On December 23, 2003, a gas-well blowout near the city of Chongqing in central China released a deadly mixture of natural gas and hydrogen sulfide. The toxic cloud of “sour gas” killed 243 people, caused the hospitalization and treatment of more than 9,000 and the evacuation of more than 60,000 nearby residents. Only two of those killed were gas field employees. The rest were residents of the surrounding area.

The nighttime accident was China’s worst oilfield disaster. The toxic cloud slowly drifted across villages, killing people in their beds or in fields or roads as they tried to flee, leaving behind what authorities described as a ten-square mile “death-zone” downwind of the blowout.

A government report issued the following week concluded the drillers improperly dismantled anti-blowout equipment, misjudged the amount of gas in the well and failed to spot the blowout. The crew failed to immediately ignite the gas as it began to escape, which would have prevented the toxic cloud from spreading. Emergency workers couldn’t approach the area until more than a day after the disaster because they lacked proper equipment.

Had local villagers been alerted through a system of sirens or public alarms, many more people might have had time to escape. Technicians finally plugged the leaking well on December 27, but the repercussions of the disaster will be felt for many years to come.

Not restricted to China

The hazards of sour gas are by no means restricted to developing nations. In 1982, a sour gas-well blew near the small town of Lodgepole in southern Alberta, Canada, and pumped sour gas into the air for 67 days. Two workers were killed and thousands of people downwind of the blowout were affected by the release. In Edmonton, over 75 miles away from the blowout, residents complained of nauseating odors, while closer to the well site residents suffered a variety of increasingly serious symptoms, including headaches, eye irritation, nosebleeds, as well as gastrointestinal and respiratory distress.

The accident could probably have been avoided, Alberta officials later concluded, if the site operator had followed a policy of more cautious drilling in the critical zone, and if they had been “better prepared to deal with unexpected developments.” It was fortunate that the blowout occurred in a remote location and not on the outskirts of Calgary, the largest city in the province.

Toxic properties

Hydrogen sulfide (H2S) is produced by the action of anaerobic, sulfur fixing bacteria on materials that contain sulfur. It is a constituent of natural gas, petroleum, sulfur deposits, volcanic gases and sulfur springs, and is especially associated with oil production, refining activities, sewers and many types of confined spaces. Fatal accidents due to H2S exposure occur far too often.

At low concentrations, H2S famously has an odor similar to rotten eggs. At higher concentrations, H2S rapidly deadens the sense of smell, leaving workers defenseless when they are depending only on their human senses to take preventive measures or escape from affected areas.

At 750 parts per million (ppm), inhalation can cause immediate collapse and unconsciousness. If exposure is very brief, for example, transitory envelopment by a passing gas cloud, the victim may awaken promptly and experience no adverse effects. In industries where hydrogen sulfide exposure is commonplace, such as oilfield work, employees often refer to this phenomenon as “knockdown.” At 1,000 ppm, a single breath results in immediate loss of consciousness, followed by cardiac arrest and death unless the individual is successfully revived.

H2S is soluble in water, explosive (its LEL concentration is 4.3% volume), corrosive, heavier than air and highly toxic. Exposure limits for H2S vary widely as a function of jurisdiction and workplace activity. The most widely recognized standards for H2S reference an 8-hour TWA of 10 ppm or 20 ppm, and a 15-minute STEL of no more than 15 ppm. Concentra-tions above 100 ppm should be regarded as immediately dangerous to life and health.

Look for it

The only way of being sure that hydrogen sulfide is not present in dangerous concentrations is to look for it with an atmospheric monitor designed for its detection.

Zero maintenance personal H2S detectors — Electrochemical H2S sensors are not consumed or affected by exposure to gas. Because they require so little power during normal operation, it is possible to package the sensor, an LCD display, audible alarm buzzer, built-in vibrator, flashing LED alarm light, and a battery capable of lasting two years all into a gas detector so compact it can be clipped into a shirt pocket or worn on a hard hat.

Multi-sensor “four-gas” monitors — The potential presence of H2S often goes hand-in-hand with the potential for combustible gas and vapor accumulations, as well as oxygen deficiency. Four-gas instrument designs have become highly compact, and the cost of a high-quality, basic four-gas monitor is only marginally more than the cost of a single-sensor H2S detector.

Fixed gas detection systems — Fixed detection systems provide alarm notification on a 24-hour-a-day basis. Alarms are activated whenever conditions become unsafe, whether or not workers are currently present in the affected areas or even if workers in the area are incapacitated. Fixed systems can be used to automatically notify local authorities in the event of an emergency.

Self-contained “stand alone” monitoring systems — Free-standing, solar-powered gas detection systems can be permanently located in remote areas. “Stand alone” systems can be equipped with radio transmitters that allow real-time monitoring results to be wirelessly transmitted to a base station or remote alarm located several kilometers away from the remotely located detectors. Because these portable systems can be powered by means of solar panels, they can easily be installed in areas that are too remote for traditional, line-powered gas detection systems.

Gas detectors used to monitor for the presence of H2S are increasingly compact, rugged, easy to use and much less expensive than ever before. Given the prevalence, dangers and human costs associated with H2S accidents, consider whether hydrogen sulfide detection should be an integral part of your workplace safety program.

SIDEBAR: Strong regs

As a result of a 1982 sour gas-well blowout, Alberta, Canada today has some of the toughest hydrogen sulfide (H2S) health and safety regulations in the world:

Oil industry standards include a comprehensive system of monitoring and reporting procedures.

Operators of “critical wells” must:

• install redundant safety equipment;
• prepare detailed emergency response plans;
• go door-to-door to warn residents of impending drilling; and
• maintain minimum separation distances from homes and public buildings.

In the event of a release, evacuation of the surrounding area becomes mandatory if the H2S concentration reaches 20 ppm.