7.2   Thermal/hydraulic design - part 2

 

The feedwater storage tank associated with the de-aerating heater on Fig 3.57 is similar in size and the provisions for leak-offs, HP drains, etc., to that described for Fig 3.56. The other features described for the first de-aerator storage tank are also provided.

A design of de-aerator currently intended for use within a CEGB power station is the 'Stork' de-aerator. This type of de-aerator employs a different approach to the task of extracting the oxygen and other non-condensable gases from the condensate. Figure 3.58 shows a section through a typical Stork de-aerator, with the steam and water paths indicated.

There is no de-aerating head as such, but in the space above the working water level in the top of the storage tank there is a series of specially designed sprayers, which spray the incoming feedwater onto a ring of plates which further breaks up the water.

The sprayers are designed to create sufficient spray area and water droplets to provide the necessary heat transfer surface to heat the incoming feed to as near to the saturation temperature (corresponding to the pressure within the de-aerator) as practicable. In practice, the temperature of the feedwater in the tank has been measured to be within 1-2°C of the saturation temperature. The stored water in the tank is therefore heated to the saturation temperature of the incoming heating steam as it enters the tank through the sprays.

A steam distribution system in the form of a 'rake' is provided, the teeth of which are vertical tubes perforated at their ends. The end which is perforated is positioned to be always below the lowest working feedwater level. The heating steam introduced into the tank content via the steam 'rake' is nearly at the same saturation temperature as the tank content and is therefore not condensed, but bubbles through the water where it absorbs any residual oxygen and non-condensable gases not liberated in the spray phase.

The oxygen and non-condensable gases are released from the steam and incoming feed in the spray area and are extracted from the air vent on the top of the shell. The 'Stork' de-aerator has no vent condenser as the loss due to the heat in the vapour/air mixture is too small to warrant its cost.

The sprayers on the top of the vessel are a Patented 'Stork' design (as shown in Fig 3.58) and create a uniform spray pattern between maximum and minimum condensate flow rates.

Section through a typical Stork de-aerator

HP drains are sprayed into the top of the tank above the water level through dispersers. The leak-offs are introduced below the water level by means of pipes of the same design and vertical attitude as those which form the teeth of the steam rake. Steam coils are provided for start-up heating with electrical heaters to maintain tank temperature.

In the event of sudden unit load reduction, the bled-steam pressure will fall and the tank content will boil. The small auxiliary connection is provided to equalise the pressure between the space above the water level in the tank and the steam rake and distribution pipework. The connection prevents the water being forced into the steam distribution system.

Vertical baffles which extend to the high water level are provided to prevent the formation of waves on the water surface and to preclude the possibility of 'sloshing'. The action of the steam bubbling through the tank content prevents the formation of stagnant areas where subcooled water could collect.

Baffles adjacent to the drain outlets shield the areas around them from disturbance by the steam discharging from the steam rake. The drain outlets are internal to the tank and have a flare to promote smooth flow into the feed pump suction pipework.

 

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