Deaerator level control

Deaerator

The deaerator is an important feed-water system component which is used to remove the dissolved gases like oxygen and carbon dioxide in the feed water. The presence of theses gases in the feed-water which will enter in boiler or steam generator can causes a rapid corrosion in the tubes. So the deaerator uses the following techniques to dissolve the gases.

 

1) If the partial pressure of the liquid reduces then gas dissolved content will be reduced. This done by spraying the feed-water in deaerator.
2) If the temperature of the solution rises  up to saturation temperature then the solubility content will reduces. This is that the feed-water is spared in the presence of high temperature steam and gets heated up so that the dissolved gases will flow through the deaerator vents. After the  above process the feed water will be collected in deaerator storage tank from which it is supplied to steam generator (boiler) through Boiler Feed water Pump (BFP).

Deaerator Storage Level

The Feed water after spraying and heating, it is collected in deaerator storage tank. The deaerator with inlet water, steam and outlet water paths is shown in below figure.

deaerator flow diagram

deaerator flow diagram

The condensated water from the hot well is fed to the deaerator through the condensate extraction pump. The steam consists of two paths, one is extraction steam from main steam and other one is the drainage of the feed water high pressure heaters (HPH 5&6). The output is connected to BFP to supply the feed water to steam generator. So the water storage in the tank depends on the input and output flows from the deaerator, i.e. if the inlet is more compared to outlet then the level is increases and in other case the level decreases. To control this level it is envisaged to control the inlet flow from condensate which gives better results.

effects of deaerator level

Deaerator high level: If the water in deaerator storage tank level increases then the pressure in the deaerator increases which effects the inlet steam flow. This affects heating of inlet water which reduces the temperature of the water. So the removal of dissolved gasses will be disturbed.

Deaerator Low level:  The low level may not have significance affect on the deaeration process but due to this low level the outlet flow may reduce which is delivered to steam generator. There will be a trip sigal to BFP on deaerator low level. So the deaerator level should be maintained.

Deaerator storage tank level control loop

deaerator level control system

deaerator level control system

In a power plant the deaerator control loop is one of the important control system which controls the CEP outlet flow by throttling the CEP outlet valve. The arrangement of control loop schematic is shown in below figure.

The control loop gets three measurement inputs which is called as three element deaerator level controller. The three elements of the controller are:

  1. Deaerator Level control
  2. Feed water to the boiler(steam Generator)
  3. Condensate water to the deaerator.

In earlier days it is only one element (deaerator level) controller i.e. The level is the measured value and controller throttles the valve according to this level to match to the set value. But during transient conditions the performance was poor so it is replaced with the three  element controller.

Three element controller

In this controller the difference between feed water and condensate water acts as main controller element and the deaerator level acts as fine control element. This combination provides the better control of level during transients. block diagram of the controller is shown in below figure.

Deaerator level three element control system

Deaerator level three element control system

Let us consider the combined inlet flow i.e. extraction steam, Heater drainage and condensate flow is raised to 120%. This affects the level of the deaerator in which the outlet is constant at 100%. The difference 20% is sense by the controller as a first element and reduces its control output which closes the CEP outlet valve so that the flow reduces. This prevents the increase in deaerator level i.e. it acts as a feed forward controller and counteracts for the increase in level. This results very small raise in level which is course controlled by the level controller as a second element controller.

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