Selection and Analysis of Axial Flow Check Valves for Long Distance Pipelines 1 OverviewThe check valve is an important part of the pipeline system and is the key equipment to ensure the safe and reliable operation of the pipeline system. With the development of petroleum and chemical industry, the working conditions of pipeline systems are becoming more and more complex and demanding, and the working parameters are getting higher and higher. It is difficult for ordinary check valves to meet the needs of pipeline system development. The axial flow check valve has the advantages of good sealing, quick closing, small closing impact, reduced pressure, low mechanical vibration, low noise, small volume, light weight and long service life, and has been widely used in recent years. However, in the field use, there is a situation in which the axial flow check valve is improperly selected and fails to perform its proper performance, and even cannot meet the requirements for on-site use. 2. Working principle of axial flow check valveThe axial flow check valve is a protection device that is installed on a device or pipeline such as a compressor or a pump to prevent backflow of the medium. When the forward flowing medium flows to the valve disc position, the flow is blocked, and some of the kinetic energy is converted into pressure energy, pushing the spool and opening the valve. As the flow and pressure of the pre-valve medium decrease, the valve closes under the action of the spring force and the back pressure of the medium to prevent the medium from flowing back. 3, the structural characteristics of the axial flow check valve3.1. Valve flap According to the valve flap structure, the commonly used axial flow check valve has a cantilever circle. Disc type, double support disc type and ring disc type. Selection and Analysis of Axial Flow Check Valves for Long Distance Pipelines Figure 1 Cantilever disc type The cantilever disc type axial flow check valve (Fig. 1) is simple to machine and easy to maintain, but has high requirements on the quality of the valve shaft and bushing. Since the moving spool is a cantilever structure, when the valve has a large diameter, the spool has a large volume and mass, and is easily jammed during movement. Compared with the ring plate of the same specification, it has greater circulation capacity. Selection and Analysis of Axial Flow Check Valves for Long Distance Pipelines Figure 2 Double support disc type The double-supported disc axial flow check valve (Fig. 2) is simple to machine and easy to maintain. The double-supported valve core structure avoids the eccentricity of the cantilever structure, has good centering, the valve core moves smoothly, has no jamming, and is reliable in operation, but there are also wear conditions of the valve shaft and the sleeve, the shaft and the shaft. The quality requirements of the sets are relatively high. Compared with the ring plate of the same specification, it has greater circulation capacity. The annular disk type axial flow check valve (Fig. 3) is a floating valve disc structure, which overcomes the shortcomings of the disc type valve shaft and the bushing. The valve core is light and flexible, and runs without wear and jam, but the structure is complicated and processed. And maintenance is difficult. The inner and outer shunts of the ring-type flow type check valve generate vibration and eddy current phenomenon, which causes the valve flap to be non-coaxially opened and closed, and the parts are loose. The inner and outer ring sealing structure requires the inner and outer sealing faces of the valve core to be closely fitted with the inner and outer sealing faces of the valve seat to achieve sealing, which is easy to cause leakage due to misalignment, especially after several non-coaxial opening, it is more difficult to achieve strict alignment. . Multiple sets of spring structures are prone to non-coaxial opening and closing due to uneven force. At the same time, the leaf spring is prone to fatigue relaxation or breakage. Selection and Analysis of Axial Flow Check Valves for Long Distance Pipelines Figure 3 ring type 3.2, sealed According to the sealed form, the axial flow check valve is divided into a metal seal and a composite seal. Metal sealing structure (Figure 4) The sealing surface has good performance, high temperature and low temperature resistance, wear resistance and long service life, but the sealing performance is relatively poor, mainly used in severe working conditions. The composite seal structure (Fig. 5) is mainly sealed with a soft seal and the metal seal is an auxiliary seal. The auxiliary metal seal prevents backflow of the medium when the primary seal fails. The composite sealing structure has good performance and reliable sealing, but the soft sealing material is easy to age and wear, the use conditions are limited, and the service life is limited by many factors. 4, working conditions In the long-distance pipeline, the axial flow check valve is mainly used in the station power equipment (compressor, pump, etc.) outlet, bypass pipeline, venting system, metering system and storage tank import and export, etc., different working conditions There are different requirements for the performance of axial flow check valves. At the exit of the power equipment, the check valve mainly plays a role in protecting large equipment. It requires the check valve to have stable reliability during use and to avoid damage to the large power equipment caused by the backflow of the medium. In the bypass system, the check valve is required to respond in time. In the venting system, it is required that the check valve can not only be opened quickly, but also has a relatively high sealing requirement, avoiding the cross flow of the venting lines. In the metering system, the check valve is required to be reliably sealed to avoid inaccurate measurement. At the inlet and outlet of the tank, the sealing performance of the check valve is very high. Selection and Analysis of Axial Flow Check Valves for Long Distance Pipelines Figure 4 metal seal Selection and Analysis of Axial Flow Check Valves for Long Distance Pipelines Figure 5 composite seal 5, choose(1) Determine the location of the check valve in the equipment, plant or piping system. Such as power equipment exports, bypass systems, venting systems, etc. (2) Determine the working conditions of the valve, applicable to the medium, working pressure, working temperature and environment. (3) Determine the medium, working pressure, working temperature and environmental conditions of the pipeline transportation, and select the shell and internal materials of the valve, such as carbon steel, alloy steel or stainless acid-resistant steel. (4) Determine the nominal diameter and connection method of the pipe connected to the valve, such as flange, thread or welding. (5) Determine the fluid characteristics of the valve, allow pressure drop, seal rating, etc. (6) Determine the requirements for the service life of the product. (7) Determine the installation dimensions and dimensions, nominal diameter, connection to the pipe, and connection dimensions, dimensions, or quality limits. 6, the conclusionDifferent structure axial flow check valves have different structural principles and characteristics. When selecting the axial flow check valve, it should be carefully analyzed and compared according to the specific working conditions and piping system requirements, and the appropriate products should be selected to achieve the best performance to meet the working conditions.