12.3 Gas Chlorination Equipment, Regulators, Injectors & Evaporators

Flowsheet of a Vacuum-Operated Gas Chlorination System

Elemental chlorine ($Cl_2$) is a heavy, greenish-yellow gas that is 2.5 times denser than air and is commercially liquefied under pressure inside 150-pound cylinders, 1-ton containers (2,000 lbs), or bulk tank cars. Modern water treatment gas chlorination systems operate almost universally under sub-atmospheric vacuum to eliminate pressurized toxic gas transport within the plant building.

[ Physical Flowsheet of Vacuum Chlorination System ]
+-------------------------------------------------------+
| Chlorine Container (150-lb Cylinder or 1-Ton Vessel)   |
| -> Container Auxiliary Valve (Pressurized Gas: ~85 psi)|
| -> VACUUM REGULATOR (Mounted directly on valve)       |
+-------------------------------------------------------+
                         |
   ================ [VACUUM BOUNDARY] =================
   (All downstream piping operates under 10-20+ in Hg vacuum)
                         |
                         v
              [ Plastic Vacuum Tubing ]
                         |
                         v
        [ Rotameter with V-Notch Orifice Valve ]
                         |
                         v
       [ Differential Pressure Regulating Valve ]
                         |
                         v
         [ Automatic Switchover Shuttle Valve ]
                         |
                         v
     [ Dual-Diaphragm Ejector Check Valve Assembly ]
                         |
                         v
          [ EJECTOR / INJECTOR VENTURI THROAT ] <--- (High-Pressure Motive Water)
                         |
                         v
[ Acidic Chlorine Solution: 500 - 3,500 mg/L HOCl ]
                         |
                         v
  [ Process Main Injection Quill (Center 1/3 of Pipe) ]

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The Vacuum-Operated Safety Design Principle

The fundamental safety breakthrough in gas chlorination is the vacuum-operated principle:

1. Positive Gas Pressure Exists ONLY at the Container Neck: Liquefied chlorine inside the cylinder exists at equilibrium pressure (approximately 85 to 100 psig at 70°F / 21°C). Positive chlorine gas pressure is strictly confined between the cylinder valve seat and the inlet poppet valve of the vacuum regulator.
2. Regulator Mounted Directly on Container: By yoke-mounting the vacuum regulator directly onto the cylinder valve using a lead gasket, pressurized gas travels less than two inches before it is throttled to a vacuum.
3. Fail-Safe Inward Leakage: The entire piping run from the discharge of the vacuum regulator through the rotameter, differential pressure regulator, and vacuum tubing to the ejector operates under negative pressure (a vacuum of 10 to 20+ inches of mercury, in Hg).
4. Automatic Spring Shutoff: If any vacuum tubing cracks, a rotameter glass shatters, or a pipe fitting breaks, atmospheric air is drawn inward into the tubing. Chlorine gas cannot leak out into the room. Crucially, the instantaneous loss of vacuum allows a heavy opposing spring inside the vacuum regulator to slam the inlet poppet valve shut, locking all chlorine securely inside the container.

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Key Components & Operational Mechanics

The Vacuum Regulator

The vacuum regulator is a diaphragm-actuated pressure-reducing valve. It contains a spring-loaded inlet valve plug (poppet) linked mechanically to an elastomeric diaphragm:

• Operation: The poppet valve is held tightly closed by a heavy spring. When the downstream ejector generates a vacuum, atmospheric air on the dry side of the diaphragm pushes the diaphragm inward, overcoming spring tension and opening the poppet valve to admit gas.
• Loss of Vacuum: If the ejector stops or vacuum piping breaks, atmospheric pressure equalizes across the diaphragm, and the internal spring immediately snaps the poppet valve shut.
• Vent to Outdoors: The regulator includes a spring-loaded safety relief valve connected to dedicated polyethylene tubing vented to the outdoor atmosphere through an insect screen. If a defective poppet valve leaks gas into the diaphragm chamber when the unit is un-vacuumed, the gas vents safely outdoors rather than into the chlorinator room.

Rotameter & Differential Pressure Regulator

• Rotameter (Flowmeter Tube): A precision-machined, vertically tapered borosilicate glass tube containing a calibrated float (silver, tantalum, or PVDF). Chlorine gas flows upward through the tube, lifting the float until upward drag balances its submerged weight. Direct-reading scales indicate chlorine mass flow rate in pounds per 24 hours (lb/day) or kilograms per hour ($kg/hr$). Manual feed rate adjustment is achieved via a precision V-notch orifice plug at the rotameter inlet.
• Differential Pressure (DP) Regulator: Installed immediately downstream of the rotameter. Fluctuations in ejector motive water pressure cause vacuum levels to bounce between 10 and 25 in Hg. The DP regulator contains a spring-loaded diaphragm that continuously adjusts an internal orifice to maintain a constant pressure differential across the rotameter V-notch orifice. This ensures the chlorine feed rate remains strictly proportional to the V-notch valve position regardless of fluctuating hydraulic conditions.

The Ejector / Injector (Venturi Nozzle)

The ejector (injector) is the hydraulic powerhouse of the chlorination system. It utilizes the Venturi principle to generate the operating vacuum:

• Hydraulic Mechanism: High-pressure motive water supplied by a dedicated booster pump enters a converging nozzle. As the water accelerates through the constricted venturi throat, static pressure energy converts into kinetic velocity energy. In accordance with Bernoulli's theorem, static pressure drops below atmospheric, generating a deep vacuum (typically 20 to 28 inches of mercury, in Hg).
• Solution Formation: The deep vacuum draws chlorine gas into the turbulent jet throat, where the gas instantly dissolves into the motive water stream, reacting rapidly to form hypochlorous acid ($HOCl$) and hydrochloric acid ($HCl$):

$Cl_2 + H_2O \rightleftharpoons HOCl + H^+ + Cl^-$

• Concentration & pH: The resulting solution is a highly acidic (pH 2.0 to 3.5), concentrated chlorine solution, typically containing 500 to 3,500 mg/L as available chlorine. This concentrated solution is piped through Schedule 80 PVC or PVDF piping to the process injection quill.

Ejector Check Valve Assembly: Preventing Water Backflood

The most common failure mode in chlorination systems is water backflooding—treated water backing up through the ejector into vacuum lines when motive water pressure drops. Concentrated chlorine gas contacting liquid water forms dense hydrochloric acid, which rapidly destroys metallic springs, rotting rotameter frames and regulators.

To prevent this, the ejector inlet contains a dual check valve assembly:

• Features two independent, spring-loaded elastomeric diaphragms and Viton/ceramic poppet valves arranged in series.
• Motive vacuum pulls the diaphragms open during normal operation. The moment motive water shuts down, spring force combined with backpressure slams both poppets shut, creating an impermeable hydraulic barrier.

Automatic Switchover Manifolds

Public water systems must maintain continuous chlorination without interruption. An automatic switchover system connects two or more vacuum regulators (each mounted on separate cylinders) to a common vacuum manifold:

• Both the primary and standby cylinder valves are fully opened.
• The primary regulator feeds chlorine gas until the operating cylinder is completely empty.
• As the cylinder empties, its internal vapor pressure collapses, causing the ejector to draw an intense vacuum spike (rising above 20 to 25 in Hg).
• This elevated vacuum trips an internal mechanical detent latch or shifts a vacuum shuttle valve, instantaneously opening the standby vacuum regulator. The switchover occurs automatically with zero pressure drop or feed disruption.

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Container Thermodynamic Limits & Chlorine Evaporators

The Thermodynamics of Gas Withdrawal

Liquefied chlorine inside a cylinder absorbs heat from the surrounding room air through the steel container wall to boil into dry chlorine gas. The latent heat of vaporization of liquid chlorine is approximately 124 BTU per pound.

Because the steel container must extract this heat from ambient air via natural convection, there are strict thermodynamic limits on maximum sustained gas withdrawal rates at 70°F (21°C) with standard room ventilation:

• 150-lb Cylinder: Maximum sustained gas withdrawal rate is 40 lb/day (1.7 lb/hr).
• 1-Ton Container (2,000 lbs): Maximum sustained gas withdrawal rate is 400 lb/day (16.7 lb/hr).

Consequences of Exceeding Withdrawal Limits: Freezing and Frosting

If an operator attempts to withdraw gas at rates exceeding these boundaries (e.g., drawing 100 lb/day from a single 150-lb cylinder), the chemical boils faster than room air can conduct heat through the steel casing. The temperature of the liquid chlorine drops below 32°F (0°C). Atmospheric moisture condenses and freezes on the exterior shell, forming a thick coat of frost and ice.

Because ice acts as an insulator, heat transfer slows further. The internal vapor pressure plummets from 85 psig down toward zero, starving the chlorinator of gas and causing loss of chlorine feed. Under no circumstances should operators apply open flames, blowtorches, or radiant heaters to warm a frosting cylinder, as this will melt the safety fusible plugs (designed to melt at 158°F to 165°F / 70°C to 74°C), triggering a catastrophic chlorine gas explosion.

[ Container Gas Withdrawal Capacities at 70°F ]
150-lb Cylinder  ----------> Max 40 lb/day (Gas Withdrawal)
1-Ton Container  ----------> Max 400 lb/day (Gas Withdrawal)
Requirement > 400 lb/day --> LIQUID Withdrawal + HEATED EVAPORATOR

Chlorine Evaporators for High-Capacity Demands

When treatment plants require chlorine feed rates exceeding 400 lb/day per ton container (or total facility demand exceeds 1,000 lb/day), withdrawing chlorine as a gas is physically impossible without manifolding dozens of containers. Instead, the facility utilizes liquid withdrawal paired with a chlorine evaporator:

1. Liquid Withdrawal: Ton containers are positioned horizontally with their two valves aligned vertically. The bottom valve draws liquefied chlorine directly from the bottom of the vessel via an internal dip tube. Liquid can be withdrawn from a 1-ton container at rates exceeding 10,000 lb/day without cooling the cylinder.
2. Evaporator Vessel: Liquid chlorine is piped through Schedule 80 seamless carbon steel piping into a heavy ASME-stamped pressure vessel inside the evaporator.
3. Thermostatically Controlled Water Bath: The pressure vessel is submerged inside an electric hot water bath maintained precisely between 160°F and 180°F (71°C to 82°C). Electric immersion heaters provide the continuous thermal energy needed to vaporize liquid chlorine into dry gas.
4. Superheater Chamber: Evaporated chlorine gas passes through a superheating dome at the top of the vessel, raising the gas temperature approximately 20°F above its boiling point to prevent gas re-condensation in downstream vacuum lines.
5. Safety Systems: The hot water bath incorporates a cathodic protection system (sacrificial anode) to prevent bath corrosion. The gas discharge line features an ASME rupture disk backed by an electronic pressure switch (calibrated at 350 to 400 psig). If extreme overpressure occurs, the rupture disk breaks, venting gas into an expansion chamber or chemical scrubber while sounding an emergency alarm.

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Comparative Technical References

Table 12.3.1: Vacuum Chlorination System Flowsheet Components

Table 12.3.2: Chlorine Container Withdrawal Limits & Operating Guidelines

| Parameter | 150-lb Gas Cylinder | 1-Ton Container (Gas) | 1-Ton Container (Liquid + Evaporator) | |---|---|---| | Max Sustained Withdrawal at 70°F | 40 lb/day (1.7 lb/hr) | 400 lb/day (16.7 lb/hr) | Up to 10,000+ lb/day (per evaporator) | | Valve Connection Configuration | Single valve (gas withdrawal) | Top valve (gas withdrawal) | Bottom valve (liquid withdrawal) | | Fusible Plug Relief Temperature | 158°F to 165°F (1 in valve/base) | 158°F to 165°F (6 to 8 in heads) | 158°F to 165°F (6 to 8 in container heads) | | Frosting / Freezing Risk | Occurs if feed exceeds 40 lb/day | Occurs if feed exceeds 400 lb/day | No container frosting; heat supplied by bath | | Required Evaporator Temperature | Not applicable | Not applicable | 160°F to 180°F (71°C to 82°C) water bath |