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The advent of significant technical advances in safety equipment has resulted in the development of a system approach to chlorine safety. Some of these advances include residual analyzers with feedback loop; non-pressurized vacuum systems; chlorine gas leak detectors; alarm systems; SCADA; automatic shut-off valves; dry scrubber systems for gas containment; positive-pressure breathing apparatus; specialized chlorine safety kits; fully automated systems to contain leaks; federal, state and local regulations; and required written emergency procedures, many more options and procedures have become available.

Recently, the use of alternate forms of chlorine, changing regulations, Homeland Security concerns and the improvement of various types of instrumentation have dictated the need for more site-specific chlorine safety plans.

Storage and handling of chlorine gas
Chlorine gas for wastewater disinfection is commonly available in 150 lb (45 kg) cylinders and one ton (907 kg) containers depending on the usage required by the facility. For facilities with very high chlorine usage, chlorine is also sometimes delivered in cargo tanks or rail tank cars. Both cylinders and containers have the tare rate and the date of the most recent hydrostatic test stamped on the container.

Chlorine gas cylinders need to be stored vertically and are designed to deliver only chlorine gas. Inversion of chlorine cylinders to withdraw liquid chlorine can cause significant safety concerns and should be avoided whenever possible. Each cylinder has only one opening located at its top into which the cylinder valve is fixed. The cylinder valve has a fusible plug below the valve packing which is designed to melt at a temperature of 70-74 degrees C.

One-ton containers, on the other hand, should be stored horizontally and each cylinder has two valves, each connected to a small draw tube inside the container. When the container is properly positioned, the top valve provides the option to withdraw gaseous chlorine while the bottom valve provides the option to withdraw liquid chlorine. Although liquid chlorine can be withdrawn at a higher rate than chlorine gas, the withdrawal of liquid chlorine requires an evaporator to convert the liquid chlorine to a gas prior to being utilized. Each container has six fusible plugs, three on each end, which are designed to melt at 70-74 degrees C.

Initially, most chlorine gas feed systems at wastewater facilities were pressure feed systems. Today, the majority of chlorine feed systems at wastewater treatment facilities, such as the Capital Controls® vacuum gas feed system from Severn Trent Services, are vacuum feed systems. In such a system, the water into which the chlorine is being injected is used to create a vacuum in a small gas feed line that pulls the chlorine gas from the cylinder through the regulating diaphragm assembly, rate indicator, rate valve and ejector and into the dilution water supply. This type of chlorine feed system is significantly safer than a pressure feed system because the entire chlorine gas feed line is under a negative pressure so the atmospheric air will leak into the line during operation rather than the chlorine gas leaking out of the line.

With normal operation, gas chlorination system leaks have a tendency to occur in similar places, so careful inspection and maintenance of these areas will help prevent leaks. There is a tendency for leaks to develop along the sides and heads of the container and at the fusible plugs and feed valves. Leaks are typically also found in similar locations for cylinders.

Automatic shut-off valves 
One way to reduce the risk of a significant chlorine gas release is to install an automatic shut-off valve system. These systems consist of a battery-powered valve actuator mounted directly on the cylinder or container, an automatic charger and a display capable of various input and output signals.

Automatic shut-off valve systems can be used on chlorine cylinders or containers and will close the cylinder or container valve within seconds of receiving an activation signal. The systems typically are activated manually by a panic button or by a signal sent from a gas detector, fire alarm or other type of alarm.

Automatic shut-off valves are relatively inexpensive, but multiple units are often required for a system using multiple cylinders or containers. It should be noted that these systems do not protect against a chlorine release if a fusible plug melts.

Chlorine containment systems
Various containment systems are available to reduce the risk of a catastrophic release of chlorine gas from gas feed cylinders and containers. The most popular chlorine containment systems consist of a steel drum into which the chlorine cylinder or container is inserted. The containment system has a hinged door which, when shut, effectively seals the chlorine cylinder or container inside the drum. There is a series of connections on the outside of the system to allow the chlorine to be removed from the system for use.

This type of chlorine containment system is very effective in controlling leaks from the cylinder or container itself and also provides some protection against blasts and fires. The systems do not, however, protect against leaks in the lines outside of the containment systems.

A disadvantage of chlorine containment systems in general is that one system is required for each cylinder or container, which can make installing the systems costly. Once installed, the systems require little repair and maintenance on an annual basis.

Emergency chlorine scrubber systems
Emergency chlorine scrubber systems treat and exhaust the air within a chlorine room after a leak. The design of a chlorine room is critical and should take into account the following: 1) chlorine gas is heavier than air; 2) the room must contain gas and liquid chlorine; 3) the pick-up duct should be 30 to 45 cm above the floor; 4) floors should be sloped toward a confined corner sump; 5) doorways should be step-down into the room with grating covered; 6) liquid escape routes should be checked, ideally by flooding the room.

In addition to the proper design of the room, proper sizing of the scrubber is extremely important. Although required sizing can vary with local building and fire codes, the scrubber should be sized to create a negative vacuum in the room by pulling approximately twice the air from the room as there is chlorine leaking into the room under a worst case scenario. The worst case scenario is generally considered to be when one fusible plug from a cylinder or container is removed and the chlorine gas is allowed to escape through the resulting orifice. In the case of a cylinder, this equates to a minimum exhaust rate of 250 SCFM for the scrubber. In the case of a container, this equates to a minimum exhaust rate of 1,380 SCFM for the scrubber. It is, therefore, typical for a scrubber for a cylinder to be sized for a minimum of 250 SCFM and the scrubber for a container to be sized for 3,000 SCFM.

Major technical advancements in the design of emergency chlorine scrubbers have made them much easier to maintain and operate. Older scrubbers are based on a wet technology in which the air to be scrubbed is exhausted upwards through a tower filled with plastic packing. Concurrently, caustic spray is sprayed from the top of the tower. The spray reacts with the chlorine gas as it travels up the tower and is exhausted to the atmosphere. Wet scrubbers are difficult and somewhat costly to maintain primarily because the caustic must be replaced on a regular basis to keep the systems functional.

The benefits of dry scrubbers
A more recent innovation, "dry scrubbers" also exhaust air from the room to be scrubbed through a tower, but these scrubbers utilize dry, impregnated, activated alumina media beads as a way to neutralize the chlorine gas instead of caustic. Dry scrubbers consist of a blower and associated ductwork, which exhausts the contaminated air through a tower full of dry media beads. An example of an effective dry scrubber is the EST™ Dry Emergency Chlorine Scrubber from Severn Trent Services.

Dry scrubbers are available in vertical and horizontal configurations and require little maintenance since the media will last many years without any attention if not used. When an event does take place, the media can be sampled through sample ports and only the exhausted media removed, sent to a landfill and replaced.

The development of effective safety products such as automatic shut-off valves, chlorine containment systems and emergency scrubber systems has helped make chlorine gas systems — for years the most commonly used disinfectant for the treatment of water and wastewater by municipalities — safer than ever.

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