CSB • Dangers of Purging Gas Piping into Buildings 1

Safety Bulletin

U.S. Chemical Safety and Hazard Investigation Board

September 2009

Dangers of Purging Gas Piping 

into Buildings

No. 2009-12-I-NC

When new fuel gas piping is put into service – or when 

existing piping is returned to service after interruptions – 

it is typically necessary to purge the lines of air.1 U.S. fuel 

gas safety codes require that new piping installations be 

pressure-tested with air or an inert gas prior to initial 

operation, and this activity requires purging during the 

introduction of natural gas.2 Purging is commonly done by 

 1 National Fuel Gas Code, NFPA 54, ANSI Z223.1 defines a piping purge as “to free a gas conduit of air or gas, or a mixture of gas and air,” at 54-15 

(2009 Ed.). The National Fire Protection Association (NFPA) develops widely recognized consensus fire protection codes and standards. Another widely 

recognized family of fire protection codes is published by the International Code Council which includes the International Fuel Gas Code (2009 Ed.). 

Both codes address safety guidance and requirements for the installation and operation of fuel gas piping and equipment. 

2 National Fuel Gas Code, NFPA 54, ANSI Z223.1 at 54-61 to 63 (2009 Ed.); the International Fuel Gas Code at 68 (2006 Ed.). The fuel gas codes require 

that piping beyond specified lengths be purged with an inert gas based upon the nominal pipe size.

one of two methods: (a) fuel gas is used to directly displace 

the air, or (b) inert gas is used to displace the air and then 

fuel gas is used to displace the inert gas. With this Safety 

Bulletin, the U.S. Chemical Safety Board (CSB) draws 

attention to serious dangers that can arise during fuel gas 

purging operations and highlights five key lessons the 

agency recommends for improving safety in the workplace.

Who’s 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

The ConAgra Slim Jim plant in Garner, North Carolina, where unsafe gas purging caused an explosion in 

June 2009 that killed three workers and sent 71 to the hospital.
CSB • Dangers of Purging Gas Piping into Buildings 2

Key Lessons

1. 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)3 and the associated danger of fire and explosion. 

Wherever practicable, 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.

2. Purging indoors should only be done in limited 

circumstances where purging outdoors is not 

practicable. In such cases:

• nonessential personnel should be evacuated;

• all ignition sources should be controlled or eliminated;

• ventilation should be adequate to maintain the gas 

concentration well below the lower explosive limit at 

all times.

 3 LEL, also known as the Lower Flammable Limit (LFL), is defined as “that concentration of a combustible material in air below which ignition will not 

occur.” Recommended Practice for Handling Releases of Flammable and Combustible Liquids and Gases, NFPA 329 (2005).

4 Natural gas is primarily composed of methane, an odorless and colorless gas. In order to heighten an individual’s ability to detect natural gas, 

small quantities of odorant are added. T-butyl mercaptan (typically described as having a “skunk-like” odor), is one such odorant. 

5 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).

6 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), stratification 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). 

3. Never rely on odor alone to detect releases of fuel gases.

An odorant4 is typically added to fuel gases, such as 

natural gas and propane, to warn workers and 

consumers of releases. 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.”

4. Always use combustible gas detectors5 to monitor 

the gas concentration during purging operations. 

To provide the most accurate information about 

combustible gas levels, sampling should be conducted 

frequently or continuously at appropriate locations.6

5. 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.

Interior of ConAgra facility following structural collapse. Aerial views of ConAgra facility after explosion.
CSB • Dangers of Purging Gas Piping into Buildings 3

Gas Purging Led to 2009 Explosion at

ConAgra Slim Jim Factory

On June 9, 2009, a major natural gas explosion heavily 

damaged the ConAgra Slim Jim meat processing factory 

in Garner, North Carolina, just south of Raleigh. Three 

workers were crushed to death when a large section of 

the building collapsed. The explosion critically burned 

four others and sent a total of 71 people to the hospital 

including three firefighters who were exposed to toxic 

anhydrous ammonia from the plant’s refrigeration system. 

Approximately 18,000 pounds of ammonia were released 

to the environment and 100,000 square feet of the plant 

were damaged. Due to the severity of the structural 

collapse, there was the potential for numerous additional 

deaths or serious injuries. 

The accident occurred during the installation of a new 

fuel gas-fired industrial water heater in an interior 

utility room of the plant. Five days prior to the accident, 

a new section of three-inch steel piping – which would 

provide natural gas to the heater – was tied into a 

six-inch natural gas supply line located on the roof. 

The new natural gas piping ran horizontally over 120 feet 

along the roof and then descended into the utility room.

On the day of the accident, a worker from Energy 

Systems Analysts (ESA), the water heater manufacturer, 

was attempting to purge the new gas line by using natural 

gas to directly displace the air. This was done by removing 

threaded fittings, creating one or more pipe openings 

near the heater. The worker then opened a quarter-turn 

valve to control the release of purged gases. ESA reported 

that it was the company’s normal practice to purge fuel 

gas piping directly into the room or area when installing 

gas-fired equipment. Code officials and other parties told 

the CSB that they believe this practice to be common. 

The purged fuel gas was vented indoors into the utility 

room, which was ventilated by an exhaust fan. However, 

no assessment was made of the adequacy of the ventilation in comparison to the rate of the gas release; whether 

a dangerous accumulation of flammable gas had occurred 

could have been most accurately verified by taking direct 

measurements inside the utility room using a combustible 

gas detector. Because of the difficulties in lighting the water 

heater, personnel perceived that the gas line was not 

effectively purged of air. Therefore, purging was conducted 

intermittently over a period of up to two-and-a-half hours.

Gas-fired water heater and piping, indicating points where gas was likely released into the building. 
CSB • Dangers of Purging Gas Piping into Buildings 4

ESA and ConAgra employees were aware of the 

natural gas purging activities inside the utility room. 

However, no appropriate combustible gas detectors 

were used to warn of a potential accumulation of gas in 

the building. Instead personnel relied primarily on the 

sense of smell to determine when the piping had been 

effectively purged of air and whether or not an unsafe 

release of natural gas occurred.

Some ConAgra employees smelled gas in the packaging 

area; others did not. Personnel who were in and out 

of the utility room noticed the gas odor, but most were 

not seriously concerned and considered the purging 

activity to be a normal part of the start-up process. 

The ESA and ConAgra employees were not aware that 

as a result of the purging, a dangerous accumulation of 

natural gas had occurred into the building, exceeding the 

lower explosive limit.

The vicinity of the utility room contained numerous 

potential ignition sources, including a number of unclassified electrical devices. Nonessential personnel were not 

aware of the water heater start-up or instructed to leave 

the plant during the gas purging activity. Over 200 people 

who had no role in the installation were in the building 

when the natural gas found an ignition source and 

exploded at approximately 11:25 a.m.

Following the June 9 explosion, ConAgra established 

new procedures for gas purging. These procedures require 

direct venting of purged gases via a hose or piping to 

a safe location outdoors, exclusion of personnel and 

ignition sources from the vicinity of the vent, continous 

air monitoring using combustible gas detectors, and 

evacuation of nonessential personnel from the facility.

Damage to vegetation hundreds of feet away from the ConAgra plant due to the ammonia release that followed the explosion.
CSB • Dangers of Purging Gas Piping into Buildings 5

Similar Incidents

Research conducted by the CSB during its investigation 

of the ConAgra explosion uncovered a number of similar 

incidents around the country that involved the purging 

of gas lines, including:

• An explosion at a 30-story hotel under construction in 

San Diego, California, on May 19, 2008, that injured 

14 workers, including three who suffered severe burns;

• An explosion at a hotel in Cheyenne, Wyoming, 

on August 7, 2007, that severely burned two plumbers;

Explosion seriously 

damages three floors 

of a Hilton Hotel 

under construction in 

San Diego in May 2008, 

injuring 14.

• An explosion that burned two plumbers at a school in 

Porterville, California, on November 16, 2005;

• An explosion on August 1, 1997, at a fitness center 

in Cary, North Carolina, a short distance from the 

ConAgra facility, which collapsed the roof, severely 

burned two people, and injured four others.

In addition, OSHA inspection records identify other 

related fuel gas purging incidents have occurred causing 

deaths and serious injuries.

The U.S. Chemical Safety and Hazard Investigation Board (CSB) is an independent federal agency charged with investigating industrial 

chemical accidents. The agency’s board members are appointed by the president and confirmed by the Senate. CSB investigations 

look into all aspects of chemical accidents, including physical causes such as equipment failure as well as inadequacies in regulations, 

industry standards, and safety management systems. 

The Board does not issue citations or fines but does make safety recommendations to companies, industry organizations, labor groups, 

and regulatory agencies such as OSHA and EPA. Please visit our website, www.csb.gov.

No part of the CSB’s conclusions, findings, or recommendations may be admitted as evidence or used in any action or suit for damages; 

see 42 U.S.C. § 7412(r)(6)(G).

Photo: Dirk Hansen

Conclusion

As a result of the findings from the ConAgra explosion 

and other recent similar incidents, this Safety Bulletin 

emphasizes five key lessons to prevent fires and explosions from purging fuel gas into buildings. These tragic 

incidents can most effectively be prevented by purging 

flammable gases to a safe location outdoors. Where this 

is not practicable, important safety precautions should 

be in place, including removing nonessential personnel, 

eliminating ignition sources, and ventilating the space 

so that the atmosphere is substantially below the LEL. 

Combustible gas detectors should always be used to 

monitor the gas concentration during purging operations 

– never rely on the sense of smell alone. To effectively 

implement these practices, workers must be fully trained 

and knowledgeable about safe purging practices and the 

hazard of odor fade in new gas piping systems.