The design daerator should consist of the dearating unit and a feed water storage vessel. A feed water storage vessel should be provided within the feed water heating system. The dissolved oxygen content in the feed water effluent from the heater should not be more than 0.007 mg/liter at any load condition, measured in accordance with the “Method and Procedure for the Determination of Dissolved Oxygen” of the Standards of the Heat Exchange Institute.
The feed water storage vessel should be integrated with a deaerating unit to fully de-aerate the feed water, if an alternative water chemistry regime is proposed the design should substantiate his provisions for control of dissolved oxygen in the feed water system.
The main functions of the feed water vessel should be to:
- Condition feed water for start-up
- Provide a reserve to compensate for fluctuating feed flows
- Ensure boiler feed pump suction requirements are met at all times
- Removal of oxygen via a steam heating/de-aerating process
Whichever arrangement of feed water / deaeration vessel in the design deaerator or feed water storage vessel to provide a full description of the normal function, including:
- Level indication and level control
- Conditioning of feedwater prior to start-up (cold start)
- Flows of condensate, bled steam, auxiliary heating steam
- System responses to transient conditions
- Any specific arrangements proposed for part load operation
- Disposal of scrubbed or vented non condensable gases
The design deaerator or feed water storage vessel should state the provision for deaeration and heating the stored water on plant starts when the LP heater is out of service. The description should also describe the provision included to monitor and control the condensate level within the storage tank and any recirculation system if necessary to ensure homogenous conditions in the stored water.
The feed water storage vessel should be located at or as close to the turbine operating floor level as possible, consistent with satisfying the feed pump net positive suction head (NPSH) requirements. The tank should store a minimum quantity of feed water corresponding to 7 minutes of rated (MCR) feed water flow or that quantity of feed water which should permit a controlled and safe shut down of the boiler, whichever is greater and assuming that the condensate is initially at the normal working level.
The feed water storage vessel should be designed to operate with freedom from condensate surging and vessel vibration. The freeboard above the top of the working level range should be sufficient to accommodate the total condenser hot well content with margin.
Boiler feed pump leak-off returns should be introduced into the feed water storage tank in a controlled manner to prevent damage from high velocity evolved steam or water impingement. At all other points when steam or water enters the deaerator / feed water storage vessel, suitable precautions such as baffles or diffusers should be provided to prevent direct impingement on the tank plates, internals or water surface, internal baffles should be arranged within the feed water storage tank to prevent surging of the condensate.
Provisions to protect the steam turbine from the risk water induction arising from any bled steam pipe work connecting the deaerator / feed water storage vessel should be as stated in Section “Bled Steam supply Lines”.
The deaerator should be of the spray/tray type and should include storage tank, supports, vent condenser and fittings. The design should be to the Heat Exchange Institute standard and suitable for full vacuum.
The deaerator should be designed and arranged for the efficient removal of non-condensable gases from the feed water under all conditions of operation, including the admission of auxiliary steam during starting and low temperature condensate under fault or restart conditions.
If a part load deaerator is offered then the design should include a full description of the start-up and operation with increasing load up to full load on the steam turbine-generator.
The design deaerator or feed water storage vessel should describe features of the deaerator head which facilitate the removal of non-condensable gases from the circulating feed water and the provision if any, for recovering heat from the vented gases and vapor. Deaerator level indicators and alarms should be provided in the CCR, these alarms should be fully functional at all times when the plant is available for operation, including periods when the plant is on standby duty.
Safety valves should be provided to protect the deaerator and feed water storage vessel from over pressure from any source. All parts of the deaerator exposed to oxygen or corrosive gases should have an adequate corrosion allowance or be of corrosion resistant materials.