4th Class Power Engineering 4B7 Practice Exam

Thermal shock happens when boiler metal is heated or cooled too quickly, causing thermal stress as different parts expand or contract at different rates. If hot surfaces are suddenly cooled by cold feedwater or if a cold shell is exposed to a rapid heat input, the resulting temperature gradients can crack tubes, welds, or the shell and lead to failures.

Prevention focuses on controlling heat input and reducing temperature differences. Heat up slowly to operating temperature, avoiding abrupt changes in firing rate or steam demand, and preheat feedwater so the water entering the boiler is closer in temperature to the boiler water. Good startup and shutdown procedures, along with maintaining proper circulation, help keep temperatures uniform and prevent localized overheating. While pressure changes can occur during startup, the main cause of thermal shock is rapid temperature change, so gradual heat-up and feedwater preheating are the key safeguards.