Process and Plant Accidents

High numbers of casualties, severe material damage, and production losses due to fires, explosions, and other accidents in recent years liave motivated all sectors of industry and responsible auUiorities to initiate scientific investigations of these deadly phenomena. Tlie ultimate goal of the investigations is to develop safety measures tliat will prevent or limit tlie effects of such accidents. 

Note Units used diroughout Part II are Uiose used in die references. No effort was made to provide consistent units. 

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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. ... 

Hazard Analysis—The determination of material, system, process, and plant characteristics that can produce undesirable consequences, followed by the assessment of hazardous situations associated with a process or activity. Largely qualitative techniques are used to pinpoint weaknesses in design or operation of the facility that could lead to accidents. The Safety Analysis Report (SAR) hazard analysis examines the complete spectrum of potential accidents that could expose members of the public, on-site woikers, facility workers, and the environment to hazardous materials. 

A characteristic of analyses of process and plant safety is that the interdependencies within the technical system, the influence of its components on one another and human interventions must be accounted for. A further characteristic is the stochastic nature of the processes to be analyzed, which renders it, for example, impossible to predict the moment of occurrence of an accident. These aspects are duly addressed. 

Eault tree analysis (ETA) is a widely used computer-aided tool for plant and process safety analysis (69). One of the primary strengths of the method is the systematic, logical development of the many contributing factors that might result ia an accident. This type of analysis requires that the analyst have a complete understanding of the system and plant operations and the various equipment failure modes. 

Control of Plant and Process Modifications Many accidents have occurred because plant or process modifications had unforeseen and unsafe side effects (Sanders, Management of Change in Chemical Plants Learning from Ca.se Histories, Butterworth-Heinemann, 1993). No such modifications shoiild therefore be made until they have been authorized by a professionally quahfied person who has made a systematic attempt to identify and assess the consequences of the proposal, by hazard and operability study or a similar technique. When the modification is complete, the person who authorized it... 

Systematic consideration of human error is neglected because of the belief that computerization of processes will make the human unnecessary. Experience shows numerous accidents in computer controlled plants. Human involvement in critical areas of maintenance and plant modification, continues even in the most automated processes. 

This section reflects on the limitations of the PSA process and draws extensively from NUREG-1050. These subjects are discussed as plant modeling and evaluation, data, human errors, accident processes, containment, fission product transport, consequence analysis, external events, and a perspective on the meaning of risk. 

Many accidents have occuiTed because changes were made in plants or processes and these changes had unforeseen side effects. In this chapter a number of such incidents are described. How to prevent similar changes in the future is discussed. Some of the incidents are taken from References 1 and 2, where others are described as well. 

Explain why a plant accident is more likely to happen during startup of a new plant or a retro-fit process. Refer to Chapter 20 and careful review the presentation or tlie bathtub curve tliat is represented by the Weibull distribution. 

The most conuiion cause of fire accidents in process plants is equipment failure. Tliis is primarily a result of poor equipment maintenance or poor equipment layout and design. Maintenance perfonned according to a detailed and well structured schedule will significantly reduce tlie occurrence of fire accidents. Tlie second largest cause of fire accidents is ignorance of tlie properties of a specific chemical or chemical process. Proper training of employees will increase tlieir knowledge of tlie properties of a specific chemical and chemical process and can prevent many of tliese chemical fire accidents. 

Control of Modifications to Plants, Processes, and Organization Many accidents have occurred when such modifications had... 

This safety audit is used for identifying inputs and material flows, processes and intermediates, and final products - but with special attention paid to human-material/process/equipment interactions that could result in (a) sudden and accidental releases/spills, (b) mechanical failure-based injuries, and (c) physical injuries - cuts, abrasions, and so on, as well as ergonomic hazards. Additional sources of adverse effects/safety problem areas are records/ knowledge of in-plant accidents/near misses, equipment failures, customer complaints, inadequate secondary prevention/safety procedures and equipment (including components that can be rendered non-operable upon unanticipated events), and inadequacies in suppliers of material and equipment or maintenance services. 

Safety and loss performance in the chemical industry is the result of the interaction of plant design, construction and maintenance with production processes, and trained people applying a well-developed operating discipline. An accident leading to personal injury, property damage, or product loss invariably is the result of the failure of one or more of these elements. Each factor involved in chemical production—equipment, process, product, and people—may be subject to a variety of failure modes which may lead to accidents. 

Construct process and storage areas so they are not close to residential areas or potential residential areas Design for total containment Redesign obsolete plants before accidents occur Store liquefied gases at low temperatures and pressures... 

In the event of a major plant accident, large quantities of nitrogen oxides could be released into the atmosphere and product nitric acid may well be released to the drains. There is little that can be done to provide for such a situation. The emphasis would be on dilution of any liquid wastes expelled. It would be left to the prevailing winds to disperse the nitrogen oxide cloud. Fortunately this gas cloud is not flammable, but it is highly toxic. Nitrogen oxide fumes either from nitric acid or from process gases may have a deceptive delayed action. The victim may feel no pain or discomfort at the time of the low-level inhalation, or for up to 48 hours thereafter, but then serious respiratory and cardiac problems can occur. 

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