1. The water-supply flow is abnormal, it is greater than the steam flow. 

2. If the pipe is seriously damaged, the steam pressure drops and the temperature rises. 

3. The negative pressure of hearth  reduces abnormally or becomes positive pressure, the current of the induced draft fan increases, in serious case, the flue gas emits from the place of the boiler wall that is not tightly sealed. 

4. The flue gas temperature at leakage position reduces or the temperature difference at both sides increases, the temperature of exhaust gas increases. 5. The leakage place has abnormal noise.