Environmental Engineering Reference
In-Depth Information
administrative delays. The problem with the leaky vent valves occurred soon after a power
failure happened during a sunny day causing the heat collecting pump to shutdown while the
water is still inside the collectors which caused partial evaporation of the water and frequent
opening and closing of the vent valves. To prevent further damage to the vent valves, it was
decided that plant re-start after the restoration of power following a power failure is made
either early in the morning or at dusk.
8.1.3. Vacuum Loss Inside Glass Tubes
As previously stated, the glass tubes operate under a high vacuum of 10
-4
mmHg. High
collector efficiency depends on maintaining this high vacuum level inside the collector. The
vacuum level is monitored every year by checking the condition of the getter. At the end of
the first year of operation, the condition of the vacuum on almost all glass tube was
essentially as new.
8.1.4. Scale Prevention
The most important item for the evaporator is scale prevention. There are two kinds of
scale that could form in seawater distillation: a hard scale and a soft scale. The hard scale
consists mainly of calcium sulfate (CaSO
4
) and very difficult to remove from the heat transfer
surfaces once formed. The only way to avoid the formation of this scale is to operate the
evaporator within the solubility limits of Ca
+2
and SO
4
2-
ions in the brine. The evaporator was
designed in such a way that in normal operation, the concentration of these ions are not
allowed to reach saturation. On the other hand, soft scale which consists mainly of calcium
carbonate (Ca CO
3
) and magnesium hydroxide (Mg(OH)
2
) can be avoided by injecting a
chemical inhibitor such as Belgard EV into seawater feed. In our evaporator 10 ppm of
Belgard EV is injected and was found satisfactory with the high-salinity seawater of the Gulf
(52,000 ppm). Scale formation was monitored on an hourly basis during plant operation by
observing the heat transfer coefficient of the heater (first effect) since it has the most
likelihood of scale formation since it operates at the highest temperature. For an emergency,
an acid injection system is used to carry out an acid cleaning procedure to remove any soft
scale formed.
8.1.5. Anti-corrosion Chemical for Use in the Heat Collecting Water
An anticorrosive chemical must be used in the heat collecting water, which circulates
through a closed system, to protect the equipment from corrosion. “High Clean CL-100”
was selected in view of the heat collector tube material being of copper and the heat
accumulator and piping material being of iron. “High Clean CL-100” was found to be
effective in protecting both materials from corrosion. This chemical is a solution of
alkanol-amino salt of nitrogen-containing condensate having the chemical symbol: [
R
-
SO
2
NH(CN
2
)
n
COOH] where
R
= C
6
H
5
and
n
= 1 to 3. The solution is a light brown
transparent liquid having a pH of 8 - 8.5. A concentration of 5,000 ppm was recommended
by the manufacturer. However, the concentration of the chemical as poured into the makeup
water tank after draining and leakage was gradually reduced as the stock of the chemical
decreased because the heat collecting water was forced to be drained more frequently than
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