How to determine the design temperature of pressure vessels

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Design temperature: Last time we discussed how to determine the design pressure of pressure vessels. As another important parameter for pressure vessels, how should the temperature be determined? Similarly, let’s first take a look at the definition of design temperature: “The metal temperature of a component, as specified under normal operating conditions (the average temperature across the metal cross-section of the component).” The design temperature, together with the design pressure, serves as the design load condition. ”--Excerpt from GB/T 150.1 \"Pressure Vessels\": Tw is the operating temperature, and T is the design temperature. Determination principle 1. Obtained through heat transfer calculations ； 2. Measure on similar containers that have been used ； 3. Determined according to the internal medium temperature ； 4. The design temperature shall not be lower than the highest temperature that the metal of the component may reach under operating conditions ； 5. For metal temperatures below 0°C, the design temperature shall not be higher than the lowest temperature that the component metal can reach ； 6. Under no circumstances shall the metal temperature exceed the allowable operating temperature of the steel ； 7. For vessels under external pressure, the maximum temperature shall not exceed that indicated on the curve of external pressure stress coefficients in GB/T 150.3 ； 8. When the metal temperatures of different parts of the container vary under operating conditions, the design temperatures for each part can be set separately ； 9. If there are two or more different operating conditions, such as the jacket being able to be heated by steam or cooled by cooling water, the design shall be carried out separately for each combination of design pressure and design temperature corresponding to those conditions, and the design pressure and design temperature for each condition shall be specified in the design documents. Method for determination: 1. When the inner wall of the container is in direct contact with the medium and there is external insulation (or cooling), in the case of steam, we can use the saturated vapor pressure at the corresponding pressure. For example, the temperature of saturated water vapor at 0.2 Mpa is 133°C; if the design pressure is set at 0.3 Mpa, then the corresponding saturated vapor temperature is 143°C, so we can use 143°C as the design temperature. If the design pressure is 0.25 Mpa, the corresponding saturated vapor temperature is 138°C, in which case we can use 138°C as the design temperature. 1) When the operating temperature Tw is less than -20°C, T is taken as the operating temperature minus 0 to 10°C, generally being slightly lower than the operating temperature ； For example, if Tw = -30℃, then the design temperature T can be set at -35℃ ； 2) When the operating temperature Tw is less than 0°C but greater than or equal to -20°C, T is taken as the operating temperature minus 5 to 10°C, with a minimum value of -20°C; for example, if the operating temperature Tw is -10 degrees Celsius, then the design temperature T can be set at -15℃ ； 3) The operating temperature Tw is greater than 0°C; T is taken as the operating temperature plus 5 to 30°C, with a minimum value of 20°C. 20°C can be considered a normal temperature that we encounter frequently ； 2. When the medium in the container is heated directly by steam or indirectly by built-in heating elements (such as heating coils, electric heating elements, etc.), the design temperature T shall be taken as the highest operating temperature of the medium. 3. When the pressure-bearing components of a container are in direct contact with media at different temperatures on both sides, the design temperature of such components should be determined based on the operating temperature of the more stringent side (such as high temperature or low temperature). For example, in the case of a container with a jacket, where steam is used to heat the medium inside the container, the design temperatures for both the interior of the container and the jacket should be determined based on the operating temperature of the steam. 4. Storage containers installed outdoors without insulation: when the minimum design temperature is determined by the local environmental temperature, 1) for storage containers holding compressed gases, the minimum design temperature can be taken as the lowest value of the monthly average minimum temperatures minus 3°C; 2) for storage containers in which the volume of liquid occupies more than 1/4 of the container’s total volume, the minimum design temperature can be taken as the lowest value of the monthly average minimum temperatures; 3) for vertical cylindrical oil tanks, the minimum design temperature can be taken as the lowest daily average temperature in the area where the tank is located plus 13°C℃ ； 5. Skirt of outdoor tower-type containers 1) Skirt without a transition section: When the design temperature of the tower (or its bottom section) is between -20~200°C, the design temperature for the skirt shell is determined based on either 50°C, or the lowest value of the average monthly minimum temperatures recorded over the years in the area of use plus 20°C ； When the design temperature of the tower (or bottom of the tower) is between 200 and 350°C, the design temperature of the skirt shell is taken as the design temperature of the tower (or bottom of the tower) ； 2) Skirt with transition section: The design temperature for the skirt shell and the skirt transition section must be determined separately. Skirt housing – two design conditions: 50°C, or the minimum value of the annual average monthly minimum temperatures in the area of use plus 20°C ； Skirt transition section – design temperature of the tower (or reactor bottom) ； 6. The inner wall is lined with insulation material; it is advisable to determine this value through heat transfer calculations, and to include a certain margin as the design temperature for the container’s metal shell (the pressure-bearing component). 7. Shell-and-tube heat exchangers 1) The design temperature of the tube side refers to the design temperature of the tube box (not the design temperature of the heat exchange tubes). 2) The shell-side design temperature refers to the design temperature of the shell side of the vessel. 3) The design temperature of the tube sheet and heat exchange tubes shall be determined based on the operating temperature of the more stringent side (such as high temperature or low temperature) as the reference for the design temperature of these components. 8. Bolts (stud bolts): 1) When the flange and associated bolts (stud bolts) have an insulating (or heat-insulating) layer on their exterior, the design temperature for these bolts (stud bolts) should be taken as the design temperature of the container. 2) When there is no insulation (or cooling) layer on the outside of the flange and associated bolts (stud), if the operating temperature is less than 65°C, the design temperature for the bolts (stud) should be taken as the design temperature of the container ； If the operating temperature is 65°C or higher, the design temperature for bolts (stud bolts) should be at least 80% of the container’s operating temperature, or it may be determined through heat transfer calculations. 3) When there is insulation material on the inner wall of the flange, the design temperature of the bolts (stud bolts) should preferably be determined through heat transfer calculations, with a certain margin added to it as the design temperature for the container’s metal shell (the pressure-bearing component). Note: When the maximum (or minimum) operating temperature approaches the allowable temperature limit for the selected material (or when the material transitions to another grade), care should be taken in selecting the design temperature margin to avoid material waste or reduced safety.