16.2 Gas Chlorination Equipment & Hypochlorite Systems
Key Takeaways
- Chlorine gas is 2.5 times heavier than air, so leaks settle into low areas and chlorine rooms require floor-level ventilation and detection.
- A vacuum-operated chlorinator holds the entire system under vacuum downstream of the cylinder valve, so a break in the line stops the flow rather than releasing gas.
- A one-ton container withdrawing gas is limited to about 400 pounds per day and a 150-pound cylinder to about 40 pounds per day before frosting and re-liquefaction occur.
- Emergency Chlorine Institute repair kits are Kit A for 150-pound cylinders, Kit B for one-ton containers, and Kit C for tank cars and trucks.
- Sodium hypochlorite avoids the acute gas hazard but degrades with time, heat, and light, and produces chlorate as it decomposes.
Properties of Chlorine Gas
| Property | Value | Operational consequence |
|---|---|---|
| Vapor density | 2.5 times heavier than air | Leaks settle to the floor; detectors and exhaust intakes go at floor level, and personnel evacuate upwind and uphill |
| Color and odor | Greenish-yellow; pungent | Detectable by odor at about 0.3 ppm, well below the danger threshold - but odor fatigue makes smell an unreliable alarm |
| Liquid-to-gas expansion | About 1 to 460 | A small liquid leak produces an enormous gas volume |
| Not flammable | - | But it supports combustion and reacts violently with many materials |
| Reaction with moisture | Forms hydrochloric and hypochlorous acid | Dry chlorine does not attack steel; wet chlorine attacks nearly everything. Never introduce water into a chlorine cylinder or line |
Exposure guidance: the permissible exposure limit is on the order of 0.5 to 1 ppm as a ceiling or 8-hour value depending on the standard applied; IDLH is 10 ppm. Concentrations above about 30 ppm cause severe respiratory injury quickly.
Gas Feed Equipment
| Component | Function |
|---|---|
| Cylinder or container valve | The primary shutoff, opened no more than one full turn so it can be closed fast |
| Yoke and lead gasket | Connects the vacuum regulator to the container; a new gasket every connection, never reused |
| Vacuum regulator | Mounted directly on the container valve; converts pressurized gas to a vacuum-only downstream system |
| Rotameter | Displays feed rate in lb/day |
| V-notch or metering orifice | Sets the feed rate |
| Injector (ejector/eductor) | A venturi in a pressurized water stream that creates the vacuum and mixes the chlorine into solution |
| Diffuser | Distributes the chlorine solution into the process stream |
| Automatic changeover | Switches from an empty container to a full one without interrupting feed |
[!IMPORTANT] The vacuum principle is the single most important safety feature in gas chlorination. Because the regulator is mounted at the container valve and everything downstream is under vacuum, a broken solution line, a cracked rotameter, or a severed vacuum tube causes the vacuum to be lost, which closes the regulator and stops the gas flow. If the same system were pressurized, that break would release chlorine gas into the room. This is why pressurized gas piping beyond the container valve is not used in modern installations.
The injector needs adequate water supply pressure and flow to create the vacuum. Loss of injector water is the most common cause of a chlorinator that "stops feeding" - check the injector water pressure, the strainer, and the injector nozzle and throat for scale before troubleshooting anything else.
Withdrawal Rate and Re-Liquefaction
Withdrawing chlorine gas from a container requires the liquid chlorine inside to evaporate, and evaporation absorbs heat from the container and the room. Withdraw too fast and the container frosts, its internal pressure drops, and the feed rate falls off.
| Container | Maximum continuous gas withdrawal at about 70 °F |
|---|---|
| 150 lb cylinder | ~40 lb/day |
| 1-ton container | ~400 lb/day |
Exceeding these rates causes frosting. Never apply direct heat, a torch, or hot water to a chlorine container - it can over-pressurize the container and lift the fusible plugs. If more capacity is needed: manifold multiple containers, or install an evaporator that withdraws liquid chlorine and vaporizes it under controlled heat.
Re-liquefaction occurs when gas leaving a warm container passes into cooler piping and condenses back into liquid. Liquid chlorine reaching a vacuum regulator damages it. Prevention: keep the piping warmer than the container, slope lines back to the container, and use a properly designed manifold and, where applicable, an expansion chamber.
Fusible Plugs
- A 150-lb cylinder has one fusible plug in the valve; a 1-ton container has six fusible plugs, three on each end.
- They melt at about 158 to 165 °F to relieve pressure in a fire.
- Never paint over, obstruct, or apply heat near a fusible plug.
Handling and Storage
- Store containers upright and secured (cylinders chained), valve protection cap in place when not connected.
- One-ton containers lie horizontally on trunnions, positioned so the two valves are vertically aligned - the top valve delivers gas, the bottom valve delivers liquid. Rotating a ton container so the valves are horizontal makes it impossible to know which phase you are withdrawing.
- Move containers with proper lifting equipment and an approved lifting beam, never by the valve or the protective cap.
- Keep the chlorine room separate, with an exterior door opening outward with panic hardware, a shatter-resistant viewing window, floor-level exhaust ventilation sized for rapid air change and discharging to a safe location, and a light switch and ventilation switch outside the door.
- Store away from ammonia, hydrocarbons, and organics; never store hypochlorite and acid where a spill could combine them.
- Keep containers away from heat and direct sun.
Leak Detection and Emergency Response
Detection: hold a swab or squeeze bottle of ammonia solution near the suspected leak - a white cloud of ammonium chloride confirms chlorine. Never spray water on a chlorine leak; water plus chlorine forms acid and accelerates corrosion, enlarging the leak.
Chlorine Institute emergency kits:
| Kit | For |
|---|---|
| Kit A | 150-lb cylinders |
| Kit B | 1-ton containers |
| Kit C | Tank cars and tank trucks |
Kits must be on site where the corresponding containers are used, inspected on a schedule, and the crew must be trained in their use.
Response:
- Evacuate upwind and uphill; account for personnel; keep everyone out of low areas.
- Only trained responders in SCBA and appropriate protective clothing approach the leak - two-person minimum, never alone.
- Attempt to close the container valve if it can be done safely.
- If it cannot be stopped, apply the appropriate repair kit.
- A leaking container is positioned so the leak is in the gas space, not the liquid space - because a liquid leak releases about 460 times the volume of a gas leak.
- Notify the fire department and hazmat, the State Water Board, and, for a reportable release, the California Office of Emergency Services and the local Certified Unified Program Agency.
- Facilities storing threshold quantities of chlorine must maintain a California Accidental Release Prevention (CalARP) program and a Risk Management Plan, plus a Hazardous Materials Business Plan filed with the CUPA.
Sodium Hypochlorite and On-Site Generation
Many California utilities have moved away from gas because of the CalARP and community risk burden.
| Chlorine gas | Sodium hypochlorite (12.5%) | On-site generation (0.8%) | |
|---|---|---|---|
| Acute hazard | Severe | Corrosive liquid; no toxic gas cloud | Minimal - very dilute |
| Regulatory burden | CalARP/RMP, HMBP, scrubbers | HMBP | Lowest; but hydrogen gas must be vented |
| Strength stability | Stable indefinitely | Degrades with time, heat, light, and metal ions | Generated on demand; no storage degradation |
| Byproducts of concern | - | Chlorate and perchlorate increase as the product ages | Low chlorate |
| Volume handled | 100 percent chlorine | ~8x the volume of gas for the same chlorine | ~15x the volume |
| Capital cost | Low | Low | High |
Hypochlorite degradation is the operational issue: strength falls, and chlorate forms. Mitigations are to buy fresh, store cool and dark, rotate stock within about 30 to 60 days, dilute for long storage, avoid metal contamination, and verify strength on receipt and periodically.
On-site generation electrolyzes a brine solution to make roughly 0.8 percent hypochlorite. It eliminates chlorine deliveries and gas hazards, but it produces hydrogen gas, which must be continuously and reliably vented, and it requires soft water and reliable power. A hydrogen accumulation in an OSG room is an explosion hazard, and ventilation interlocks are a critical safety system.
Where should chlorine gas detectors and exhaust ventilation intakes be located in a chlorine room, and why?
A plant needs 600 lb/day of chlorine gas and is feeding from a single one-ton container at about 70 degrees F. What will happen?
An operator suspects a chlorine leak at a one-ton container connection. What is the correct detection method?