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PT Notes

Calibration of Risk Matrices

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

Risk matrices are used widely in process safety to rate and rank risks posed by processes to help with decision making on risk reduction. For example, commonly they are used in process hazard analysis (PHA) to rate the risks of hazard scenarios and determine the need for risk reduction measures.

Use of risk matrices finds favor because they appear to be simple to understand, do not require specialized expertise, and are graphically appealing. However, there are no industry or government standards for risk matrices for process safety. Unfortunately, risk matrices developed by individual companies often are flawed in various ways, possibly because their development appears to be deceptively simple but is actually more complicated than it seems. Poorly designed risk matrices make the process of risk ranking difficult and produce risk estimates ill-suited for decision making. In particular, there are pitfalls in the allocation and calibration process for risk matrices that often are unrecognized.

Frequently, risk matrices are defined using numerical values for event severity and likelihood levels. Therefore, the resulting risk levels denote numerical values of risk. Consequently, such risk matrices must be calibrated with reference to appropriate numerical risk tolerance criteria to define appropriate risk reduction requirements. Use of improperly calibrated risk matrices produces incorrect decisions on risk reduction for processes. 

Risk matrices are used with events for which a severity and likelihood can be assigned. In PHA, the event is a hazard scenario and the risk tolerance criteria built into risk matrices must be for single hazard scenarios. Such criteria can only be determined by allocating overall facility risk tolerance criteria to individual scenarios. It is erroneous to use overall facility risk tolerance criteria for individual hazard scenarios which would result in tolerating far more risk than intended. Also, many overall facility criteria are intended to include risk from all sources of all hazards in a facility, not just process safety hazards. Thus, such criteria must be offset for contributions made by non-process-safety hazards when used in process safety studies.

Allocation involves estimating the number of hazard scenarios possible and dividing an appropriate overall facility risk tolerance criterion by that number. The estimation of the number of hazard scenarios depends on the level of detail used to define scenarios; the nature, scale, and complexity of the process; and the range of hazards addressed. These factors can be highly variable from one situation to another. Also, many process facilities contain multiple processes and the overall facility risk tolerance criteria must be allocated using the total number of scenarios for the facility, not any one individual process.

The tolerable risk for each facility and process may vary and the number of hazard scenarios will vary for each facility and process. Consequently, risk matrices must be calibrated for each facility and process for which they will be used. Thus, each facility and process needs its own customized risk matrices.

Both risk to individuals and to groups of people are important. Individual risk criteria protect any single individual from bearing too large a share of the risk. Group risk criteria protect populations of people from bearing undue risk. Separate risk matrices are needed for each type.

The goal for companies should be to develop risk matrices that produce consistent risk ratings across all their processes and facilities to encourage consistent decisions on risk reduction.

The calibration of risk matrices is discussed in greater detail in the article:

Calibration of risk matrices for process safety, Journal of Loss Prevention in the Process Industries, Vol. 38, pages 163-168, 2015.

The article can be accessed for free on ScienceDirect until November 17, 2015 by clicking on the link below. No sign up or registration is needed - just click and read.

http://authors.elsevier.com/a/1RnFW3O6UQDfq6

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Click on the links below for information on related training courses:

Process Hazard Analysis (PHA) for Team Leaders

Risk Tolerance Criteria for Process Safety

Safety Integrity Level (SIL) Determination Using LOPA and Other Methods to Comply with IEC 61511 and ISA 84

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