The evaporation capacity of boiler includes rated evaporation and maximum continuous evaporation.  When the boiler load is higher the maximum continuous evaporation, it is called operating beyond output or overload operation. Some problems may occur when the boiler is operating beyond output. 

1. Due to increased consumption of fuel, the volumetric thermal load of hearth is increased accordingly, the flue gas temperature inside the boiler and at hearth outlet both rise, which will cause the overheating steam temperature, superheater and reheater pipe wall temperature to  rise, therefore the temperature shall be strictly monitored and adjusted and long –term over-temperature shall be avoided as possible. For the coal-fire boiler, due to the increased volumetric thermal load of hearth, the possibility of coking inside the boiler will be enlarged. 

2.The working medium flow velocity inside the vaporization system of boiler rises and the flow resistance increases, which is harmful to water circulation, therefore it is required to pay special attention to monitoring the part with poor water circulation. 

3. The working medium flow inside the superheater increases, the flow resistance will rise too, the differential pressure between the boiler drum to the superheater outlet is increased, making the pressure of boiler drum and header rise, the strength of these parts must be considered. 

4. The steam space load and evaporation load of the boiler drum are both enlarged, the water carried by saturated steam will be increased, thus affecting the steam quality. 

5. The total displacement of boiler safety valve is designed as per the maximum continuous evaporation, if the boiler is operating beyond the output, in case the load shedding occurs suddenly, although the safety valve is fully opened, it is hard to ensure the steam pressure to be reduced quickly, at this moment the steam bleeding valve shall be opened to release the steam to ensure the safety of boiler. 

6. The air volume required by combustion and the flue gas generating are increased both, therefore in case the induced draft fan and forced draft fan are both opened and the air amount is still insufficient, the working condition of combustion in boiler will be affected and the possibility of slagging will be increased; in addition, due to quickly rising of fume gas flow velocity, the wear degree of heating surface by flying ash will be aggravated.   

7. When the boiler is operating beyond output, the exhausting gas temperature will rise, the heat loss from exhausting gas will increase; the remaining time of fuel inside the boiler will be shortened, the heat loss from mechanically imperfect combustion and chemically imperfect combustion will be increased, thus the heat efficiency of boiler will be reduced.  

To sum up, the boiler operation beyond output will bring adverse effect to safety and economical efficiency, generally the boiler shall not operate beyond output, if it is urgently required specially, the range beyond output and the time of operating beyond output shall be strictly restricted.