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COMMERCIAL SAFETY PRODUCTS |
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The Use of Combination Oxygen and Combustible Gas Detectors.
- INFORMATION DATE : 19900118
- RECORD TYPE : Hazard Information Bulletin
- SUBJECT : The Use of Combination Oxygen and Combustible Gas Detectors.
JAN 18 1990
MEMORANDUM FOR: REGIONAL ADMINISTRATORS
THRU: LEO CAREY
Director
Office of Field Programs
FROM: BARRY WHITE
Director
Directorate of Technical Support
SUBJECT: Hazard Information Bulletin on the Use of Combination
Oxygen and Combustible Gas Detectors
The purpose of this bulletin is to alert field personnel to some of the
shortcomings/limitations in the use of combination oxygen and combustible
gas detectors. Recently the Boston Regional Office brought to our
attention a fatality in which an employee, from a small welding firm, was
cutting up a 4,000 gallon tank that had contained leaded gasoline, when
the tank exploded.
The ensuing investigation uncovered that the tank had been repeatedly
steam cleaned and purged with nitrogen gas prior to the explosion.
Although combustible gas readings were taken prior to welding, with the
last reading indicating below the LEL, it was evident that improper use of
the instrument and an incomplete understanding of its limitations by the
operator were the cause of the fatality.
In tank removal operations, such as the above, it is common practice to
purge a tank containing flammable vapors with either carbon dioxide or an
inert gas, such as nitrogen. When the oxygen content falls to about 10% or
below, a false combustible gas indicator reading can occur.
The combination oxygen and combustible gas meter is used to test
atmospheres for sufficient oxygen content for life support and/or the
presence of combustible gases or vapors posing a potential
flammability/explosion hazard. Common examples of locations where this
instrument is used include storage tanks, confined spaces, manholes, tank
cars, ships and shipyards, tunneling, pumping stations and hazardous waste
sites.
___________
The Directorate of Technical Support Issues Hazard Information
Bulletin (HIS) in accordance with (OSHA Instruction CPL 2.65 to
provide relevant information regarding recognized or misunderstood
health hazards, inadequacies of materials, devices, techniques, and
engineering controls. HIB's are initiated based on information
provided by the field staff, studies, and reports and concerns
expressed by safety and health professionals, employers, and the
public. Information is compiled based on a thorough evaluation of
available facts, literature and in coordination with appropriate
parties. HIB's are used as vehicles for the dissemination of
technical information to OSHA.
The combustible gas indicator is designed to measure combustible gas or
vapor content in air. This instrument is capable of detecting the
presence of any gas or vapor which, when combined with oxygen in free air,
presents a potential hazard due to flammability/explosion. The
combustible gas indicator will not indicate the combustible gas content in
atmospheres containing less than 10% oxygen.
It should be noted that each instrument has its own set of operating
procedures and instructions. Consequently, it is impractical to list, in
this memo, all precautions and warnings for each oxygen/combustible gas
detector in use. However, by way of illustration, other precautions and
warnings not already discussed above, and specifically applicable to the
MSA MicroGard Portable Alarm, are as follows:
1) The instrument should not be used where the oxygen concentration
exceeds that of fresh air (oxygen enriched atmosphere) when
sampling for gases like acetylene and hydrogen.
2) Certain materials such as silicon, silicates (such as in certain
hydraulic fluids) and organic lead (such as in leaded gasoline)
will poison the combustible gas sensor thereby giving
erroneously low readings.
3) Combustible gas readings, either negative or greater than 100%
LEL, may indicate an explosive concentration of gas beyond the
accurate response range of the combustible gas sensor.
4) Pressurized or low pressure samples will give erroneous oxygen
percent readings.
5) Acid gases, such as carbon dioxide, will shorten the service
life of the oxygen sensor.
6) The instrument will not indicate the presence of combustible
airborne mists or dusts such as lubricating oils, coal dust or
grain dust.
The above caution list is by no means comprehensive. For a more complete
listing of cautions and warnings regarding the MSA MicroGard Portable
Alarm the instruction manual must be referenced.
The safe and effective performance of any oxygen/combustible gas detector
requires that the operator know the correct use of the instrument to
detect explosive concentrations of combustibles. It is important that the
instrument response be appraised in light of the limitations and
guidelines given in the instrument manual. The instrument should be
operated only after the instructions, labels, cautions and warnings, and
any other literature accompanying the instrument are carefully read and
understood.
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