Preventing Chlorine Gas Accidents

For portable detectors, think about ease of use, training, battery life, and service life. For fixed detectors: ease of installation, calibration, and maintenance.

• By Alan Austin
• May 01, 2005

WHILE you might not realize it, the chemical chlorine (Cl₂) and its compounds are part of almost everyone's daily life. The water you drink, the food you eat, the medicine you take, the clothing you clean, the pool you swim in, the car you drive, and thousands of other products are sanitized or manufactured with chlorine.

Chlorine is found in many industrial processes, including those used to make plastics, vinyl, and nylon, as well as pharmaceuticals and the food/beverage industry, too. The electronics industry relies on chlorine in the production of microprocessors and computers. You'll find chlorine supports the manufacture of gasoline additives, brake fluid, and antifreeze, as well as popular metals such as titanium, magnesium, and aluminum.

How can something that is so common also be so dangerous and potentially deadly when used improperly? Chlorine gas is toxic and green-yellow in color at room temperature. It is also water soluble. At 0.3 to 0.5 parts per million (ppm), you can start to smell chlorine gas, and trouble soon follows in higher concentrations.

Chlorine burns the eyes, nose, and throat--eventually causing bronchial inflammation, respiratory tract damage, and death. Chlorine is heavier than air, which means that it remains near the ground and exposure victims need to be moved away from the contaminated environment. In World War I, chlorine gas also was used as a weapon of mass destruction on U.S. soldiers.

Accidental exposure to chlorine gas need not be deadly when the proper safety procedures and equipment are in place. Both EPA and OSHA have strict regulations for the use of chlorine. Many accidents are preventable with the proper training and toxic gas monitoring safety equipment.

In the workplace, both portable gas detectors and fixed gas detection systems are employed to help protect workers from chlorine. Depending on the type of workplace hazard, one or both types of gas detectors may be in use. The use of chlorine is prevalent in so many industries that it is helpful to have a broader understanding of its applications before developing a chlorine gas safety program and determining the requirements for portable and/or fixed gas detectors in any plant.

Oil Refineries
Oil refineries inject chlorine directly into stacks to reduce sulfur emissions (H₂S, for example) and require monitoring. The sulfur monochloride compound that forms in the stack is a solid and can drop out of the air flow in the stack and coat the inside of the stack itself or drop down to the bottom of the stack, where it can be collected. Portable and fixed toxic gas detectors also can be used to monitor potential chlorine leakage around the bulk storage tanks.

Clean Water Treatment
Chlorine is added to treat drinking water to destroy bacteria and other harmful micro-organisms. It also controls algae or slime and helps improve the taste and smell of fresh water. Chlorinating basins and well sites have small buildings enclosing chlorine storage tanks with mixing systems. Fans provide a ventilation system, with gas detectors installed for monitoring and alarm purposes in case of a system leak. Portable gas detectors often are carried into these confined space areas.

Sewage Wastewater
For neutralization of effluent, a complete (Cl₂) chlorination system is often used at waste treatment facilities. Toxic gas detectors are used to detect chlorine gas at these locations: the chlorine tanks, the chlorine dosing pump, the chlorine mixer, and the sampling area. In open settling pond areas, portable gas detectors are carried to warn workers of excessively high concentrations of gas.

Pharmaceuticals Production
Most prescription and over-the-counter pharmaceuticals contain or are manufactured with chlorine. Some of the more common medications that are made using chlorine include pain relievers, decongestants, antihistamines, and antibiotics. Toxic gas detectors are utilized both within the areas where pharmaceuticals are processed and in storage tank areas.

Pulp/Paper Bleaching
Bleaching of pulp is a series of chemical reactions conducted in stages. These reactions are carefully controlled to bleach the pulp without destroying the strength of the fiber. Chlorine gas and water are added to the eductor prior to the mixing stage. Toxic gas detectors can be used to monitor for chlorine gas leakage during the bleaching process.

Agricultural Pesticides
With world hunger continuing to be a pressing problem, chlorine is used in the manufacturing process for crop protection chemicals. Such chemicals help crops resist disease, insects, and weeds. In large pesticide manufacturing operations, toxic gas detectors are found in the mixing and blending areas, as well as in storage tank areas for both raw materials and finished products.

Food & Beverage
From meat to produce to beverages, chlorine is applied as a disinfectant and antimicrobial agent. Chlorine is used as a sanitizer in cleansing employees' hands and footwear, in the washing of beef carcasses, in fruit/vegetable spray washing or flume systems, and in the processing of eggs, seafood, and much more. Wherever chlorine is stored in tanks or dispensed with injection systems in heavy concentrations, portable and fixed toxic gas detectors can protect people and equipment.

Protecting Workers from Chlorine Gas
Every year, workers are injured needlessly in plant accidents caused by leaking toxic gases. These gases include chlorine and others such as ammonia, carbon monoxide, hydrogen sulfide, and more. Most of these accidents are preventable with personal safety or plant gas monitoring equipment. Knowing when to utilize portable gas detectors versus fixed gas detection systems is important.

Portable and/or fixed-point gas detectors are essential to protect workers and equipment from chlorine gas, as well as an OSHA and EPA requirement. Portable detectors rely on electrochemical cell (EC) sensors, and they are utilized in confined spaces or large areas where fixed systems are impractical or cost prohibitive.

In confined spaces, the access is often infrequent and the danger may be more from oxygen deficiency than from a toxic gas such as chlorine or a combustible gas requiring continuous fixed monitoring. When portable detectors sense a toxic or combustible gas, they immediately warn the technician through visual, audible, and vibrating alarms.

A fixed system typically consists of one or more electrochemical cell sensors placed in key locations. When toxic or combustible gas is present outside of acceptable limits (an alarm), gas exposure data are communicated via hard wiring to a control station. Workers are warned by audible signals, flashing lights, and often by monitoring screens that a hazardous gas is present.

Electrochemical Sensing
Electrochemical sensors are widely used for both fixed gas detection systems and in hand-held portable devices for personal use where fixed gas detection systems are impractical. Such use is typically found in confined space areas where access is infrequent or in large outdoor areas where the use of fixed gas detection systems is cost-prohibitive.

Electrochemical sensors consist of a sensing electrode, a counter electrode, and a reference electrode separated by electrolyte. The gas to be detected diffuses through a capillary diffusion barrier, which controls the amount of gas reaching the sensing electrode. The target gas reacts at the surface of the sensing electrode by either oxidation or reduction. Reactions are catalyzed by specifically developed electrodes.

Gas detection with electrochemical cell sensing is reliable, accurate, responsive, and relatively low in cost. The detectors require monthly recalibration of the cells, which typically last about one to two years--sometimes longer, depending on the gas sensor type and the operating environment.

Advantages:

• Detect a wide range of toxic gases.
• Are accurate, perform reliably.
• Are easy to use.
• Are small and lightweight for personal use.
• Are affordable.

Disadvantages:

• The detection range can be limited.
• They are not suitable for the harshest of environments.
• They require frequent recalibration.

Workplace Safety Planning
When planning a toxic gas safety program to protect against chlorine and other hazardous substances, there are many important factors to consider no matter whether you need a portable or a fixed gas detector. Some of the factors are:

• Walk the plant, check potential leak sources, think over placement of fixed detectors, and identify confined space permit hazard areas for portable detectors.
• Review the plant's ambient operating environment (watching for temperature and humidity extremes).
• Consider the various portable and fixed detector sensing technologies for performance and reliability.
• For portable detectors, think about ease of use, training, battery life, and service life.
• For fixed detectors, look at ease of installation, calibration, and maintenance.
• Evaluate the detector's maintenance and repair needs.

Protecting plants where people are continuously in the presence of processes requiring chlorine gas, for example, is often best achieved with fixed gas-sensing systems using fixed gas detectors placed throughout the facility. You could give every employee a portable gas detector, but how would you guarantee they would always wear and maintain their monitors in good working order?

In confined spaces, on the other hand, the technology of choice is EC sensors in a portable gas detector. Chlorine rooms need to be cleaned, pumps require maintenance, and toxic gas or oxygen deficiency can be deadly in such environments. Placing fixed gas detectors inside tanks, though, is both impractical and too costly given their infrequent access by technicians. The best answer in this case is a portable detector in the many plant environments where people are only present to perform specific tasks on a limited basis.

This article appears in the May 2005 issue of Occupational Health & Safety.

This article originally appeared in the May 2005 issue of Occupational Health & Safety.