Toxic Exposure Subindex

Flammability, Explosiveness and Toxic Exposure Subindices (Ifl, Iex and Itox) are determined for each substance present in the process. These indices are summed for every substance separately. The maximum sum is used as the subindex value. Corrosiveness Subindex is determined on the basis of the most corrosive material in process. 

The subindex of flammability describes the flammability of liquid e.g. in the case of a leakage. Flammability of liquids is measured by their flash points and boiling points. The classification used is based on the EU directive (Pyotsia, 1994). Substances are divided into non-combustible, combustible, flammable, easily flammable and very flammable (Table 11). 

Very flammable (flash point 0°C boiling point 35°C) 4 

The explosiveness is determined by the difference between the upper and the lower explosion limits of the substances. The range of explosion limits has been divided into four steps. The subindex values are shown in Table 12. 

Health hazards caused by chemicals are represented by the Toxic Exposure Subindex (ITox). hi the ISI the evaluation of toxic exposure is based on the Threshold Limit Values (TLV) because TLV data is readily available for most substances in process industry. TLV values express the harmful exposure limits of substances in the threshold time of 8 hours. The index value is higher, when the TLV is lower i.e. the substance is more toxic. It is important to use TLVs with same threshold time so that the results are comparable. Score limits in Table 13 are based on Mond Index (ICI, 1985). 

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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. 

Table 13. Determination of the Toxic Exposure Subindex ITox-...

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