Water Softening - Public Water Systems - page 435

Gas
Air

Igniter

Recarbonation Chamber

Water Level

Submerged Burner

Direction of CO 2 Flow

Fig. 13-7. Submerged gas burner schematic (From Culp, Gordon, and Williams, Robert, Hand-
book of Public Water Systems. Copyright 1986 by John Wiley & Sons, Inc. Reprinted by
permission of John Wiley & Sons, Inc.)

tanks is 350 psi (2,400 kPa), and the ASME Code for Unfired Pressure Vessels requires
hydrostatic testing to 525 psi (3,600 kPa), or 1.5 times the working pressure. The tanks
may be insulated with pressed cork or polyurethane foam. The cooling and vaporizing
systems are designed to maintain the liquid CO 2 at about 0 F( 17.8 C) and 300 psi
(2,000 kPa). If temperature and pressure rise, the cooling system comes on; and with
falling pressure, the vaporizer comes into service. In the event either of these systems
fails, or in the event of fire or other accident, the storage tanks are fitted with high
and low pressure alarms, two safety pop valves, a manual bleeder relief valve, and a
bursting disk.
Either liquid or gas feed systems may be used to apply the liquid CO 2 to the water
to be treated. As CO 2 is withdrawn from the storage tank, the pressure is reduced.
This pressure reduction cools the CO 2 , with the danger of dry ice formation if the
expansion is too rapid. Consequently, it is common practice to reduce the pressure in
two stages, from the 300 psi (2,000 kPa) tank pressure to the 20 psi (140 kPa) pressure
ordinarily required for feeding the CO 2 . Vapor heaters may also be used just ahead of
the pressure-reducing valves.
For CO 2 gas feed, an orifice plate in the feed line with a simple manometer may
be used to measure flow, and a manual valve installed downstream may be used to
regulate or control the amount of CO 2 applied. Automatic control to a manual set point
could be provided by using a differential pressure transmitter on the feed line orifice,
and connecting it to an indicating controller that would operate the control valve.
Optionally, the CO 2 feed could be made fully automatic by providing pH control. In
this case, an electrode would be installed to measure the pH of the recarbonated water.
This signal would be amplified and sent through a controller that would throttle the
control valve on the feed line at low pH and open it at high, as set on the controller.