Deaerator pressure control system is one of the important control system in a power plant. It controls the extraction steam to the deaerator which directly impacts the temperature of the deaerator. I.e. this pressure impacts on heat transmission of steam to water to remove the dissolved gases. It also helps in venting the dissolved gases i.e. it avoids mixing of separated gases once again with the water.
Deaerator Steam Lines
Normally deaerator is provided with two paths of steam lines to maintain the redundancy. i.e. if any of the path fails the next will provide the required stream. The diagram shows the possible steam extraction lines to deaerator.
The first steam line is from IPT (intermediate Pressure turbine) extraction point. This line provides steam during normal turbine operating conditions. The second steam path is from the turbine bypass which provides the steam to the deaerator in the case of turbine trip. An auxiliary steam supply is also provided to maintain the pressure during starting or in the case of unavailable of two steam paths. This path is controlled manually by the operator.
Two Pneumatic valves will be used to control the steam in the two live steam paths. An automatic logic will be provided to open the valve in one path if the second path steam is not available. Once the valve opens it is modulated to control the pressure by the pressure control system.
Deaerator pressure controller
Two valves are modulated pneumatic type which are operated by the digital control system. normally this PI controller is used to modulate the valve. The basic arrangement of control system is shown in figure.
The deaerator saturation pressure is measured with three pressure transmitted. 2/3 voting logic is used to consider the pressure logic at the input of the control system. An Auto/Man station is also provided to take the control in to Manual in abnormal condition. The selection and manual throttling process is normally done by the plant DCS (Distributed Control System).
The actual pressure is compared with the set-point fed by the operator by OWS (operator work station) and generates an error. The error is fed to the digital PI controller to produce new demand according to the demand. This is conventional PI controlled with the Kp and Ti constants. There will be an option to tune the controller also by changing the parameters.
The output digital demand is converted to pneumatic pressure control to modulate the control valve.