Undesirable event

Fault Tree Analysis. Fault trees represent a deductive approach to determining the causes contributing to a designated failure. The approach begins with the definition of a top or undesired event, and branches backward through intermediate events until the top event is defined in terms of basic events. A basic event is an event for which further development would not be useful for the purpose at hand. For example, for a quantitative fault tree, if a frequency or probabiUty for a failure can be deterrnined without further development of the failure logic, then there is no point to further development, and the event is regarded as basic. 

ETA breaks down an accident iato its contributing equipment failures and human errors (70). The method therefore is a reverse-thinking technique, ie, the analyst begias with an accident or undesirable event that is to be avoided and identifies the immediate cause of that event. Each of the immediate causes is examined ia turn until the analyst has identified the basic causes of each event. The fault tree is a diagram that displays the logical iaterrelationships between these basic causes and the accident. 

The TOP event s the specific accident or undesired event that is the subject of the ETA. 

Safety Devices Pressure relief devices, flame arresters, and methods for handhng effluent from controlled releases provide control of accidental undesirable events. Special equipment should be considered for highly toxic chemical service. The following matters are considered ... 

Hazard Analysis The identification of undesired events that lead to the materialization of a hazard, the analysis of the mechanisms by which these undesired events could occur and usually the estimation of the consequences. 

Incident investigation The management process by which underlying causes of undesirable events are uncovered and necessary steps are taken to prevent similar occurrences. 

The reasons (i.e., the motivations, concerns, activators, and needs) for considering the use of QRA define the requirements for information. The next question is, can QRA supply the appropriate information to satisfy the need By definition, QRA studies generate numerical estimates of the expected frequency and/or consequence(s) of undesired events. The results of the QRA can be formulated and used on two bases (1) an absolute basis and (2) a relative basis. 

Risk The likelihood of a specified undesired event occurring within a specified period or in specified circumstances. 

The technique is particularly useful in evaluating the effect of alternative actions on reducing the probability of occurrence of the undesired event. 

A simple example of fault tree analysis applied to an internal combustion engine (Figure 3.4.4-2) is the Figure 3.4.4-3 fault tree diagram of how the undesired event "Low Cylinder Compression" may occur. The Boolean equation of this fault tree is in the caption of Figure 3.4.4-3. Let the occurrence of these events be represented by a 7, non-occurrence by 0, and consider that there may he a long history of occurrences with this engine. Several sets of occunrence.s (trials) are... 

The application of this procedure is best seen by performing an FMEA on a simple two-phase separator. Table 14-3 lists those process upsets that can be sensed before an undesirable event leading to a source of condition occurs. For overpressure, primary protection is provided by a high pressure sensor that shuts in the inlet (PSH). If this device fails, secondary protection is provided by a relief valve (PSV). 

Undesirable Event Cause D l ctable Condilion At Component... 

Human error Physical and cognitive actions by designers, operators, or managers that may contribute to or result in undesired events. 

Risk is defined as tlie product of two factors (1) tlie probability of an undesirable event and (2) tlie measured consequences of the undesirable event. Measured consequences may be stated in terms of financial loss, injuries, deatlis, or Ollier variables. Failure represents an inability to perform some required function. Reliability is the probability that a system or one of its components will perform its intended function mider certain conditions for a specified period. Tlie reliability of a system and its probability of failure are complementary in tlie sense tliat the sum of these two probabilities is unity. This cluipler considers basic concepts and llieorenis of probability tliat find application in tlie estimation of risk and reliability. 

A fault tree is a grapliic teclmique used to analyze complex systems. The objective is to spotlight conditions tliat cause a system to fail. Fault tree analysis attempts to describe how and why an accident or otlier undesirable event lias occurred. It may also be used to describe how and why an accident or otlier undesirable event could take place. Thus fault tree analysis finds wide application in hazard analysis and risk assessment of process and plant systems. ... 

Risk analysis. The risk of accidents can be assessed in terms of two factors severity and probability of the accident. Severity is high if consequences of the accident to employees, the public, the environment, and the plant are significant. Severity is related to the amount and properties of hazardous (toxic, flammable, explosive) substances that can escape to the surroundings during the accident, and to the energy that is released during the accident. Probability is associated with the likelihood of the occurrence of unwanted chains of events and the time of development of undesired events starting from the disturbance. If the time... 

Changing process or mechanical conditions to reduce the potential for runaway reactions, accelerated corrosion or erosion, or other possible causes of undesirable events... 

The what-if analysis is a creative, brainstorming examination of a process or operation conducted by a group of experienced individuals able to ask questions or voice concerns about undesired events. It is not as inherently structured as some other methods, such as the hazard and operability (HAZOP) study or a failure mode and effects analysis (FMEA). 

FTA is a systematic, deductive failure analysis that focuses on a particular accident or undesired event called the "top event" and develops the underlying sequence of events leading to the top event. A separate FTA must be performed for each top event. 

The most undesirable event is a power failure. In that case, the valve between vacuum pump and the condenser has to close automatically. If the power failure lasts only a few minutes, the condenser has a certain heat capacity to maintain the sublimation of ice. The tolerable time of power failure depends from the plant design. 

An other undesirable event, is an air pressure by a leak in the plant which reduces or stops the sublimation. The counteractions could be ... 

In Section 2.2.8, possible failures during the freeze drying process are classified in four categories, and the preventions and necessary actions briefly discussed. In this chapter, some unexpected or undesirable events are studied which, by experience may happen. The problems listed here are selected from the course of the freeze drying process. A breakdown of single components, for example pumps, compressors or valves are not included in this chapter. The list will be incomplete, but an attempt has been made to mention some of the more frequent events. The problems with leaks and their hunting is discussed in Section 2.2.8. 

The best known measure for safety is risk, which is defined as the possibility of loss (Taylor, 1994). The problem of awareness of risk can be seen as one of failure of communication and of mismanagement (Kharbanda and Stallworthy, 1988). Risk by the Chartered Insurance Institute (1974) is the mathematical probability of a specified undesired event occuring, in specified circumtances or within a specified period. In a process plant the losses may be such as a damage to equipment, a loss of production or an environmental damage as well as an injury or a death. Risk involves two measurable parameters (Taylor, 1994) consequence and probability. Some events are more probable to occur than others, but a unique consequence of the sequence of events cannot be predicted. 

Layer-of-protection analysis (LOPA) A method, based on event tree analysis, of evaluating the effectiveness of independent protection layers in reducing the likelihood or severity of an undesired event. 

Drug-related problems can be defined as Any undesirable event experienced by the patient that involves or is suspected to involve drug therapy and that actually or potentially interferes with a desired patient outcome (Strand et al. 1990). This is a vital component of Pharmaceutical Care and Clinical Pharmacy and will be described more in detail in another chapter. It should however be noted that there are several definitions and classification systems for DRPs. A literature review (van Mil et al. 2004) identified fourteen classifications and their critical elements. In the presented definition a potential problem is a DRP but this is not the case in all definitions and classifications. This is also the case for unavoidable adverse drug reactions (e.g. with cytotoxic agents). 

Fault tree analysis (FTA) and event tree analysis (ETA) are the methods most commonly applied quantitatively. Since they only address the likelihood of undesired events, these methods are often combined with consequence severity calculations in a quantitative risk analysis, as described by CCPS (1999b). Layer of protection analysis (LOPA) uses a semiquantitative, order-of-magnitude approach. It is documented with worked examples in CCPS (2001b). 

Fault Tree Analysis (FTA) Scenario- based Deductive By undesired event Can analyze complex processes with multiple safeguards and operator interactions Only looks at events that precede the selected top event Highest... 

Many materials in common use today have obvious reactivity hazards, for example, explosives, laboratory chemicals, and raw materials to make plastics and other useful products. Yet they are handled safely every day. How Their hazards have been recognized and controlled so that undesirable events (those which can cause loss and harm) do not happen. Your first source of information for controlling hazards should always be your material supplier. 

A process or a facility satisfies the single fault tolerance principle when it has been designed or equipped in such a way that a single fault does not lead to the occurrence of the undesired event. This applies also to safety-related systems. 

Layers of protection—a concept whereby several different devices, systems, or actions are provided to reduce the likelihood and severity of an undesirable event. 

Prevention—The process of eliminating or reducing the probability of the hazards or risks associated with a particular activity. Sometimes used to denote actions taken in advance to reduce the likelihood of an undesired event. 

Another type of logic tree, the event tree, is an inductive technique. Event Tree Analysis (ETA) also provides a structured method to aid in understanding and determining the causes of an incident.(i) While the fault tree starts at the undesired event and works backward to identify root causes, the event tree looks forward to display the progression of various combinations of equipment failures and human errors that result in the incident graphically. 

The undesired event appears as the top event and the trees are drawn from top to bottom. Two basic logic gates connect event blocks the AND-gate and the OR-gate. The facts dictate the structure of the incident diagram and limit the influence of presupposed conclusions invariably drawn by team members before all of the facts are identified and logically matched. Logic rules are used to test the tree structure. 

---