Water Management; The Sc System Of The Nordmann At4 D - Nordmann AT4 D Service Manual

5

Water management

5.1

The SC system of the Nordmann AT4 D

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Lime deposition in steam generators is a well-known process that can't be
avoided if tap water is used. Lime scaling on steam cylinder walls, valves
and heating electrodes prevent reliable operation of the steam generator,
if the steam generator is not regularly maintained.
The SC system of the Nordmann steam generators does not prevent lime
scaling, however it does lead to substantially longer operation cycles be-
cause the lime deposit at surfaces is drastically reduced.
How does it come to lime scaling?
Calcium occurs in the environment only in bound form as constituent of
minerals. Calcium is a shining silver-white, soft metal, which reacts with
water strongly. It is the relevant element for the formation of lime.
In water lime (CaCO ) is present in solve form, i.e. as ions Ca++ and CO
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Even with small physical changes of the water such as heating up or swirl-
ing lime precipitates.
An important role on binding and precipitation of lime plays the carbonic
acid (H CO ) contained in the water, and the so-called lime carbonic acid
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equilibrium. Carbonic acid is a reaction product of carbon dioxide (CO
with water (H O). In the water a lime carbonic acid equilibrium prevails, if it
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contains even so much carbon dioxide that it does not precipitates lime but
no lime can be solved. If carbon dioxide is withdrawn from such a water,
lime precipitation occur.
Since the solubility of gas decreases with increasing water temperature, the
carbon dioxide escapes when the water is heated up. Through this interven-
tion into the lime carbonic acid equilibrium lime precipitates and settles in
the steam cylinder. The lime precipitation increases very strongly with water
temperatures higher than 60°C.
A additional point, which influences the precipitation of lime in an electrode
steam generator is the current flow in the water. Due to the electron activity
in the water resulting from the current flow, the molecular structure of the
lime is disturbed. Therefore no lime wafers can build up in the zone where
the current flows and the lime is precipitated only as small particles.
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