A MINI-SYMPOSIUM ON FLAMMABILITY DETERMINATION
To be Held in conjunction with the ASTM E27.04 Spring Meeting
May 7, 2008, Denver, Colorado, USA
A Method for Determining the Flammable Limits of Gases in a Spherical Vessel
Prof. Daniel Crowl, Michigan Technological University, USA and
Dr. Young-Do Jo, Korea Gas Safety Corporation, Korea
The flammable limits of gases are very important to prevent fires or explosions in
industrial and domestic applications. When a flammable or combustible gas is mixed with an
oxidizer, such as air, in the proper concentration in the presence of a source of ignition, rapid
combustion or an explosion will occur. The flammable limits are the boundary concentrations of
flammable gas that will support the propagation of flame through the volume of the mixture after
ignition. Flammable limits are also defined for both upward and downward flame propagation
and are, for most chemical species, found at different concentrations.
The current method for determining the flammable limits for a gas in a closed spherical
vessel is based on a specification of the maximum pressure increase during the combustion,
usually from 5 to 10% of the initial ambient pressure. This approach is completely arbitrary and is
not fundamentally based. For most hydrocarbons this pressure boundary and hence the flammable
limit is easy to determine experimentally since an abrupt pressure drop occurs at the flammable
limit as the fuel concentration in air is adjusted. However, for some species, particularly hydrogen
mixed with air, the drop in maximum combustion pressure is not very abrupt and the fuel
concentration can range several percentage points depending on the arbitrary criterion used for
the flammable limit.
This paper will discuss a new approach for determining the flammable limits for a gas in
a spherical vessel. The approach is based on the maximum second derivative of pressure rise. The
second derivative is indicative of an acceleration of the combustion rate and is, hence,
fundamentally based. Furthermore, we have identified a new approach to determine the
downward propagating flammable limits based on the combustion time, that is, the time that the
gas actually burns in the vessel.
Towards a Global Standard for Flammability Determination
Dr. Elisabeth Brandes, PTB (Physikalisch-Technische Bundesanstalt), Germany
and Dr. Erdem A. Ural, Loss Prevention Science and Technologies, Inc., USA
Lower Flammability Limit (LFL), Upper Flammability Limit (UFL), and the Limiting
Oxygen Concentration (LOC) are used frequently in fire and explosion hazard evaluation and
protection designs. Unfortunately, different countries use different test standards which may result
in different flammability limit values. Global companies are often faced with the difficult decision
of selecting the most appropriate values for their applications. The problem has recently been
compounded with the discovery that some of the old US Bureau of Mines LOC data published in
reference books can be non-conservative. Consequently, the 2008 edition of NFPA 69, “Explosion
Prevention Systems,” requires either using new LOC data or subtracting 2 volume % from the old
LOC values.
This paper presents a critical comparative review of the modern flammability standards
currently used in Europe and in United States, and the data obtained in accordance with them.
There is significant room for improving these standards. Visual test methods employing small tube
or flask tend to make the flammable range appear wider than it is. They can also conceal
flammability. The pressure rise criterion methods, on the other hand, are much less subjective
than the visual methods and are becoming more popular. This paper presents a simplified analysis
which links the visual criteria to the pressure rise criteria. The analysis also identifies a technical
flaw inherent in the fixed pressure rise criteria. This paper also proposes a new pressure rise
criterion which is volume dependent for smaller test enclosures.
A comparison of how US and European safety professionals use the flammability data,
and a fundamental philosophical difference between the US and European methods for
flammability determination are also provided.
Both papers will also be presented at the 4th Global Congress on Process Safety
For additional information contact: erdem.ural@lpsti.com or (781) 818-4114