Tolerable risk

The core function of QRA is to provide information for decision making. QRA results in and of themselves cannot prove anything. However, decision makers can compare QRA risk estimates to their own risk tolerance criteria to decide whether a plant or operation is safe enough. The same QRA results can support both the plant manager s contention that the plant is safe, as well as the community activist s claim that the plant is unsafe. The difference lies in the individual s risk tolerance, not the QRA. 

If both frequency and consequence values are calculated and reported on an absolute basis, then they may be reported graphically in combination with one another (Chapter 3), or simply as the product of frequency and consequence. Table 5 contains some examples of typical risk estimates (frequency and consequence products). Based on absolute risk estimates, you can decide whether the risk of a specific activity exceeds your threshold of risk tolerance (risk goal). If so, analysts can estimate the reduction in risk, given that certain improvements are made, assumptions changed, or operating circumstances eliminated. 

Since derivations and literature references are typically not given in detail (often not at all) it is difficult for users to recognize errors or assess the validity of some statements when applied to a particular situation. It would be helpful for the user to appreciate limitations of applicability and to consider risk tolerance when adopting a specific practice based on minimum safe practices given in codes. 

Since velocity varies with the inverse square of pipe diameter d, an important consideration is the selection of pipe diameter. For any given velocity-diameter product, larger pipe diameters allow larger flow rates. Since occasional static ignitions in road tankers may occur at nr/ = 0.38 mVs, smaller values might be considered for nonconductive liquid transfer depending on risk tolerance. 

It is moderately toxic, flammable, and a moderate fire risk. Tolerance, 25ppm in air. 

Risk screening (Section 4.3) Information used in consequence modeling I Generic frequency data on events of concern from similar plants I Risk tolerance criteria or methodologies ... 

Recommending risk tolerance criteria is beyond the scope of this book. Further, risk tolerance is company-specific and each company should consider establishing criteria that reflect company goals and objectives. Establishing risk tolerance criteria unavoidably involves making subjective decisions, and little guidance is available on this topic. This section discusses risk tolerance criteria that have been applied elsewhere to provide a perspective that might assist individual companies in this task. 

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. 

Development of Company-Specific Risk Tolerance Criteria 129... 

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 ... 

Figure 4.1 Presentation of risk tolerance levels and ALARP region (Ref. 46).

Internal company risk tolerance criteria had been established prior to performing the risk screening. When Figures 6.3 and 6.4 were compared to these internal criteria, it was concluded that Building 3 did not present sufficient risk to occupants to be of concern, while additional evaluation was required for Building 2. 

Development of company-specific risk tolerance criteria... 

For companies to make risk-based decisions from risk screening or quantitative risk assessments, company-specific risk tolerance criteria or methodologies should be developed for both individual risk and aggregate risk. Chapter 4 provides a discussion of risk criteria and methodologies, and offers guidance for companies to consider in developing their own approaches to risk tolerability. 

Risk tolerability is a complex subject. Risk tolerance in society generally has changed over time and will continue to do so. Tolerance criteria may vary from company to company. Further, the reasons for the variations can differ from company to company. Many companies, in fact, have not developed explicit risk tolerance criteria. 

The methodologies offered in this book allow some degree of building evaluation without the development of explicit risk tolerance criteria. However, additional guidance in the development and selection of risk tolerance criteria will benefit those companies that wish to fully utilize risk screening and quantitative risk assessment as evaluation tools. 

LOPA is a semi-quantitative tool for analyzing and assessing risk. This method includes simplified methods to characterize the consequences and estimate the frequencies. Various layers of protection are added to a process, for example, to lower the frequency of the undesired consequences. The protection layers may include inherently safer concepts the basic process control system safety instrumented functions passive devices, such as dikes or blast walls active devices, such as relief valves and human intervention. This concept of layers of protection is illustrated in Figure 11-16. The combined effects of the protection layers and the consequences are then compared against some risk tolerance criteria. 

Environmental impact is part of a facility s risk tolerance criteria. First, there is the direct cost of clean-up, monitoring costs, and environmental costs. There could be potential fines from local, state, or federal authorities. Finally, there may be potential civil actions that could result in millions of dollars of damage. 

There have been attempts by companies to assign quantitative risk numbers to these regions. It is important to note that different values should be considered for workers vs. for the public near the facility. Generally, the risk to the public is one order of magnitude lower than for workers. The workers accept the risk, whereas the public wants a lower risk. Each company should establish their own risk tolerance values. This may be used to assess financial and business risk. 

Increasingly, newer fired process heater installations are adding more fuel-air combustion controls and safety instrumentation systems. However, the decision on the extent of fired heater combustion controls, instrumentation, and safety systems to employ is fundamentally a loss prevention and risk tolerance issue, rather than a fire protection one. The following recommended practices, codes and standards apply to fired heater and dryer controls and instrumentation ... 

Safety Critical Actions—Specific steps humans take that provide layers of protection to lower the risk category of a specific scenario or scenarios from unacceptable to acceptable as defined by organizational risk tolerance criteria. Sometimes called administrative control. Such steps... 

What is your risk tolerance Investors can be labeled as either conservative, moderate, or aggressive depending on the amount of risk they can comfortably handle. The rule Don t take on more risk than you can sleep with. Lying awake nights worrying about your savings is no way to either anticipate or enjoy your retirement. 

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