Double-eccentric butterfly valve is also called double-offset high-performance butterfly valve. It is an upgraded pipeline block and control valve based on the midline butterfly valve. The opening and closing friction is optimized by the two eccentric structures. It is widely used in water supply, sewage, industrial circulating water and other medium pipelines. It is also a very common valve type in the water supply and drainage system of the foreign trade GOST project. Its core structure is characterized by two eccentricities. The first one is that the valve stem axis deviates from the centerline of the butterfly plate sealing surface, and the second one is that the valve stem deviates from the centerline of the valve body. At the moment when the valve opens, the butterfly plate sealing surface can quickly separate from the valve seat. During the opening and closing process, the sliding friction between the sealing pairs is greatly reduced, and the wear is reduced. Compared with the ordinary midline rubber butterfly valve, the service life is longer, the opening and closing torque is smaller, and the driving configuration cost of large-diameter model is lower. The double eccentric butterfly valve has a variety of sealing forms, mainly including rubber soft seal and PTFE seal, and can also be made into metal hard seal, which is suitable for different pressure and temperature conditions. The valve body is made of nodular cast iron, carbon steel and stainless steel, and the connection modes include wafer type and flange type. The flange type can be manufactured according to ГОСТ 33259 standard, which is suitable for supporting pipeline requirements in Russia and Eurasia. The driving mode shall be flexible. The manual worm gear operating mechanism can be configured for small-diameter pipelines. Pneumatic and electric actuators can be selected for large-diameter pipelines or automatic pipelines, which can support local switch and remote flow regulation and adapt to the automatic control system of pump station, water plant and heat exchange station. Compared with the midline butterfly valve, the double eccentric butterfly valve has stronger pressure resistance, better sealing stability and is suitable for the pipeline with pressure fluctuation; Compared with the triple eccentric butterfly valve, the double eccentric structure has simple process and higher cost performance, which is suitable for the normal working conditions such as normal temperature clean water, sewage and weak corrosive medium. However, the triple eccentric butterfly valve is more suitable for the severe working conditions such as high temperature flue gas and high temperature steam. Therefore, the selection can be comprehensively distinguished according to the medium temperature, impurity content and budget. In the application scene, the main municipal water supply pipe, water intake and delivery pipe of water plant, outlet pipe of sewage treatment plant, circulating cooling water system of power plant, HVAC chilled water and heating return water pipe and fire water system are the main application scenes. The medium is mainly clean water, sewage and neutral circulating fluid, which is not suitable for the working condition of long-term high temperature and large amount of dust particle washing. The double eccentric butterfly valve supports two-way sealing, without strictly distinguishing the flow direction during installation. It is convenient for site construction, compact in structure and light in weight. Compared with gate valve and globe valve in large-diameter pipeline projects, it saves installation space, and has advantages in procurement and operation&maintenance costs. In the process of type selection, it is required to confirm the nominal pressure, diameter, medium temperature, medium type, flange standard and drive mode. If it is an export project, it is required to confirm the inspection standard and tightness class required by the customer in advance, and achieve Class A sealing according to the requirements of ГОСТ 9544. During installation, it is suggested to clean welding slag and sundries inside the pipeline to avoid foreign matters blocking and scratching the sealing ring. Priority shall be given to horizontal installation. After installation, hydraulic pressure test shall be carried out to check the opening and closing action and sealing performance. Routine maintenance is simple. Regularly check the operating condition of the actuator and the position of the valve stem for leakage. If the sealing element is aged or damaged, the sealing ring can be replaced separately, without replacing the valve body as a whole, so as to reduce the maintenance cost in the later period. For normal temperature water supply and drainage, circulating water and other working conditions, the double eccentric butterfly valve combines the sealing performance, cost and use stability. It is an economical and practical cutoff and regulation scheme for industrial and municipal fluid pipelines, and also a high-performance butterfly valve product frequently selected in the bidding list of Eurasia Water Supply and Drainage Project.