Analysis and application of safety valve spring working condition characteristics (HY-industry technical centre)
Summary:
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Aiming at the problems of safety valve springs under severe working conditions, from the detection of the temperature of the safety valve under high-temperature working conditions, especially the temperature analysis of the spring part, the effect of different temperatures on the spring stiffness of the safety valve is studied, so as to determine the safety of different working temperatures. The valve cold test differential pressure provides a reference for the design and manufacture of safety valves.
01 Overview
The spring is the most critical component of the safety valve. Once it fails, the entire safety valve will no longer be able to be used. In this issue, we will briefly talk about the failure of the safety valve spring. It should be particularly pointed out here: the failure of the spring not only refers to the fracture of the spring, but also includes the failure caused by the change of the spring. The main reasons for spring failure are as follows:
1. Internal defects of the spring cause fracture
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In the production process of reed wire, especially in the production process of large-diameter high-strength material reed wire, there may be certain physical defects inside the reed wire, such as voids, cracks, etc., which are extremely affected by the shear force of the spring. Prone to crack propagation, and the most cause the spring to break. In addition, the internal structure of the reed wire material is not uniform, and the high content of trace elements such as S and P can easily lead to the generation and expansion of internal defects in the spring.
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For this type of failure event, the cause of failure can be determined by the method of final chemical composition analysis of the spring material and port analysis. With the development of material processing level and spring coiling technology, the situation of spring fracture due to unqualified materials or internal defects is decreasing year by year.
2, Spring is broken due to environmental corrosion
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Under normal circumstances, the spring in the safety valve does not contact the medium before the safety valve is opened. If there is a back pressure corrosive environment at the outlet of the safety valve, the spring can also be protected by adding a bellows. The presence of corrosive media in the air environment mostly occurs in chemical systems, and is mainly divided into uniform corrosion and local corrosion. Uniform corrosion of the spring can be avoided by regular inspection. Local corrosion is more difficult to find and the damage is greater. Among them, the presence of chloride ions and sulfide ions have the greatest impact on the spring, because these two media are very easy to induce stress corrosion cracks in the spring. Corrosion and fracture of springs caused by environmental factors can be avoided by reasonable selection of materials. In addition, the reasonable surface treatment process when the spring leaves the factory: painting, metal coating, etc. can also avoid spring corrosion.
3. Decrease of spring stiffness leads to failure
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In fact, the third point has received the lowest degree of attention in practice, but has the widest scope of influence. In the previous content, we have mentioned that a very important index of the safety valve spring is the spring stiffness. Unreasonable spring stiffness will directly lead to abnormal performance such as overpressure and seat return of the safety valve. After the safety valve is online, the spring stiffness may change (mainly decrease) due to the influence of the use time and the use temperature. This process is very slow and difficult to detect. During the periodical calibration process, the impact of the spring stiffness decrease in the previous period on the setting is continuously corrected through the calibration process. Only when the safety valve takes off early will people realize the decrease in spring stiffness. The impact of the decrease in spring stiffness on the action performance cannot be measured and corrected during the periodic calibration process. The spring stiffness of the safety valve with a long service time (especially in high temperature conditions) may have been reduced to the range that cannot meet the performance requirements. , But it could not be found in time.
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The long-term use stability of springs can be effectively improved by means of strong pressure and thermal pressure. However, whether the relevant process can meet the stiffness requirements of long-term service springs (serving for more than 5 years and using in high-temperature environments) is still lacking corresponding data. It should be pointed out that the selection of reasonable spring materials has a great influence on the spring stiffness and stability, and it also needs to be considered when selecting the spring.
Action performance of the spring-loaded safety valve is mainly controlled by the spring. To be precise, the stiffness of the spring has the greatest influence. If the spring stiffness is not selected properly, the safety valve may not work normally. Due to this particularity, when designing the safety valve spring, the stiffness of the spring must first be determined, and strive to be close to the actual discharge conditions.
With the development of modern industry, the working conditions of safety valves are becoming more and more demanding. The safety valve springs work under severe working conditions, especially in the working conditions of high-temperature media, which is a severe test for the springs. Analyze the influence of different temperatures of the safety valve spring on the stiffness, and carry out the research on the differential pressure of the safety valve cold test, which provides an important reference for the design and manufacture of the safety valve.
02 The relevant standards stipulate the differential pressure of the safety valve cold test
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The API5201-2000 standard stipulates that the cold test pressure difference (CDTP) includes corrections for back pressure and/or temperature under actual operating conditions. When the pressure relief temperature exceeds 250 (12111°C), a temperature correction factor (magnification factor) is typically required. This factor is used to compensate for changes in spring load caused by increased heating of valve components and changes in spring material properties. For low temperature applications below -75 (-5944℃), temperature compensation is also required. When temperature compensation is required, the temperature correction factor should be obtained from the pressure relief valve manufacturer. When the cold test pressure difference includes corrections to back pressure and temperature, first calculate the pressure difference, and then multiply the pressure difference by the temperature correction factor to determine the cold test pressure difference. When used under high temperature or back pressure, the pilot operated pressure relief valve will also require a cold test differential pressure. Consult the valve manufacturer regarding back pressure and temperature limits, as well as the required correction factors.
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GB/T249211-2010 stipulates that the cold test differential pressure includes corrections to operating conditions such as temperature and back pressure. When the conventional pressure relief valve is used for high temperature working conditions or under constant back pressure for setting pressure test on a test bench at room temperature, the setting pressure needs to be corrected. For the adjustment of the differential pressure in the cold test, for pressure relief valves whose discharge temperature exceeds 120°C or below -59°C, a temperature correction coefficient for the set pressure is required for correction, and the manufacturer should be consulted.
03 The influence of different temperatures on the spring stiffness of the safety valve
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Under the operating conditions of the safety valve, the temperature of the medium will be transmitted to all parts of the safety valve through the valve parts. After analyzing the data obtained by setting the detection points of the various parts of the safety valve on site, it can be known that as the medium temperature rises, the safety valve spring is The temperature of the spring also rises, and the temperature of the spring is different. The temperature of the upper part of the spring is lower. The lower the temperature, the higher the temperature, the larger the diameter of the safety valve, the more obvious this situation. As the temperature of the spring increases, its stiffness decreases, causing the safety valve to leak, and even the set pressure drops, causing the safety valve to open early.
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In order to reduce the number of samples tested, 50CrVA and 30W4Cr2VA, which are commonly used materials for safety valve springs, were selected for thermal stiffness testing. It can be seen from the test data that as the spring test temperature increases, the spring stiffness shows a downward trend, and the higher the temperature, the more obvious the spring stiffness decreases. Compared with the 50CrVA spring, the stiffness of the 30W4Cr2VA spring has a smaller drop.
04 Determination of differential pressure of safety valve cold test
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Relevant standards of safety valves stipulate that the cold test differential pressure is a correction for the temperature operating conditions, and also stipulates that the specific temperature correction coefficient should be consulted with the manufacturer. According to the temperature values measured at various parts of the safety valve and the relative spring stiffness values, the corresponding relative stiffnesses at different temperatures can be obtained.
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According to the spring temperature corresponding to different medium temperature, the relative value of spring stiffness of different materials is obtained by interpolation method according to the detection data of the relative value of spring stiffness. From the relative stiffness value, the differential pressure of the cold test of the safety valve at different working temperatures can be obtained. The maximum correction value of the differential pressure in the cold test of the safety valve should generally not exceed 3%.
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High temperature safety valve spring is made of 50CrVA material and the temperature should be controlled at 200~250℃, the heat preservation time is 24h, and its use temperature should not exceed 400℃. For 30W4Cr2VA material, the heating and pressure treatment temperature should be controlled at 300~350℃, the holding time should be 24h, and its use temperature should not exceed 540℃. When the working temperature exceeds 540℃, the safety valve spring material should be nickel-based alloy materials such as INCONEL718 or Inconel X-750.
05 Conclusion
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Based on the analysis of the influence of temperature in different working conditions on the spring stiffness of the safety valve, combined with the research and testing of the differential pressure of the safety valve cold test, it is determined that the correction value of the safety valve cold test differential pressure is related to its temperature, spring material and design structure. The structure design of the high temperature safety valve should consider the influence of the medium temperature. When the medium temperature is higher than 350°C, a heat insulation rack or a heat dissipation rack structure should be set between the valve body and the bonnet. The spring cavity is designed as an open structure to facilitate heat dissipation and reduce the temperature of the spring cavity. For different spring materials and different medium temperatures, the correction values are also different.
