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.
Figure 1: Thermal Shock Crack in Hot Reheat Bypass Piping Downstream of Spray Valve
Good control logic and operator awareness is crucial to ensure that spray water doesn't build up when valves are closed, that sprays are not exceeding their proper flow range or that there is over-spraying into the steam. Since these thermal impingement/shock induced cracks initiate on the steam-side (ID), a volumetric crack detection using ultrasonic testing (UT linear phased array or shear wave) inspection is the best NDT inspection technique. Magnetic Particle (MT) inspection can be used as a supplement or independent method although it is not as likely to identify small i.d. initiating flaws.
Figure 2: Thermal Shock Induced Mid-Wall Cracking – Inter-Stage Attemperator Piping