Safety Index

If the material in two process alternatives is both flammable and highly toxic, then they can be compared on both bases separately. If the assessments of the relative flammability and toxicity are in conflict, then we can only resort to a safety index. 

The approach to developing metrics for process safety is analogous to those that might be used to assess Occupational Exposure risk. One can cite as well several indices that have been developed as metrics for estimating and ranking the safety of a given process or chemical reaction, such as the DOW fire and explosion index,the Stoessel index ° for hazard assessment and classification of chemical reactions, the Inherent Safety Index, the Prototype Index for Inherent Safety, amongst others. ... 

In this thesis an Inherent Safety Index for conceptual chemical process design is presented. This is required, since inherent safety should be considered in the early phases of design when the major decisions on the chemical process are made. The presented methodology allows such a consideration since the index is based on the knowledge available in the preliminary process design stage. 

The total index is divided into Chemical and Process Inherent Safety Index. The previous is formed of subindices for reaction heats, flammability, explosiveness, toxicity, corrosiveness and chemical interaction. The latter is formed of subindices for inventory, process temperature, pressure and the safety of equipment and process structure. 

A new approach for computerized Inherent Safety Index is also presented. The index is used for the synthesis of inherently safer processes by using the index as a fitness function in the optimization of the process structure by an algorithm that is based on the combination of an genetic algorithm and case-based reasoning. Two case studies on the synthesis of inherently safer processes are given in the end. 

Edwards and Lawrence (1993) have developed a Prototype Index of Inherent Safety (PIIS) for process design. The inherent safety index is intended for analysing the choice of process route i.e. the raw materials used and the sequence of the reaction steps. This method is very reaction oriented and does not consider properly the other parts of the process even they usually represent the majority of equipment. 

In the Chapter 7 the selected inherent safety parameters for conceptual process design were presented. From these parameters an inherent safety index is formed in this Chapter. There is a straight link between inherent safety principles and the inherent safety index as discussed earlier (see Figure 5). 

Table 7. Inherent safety index and its subindices (Hurme and Heikkila, 1998).

The ISI is calculated by Equation 3, where the Total Inherent Safety Index (In) is the sum of the Chemical Inherent Safety Index (Ici) and the Process Inherent Safety Index (IPI). These indices are calculated for each process alternative separately and the results are compared with each other. Table 8 describes the symbols of the subindices. 

The Chemical Inherent Safety Index Ici (Eq.4) contains chemical factors affecting the inherent safety of processes. These factors consist of chemical reactivity, flammability, explosiveness, toxicity and corrosiveness of chemical... 

The Process Inherent Safety Index IPI (Eq.5) expresses the inherent safety of the process itself. It contains subindices of inventory, process temperature and pressure, equipment safety and safe process structure. 

The calculations of the Inherent Safety Index (ISI) are made on the basis of the worst situation. The approach of the worst case describes the most risky situation that can appear. A low index value represents an inherently safer process. In the calculations the greatest sum of flammability, explosiveness and toxic exposure subindices is used. For inventory and process temperature and pressure the maximum expected values are used. The worst possible interaction between chemical substances or pieces of equipment and the worst process structure give the values of these subindices. 

The way of using the index is flexible. Comparisons can be made at the level of process, subprocess, subsystem, or considering only part of the factors (e.g. only process oriented factors). Different process alternatives can be compared with each other on the basis of the ISI. Also the designs of process sections can be compared in terms of their indices in order to find the most vulnerable point in the design. Sometimes a comparison based on only one or two criteria is interesting. E.g. a toxicity hazard study can be done by considering only the toxic exposure subindex. Because its flexibility the total inherent safety index is quite easily integrated to simulation and optimization tools. 

The Weighting between Subindices of Inherent Safety Index... 

Chemical inherent safety index, ICi Symbol Score... 

The Chemical Inherent Safety Index deals with the hazards which are related to the chemical properties of substances in the process. The Index has been divided into subindices for reaction hazards and hazardous substances. 

The Inherent Safety Index (ISI) deals with both main reaction(s) and those side reactions taking place in the reactor and which are meaningful. It also deals with chemical interaction which describes unintentional chemical reactions which can take place among chemicals in the plant. 

Chemical interaction considers the unwanted reactions of process substances with materials in the plant area. These reactions are not expected to take place in the reactor and therefore they are not discussed in the side reaction subindex. The Inherent Safety Index has utilized EPA s matrix (Hatayama et al., 1980) to classify the hazards of the chemical interaction in a process. The worst interaction that appears between the substances present in the plant area is used in the calculations for the Chemical Inherent Safety Index. 

In the Inherent Safety Index corrosiveness is determined on the basis of the required construction material (Table 14). If carbon steel is enough, the index value is 0. For stainless steel the value is 1, but for all special materials the index is 2. The estimation is made for each process stream separately, and the worst case gives the final index value. 

Process temperature for the Inherent Safety Index (ISI) is determined on the basis of the maximum temperature in the process area under investigation. This is feasible since in the early stage of process design preliminary estimates of temperatures and pressures are available. 

A frequently used safety index in process industry is for instance the Dow Fire Explosion Hazard Index (1987). The Dow F E Index gives penalties for fired equipment and certain specified rotating equipment. These are a part of the Special Process Hazards, within which the penalties of a process unit are summed. 

For offsite equipment the scores 0-3 have been used instead of scores 0-4 for ISBL equipment (HeikkilS and Hurme, 1998b), since the offsites represent only one third of all losses (Instone, 1989) and are therefore not as essential as ISBL. Also much of the risks of OSBL are due to the large inventory of flammable or toxic chemicals, which are discussed by the inventory, flammability and toxicity indices, not by the Equipment Safety Index. Also the equipment of same size is probably safer in OSBL than in ISBL because of the larger spacings in layout. 

Table 24. The scores of Equipment Safety Index IsQfor OSBL. 

Inherent Safety Index of Safe Process Structure... 

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