Postrelease mitigation measures

The techniques that can be applied as postrelease mitigation measures in many cases are not based on mature science. Only in recent years has attention been focused on the design of postrelease mitigation systems and the development of needed fundamental knowledge through testing and data correlation. 

Chapter 2—Overview of Release Scenarios and Postrelease Mitigation This chapter provides an overview of the different types of releases that can occur, the consequences of the discharges, and a general description of available postrelease mitigation measures. 

Mitigation techniques are divided into two categories prerelease and postrelease mitigation measures. Prerelease mitigation measures take effect... 

Flare stacks that safely bum organic material released into vent headers are an example of a prerelease mitigation measure. The flare destroys the hazardous organic material before it reaches the environment. A dike around a storage tank is an example of a postrelease mitigation measure. The dike contains the release in a small area, reducing the total evaporation rate from the spill and so reducing the impact of the release. 

Those who respond to a release are usually heading into an area from which everyone else is trying to escape. Well-designed postrelease mitigation measures can provide response personnel with safer access to the hazard zone. Off-site consequences can be significantly reduced, as well. 

Containment uses a physical barrier to prevent an uncontrolled release of materials to the environment. The walls of a vessel or pipe serve as the primary containment barrier that encloses harmful materials. Redundant (secondary) containment serves as a safeguard if the primary barrier fails, and is considered a postrelease mitigation measure. Containment can take many forms, depending on factors such as the system or process to be contained, the risks involved with a release, and the cost benefit of the additional secondary containment. 

To demonstrate the effectiveness of diking as a postrelease mitigation measure, an accidental release of carbon disulfide from a vertical storage tank will be evaluated. In the example, isopleths to the ERPG-2 concentration of 50 ppm (AIHA, 1992) will be considered. 

As discussed in Section 3.1.2.1, a liquid that is uncontained is one over which there is no control and which will result in potentially severe consequences. If a dike is placed around the tank containing the refrigerated liquid ammonia and the ammonia spill is confined within it, a much reduced hazard zone can be obtained because we have limited the surface area available for vaporization and additional postrelease mitigation measures can be applied. As pointed out in Chapter 3, combinations of postrelease mitigation measures will provide the best overall response to an accidental release. 

In this section we will demonstrate the use of water sprays as a postrelease mitigation measure. The theory and design of water spray systems has been detailed in Chapter 4. The scenario will involve the accidental release of hydrofluoric acid (HF) from a storage tank. Again we will be considering the distance to reach the ERPG-2 concentration for HF of 20 ppm (AIHA, 1992). 

In the analysis it was assumed that the water-spray curtain was activated within one minute of the start of the incident and that 86% of the HF vapor from the evaporating pool that reached it was removed. The balance of the HF not removed by the water sprays became the material that formed the reduced hazard zone and the input for the dispersion modeling. For the F/l meteorological conditions the mitigated hazard zone for the centerline concentration of 20 ppm was 750 m. The effectiveness of water spray as postrelease mitigation measure is shown in Table 7.6. 

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