration, etc., the actuator will not be able to obtain sufficient air pressure to push the piston or diaphragm to move. In addition, there are several key auxiliary components in the control air circuit, such as air filters, pressure regulators, lubricators, etc. If these components cause a drop in downstream pressure due to contamination, blockage or aging failure, it will also cause insufficient driving air pressure actually received by the actuator. When the pressure drops to a critical point that cannot overcome the friction of the valve body or the preload of the spring, the actuator will automatically release the original air pressure, and the spring mechanism will intervene to initiate a fail-safe action.
Secondly, the loss of control signals from the electronic control part is also a trigger path that cannot be ignored. In most pneumatic control systems, the opening and closing of the actuator air source is controlled by a solenoid valve. When the solenoid valve works, it relies on current to drive the coil to generate a magnetic field, so that the valve core is in an open or reversing state to control whether compressed air enters the working chamber of the actuator. Once the solenoid valve loses power due to power outage in the control system, abnormal relay operation, poor wire contact or module failure, its internal return spring will force the valve core to return to the default position. At this time, the solenoid valve will not only close the air inlet channel, but may also open the exhaust channel, causing the internal pressure of the actuator to drop rapidly. In this state, the working chamber in the actuator loses pressure, and the originally compressed spring inside it releases the stored energy, pushes the piston back, and drives the valve to the pre-designed "fail-safe position".
