HAZARD
Cherie Berry, Commissioner of Labor
1- 800- 625- 2267 w www. nclabor. com
• Feb. 7, 2010: An explosion at the Kleen Energy Systems
power plant construction in Middletown, Conn., killed six
workers and injured 50 others.
• June 9, 2009: An explosion at the ConAgra Slim Jim plant in
Garner, N. C., killed four workers and injured 70 other workers.
In addition, an ammonia refrigeration line was ruptured,
forcing the evacuation of nearby schools.
• May 19, 2008: One firefighter and 13 construction workers
were injured following an explosion at a hotel under con-struction
in downtown San Diego, Calif.
All of these incidents had two things in common. First, natural
gas was allowed to vent into the interior of a building during
the purging of gas pipelines.
Second and most important,
each of these incidents could
have been prevented.
What is natural gas?
Natural gas is a fossil fuel
consisting of a mixture of
hydrocarbons, mostly methane,
which is found underground or
produced by the gasification of
coal. Natural gas is colorless and
odorless and is doped with small
quantities of odorants, such as
t- butyl mercaptan ( described as
having a “ skunk- like” odor), to
heighten an individual’s ability to detect its presence.
Why is natural gas hazardous?
Natural gas is a highly flammable gas that readily burns in
air and can explode when it accumulates in a confined space.
The flammable limits in air for natural gas range from 3.8- 6.5
percent ( lower explosive limit, “ LEL”) to 13- 17 percent ( upper
explosive limit, “ UEL”).
Natural gas is also a simple asphyxiant and will readily displace
oxygen in air at higher concentrations. The immediately dangerous
to life and health ( IDLH) level is 1,200 parts per million ( ppm).
Please note that 1 percent is equivalent to 10,000 ppm.
What industries and occupations are at risk?
Personnel who manage, install, maintain, repair, inspect, or
place into operation fuel gas piping and equipment, including:
• plumbers
• gas installers
• maintenance workers
• contract supervisors
• industrial facility managers
How can natural gas explosions be prevented?
• Purging new or existing gas
piping into buildings can be
highly hazardous due to the
possible accumulation of gas
above the lower explosive limit
( LEL) and the associated danger
of fire and explosion. Wherever
possible, directly vent purged
gases to a safe location outdoors,
away from people and ignition
sources. This can be done using
a temporary hose or piping or
permanently installed vent pipes,
depending on the facility design.
• Purging indoors should only be
done in limited circumstances where purging outdoors is not
practicable. In such cases:
v nonessential personnel should be evacuated;
v all ignition sources should be controlled or eliminated;
v ventilation should be adequate to maintain the gas con-centration
well below the lower explosive limit at all times;
• Never rely on odor alone to detect releases of fuel gases.
An odorant is typically added to fuel gases, such as natural
gas and propane, to warn workers and consumers of releases.
Occupational Safety and Health Division
1101 Mail Service Center, Raleigh, NC 27699- 1101
Dangers of Natural Gas Pipe Purging Into Buildings
ConAgra Slim Jim plant in Garner, N. C., where unsafe gas purging
caused an explosion in June 2009.
continued on page 2
However, the perception of odor is highly subjective and varies
from one person to another. People also become desensitized
to odor during prolonged exposures. Additionally, new gas
pipes and containers can react with or otherwise remove the
odorant, an effect known as “ odor fade.”
• Always use combustible gas detectors to monitor the gas
concentration during purging operations. 1 To provide the
most accurate information about combustible gas levels,
sampling should be conducted frequently or continuously
at appropriate locations. 2
• Ensure personnel involved in gas purging operations are
fully trained and knowledgeable about safe gas venting
practices, the proper use of gas detectors, and the danger
of relying on the sense of smell alone to detect gas releases.
Include training on the problem of odor fade in new gas
piping systems.
Notes:
1. Combustible gas detectors measure combustible/ flammable
gas concentration in the atmosphere, which is indicated on
the device as a percentage of the lower explosive limit ( LEL).
2. The evaluation of appropriate locations for combustible gas
monitoring should include consideration of the purge location,
characteristics of the gas ( lighter or heavier than air), strati-fication
or mixing of the gas, and existing ventilation. See the
International Society for Automation ( ISA) RP 12.13,
Recommended Practice for the Installation, Operation, and
Maintenance of Combustible Gas Detection Instruments
( 2003) and the American Petroleum Institute ( API) 2009, Safe
Welding, Cutting, and Hot Work Practices in the Petroleum
and Petrochemical Industries ( 2002).
Available Resources
The North Carolina State Building Code was amended in
September 2009 to generally prohibit indoor venting during fuel
gas purging operations unless venting outdoors is not possible.
In addition, the new rules require training for personnel involved
in gas purging and prohibit the reliance on odor as a determinant
of gas concentration. See Volume: Fuel Gas, Section 406.7:
www. ncdoi. com/ OSFM/ Engineering/ BCC/ Documents/
TempRules/ 090915_ FuelGas406.7. pdf.
Section 8.3 of NFPA 54 ( National Fuel Gas Code) specifically
addresses the purging of fuel gas lines prior to placing or
returning them in service. NFPA standards are copyrighted and
can be purchased or viewed online by subscription through
their website at www. nfpa. org.
Information regarding the investigation of the ConAgra Plant
Explosion and other natural gas explosions can be accessed through
the U. S. Chemical Safety and Hazard Investigation Board at
www. csb. gov/ UserFiles/ file/ CSB_ Safety% 20Bulletin_
Final_ Embargoed_ 10_ 2_ 09. pdf.
Disclaimer
This industry alert provides general information about the
hazards associated with the use of natural gas to purge gas
lines and ways to prevent natural gas accumulations inside
buildings during this procedure. This document is not intended
to be a substitute for compliance with the requirements of
applicable OSH standards.
Printed 9/ 10
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