During my experience in power plant construction, an issue was raised regarding mechanical equipment design criteria. Unfortunately, the project specification stated that each equipment shall suffer at least 20.000 thermal cycles in 25 years of operation. However, the calculation of the number of the thermal cycle depends directly on the definition of the thermal cycle. For instance, as per ASME Codes, the mentioned cycle is described as follow:
"15. Operational Cycle: An operational cycle is defined as the initiation and establishment of new conditions followed by a return to the conditions that prevailed at the beginning of the cycle.
Three types of operational cycles are considered:
- the startup-shutdown cycle, defined as any cycle which has atmospheric temperature and/or pressure as one of its extremes and normal operating conditions as its other extreme;
- the initiation of, and recovery from, any emergency or upset condition or pressure test condition that shall be considered in the design;
- and the normal operating cycle, defined as any cycle between startup and shutdown which is required for the vessel to perform its intended purpose."
Therefore, the ASME Codes are referring to the Operational Cycles. I hope that InnoCommunity can provide a statement regarding this issue and specify which standard is giving a clear definition of a thermal cycle.