1. The electric governing valve of water intake is in failure. 

2. The water intake amount is too less or the pressure is too low. 

3. The water draining valve is opened by mistake or is not closed tightly. 

4. The water supply valve is opened by mistake or the water supply system and boiler economizer etc. leak. 

5. The load of the unit suddenly increases. 

6.The condensation pump and relay pump are in failure or the valves of the condensation water system and relay water system is closed by mistake or the valve spool falls off( applicable to water level decreasing of high pressure of deaerator).