1.External disturbance:

Change of steam pressure caused by external load change is called external disturbance. When external load increases, the steam consumption of the unit increases, but the boiler has not been adjusted to adapt to the new conditions, the boiler  evaporation capacity will be less than the demand for steam from outside, the mass balance is broken, the steam pressure drops. 

2.Internal disturbances: 

Change of steam pressure caused by the working condition change of boiler itself is called internal disturbances. In the operation, the external demands to steam is does not change, but due to changing of the working condition of boiler combustion (such as combustion instability or fuel quantity and airflow changes) and the changing of the working condition inside the boiler( such as heat transfer conditions change), making the steam production quantity from the evaporation area change, the mass balance between the boiler evaporation and steam demands is destroyed, so that the steam pressure changes.