OSHA Hazard Information Bulletins
January 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 Directorate of Technical Support Issues Hazard Information
Bulletin (HIB) 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 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 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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