On the practical application of electric actuators in thermal power plants

Abstract: Electric actuators with integrated variable frequency intelligent control units save space and have accurate positioning, but have poor resistance to harsh environments; traditional electric actuators have slightly poor positioning accuracy and are relatively large in size, but have strong resistance to harsh environments. So what kind of electric actuators are more suitable for use in thermal power plants? In fact, the actuators suitable for practical application in thermal power plants are not based on how high the scientific and technological content is, nor on their gorgeous data at normal temperature and normal conditions, but on whether they can meet the requirements of no refusal to move, no false movement, stable signal transmission, and no failure for a long time under extremely harsh environmental conditions such as high temperature, high pressure, large vibration, and much dust. As long as the above requirements can be met, they are electric actuators suitable for practical application in thermal power plants.

Keywords: electric actuator, thermal power plant, practical application

Electric actuators are devices that drive valves through electric motors to make the valves reach the fully open or fully closed position. Electric actuators are widely used in thermal power plants and are an indispensable part of thermal power plant automation control. For example, a medium-sized thermal power plant with two 300MW units uses 400 to 500 electric actuators. Whether these actuators can operate stably without refusal or malfunction is also directly related to the safety and stability of the units. In a thermal power plant, operators send switching commands to the DCS through a computer. After receiving the command, the DCS transmits the command information to the electric actuator, causing the actuator to drive the valve to switch according to the operating requirements of the unit. At the beginning of the commissioning of thermal power plants, electric actuators of different models from different manufacturers are generally installed. Although each electric actuator has its own characteristics and advantages, in the actual operation of thermal power plants, some actuators will frequently fail after long-term operation, and will refuse to operate, malfunction, and fail to transmit switch signals, which greatly reduces the reliability and economy of the operation of thermal power units, and even brings certain dangers. Therefore, it is very important for thermal power plants to choose the appropriate type of electric actuator. Take a thermal power plant as an example. At the beginning of its construction, there were more than 400 electric actuators installed in two 300MW units, including 6 types of electric actuators. However, some of these actuators could not operate stably in their locations, and because there were too many types of actuators, daily maintenance work was extremely difficult. Therefore, according to the actual operating conditions on site, the relevant technical personnel of the power plant conducted a follow-up investigation on the operating conditions of the above types of electric actuators to determine how to make these actuators operate stably to ensure the stable operation of the power plant units.

AUKOMA AKZ series intelligent electric actuators are patented products of AUKOMA. Because they have an integrated variable frequency intelligent control unit inside, AUKOMA AKZ has eliminated the complex motor steering control box required by traditional actuators. They have the advantages of small size, precise positioning, and reliable operation. Therefore, this type of electric actuator is installed at some of the more important valves in power plants. However, after long-term field use, relevant technical personnel found that:

① Since many valves on site are located on high-temperature and vibrating pipelines, this type of electric actuator integrates an integrated variable frequency intelligent control unit, which is what we usually call an electronic board, so it has poor high temperature and vibration resistance. After long-term operation, the control board and power board will be damaged, causing the actuator to fail to operate correctly, which poses a hidden danger to the safe and stable operation of the unit:

② The positioning work of this type of electric actuator requires the actuator to travel more than one-third of its stroke according to the established valve opening (that is, the counter must rotate 120 degrees or more), while some valves are not allowed to be set to the switch position in this way when the unit is running, such as the high-pressure cylinder vacuum valve of the steam turbine. Once during the operation of the unit, due to the valve positioning being too tight after the previous maintenance work, the valve did not reach the preset closing position during the valve closing process. Even if the actuator torque was actuated, the operator could not observe the valve opening and closing status in time, and the valve did not have the conditions for repositioning, which to a certain extent left hidden dangers for the safe and stable operation of the unit. If the valve cannot be opened and closed normally, the vacuum degree of the steam turbine condenser will decrease, and even cause the unit to be shut down unplanned. serious consequences. AUKOMA ZL series electric actuator is a kind of electric actuator widely used in boiler pipelines. This kind of electric actuator is simple and convenient to position, easy to operate, and flexible to switch. From the beginning of the unit commissioning, this kind of actuator is more suitable for use in the environment of thermal power plants. However, after a period of operation, due to the harsh environment on site, there is not only a lot of dust, but also large pipeline vibration and high temperature. Sichuan Electric Actuator is also an electric actuator with electronic boards. In this environment, the life of the electronic board will be greatly shortened. The time required for the electric actuator to operate is greatly shortened, so this type of electric actuator will frequently malfunction after running for a period of time, which brings great difficulties to daily inspection and maintenance work and is not economical. The traditional type of electric actuators controlled by contactors, represented by AUKOMA Z series electric actuators. These electric actuators are not as advanced as the two actuators mentioned above at sight, and they seem simple, but after actual application on site, it is found that this type of actuator can operate stably for a long time in an environment with a lot of dust, high vibration and high temperature. The control cable is connected to the actuator in the form of a plug-in. During the unit maintenance work, when the valve is repaired, it is only necessary to unplug the plug-in. Moreover, the price is relatively low, which can be said to be good value for money. The only disadvantage is that The point is that the actuator uses gear positioning and counting, and its accuracy is not as high as that of the actuator with electronic board. It cannot be used for some valves that require precision. It can be seen that each type of electric actuator has its own advantages and disadvantages. So how can we choose the actuator model appropriately to meet the safe and stable operation conditions of the thermal power plant?

Through the follow-up investigation of these actuators and many years of practical application, the relevant technical personnel of the power plant have summarized some experience methods for the use of electric actuators to ensure the safe and stable operation of the unit.

First, regarding the AUKOMA ZL series electric actuators used in the boiler: by carefully observing the working conditions of the actuators, the technicians found that the valves at the same position of the boiler are equipped with electric actuators with full-open and full-close functions as well as electric regulating actuators with regulating functions. Therefore, the application of this position will not affect the stability and economy of the equipment operation. Therefore, it was decided to replace some of the AUKOMA ZL series electric actuators with AUKOMA Z series actuators. After a period of observation, the replaced actuators are completely effective. It can not only meet the conditions for stable operation of the boiler, but also the actuator itself can operate safely and stably for a long time, which greatly reduces the maintenance rate of the actuator and saves a lot of money for replacing panels. Secondly, for valves that have strict requirements on the accuracy of electric actuators, AUKOMA AKZ actuators are still used. However, electric actuators with electronic boards cannot operate safely and stably for a long time in high temperature and high vibration environments. So, can the control unit of the actuator be installed separately from the drive parts such as motors? Through communication with the AUKOMA actuator manufacturer, the electric actuator was modified accordingly, that is, the electronic board drive part of the electric actuator was installed in a place with normal temperature and no vibration, and the actuator was driven by a control cable. The motor of the actuator is used to open and close the valve. After a period of observation, the failure rate of the actuator has been greatly reduced, which can meet the conditions for safe and stable operation of the unit. As for some valves, they are not allowed to be fully opened or fully closed under normal operation of the unit. Therefore, when the unit is stopped, the expansion of the valve pipeline under high temperature conditions must be carefully considered. Through experience, after the valve is manually closed, it is rotated three times to make the setting position just appropriate to ensure that the torque protection action does not occur under high temperature conditions. After making the above adjustments to the electric actuators, the electric actuators installed in the power plant can meet the actual production requirements on site, and the safe and stable operation time is greatly extended. It has been nearly four years since the transformation was completed, and only a few actuators occasionally fail. More than 90% of the actuators have not yet failed. This not only provides a strong guarantee for the safe and stable operation of the unit, but also saves a lot of money in the maintenance cost of the actuators. It can be seen that this is a very successful equipment transformation project.

It can be seen that what kind of electric actuator is suitable for use in thermal power plants does not depend on how high the technological content is, nor on its gorgeous data under normal temperature and normal conditions, but on whether it can meet the requirements of no refusal to move, no false movement, stable signal transmission, and no failure for a long time under extremely harsh environmental conditions such as high temperature, high pressure, large vibration, and much dust. As long as the above requirements can be met, the electric actuator is suitable for use in the actual environment of a thermal power plant.