Design conditions in ASME B31.3 are specifically intended for pressure design. The design pressure and temperature are the most severe coincident conditions, defined as the conditions that result in the greatest pipe wall thickness or highest required pressure class or other component rating. Design conditions are not intended to be a combination of the highest potential pressure and the highest potential temperature, unless such conditions occur at the same time. Although it is possible for one operating condition to govern the design of one component in a piping system (and be the design condition for that component) and another to govern the design of another component, this is a relatively rare event. If it is encountered, the two different components in the piping system should have different design conditions.
It is the metal temperature that is of interest in establishing the design temperature. Thus, the design temperature does not necessarily coincide with the temperature of the process fluid. In addition to fluid temperature, other considerations include ambient cooling, ambient heating, solar radiation, and maximum heat tracing temperature.
For an insulated pipe, the metal temperature is generally considered to be the fluid temperature, unless some other means of heating is a consideration, such as the presence of steam tracing, which could potentially heat the pipe to the steam temperature. For uninsulated pipe, the metal temperature is taken as the fluid temperature if the temperature is below 65°C (150°F), unless some other effect, such as solar heating of the pipe, could result in a greater temperature. A commonly assumed potential temperature due to solar heating for piping exposed to the sun is 50°C (120°F); local conditions may dictate a higher temperature. For an uninsulated pipe, the Code permits consideration of ambient cooling effects if the fluid temperature is 65°C (150°F) or greater. Of course, if solar radiation can bring the metal temperature to higher than this temperature, as in Saudi Arabia, ambient cooling effects could only be considered at some higher temperature. The metal temperature of the pipe can be determined via heat transfer calculations or testing, such as measurement of the temperature on a similar system, or based on presumptive values given in the following table, which provides conservatively high estimates of wall temperature: