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OXIDATION AND DISINFECTION 10.23

ing, pressurized gas chlorine piping, vacuum piping between the vacuum regulator and
chlorine feeder, vacuum piping between the chlorine feeder and the injector or gas in-
duction unit, and the chlorine solution piping between the injector and the feed point.
Sizing of the pressurized chlorine piping is trivial and will result in very small-
diameter piping for pressure drop requirements. One-inch piping minimum should be used
for these installations. It is very unlikely that larger piping will be required.
Sizing of the vacuum lines is more complicated. As the chlorine is under vacuum
and the pressure is very small, any pressure drop of any magnitude results in a signif-
icant change in the density of the compressible chlorine gas. As a result, the use of the
Darcy equation is unsuitable for the calculation of pressure drop in vacuum lines, as
the chlorine gas is not incompressible (an assumption built into the Darcy equation).
Rather the use of either the isothermal or adiabatic equations is recommended for this
application as both are suitable for compressible flow. The isothermal equation is the
simpler of the two and can be found in Crane's Technical Paper No. 410, 1988, p. 1-8
(equation 1-6). However, manufacturers of chlorine feed equipment typically supply
charts for selection of pipe sizes for different lengths of system piping. These tables
should be used preferentially.
Another concern for chlorine system design is appropriate material selection. In gen-
eral, on the pressure side of the vacuum regulator, metallic piping materials should be se-
lected; and on the vacuum side, thermoplastic piping materials should be selected.
Dry gaseous and liquid chlorine will not attack carbon steel at normal temperatures;
as a consequence, liquid chlorine is packaged in steel containers. On the other hand, like
liquid oxygen, liquid chlorine will sustain combustion of steel once any portion of the
steel-chlorine contact surface has been heated to the kindling point of 438 ° F (225 ° C).
Because of this potential danger, heat should never be applied to a chlorine container or
piping containing chlorine. If a steel pipe containing liquid chlorine or even chlorine gas
at reduced pressure is accidentally cut with a welder's torch, the pipe will ignite and con-
tinue to burn as long as there is a chlorine supply available. Small amounts of moisture
will also cause chlorine to attack steel. Because a trace amount of moisture is unavoid-
able, some of the corrosion product (FeC13) is always found in chlorine containers and in
chlorine lines. Some may wish to use enhanced metallurgies for pressurized chlorine ser-
vice. Stainless steels are acceptable for this service but supply very little advantage. Ti-
tanium piping will spontaneously ignite in the presence of dry chlorine (although not with
wet chlorine) and so should be avoided at all costs.
As stated earlier, there are three distinct sections of pressurized chlorine piping: the
flexible connection, the pressurized liquid chlorine piping, and the pressurized gaseous
piping. Often as a chlorine container is emptied, the vacuum generated at the injector is
pulled all the way back to the chlorine container. It must be noted that as the chlorine
drops in pressure, the temperature of the chlorine will drop below -20 ° F, which is the
lower limit at which carbon steel can be used because of brittle fracture phenomena. For
the gaseous chlorine piping, the mass of the piping is so much greater than the mass of
the gas that the piping never gets sufficiently cold for brittle fracture to be a concern. On
the liquid side, the liquid will get much colder than -20 ° F, and the relative mass of the
liquid and piping is such that the piping will achieve these low temperatures as well. There
are two solutions to this problem: Either use an enhanced metallurgy, or design the liq-
uid piping such that it is either free draining to the evaporator or chlorine container. If
the second methodology is selected, a 2% minimum slope should be maintained in the
piping.
For the metallic piping in the pressurized section, Schedule 80 seamless steel piping
should be used (as appropriate). Reducing fittings should be used rather than bushings,
and ammonia-type unions with lead gaskets should be used rather than ground joint unions.
All parts should meet Chlorine Institute standards. Isolation valves should be ball type or
rising-stem type, having cast iron or steel bodies, Hastalloy trim, Teflon seats, and stain-