Temperature Field Calculation & Thermo-solid Coupling Analysis of High-temperature Forged Gate Valves (Part Three)
Oct 26, 2021
5. The calculation of the thermo-solid coupling stress field of forged gate valves under high-temperature conditions
The temperature field obtained by the steady state thermal analysis has a significant influence on strain and stress in the structural analysis, but the structural response will not have a great influence on the thermal analysis results. Therefore, the thermal stress analysis involved in the thermo-solid coupling calculation of the forged gate valve is an indirect coupling. Given that the thermal-stress analysis only involves the continuous interaction between the temperature field and stress field, manual sequential coupling is adopted in this article. The total stress field calculation of the gate valve uses the same model as the previous steady-state thermal analysis, considering the working pressure. The working pressure, hand wheel torque as well as thermal stress caused by temperature gradient are considered. Calculate the total stress when the pressure component of the gate valve is subjected to the combined action of temperature loads and mechanical loads.
5.1 Boundary conditions and loads
In the setting of boundary conditions, the calculation results of the temperature field obtained from the above steady-state thermal analysis are inserted into the structural finite element model. The value is applied to the entire model as the boundary condition of the body load. The working pressure of the inner surface of the gate valve in contact with the medium is applied, that is, 0.72MPa, and the torque of 11.2Nm is applied on the valve stem. The application of the displacement boundary condition and that of the stress calculation under normal temperature conditions are the same. A section of pipe is added to the inlet and outlet ends of the valve body, and then a fixed constraint displacement boundary condition is applied to both ends of the pipe.
5.2 Calculation results and stress evaluation
According to the above boundary conditions and initial conditions, through finite element numerical simulation calculations, the total stress field of the gate valve composed of the thermal stress and structural stress is shown in Figure 10. For valves, strength analysis is an important guarantee for the integrity of the valve's pressure boundary and a necessary part of safety assessment. Figure 11 is the total stress cloud diagram of the valve body composed of thermal stress caused by temperature effects and structural stress caused by internal pressure. The greatest equivalent stress is 23MPa, which is the partial stress caused by the contact between the components, and belongs to the secondary stress. From the previous temperature field calculation, it can be seen that the temperature difference between the inner and outer surface of the valve body is small, so the thermal stress caused by this is small. It can also be seen from Figure 11 that the stress in most areas of the valve body is less than 10MPa, except for a few stress concentration points. The film stress is less than 1.5S, so it will not cause overall plastic deformation of the valve body. For the stress concentration point, the stress value is also less than 3s, which will not cause fatigue failure.
Figure 10 The cloud diagram of the equivalent stress of the total stress field of gate valves
Figure 11 The cloud diagram of the equivalent stress of the total stress field of the valve body
Figure 12 The cloud diagram of the equivalent stress of the total stress field of the bonnet
Figure 12 is the cloud diagram of the equivalent stress of the total stress field for the extended cylinder valve bonnet. From the previous calculation of the temperature field, it can be seen that there is a great temperature gradient along the vertical direction of the valve bonnet, so great thermal stress is generated. The maximum stress occurs at the gradient, which is as high as 252MPa. It is recommended to adopt filleted corner transition or arc transition here. In order to make the valve bonnet meet the strength requirements, the structure needs to be optimized to reduce thermal stress.
6. Conclusion
1) Establish a three-dimensional solid model for the high-temperature forged steel gate valve. Consider the characteristic of heat transfer under actual working conditions and set boundary conditions of temperature fields. Use Ansys steady-state thermal analysis function to calculate the temperature field of the gate valve, and obtain its heat transfer law and distribution of temperature field. The results show that the height of the valve bonnet's cylinder is sufficient, and the design area of the heat dissipation plate is reasonable, so that the temperature at the bottom of the stuffing box set and hand wheel is within the allowable range.
The temperature field obtained by the steady state thermal analysis has a significant influence on strain and stress in the structural analysis, but the structural response will not have a great influence on the thermal analysis results. Therefore, the thermal stress analysis involved in the thermo-solid coupling calculation of the forged gate valve is an indirect coupling. Given that the thermal-stress analysis only involves the continuous interaction between the temperature field and stress field, manual sequential coupling is adopted in this article. The total stress field calculation of the gate valve uses the same model as the previous steady-state thermal analysis, considering the working pressure. The working pressure, hand wheel torque as well as thermal stress caused by temperature gradient are considered. Calculate the total stress when the pressure component of the gate valve is subjected to the combined action of temperature loads and mechanical loads.
5.1 Boundary conditions and loads
In the setting of boundary conditions, the calculation results of the temperature field obtained from the above steady-state thermal analysis are inserted into the structural finite element model. The value is applied to the entire model as the boundary condition of the body load. The working pressure of the inner surface of the gate valve in contact with the medium is applied, that is, 0.72MPa, and the torque of 11.2Nm is applied on the valve stem. The application of the displacement boundary condition and that of the stress calculation under normal temperature conditions are the same. A section of pipe is added to the inlet and outlet ends of the valve body, and then a fixed constraint displacement boundary condition is applied to both ends of the pipe.
5.2 Calculation results and stress evaluation
According to the above boundary conditions and initial conditions, through finite element numerical simulation calculations, the total stress field of the gate valve composed of the thermal stress and structural stress is shown in Figure 10. For valves, strength analysis is an important guarantee for the integrity of the valve's pressure boundary and a necessary part of safety assessment. Figure 11 is the total stress cloud diagram of the valve body composed of thermal stress caused by temperature effects and structural stress caused by internal pressure. The greatest equivalent stress is 23MPa, which is the partial stress caused by the contact between the components, and belongs to the secondary stress. From the previous temperature field calculation, it can be seen that the temperature difference between the inner and outer surface of the valve body is small, so the thermal stress caused by this is small. It can also be seen from Figure 11 that the stress in most areas of the valve body is less than 10MPa, except for a few stress concentration points. The film stress is less than 1.5S, so it will not cause overall plastic deformation of the valve body. For the stress concentration point, the stress value is also less than 3s, which will not cause fatigue failure.
Figure 10 The cloud diagram of the equivalent stress of the total stress field of gate valves
Figure 11 The cloud diagram of the equivalent stress of the total stress field of the valve body
Figure 12 The cloud diagram of the equivalent stress of the total stress field of the bonnet
Figure 12 is the cloud diagram of the equivalent stress of the total stress field for the extended cylinder valve bonnet. From the previous calculation of the temperature field, it can be seen that there is a great temperature gradient along the vertical direction of the valve bonnet, so great thermal stress is generated. The maximum stress occurs at the gradient, which is as high as 252MPa. It is recommended to adopt filleted corner transition or arc transition here. In order to make the valve bonnet meet the strength requirements, the structure needs to be optimized to reduce thermal stress.
6. Conclusion
1) Establish a three-dimensional solid model for the high-temperature forged steel gate valve. Consider the characteristic of heat transfer under actual working conditions and set boundary conditions of temperature fields. Use Ansys steady-state thermal analysis function to calculate the temperature field of the gate valve, and obtain its heat transfer law and distribution of temperature field. The results show that the height of the valve bonnet's cylinder is sufficient, and the design area of the heat dissipation plate is reasonable, so that the temperature at the bottom of the stuffing box set and hand wheel is within the allowable range.
2) Use the same grid model and obtain temperature field results. Consider the working pressure, handwheel torque, pre-tightening force of bolts as well as thermal stress caused by the temperature gradient. Calculate the total stress when the gate valve is subjected to the temperature load and the mechanical load. The stress evaluation of the pressure-bearing parts shows that the valve body can prevent elastic failure and partial failure. The valve bonnet has a large area of partial thermal stress, which will cause partial plastic deformation. From the temperature field results, it can be seen that the temperature difference between the inner and outer wall surfaces of the extended cylinder is very small, which is not the cause of the great thermal stress. It is difficult to reduce the thermal stress by changing the wall thickness. The main reason is that there is a great temperature gradient in the cylinder direction. Therefore, the structure of the cylinder direction needs to be changed.
3) In the design stage, the overall temperature and stress distribution of the gate valve are obtained by the finite element, which provides the structural design and improvement of the gate valve with accurate data and theoretical basis, reduces the cost of subsequent prototype trial production and testing, and shortens the product's development cycle.
Next: The Structural Design of Forged Slab Gate Valves without Welding Seams
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