The longitudinal stress due to sustained loadings such as pressure, weight and other mechanical load is generally limited to the basic allowable stress at design temperature, Sh. Where Sc is the basic allowable stress at ambient temperature. If SE1 and SE2 have different signs, then the stress range will be the absolute sum of SE1 and SE2. This very often is not evaluated properly by some of the computer programs. Therefore the combination is quite complex.
Many programs just calculate the stress for each occasional load, and compare it with the allowable.
This is inadequate in most of the cases. Another purpose of piping stress analysis is to maintain the system operability. This function is very often overlooked by stress engineers. In reality, this is very important and is much more difficulty to achieve than to ensure the structural integrity. Although, a pipe may not break under design operating conditions, if it is stressed to the extent that will cause the connecting equipment to deform excessively or the shafts to go out of alignment then the operation of the system can not be maintained.