Selection principle of electric actuator for emergency shut-off valve

Selection principle of electric actuator for emergency shut-off valve!

The basis for correct selection of electric actuator of emergency shut-off valve is as follows:

1. Operating torque: the operating torque is the main parameter for selecting the electric device of the emergency cut-off valve, and the output torque of the electric actuator shall be 1.2~1.5 times of the operating torque of the valve.

2. There are two main engine structures for operating the thrust valve electric device: one is to output torque directly without a thrust disc; The other is to configure a thrust disc, and the output torque is converted into output thrust through the stem nut in the thrust disc.

3. Number of rotation turns of output shaft: the number of rotation turns of the output shaft of the electric device of the emergency cut-off valve is related to the nominal diameter of the valve, the valve stem pitch, and the number of thread heads, which should be calculated as M ＝ H ／ ZS (M is the total number of rotation turns that the electric device should meet, H is the valve opening height, S is the valve stem drive thread pitch, and Z is the number of valve stem thread heads).

4. For multi turn rising stem valves, if the electric device is allowed to pass through the valve stem diameter but cannot pass through the valve stem of the valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric device must be greater than the outer diameter of the stem of the rising stem valve. For some rotary valves and non rising stem valves in multi rotary valves, although the passage of the valve stem diameter is not considered, the valve stem diameter and the size of the keyway should also be fully considered when selecting, so that they can work normally after assembly.

5. Output speed: if the opening and closing speed of the shut-off valve is too fast, water hammer is likely to occur. Therefore, proper opening and closing speed shall be selected according to different service conditions.

6. The electric actuator of emergency shut-off valve has special requirements, that is, it must be able to limit torque or axial force. Usually, the valve electric device adopts the coupling that limits the torque. When the specification of the electric device is determined, its control torque is also determined. Generally, the motor will not overload when it operates within a predetermined time. For example, Shanghai Yonglong The electric emergency cut-off valve and liquid ammonia emergency cut-off valve produced by the valve factory can provide overload protection. When the valve is jammed and foreign matters are found, the motor power supply will be automatically disconnected, which will more effectively protect the electric cut-off valve and electric actuator from damage.

Electric emergency shut-off valve - Shanghai Yonglong Valve Factory

However, overload may be caused if:

1. The power supply voltage is low, and the required torque cannot be obtained, so that the motor stops rotating;

2. The torque limiting mechanism is incorrectly adjusted to make it greater than the stop torque, resulting in continuous excessive torque and stopping the motor;

3. Intermittent use, the heat accumulation generated exceeds the allowable temperature rise of the motor;

4. For some reason, the circuit of the torque limiting mechanism breaks down, causing excessive torque;

5. The use environment temperature is too high, which will relatively reduce the thermal capacity of the motor.

Liquid ammonia emergency shut-off valve - Shanghai Yonglong Valve Factory

In the past, fuses, overcurrent relays, thermal relays, thermostats, etc. were used to protect motors, but each of these methods has its advantages and disadvantages. There is no absolutely reliable protection method for electric devices, which are load changing equipment. Therefore, various combinations must be adopted, which can be summarized into two types: one is to judge the increase or decrease of motor input current; The second is to judge the heating condition of the motor itself. The time allowance given by the motor thermal capacity shall be taken into account for both modes.

Generally, the basic protection method for overload is:

1. Thermostat is used for overload protection of continuous operation or inching operation of motor;

2. Thermal relay is used for motor stall protection;

3. In case of short circuit accident, fuse or overcurrent relay shall be used

The electric actuator of the valve is an indispensable equipment for realizing valve program control, automatic control and remote control. Its movement process can be controlled by the stroke, torque or axial thrust. The correct selection of the electric actuator of the emergency cut-off valve is crucial to prevent overload (the working torque is higher than the control torque).

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