Daily maintenance of control valves and selection recommendations

Thread Content

The most fundamental requirement for selecting a control valve is to meet the requirements regarding temperature, pressure, liquid level, flow rate, and sealing performance in the production process; the operating pressure difference of the valve must be less than the required pressure difference. Other factors to consider include the valve’s service life, reliability, actuation speed, flow characteristics, mode of operation, flow direction, type of actuator, output torque, stiffness, spring range, material, and cost-effectiveness. While meeting the requirements of process control, the selected valves should be simple, reliable, inexpensive, long-lasting, easy to maintain, and have timely and reliable access to spare parts. Avoid focusing solely on good structure, materials, and accessories while neglecting considerations of reliability and cost-effectiveness. 　　 　　The process parameters that should be provided for the selection of control valves include temperature and pressure, as well as the pressure difference at normal flow rates and at shutdown. Regarding the properties of the fluid, factors such as corrosivity, viscosity, and the impact of temperature changes on these properties are important. System requirements involve parameters such as leakage rate, control ratio, response speed, frequency, linearity, and noise level. 　　 　　2 Introduction to Control Valves 　　　In power plant automatic control systems, control valves are a common type of actuator. In automatic control systems, advanced control theories, excellent design concepts, unique control algorithms, and complex control strategies are all implemented through the actuators acting on the controlled object. Therefore, selecting the appropriate structure, flow characteristics, and capacity of the control valve, as well as the output torque, thrust, and stroke of the actuator, plays a crucial role in ensuring the safety, stability, economy, and reliability of automatic control systems. Compared to calculation, selection is more difficult and presents more problems, so special attention should be paid to this. 　　 　　3 Classification of control valves 　　　Control valves can be divided into 9 major categories based on their structural characteristics. 　　 　　①The straight-through single-seat control valve is the most widely used type. It features low leakage, a small allowable pressure difference, a complex flow path, and a simple structure; therefore, it is suitable for applications with strict sealing requirements and low operating pressure differences. However, for smaller sizes (for DN300 mm, butterfly valves are usually chosen). 　　 　　⑧The ball valve, specifically the O-ring ball valve, functions as a non-obstructive control valve when fully open; it has the best \"self-cleaning\" properties and is suitable for two-position on/off applications involving media with many impurities and fibers. The V-shaped ball valve features approximately equal percentage control characteristics, making it suitable for applications with impurities and fibrous media where significant regulation is required; however, it is relatively expensive. 　　 　　⑨The eccentric rotary valve falls between butterfly valves and ball valves; it has good \"self-cleaning\" properties and excellent control capabilities, and can also be used for shut-off purposes. Therefore, it is suitable for applications with contaminated media and where low leakage levels are required, although its price is relatively high. 　　 　　Among these 9 types of products, the first 6 are linear stroke control valves, while the last 3 are rotary stroke control valves. Users must understand their features and precautions. All types of variant and improved products are based on these 9 categories; by understanding these 9 main categories, it becomes easier to grasp the applications of other products. 　　 　　Sealing of the valve cover and valve stem 　　　 Generally, the medium temperature should be 10 times the pipe diameter, and the straight section downstream of the outlet should be 3 to 5 times the pipe diameter. The pressure tapping points for the inlet and outlet of the valve are located at 2 times the pipe diameter in front of the valve and 3 times the pipe diameter behind the valve. Control valves must be installed on pipelines in accordance with the flow direction arrow to avoid excessive installation stress. 　　 　　④The installation location of manually operated valves should facilitate manual operation, and the operator should be able to see the parameters displayed on indicators such as level gauges. Side space for removing accessories such as the control valve handwheel mechanism and positioner should also be taken into account. 　　 　　For large-diameter and high-mounted control valves, it is necessary to consider a location where the operator can remove the connection bolts during maintenance, as well as the operator’s working position. 　　 　　⑤The piping and wiring scheme for the control valve signals of the signaling accessories should meet the requirements of the control system. Piping should preferably use