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December 2013: VCE Basics

Introduction

The following article is written as a simple primer of VCE phenomenon and criteria. The intent is to discuss some basic concepts and terminology, while excluding highly technical and mathematical concepts.

A Vapor Cloud Explosion (VCE) is a very special class of catastrophic explosion occurring with an extraordinary release of a large volume of ignitable chemicals under certain conditions. VCE incidents are characterized by extremely high overpressures generated by ignition of an airborne vapor cloud of flammable materials. Overpressures, and thus damages, are greatly in excess of those for a "normal" air/vapor explosion occurrence.

VCE incidents are among the most dangerous and destructive losses which can befall chemical process and transportation industries. A VCE can destroy large areas, including buildings, heavy process structures and equipment. Pressure effects from a VCE incident may be felt many miles from the explosion epicenter. Secondary fires and explosions can result in further damage.

Three things must happen sequentially for a VCE event to occur:

  1. Release of large quantities of flammable materials in vapor format, or liquid format with pressure and temperature conditions allowing for rapid vaporization.
  2. Dispersion and mixing of released material to form an outdoor cloud in air.
  3. Delayed ignition (allows for dispersion and mixing.)

Example

As we are all familiar with gasoline and to a degree with its flammability and explosive characteristics, it's helpful to explore some basic examples. To begin to look at what might happen, we need to know some basic physical characteristics with respect to gasoline and the particular conditions and surroundings in which it is being processed.

Gasoline is actually composed of many chemical constituents, but a major component is hexanes, and gasoline has characteristics principally of hexanes, of which there are six possible isomers. As this gets seriously complex with unnecessary details, to simplify, we will consider gasoline to have the characteristics of n-hexane, with a flash point of -15°F and boiling point of 156°F.

From this physical information we understand that putting a match (source of ignition) to gasoline at any temperature above -15°F will cause it to burn. Additionally, at any temperature above 156°F we see that gasoline will boil, or evaporate vey rapidly.

Less obvious are the effects of pressure. Taking an excursion from VCE's, it is instructive to momentarily consider something more benign than gasoline, namely chicken soup. If we place chicken soup in an open pan and on the stove at a temperature of 212°F (the temperature at which water boils), it will boil very rapidly with the vapors dispersing into the atmosphere of the kitchen - it smells good! If we place the chicken soup inside a pressure cooker instead and heat it to 300°F, it will no longer boil, but the pressure inside will increase to compensate. If the pressure cooker then fails and the lid blows off, the contents are released all at once. Suddenly, chicken soup - still at 300°F but now released to atmospheric pressure - boils (vaporizes) rapidly forming a chicken soup cloud. Fortunately, chicken soup is neither flammable nor explosive, and all we are left with is a mess. However, if we replace chicken soup in the pressure cooker with gasoline the outcome would be different, as the failed pressure cooker's contents are released and cease to be a liquid under pressure, to become a rapidly expanding vapor cloud. With the addition of a source of ignition (e.g., the gas flame for the stove), we have a mini explosion.

To put this physical information into a real scenario, suppose we have a 100,000 gallon diked gasoline tank at atmospheric pressure on an 85°F day. Then let's say it is a particularly BAD day and the tank catastrophically fails, releasing its entire contents into the dike. Experience shows that this type of day can only get worse, so after a few minutes a bolt of lightning provides ignites the spill. We've all seen enough movies to envision this being one tremendous fireball explosion (flash fire), however it should not be a VCE 1. The gasoline released is well below its boiling point and is not pressurized, and under these conditions it is not likely to vaporize enough gasoline quickly enough to create a significantly large vapor cloud. Additionally, in a tank farm environment congestion of structures and equipment needed to enhance VCE effects is not typically present.

Let's change the conditions and see what happens. We'll reduce the liquid volume to 10,000 gallons (a mere 10% of what we had above), increase the temperature to 250°F, the pressure to 100 psi, and located the tank at a congested refinery facility. Again, this is a BAD day, and the tank fails catastrophically. However this time, pressure and temperature conditions are present for the contents of the tank contents to vaporize and form a cloud. Uh-oh, see it coming now?

Using the Ideal Gas Law (PV=nRT) we can calculate that if all the hexane vaporized (this would not really happen in real life) it could form a cloud of about 9,300 cu. meters volume. That doesn't sound too bad, does it? But wait, remember the fire triangle? We need oxygen to complete combustion (no O2, no boom). So let's mix the vaporized gas with the perfect volume of air containing oxygen (stoichiometric mixture) to complete combustion. Now the volume calculations show a cloud with a volume of about 430,000 cu. meters. Take this cloud, flatten it to hug the ground, extend it to account for the light breeze, intermingle it with a lot of heavy chemical process structures, provide that lightning bolt, and NOW the bad day gets much, much worse. This is going to be a VCE.

Results

The results of a VCE event will be utterly devastating, with nearly total destruction at the epicenter and damage reducing concentrically as distance from the epicenter increases.

When VCE's are modeled, the results are typically shown as concentric overpressure rings with highest pressure at the center and pressures decreasing as distance from the center of the event increases. This is, of course, idealized and the reality is that pressure effects, while generally concentric, can vary significantly with local conditions. Pressure impulse within, or under the vapor cloud area are highest. Outside the vapor cloud overpressures diminish rapidly with distance, but can be sufficient to result in structural damage over 1,000 ft. away. Complete damage is anticipated within the perimeter of the vapor cloud, and likely through the range of the 10 psi over pressure demarcation.

For reference, glass shattering and window frame damage can occur at as low as 0.5 psi, and collapse of structures at as little as 2 psi. Heavier and reinforced structures may survive higher pressures up to 5-10 psi, but will likely incur sufficient damage so as to not be salvable. At pressures above 10 psi, damage is anticipated to be effectively near complete.

Prevention, Mitigation and Control

Once it has been established that a VCE is possible, there are no control or mitigation measures as there are for fires (e.g., sprinkler protection for storage hazard), or more limited explosions (e.g., explosion suppression systems for dust hazard). These avenues of control are simply not possible for a VCE event. If the VCE event occurs, it cannot be controlled or mitigated.

The only way to prevent a VCE event where the possibility exists is to keep the "bad stuff" inside the pipes and vessels where it is supposed to be. As plants and facilities handling large amounts of flammable chemicals are designed to maintain chemicals without unexpected release, the fact that VCE events occur can (arguably) nearly always be considered a Human Element failure. This statement in itself is a singularly unhelpful; however it shows the importance and criticality of maintaining Human Element programs to the highest possible standards at these facilities.

Prediction, Calculation and Other Considerations

VCE mechanisms are not fully understood, but computers and the ability to create complex models is narrowing the gap between observed and predicted behaviors. The computation of VCE effects and results are extraordinarily complex, rivaling those of weather prediction. There are many modeling programs available, and some may provide better approximations than others. Whatever model is used, it is only an approximation and not a guarantee that a VCE event could not occur under lesser conditions, or would actually occur if the model conditions were met.

To some extent it is only necessary to predict that a VCE event is possible, and it is not necessary to accurately predict the precise results of overpressure effects. Armed with the knowledge that a VCE event is possible, we know that the results will be near total destruction at the epicenter and lesser degrees outwards. For small to medium sized facilities, this often translates to the entire facility. Therefore, the degree of accuracy can be relatively academic in some instances.

VCE propagation occurs when flame fronts develop and accelerate at speeds much greater than "normal" vapor explosions. Flame fronts in a VCE can travel at detonation speeds (in excess of the speed of sound) versus a more common deflagration (less than the speed of sound). Just as in most accidents involving increased speed, damage increases proportionally as well. Flame fronts developing at lower speeds will result in a flash fire, and not in a VCE.

A unique phenomenon of VCE's is that the degree of congestion, or confinement, within the vapor cloud prior to ignition has a significant influence on the overpressure forces created during a VCE event. Typically, the higher the congestion, the greater the overpressure forces, and the more likelihood of a VCE event. Where congestion is absent, VCE events are generally not predicted.

Congestion in conjunction with large amounts of flammable materials at pressure and/or temperature is most commonly found in outdoor chemical process units where process structures, vessels, piping, etc. can create a highly congested environment. This behavior with respect to congestion is not fully explained, and is predictable only in a general sense rather than to a specific degree.

Summary

Thankfully, VCE events are a relatively rare occurrence, but when they do occur the effects are devastating to buildings, equipment, personnel, and economic livelihood. Although there is nothing that can be done to mitigate a VCE occurrence once it has initiated, there is much that be done to minimize the likelihood of one occurring in the first place. If you feel you have conditions which have the potential for a VCE event, Risk Logic Inc. is available to provide evaluation and further assessment.

Further Reading and Calculation Methods with Notes 2

Center for Chemical Process Safety (CCPS), (2010) Guidelines for Vapor Cloud Explosion, Pressure Vessel Burst, BLEVE and Flash Fire Hazards - 428 pages with lots of information in a relatively readable manuscript.

EPA, RMP*Comp - The EPA's Risk Management Program (RMP) tool is an free online (also downloadable) calculation program which requires very minimal input, but also supplies very minimal output. Data generated by this program should be used with care.

FM Global Data Sheet 7-42 (May 2008 Ed.) Guidelines for Evaluating the Effects of Vapor Cloud Explosions Using a TNT Equivalency Method. Provides a relatively simple set of criteria enabling the user to predict if a VCE is, or is not, possible. Also includes information on what materials, amounts, and conditions might allow for a VCE event, as well as information for completing calculations using TNT modeling methodology.

FM Global Data Sheet 7-42 (Oct. 2012 Ed.) Evaluating Vapor Cloud Explosions Using A Flame Acceleration Method. Completely revised data sheet replacing TNT analysis methodology with Flame Acceleration methodology to categorize materials and set threshold limits. The information in this data sheet does not supply information allowing the user to calculate a VCE model, but suggests the use of FMG's BlastCalc, a proprietary software modeling system.

GexCon FLACS (FLame ACceleration Simulator) - modeling software based Computational Fluid Dynamics (CFD). Proprietary software modeling of explosion events, including VCE. Proprietary software available in consideration of licensing fees.

Sedgewick Energy Ltd., SLAM (Sedgwick Loss Assessment Model) - modeling software based on the Congestion Assessment Method. No additional information or currency known.

Swiss RE, Ex Tool - modeling software based on the TNT equivalency method. No additional information or currency known.

The Netherlands Organization (TNO), EFFECTS - modeling based on the Multi-Energy Method. Details, calculations, and examples can be found in the "Yellow Book"

The Netherlands Organization (TNO), Methods for the Calculation of Physical Effects - due to releases of hazardous materials (liquids and gases) "Yellow Book" (3rd Ed. Second revised print 2005)

US Chemical Safety Board (CSB) http://www.csb.gov/ - The CSB is an independent federal agency charged with investigating industrial chemical accidents. The website contains in depth investigation of both ongoing and past accidents, including VCE events.

NOTES:

1: VCE events and effects have been attributed to very similar incidents, even though they were not anticipated to have been likely. The Buncefield Oil Storage Depot event of 2005 is worth note and study.

2: By virtue of the simplicity of this review, the references are also simplified and non-comprehensive. A good review of modeling methods can be found in a publication of, Health & Safety Laboratory, A Review of the State-of-the-Art in Gas Explosion Modeling (2002).