Focus 2009 - No. 9
Is the Liquid Flammable or Combustible?....
Does it Matter?
Peter Engstrom, Process Safety Specialist
Chilworth Technology, Inc.
250 Plainsboro Rd, Bldg #7
Plainsboro, NJ 08536
Tel: 609-799-4449
Fax: 609-799-5559
http: www.chilworth.com
email: safety@chilworth.com
Those of us who have worked with hydrocarbons, particularly in the petroleum industry, have been taught that the words
“flammable” and “combustible” are not interchangeable. In addition, we’ve been taught that combustible liquids do not
burn as readily as flammable liquids. So how do we determine whether a liquid is flammable or combustible?
Furthermore, does it matter when it comes to designing facilities to minimize the safety risk to personnel?
Years ago when folks were writing standards for safely handling hydrocarbon liquids, they recognized that the chance and
severity of a fire was higher for hydrocarbon liquids that readily vaporized at ambient temperature than for hydrocarbon
liquids that needed additional heat to vaporize. So they decided to categorize hydrocarbon liquids into two categories: 1)
liquids that vaporized sufficiently to ignite at ambient temperature were classified as Flammable, while 2) liquids that
needed additional heat to vaporize sufficiently to ignite were classified as Combustible.
Of course, before the terms “flammable” and “combustible” could be included in the standards, two parameters had to be
agreed on: 1) What physical property would be used to determine how “readily” a liquid burned, and 2) What
temperature would be used to distinguish between the two terms (in other words, what temperature would represent the
maximum “ambient” temperature).
The property used to determine how readily a liquid burned was agreed to be the Flash Point, defined as the lowest
temperature that will vaporize enough liquid to ignite the vapor in the presence of an ignition source. Other properties
could have been used, such as Fire Point or Lower Temperature Limit, but Flash Point won the title.
Deciding on what ambient temperature to use to determine whether a liquid is flammable or combustible turned out to be
trickier. To this day, the temperature cutoff varies by organization. Furthermore, the temperature has changed within an
organization over the years. For instance, the National Fire Protection Association (NFPA) used 140 F (60 C) for the
cutoff until 1973, when it changed to the current temperature of 100 F(37.8 C) to conform to the then recently published
OSHA 1910.106 Flammable and Combustible Liquid Standard. The Department of Transportation (DOT) initially used
80 F (27 C), then changed to 100 F when the OSHA standard was published, then changed again in 1991 to 141 F(60.5C)
to conform to the United Nations rules for transporting liquids, though they maintain a “domestic” exemption at 100 F.
Temperature cutoff notwithstanding, the terms “Flammable” and “Combustible” were incorporated into the various design
standards being written at the time, such as those from NFPA and American Petroleum Institute (API), to help companies
design safe operating facilities. The philosophy within these standards was that facilities that handle flammable liquids
should have more safety systems than those that handle combustible liquids. Eventually, some of these standards were
included into United States regulations, such as OSHA 1910.106, DOT Title 173.120, and EPA RCRA regulations.
Unfortunately, problems started to arise with the terms “Flammable” and Combustible”. For instance, a combustible
liquid at a temperature greater than its Flash Point could burn as readily as a Flammable Liquid, so should a distinction be
made between flammability and combustibility in this case?
This is a common occurrence in industry. For example, petroleum refineries--by the nature of their processes, including
distillation, hydrotreating, and cracking--frequently have situations where combustible liquids are at temperatures above
their Flash Points. So do ball mills or steam jacketed batch reactors that use petroleum solvents, such as mineral spirits,
where the temperature inside the equipment is higher than the solvent’s Flash Point. For that matter, it’s possible that a
combustible liquid could be stored in an area where the ambient temperature is higher than the liquid’s Flash Point, such
as in unventilated buildings during the summer.
Focus 2009 - No. 9
Treating the liquids in these examples as combustible liquids during the facility design and operation would be underestimating the potential fire hazard that these liquids could create if exposed to air. In addition, should these liquids spill,
they would still be at their elevated processing temperature for some period of time before they cooled to ambient
temperature, meaning they could still be above their Flash Points.
Another problem is perception: The fact that a combustible liquid does not burn as readily as a flammable liquid
sometimes is extrapolated to say that a combustible liquid does not burn at all. This has happened in Process Hazards
Analysis (PHAs) that I have facilitated, where a liquid at a temperature above the Flash Point has spilled in large
quantities. However, because the liquid was designated as combustible on the MSDS, some of the PHA Team members
assumed there was no fire risk associated with the spill.
So what should a process safety professional do?
When the discussion turns to a debate whether a liquid is flammable or combustible, I like to refocus the group towards
the Laws of Physics and away from manmade terminology. In this case, the Law of Physics of interest is that some
liquids can burn under the right conditions. Therefore, the group, whether a design team or PHA Team, should focus on
identifying what those conditions are and whether the liquid being discussed could be at those conditions.
To help with this “Laws of Physics” discussion, I don’t use the terms “Flammable” and “Combustible” when describing a
liquid because over the years these terms have collected a lot of baggage, i.e., different, and sometimes emotional,
meanings to different people.
Instead, I use the term “ignitable mixture”. This is the term used in NFPA 497- Recommended Practice for Flammable
Liquids Electrical Classification. The term is defined as either “….flammable liquid produced vapors or (emphasis
added) combustible liquid produced vapors mixed with air that may burn or explode”.
While NFPA 30 - (Flammable and Combustible Liquids Code) - does not use this term, it uses a similar phrase as the
definition above. The phrase used in NFPA 30 is: “….flammable liquids or (emphasis added) combustible liquids
[stored/handled/heated] above their Flash Point”. Dow’s Fire & Explosion Index Hazard Classification Guide uses a
similar phrase when calculating risk (referred to as penalties) associated with a liquid.
I have found that once the group uses the term “ignitable mixture”, they begin to focus on the operating conditions that the
liquid is exposed to, such as temperature, pressure, and composition, and not on terminology. During a PHA, this also
causes the PHA Team to identify more Hazard Scenarios where a fire hazard could be present, as well as identify more
robust engineering controls to reduce the likelihood of a fire.
For example, one client used a petroleum solvent with a Flash Point of 105 F in a ball mill. Because the solvent was
“combustible” per the definition, previous PHAs had not identified a fire hazard and, as a result, the mill was not inerted.
But once we started discussing the operating conditions inside the ball mill, it was discovered that the temperature inside
the mill was as high as 120 F, meaning an “ignitable mixture” was present inside the mill. In addition, it was pointed out
that the temperature inside the building regularly exceeded 100 F in the summer because the building had an inadequate
ventilation system. Both these observations led to the following recommendations to reduce the fire hazard: 1) inert the
mill below the Limiting Oxygen Concentration (LOC) of the solvent, and 2) install building ventilation, along with Lower
Flammable Limit (LFL) monitors and alarms.
Of course, no discussion on flammability and combustibility would be complete without pointing out the problems
associated with obtaining an accurate Flash Point. There are several ASTM test methods for measuring Flash Point based
on the liquid’s viscosity, solid suspensions and tendency to coat the test apparatus. Unfortunately, these tests do not
always duplicate actual operating conditions due to the test apparatus setup as well as differences between testing and
operating conditions. For example, Flash Point data commonly found in Material Safety Data Sheets (MSDSs) is based
on pure liquids at atmospheric pressure. However, the Flash Point varies when mixed with another liquid, gas, or a
combustible dust (creating a hybrid mixture).
Focus 2009 - No. 9
As a result, many clients incorporate two Best Engineering Practices when designing and operating a facility: 1) have the
Flash Point tested on the liquid under actual operating conditions, and 2) use a temperature safety factor, i.e., operate at a
temperature that is well below the Flash Point. What safety factor should be used? Two documents that can provide
some guidance are: 1) NFPA-77- Recommended Practice on Static Electricity- states that a safety margin of 7 F(4 C)
should be used for pure liquids and 16 F (9C) should be used for mixtures, such as hydrocarbon liquids, and 2) OSHA
1910.106 specifies that a “combustible liquid heated to within 30 F of its Flash Point shall be handled in accordance with
the requirements of the next lower class of liquids”.
Note, the idea of using a temperature safety factor is analogous to the common use of safety factors in other safety
systems, such as: 1) LFL Monitoring which uses 25% of the LFL as an alarm or interlock point (reference API-14C &
NFPA-69), and 2) oxygen monitoring which uses 50-60% of the Limiting Oxidant Concentration (LOC) as an alarm or
interlock point (reference NFPA 69).
So, does it matter whether a liquid is categorized as flammable or combustible? OK, yes, it is an important characteristic
of a liquid to know. Just don’t forget that a combustible liquid can have the same fire and explosion risk as a flammable
liquid; it all depends on whether it has the potential to create an “ignitable mixture”.
How Chilworth Technology Can Assist You
Chilworth Flammability Laboratory not only conducts the whole range of gas, vapor and dust flammability tests,
such as flash point, auto-ignition temperature, lower and upper flammable limits, minimum oxygen concentration,
and thermal stability, which are required to develop a Material Safety Data Sheet (MSDS) and process safety
programs for specific processes, but also develops and conducts special flammability tests under actual process
conditions. Very limited data on the flammability properties of chemicals or chemical mixtures at adverse conditions
of temperatures and pressures are currently available. In such cases, the flammability properties of chemicals must
be experimentally determined at their process conditions to ensure safety of a chemical process. Chilworth
Technology has highly specialized flammability laboratory and full-scale testing facilities and recognized expertise
for the determination/study of the flammability characteristics of chemicals under process conditions. Our testing
and research facilities include four “barricade cells” for the study/testing of materials under various temperature and
pressure conditions. The “barricade cells” are designed to withstand explosions of 10LBS of TNT equivalent and
have the capability to monitor the system control parameters from a safe control room area. Blast and missile
resistant windows allow direct observation of the experiments.
Photo-1 High Initial Pressure and Temperature Gas/Vapor Explosion Test Set-up in the Pressure Barricade Area
Chilworth Technology possesses a wide variety of pressure vessels ranging from 75mL to 5.0 gallons and materials
of construction ranging from Stainless Steel to Hastelloy to glass-lined to Teflon-lined. This gives us the flexibility
to work with a wide range of chemicals (toxic or corrosive) under actual process conditions (temperatures and
pressures). Chilworth Technology’s engineers and specialists have extensive experience in designing experiments
dealing with chemicals (toxic/energetic/corrosive) under unusual process conditions of temperature and pressure.
Chilworth Technology can be your one stop to solve all your process safety issues/problems.
For assistance regarding if your liquid is flammable or combustible, please contact us at Tel: 609-799-4449, Fax: 609799-5559, email: safety@chilworth.com. You may also visit our website at: www.chilworth.com.