Postrelease Mitigation

First we will look at a release scenario that is unmitigated, and then at the modification of a scenario to include a postrelease mitigation technique. The effect of the postrelease mitigation technique will be evaluated by applying the consequence modeling techniques described above. It is important to note 

The determination of the consequences for a group of selected release scenarios may be found in the CCPS document entitled Guidelines for Use of Vapor Cloud Dispersion Models, Second Edition (CCPS, 1996). 

The examples that are used in the following sections, and the conditions that have been selected to illustrate the effectiveness of the postrelease mitigation techniques, may not represent standard industrial practices for the materials selected. For detailed engineering information the manufacturers or suppliers of the materials should be consulted. They can also supply information on the most effective postrelease mitigation techniques to use in the event of an accidental loss of containment. 

Also bear in mind that the meteorological and other information described in the following section has been selected as representative conditions only. When an actual situation is being considered the meteorological conditions unique to the site under consideration should be used. 

For each of the postrelease mitigation techniques examined a description of the consequences of the unmitigated release will be presented first, followed by the mitigated release. The results have been tabulated and shown in figures. 

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CCPS G-24. Guidelines for Postrelease Mitigation Technology in the Chemical Process Industries. American Institute of Chemical Engineers, Center for Chemical Process Safety, New York. 

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. 

Empirical data are available for the design of some postrelease mitigation systems but these cases are limited in scope and only applicable to specific situations. 

The purpose of this guideline is to present methods and design examples of postrelease mitigation methods which are in current industrial use, and to assess areas where further research is needed. 

This guideline has been organized to meet the needs of readers who are new to the concept of postrelease mitigation and who are interested in learning about techniques available for mitigating specific scenarios. To help each reader determine where to look in the guideline, a short description of each chapter s contents is provided below. 

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. 

This chapter introduces the various methods of postrelease mitigation. First, however, prerelease mitigation techniques, different types of releases, and the potential consequences of hazardous material releases are discussed. These discussions are intended to provide the reader with some background into postmitigation concepts covered in detail later. An experienced engineer can use this chapter as a quick reference to the release mitigation techniques available. For someone new to this subject, this chapter will help to focus on appropriate strategies which are explored more fully in later chapters. 

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. Keeping a released material confined is the next essential step in postrelease mitigation. This action limits the surface area and allows other mitigation techniques to be used in a more effective and efficient manner. 

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. 

Chapter 6 addresses issues associated with postrelease mitigation, namely, the detection of a leak and the response of an organization to the event. Chapters 3, 4, and 5 cited numerous needs to detect releases and activate postrelease mitigation systems. In this chapter, the hardware that is available to perform this function, their operating principles, and maintenance requirements to ensure maximum system reliability are reviewed in some detail. 

Following a release, the plant staff must respond and alert the surrounding population that an emergency exists on the site. This is facilitated by having a well-thought-out and rehearsed emergency response plan in place. The essential elements of this plan are associated with postrelease mitigation. 

For the systems described in the previous sections, there is a wide variation in response times. As some of the examples in Chapter 7 will show, quick detection of a release and timely activation of postrelease mitigation systems is essential. 

Calibration checks also serve to minimize nuisance alarms and false trips of postrelease mitigation systems. This ensures that the personnel monitoring the system will have confidence in it and will respond aggressively when an alarm is received. If there are many nuisance alarms or false trips of postrelease mitigation systems, it is highly likely that an alarm will be ignored and the system turned off, or the response time will be slow because someone will go to the location to determine whether there is really... 

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