Quantitative Risk Analysis

Quantitative Risk Analysis

By

David L. Russell, PE, ASP

President, Global Environmental Operations, Inc.

4797 Dean Lane, SW, Lilburn, GA, 30047

Introduction

Risk analysis is a part of financial and business communities. Quantitative Risk Analysis (QRA) represents a change in risk perception from a feeling about a project to something quantifiable. Quantitative Risk Assessment is becoming more important in industrial applications. In the US, OSHA’s Process Safety Management Rules (29 CFR 1910.119) have been in effect since 1993. Part of PSM is the risk analysis, whether from process change or other sources. The regulations are broadly applicable to the petrochemical and chemical industries.

Quantitative Risk Analysis is a requirement in the EU and UK but not in the US. This article will delve into the history and rationale behind QRA, provide an oversimplified example of a QRA, and provide some usable data on failure rates for QRA preparation.

QRA and preventive maintenance (PM) are related because good PM reduces failure rates and extends equipment life. Preparing a QRA is often complex, but we will highlight some important elements to simplify it. The QRA requirements from Singapore are excellent and are used as an example[i].

The History

“QRA is a formal and systematic approach to estimating the likelihood and consequences of hazardous events and expressing the results quantitatively as risk to people, the environment, or your business. It also assesses the robustness and validity of quantitative results, by identifying critical assumptions and risk-driving elements” – source: DNV.com

The nuclear industry has widely used QRA to evaluate critical component life. QRA is a mature technology, widely used and refined, and its limits are well understood[ii]. Current risk assessment methodologies were developed from incident histories. Major incidents included Loss of Containment in the reactor or storage tank; minor incidents included spills, pump failures, heat exchanger failures, and instrumentation malfunctions. If a pump did not start when it was supposed to, or if a heat exchanger developed a leak, those were often coded and counted in the failure databases.

Significant Chemical and Petrochemical Incidents

In Sevesto, Italy on July 10, 1976, a runaway chemical reaction involving trichlorophenol, there was a released of 2,3,7,8 Tetracholordibenzo p dioxin (TCDD), a carcinogenic chemical that is fatal to exposed animals. The Dioxin release poisoned over 6 hectares, killed livestock, caused chloracne in 640 individuals, and hospitalized 19 children in the nearby downwind communities. The chemical was so hazardous that it required 5 years for the Italian government to remediate the site, and the contaminated soils were placed in a concrete vault with a lifetime of several thousand years.

The European Parliament issued the Sevesto Directive on June 24, 1982 (Sevesto 1 or Directive 82/501/EECD), introducing and requiring risk assessment for chemical and petrochemical industries.  The Sevesto II (1996) and Sevesto III (2012) directives soon followed. The directives require proactive control measures, safety management systems, emergency preparedness plans, and regulated packaging, shipping, and labeling of dangerous chemicals.

In Bhopal, India, the release of Methyl isocyanate from a Union Carbide subsidiary on December 2-3, 1984, killed approximately 2260 and injured 558,125 people[iii]. The Bhopal Incident caused the dissolution and sale of Union Carbide (Chemical) Company, and the Indian criminal indictment of UCC’s Chairman of the Board.

In 1996 the US Department of Energy issued the DOE HANDBOOK CHEMICAL PROCESS ANALYSIS[iv], (2^nd Revision, 2004). The handbook is applicable to all contractors with DOE contracts but does not consider statistical probabilities.

Sunday, December 11, 2005, a fire and related fuel-air explosion at Hertfordshire Oil Terminal (Buncefield) in England registered 2.4 on the Richter (earthquake) scale and injured 43 people in the surrounding community. Since then, other chemical and petrochemical incidents have involved their local communities, with and some without loss of life.

The US is not without its own hazardous incidents. In 1975, a fire and explosion at the Gulf Refinery in Philadelphia injured 14 and killed 8 firefighters working in an adjacent diked area. In the same year, a crude oil tanker was rammed in the Delaware River, causing a large explosion and damage to nearby communities. There have been other releases, some related to the transportation of hazardous chemicals[v].

A good QRA requires a good database for risk assessment. With good data, we can answer the question, “How safe is my facility and community?” Any facility and surrounding community’s safety depends on proactive maintenance, operator training, and good proactive management. The probability of chemical release incidents is very low[vi], which often makes installing expensive preventive measures very difficult for any industry.  So how do we judge risk and justify safety expenses?

One of the initial developments in defining risk was the colored risk management matrix. The matrix is a simple color-coded chart that attempts to quantify risks. There are several versions of the risk matrix, including one that attempts to include economic damage and risk, as shown below:

Figure 1 Risk Management Matrix example

Source:  https://safetyculture.com/topics/risk-assessment/5×5-risk-matrix/

Some versions of the matrix use Millions of Dollars as cost headers in the top row to quantify the severity of the incident. The cost considerations can include loss of inventory, loss of sales, facility reconstruction etc. The risk assessment matrix can be dollarized and frequency-coded. The Monetized Risk Table can look something like that below:

Figure 2 Monetized Risk Table

The EU, UK and Singapore Approach to QRA

A guide to the Application of QRA is provided by the UK’s Health Safety Executive, and is available on the Internet. [vii]

 Table 1_ Guide to the Content of the Colored Books

The Economic Commission for Europe, the UK government and the Dutch government responded to the challenge by issuing a series of “Colored Books” in the 2004-2008 time frame. Their titles and contents are shown below[viii]:

The Colored Books contain the formulas and some of the data for QRA. They are available by title search on the Internet. Below, are some of the data from the various studies on industrial incidents and failures.

The Economic Commission for Europe Inland Transport Committee’s Working Party on the Transport of Dangerous Goods, in 2006 published a guideline for Risk Assessment^viii. It addresses the Calculation of Risk of Rail Transport of Dangerous Goods.  The publication addresses the value of an event tree approach to QRA combined with statistical analysis of railway incidents where there was a release of hazardous materials and evaluation of the potential consequences of a release.  They suggested two types of information be assembled based on Dutch experiences. The first was a frequency-probit graph for chemicals in transport. The second was an analysis of the population densities adjacent to the railroad lines. From these two graphs, one can understand the population at risk from hazmat rail incidents.

Figure 3 Dutch approach to population exposures from rail transport of Hazardous Materials. 

Figure 4 Example of iso-risk curve graphic for individual risk (source: MINVENW, NL)

The scale on both axes is distance. The hatched areas represent population densities. The curves indicate the risk levels per year. Green is the lowest risk frequency.

There are other approaches to the QRA Process.

Singapore’s Approach

The Government of Singapore issued very comprehensive guidance documents and requirements for preparation for QRA. The requirements include analysis of consequences from total and partial Loss of Containment, Boiling Liquid Expanding Vapor Explosions (BLEVE), operational and process errors (mischarging or overfilling reactors or tanks), bulk transport and pipeline incidents, and alarm failures leading to incidents. One must assess the potential number of casualties from pool fires, jet fires, flash fires, vapor cloud explosions, pressure vessel failures, runaway reactions, failures of firewalls, blast walls, and toxic releases. Many of these cases can be ruled out based on the material properties.

Most toxic release incidents generate air pollutants. The Singapore QRA requirements include an evaluation of upwind (back spreading) contamination and downwind toxic profiles, and the provisions to reduce emission exposure to levels that are “As Low As Reasonably Possible”, (ALRAP).

A QRA requires evaluating the consequences of a release scenario. Several consulting firms have developed models that automate this process, and they advertise that they can provide complete services for QRA preparation through their use. If the project is complex, the cost will be several thousand dollars.

 In the US, a popular scenario generation site is ALOHA, developed by the USEPA as part of the CAMEO Suite[x]. ALOHA is (Areal Locations of Hazardous Atmospheres). Some states use it for hazard assessment in school sites. ALOHA can predict the downwind effects of toxic chemicals and the associated exposure hazard, but it does not quantify the likelihood of fatalities.

The USEPA has risk assessment guidelines that include specific health-based recommendations, remediation, ionizing radiation, radon, air pollution, and other factors. However, the guidelines do not address frequency issues for industrial incidents or provide much guidance on risk factors and frequencies[xi].

RAST by AIChE  and CCPS for Hazard Identification

The EPA, the Center for Chemical Process Safety and AIChE have Developed RAST, an Excel-based hazard identification system for screening for process hazards. The program requires user input and significant chemical data.  An associated program, Chemical Hazards Engineering Fundamentals (CHEF), is available only with membership in CCPS.  The programs can identify the hazards of specific chemical processes. The program opens with Excel^®, but the program does not estimate the type or frequency of process incidents. The RAST tutorial is excellent and free. The download of RAST can be accomplished by linking to the AIChE website.

CHEF is available through AIChE. The CHEF program includes the identification of chemical and operational hazards and suggestions for scenarios; source models for estimation of leak rate, evaporation rate, vapor dispersion, and explosions; effect models for estimation of human harm from toxic releases, flammable releases, and explosions; techniques for evaluation of likelihood (historical data, fault tree, and event tree); and quantification of risk. The demonstration slides that discuss CHEF are quite informative, but heavily annotated for information.  The dues for CCPS start at approximately $8,200 per year, and it is structured so that large corporations pay proportionally higher fees.

As a matter of practice, the US is a litigious society. One possible reason that QRA in the US does not include the possibility of fatalities is Lawyers! It would be difficult to imagine a corporate executive attempting to defend the operation of any facility where there was the statistical possibility of a fatality to a non-employee. Even the slightest statistical possibility for an incident or accident could bring about extreme consequences.  Ignorance is sometimes better, especially in a courtroom setting in front of a jury.

HOW TO PREPARE A QRA

There are a number of steps in the preparation of a QRA before one can assign probabilities. The Government of Singapore has a step-wise guide (see endnote i), which we will highlight because they have good graphics. The first task is to identify the hazard, as below:

Figure 5 QRA process steps from Singapore Government

As an initial evaluation one might want to consider the DOW Chemical Fire and Explosion Index (F&EI) for an initial evaluation of your risk. The DOW F&EI program is available from AIChE for $50.00.  It will give an initial screening.

The first, identify the hazard. Here are two cases on opposite sides of the US. The considerations are an ammonia cloud and a chlorine cloud that could impact a metropolitan area. The consequences include injury to people, fatalities (on and offsite), and potential escalation into other events. The ammonia cloud can be flammable if conditions are right, and the chlorine cloud is highly toxic.

The second step is identifying a scenario(s):  A typical scenario might include a release of a large ammonia cloud from the failure of a refrigerated storage tank, about a mile from a populated area in Central California or a failure of a valve on a railway tanker of chlorine. If the ammonia tank loses containment, it will release substantial quantities of liquid ammonia into a 250’x 510’ dike approximately 5’ high.  The chlorine leak is from a rail tank car on a siding at a transfer yard. Both will be addressed in the last section. The QRA, must provide consequence-based results, including the distances and specific harm levels and frequencies in the harm zones.

The third step is an event tree and frequencies. The event tree shown below is from the Singapore guidance document.

Figure 6 QRA Event Tree

Figure 7  Compressed Event Tree for example — Ammonia

The event tree has been reduced and edited to eliminate fire as a possibility. The concern is asphyxia and inhalation.

Expressed in tabular form, the graphic would look like the following:

Table 2  QRA– Event Tree discussion

Modeling the consequences of individual exposures is required for each case.

Potential Incident Effects Modifiers

The potential modifiers to any accident scenario include mechanical provisions, equipment, and alarms that will provide early warning of a hazard. These provisions are generally considered in the development of a bow tie analysis as shown below:

Figure 8 Bow Tie Analysis of Hazard

The diagram tends to be self-explanatory, and the overall format is hazards and problems go on the left, with solutions on the right.

Models and Modeling:

Several models can be used to evaluate disaster consequences. There are some pollution programs for air releases from the USEPA[xii]. Some of the EPA environmental models are shown below:

https://www.epa.gov/rsei   a model for screening environmental indicators

https://www.epa.gov/scram/air-quality-dispersion-modeling-screening-models  includes: AERSCREEN, CALC3QHC, COMPLEX1, CTSCREEN, TSCREEN, VALLEY AND VISCREEN  The page provides access and documentation for the models.

https://www.epa.gov/tsca-screening-tools  a set of tools to be useful in screening chemicals listed under TSCA.

https://www.sciencedirect.com/science/article/abs/pii/S0950423017308203 contains a description of BLEVE and Fireball Dynamics using CFD .

https://pages.nist.gov/fds-smv/ links to the Fire Dynamics Simulator focusing on smoke and heat transport.

https://www.epa.gov/cameo/what-cameo-software-suite contains several programs for releases to air and planning.

https://www.epa.gov/hydrowq/water-quality-analysis-simulation-program-wasp#:~:text=WASP%20is%20one%20of%20the,in%20the%20Savannah%20River%2C%20GA.  WASP is a water quality model for multiple pollutants.

https://www.questconsult.com/papers/radiant-heat-from-fireballs/  Quest Consultants have a program for modeling the heat flux from fireballs.  Commercially available program.

The Government of Singapore mentions the following models: DNV PHAST; TNO Effects; CERC ADAMS; Shell Global Solutions FRED; BP CIRRUS; and SAFER TRACE as consultant firms whose software will meet the Singapore QRA standards.

For toxic probits, it is suggested that one use the Green Book, and consider 10% and 50% fatalities. Considerations must include potential fatalities unsheltered (outside).

Air Modeling considerations should consider nighttime and daytime conditions, including high stability and high humidity considerations, and wind rose considerations to determine the probability of wind variability in different directions. Generally high stability, high humidity, nighttime conditions are the most severe. PREVAILING WINDS CAN NOT BE COUNTED ON TO KEEP THE PUBLIC SAFE WHEN THERE IS AN INCIDENT! They are acceptable for planning purposes only.

Singapore Harm Criteria

Harm Footprints must consider: 3%,10% and 50% fatalities from toxic releases, Radiation Exposures range from 4kW/m² to 37.5kW/m²; Acceptable level 3*10^-7/year

Lower Flammable Limits must be considered;

Overpressures from an explosion must be evaluated at 5 and 10 psi.

Harm footprints must be modeled for all incident and weather conditions, including fireballs and vapor cloud explosions.

Cumulative Outcomes for all categories for fixed installations — 10^-4/yr

Occupied Buildings onsite: consider the time the structure is occupied:: — 10^-3/yr

Potential Loss of Life occupied buildings: factor in Radiation Value * occupancy

Rank Harm Criteria Scenarios based on frequencies of 10^-5 to <10^-8/per year, and Measure the offsite damage in 10⁶ dollar increments from $2M to over $2000M (1.524M USD to $1542M USD)

The harm/risk criteria for exposures should be ALARP (As Low As Reasonably Possible) for each scenario.

Alternatives for Risk Reduction must be identified, including the substitution of alternative substances for hazardous or flammable materials.

Further Risk Reduction Measures should include shutdown systems, improved specifications, and physical controls similar to those in PSM alternatives.

Emergency Action Plans and Response Plans should include evacuation, prepositioning of supplies, and emergency response services should be developed and implemented.

Modifiers based on Cost

The estimate of the cost to achieve ALARP exposure can be estimated from:

Cost Multiplier= A multiplier that ensures that cost outweighs the benefits 1<CM<10

The benefit of injuries may be factored in considering the remaining lifetime of injured persons and the benefits of avoided damage and escalation costs as described in the Singapore QRA Guide.

QRA Frequency Data

Loss of Containment:

Frequency Data on Failure Rates for pressurized tanks containing LPG or Chlorine:

Table _3_ Failure Data for Pressure Vessels (from the UK HSE)[xiii]  incidents/year

Table 3a  HAZMAT Accident and Non-Accident Release Rates for US Rail

Source: Railway Supply Institute:

Pressurized Tank Cars represent ~19.6% of railway fleet.

HAZMAT SHIPMENTS BY CATEGORY:

21.2% of shipments are flammable gases

14.0% of shipments are corrosive materials

48.4% of shipments are flammable liquids (petroleum and other materials)

  2.4% of shipments are non-flammable gases and poisonous materials

HAZMAT NON-ACCIDENT RELEASE RATES PER 1000 ORIGINATIONS 2022: 0.16

HAZMAT ACCIDENT RELEASE RATES PER 1000 CARLOADS 2022:                0.00793

Table 4 Failure Rates for Pressurized Storage Tanks[xiv]   incidents per YEAR

BEVI is the acronym for the Dutch Regulation authorizing the development of data

Table 5 Failure Rate for Pressurized Aboveground Pipework (HSE Data 2010)

Table 6 Failure Rate and Frequency for above ground pipelines up to the first flange and welded stumps (Dutch Study BEVI)

Dominant Causes of Loss of Containment Failures in the Dutch Study were:  

·               Incorrect materials used for installation parts;

·                Inadequate control of process streams;   

·               Making safe items in the plant prior to maintenance;

·           Errors in the assembly of components, for example loose connections;

·           Failure to recover process deviations;

·           Opening containment with direct links to the atmosphere (containment is bypassed but does not fail structurally);

Most failures occurred during normal operations, startup, or maintenance operations

Approximately 52% of the failure accidents occurred despite the system having at least one safety barrier (preventative control system). In the study of the failures,

65% were due to bypass,

17% due to leakage and catastrophic failures

16%. In the accident study,

In the study 20 of the incidents the quantity of material released was above 1 ton.

In 22 accidents where there were injuries 46 were casualties,

17 were hospitalized with burns, chemical burns or poisoning;

Approximately 1/3 of the injured were contractors.

Key components in the accidents with loss of control included:

deviation from normal operating conditions that increased temperature or pressure or both, causing piping failure either by rupture, leak, or catastrophic explosion.

In a UK study of 104 accidents where facilities had failures of their Safety Management Systems with one or more defects, the leading causes of the accidents were:

1) Operational Control failures (30%)

2) Failure to identify and evaluate major hazards (20%) 

3) Management of change (16%)

4) Failure of monitoring performance equipment (11%) and

5) Organization and personnel issues (8%). 

6) Unknown causes and plants with no relevant defects in their safety management system (17%)

This suggests that critical review of major hazards and better operator training could significantly reduce the number of incidents by as much as one-half.

The type of incidents created by the failures is shown below:

Figure 9 Direct Effects of Safety Management Systems Failures

In the EU and UK the Sevesto directive established MARS or the Major Accident Reporting System. The definition of a major reportable incident includes:

·      At least 5% of total inventory onsite involved

·      Injury or damage to people or real estate

·      Environmental Damage

·      Cross Border Damage

The following analysis of accidents is classified by product type:

Table 7 Accident Frequency of 56 accidents in the Industrial Sector under the MARS System

QRQA -Taylor Tables

JR Taylor, an Danish Safety Engineer has written Risk Analysis for Process Plants, Pipelines, and Transport (1994).  Since that time modeling improvements and additional volumes of analyzed safety incidents and accidents have been published. One of the most recent publications is Volume 33, (2020). It is available through Academia.edu, and contains an analysis of over industrial chemical industry 80 accidents. 

Also available is QRAQ Project Volume 4 (2010) Frequency of Releases and Accidents it is available on the Internet through scribid.com.  As might be expected, the piping failure rates per meter per year for are different for different processes. The petroleum industry piping (crude unit) has a failure rate of 9.0×10^-5; the Chlor-Alali plant failure rate is 3.0x 10^-5 per meter per year. By comparison, the LPG storage piping has a failure rate of 2.0 x 10^-5 per meter per year, while Ammonia and Fertilizer, and Refinery plants have low failure rates of  <10^-5/meters/yr.

OREDA Offshore Reliability Handbook Vols 1 and 2.

The handbooks are for the offshore petroleum industry and have been used since 1984. The copies can be borrowed from special technical libraries, or both volumes can be purchased for approximately $1,200.

Two tables and accompanying scenarios have been prepared using the EPA’s Cameo Suite—ALOHA and MARPLOT—to illustrate the availability of existing information and tools. There is NO accompanying probability data for the likelihood of occurrence, but the frequencies are very low. The graphical tables show the ALOHA input data describing the release scenario, and the pictorial data show the potential exposures from the incidents.

The first analysis shows the possible consequences of an Ammonia Tank failure in Stockton, California. The second analysis shows the possible consequences of a 3” diameter hole in a chlorine tank car on a rail siding in Ackworth, Georgia. The figures and data are presented after the conclusions. No attempt was made to model the East Palestine incident of February 2023 because the full extent of the data is unavailable.

Conclusions:

The EU Government and the Government of Singapore (GoS) have taken a different approach to the hazards posed by the US Chemical Industry. Much of their data are public and readily accessible, whereas similar information from US sources is much more difficult to obtain, and not readily available[xv] without significant expense or effort. With greater access to data, the public can be better informed and participate in the siting process for new facilities and improvements to existing facilities.

The EU and GoS have mandated public restrictions on where industrial facilities can be built, and the US may need to consider something similar. Acceptable precautions for neighboring residents are an idea whose time has come, and some companies are already taking proactive steps to be responsible for good corporate behavior.

Table 8  Hazard Study for Stockton, CA from ALOHA/MARPLOT SITE DATA:

   Location: STOCKTON, CALIFORNIA, CALIFORNIA

   Building Air Exchanges Per Hour: 0.41 (unsheltered single storied)

   Time: November 4, 2024  1349 hours PST (using computer’s clock)

 CHEMICAL DATA:

   Chemical Name: AMMONIA                 Molecular Weight: 17.03 g/mol

   AEGL-1 (60 min): 30 ppm   AEGL-2 (60 min): 160 ppm   AEGL-3 (60 min): 1100 ppm

   IDLH: 300 ppm      LEL: 150000 ppm     UEL: 280000 ppm

   Ambient Boiling Point: -28.2° F

   Vapor Pressure at Ambient Temperature: greater than 1 atm

   Ambient Saturation Concentration: 1,000,000 ppm or 100.0%

 ATMOSPHERIC DATA: (MANUAL INPUT OF DATA)

   Wind: 4 miles/hour from WNW at 3 meters

   Ground Roughness: urban or forest      Cloud Cover: 10 tenths

   Air Temperature: 60° F                 Stability Class: D

   No Inversion Height                    Relative Humidity: 75%

 SOURCE STRENGTH:

   Evaporating Puddle (Note: chemical is flammable)

   Puddle Area: 70684 square feet         Average Puddle Depth: 5 inches

   Ground Type: Sandy dry soil            Ground Temperature: 60° F

   Initial Puddle Temperature: -28.2° F

   Release Duration: ALOHA limited the duration to 1 hour

   Max Average Sustained Release Rate: 12,200 pounds/min

      (averaged over a minute or more)

   Total Amount Released: 172,134 pounds

 THREAT ZONE:

   Model Run: Gaussian

   Red   : 1.3 miles — (300 ppm = IDLH)

   Orange: 1.8 miles — (160 ppm = AEGL-2 (60 min))

   Yellow: 4.2 miles — (30 ppm = AEGL-1 (60 min)):

Figure 10  Stockton, CA Location Map

Figure 11 Stockton, CA, Hazard Map from ALOHA/MARPLOT

Red   : 1.3 miles — (300 ppm = IDLH)

Orange: 1.8 miles — (160 ppm = AEGL-2 (60 min))

Yellow: 4.2 miles — (30 ppm = AEGL-1 (60 min)):

Table 9  Possible Transportation Hazard Assessment using ALOHA and MARPLOT

Austell, GA Rail Yard: Incident to a pressurized tank of chlorine at ambient temperature (under pressure) 3” bottom valve broken off

SITE DATA:

   Location: AUSTELL GA RAIL YARD, GEORGIA

   Building Air Exchanges Per Hour: 2 (user specified)

   Time: November 4, 2024  1530 hours EST (using computer’s clock)

 CHEMICAL DATA:

   Chemical Name: CHLORINE                Molecular Weight: 70.91 g/mol

   AEGL-1 (60 min): 0.5 ppm   AEGL-2 (60 min): 2 ppm   AEGL-3 (60 min): 20 ppm

   IDLH: 10 ppm

   Ambient Boiling Point: -30.6° F

   Vapor Pressure at Ambient Temperature: greater than 1 atm

   Ambient Saturation Concentration: 1,000,000 ppm or 100.0%

 ATMOSPHERIC DATA: (MANUAL INPUT OF DATA)

   Wind: 4.2 miles/hour from wnw at 3 meters

   Ground Roughness: urban or forest      Cloud Cover: 7 tenths

   Air Temperature: 70° F                 Stability Class: C

   No Inversion Height                    Relative Humidity: 75%

 SOURCE STRENGTH:

   Leak from short pipe or valve in horizontal cylindrical tank

   Non-flammable chemical is escaping from tank

   Tank Diameter: 10.5 feet               Tank Length: 35 feet

   Tank Volume: 3,031 cubic feet

   Tank contains liquid                   Internal Temperature: 70° F

   Chemical Mass in Tank: 126 tons        Tank is 95% full

   Circular Opening Diameter: 3 inches

   Opening is 0 feet from tank bottom

   Release Duration: 45 minutes

   Max Average Sustained Release Rate: 6,040 pounds/min

      (averaged over a minute or more)

   Total Amount Released: 252,000 pounds

   Note: The chemical escaped as a mixture of gas and aerosol (two phase flow).

 THREAT ZONE:

   Model Run: Heavy Gas

Red   : 5.3 miles — (10 ppm = IDLH)

Orange: greater than 6 miles — (2 ppm = AEGL-2 [60 min])

Yellow: greater than 6 miles — (0.5 ppm = AEGL-1 [60 min])

Figure 11  ALOHA and MARPLOT assessment of a Chlorine Leak

Red   : 5.3 miles — (10 ppm = IDLH)

Orange: greater than 6 miles — (2 ppm = AEGL-2 [60 min])

Yellow: greater than 6 miles — (0.5 ppm = AEGL-1 [60 min])