The design minimum temperature is required to determine the impact test requirements of ASME B31.3. As the temperature is decreased, the notch toughness of the material drops. Thus, impact tests are required under conditions of sufficiently low temperature and high stress to ensure that the material has adequate toughness to avoid a brittle fracture.
In establishing the minimum temperature, one must consider the lowest component temperature that can be expected in service. This can include effects such as auto refrigeration (the cooling that occurs due to depressurization) and low ambient temperatures.
With the inclusion of stress as a consideration in the need to perform impact testing, it is possible that more than one minimum temperature, each with different stress conditions, will require consideration. This is because the loads may be different at different low-temperature conditions, and lower temperatures with-out impact testing may be permitted at lower stress conditions. #Little_PEng