In pipeline engineering, the correct selection of electric valves is one of the guarantee conditions to meet the use requirements. If the electric valve used is not selected properly, it will not only affect the use, but also bring adverse consequences or serious losses, therefore, the correct selection of electric valves in the pipeline engineering design.
The working environment of the electric valve
In addition to paying attention to the pipeline parameters, special attention should be paid to the environmental conditions of its operation, because the electric device in the electric valve is an electromechanical equipment, and its working condition is greatly affected by its working environment. Normally, the working environment of the electric valve is as follows:
1. Indoor installation or outdoor use with protective measures;
2. Outdoor installation in the open air, with wind, sand, rain and dew, sunlight and other erosion;
3. It has a flammable or explosive gas or dust environment;
4. Humid tropical, dry tropical environment;
5. The temperature of the pipeline medium is as high as 480°C or above;
6. The ambient temperature is below -20°C;
7. It is easy to be flooded or immersed in water;
8. Environments with radioactive materials (nuclear power plants and radioactive material test devices);
9. The environment of the ship or dock (with salt spray, mold, and moisture);
10. Occasions with severe vibration;
11. Occasions prone to fire;
For the electric valves in the above-mentioned environments, the structure, materials and protective measures of the electric devices are different. Therefore, the corresponding valve electric device should be selected according to the above-mentioned working environment.
Functional requirements for electric valves
According to the engineering control requirements, for the electric valve, the control function is completed by the electric device. The purpose of using electric valves is to realize non-manual electrical control or computer control for the opening, closing and adjustment linkage of valves. Today’s electric devices are not just used to save manpower. Due to the large differences in the function and quality of products from different manufacturers, the selection of electric devices and the selection of valves are equally important to the project.
Electrical control of electric valves
Due to the continuous improvement of the requirements of industrial automation, on the one hand, the use of electric valves is increasing, and on the other hand, the control requirements of electric valves are getting higher and more complex. Therefore, the design of electric valves in terms of electrical control is also constantly updated. With the progress of science and technology and the popularization and application of computers, new and diverse electrical control methods will continue to appear. For the overall control of the electric valve, attention should be paid to the selection of the control mode of the electric valve. For example, according to the needs of the project, whether to use the centralized control mode, or a single control mode, whether to link with other equipment, program control or the application of computer program control, etc., the control principle is different. The sample of the valve electric device manufacturer only gives the standard electrical control principle, so the use department should make a technical disclosure with the electric device manufacturer and clarify the technical requirements. In addition, when choosing an electric valve, you should consider whether to purchase an additional electric valve controller. Because in general, the controller needs to be purchased separately. In most cases, when using a single control, it is necessary to purchase a controller, because it is more convenient and cheaper to purchase a controller than to design and manufacture it by the user. When the electrical control performance can not meet the engineering design requirements, the manufacturer should be proposed to modify or redesign.
The valve electric device is a device that realizes valve programming, automatic control and remote control*, and its motion process can be controlled by the amount of stroke, torque or axial thrust. Since the operating characteristics and utilization rate of the valve actuator depend on the type of valve, the working specification of the device, and the position of the valve on the pipeline or equipment, the correct selection of the valve actuator is essential to prevent overload (the working torque is higher than the control torque). In general, the basis for the correct selection of valve electric devices is as follows:
Operating torqueThe operating torque is the main parameter for selecting the valve electric device, and the output torque of the electric device should be 1.2~1.5 times of the valve’s operating torque.
There are two main machine structures for operating the thrust valve electric device: one is not equipped with a thrust disc and directly outputs torque; The other is to configure a thrust plate, and the output torque is converted into output thrust through the stem nut in the thrust plate.
The number of rotational turns of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the stem and the number of threads, which should be calculated according to M=H/ZS (M is the total number of rotations that the electric device should meet, H is the opening height of the valve, S is the thread pitch of the valve stem transmission, and Z is the number of threaded heads of the valve stem).
If the large stem diameter allowed by the electric device cannot pass through the stem of the equipped valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow output shaft of the actuator must be larger than the outer diameter of the stem of the open rod valve. For the dark rod valve in the partial rotary valve and the multi-turn valve, although the passing problem of the valve stem diameter is not considered, the diameter of the valve stem and the size of the keyway should also be fully considered when selecting, so that it can work normally after assembly.
If the opening and closing speed of the output speed valve is too fast, it is easy to produce water hammer. Therefore, the appropriate opening and closing speed should be selected according to different use conditions.
Valve actuators have their own special requirements, i.e. they must be able to define torque or axial forces. Usually valve actuators use torque-limiting couplings. When the size of the electric device is determined, its control torque is also determined. Generally run at a predetermined time, the motor will not be overloaded. However, if the following situations occur, it may lead to overload: first, the power supply voltage is low, and the required torque cannot be obtained, so that the motor stops rotating; the second is to mistakenly adjust the torque limiting mechanism to make it greater than the stopping torque, resulting in continuous excessive torque and stopping the motor; the third is intermittent use, and the heat accumulation generated exceeds the allowable temperature rise value of the motor; Fourth, the circuit of the torque limiting mechanism fails for some reason, which makes the torque too large; Fifth, the ambient temperature is too high, which reduces the heat capacity of the motor.
In the past, the method of protecting the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. There is no reliable protection method for variable load equipment such as electric devices. Therefore, various combinations must be adopted, which can be summarized into two kinds: one is to judge the increase or decrease of the input current of the motor; The second is to judge the heating situation of the motor itself. In either way, either way takes into account the given time margin of the motor’s heat capacity.
Generally, the basic protection method of overload is: overload protection for continuous operation or jog operation of the motor, using a thermostat; For the protection of motor stall rotor, thermal relay is adopted; For short-circuit accidents, fuses or overcurrent relays are used.
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Post time: Nov-26-2024
