that way,” said Lee. He added that gas was his preferred option for chlorine addition since the plant is too big for tablets
or powder and bleach would likely cost twice as much.
Two 1-ton cylinders of chlorine are stored in a covered
building. Sulfur dioxide, used at the end of the plant process
for dechlorination, is stored inside an enclosed building.
Each chlorine and sulfur dioxide cylinder was placed inside
a special vessel apparatus in 2010 and 2012, respectively, to
contain and process any leaks. These ChlorTainer™ cylinder
containment vessels, supplied with loaders, scale systems, and
instrumentation, were manufactured by TGO Technologies of
Santa Rosa, Calif.
“That vessel containment was much preferred to a
scrubber-type solution, which would require major building
modifications,” Lee noted. “A scrubber system would also
have required a special six-member, trained response team to
be available to deal with a leak within the building.”
“With the vessels, it is all done automatically, with minimal
personal exposure. And with two tanks for each gas, we don’t
have to suffer any plant process interruption if there is leak
containment going on in one of them.”
Lee said the procedures for leak management were not dif-
ficult and that the container manufacturer was very helpful
with training. “We got used to it during some minor episodes
that occurred during cylinder deliveries,” he recalled. “They
had been placed in the containment, so we just bypassed to
the other cylinder while the gas leak was vacuumed out.”
The containment vessels were positioned on a concrete pad
and bolted down. With any accidental leaks of chlorine kept
within the containment vessel, no atmospheric venting is gen-
erated. The vessels are ASME-rated pressure tanks and any
leaks are recycled to the injection system at
a normal flow rate. A failsafe valve ties into
the chlorine sensor, so that in the event of
an external release, the failsafe valve will
close and sense it is under vacuum.
Any leak or release of chlorine gas from
the vacuum line downstream of a vacuum
regulator will compromise the vacuum condition and cause the vacuum regulator to
close, stopping the flow of chlorine gas to
the vacuum line. The maximum release of
chlorine gas will be the amount of chlorine
gas that is the length of the vacuum line to
the chlorine injector and not drawn into the
water solution by the suction of the injector.
There can be a question of the integrity of the system from
the outlet line, where chlorine gas penetrates the wall of the
pressure vessel, to the location where the vacuum line leaves
the outlet side of the pressure regulator. A study has shown
this could release less than 0.0009 lbs of chlorine gas. Should
there be a gas leak inside a vessel, there is no waste, as the
gas is processed at a normal rate until everything is used.
WATER TREATMENT PLANT APPLICATION
At a 12-MGD water treatment plant (WTP) operated by
the City of Benicia, Calif., Superintendent Scott Rovanpera
reports that abandoning a plan to switch from chlorine gas
to bleach for primary disinfection was enabled through a
well-engineered risk management plan for any gas leak risk,
thereby allowing him to continue receiving much appreciated
benefits from using gas.
“When I started here in 1997, there was already a design
engineering study underway toward installing a sodium hypochlorite (bleach) system to replace chlorine gas,” recalled
“But I knew of significant problems with using sodium
hypochlorite in a treatment plant nearby; it was leaking at
pipe unions and ball valves,” he noted. “I also saw that our
location was in a rural area of north Benicia, with few people anywhere within a mile of the plant, thereby creating a
huge buffer in the event of a gas leak.” Rovanpera added that
there hadn’t been a major gas leak since the plant opened in
1971; just a few minor ones with transfer hoses when storage
tanks were changed.
“We also much appreciated the chlorine gas as an exceptionally strong oxidant that could break down the walls
of bacteria and viruses, and it was far less expensive than
Chlor Tainer™ cylinder containment vessels, shown
here at Benicia’s water treatment plant, are designed
for not less than 40 years of service.