Safety analysis in conceptual design of process control: Dissertation

Conceptual design determines the detail design procedures to be followed and has great impact on the result of the design, the product. The costs of the design as well as the total life-cycle cost of the product are substantially fixed during these early phases of design. Safety is one of the design parameters that affect these costs and has to be taken into account as early as possible in the design. Major accidents, such as Seveso, Flixborough, Bhopal and Mexico City, have raised many interesting questions of safety and resulted in much new legislation. However, the lack of methods with which to study the system without undue effort limits design for safety during the early design phases. This thesis focuses on the conceptual design of process control systems, and on the problem of safety requirements determination. Safety analysis methods of process design are applied to tackle the problem and the information gap between process designers and control system designers. The major part of the work was to develop a new two-phase method, SCRAM (Safe Control Requirements Analysis Method), for analyzing the safety aspects of large processes within reasonable time. The first phase is an Initial Hazard Identification that studies potential accidents on the basis of materials handled in the process circumstances. The second phase is a modified Hazard and Operability Study that studies ways in which the process is led to unsafe states and how accidents may subsequently develop. The basic idea is to find the hazardous states of the process, the critical process parameters and the accident chains. These determine the desired operating conditions and parameters to be controlled, so that safety related control actions will be identified and their requirements defined. The use of the method has been studied by applying it to three cases. The aspects that were studied in them are: first, the support that the method gives to the control system design; second, the usefulness of the method as a safety analysis; and third, the support that the method gives for identifying the most critical control systems for more detailed analyses. On the basis of the case studies, the method is evaluated with respect to what is achieved and what is missed. It is concluded that the method identifies safety-critical process parameters and control actions and is usable as a Preliminary Hazard Analysis of the process for control system designers' purposes. Many design problems that are not covered by the method and that need further research and development work were detected during the evaluation. However, this method gives a starting point for better requirement analysis in conceptual design of process control systems.