The significance of the minimum design metal temperature for the equipment
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I would like to ask the experienced colleagues: In what main aspects does the minimum designed metal temperature affect the equipment? I don’t understand why this value is set as an input in the ASME calculation software; I’m not aware of what impact this minimum design temperature has on the calculations for the equipment.
At different temperatures, it has an impact on the use of metals, as it can easily lead to instability! Instability leads to the failure of the container!
Reply to 2# jindawei2: I still don’t quite understand, as the range of applications for the materials I selected is much greater than the minimum designed metal temperature. Why does it become unstable?
It is not a problem of equipment stability, but rather a problem related to the toughness of the material. For carbon steel and low-alloy steel, the low-temperature toughness of such materials is directly related to the material’s grade and thickness; varying these factors and conditions results in different minimum temperatures at which brittle transition occurs. For example, the requirement for the temperature during hydrostatic testing in our standards is based on such considerations. It is recommended that the original poster give more consideration to this aspect.
Many provisions in ASME are related to the Minimum Design Metal Temperature (MDMT). The MDMT is specified to prevent brittle fracture of the material at low temperatures; generally, impact tests must be successful at temperatures below the MDMT in order to indicate that the material has sufficient toughness.
What was said on the 5th floor is correct; MDMT is provided mainly to prevent the material from fracturing at low temperatures. However, if an impact test is required, the test temperature must be ≤ MDMT, rather than being “generally required”. Of course, UCS-66 provides a method for determining whether each stressed component requires impact testing; impact testing can also be avoided by increasing the wall thickness (i.e., reducing stress). In the calculation manual, MDMT serves as a criterion for determining whether an impact test is required ； Marking on the diagram indicates that when the equipment is operating at its design pressure, the operating temperature must not be lower than MDMT.
At low temperatures, the toughness of the material needs to be considered, not its stability
According to ASME regulations, the MDMT must be clearly indicated on the equipment nameplate. The significance of the MDMT parameters is to prevent brittle failure accidents of the equipment during use or hydrostatic testing. It cannot be assumed that the toughness of ferritic steels is solely related to temperature; therefore, the MDMT is related only to material selection. The determination of MDMT is related to factors such as plate thickness, design temperature, and stress levels; therefore, it is an essential parameter in vessel design.
It is recommended to read these materials carefully: ASME Section VIII-Ⅰ, Volume UG20(f) and UCS66. The regulations regarding the minimum design metal temperature are all based on painful accident lessons. The toughness of a material decreases as the temperature drops; when the operating temperature falls to a certain level, the toughness of the material drops sharply, leading to brittle fracture. Additionally, the thicker the “thickness” of the material, the lower its toughness becomes. This “thickness” is the control thickness; Volume VIII-I’s UCS66(a) provides a rigorous calculation method for this thickness.
It is mainly to consider low-temperature impact performance