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There is a 'knowledge' in solving the internal leakage of electric valves

by:Sino Valves     2021-03-06
In our daily use of electric valves, we often encounter various failures. The leakage of electric valves is a common one among many failures, and the leakage of electric valves generally includes internal leakage and external leakage! So, what is the difference between the two? The internal leakage of an electric valve generally refers to the situation that the medium still flows out, leaks or drips from the flow outlet of the electric valve after the electric valve is completely closed. Generally speaking, it is not tightly closed and the seal is not good. ! The cause of this failure is usually due to the erosion and wear of the medium or the inclusion of impurities. Even if the electric valve screw (valve stem) is tightened in place, since there is still a gap between the valve core and the valve body, the medium can still flow through. The external leakage of the electric valve generally refers to the leakage of the medium in the external sealing part of the electric valve packing after the electric valve is completely closed or opened. That is, between the electric valve and the connecting flange or the connecting thread, the sealing is not tight, which causes the medium to flow out of the connecting sealing surface, or the medium is caused by the poor sealing between the valve stem and the gland (ie the gland on the valve stem) Leakage, or external leakage caused by trachoma on the valve body blank, and external leakage caused by the wear of the valve body due to medium erosion. The main factors affecting the dead zone of the electric valve: friction, wandering, torsion of the valve shaft, and the dead zone of the amplifier. Various control valves are not the same as sensitive to friction. For example, rotary valves are very sensitive to friction caused by high seat load, so pay attention to this when using. But for some seal types, high seat load is necessary in order to obtain the closing level. In this way, the design of this kind of valve is very poor, and it is easy to cause a large dead zone. The effect of this on the process deviation is obvious and is simply decisive. 2. Solution to the internal leakage of the electric valve of the newly-built unit. The internal leakage of the electric valve of the thermal system of the power plant will not only cause the waste of working fluid and heat of the unit, affect the economy and safety of the unit, but also cause the system to be unable to be isolated and bring difficulties to maintenance. The valve internal leakage management should be comprehensively considered from the three aspects of electric valve selection, installation, commissioning and operation. 1. Selection of electric valves 1. Incorrect selection will cause internal leakage of valves due to cavitation corrosion. Electric valves for different purposes have specific technical requirements. Electric valves must be selected reasonably according to the system process to prevent cavitation corrosion. Valve internal leakage, maintenance professionals should focus on this. 2. The electric valve pushes down to close. If the thrust of the electric actuator does not match the valve, it is easy to reach the full closed position when there is no medium pressure in the pipeline. When there is pressure in the pipeline, if the electric If the torque of the actuator is too small, it cannot overcome the upward thrust of the medium, which will result in improper closing, and long-term scouring of the medium will cause the valve to leak. The thermal workshop should make clear requirements for the torque of the electric actuator during the bidding of the valve. When the electric actuator is matched with the valve, the rated torque should not be less than 1.2 to 1.5 times the rated torque of the valve. The electric actuator must reserve enough torque. margin. 3. Internal leakage caused by the quality of electric valve manufacturing. The valve manufacturer does not strictly control the material, processing technology and assembly technology of the valve during the production process, resulting in unqualified grinding of the sealing surface, and no products with defects such as pitting and blisters. Thorough removal caused internal leakage of the electric valve. Bidding for professional machinery equipment should strictly follow the company's valve selection requirements, and adopt methods such as inspections of electric valve manufacturers and performance surveys of the same type of units, and select well-known manufacturers with good valve quality, high reputation and complete quality assurance. 2. Installation and commissioning of electric valves 1. Strengthen valve maintenance during unit commissioning to prevent internal leakage of valves. For newly installed and commissioned units, in addition to requiring the construction unit to store and maintain the valves, check whether the valves have appearance defects during installation. Check whether the switch is flexible, whether the packing (packing) is missing, whether there is any foreign matter in the valve body, whether the installation direction is correct, etc. to check. When the valve is installed and the pipeline and system are steam purged or flushed with water, it is necessary to check whether the valve is in the fully open position. After purging or flushing the flanged electric valve, the valve flange can be removed at the end of the pipeline for internal inspection , Check whether there is any blockage of foreign matter or wear of the valve core joint surface, and replace or grind as appropriate. 2. Strengthen the accuracy of electric valve limit setting during commissioning, invite valve manufacturers to participate in the electric valve manual (such as torque, opening/closing stroke or time requirements, etc.) or under the guidance of the manufacturer’s personnel to avoid Due to the inaccurate stroke setting of the electric valve and the failure to reach the full closed position of the valve, the internal leakage of the valve was flushed during the commissioning of the unit. However, the determination of the full closed limit of the valve is difficult to grasp and is affected by the processing and assembly process. Electric valves generally cannot be opened or stuck in hot state after being manually closed in cold state. If the stroke of the electric valve is adjusted to be smaller by the position of the upper and lower limit switches, it will cause the electric valve to close or incompletely open. If the stroke of the electric valve is adjusted larger, it will cause the over-torque protection action. If the torque is adjusted larger, the deceleration mechanism or the valve will be damaged, or even the motor of the electric actuator will be burnt.
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