4.3 Chlorination Equipment: Gas Systems, Vacuum Regulators, Hypochlorite Storage & Onsite Generation

Three ways to put chlorine in water

Regulation 100 classifies gas chlorine, onsite hypochlorite generation, and sodium or calcium hypochlorite as separate treatment types with different facility classes, because the equipment and the hazards genuinely differ. WPI's Equipment Operation and Maintenance outline lists the "chlorine disinfection system" as an inspect, maintain, and operate item. This section is about the hardware; the chemistry and CT calculations appear in the disinfection chapter.

Gas chlorine systems

Chlorine gas is shipped as a liquefied gas under pressure in 150-pound cylinders and 1-ton containers. Because chlorine gas is about 2.5 times heavier than air, a release settles into low areas: basements, pits, trenches, and valve vaults. That single fact governs the entire building design.

The vacuum principle

Modern systems are all-vacuum downstream of the container, and this is the most important safety concept in the subject. A vacuum regulator mounted directly on the container valve holds the gas at pressure only inside the container. Everything past the regulator, including all the tubing running through the plant, is under vacuum created by the ejector. If a vacuum line breaks, air is drawn in and the regulator closes, so no gas escapes. Pressurized gas piping outside the chlorine room is not acceptable practice.

The flow path is:

1. Container valve with a fusible plug that melts near 158 to 165 degrees Fahrenheit to relieve pressure in a fire.
2. Vacuum regulator / pressure-reducing valve, with an inlet lead gasket that is replaced at every container change.
3. Rotameter and rate valve, manual or automatic.
4. Ejector (injector), where a motive water stream through a venturi creates the vacuum and dissolves the gas.
5. Diffuser at the application point.

Withdrawal rates and reevaporation

Chlorine withdrawn as a gas absorbs heat of vaporization from the liquid, chilling the container. Exceeding the withdrawal rate causes frosting, then a falling withdrawal rate, then loss of feed. Rule-of-thumb maximum continuous gas withdrawal at about 70 degrees Fahrenheit is roughly 40 lbs per day per 150-lb cylinder and 400 lbs per day per ton container. Larger demands require liquid withdrawal through an evaporator, which supplies external heat. Never apply direct heat, torches, or steam to a container.

A ton container has two valves on the end, arranged vertically when properly positioned: the upper valve draws gas, the lower valve draws liquid. Orienting the container so the valves are one above the other is not cosmetic; it determines what you get.

Chlorine room requirements

• Located at grade with direct exterior access, never below grade.
• Ventilation at floor level, roughly one complete air change per minute, with the switch outside the door and a window in the door so you can look in before entering.
• Gas detection with audible and visual alarm inside and outside the room.
• Separate room from the ejector, controls, and other chemical storage; never store ammonia in the same room.
• Emergency repair kits: Kit A for 150-lb cylinders, Kit B for ton containers, Kit C for tank cars.
• Self-contained breathing apparatus stored outside the chlorine room, inspected monthly, with trained users.
• Containers secured upright with chains, and valve protection hoods in place whenever not connected.
• Leak detection with a household ammonia solution on a swab, which produces a white cloud of ammonium chloride at a leak. Never use water, because chlorine plus moisture forms hydrochloric and hypochlorous acids that accelerate corrosion and make the leak worse.

Facilities storing chlorine gas above threshold quantities also fall under the EPA Risk Management Program and OSHA Process Safety Management, and under the America's Water Infrastructure Act risk and resilience assessment requirements.

Sodium hypochlorite

Liquid bleach at 10 to 15 percent available chlorine is the most common alternative and eliminates the toxic gas hazard entirely. It brings its own problems:

• It decomposes. Strength falls with time, heat, light, and metal contamination. Decomposition roughly doubles for each 18 degrees Fahrenheit rise. Product delivered at 12.5 percent may be 10 percent within weeks in a hot room. Store cool, dark, and turn over inventory, and test the strength of what you actually have rather than what the label says.
• It off-gasses oxygen, causing vapor lock in feed pumps. Tanks need vents; feed pumps need degassing heads or peristaltic designs.
• It raises pH. Sodium hypochlorite is strongly alkaline, so it raises the treated water pH, which reduces the fraction of disinfectant present as hypochlorous acid and therefore reduces disinfection efficiency at a given residual.
• Decomposition produces chlorate, an emerging contaminant of concern that increases with storage time, temperature, and higher product strength.
• It is corrosive and reacts violently with acid. Dedicated containment, dedicated fill connections, and PPE at every connection break.

Calcium hypochlorite is a dry solid at about 65 to 70 percent available chlorine, supplied as granules or tablets. It stores far longer than liquid, but it is a powerful oxidizer: contact with organic material, oil, grease, or rags can start a fire, and contact with even small amounts of water in a closed container can generate heat and gas. Store it dry, separate from organics and acids, in its original container.

Onsite hypochlorite generation

An onsite generation (OSG) system electrolyzes a brine solution to produce a dilute sodium hypochlorite of roughly 0.8 percent available chlorine. Because there is no shipment or storage of hazardous chlorine, OSG removes the gas hazard and the strength-decay problem of bulk bleach.

Components are a water softener (essential, because hardness scales the electrolytic cell), a brine tank and saturator, a rectifier converting AC to low-voltage DC, the electrolytic cell, a hypochlorite storage tank, and hydrogen dilution and venting.

Two operational realities dominate:

1. Hydrogen gas is a by-product. Electrolysis of brine liberates hydrogen, which is explosive between roughly 4 and 75 percent in air. The system must have forced dilution air and continuous venting to the outside, with no ignition sources and interlocks that stop generation on ventilation failure. Hydrogen accumulation in an OSG room is the principal hazard of the technology.
2. The cell must be acid cleaned. Even with softening, calcium and magnesium scale accumulates on the electrodes and reduces output. Periodic acid cleaning per the manufacturer's procedure restores capacity, and falling output at constant current is the indicator.

Because the product is only about 0.8 percent, the volume fed is roughly 15 times that of 12.5 percent bulk bleach, so feed pumps and piping are correspondingly larger.

Comparing the three

Whichever system a plant uses, the operator's daily checks are the same in spirit: verify the residual being produced, verify the feed equipment is delivering the rate it is set to, verify the safety systems (ventilation, detection, alarms, SCBA, eyewash) are functional, and record what you found.