Many combined cycle power plants have experienced thermal shock damage to bypass power piping, especially in the HP steam and Hot Reheat systems. Severe shocks occur when inadequate drainage exists and liquid water accumulates in bypass piping, for example during periods of operation when the bypass is closed or during shutdowns. Startups where the bypass is opened then allows the accumulated liquid to be picked up and carried at high velocity downstream where it impacts on the internal surface at a change in piping direction, imparting a significant load and associated stress. Low load operation is another means by which very high attemperator spray can fail to be absorbed by the steam and consequently come in contact (in liquid form) with the pipe wall.
As combined cycle gas turbine plants are called upon to play a larger and more flexible role in the generation mix, it’s important to schedule a comprehensive assessment of major components at key intervals to ensure reliable operation.