Problems Occurred in High-temperature and High-pressure Valves

With the rapid development of thermal power, petrochemical, nuclear power, aerospace and other fields, the application of high-temperature and high-pressure valves has become more and more extensive, but the following problems have also appeared.
(1) Long-term operation under high temperature and high pressure and alternating working conditions will cause the valve to creep, resulting in thermal fatigue and plastic deformation, which will lead to the failure of the valve's moving parts and sealing structure.
 

Figure 1 The distribution of hardness of samples under different cryogenic treatments


Figure 2 The distribution of elongation and yield strength of samples under different cryogenic treatments
 
(2) The transient thermal stress may exceed the maximum limit of the steady-state design, resulting in insufficient strength of the valve.
(3) Under the working conditions of high-temperature and high-pressure difference, the liquid will gasify and generate bubbles. When the bubbles burst, air hammer will be generated, resulting in great pressure, cavitation, strong vibration and noise; the performance and service life of the valve will be worse and decreased.
In response to the above problems, domestic and foreign workers have carried out a lot of research and improvement.
 
1. Improvement in materials and structures
Starting from the valve's material and structural design, the moving parts and sealing structure of the valve are improved and perfected. Some scholars have carried out finite element analysis on the valve at high temperatures and found that the deformation of the parts with high temperatures is greater, while the deformation of the parts with low temperatures can be ignored. When conducting thermal stress analysis, WU and others found that the valve core and valve seat will deform greatly under high temperatures, resulting in insufficient fitting clearance, thereby increasing the friction between the two and causing the valve to fail to work properly. It can be seen that in the environment of the high-temperature medium, the valve is prone to thermal expansion, which causes uneven changes in the dimensions of each component, and reduces the fitting clearance between the various components of the valve, resulting in wear of moving parts, loose bolts, blockage and leakage. At the beginning of the design, the thermal expansion of the material at high temperatures should be fully considered.
 
First, select materials with small expansion coefficients, and try to select materials with similar expansion coefficients for each component of the entire valve to ensure that under high temperatures, the amount of deformation of each component tends to be the same; avoid excessive deformation of individual components. Second, attention should be paid to the high-temperature creep, hardness, impact resistance and other mechanical properties of the valve material to cope with the long-term wear and impact by media for the valve under high temperatures, and improve the reliability and safety of the valve. Some scholars have summarized the materials that can be used for high-temperature valves at different temperatures, as shown in Table 3.
 
Table 3 Optional materials for high-temperature valves at different temperatures

 
Please note that when the operating temperature exceeds 1000℃, it is only used when the carbon content is greater than or equal to 0.04%.
 
For sealing structures, graphite can be used instead of traditional rubber sealing, such as expanded graphite packing (Figure 4) and reinforced graphite packing, and the maximum working temperature can reach 600℃. They can still play a good sealing effect even if they are deformed at high temperatures due to their soft texture, and their chemical properties are stable. Graphite gaskets with metal inner reinforcing rings and outer locating rings can be selected to improve the locking force and sealing strength. The elastic seal can be selected, and the corresponding compensation adjustment is given by the spring to ensure the safe operation of the valve. Stellite alloy and chromate boron alloy can be selected as the sealing surface of the valve. The variation law of hardness changed with temperatures is shown in Figure 5. At a temperature of 500℃, the hardness of Cr14 and 9Cr10Mo decreases sharply, while the hardness of Stellite alloy and chromate boron alloy can still be kept at a temperature of 700℃, which can effectively avoid the failure of the valve due to the plastic deformation of the sealing surface.


Figure 4 The expanded graphite packing
 
2. The simulation analysis of working conditions
The three-field coupling of fluid, temperature and structure is analyzed based on the finite element analysis software, and the actual working state of the valve is restored for analysis. The finite element analysis method is mainly divided into single field analysis and coupled field analysis. The single field only considers the temperature field to analyze the valve. Weishu Cheng used ANSYS to analyze the temperature field of the right-angle globe valve under high temperatures and high pressure, obtained the temperature field distribution of the dangerous part of the valve, and adjusted the material and wall thickness of the valve according to the thermal position and peak value to improve the safety of the valve. Yujie Li analyzed the different parts of the electric control valve from three perspectives such as the steady-state temperature field, thermal stress field and thermal deformation field. The thermal stress at the packing is the greatest, followed by the copper guide sleeve. The bolt head material with better mechanical properties should be used, and the heat sink structure can effectively reduce the valve temperature. Qingzhong He and others used the Singhal cavitation model and the mass transfer equation to analyze the effect of thermodynamics on the cavitation of the control valve. The study found that the pressure reduction process of the control valve will cause part of the energy conversion and the absorption of latent heat of vaporization. As the temperature of the medium increases, the temperature rise in the throttling cavitation area increases, and the influence range of cavitation expands, as shown in Figure 6.
 

Figure 5 The variation trend of the temperature of each component at different temperatures
 
Although the temperature field can obtain the temperature distribution and the micro-deformation of the valve caused by thermal stress, it is only analyzed for a single factor, and the influence of factors such as structure and fluid are ignored. At present, most of the temperature field analysis is a steady-state thermal analysis, that is, maintaining a fixed temperature and obtaining the distribution of the valve temperature field only, but cannot obtain the temperature changed with time. Therefore, for the temperature under special conditions such as opening and closing of the valve, the field distribution cannot be quantitatively analyzed, and the influence of the actual comprehensive stress on the structural strength of the valve cannot be obtained.
 
The coupling field involves a two-field coupling analysis of heat-solid and a three-field coupling analysis of heat-fluid and solid, that is, after the distribution of the temperature field is obtained, it is used as a boundary load for stress field analysis. Jinliang Liu used Workbench to carry out the coupled stress under the pressure field and temperature field, and determined the minimum wall thickness under the combined action of thermal stress and compressive stress. Peng Lin analyzed the temperature field and stress field of the valve body of the steam turbine bypass valve under the standby and operating conditions, and the comprehensive stress and temperature gradient of the valve body at the valve inlet under the standby condition were the greatest. The distribution of the comprehensive stress value under operating conditions over time is shown in Figure 7. The comprehensive stress of the valve body first decreased, then increased; it decreased again until it was stable. The maximum stress was 85.6MPa. Shuxun Li used Fluent and ANSYS software to analyze the valve strength and fatigue under transient thermal shock using the heat-fluid-structure coupling method. The results showed that the transient thermal shock under pressure has a great influence on the temperature field, structural strength, fatigue life and sensitivity of the valve body. The variation of valve stress with time under thermal shock is shown in Figure 8.
 

Figure 6 Vaporization volume fraction under different pressure


Figure 7 The distribution of comprehensive stress value changed with time under operating conditions
 

Figure 8 Valve stress changed with time under the thermal shock
 
Compared with single field analysis, coupled field analysis can combine the actual situation, consider multiple issues such as fluid impact, thermal stress and structural strength, and conduct a more accurate analysis of service life and fatigue strength of valves. However, it is easy to have no solution due to the complex superposition of the composite field, the problem of setting boundary conditions, and a large amount of calculation. However, it is still undeniable that the heat-fluid-structure coupling has a great role in promoting the design and application of high-temperature and high-pressure valves.