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Correct choice of valve electric device to prevent overload occurs
Valve actuators are essential components used to control and operate valves, connecting them to the system. These devices are typically powered by electricity and can be controlled based on stroke, torque, or axial force. The performance and suitability of a valve actuator depend on factors such as the type of valve, its operational specifications, and its position in the pipeline or equipment. Therefore, selecting the right electric valve actuator is crucial, especially when considering the risk of overloading—when the working torque exceeds the control torque. Proper selection should take into account several key parameters:
1. **Operating Torque**: This is the most critical factor when choosing an electric actuator. The actuator’s output torque should be 1.2 to 1.5 times the maximum operating torque of the valve.
2. **Operation Thrust**: There are two main structures for valve actuators: those without a thrust plate, which directly output torque, and those with a thrust plate, where the torque is converted into thrust through the stem nut.
3. **Output Shaft Rotation**: The number of revolutions required by the actuator depends on the valve's nominal diameter, stem pitch, and thread count. The formula M = H / (Z × S) can be used, where M is the total revolutions needed, H is the valve opening height, S is the thread pitch, and Z is the number of threads.
4. **Stem Diameter**: For multi-turn open-stem valves, the actuator must allow the stem to pass through its hollow output shaft. The inner diameter of the actuator’s shaft must be larger than the outer diameter of the valve stem. Even for rotary or dark rod valves, the stem size and keyway dimensions must be considered to ensure proper operation after installation.
5. **Output Speed**: Fast opening and closing can cause water hammer effects. Therefore, it's important to choose an appropriate speed based on the application conditions.
6. **Installation and Connection**: Actuators can be mounted vertically, horizontally, or on the floor. Connections vary depending on whether the valve uses a thrust plate, stem-through design, or not. Some actuators are designed for wide applications, enabling automated and remote control of valves.
Special attention must be given to torque and axial force limits. Most actuators use torque-limiting couplings. Once the actuator’s specifications are set, the control torque is also defined. Under normal conditions, the motor won’t overload. However, overloading can occur due to low voltage, incorrect torque settings, frequent jogging, circuit failures, or high ambient temperatures.
Overload protection methods have evolved from traditional approaches like fuses, overcurrent relays, and thermal relays. While these have their pros and cons, no single method is fully reliable for variable-load devices. A combination of strategies is often necessary. Two primary methods include monitoring motor current and detecting temperature rise. Both require consideration of the motor’s thermal capacity and time margins.
One effective solution is using a thermostat embedded in the motor winding, which cuts off power when the rated temperature is reached. This method is reliable because it aligns with the motor’s thermal characteristics. Common protection strategies include using thermostats for continuous or intermittent operation, thermal relays for stall protection, and fuses or overcurrent relays for short circuits.
In conclusion, selecting the correct valve electric device and implementing proper overload protection are vital to ensuring safe and efficient operation. These considerations should not be overlooked in any application involving valve actuators.