Valve electric actuator is an indispensable device to realize valve program control, automatic control and remote control. Its motion process can be controlled by stroke, torque or axial thrust. Because the working characteristics and utilization of valve electric actuators depend on the type of valve, the working specifications of the device and the position of the valve on the pipeline or equipment, the correct selection of valve electric actuators can prevent overloading (high working torque) (For controlling torque) is very important.
In general, the basis for the correct selection of valve electric actuators is as follows:
Operating torque Operating torque is the most important parameter for selecting a valve electric actuator. The output torque of an electric device should be 1.2 to 1.5 times the maximum torque of the valve operation.
There are two types of host structure for operating thrust valve electric actuators: one is to directly output torque without a thrust disk; the other is to configure a thrust disk, and the output torque is converted to output thrust through the valve stem nut in the thrust disk.
The number of rotations of the output shaft of the valve electric actuator depends on the nominal diameter of the valve, the pitch of the valve stem, and the number of thread heads. It must be calculated according to M = H / ZS (M is the total rotation that the electric device should meet Number of turns, H is the valve opening height, S is the thread pitch of the stem transmission thread, and Z is the number of stem thread heads).
For stem diameters of multi-turn type open stem valves, if the maximum stem diameter allowed by an electric device cannot pass through the 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 larger than the outer diameter of the stem of the bright stem valve. For the part of the rotary valve and the multi-turn valve of the dark-stem valve, although the passage of the stem diameter need not be taken into consideration, the diameter of the stem and the size of the keyway should be fully taken into consideration when matching, so that it can work normally after assembly.
If the opening and closing speed of the output speed valve is too fast, water hammer will easily occur. Therefore, the appropriate opening and closing speed should be selected according to different use conditions.
Valve electric actuators have special requirements, that is, they must be able to limit torque or axial force. Valve electric actuators usually use couplings that limit torque. When the size of the electric device is determined, its control torque is also determined. The motor is generally operated within a predetermined time, and the motor will not be overloaded. However, the following situations may cause overload: First, the power supply voltage is low and the required torque cannot be obtained to stop the motor; second, the torque limit mechanism is incorrectly adjusted to make it larger than the stopped torque. It causes continuous excessive torque to stop the motor. Third, intermittent use, the accumulated heat generation exceeds the allowable temperature rise of the motor; fourth, the circuit of the torque limit mechanism fails for some reason, causing the torque to be excessive. Fifth, the use of ambient temperature is too high, which relatively reduces the motor's thermal capacity.
In the past, the methods for protecting the motor were to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have advantages and disadvantages. There is no absolutely reliable protection method for load-changing equipment such as electric devices. Therefore, various combinations must be adopted. There are two types of induction: one is to judge the increase or decrease of the motor input current; the other is to judge the heating of the motor itself. In these two methods, regardless of the type, the time margin given by the thermal capacity of the motor must be considered.
Generally, the basic protection method for overload is to use a thermostat for overload protection of the motor during continuous operation or jog operation; to protect the motor from stalling, use a thermal relay; for short-circuit accidents, use a fuse or an overcurrent relay.
The distribution of the valve market is mainly based on the construction of engineering projects. The largest users of valves are the petrochemical industry, the power sector, the metallurgical sector, the chemical industry and the urban construction sector. The petrochemical industry mainly applies API standard gate valves, globe valves and check valves; the power sector mainly uses high-temperature pressure gate valves, globe valves, check valves and safety valves for power stations and low-pressure butterfly valves and gate valves for water supply and drainage valves; the chemical industry mainly uses Stainless steel gate valve, globe valve, check valve; metallurgical industry mainly uses low-pressure large-diameter butterfly valves, oxygen shut-off valves and oxygen ball valves; urban construction departments mainly use low-pressure valves, such as large-caliber gate valves for urban water pipes, and mid-line buildings Butterfly valves, urban heating mainly use metal-sealed butterfly valves; oil pipelines mainly use flat gate valves and ball valves; the pharmaceutical industry mainly uses stainless steel ball valves; the food industry mainly uses stainless steel ball valves.
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