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What should we pay attention to when designing pneumatic ball valves in harsh working conditions:

  Pneumatic ball valve control is the most commonly used terminal control element in automation control in industry. Pneumatic ball valve regulates the flowing fluid (medium) to compensate for the load flow and make the controlled process as close as possible to the required set point. Based on its application in industry The importance in the field of automation makes the design and manufacturing of pneumatic ball valves particularly important, especially for certain strict and harsh working conditions, such as high temperature, high pressure difference, high flow rate, cavitation, etc., which will be carefully considered in terms of materials, structure, manufacturing, etc. Discussion.

  1. Selection of pneumatic ball valve materials:

  1. Metal materials; materials are crucial factors, such as material performance, creep, thermal expansion rate, oxidation resistance, wear resistance, thermal scratch resistance and heat treatment temperature, etc. These are the first things that should be paid attention to. Under high temperature conditions, creep and fracture are one of the main factors of material damage, especially for carbon steel. When exposed to above 425°C for a long time, the carbonization phase in the steel may transform into graphite, while for austenitic stainless steel Only when the carbon content exceeds 0.4% can it be used above 528°C. Therefore, when used at high temperatures, the tensile strength, creep, high temperature aging and other parameters of the valve body material should be calculated separately. For the design of valve trim, additional consideration should be given to the hardness of the material at high temperatures, the thermal expansion coefficient of the mating parts, the difference in thermal hardness of the guide parts, elastic deformation, plastic deformation, etc. In the design, corresponding safety factors and reliability factors should be given to ensure that damage caused by multiple factors is avoided. And be familiar with the creep rate of materials at high temperatures to select appropriate stress so that the total creep of the material does not extend to fracture within the normal service life range or allow micro deformation without affecting the normal use of the guide parts.

  In order to avoid wear, erosion and cavitation on the surface of the pneumatic ball valve's internal parts (valve core, valve seat), the thermal hardness of the material must be considered under high temperature conditions to prevent changes in metal hardness. Under high pressure difference, most of the energy of the fluid is concentrated in the valve internal parts for release, which may overload the valve internal parts. At high temperatures, the mechanical properties of most materials deteriorate and the materials become soft, which greatly affects the valve internal parts. service life of the parts. Therefore, appropriate materials should be selected correctly to extend the service life of the valve. In addition, the impact of high-temperature aging on the physical properties of the material, such as changes in toughness and intergranular corrosion, must also be considered. When the operating temperature reaches or exceeds the heat treatment temperature, the valve trim will cause problems such as annealing and reduced hardness. In order to prevent changes in material hardness, the maximum temperature limit must be selected within a safe range. For the same medium, its molecules are relatively active under high temperature conditions. Some generally corrosive media may cause serious corrosion damage to the metal materials of the valve body and valve trim. The medium penetrates in a high-speed ion state. The interior of the metal changes the characteristics of the material, such as thermal expansion, intergranular corrosion, etc. Therefore, in addition to cost performance, the selection of materials should also consider the failure caused by multiple factors.

  In the case of high pressure difference and high flow rate, even if the temperature is normal temperature, the characteristics of the material should be evaluated so that the material can meet the working conditions. Generally speaking, at normal temperature, when the pressure difference exceeds 15bar, the material of the valve core and valve seat should be adjusted from 316 to Stellite alloy surfacing or higher requirements alloy. For weakly corrosive media, 420QT can be used (quenching + tempering), 440QT, etc. High pressure difference and high flow rate will cause severe erosion or cavitation, which will cause great damage to the valve trim materials. Therefore, the material requirements for the valve body and valve trims are very high. Stainless steel surfaces should be considered for the valve core. Nitriding (HRC70) treatment makes it have strong erosion resistance and improves the flow accuracy and service life of the valve. The oxidation resistance of materials at high temperatures is also a very important parameter. During temperature cycle changes, the selected materials should avoid repeated oxidation of the material surface and the generation of oxide scale. In general, austenitic stainless steel, cemented carbide and special alloy materials have good high temperature stability, and appropriate materials can be selected according to different high temperature working conditions.

  2. Non-metallic materials

  General non-metallic materials cannot withstand high temperatures (above 300°C), but flexible graphite can withstand high temperatures above 700°C. Therefore, under high temperature conditions, whether it is a static seal or a dynamic seal, flexible graphite or composite materials can generally be selected. However, it should be noted that the friction coefficient will increase.

  2. Selection of structure and thermal conductivity of pneumatic ball valve components:

  In the design of high-temperature and high-pressure differential pneumatic ball valves, the impact of thermal expansion of different components on the action of the valve trim must be carefully considered. When high-temperature media flows through the valve, since the linear expansion coefficient of the valve body is often smaller than that of the valve seat, the valve body limits the radial expansion of the valve seat, and the valve seat can only expand toward the inner diameter, making the valve The working clearance between the core and the valve seat is smaller than the standard valve design clearance at normal temperature, causing the valve trim to become stuck. The same phenomenon will occur with the valve core and guide sleeve. Therefore, when the valve is used at high temperature, the design clearance of the standard valve at normal temperature (including between the valve core and the valve seat; between the guide sleeve and the valve stem) should be appropriately increased, so that it will not get stuck even when working at high temperature. . Therefore, the design of the gap is very important. The confirmation of parameters such as material, size and temperature difference is very important to the designer. At present, the corresponding data can be obtained from the "ASME Boiler and Pressure Vessel Code II Materials Part D Performance".

  In situations with high leakage requirements, the valve body and valve seat should be made of the same alloy steel as much as possible, and adopt a single-seat or cage structure. Try to avoid using a double-seat valve structure, and the sealing surface should be hardened to avoid high temperatures. The leakage of the lower valve increased significantly. In addition, the damage caused by the additional load caused by high temperature should also be considered.

  Cyclic changes in temperature will cause the valve seat and guide sleeve to loosen, so seal welding and lap welding must be used to prevent looseness or compress the structure. The sealing of the valve seat gasket can only be obtained when the sealing force is greater than the yield limit of the gasket. However, under high temperature, high pressure and thermal cycle conditions, the sealing material creeps and causes leakage. The integral valve seat can be used, which is composed of the valve. The valve seat is directly formed on the body and hardened. For large-diameter valves, the valve seat can be welded on the valve body and the gasket removed to avoid unnecessary leakage. Depending on the temperature of the medium, the temperature that the stuffing box can withstand and the temperature that the actuator can withstand should also be considered.

  The relationship between the stuffing box structure and operating temperature:

  3. The sealing structure of the pneumatic ball valve under the periodic changes of high temperature and high pressure difference:

  The valve seat sealing surface structure used for periodic changes in high temperature can adopt a self-centering wedge-shaped structure. This structure is used for the out-of-round sealing line and wear of the valve seat caused by the expansion of parts, and has automatic centering and compensation functions. It can have a good sealing effect under high temperature, high pressure difference and temperature cycle changes. The sealing is achieved by relying on the elastic deformation of the flexible valve seat sealing part.

  When calculating the sealing specific pressure of a material under high temperature conditions, reasonable values ​​should be selected considering that the strength limit and yield limit of the sealing material decrease under high temperature conditions.

  4. Changes in material hardness under high temperature conditions:

  Under high temperature conditions, the hardness of various materials decreases to varying degrees. The decrease in hardness increases the possibility of plastic deformation and scratches of the material. Comparison of hot hardness of surface hardening materials tungsten-chromium carbide, chromium-boron alloy and some stainless steels.

  5. Plastic changes of materials:

  Plastic deformation refers to a metal surface being scratched by other materials, bonding together, or the surface rolling into a ball. It is related to temperature, material, surface finish, hardness, and load, and will be affected by fluids. High temperatures will soften the metal and increase its plastic deformation tendency. Plastic deformation will cause: the valve to be stuck; damage to the sealing surface; increased friction, causing inaccurate positioning of the valve core. If there are larger and harder particles in the pipeline fluid, the valve internal parts will be worn rough and uneven, resulting in plastic deformation.


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The valves are not maintained in a timely manner, resulting in disrepair, leakage or malfunctioning switches; the valves are not regularly inspected and tested for pressure, or even used for many years without cleaning, pressure testing and technical appraisal, resulting in debris deposited in the valve, lax closure, and serious leakage. Oil, oil leakage; the valve is not closed after maintenance, or the pipe opening is not blocked after the valve is removed.

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In fluid pipeline systems, valves are control components whose main functions are to isolate equipment and pipeline systems, regulate flow, prevent backflow, regulate and relieve pressure. Since it is very important to select the most suitable valve for the pipeline system, it is also important to understand the characteristics of the valve and the steps and basis for selecting the valve.

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For a single core, when the medium is flow-open, the valve has good stability; when the medium is flow-closed, the valve has poor stability. The double-seat valve has two valve cores. The lower valve core is closed and the upper valve core is open. In this way, when working at a small opening, the closed-flow valve core will easily cause vibration of the valve. This is a double-seat valve. The reason why it cannot be used for small opening work.

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