2.3. Low personnel and economic losses
In a plant, some units which are of high-personnel density, high-economy density or of high process importance, such as the office building, central control room, laboratory, fire station, telecommunications station etc., are the key protection facilities and are expected to be least or last units to be damaged in the plant. Reasonable layout shall select the areas with lowest risk as the location of these units by reference to the risk field of the whole plant zone, so that to lower the personnel and economic losses under accident conditions.
2.4. Escape convenience under emergency condition
Once fatal accident occurs, in order to prevent the expansion and worsen of accident, personnel in the plant must be promptly evacuated and transferred to safety belt, and reasonable layout can guarantee a fast and safe evacuation. To make it possible, a rational evacuation route should been set as short as possible for each high-personnel density unit, and one essential rule is that the evacuation route should not pass through the high risk regions.
3. safety performance metrics and mathematical representation
Based on above analysis, four metrics were proposed to evaluate the safety performance of the layout. The mathematical representations of the four metrics were also preliminary studied.
3.1. Accident triggered likelihood degree
Accident triggered likelihood degree is used to evaluate the safety rationality of the relative position relationship of ignition source unit and release source unit. From analysis, it can be known that the probability of fire and explosion will be
lower if the following principles can be fulfilled.
1 The ignition source unit should be located at the upwind direction of all release source units (the less the amount of release source that located at the upwind direction, the better the safety performance);
2 Keep ignition source unit away from release resource unit (the larger the distance between ignition source unit and release source unit, the better the safety performance);
3 As gas diffusion under wind direction is easier than under crosswind direction, keep upwind release source far away from the due upwind direction of ignition source (the smaller in Figure 1, the better the safety performance).