Tuesday, February 10, 2009

Hazard Analyses

Introduction The meaning of hazard is often confused with risk. Hazard is defined as the inherent potential of a material or activity to harm people, property, or the environment. Hazard does not have a probability component.

There are differences in terminology on the meaning of risk in the published literature that can lead to confusion. Risk has been defined in various ways (CPQRA, 1989, pp. 3, 4). In this edition of the handbook, risk is defined as: “A measure of economic loss or injury in terms of both the incident likelihood and magnitude of loss or injury.” Risk

implies a probability of something occurring.
Definition of Terms Following are some definitions that are useful in understanding the components of hazards and risk (CPQRA, 1989, pp. 3, 4).

acceptable risk
The average rate of risk considered tolerable for a given activity. accident A specific combination of events or circumstances that leads to an undesirable consequence. acute hazard The potential for injury or damage to occur as a result of an instantaneous or short-duration exposure to the effects of an accident.

chronic hazard
The potential for injury or damage to occur as a result of prolonged exposure to an undesirable condition. Cause-Consequence A procedure using diagrams to illustrate the causes and consequences of a particular scenario. They are not widely used because, even for simple systems, displaying all causes and outcomes leads to very complex diagrams. Chemical Exposure Index (CEI) The CEI provides a method of rating the relative potential of acute health hazard to people from possible chemical release incidents. consequence The direct, undesirable result of an accident, usually measured in health and safety effects, loss of property, or business costs, or a measure of the expected effects of an incident outcome case. For example, an ammonia cloud from a 10-lb/s leak under stability class D weather conditions and a 1.4-mi/h wind traveling in a northerly direction may injure 50 people.

consequence analysis
Once hazards have been established, methods exist for analyzing their consequences (size of vapor cloud, blast damage radius, overpressure expected, etc.). This is independent of frequency or probability.

domino effect
An incident which starts in one piece of equipment and affects other nearby items, such as vessels containing hazardous materials, by thermal blast or fragment impact. This can lead to escalation of consequences or frequency of occurrence. This is also known as a knock-on effect.

An occurrence involving equipment performance or human action or an occurrence external to the system that causes system upset. An event is associated with an incident, either as a cause or a contributing cause of the incident, or as a response to an initiating event.

event sequence
A specific, unplanned sequence of events composed of initiating events and intermediate events that may lead to an incident.

event tree
Seeks to identify the ultimate consequence of an event, while fault tree analysis aims to identify the basic causes of a specific event. Event trees can grow quite large very quickly.

failure mode and effect analysis (FMEA)
A hazard identification technique in which all known failure modes of components or features of a system are considered in turn and undesired outcomes are noted. It is usually used in combination with fault tree analysis. It is a complicated procedure, usually carried out by experienced risk analysts.

fault tree
A method for representing the logical combinations of various system states which lead to a particular outcome, known as the top event.

Fire and Explosion Index (F&EI) The F&EI is used to rate the potential of hazard from fires and explosions.

The rate at which observed or predicted events occur.

HAZOP stands for “hazard and operability studies.” This is a set of formal hazard identification and elimination procedures designed to identify hazards to people, process plants, and the environment.

The loss of containment of material or energy; for example, a leak of a flammable and toxic gas.

incident outcome
The physical outcome of an incident; for example, a leak of a flammable and toxic gas could result in a jet fire, a vapor cloud explosion, a vapor cloud fire, a toxic cloud, etc.

The likelihood of the occurrence of events or a measure of the degree of belief, the values of which range from 0 to 1.

probability analysis
Evaluates the likelihood of an event occurring. Using failure rate data for equipment, piping, instruments, and fault tree techniques, the frequency (events/year) can be quantified.

quantitative risk assessment (QRA)
The systematic development of numerical estimates of the expected frequency and/or consequence of potential accidents associated with a facility or operation. Using consequence and probability analyses and other factors such as population density and expected weather conditions, QRA predicts the fatality rate for a given event. This methodology is useful for evaluation of alternates, but its value as an absolute measure of risk should be considered carefully.

risk analysis
The development of a quantitative estimate of risk based on engineering evaluation and mathematical techniques for combining estimates of incident consequences and frequencies.

risk assessment
The process by which results of a risk analysis are used to make decisions, either through a relative ranking of risk reduction strategies or through comparison with risk targets. The terms risk analysis and risk assessment are often used interchangeably in the literature.

worst credible incident
The most severe incident, considering only incident outcomes and their consequences, of all identified incidents and their outcomes.

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