As I
mentioned on Thursday the Chlorine Institute has published an updated
version (Edition 6) of their Pamphlet
74. That pamphlet used to be titled “Guidance on Complying with EPA
Requirements Under the Clean Air Act by Estimating the Area Affected by a
Chlorine Release”. The new version
removes reference to the EPA and is more simply called the “Guidance on
Estimating the Area Affected by a Chlorine Release”. This reflects the more
widespread use of the modeling data.
I do not have a
copy of Edition 5 of this pamphlet, but I do have Edition 4 that I have used
for a reference in a number of blog posts on chlorine releases that I have done
over the years. So I am going to be comparing the downwind hazard predictions
from these two versions of the Pamphlet.
Background
Chlorine is a very
common hazardous material and it is widely used in this country. It is
certainly the largest volume toxic inhalation hazard (TIH) chemical shipped in
the United States. It is typically shipped in the following types of
containers:
∙ 150-lb. Cylinder;
∙ 1-Ton Container;
∙ 17-Ton Tank Truck Trailer; and
∙ 90-Ton Rail Tank
Car
Chlorine is used as a disinfectant for water treatment and
waste water treatment facilities. It is also used in the manufacture of a large
number of industrial chemicals and pharmaceuticals. It is generally produced
via the electrical disassociation of common salt, NaCl; producing both chlorine
(Cl2) and caustic soda (NaOH). A less common, but still important,
source of chlorine is as a byproduct of the production of magnesium from
natural ores.
Chlorine gas was used as the first modern chemical weapon in
World War I. The CDC
reports that the immediately dangerous to life and health (IDLH) concentration
of chlorine gas is 10 ppm while reporting that lethal effects have been seen at
34 to 51 ppm when exposed for over an hour. The OSHA personal exposure limit is
1 ppm with a short term exposure limit of 3 ppm.
The Chlorine Institute has been using the values of 3 ppm
and 20 ppm as their standards for downwind exposure reporting in Pamphlet 74.
In a release scenario exposure levels at less than 3 ppm are considered to be
relatively safe in the short term. Exposures between 3 and 20 ppm will have
some immediate medical effects that will probably require medical attention,
but will probably still allow an otherwise uninjured person to self-evacuate
from the affected area. Exposures over 20 ppm will almost certainly cause
immediate health effects of a severity that would stop a person from
self-evacuating from the affected area and may result in death.
Worst Case Scenarios
The table below describes the worst case scenario reporting
from both versions of Pamphlet 74 and the RMP*Comp tool used to report
downwind distance of concern for both the EPA Risk Management Program (RMP) and
the DHS Chemical Facility Anti-Terrorism Standards (CFATS) program. All
distances have been converted to feet.
Pamphlet
74 – Ed 6
|
Pamphlet
74 – Ed 4
|
RMP*Comp
|
|||
20
ppm
|
3
ppm
|
20
ppm
|
3
ppm
|
.0087
ppm
|
|
150 lbs
|
108
|
210
|
3,168
|
8,976
|
4224
|
1 ton
|
331
|
988
|
11,616
|
35,376
|
15840
|
17 ton
|
1,152
|
1,857
|
41,712
|
121,440
|
58080
|
90 ton
|
1,184
|
1,765
|
78,144
|
219,920
|
>132000
|
This obviously marks a serious departure from the previous
guidance on downwind hazards from chlorine gas releases. According to both
publications (pgs 16-17 in both Editions) the following conditions of the
release are the same in both studies:
• The ambient relative humidity is
50%
• The ambient temperature is 77°F
(25°C)
• Liquid chlorine is at 77°F and
100 psig before the release
• Surface roughness is 3.94 inches
(0.1 meters). Such a surface roughness corresponds to a relatively flat,
grassy, rural setting. A release in hilly terrain, forested area, and urban
environment or over water may have significantly different terrain results.
• The wind speed is 3.36 miles/hour
(1.5 meters/sec). The reference height for measuring the wind speed is 32.8
feet (10 meters).
• The contents of the container are
released instantaneously and evaporated at a constant rate over a ten-minute
period
• The atmospheric stability is
class F (night, < 50% cloud cover)
• The release occurs at ground
level
• Solar radiation is assumed at 0
Btu/hr/ft2
• Averaging time is 10 minutes
• Receptor height is 0 feet (ground
level)
Modeling Differences
What is clearly different is that the two Editions are
reporting results from different dispersion models. Edition 4 uses TRACE 8.0
developed by Safer Systems, Inc. Edition 6 uses Hazard Prediction and Analysis
Capability (HPAC) 5.0 developed by DOD’s Defense Threat Reduction Agency (DTRA).
There is a brief discussion about the model used in each report as the first
appendix. What is missing, however, is any discussion in the latest edition
about why the model changes were made and how that impacted the huge change in
predictions reported.
What is mentioned in the press release announcing the latest
version of the Pamphlet is that it “incorporates information obtained from
the DHS ‘Jack Rabbit I’ chlorine release field tests”. I think that
it would be safe to assume that TSA did not run tests meeting exactly the conditions
specified in the report. I have not seen the final report on the Jack Rabbit I
trails (and I suspect it is classified and thus I will never see it), but with
only a total of ten releases split between chlorine and anhydrous ammonia, I
would expect that the researchers would have varied the environmental situation
as much as possible to obtain the type of results necessary to develop a robust
model.
Without a robust discussion about the differences in the two
models used to report the data in these two different editions of Pamphlet 74,
it is hard to make a judgement as to which is the more appropriate model upon
which to base regulatory and emergency response decisions. And make no mistake,
with the wide discrepancy between the two sets of predictions provided in the
two versions of the Chlorine Institute report, it is absolutely assured that
that the manufacturers and users of chlorine will press for changes in safety
and security regulations to reduce their costs of operations. And just as
surely they will be vocally and vigorously opposed by any number of
environmental activists.
Modeling Problems
I have discussed the modeling problems associated with
chlorine dispersion modeling in a couple of earlier blogs. Back in March of
2010 I reported on the exposure
estimates from Edition 4 of Pamphlet 74; saying:
“Looking at the charts on pages 24
and 25 of Pamphlet 74 it looks like anyone inadequately protected in the cloud
for up to a couple of miles away from the catastrophic release from a full
railcar is at serious risk of being killed by the cloud. Inadequate protection
in the cloud at distances of up to 15 miles from the release could have very
serious medical consequences.”
In my blog post on what we now know as the
start of the Jack Rabbit Project I noted that:
“It seems that the chlorine cloud
from the two most recent catastrophic releases [Graniteville, SC and Macdona,
TX] of chlorine from rail cars did not come anywhere near following the
dispersion model for the release. TSA is initiating a two phase study of
chlorine gas dispersion to clarify these discrepancies.”
Based upon the observed chlorine dispersions seen in these
two accidents, it would seem obvious that the model used in the 4th
Edition of Pamphlet 74 is more than a little exaggerative in it prediction of
downwind areas of concern. And that, of course, was the reason for initiating
the Jack Rabbit project in the first place. It would be interesting to see if
anyone has gone back and used the observed data from those two real-world
incidents to see how well the new model used in Edition 6 fits the observed
conditions.
Of course, I would be very surprised if any reasonable model
could accurately predict the dispersion cloud in a real incident. There are
just too many variables that would affect both the macro and micro dispersion
effects of the cloud. I discussed some of these in an earlier
blog post.
The Use of the
Dispersion Model
Practically speaking we do not need for an absolutely
accurate prediction of gas cloud dispersions patterns. What we need is
something that will allow facility safety personnel or emergency response
personnel (depending on where the release takes place) to develop an initial
plan for responding to a catastrophic chlorine release. With that in mind it
would seem to me that we would want a plan that errors on the side of a larger
downwind hazard area rather than one that underestimates the area.
From the new model, if it is appropriately supported by the
Jack Rabbit data (including the upcoming Jack Rabbit II later this year), we
have been grossly overestimating the potential exposure to chlorine releases
from industrial facilities and transportation incidents. If that is true we may
have overburdened producers and users of chlorine in the protective measures
that we have required them to take.
Having said that, the relatively small downwind hazard areas
described in the latest version of Pamphlet 74 feel like they are too small. This
may lead us to reduce the protective burden to the point where we put people
needlessly at risk near chlorine facility storage areas. Before we take that
risk it would be prudent to ensure that there was a solid peer review of the
model construction and methodology.
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