1. That the feed water regulating system is in failure or the feed water control valve breaks down are not found by the operator and are not handled by the operator in time. 

2. The incorrect indication of water level gauge, steam flow meter or feedwater flow meter makes the operator misinterpret and incorrectly operate. 

3. The boiler load drops suddenly and the feed water is not adjusted timely. 

4. The feed water pressure suddenly rises. 

5. The operators are negligent, they do not monitor the water level very well, do not adjust it in time or make incorrect operation.