2 Worst-case release scenarios

Definition

The Environmental Protection Agency (EPA), USA has defined a worst-case release as the release of the largest quantity of a regulated substance that results in the greatest distance from the point to a specified end-point. For substances in vessels, release of the largest amount in a single vessel; for substances in pipes, release of largest amount in a pipe should be assumed. Actually the largest quantity should be determined taking administrative controls into account .

Administrative controls are written procedures that limit the quantity of a substance that can be stored or processed in a vessel or pipe at any one time, or alternatively, occasionally allow a vessel or pipe to store larger than usual quantities (e.g. during turnaround). It is not necessary to consider the possible causes of the worst-case release or the probability that such a release might occur; the release is simply assumed to take place.

Worst-case releases of toxic substances

For the worst-case release analysis for toxic substances, several assumptions are used. These assumptions are very conservative; the results likely will be very conservative.

Modeling assumptions

  • End-points: in the definition of worst-case analysis for toxic substances, the endpoints are the concentrations below which it is believed nearly all individuals could be exposed for one-half to one hour without any serious health effects. The distance to the end-point estimated under worst-case conditions should not be considered a zone in which the public would likely be in danger, instead, it is intended to provide an estimate of the maximum possible area that might be affected in the unlikely event of catastrophic conditions.
  • Release height: all releases are assumed to take place at ground level for the worst case analysis. Even a ground-level release is unlikely at the site, we must use this assumption for the worst-case analysis.
  • Wind speed and atmospheric stability: meteorological conditions for the worst case scenario are defined as atmospheric stability class F (very stable atmosphere) and wind speed of 1.5 m/s. If we can demonstrate a higher minimum wind speed or less stable atmosphere over three years, these minimums may be used.
  • Temperature and Humidity: The highest daily maximum temperature that occurred in the previous three years and the average humidity for the site should be used. Small differences in temperature and humidity are unlikely to have a major effect on results.
  • Topography: Two choices are provided for topography for the worst-case scenario. If the site is located in an area with a few buildings or other obstructions, we should assume open (rural) conditions. If the site is in an urban location, or is in an area with many obstructions, we should assume urban conditions.
  • (f) Gas or vapour density: For the worst-case scenario analysis, we must use a model appropriate for the density of the released gas or vapour. Generally, for a substance that is lighter than air or has a density similar to that of air, we would use a model for neutrally buoyant vapours. For a substance that is heavier than air, we would generally use a dense gas model.

Required parameters for modelling worst-case scenarios

  • For toxic substances, use the endpoint of STEL.
  • For explosive substances, use the endpoint of an over pressure of 1 pound per square inch (psi) for vapour cloud explosions.
  • For flammable substances, use the endpoint of an heat flux of 4.5 kW/sqm.

Wind speed/stability

  • Use wind speed of 1.5 meter per second and F stability class unless the local meteorological data applicable to the site show a higher minimum wind speed or less stable atmosphere at all times during the previous three years. If the site demonstrates a higher minimum wind speed or less stable atmosphere over three years, these minimums may be used.

Ambient temperature/humidity

  • For toxic substances, use the highest daily maximum temperature during the past three years and average humidity for the site.
  • Height of release
  • For toxic substances, assume a ground level release.

Topography

  • Use urban or rural topography, as appropriate.

Temperature of released substance

  • For liquids (other than gases liquefied by refrigeration), use the highest daily maximum temperature, based on data for the previous three years, or at process temperature, whichever is higher.
  • Assume gases liquefied by refrigeration at atmospheric pressure are released at their boiling points.

Identification of Worst-case release scenario

For the identification of worst-case scenario of toxic substances, we have to analyse more than one scenario, because the distances depend on more than simply the quantity in a process. For example, for toxic liquids, distances depend on the magnitude of the toxic end point, the molecular weight, volatility of the substance and the temperature of the substance in the process, as well as quantity. A smaller quantity of a substance at an elevated temperature may give a greater distance to the end-point than a larger quantity of the same substance at ambient temperature. In some cases, it may be difficult to predict which substance and process will give the greatest worst case distance.

The following cases (1-4) will throw more light to understand the worst case scenarios:

Worst-case releases of Flammable substances

For the worst-case scenario involving a release of a regulated flammable substance, the following aspects are considered:

  • Quantity of the flammable substance is released into a vapour cloud and that a vapour cloud explosion results. Generally we estimate the distance to an end point to an overpressure level of 1 psi from the explosion of the vapour cloud and heat radiation intensity of 4.5 kW/sqm.
  • If the flammable substance is normally a gas at ambient temperature and handled as gas or liquid under pressure or, if the flammable substance is a gas handled as a refrigerated liquid and is not contained when released or the contained pool is one centimeter or less deep, we must assume the total quantity is released as a gas and is involved in a vapor cloud explosion.
  • If the flammable substance is a liquid or a refrigerated gas released into a containment area with a depth greater than one centimeter, we may assume that the quantity that volatilises in 10 minutes is involved in a vapour cloud explosion.

The following cases (5-10) will throw more light to understand the worst case scenarios:

 

 
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