Using the PEAC tool
for Industrial Vapor Cloud Explosion and Fire Accidents
BP Texas City
AP aerial photo from
CBS 60 Minutes website
On March 23, 2005, the worst U.S. workplace accident in 16
years occurred at the BP Texas City refinery when flammable vapors from a blow
down vent ignited; the resulting explosion and fire killed 15 workers and
injured 180. The accident occurred
during startup of the refinery’s octane-boosting isomerization unit (boosts
octane content of gasoline), when a distillation tower and attached blowdown
drum were overfilled with highly flammable liquid hydrocarbons (the major part
of gasoline). Because the blowdown drum
was vented to the atmosphere, there was a geyser-like release of flammable
liquid and vapor onto the grounds nearby causing a series of explosions and
fires that killed workers in and around nearby trailers. The blast was felt up to 5 miles away. The accident was covered in the news media,
including CBS 60 Minutes. It was also
investigated by the U.S. Chemical Safety Board (CSB) and discussed in various
journals, popular magazines, including the Nov 6 and 13, 2006, issues of Chemical
and Engineering News
published by the American Chemical Society.
U.S. Chemical Safety and Hazard Investigation Board
The U.S. Chemical Safety and Hazard
Investigation Board is an independent federal agency charged with investigating
industrial chemical accidents at fixed facilities. The Board members are appointed by the President and confirmed by
the Senate. The agency does not issue
fines or citations but does make recommendations to the industry involved and
to regulatory agencies and labor groups.
It is designed to conduct scientific investigations as to the root cause
of chemical accidents and is not an enforcement or regulatory body. Most of the Board members and staff have
degrees in chemical or mechanical or other engineering disciplines, have
Professional Engineer licenses, have chemical process industry experience, or
are health or safety professionals.
Congress in establishing CSB specifically stated (see 42 U.S.C. section
7412(r)(6)(G)): “No part of the
conclusions, findings, or recommendations of CSB relating to any chemical
incident may be admitted as evidence or used in any action or suit for damages
arising out of any matter mentioned in an investigation report”.
The Board’s final report on the BP Texas City refinery
accident is expected in March 2007, but because the investigation has been
ongoing, various interim reports are available to the general public at the CSB
website, http://www.csb.gov/. AristaTek
visited the website to obtain information about the flammable vapor release for
the purpose of modeling in the PEAC tool.
The CSB has already made three major recommendations as a
result of investigating the BP Texas City refinery incident:
first recommendation, made immediately after the accident, was for BP (all
of the BP refineries) to remove trailers from locations near process
CSB recommended that the American Petroleum Institute (API), the leading
oil industry trade association that develops widely used safety practices,
should revise its “Recommended Practice 521, Guide for Pressure Relieving
and Depressuring Systems” to warn against using blowdown drums such as at
the Texas City refinery, and to use the inherently safer flare systems
instead. Companies must plan
effectively for large-scale flammable liquid releases from process
U.S. Occupational Safety and Health Administration (OSHA) should establish
a national program promoting the elimination of unsafe blowdown systems in
favor of safer alternatives such as flare systems. OSHA should also emphasize the need for
companies to conduct accurate relief valve studies and use appropriate
equipment for containing liquid releases.
The CSB upon accident investigation also documented eight
previous releases from the same blowdown drum during the period 1994 to
2004. In six of the releases,
dangerous flammable vapor clouds formed at ground level but did not
ignite. In the other two cases, the
blowdown stack caught fire. The lead
investigator for the CSB investigation noted a number of safety problems, and remarked: “This drum simply wasn’t large enough to
hold all of the liquid released from the distillation tower if it flooded. Not only could the blowdown drum not hold
enough liquid, but it could not assure safe dispersion of flammable vapors
through the vent stack”.
Since the accident, BP has pledged to eliminate all of the
blowdown drums from its five U.S. refineries (a total of 17 blowdown drums) and
use a safer flare system (an adequately sized vessel for containing liquids and
a stack with a flame for safely burning flammable vapors).
In 1992, the same Texas City refinery, then owned by Amoco
Corporation, was cited by OSHA for operating an unsafe blowdown drum. But Amoco succeeded in having the citation
and fine withdrawn, citing that it had complied with the API Recommended
Practice 521. The Amoco refinery was
acquired by BP in 1998.
Another accident, on July 28, 2005 at the same refinery,
prompted CSB to investigate safety practices in general at the refinery. In this case a pipe elbow failed at a
residual hydrotreater unit releasing hydrogen gas at high temperature and pressure. The hydrogen immediately ignited; the
resulting fireball burned for two hours causing an estimated $30 million in
damages. According to the CSB lead
investigator, the root cause was inadvertent substitution of a carbon steel
elbow (instead of a specialized alloy which can hold up under the conditions of
use) when the system was disassembled and reassembled during an earlier
maintenance. Verification of the piping
material should have been done using an x-ray fluorescence test device. Overall, CBS found that BP’s global
management were aware of problems with maintenance and responded to safety
concerns, but the emphasis was on improving procedural compliance and reducing
occupational injury rates. Catastrophic
safety risks remained. Unsafe and antiquated
equipment designs were left in place, and unacceptable deficiencies in
preventive maintenance were tolerated, according to CSB chairman Carolyn W.
The magazine, Chemical and Engineering News, Nov 13, 2006,
carried an article on lectures made by CSB chairman Carolyn W. Merritt to the
chemical processing industry throughout the United States criticizing
“corporate culture behavior”, which in order to improve the immediate bottom
line for shareholders results in a history of corporate cost cutting. The cost cutting results in maintenance,
safety, and plant infrastructure deterioration. The problems at BP are not unique to one refinery or one
corporation. At BP, there was a culture
where people at the refinery accepted higher levels of risk than they should
have, and there was no feedback loop from past incidents. Merritt has given her speech to company
corporations “hundreds of times”, and she says after giving her speech, someone
comes up invariably and says to her, “You’ve got to come to our company and
give this presentation”.
BP has set aside $1.6 billion to settle more than 1000
claims against the company.
Analysis of BP
Vapor Cloud Explosion at BP
A video description of the BP accident (produced November
2005 by the CSB) is available at
This video has been viewed about 350,000 times since CSB
first released it.
The startup of the isomerization unit began overnight, when
light hydrocarbon liquid (a major component of gasoline) was added to the
distillation tower. The video makes
mention of a faulty level indicator on the tower which showed a liquid level of
only 10 feet when in reality there was much more liquid in the tower, causing
operators to overfill the tower. When a
heat exchange system was started the next morning for heating the hydrocarbon,
a valve on a pipeline leading from the tower should have been open as per
normal procedure but was closed resulting in further overfilling the tower to a
depth of 138 feet. The heating of the
hydrocarbon liquid increased the pressure inside the tower resulting in excess
liquid to be diverted to a nearby blowdown drum fitted with a 100 foot tall
vent stack. The heating also expanded
the liquid volume in the tower. An
observation was made just before the vapor cloud explosion; there was a
geyser-like fountain of liquid and vapor coming from the top of the vent stack
for at least a minute before the explosion.
The hot, liquid droplets fell to the ground and evaporated forming a
vapor cloud near the ground; some liquid entered a nearby storm sewer. Plant records for earlier years showed
overfilling and at least a start of vapor cloud formation eight times before. For two of the times, a fire occurred in the
vent stack, and the other six times the vapor cloud dispersed and did not
ignite. But this time considerable
vapor formed and was ignited (1:20 PM) apparently by a diesel pickup truck
parked 25 feet away from the blowdown drum.
The blast killed 16 people located in nearby trailers. The blast also set off secondary fires and
explosions at processing equipment injuring more people and damaging over 50
chemical storage tanks. Some of the
injuries occurred when workers cut their hands on the barbed wire fence
surrounding the refinery when they tried to escape the fires (the fence was not
Perhaps most interesting is an analysis of the blast
structural damage of the approximately 44 trailers located within 1000 feet of
ground zero (the blowdown drum location) at BP refinery as investigated by
CSB. Not all of the trailers housed
people; many were used for storage and were unoccupied. A detailed analysis of trailer damage is
presented at http://www.csb.gov/completed_investigations/docs/CSB_BPTrailerData6-30.06.pdf. All of the 15 deaths occurred in occupied
trailers located closest to ground zero.
At distances less than 200 feet, trailers were essentially demolished. Major structural damage occurred between 200
and 500 feet. All windows were broken
at 500 feet. The roof of one trailer at
597 feet collapsed, and its walls were heavily damaged. The threshold distance for window breakage
appeared to be around 700 feet. Most
trailers located beyond 700 feet were relatively undamaged by the initial
Table 1. Summary of Injuries and Fatalities in
Trailers Near Ground Zero
Distance from Ground Zero, ft
# People in Trailer
12 fatalities, 10 injuries, trailer essentially demolished
3 fatalities, trailer demolished by blast
2 injuries from flying debris in trailer, trailer heavily
1 injured from flying glass, 1 uninjured, wall damage
2 injured, major blast damage to trailer
1 injury from flying glass, 1 uninjured
The reason that the company had located trailers at the site
was to accommodate staff and contractors who were doing maintenance and
renovations on nearby process units, and that the trailers were said to be
temporary. After the accident, BP
developed a new corporate trailer siting policy that provided exclusion zones
for areas where explosions are possible.
PEAC Tool Use by
The PEAC tool is primarily developed for use by first
responders in the event of a toxic chemical release, fire, or explosion. It also contains an extensive database on
the physical properties, reactivity, and health hazards of many thousands of
chemicals. There are over 100,000
chemical names in the database. There
is also information on cleanup in case of a spill. Evacuation zones are presented in case of a toxic chemical
release, explosion, or fire. The tool
is designed to give results quickly with the user supplying only a minimal bit
A safety inspector taking a walk through the facility might
input information on flammable chemicals and establish exclusion zones for
vehicle traffic or location of trailers, or for clearing the area of
nonessential personnel during critical operations. The user might input information on the size of the vessels and
assume a worst case where the vessels are nearly full and all of the flammable
chemical is released, vaporized, and ignites in a vapor cloud explosion.
In the case of a non-flammable, toxic chemical release the
action taken will be different. The
chemical facility should have in place a plan for evacuation and/or shelter in
place to minimize exposure from an airborne chemical cloud. The PEAC tool can provide information on
exposure to the chemical as it travels downwind from the release site. The tool is designed for responders to go
through a variety of different release or weather scenarios quickly.
The Environmental Protection Agency requires industry to
have in place a facility off-site consequence analysis in the case of release
of flammable or toxic chemicals. The
analysis should include a worst-case scenario, where all of the chemical stored
or used is released at once and explodes or becomes airborne as a toxic cloud. The worst-case weather condition for a toxic
cloud for this type of analysis is the clear nighttime, low wind speed
condition (the so-called “F stability”).
About 200 chemicals are covered by the regulations. This is in addition “Community
Right-to-Know” regulations where companies are to provide information to fire
departments, local emergency planning committees, etc., on what hazardous
chemicals are stored or used on site.
There are also Occupational Safety and Health Administration (OSHA)
regulations of worker safety including limiting exposure to toxic chemicals.
Let’s see how this might work. A safety inspector going through a refinery notes a piece of
process equipment which contains heated light end hydrocarbons which is a major
component of gasoline. Assume a
worst-case situation where level controls fail, the vessel is overfilled, and
the vessel contents spill and vaporize.
Assuming the vessel is four feet in diameter and is 50 feet tall, what
is the maximum blast distance if the entire vessel contents vaporize and
ignite. The PEAC user types in the word
“gasoline” under “lookup” and then clicks on the red and yellow blast/fireball
pictured at left to initiate the calculation for blast effects and the
fireball. There is also a scroll down
bar where the user can also get information on the physical properties and
health effects on exposure to gasoline, but we are not interested in this now.
Specifying the container size
Specifying damage thresholds
A screen pops up as shown above (left side). The user types in the vessel dimensions, and
then clicks on the black arrow [
] at the
top left to proceed further. The PEAC
tool then asks the user to input a blast overpressure and a heat damage choice
(above, right side). For this example,
we will chose 1 psi overpressure (windows shattered, most structures still
standing but trailers may be severely damaged.
We will ask the tool to also calculate the threshold distance for second
degree burns (ignoring secondary fires and any pool fires).
The PEAC tool estimates the mass inside the container
(ignoring end curvature) and the blast distance corresponding to a 1 psi
overpressure. Assuming that a fireball
immediately follows, the PEAC tool also calculates the distance to threshold of
second degree burns for exposed flesh (ignoring any secondary fires or
explosions). Because of uncertainties
of the vapor cloud shape a safety factor of two is applied to all distances
from ground zero. At 1 psi
overpressure, windows break, and there is structural damage to trailers. The overpressure distance calculated is
greater than experienced in the BP accident probably because less hydrocarbon
(gasoline) was involved in the BP explosion, and the PEAC tool has a safety
factor of 2 on distance.
The PEAC tool is not limited to distance calculations linked
to 1 psi overpressure or to the threshold for second degree burns. Other overpressures and damage estimates can
be entered. The PEAC calculator can
generate a list of damage estimates as a function of distance from ground zero,
store the information, and print it later.
A safety inspector can generate his lists as he/she walks through the
facility, and use this information for developing exclusion zones for trailers
and for possible ignition sources such as vehicles. The PEAC tool also automatically will display the standoff
distances on a street map.
The Lower Explosive Limit for gasoline is 1.4%. This is equivalent to 14,000 parts per
million by volume in air. By clicking
on the PAD icon on the first screen, the user can estimate a distance downwind
corresponding to 14000 parts per million.
The worst-case scenario is for all of the gasoline to be released at
once, and a clear nighttime sky at under a low wind condition. We will set the PEAC internal clock to a
nighttime, specify clear skies, urban condition, a wind speed of 2 miles per
hour, and a sudden mass release of 26829 lbs as calculated previously. The PEAC tool calculates that the concentration
< 14000 ppm for vapor cloud explosion extends out to 217 yards (651
feet). Under daytime conditions or
windy conditions, the distance will be less.
This type of analysis may be useful in estimating ignition exclusion
zones. The 328 foot initial isolation
zone distance came from the 2004 Emergency Response Guidebook, published by the
Department of Transportation, and does not apply here.
The Protective Action Distance
can, of course, be based on toxicity and not on the Lower Explosive Distance.
The important thing is that a tool is available for
developing exclusion zones and dealing with safety issues.