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3. Alternative-case release scenarios

The acceptable alternative scenario for a covered process must be one that is more likely to occur than the worst-case scenario and that reaches an endpoint off-site, unless no such scenario exists. It is not necessary to demonstrate greater likelihood of occurrence or carry out any analysis of probability of occurrence; we only need to use reasonable judgment and knowledge of the process. If, using a combination of reasonable assumptions, modelling of a release of a regulated substance from a process shows that the relevant endpoint is not reached off-site, we can use the modelling results to demonstrate that a scenario does not exist for the process that will give an endpoint off-site.

Generally five-year accident history and failure scenarios are considered in process hazard analysis in selecting alternative release scenarios for regulated toxic or flammable substances (e.g., we might choose an actual event from our accident history as the basis of our scenario). We also may consider any other reasonable scenarios.

The alternative scenarios should be such for which we can explain to emergency responders and the public as a reasonable alternative to the worst-case scenario. For example, we could pick a scenario based on an actual event, or we could choose a scenario that we worry about, because circumstances at our site might make it a possibility. If we believe that there is no reasonable scenario that could lead to off-site consequences, we may use a scenario that has no off-site impacts for our alternative analysis. We should be prepared to explain our choice of such a scenario to the public.

Number of release scenarios

A few release scenarios may be as follows:

  • Transfer hose releases due to splits or sudden uncoupling;
  • Process piping releases from failures at flanges, joints, welds, valves and valve seals and drains or bleeds;
  • Process vessel or pump releases due to cracks, seal failure, drain bleed, or plug failure;
  • Vessel overfilling and spill, or over de-pressurization and venting through relief valves or rupture disks; and
  • Shipping container mishandling and breakage or puncturing leading to a spill.

Mitigation systems for alternative release scenarios

We may consider active mitigation systems, such as interlocks, shutdown systems, pressure relieving devices, flares, emergency isolation systems, etc. Mitigation systems considered must be capable of withstanding the event that triggers the release while remaining functional.

Alternative releases of toxic substances

Although alternative scenarios are intended to be more likely than worst-case scenarios, the analysis of alternative scenarios should not be expected to provide realistic estimates of areas in which the public might be endangered in case of a release. The same conservative, protective endpoints are used for alternative release analysis as for worst-case analysis. These endpoints are intended to represent exposure levels below which most members of the public will not suffer any serious health effects. The endpoints are based on exposures for longer periods than may be likely in an actual release. In addition, modelling carried out to estimate distances to these endpoints, even when based on more realistic assumptions than used for the worst-case modelling, likely will provide results with a high degree of uncertainty. These estimated instances should not be considered a necessarily accurate prediction of the results of an actual release.

Modelling assumptions

  • Quantity: EPA has not specified any assumptions we must make concerning quantity released for an alternative release scenario. We could consider any site-specific factors in developing a reasonable estimate of quantity released (e.g., The quantity that could be released from a sheared pipe in the time it would take to shut off flow to the pipe).
  • Release Height: We may assume any appropriate release height for our alternative scenarios. For example, we may analyse a scenario in which a regulated substance would be released at a height well above ground level.
  • Wind Speed and Atmospheric Stability: We should use typical meteorological conditions at our site to model alternative scenarios. To determine typical conditions, we may need to obtain local meteorological data that are applicable to our site. If we do not keep weather data for our site (most sources do not), we may call another nearby source, such as an airport to determine wind speeds for our area. Our airport or other source will be able to give information on cloud cover.

The following cases (11-12) will throw more light to understand the worst case scenarios:

Alternative releases of flammable substances

Alternative release scenarios for flammable substances are somewhat more complicated than for toxic substances because the consequences of a release and the endpoint of concern may very. For the worst case, the consequence of concern is a vapor cloud explosion, with an over pressure endpoint. For alternative scenarios involving fires rather than explosions, other endpoints than over pressure (e.g., heat radiation) may need to be considered. The rule specifies endpoints for fires based on the heat radiation level that may cause second-degree burns from a 40-second exposure and the lower flammability limit (LFL), which is the lowest concentration in air at which a substance will burn.

Some possible scenarios

Some possible scenarios involving flammable substances are discussed below.

  • Vapour cloud fires (flash fires): may result from dispersion of a cloud of flammable vapor and ignition of the cloud following dispersion. Such a fire could flash back and could represent a severe heat radiation hazard to anyone in the area of the cloud. Vapour cloud fires may be modeled using air dispersion modeling techniques to estimate distances to a concentration equal to the LFL.
  • A pool fire, with potential radiant heat effects, may result from a spill of a flammable liquid. The endpoint for this type of fire is a radiant heat level of 5 kilowatts per square meter (kW/m2) for 40 seconds; a 40-second exposure to this heat level could cause second degree burns.
  • A Boiling Liquid Expanding Vapour Explosion (BLEVE), leading to a fireball that may produce intense heat, may occur if a vessel containing flammable material ruptures explosively as a result of exposure to fire. Heat radiation from the fireball is the primary hazard; vessel fragments and over pressure from the explosion also can result. BLEVEs are generally considered unlikely events. However, if we think a BLEVE is possible at our site, we should estimate the distance at which radiant heat effect can cause second degree burns. The point of off-site consequence analysis is to determine how far away from the point of release effects of concern could occur, so we should estimate the distance for BLEVEs even if they do not last for 40 seconds. We also may want to consider models or calculation methods to estimate effects of vessel fragmentation, although we are not required to analyse such effects.
  • For a Vapor Cloud Explosion (VCE) to occur, rapid release of a large quantity of flammable material, turbulent conditions (caused by a turbulent release or congested conditions in the area of the release, or both), and other factors are generally necessary. Vapour cloud explosions generally are considered unlikely events; however, if conditions at our site are conducive to vapor cloud explosions, we may want to consider vapour cloud explosion as an alternative scenario. The 1 psi over pressure endpoint still applies to a vapor cloud explosion for purposes of analysing an alternative scenario, but we could use less conservative assumptions than for the worst-case analysis, including any reasonable estimate of the quantity in the cloud and the yield factor. A vapour cloud defloration, involving lower flame speeds than detonation and resulting in less damaging blast effects, is more likely than a detonation. We may assume a vapor cloud deflagration for the alternative scenario, if we think it is appropriate, and use the radiant heat endpoint (adjusted for duration).
  • A jet fire may result from the puncture or rupture of a tank or pipeline containing a compressed or liquefied gas under pressure. The gas discharging from the hole can form a jet that “blows” into the air in the direction of the hole; the jet then may ignite. Jet fires could contribute to BLEVEs and fireballs if they impinge on tanks of flammable substances. A large horizontal jet fire may have the potential to pose an off-site hazard. We may consider a jet fire as an alternative scenario, if appropriate for our site.

Modeling Assumptions

  • Quantity: EPA has not specified any assumptions we must make concerning quantity released for an alternative scenario analysis for flammable substances. We may consider any site-specific factors in developing a reasonable estimate of quantity released, as for toxic substances (e.g., the quantity that could be released from a ruptured pipe in the time it would take to shut off flow to the pipe).
  • Release Height: We may assume any appropriate release height for our alternative scenarios for flammable substances.
  • Wind Speed and Atmospheric Stability: Meteorological conditions may have little effect on some scenarios for flammable substances (e.g., vapour cloud explosions and BLEVEs scenarios but may have a relatively large effect on others (e.g., a vapor cloud fire resulting from down wind dispersion of a vapor cloud and subsequent ignition). We should use typical meteorological conditions at our site to model appropriate alternative scenarios. To determine typical conditions, we may need to obtain local meteorological data that are applicable to our site, as discussed above.

The following cases (13-14) will throw more light to understand the worst case scenarios:

Estimating distance to the endpoint

We may use any appropriate model to estimate the distance to the specified endpoint for alternative scenarios for regulated flammable substances. Several possible consequences of releases of flammable substances are discussed below.

  • Vapour cloud fire: We may use any appropriate model to estimate distances for a vapor cloud fire. The LFL endpoint would be appropriate for vapour cloud fires. We may use air dispersion modelling to estimate the maximum distance to the LFL. We may want to consider, however, whether it is likely that a flammable gas or vapor could disperse to the maximum distance to the LFL before reaching an ignition source. The actual dispersion distance before ignition might be much shorter than the maximum possible distance.
  • Pool fire: Any appropriate model may be used for pool fires of flammable liquids. The applicable endpoint for the heat radiation level of 4 kW/m .
  • BLEVE: If a fireball from a BLEVE is a potential release scenario at our site, we may use any model or calculation method to estimate the distance to a radiant heat level that can cause second degree burns (a heat “dose” equivalent to the specified radiant heat endpoint of 5kW/m for 40 seconds).
  • Vapour cloud explosion: If we have the potential at our site for the rapid release of a large quantity of a flammable vapor, particularly into a congested area, a vapour cloud explosion may be an appropriate alternative release scenario.

Number of Scenarios

It is necessary to analyse at least one alternative release scenario for each listed toxic substance above its threshold quantity. Even if we have a substance above the threshold in several processes or locations, we need only analyse one alternative scenario for it. Similarly, to analyse one alternative release scenario representing all regulated flammable substances; we do not need to analyse an alternative scenario for each flammable substance above the threshold.

For example, if we have five listed substances chlorine, ammonia, hydrogen chloride, propane, and acetylene above the threshold quantity, we will need to analyse one alternative scenario each for chlorine, ammonia, and hydrogen chloride (toxic substances) and a single alternative scenario to cover propane and acetylene (flammable substances). In addition, no alternative scenario analysis is required for any process that does not contain more than a threshold quantity of a regulated substance, even if we believe such a process is a likely source of a release.


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