Grid-based individual risk calculation in the classification of hazardous area with a risk-based approach

• A risk-based approach was adopted for hazardous area classification of secondary grade sources of release.
• The hole sizes from secondary grade release sources were calculated for a typical gas processing unit by using EI15 approach.
• Appropriate hole size for secondary grade releases was determined in different areas of the intended unit by proposed grid-based approach.
• Utilization of multiple Nrange value for increasing accuracy of EI15 approach was proposed.
• The resultant zone 2 of all approaches was compared and error rates were calculated.

The energy institute model code of safe practice, part15, 2005 (EI15) is one of the most widely-used practices in studies of hazardous area classification, which adopts a risk-based approach in order to determine the hole size of secondary grade sources of release (such as valves, flanges, and pumps). Employing an average parameter for the whole unit, the approach determines the number of effective release sources, affecting an individual in hazardous area, within its risk calculation. In case of high dissipation among the number of secondary grade sources of release in different locations of a plant, such an average value ends up overestimating the risk for hole sizes where there are few release sources or in case of places in which there are many release sources, it results in an underestimation of the risk. Therefore, this paper aims to propose a grid-based approach to calculate the exact value of the risk from secondary grade sources of release, in order to determine the accuracy of such a practice. In its risk calculations, the proposed approach uses the accurate number of effective secondary grade sources of release in each location of the plant, instead of an average number. For so doing, the hole sizes from secondary grade sources of release are calculated for a gas dehydration and dew point control of natural gas unit by means of EI15 approach for risk calculation. Afterwards it calculates the risk of the releases with different hole sizes by means of the grid-based approach on the intended unit, specifying the appropriate hole size for secondary grade sources of release in different areas of the intended unit. It then compares the results of zone 2 from both approaches, in turn measuring the error rate of EI15 approach as it determines the hole size of secondary grade sources of release. Eventually, it is proposed that utilizing multiple average values for the number of effective release sources in different parts of the unit will increase the precision of EI15.