Risk to Process Plant Buildings

As discussed in the introduction to this chapter, prior accidents have suggested the need for an improved approach in addressing the risks to process plant buildings and their occupants. Table 1.1 provides a selected list of some serious incidents involving buildings in process plants. 

Process and Plant Documentation for Evaluating Explosion and Fire Risks to Process Plant Buildings... 

The results of previous process hazards evaluations may be used if it can be confirmed that the reviews adequately addressed explosion and fire risks to process plant buildings. ... 

This section discusses some of the measures available for reducing risk to process plant building occupants. Determining which actions to take is an iterative process. Proposed risk-reduction measures should be evaluated to confirm feasibility and to determine that risk will be sufficiently reduced. 

Section 6.2 discussed various options to reduce risk to process plant buildings and occupants. Some of these options, singly or in combination, may be applicable to a specific building under study. 

Evaluating risk to process plant building occupants can be accomplished through detailed qualitative and/or quantitative risk assessment. However, because of the large numbers of buildings and varying plant situations involved, these types of studies could be costly and time-consuming if applied in all cases, and should be reserved for those situations for which cost-effective solutions cannot otherwise be identified. 

The benefits of a modification include factors indirectly related to the reduced risk to process plant buildings and occupants. An explosion or fire can result in losses in one or more of the following categories ... 

As will be shown in Section 4.2, published data are available on the application of societal risk measures, including the development of risk tolerability limits for F-N curves. However, much of this guidance has been developed for characterizing risks to the general public and would not normally be considered as a basis for assessing risks to on-site personnel. It is appropriate, therefore, to suggest another risk measure, similar in concept to societal risk, for on-site applications to process plant buildings ... 

Process plant buildings can be subjected to a range of impacts, depending upon the type of building construction, the location, the process conditions, and the materials being handled. From a risk standpoint, the more favorable situation is where the building locations and types of construction are such that no injury to building occupants could occur as a result of a plausible or reasonably believable event. The less favorable situation is one... 

Note that most societal F-N curves extend out to hundreds or even thousands of fatalities. Such events involving process plant buildings are extremely unlikely, since few buildings, if any, within process facilities have such large concentrations of people. Aggregate risk curves should reflect site-specific conditions and realistic events. 

The first step in a process plant building risk assessment is to identify specific accident scenarios that endanger building occupants. As discussed in Chapter 2 and illustrated in Table 2.1, accident scenarios are sequences of events that lead to an outcome of concern. The specific outcomes of concern are those involving explosions or fires that could impact buildings in process plants. 

Key Factors to Consider in Process Plant Building Risk Assessments... 

Population and individual risk can be determined in the same manner as discussed in Chapter 4. These risk measurements can then be compared with risk tolerance criteria, or decision methodologies can be used, to assist in making risk-reduction decisions about process plant buildings. 

One risk-reduction option is to construct a new blast-resistant building in the same location as the existing building. This involves demolishing the existing building, which may cause significant disruption to process plant operations. 

When any decisions regarding process plant buildings are to be made, it is important that the uncertainties in the risk assessment process be clearly understood and that these uncertainties be a consideration in the decision making. 

API RP 752, Management of Hazards Associated With Location of Process Plant Buildings, 2d ed. (American Petroleum Institute, Washington, 2003), which gives a risk-based approach to evaluating protection afforded by occupied structures... 

Evaluation of risk to buildings in process plants will usually identify a wide range of possible risk values. This is because of the wide range of building types, occupancy, function, and location, as well as the variety of processes and materials that may be involved. Those buildings that meet applicable... 

These process safety management systems help ensure that facilities are designed, constructed, operated, and maintained with appropriate controls in place to prevent serious accidents. However, despite these precautions, buildings close to hazardous process plants have presented serious risks to the people who work in them. This observation is prompted by the fact that some buildings, because of their design and construction, have collapsed when subjected to comparatively moderate accidental explosions, with serious injury or fatality to the occupants. Conversely, experience indicates that personnel located outdoors and away from such buildings, if subjected to the same blast, may have a lower likelihood of serious injury or fatality. 

It should be noted that consequence screening is performed without regard to the likelihood of an event s occurring. As a result, consequence screening does not determine risk. Furthermore, the consequence evaluation performed may not represent a detailed evaluation of consequences to the process plant. Instead, it is an approximation of expected consequences, given an estimate of potential blast overpressure and anticipated response of representative building types. The user should not mistake this evaluation for a detailed consequence assessment. 

While societal risk is generally applied to events that can impact the public, major accidents in chemical processing plants may also have the potential to affect large numbers of people. In particular, a single major event could affect multiple buildings and many individuals inside each building. Thus, the concept of societal risk can be applied to on-site risk evaluations as well as off-site evaluations. 

Chapters 3 and 4 described a series of screening tools that can be used to evaluate the design and siting of buildings in process plants. The results from the screening for a particular building will fall into one of three risk categories ... 

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