In pipeline engineering, the correct selection of electric valves is one of the guarantee conditions for meeting usage requirements. If the selection of electric valves used is improper, it will not only affect the use, but also bring adverse consequences or serious losses. Therefore, electric valves should be selected correctly in pipeline engineering design.
Working environment of electric valves
In addition to paying attention to pipeline parameters, special attention should also be paid to the working environmental conditions of electric valves, because the electric device in electric valves is an electromechanical equipment, and its working condition is greatly affected by its working environment. Under normal circumstances, there are several working environments in which electric valves are located:
1.Indoor installation or outdoor use with protective measures;
2.Outdoor outdoor installation, subject to erosion from wind, sand, rain, dew, sunlight, etc;
3.Having environments with flammable, explosive gases or dust;
4.Environment in humid and dry tropical regions;
5.The temperature of the pipeline medium is above 480 ℃;
6.The ambient temperature is below -20 ℃;
7.Easy to be submerged or submerged in water;
8.Environment containing radioactive materials (nuclear power plants and radioactive material testing devices);
9.The environment on ships or docks (with salt spray, mold, and humidity);
10.In situations with severe vibrations;
11.Places prone to fire hazards;
For electric valves in the above environment, their electric device structure, materials, and protective measures are all different. Therefore, the corresponding valve electric device should be selected based on the above working environment.
Functional requirements for electric valves
According to engineering control requirements, the control function of electric valves is completed by electric devices. The purpose of using electric valves is to achieve non manual electrical or computer control of valve opening, closing, and adjustment linkage. The current use of electric devices is no longer just about saving manpower. Due to the significant differences in functionality and quality among products from different manufacturers, choosing an electric device and selecting the corresponding valve are equally important for the project.
Electrical Control of Electric Valves
Due to the increasing demand for industrial automation, on the one hand, the use of electric valves is increasing, and on the other hand, the control requirements for electric valves are also becoming higher and more complex. So the design of electric valves in terms of electrical control is constantly being updated. With the advancement of science and technology and the widespread application of computers, new and diverse electrical control methods will continue to emerge. When considering the overall control of electric valves, attention should be paid to selecting the control mode of electric valves. For example, according to engineering needs, the control principles vary depending on whether to use centralized control or single control, whether to link with other devices, whether to use program control or apply computer program control, and so on. The sample provided by the valve electric device manufacturer is only the standard electrical control principle, so the using department should conduct technical disclosure with the electric device manufacturer to clarify the technical requirements. In addition, when choosing electric valves, consideration should be given to whether to purchase additional electric valve controllers. Because in general, controllers need to be purchased separately. In most cases, when using a single controller, it is necessary to purchase a controller because purchasing a controller is more convenient and cheaper than designing and manufacturing it by the user themselves. When the electrical control performance cannot meet the engineering design requirements, modifications or redesigns should be proposed to the manufacturer.
Valve electric device is an indispensable equipment for achieving valve program control, self-control, and remote control. Its movement process can be controlled by the magnitude of stroke, torque, or axial thrust. Due to the fact that the working characteristics and utilization rate of valve electric devices depend on the type of valve, the operating specifications of the device, and the position of the valve on the pipeline or equipment, it is crucial to correctly select valve electric devices to prevent overload phenomena (working torque higher than control torque). Generally, the basis for correctly selecting valve electric devices is as follows:
Operating torque is the most important parameter for selecting valve electric devices, and the output torque of the electric device should be 1.2-1.5 times the maximum operating torque of the valve.
There are two main structures for operating the thrust valve electric device: one is to directly output torque without a thrust disc; Another way is to configure a thrust disc, and the output torque is converted into output thrust through the valve stem nut in the thrust disc.
The number of rotations of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the valve stem, and the number of thread heads. It 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 pitch of the valve stem transmission thread, and Z is the number of thread heads of the valve stem).
For multi turn open stem valves, if the maximum valve stem diameter allowed by the electric device cannot pass through the valve stem of the matched 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 valve stem of the rising stem valve. For some rotary valves and concealed stem valves in multi rotary valves, although the issue of valve stem diameter passing does not need to be considered, the size of the valve stem diameter and keyway should also be fully considered when selecting, so that they can work normally after assembly.
If the opening and closing speed of the output speed valve is too fast, it is easy to cause water hammer phenomenon. Therefore, the appropriate opening and closing speed should be selected according to different usage conditions.
Valve electric devices have special requirements, which must be able to limit torque or axial force. Usually, valve electric devices use couplings that limit torque. After the specifications of the electric device are determined, its control torque is also determined. Generally, the motor operates within a predetermined time and does not overload. But if the following situations occur, it may lead to overload: first, the power supply voltage is low, and the required torque cannot be obtained, causing the motor to stop rotating; Secondly, the torque limiting mechanism was incorrectly set to exceed the stopping torque, resulting in continuous excessive torque and causing the motor to stop rotating; Thirdly, intermittent use generates heat accumulation that exceeds the allowable temperature rise of the motor; Fourthly, due to some reason, the circuit of the torque limiting mechanism malfunctioned, resulting in excessive torque; The fifth reason is that the operating environment temperature is too high, which relatively reduces the thermal capacity of the motor.
In the past, the methods for protecting motors included using fuses, overcurrent relays, thermal relays, thermostats, etc., but each of these methods has its own advantages and disadvantages. There is no absolutely reliable protection method for variable load equipment such as electric devices. Therefore, various combinations must be adopted, which can be summarized into two types: one is to judge the increase or decrease of the motor input current; The second is to assess the heating condition of the motor itself. Both of these methods require consideration of the time margin given by the motor's thermal capacity.
Usually, the basic protection method for overload is to use a thermostat for overload protection during continuous or incremental operation of the motor; The protection against motor stalling is achieved through the use of thermal relays; For short-circuit accidents, fuses or overcurrent relays are used.
