Steam pressure and temperature are the key parameters of power generation for thermal power plants and also for process application. Steam pressure and temperature decide the amount of heat available, in the form of enthalpy, to do work in turbine. A drop in both will reduce the plant heat rate in different magnitude. Many boiler operators prefer to operate the boiler at slightly lower pressure, when they have load variation, fuel quality variations, etc, as they get more margins between the safety valve set pressure and the operating pressure. Main steam pressure is used as the input to master pressure controller to change the fuel input to the boiler.
The main reasons for variation in steam pressure when the boiler operates with steady conditions are:
- Sudden increase of generator load
- Mal-operation of steam pressure controller
- Coal hang up in mills
- Disturbed combustion condition in furnace
- Tripping of one or more mills
Sudden increase in load can be due to grid demand or due to process requirement in the plant. Coal hang up in coal mills is is not uncommon due to the presence of foreign material in coal or even due to high moisture coal. Disturbed furnace condition can be due to many reasons.
The plant responses due to these conditions give indication to the boiler desk operator and local operator to take immediate action. Normally the turbine will slow down due to the braking effect of the generator. During this time the steam demand signal will go up reducing the pressure further if the fuel input does not increase. As the main steam pressure drops, there is a good reason for the auxiliary steam pressure to drop. This will depend on many other operating conditions from the turbine and the boiler side.
Immediate operator action will depend upon the cause that resulted in the reduction of steam pressure. The first action of the operator will be to check and reduce the turbine generator load so that the steam pressure does not drop to a very low level. The operator will switch to manual mode for pressure control if he suspects the pressure controller is misbehaving. This he can infer from the erratic way in which the pressure control is behaving even before the main steam pressure drops continuously or otherwise. Starting of the spare mill will be required if any of the operating mills have tripped. He first will reduce the load if possible and bring in the new mill immediately. During this disturbance the boiler opreator will have to keep vigil on the water level in the boiler drum. If this level drops more and reaches the trip level, then he will lose the unit and will have to line-up for start-up again.
The local operator will have to quickly check if the coal mill feeder is delivering the coal in all the operating mills. Check for any coal hang up in any of the operating mills. He has to also check the furnace conditions and inform the desk operator. If he has to line up another mill to start, he has to inform the boiler control room and prepare the mill for starting.