Natural Gas and Explosive Limits
9/20/2010
Because many of us have sat through an OSHA HAZWOPER class or similar safety and health instruction course, the terms lower explosive limit (LEL) and upper explosive limit (UEL) should conjure up some recollection. In simple terms, these are chemical-specific air concentrations that, when subject to an ignition source, can result in an explosion. Just about every substance has its unique LEL and UEL. Airborne concentrations of specific materials that are less than its LEL will not cause an explosion, as the mixture of chemical to air (oxygen) is too lean. On the other hand, when the air concentration is too rich, having too great a concentration of a chemical within air, again, the atmosphere will not support an explosion. During the disaster a few weeks ago in San Bruno, California, there was a break in the natural gas line within a residential neighborhood, allowing the immediate area to accumulate with natural gas, reaching and even exceeding the LEL for natural gas. The result was the destruction of nearly 40 homes and at least four deaths. Natural gas is comprised mostly of methane (approximately 95%) with traces of other hydrocarbons (ethane, propane, iso-butane, etc). These gases are a light-weight and odorless, with explosive ranges of 4% – 16%. For safety purposes, a small amount mercaptan is included. Typically referred to as an odorant, mercaptans have a very distinctive odor that can be detected by the human sense of smell at the low concentration range of 1 -2 parts per billion (a combination of mercaptans comprise the chemical spray skunks emit to ward off would-be attackers). While mercaptans have a very low odor threshold, their occupational exposure limits are much higher (10 parts per million). Therefore while we may detect mercaptans, they do not represent a health hazard. For all of you chemists, mercaptans are basically a thiol group (S-H group) attached to an organic chain. It’s the thiol, or more specifically, the sulfur in the thiol group that produces the chemicals’ displeasing odor.
While the ignition source for the San Bruno disaster is still yet to determined, it is evident that airborne natural gas did escape into the atmosphere and was ignited by some unknown flame source (spark from a car, tossed cigarette, etc.); What we do know is that residents did smell natural gas (or the odorant) and reported the incident to the local power company, Pacific, Gas and Electric. According to recent sources, the responses to customer complaints were not addressed in a timely manner. Quite possibly if a proper response was initiated, the location of the gas leak could have been identified and the area isolated so that ignition sources would have been removed; thus preventing the catastrophe.
Many compounds/chemicals have explosive ranges. While we may use an instrument to detect ignitable gases (i.e. explosimeter), we still need to know what chemicals we are concerned about. For instance, while it is common to calibrate an explosimeter with methane (LEL = 5%), we may be working in an environment where a chemical with a very stringent lower LEL may be present (ex. MEK = 1.4%, or Benzene = 1.2%, or 1,2,3 –trimethylbenzene =0.8%). Therefore, when the meter read 20% of the LEL, we could already be in an explosive atmosphere! This is one reason why it is important to have qualified persons operate field equipment.
Where can we go to find out about explosive limits for specific gases? The National Institute for Occupational Safety and Health (NIOSH) publishes the “Pocket Guide to Hazardous Chemicals:” a book that provides an abundance of information for hundreds of chemicals. It is free by just contacting NIOSH (1-800-CDC-INFO) or go to http://wwwn.cdc.gov/pubs/niosh.aspx. A CD-ROM that contains the pocket guide as well as other chemical information can also be ordered at no cost. Another way to get chemical information from NIOSH is to simply visit their web page at http://www.cdc.gov/niosh/npg/. This site allows one to view chemical-specific information at any time without any limitations.
Safety Challenge #1: There are many times when a photoionization detector (PID) is used to detect airborne contaminants. Even though a PID is much more sensitive than an explosimeter (providing concentrations in the parts per million range as opposed to percentages), why wouldn’t a PID be useful for natural gas leaks?
Answer: A typical PID has a 10.6 eV lamp, meaning it can ionize materials with an ionization potential (IP) when subjected to that energy. One example is benzene, that has an IP of 9.24. Some PIDs have the capability to use an 11.7 eV Lamp, allowing for additional molecules to be ionized; providing their IP is equal to or less than 11.7eV. However, methane, the main constituent of natural gas, has an IP listed at 12.615; so, a typical PID would be able to ionize methane (or natural gas). An explosimeter or Flame ionization detector would be the appropriate instrument to use in this case.
Safety Challenge #2: Even though hydrogen has a LEL that is very similar to natural gas (approximately 5%), why is hydrogen considered a very dangerous explosive?
Answer: Hydrogen has a very broad flammable range. As an example, methane has a lower and upper explosive concentration of 5% – 15% (respectively), Hydrogen has a range of 4% – 75%). And the upper range increases to 90% when in an atmosphere of pure oxygen. Therefore, there is a much greater risk for hydrogen to react when subject to an open flame.
Life is a succession of lessons which must be lived to be understood.
Ralph Waldo Emerson