While the reasons for gas detection are well-known, what’s less certain is which methods and technologies are most useful for individual process applications, and where and how much to apply them for the best result and greatest efficiency.
These reasons include safeguarding life and property by providing early warnings of hazardous conditions; enabling personnel to evacuate and notifying others to avoid reentry; and allowing time for intervention and correction by triggering protection systems, such as ventilation, water mist and fire suppression. “Gas detection systems also help satisfy local and fire codes, enable facilities and companies to be insured, and address both real and perceived safety concerns,” says John Greivell, vice president of Lesman Instrument Co. in Bensenville, Ill., who presented “Gas detection system—considerations for design” at a recent ISA Will-DuPage chapter meeting. Lesman distributes, integrates, services and repairs, and builds panels for gas detection.
Greivell reports there are three primary types of gas hazards:
- Flammability includes risks of fire and/or explosion due to gases and solvents like methane, butane, propane or gasoline. As always, the three factors required for combustion are an ignition source, oxygen and fuel in gas or vapor form. However, flammable gases are only ignitable over their flammable range from their lower explosive limit (LEL) to their upper explosive limit (UEL), and tend to be measured as a percentage of their LEL.
- Toxic with risks of poisoning due to gases like carbon monoxide, hydrogen sulfide and chlorine. Toxicity is based on acute or chronic exposure levels, while toxic risk is measured in parts per million (ppm) concentrations.
- Asphyxiant with risks of suffocation caused by oxygen deficiency when it’s normal concentration is displaced by nontoxic or minimally toxic gases, such as nitrogen, carbon dioxide, argon, helium and others. Accidental gas leaks in confined spaced can deplete normal oxygen levels.
Project goals begin with failure
To confront and prevent these hazards, Greivell explains that gas detection projects should begin by answering four main questions: What does a failure look like? How do you want to communicate? What behaviors do you want? And, what’s the design life?
“Defining what a failure looks like begins with identifying leak scenarios, including what gas or chemical is involved, its hazard characteristics, and its vapor density, which determines where it will migrate to because heavy gases sink, while light gases float,” says Greivell. “We also need to know a leak’s source location, and under what conditions would a failure occur, such as temperature, humidity, release pressures and other environmental conditions. This also means identifying physical features of plants and facilities, airflow and wind directions, and engaging everyone who might be affected.”
Read more: Gas detection design basics