Inherent safety assessment technique for preliminary design stage

Abstract

Safety is defined as accidents prevention through appropriate technologies to identify and eliminate hazards before an accident occurs. Safety program that prevents hazards in the first place is preferable compared to the typical approaches of controlling the hazards upon being detected. Plants should be designed so that they are user-friendlier to the workers. This can be done by preventing the presence of the hazards in the process during its design stages. The concept of preventing hazards existence early during the design stage is called inherent safety. This paper introduces an inherent safety assessment technique for preliminary design stage (ISAPEDS) of chemical process. ISAPEDS focuses on assessing inherent safety using information available from the process flow sheet diagram (PFD). ISAPEDS is a technique that is an extension of the Numerical Descriptive Inherent Safety Technique (NuDIST) and the Graphical and Numerical Descriptive (GRAND) techniques developed by the same authors for application during research and development (R&D) design stage. There are five inherent safety parameters considered in this assessment which are operating temperature, operating pressure, flammability, explosiveness and toxicity. ISAPEDS introduces four new features as an improvement to the previous NuDIST and GRAND methods. First, instead of assessing these parameters as a standalone hazard factor, the parameters are assessed in relations to each other. For example the flammability parameter is evaluated in relations to the operating pressure while explosiveness parameter is evaluated in relations to the operating temperature. The second feature is assessing the chemicals as a mixture instead of as individual substance, which is a very rare scenarios in a typical chemical processes. Thirdly, the evaluation will be done comprehensively on each of the process equipment in the process flow diagram (PFD). The last one is the score of the safety hazards assessment in the whole process will be weighted based on the chemicals composition in the mixture. In this technique, a higher score is indicated as more hazardous compared to a lower score. A simple case study is performed on the hydrodealkylation process of toluene (HDA) focusing on three equipment which are the reactor (R101), the toluene feed drum (V101) and the low pressure phase separator (V103). The overall assessment shows that R101 is the most hazardous equipment in the process with the highest ISAPEDS score of 298.6 while V101 is the least hazardous equipment in the process with the lowest ISAPEDS score of 117.1. Among the three equipment, R101 was found to be the most hazardous in all five parameters resulting it to be the most hazardous equipment in the process section.