Safe Process Structure

The chosen meaningful parameters are the following reaction heat, flammability, explosiveness, toxicity, corrosiveness, chemical interaction, inventory, process temperature and pressure, equipment safety and safe process structure (see Table 5). This does not mean that other factors affecting the inherent safety of a process are meaningless. On the contrary they should be considered more detailed in further design stages. 

From Table 6 it can be seen how the selected parameters have a connection to the basic principles of inherent safety. For instance the subindices of equipment safety and safe process structure contain several characteristics of inherent safety such as limitation of effects or tolerance to maloperation. It is practical to include several characteristics into few parameters, since the inherent safety principles are both very broad and overlapping. The philosophy behind them cannot be described just by one process parameter. The selected parameters are discussed in more detail on the following pages. 

Limitation of Effects - safer technical alternatives - safer reaction conditions temperature pressure equipment safety safe process structure pressure temperature chemical interaction... 

Simplification - simplify process facilities safe process structure... 

Making Incorrect Assembly Impossible -choice of equipment, piping and fittings correctly equipment safety safe process structure... 

Ease of Control safe process structure heat of reaction... 

Equipment safety tries to measure the possibility that a piece of equipment is unsafe (Heikkila and Hurme, 1998a). Here equipment includes all major pieces of equipment such as pumps and vessels etc. but not piping, valves or instruments as separate entities. Equipment safety considers the safety of the equipment as such without interactions through the process with other equipment. This latter aspect is described by the safe process structure (Ch. 7.9). However interactions through layout, such as a furnace can be a source of ignition for a leak from other piece of equipment, are considered by the equipment safety. 

Safe process structure describes the inherent safety of the process configurations. The safe process structure means which operations are involved in the process and how they are connected together. Therefore the safe process structure... 

Most factors affecting inherent safety are quite straightforward to estimate since they are e.g. based on the physical and chemical properties of the compounds present. An inherently safe process structure is not possible to define by explicit rules, but one has to rely on standards, recommendations and accident reports. This information is based on the experience gained in the operation practice of different processes (Lees, 1996). For example accident reports, which are made after accidents, give valuable information of the possible weaknesses in the different process solutions. Also extensive databases have been collected from accident reports (Anon, 1996). From this data a database of good and bad designs can be collected. 

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. 

Inherent Safety Index of Safe Process Structure... 

Table 25. Values of the Safe Process Structure Subindex 1st-... 

The Safe Process Structure Subindex for the acetic acid process is discussed in more detail in Chapter 13.2. 

All stored cases are validated on the basis of the Safe Process Structure Subindex. The validations are given for every case and included in the output. Further information on the cases is given as appendices, which describe the case in more detailed. 

For the evaluation of the safe process structure of the acetic acid process (Ch. 12, Fig. 7) CBR database searches were done on two levels (Heikkila et al., 1998). First level was the acetic acid process as a whole. On the second level the reactor system was studied in more detail. 

The final score of the Safe Process Structure Subindex for the carbonylation process would be 2 based on this limited reasoning, since the final score of 1st is chosen on the basis of the worst case. Of course in practice one should do the reasoning on all the systems and subsystems in the process. This case study was given only to represent the principle of CBR. in reasoning the value of the Safe Process Structure Subindex. 

Case-based reasoning on the safe process structure subindex... 

The chemical and most process factors affecting the index are quite straightforward to estimate. More problematic are the equipment safety and the safety of process structure. The equipment safety subindex was developed based on evaluation of accident statistics and layout information. The evaluation of the safe process structure subindex is based on case-based reasoning, which requires experience based information on accident cases and on the operation characteristics of different process configurations. 

Safe process structure 1st Total inherent safety index ixi... 

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