Selection and Failure Analysis of Valves Used in Petrochemical Engineering
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Keywords: valve; selection; fault analysis In the petrochemical industry, its large-scale devices and piping systems are widely used in this valve component. The valve can be opened and closed by its opening and closing, it can realize pressure-reducing pressurization, and can also change the direction of medium flow. . The petrochemical industry itself has a certain degree of particularity. It often withstands high temperature and high pressure during the operation process, and poisonous and harmful gases and flammable and explosive materials are very common. In order to maintain the continuity of production, this also gives a comparison to the quality of the valve itself. High demand. The use of valves is very large, and the wear and tear situation is very serious in the specific work, so the choice of valve type and the control of quality are very important contents.
1 The basis of the valve equipment selection Because the production process is different, and the specific requirements of the piping system for the valve are not the same, so the valve is in a variety of types and structural forms, equipment selection has always been very important in engineering design One of the content. Selecting the most suitable valve device can extend the service life, and it is very obvious for the long-term stable operation of the device.
1.1 The temperature and pressure of the valve At a certain temperature, the maximum pressure that can be assumed is the pressure-temperature rating of the valve. If the temperature rises, the pressure threshold will be reduced. This content is also the main basis for the selection of valve equipment. . In many countries, pressure-temperature rating thresholds have been established for valves and flanged fittings, and some manufacturers have adopted both standards.
1.2 About valve materials For a valve component, the choice of quality, in addition to temperature and pressure, often directly determines the important factors of reliability and service life, and also has a certain influence on its corrosion resistance. Valves vary in their type and purpose, but also in materials. Therefore, different valves require different materials. In addition, the heat resistance and resistance of such materials should also be considered. Corrosion performance.
1.3 Closedness of valve components Sealing property is also an important measure of the quality of a valve. Sealing locations in a valve include the following types: opening and closing parts, connection locations between valve body caps, connection between valve pipes, and packings. Between the valve stem and so on. The leakage of the opening and closing parts is inner leakage, which is not easy to find, and the rest is leakage. Sealing property is also the main influencing factor of the quality of valve fittings, and it is an important parameter for valve selection.
1.4 Fire safety issues After a fire occurs, if the valve becomes the failure point of the entire pipeline system, it will cause great damage to the entire pipeline. It is a soft-seal valve, and its use of material strength is much lower than the valve body metal, and As the temperature around the valve rises, the soft seal structure is likely to become a destructive point, directly increasing the damage caused by the fire. Although many soft-seal valves have better sealing properties, if the pipelines used are used to transport flammable and explosive materials, fire protection design work should be emphasized.
2 Common Fault Analysis in Engineering Application 2.1 Improper valve selection The main cause of valve accident is caused by improper selection.
2.1.1 After the ball valve is used in the normally closed state for a long period of time, impurities such as scales are likely to adhere to the surface of the valve ball, so that the required torque increases when the valve is opened, causing the valve not to open properly. Therefore, when selecting, normally closed occasions should choose butterfly valve, normally open occasions should choose ball valve. Because the butterfly valve is in the fully closed position, the sealing surface of the valve seat is in contact with the valve flap, impurities are not easily adhered, and the same situation occurs when the ball valve is fully opened.
2.1.2 For valves with double seal seats such as gate valves and ball valves, the fluid medium is likely to be trapped inside the middle chamber. When the ambient temperature rises or the temperature of the system increases, the retained fluid medium can easily expand or evaporate, causing the valve body to move. Excessive pressure will cause the sealing ring to deform or break, causing leakage. At this time, valves with a pressure relief structure should be used.
2.1.3 Small-diameter gate valves for steam boilers and other sewage pipelines. When the steam passes through, the valve is heated. After the cooling is turned off, the gate plate is easily wedged and it is difficult to open again. At this point, the selected gate valve should be considered to set the bypass or fully closed and then rewind half a circle, as the closed position of the valve or change the cut-off valve.
2.1.4 When a swing check valve is installed next to a bend, the vortex action will cause the valve flap to rotate, causing flutter and wear and even shedding of the valve flap and the valve pin. In this case, it is advisable to design a straight pipe section with a certain length before and after the check valve, or to use a check valve valve structure that prevents the valve from rotating.
2.2 Fluid Media Velocity 2.2.1 Overflow Velocity When the shut-off valve is used in a partially open position, the impact of a high-speed fluid on the valve flap causes the valve flap to rotate, causing the valve flap and valve stem connection to wear out. At this time, it is advisable to consider a stop valve with a bellows to prevent the flap from rotating. If the flow rate of the swing check valve is too high, the valve flap is suspended in the fluid near the fully open position, which may cause flutter.
2.2.2 Flow rate too low When the swing check valve is used under low pressure and low flow speed (valve opening height is about 10°), the valve disc is close to the fully closed position, which easily causes chattering and flaps with the valve seat and gives off.咔嗒咔嗒 的 的 的 的 的 的 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 声音 。 。 。 。 。 。
2.3 Check valve The check valve is sensitive to the conditions of use (with or without pulsation, water hammer, etc.) and there are many cases of failure during use. The selection should consider the maximum and minimum use of flow, pressure fluctuations, the minimum flow rate and size of the check valve, etc., according to the valve pressure difference before and after the determination of the valve opening height.
2.3.1 Swing type valve is used at low pressure and low flow rate. The valve flap is easy to produce vibration. At this moment, the valve can only be partially opened, and the valve disc keeps on switching continuously to vibrate. Preventing chattering can be considered from the valve structure (such as a structure that uses a double lever to close the valve) and piping (such as increasing the flow rate or closing the valve downstream of the check valve to reduce the fluctuation of the flow rate downstream of the check valve). .
2.3.2 When the flow direction of the fluid changes abruptly, the valve flap of the check valve will violently close on the valve seat, causing great wear of the valve seat and generating water hammer along the pipeline. In the project selection, full consideration should be given to the structure of the valve (such as the damper installed on the valve disc or the use of a metal seat), switching speed, and buffer tank settings.
Concluding remarks The types of valves are numerous and the amount of use is large, but the quality is uneven. Therefore, accidents caused by improper valve selection or unqualified quality have always been a problem faced by the petrochemical industry. In addition, improper operation in actual use, or parts aging, valve leakage, etc., are very common situations and should be noted. This article first introduced the valve type, and made some explanations for its selection. During the use process, the staff should have a good idea of ​​the structure of the valve and pay attention to the development of new technologies and new materials. Valves are key components in the petrochemical industry. Only by increasing the degree of emphasis on them can the petrochemical industry ensure safety and stability in operation and contribute to the economic development of China.
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