Process 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. ... 

The first step in the procedure is to conceptually divide the process into separate process units. A process unit is a single pump, a reactor, or a storage tank. A large process results in hundreds of individual units. It is not practical to apply the fire and explosion index to all these units. The usual approach is to select only the units that experience shows to have the highest likelihood of a hazard. A process safety checklist or hazards survey is frequently used to select the most hazardous units for further analysis. 

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). 

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 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. 

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. 

Inherent Safety Index of Safe Process Structure... 

Application of Inherent Safety Index for Computerized Process Synthesis... 

Heikkila et al. (1996) have expanded the work of Hurme and Jarvelainen (1995) with environmental and safety aspects (Fig. 11). The alternatives are simulated to determine the material and heat balances and to estimate the physical properties. Then the alternatives are assessed in economic terms for which the internal rate of return is used. The environmental effects are estimated by equivalent amount of pollutant that takes into consideration the harmfullness of the different effluent substances. With environmental risks are also considered aspects of occupational health to choose inherently healthier process. Even though most health related rules are considered later in the work instructions, health effects should also be a part of the decision procedure. The inherent safety is estimated in terms of the inherent safety index as described later. 

The Inherent Safety Index, ISI, was developed to consider a wider range of factors affecting the inherent safety of the process (Ch. 8). The ISI allows the evaluation of inherent safety of process alternatives to be done in a computerized process synthesis environment. The represented synthesis approach allows the inherent safety comparison of process alternatives to be done flexibly and interactively in the conceptual design phase. 

In the approach by Hurme and Heikkila (1998) inherent safety is the only objective function considered in the process synthesis. It is estimated by the Inherent Safety Index (Heikkila et al., 1996) which has been developed for... 

A synthesis problem on purification of butenes is discussed (Hurme and Heikkila, 1998). The aim is to synthesize the separation process by maximizing the inherent safety. The inherent safety is measured as the Inherent Safety Index. 

The fitness function was based on the inherent safety index, which was simplified It was noticed that there are only minor differences in the safety properties of the compounds in the process. Therefore most subindices are the same for all configurations. The equipment type used in all the configurations is the same (i.e. distillation). Therefore the subindex of equipment safety is constant too. Also the safety of process structures is quite the same since the distillation systems used are rather similar in configuration. Therefore the subindex for process structure was not evaluated and case-based reasoning was not needed. 

The aim of the reasoning is to determine the inherently safe process configuration for this process by using the Inherent Safety Index as an objective function in the genetic optimization. The index can be once again simplified, since the compounds present are fixed by the product produced. Therefore also the physical and chemical properties are fixed and the related subindices are... 

Calculation of fitness by using the inherent safety index. Only inventory, equipment safety, heat of reaction and process configuration subindices were considered. 

---