Earlier this week there was a chemical spill at a
manufacturing facility in in the Pittsburgh, PA area. News reports (here
and here)
did not name the flammable liquid chemical involved, but did note that nearby
businesses and a local college were evacuated in response to the spill of an
estimated 200 gallons.
Were the evacuations an expensive over-reaction? Without
knowing the chemical involved and what information was initially available to
the local emergency response agencies I really cannot say. But, there are
certainly situations where even such a small spill might require at least this
level of response.
Flammable Liquids
The term ‘flammable liquids’ covers a wide variety of
chemicals with a significant spread of potential hazards. The US Department of
Transportation, for instance describes a flammable liquid as any liquid with a
flash point of less than 140°F.
Flashpoint describes the temperature above which a liquid will ignite if an
ignition source (open flames, sparks, or electrical arcing) is placed above the
liquid surface.
Generally speaking, the lower the flashpoint, the lower the
energy needed in the ignition source sources to ignite the flammable liquid.
For some liquids, an actual flame source will be required, while others only
require a static electricity discharge to ignite the liquid.
So the most obvious concern with the spill of a flammable
liquid is the potential for a fire. In a situation where the liquid is confined
within a diked area and there are no ignition within that contained area, the
risk of a fire is relatively low; unless….
Another area of concern with flammable liquids is the
evaporation rate and the ‘explosive limits’ for the vapors. As the liquid evaporates
it forms a gas cloud above the liquid surface. The size of that gas cloud
varies with the local wind speed and obstructions around the spill site, but
unless it is in an enclosed and sealed area, the gas cloud will spread
generally downwind from the spill site.
The gas cloud is also flammable and could burst into flames
if it comes into contact with an ignition source. The likelihood of it bursting
into flames is determined by its explosive limits, the range of concentration
of the chemical fumes in air at which those fumes ignite. These limits are
called the ‘lower explosive limit’ (LEL) and the ‘upper explosive limit’ (UEL)
and are expressed as percent of the chemical fumes in air. Generally speaking,
the wider the spread between the LEL and UEL, the more of a potential ignition
problem that you have.
The reason that this range of concentrations is called the ‘explosive
limits’ is that when a gas cloud ignites, almost the entire content of the
cloud (within those limits) burns at the same time, releasing all of the energy
of combustion in one explosive event. The effects of that explosion can be felt
over a much wider area than that of a liquid spill fire. Those effects can
include spreading the fire by igniting other flammable materials or just the
overpressure shock wave causing physical damage surrounding surfaces and
structures.
Other Chemical Effects
All chemicals can potentially have varying negative effects
on living organisms, depending on the way the chemical is introduced into those
organisms and the amount of the chemical introduced. The degree of the hazard
of these negative effects can grossly be described as the toxicity of the
chemical.
The important thing to remember is that the degree of
toxicity is related to the amount of the chemical entering the body (dose). For
chemical vapors the dose is related to the concentration of the chemical in the
air and the time of exposure. For toxic inhalation hazard (TIH) chemicals very
small/short exposures to the chemical vapors can be life threatening. For most
chemicals, however, vapor concentrations have to be very high to be life
threatening.
For non-TIH chemicals the potential negative effects of
exposure to chemical fumes are quite wide and are often variable for different
people. More importantly, those effects will be reduced as the concentrations
decrease as the vapor cloud disperses downwind. The further away you are from
the initial release the lower the chance that there will be a noticeable
negative effect. The safe distance downwind will vary according to the size of
the vapor cloud, the wind speed, and, of course, the chemical involved.
Emergency Response
From the discussion above, it is quite obvious that
emergency response personnel have to know a lot of information about the
chemical involved in a release situation to know what precautions have to be
taken to protect both themselves and the public. Simply knowing that a ‘flammable
liquid’ has been released does not provide enough information. Unfortunately,
knowing the ‘name’ of the chemical may not provide sufficient additional information
because you can commercially name a chemical product anything that you want.
The Federal government has established a requirement that
every manufacturer of chemicals must produce and make available a Safety Data
Sheet (SDS) for every chemical they manufacture. The requirements for a SDS are
fairly detailed, but, suffice to say, they are required to provide information
on flammability and toxicity that would allow emergency response personnel to
understand the hazards associated with a spill. This is the reason that
businesses are required to provide copies of SDS to local fire departments for
all chemicals stored on site.
The idea behind providing SDSs to local fire departments was
that those departments could then plan their responses to incidents at these
businesses. Unfortunately, for all but the smallest fire departments, the
number of SDS that should be provided to them would number in the hundreds or
thousands. I would be surprised if more than a cursory glance is given to any
of these SDSs.
No one should expect a local fire department to have a
response plan for every chemical that is stored in their area of responsibility.
There are just too many chemicals involved and most of them are relatively
innocuous without any special hazard.
Even for chemicals with a known hazard (‘flammable liquid’)
most fire departments are going to rely on tools like the DOT’s Emergency
Response Guide (ERG) to provide the information for the initial response to
chemical spills. Only the largest chemical inventories or most hazardous
chemicals are going to draw the attention required for advance emergency
response planning.
An Over-Reaction?
So the question remains, was this an over-reaction? Maybe a
little. Lacking detailed information about the chemical involved and the size
of the leaking storage tank one should probably turn to page 194 of the 2016
ERG and equating a storage tank with the guide’s “tank, rail car or tank truck”,
a half-mile evacuation would be recommended for a flammable liquid fire. For a
spill the recommended ‘initial downwind evacuation’ is only 1000 feet.
Having said that, it is never really a good idea to second
guess the people who are making on-the-scene decisions, especially when they
are erring on the side caution. Nothing happened this time; no fire and no
injuries. If something had happened and there were off-site consequences the decision
would have provided more maneuver room for the emergency response personnel.
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