During operation, the steam pressure changes, it is required firstly to determine that the reasons belong to internal or external disturbances, then the changes of steams pressure can be properly adjusted. The change of steam pressure is always closely related to the steam flow, therefore, according to the relationship of steam pressure and steam flow changing, it can be determined the reasons of the change of the steam pressure are internal or external disturbances. 

1. When the steam pressure reduces, at the same time the steam flow meter indication increases, showing that  the demands on steam from outside world is increased; When the steam pressure rises, at the same time the steam flow rate decreases, showing that the demands on steam from outside world is reduced, these all belong to external disturbance. That is to say, when the changing direction of steam pressure and steam amount is opposite, the reasons of steam pressure changing belongs to external disturbance. 

2. When the steam pressure reduces, at the same time, the steam flow rate decreases, showing the heat supplied by fuel combustion inside the boiler is not enough , thus causing the evaporation to decrease; When the steam pressure rises, at the same time, the steam flow rate increases, showing that the heat supplied by fuel combustion inside the boiler is slightly large, thus causing the evaporation to increase, these belong to internal disturbances, that is to say, when the changing direction of steam pressure and steam amount is the same, the reasons of steam pressure changing belongs to internal disturbance. 

It is required to note that for the unit, the methods above mentioned for judging internal disturbance applies only to early working condition changes, that is to say, it is only suitable for steam turbine speed control valve before action, and after action of speed control valve, the changing direction of boiler steam pressure and steam flow direction is opposite, so during operation attention shall be paid. Causes for the above special cases are: The external load does not change, but the boiler combustion capacity suddenly increases( internal disturbance) ,initially while the steam pressure rises, the steam flow also increases. In order to maintain the rated speed of steam turbine, the speed regulating valve will be turned down, at this time, the steam pressure will continue to rise, and the steam flow rate decreases, and that is to say the changing direction of steam pressure and flow is opposite.