Although hazardous materials incidents may account for only one to two percent of total responses, they are rarely simple or straightforward operations, and a significant amount of planning, training, and equipment are involved in developing and maintaining a hazardous materials team. In order to avoid being caught off guard by an incident, the parties involved with the response need to be adequately prepared. Preparation includes identification and assessment of potential hazards in a community. Consequence models, if used properly, can be an invaluable tool for organizations to give them accurate estimates of the potential consequences that may result from hazardous materials incidents.

Consequence Analysis

Consequence models are best used in the process of scenario development and evaluation. Potential scenarios would be developed based on information provided on the amounts and types of hazardous materials stored and transported in a given area. Once credible scenarios are developed, they can be evaluated to see what the potential consequences are. Assumptions must be credible and scenarios realistic or the output from the model will not be representative of what would happen during an incident. An example of this would be the loading of Liquefied Petroleum Gas (LPG) at a facility. Usually, the tanker delivering LPG contains a larger amount of the material than is stored on-site. In addition, incidents are more likely to occur during transfer operations (usually due to human error). Therefore, analysis of potential releases at the site should include analysis of the potentially larger release from the tank truck or railcar that is unloading the material. Consequence models would allow a risk analyst to estimate the potential consequences resulting from a release of a hazardous material.

Consequence modeling can be defined as the use of mathematical representations of conservation and physical laws to analyze and quantify potential damaging effects of hazardous events, usually by loss of containment. There are several types of models that can be used for many potential scenarios. The goal of using a consequence model is to estimate the potential impacts of a hazardous release. In order to do this, a risk analyst must determine the impact of interest (fire, explosion, toxic dispersion, etc.), identify factors affecting its impact, and identify an appropriate model to estimate the magnitude of those impacts. There are also several different models of varying complexity that can be used for the same events. Both screening level and detailed models are available. Good models for the situation at hand will accurately describe the phenomena being studied, not require the use of unavailable data, and yield results in a useful form.

Liquefied Petroleum Gas Example

According to NFPA standards, fire water is mandatory in any operational refinery or plant. Any plant emergency may not be adequately reacted to and overcome if there is no fire protection and fire water system. Facilities can prepare the appropriate amount of fire water by deciding on the worst-case scenario in case of a plant emergency.

Read this publication, A Risk Based Approach to Calculating Fire Water Demand, for an example on calculating what the maximum required fire water demand rate is for a liquefied petroleum gas (LPG) storage facility with Process Safety Office^® SuperChems™ software. The intent is on reducing the risk level, i.e. measures intended to prevent and/or mitigate the identified hazardous scenarios, of a facility. Using this method, the optimal amount of fire water can be obtained, and a more focused emergency response plan can be developed.

Thermal Radiation Risk Contours example from Process Safety Office® SuperChems™

Types of Models

Source Term

Source term models characterize the important features of the initial release and provide necessary input to the other models. Important information that is used as input for source term analysis are such things as the amount of material discharged, the rate of discharge (instantaneous or continuous), temperature, and pressure. This information will allow a risk analyst to accurately characterize the release. Source term models address such topics as the formation and spreading of liquid pools and resultant evaporation and boiling off of material, the actual amount released, two-phase flow, and temperature and pressure inside the pipe or vessel at the release point and immediately outside the pipe or vessel. An example of this would be the initial drop in temperature resulting from the release of a liquified gas. Source term models also address intermediate phenomena such as gas expansion, two-phase flow, aerosolization, rainout, and entrainment.

Dispersion

Dispersion models enable the risk analyst to predict the path and size of a plume of a cloud of a toxic or flammable gas to the vapors evaporating off of a spreading liquid pool. The models can predict downwind travel to such concentrations as the lower flammable limit of the material or specified toxic concentrations (e.g. the IDLH) at various times after the spill has taken place. The resultant plume can then be superimposed over a map of the area of concern to determine affected areas (e.g. sensitive population centers such as schools and hospitals). When using a dispersion model, the risk analyst must be careful to choose the limiting concentration. Values such as the lower flammable limit (LFL) are straightforward, but choosing a limiting concentration for toxic hazards can be more complex. Toxicity data can be limited on many materials. In addition, the duration of potential exposure also needs to be considered. Dispersion models are also sensitive to such factors as temperature, pressure, humidity, etc.

Fire and Explosion

Fire and explosion models can predict other consequences of hazardous materials incidents. Releases of hazardous materials often result in the potential for a fire or explosion. These models can predict distances of concern for such consequences as thermal radiation from liquid pools that have ignited, distances to specified radiant heat values for fireballs, and overpressure distances from vapor cloud explosions. Important material characteristics used in fire modeling are flash point, autoignition temperature, ignition energy, flammability limits (LFL and UFL), and the heat of combustion. The types of models used for fires are pool fire, flame jet, fireball, and vapor cloud fire models. Damage criteria used in fire models are injury to people, ignition of combustibles, and damage to structures or equipment. Damage can be caused by steady state radiation from pool fires or flame jets or unsteady radiation from fireballs and vapor cloud fires.

Explosion

Explosion models allow the risk analyst to model deflagrations, detonations, and boiling liquid expanding vapor explosions (BLEVEs). These results can be used in the same way as the dispersion models as well as to gauge the potential for damage to buildings and other infrastructure based on historical information as to what they can stand (e.g. the minimum amount of overpressure to break the glass in a car windshield).

Invaluable Tool for Risk Management

Making intelligent decisions regarding planning for hazardous materials emergencies is important. The advantages of consequence modeling software such as Process Safety Office^® SuperChems™ should not be overlooked. Proper use of this software will facilitate timely and efficient response to hazardous materials emergencies and reduce risk to personnel, emergency response, and the general public.

ioMosaic professionals have decades of experience working with consequence modeling systems, which means you will have unrivaled efficiency and expertise at your disposal. Call us today at 1.844.ioMosaic or send us a note. We'd love to hear from you.