Severn Trent VAX4600 Series, Инструкция по эксплуатации

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115.6001.10 - 18 -
3.3.2 Abnormally High Chemical Supply Pressure
CAUTION: Vaporizer performance will be restricted if ton containers and tank cars produce
abnormally high liquid pressure. Normal ton container pressure ranges (unpadded) are 85-100 psig
(586-690 kPa). Liquid pressure in a padded tank car is normally 125 psig (862 kPa). Vaporizer
capacity is reduced by padding and very low liquid chemical temperature.
Restricted Performance Example
Assume that the liquid chlorine being fed from the tank car is 60°F (15.6°C) and that the vaporizer
water bath temperature is 180°F (82.2°C). The vapor pressure of the liquid chlorine at 60°F (15.6°C)
is approximately 70 psig without any padding, and the thermal difference through the vaporizer
chamber wall between the heated water and the liquid chlorine will be 120°F (49°C). With this
temperature difference, the vaporizer can transfer heat and thereby vaporize the liquid chlorine to
gaseous chlorine.
The Chlorine Institute Pamphlet #54 permits maximum allowable padding pressure to be 190 psig
(1310 kPa) for liquid chlorine at 60°F (15.6°C). In this case, the liquid pressure in the vaporizer tank
then becomes 190 psig (1310 kPa), instead of 70 psig (483 kPa), because of the padding.
From the vapor pressure curve of chlorine, a 190 psig (1310 kPa) vapor pressure is equivalent to
120°F (49°C) so that the vaporizer must heat the liquid chlorine to 120°F (49°C) prior to vaporization.
With the vaporizer water bath at 180°F (82.2°C), a 60°F (15.6°C) change in temperature is available
to vaporize the chlorine instead of a change in temperature of 120°F (49°C) without padding. The
increased heat required to raise the liquid chlorine temperature 120°F (49°C) reduces the vaporizer
capacity and superheat.
To reduce abnormally high liquid chlorine pressure induced by heat (ton containers) or induced by
padding pressure (tank cars), operate on the gas phase of the source (top container valve) until the
pressure drops to approximately 70 psig (483 kPa). The chlorinator can operate to its maximum
capacity at 20 psig (138 kPa) gas pressure at the inlet to the vacuum regulator but 40 psig (276 kPa)
is preferred. Padding of a tank car will only be required when the liquid chlorine temperature is below
30°F (-1.1°C), and then only a small amount of padding is required.
If padding is used and pressure is maintained above 80 psig (552 kPa), chlorine liquid carry-over can
occur, and must be prevented for proper performance.
3.3.3 Shutdown
The vaporizer can be shut down using two methods:
Caution: Liquid chemical must not remain trapped in the vaporizing chamber when the liquid inlet
shutoff valve and gas discharge valve or process equipment is shut off. If liquid is trapped in the
heated vaporizer, the pressure will rise, causing the rupture disc to burst and the pressure relief valve
to open.
a. Method A (preferred)
Shut off the container liquid supply inlet valve at the vaporizer while maintaining the water bath
temperature. The remaining liquid will vaporize and discharge to the process.
Note: The process equipment must be operating. When the gas pressure gauge indicates the
rapid drop in pressure, all the liquid is vaporized. The manual gas discharge valves and the
process equipment may then be shut off.
b. Method B
Shut off the manual gas discharge valves to the process while maintaining the water bath
temperature. The pressure buildup in the vaporizing chamber will cause the liquid chemical to
return to its source. Shut down the process equipment, then shut off the container liquid supply
valve at the vaporizer.
After completing Method A or B, residual pressurized chemical gas may still be in the vaporizer
chamber. Before disconnecting any pipes, open the manual vent valve to make sure all gas is
vented.