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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 effectiveness of a valve actuator depend on the type of valve, its operational specifications, and its location within the pipeline or equipment. Therefore, selecting the right electric valve actuator is critical, especially when considering the risk of overloading—when the operating torque exceeds the control torque.
To ensure proper selection, several key factors must be considered:
1. **Operating Torque**: This is the most important parameter when choosing an electric actuator. The actuator's output torque should be 1.2 to 1.5 times the maximum torque required by the valve.
2. **Operation Thrust**: There are two main types of valve actuators: those without a thrust plate, which directly output torque, and those with a thrust plate that converts torque into thrust through the stem nut.
3. **Output Shaft Rotation**: The number of rotations the motor makes is related to the valve’s nominal diameter, stem pitch, and thread count. The formula M = H / (Z × S) helps determine the total revolutions needed, where M is the required revolutions, H is the valve opening height, S is the stem 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 shaft. The inner diameter of the actuator's output shaft must be larger than the outer diameter of the valve stem. Even for rotary or dark rod valves, the stem diameter and keyway size must be considered to ensure proper assembly and function.
5. **Output Speed**: Fast opening and closing can cause water hammer effects. Therefore, the speed should be selected based on the application to minimize such risks.
6. **Installation and Connection**: Actuators can be mounted vertically, horizontally, or on the floor. Connections vary depending on whether the valve has a thrust plate, stem-through design, or not. Some actuators are designed for versatile use in automated systems like remote or program-controlled valves.
Special attention must be given to torque and axial force limits. Most actuators use torque-limiting couplings. Once the device is specified, the control torque is set. Under normal conditions, the motor won't overload. However, overloads can occur due to low voltage, incorrect torque settings, intermittent jogging, circuit failures, or high ambient temperatures.
Common protective measures include fuses, thermal relays, and thermostats. However, each method has limitations, so a combination approach is often necessary. Overload protection can be based on either monitoring motor current or detecting internal heat. Using both methods ensures better reliability, as neither alone perfectly matches the motor’s thermal characteristics.
Rotork actuators, for example, integrate a thermostat within the motor winding, which cuts off power when the rated temperature is reached. This method is reliable because it reflects the motor’s actual thermal capacity.
In summary, the correct selection of a valve electric device and effective overload prevention are crucial. They require careful consideration of various parameters and protective strategies to ensure safe and efficient operation.