The capital projects manager for the City of El Paso, Texas water utilities’ extensive network of treatment plants and booster stations reports long-term, readily accomplished safety enhancement for its employees, as well as the public it serves, via various deployments of special secondary containment vessels for chlorine gas cylinders.
Safety
“We have had no incidents of chlorine gas exposure for our workers or surrounding communities,” said Warren Marquette, P.E. and capital projects manager. “Our facilities are right up against our population in many locations, and we’ve conducted simulated plume escape studies as part of our hazard operations program."
Marquette has seen cylinder containments used in other cities as well as El Paso, and the safety of the systems during chlorine dosing and their reliability make a difference.
"We’re pleased to have enhanced workplace safety, while also enhancing public safety," he said.
Marquette's experience with chlorine dates back 25 years, including when chemical production processes kept it in use. He said his first installation in 2002, when he served as a consultant to the utility was at a booster station.
“We’ve since expanded deployment considerably, including after I became an employee in 2015, first as utility engineer, and a year later as capital projects manager," he said. "We now have them installed at numerous other booster stations, as well as at two of our wastewater treatments plants (WWTPs), and a blended desalination/well water drinking water treatment plant (WTP).”
Secondary containment vessels
Due to available foot print and the consumption rate of the plant, pressurized chlorine systems were impractical at the other plants, Marquette said. But as the facilities are built out, he foresees more secondary containment technologies will become more common place, particularly in advanced water treatment and direct potable reuse plants currently in the design phase.
“We’ve been using this technology for over 20 years now, including many units," he said. "From the start, they’ve gone incredibly smoothly, including the commissioning, especially compared to other systems we’ve put in. We dose by weight, and the units come with a built-in weigh scale, as well as nitrogen fail-safe isolation valve, weight indication, safety valve, regulator connection; everything well packaged, for simple, seamless integration into our SCADA system; basically plug and play.”
The special secondary containment vessels for chlorine gas cylinders, manufactured by ChlorTainer/TGO Technologies Inc. of Santa Rosa, California, are currently in use at three plant locations — a 17.5 MGD WWTP, a 12.5 MGD WWTP, and the 27 MGD blended WTP — where chlorine is dosed into reclaimed water at the clearwell and again at the discharge pipe.
They are also currently in use at numerous booster stations, in both 150-pound and 1-ton sizes, where chlorine is further added to maintain free chlorine residuals as regulated throughout the distribution system.
“We dose chlorine into the discharge of the booster stations using a quill injector fitting,” Marquette said. “In the plants, we’re dosing into whatever clearwell we have, and then again at discharge. For the booster stations, we’ll typically have an electrical room, a pump room, and a separate room for the ChlorTainers; two 1-ton units, with drawbridge loading, side by side. Chlorine solution is injected to the pump station distribution discharge to maintain chlorine residual.”
Delivery trucks bring in the cylinders, which are placed on a roller conveyor. The empty container is removed and the new one is put in its place. Marquette said this process takes at most 15 to 20 minutes, including inspection.
“We trained people very easily. The units are simple, easy maintenance; operators love ‘em, no safety issues at all," he said. "We’d be happy to share our experience, through our people in charge of water production, water and wastewater; go through all plants and booster stations that have them; I would definitely recommend it."
Installation with contractor
Marquette also noted that “there haven’t been any installation contractor issues; they’ve gone very smoothly.”
The installation contractor for El Paso Water Utilities’ secondary containment vessels for chlorine gas cylinders has been Smithco Construction Inc., headquartered in Caballo, New Mexico, since 1969. Smithco Construction serves all of that state, plus parts of Arizona and West Texas, particularly the Odessa/Midland area and El Paso.
The installations have been “probably as straightforward as you can be,” reported Robby Sanders, a Smithco project manager, for a recent installation.
“We installed a 1-ton ChlorTainer as part of a booster station for El Paso drinking water that was completed in 2019," Sanders said. "Commissioning was then delayed due to Covid, but we expect that to be completed soon. We put it in the utility’s own concrete black (CMU) building that we built for that project. Due to the delay between installation and commissioning, we will probably have to revisit the warranty situation, but I have no concern about that.”
Smithco Construction was also awarded contracts with the utility's airport and the Ranchos Real booster stations to install ChlorTainer units. Sanders first installed a unit in 2018 in Carlsbad, New Mexico, which gave him useful experience.
“I knew about the product from one of our other project managers who had already done similar work in El Paso," he said. "Our scope of service in Carlsbad also included installation of 20,127 linear feet of 24-inch waterline; 9,650 linear feet of 16-inch waterline; and supervising a subcontractor for a 5 million gallon concrete pre-stressed water reservoir.”
In each of these installations, he said communication with field crews has been effortless and seamless. Deliveries were on time and the instructions for installation ensured everything was smooth. Submittal reviews, while complex, have been easy to get approved with the firm, vendor and utility's consulting engineer.
"Compared to other kinds of equipment, this one is more detailed, but it’s been easy to get approved, by the second time around; we get it released, and fabricate right away," Sanders said. "That’s not always the case with water plant equipment. The vendor has been very responsive to any question or comments; they get back to us right away.”
The secondary containment 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 leak detection sensor, so that in the event of an external release, the nitrogen failsafe valve will close, stopping it completely.
The vessels’ life expectancy is stated at no less than 100 years, given proper maintenance. This features annually changing out the Viton O-ring on the door, which takes about half an hour of time and approximately $200.
