چكيده لاتين :
Inherent safety (IS) refers to set of measures which increases the level of safety in industries without adding any safety equipment. In general; the strategies for improving safety in the industries can be subdivided into conventional and inherent categories. In conventional safety; the process safety level will be enhanced by adding a variety of safety equipment to processes or/and plants (which named Add-on) also engineering techniques and strategies. In conventional safety methods and strategies; maintaining a high level of safety requires constant maintenance. On the other hand; in case of perturbation or failure in any of the safety layers, the safety level will back to the basic level that was before doing any safety measures.
Inherent safety strives to eliminate risk rather than controlling or accepted them. In recent decades; many studies have been done on inherent safety. Many researchers have present different principles as inherent safety principles. For instance; Faisal Khan et al., in the integrated inherent safety index (I2SI) have proposed five principles of minimization, substitution, attenuation, limitation of Effects and simplification as inherent safety principles. All of the principles named inherent safety are essentially aimed to eliminating risk, hazardous substances or hazardous and/or complex processes in industries. In this study, the I2SI used to assess the inherent safety status of a process plant. This index can evaluate the inherent safety status at all stages of the system life cycle, especially the operational phase. It is also capable of evaluating the inherent safety for each equipment and process as well as the whole system and quantifying the results. In this aspect, this index can be used to prioritize control measures. In addition to the above, this index can compare the costs of implementing inherent safety principles with conventional safety costs and quantify the economic justification for implementing inherent safety.
This study aimed to evaluate the inherent safety status of a C2 + recovery unit in a petrochemical plant and justify the cost of implementing the principles of inherent safety and its impact compared to conventional safety.
Methods: First, the inherent safety status of each equipment and eventually the whole system were evaluated using the I2SI approach. In addition, I2SI could calculate the cost of implementing both conventional and inherent safety of the processes. Hence, by comparing cost indices together; the economic justification for implementing the inherent safety was examined. The I2SI consists of two main sub-indicators, including the hazard index (HI) and the inherent safety potential index (ISPI). Moreover, this sub-indicator calculated by other sub-indicators. To calculate the HI and ISPI, first, the damage index (DI) was determined, then the degree of need for process hazard control index (PHCI) was calculated for each case. Next, to calculate the ISI, each of the inherent safety principles was evaluated based on the applicability according to the process engineers and process designer’s viewpoint, also the application of the inherent safety principles to control each of the process condition parameters was obtained separately.
Finally, the I2SI was calculated for each equipment as well as the whole unit. In general, the range of score for HI and ISPI can from 1 to 200, which provides the flexibility to quantify the calculation of the I2SI to inherent safety status. Scholars have shown the processes or equipment which obtain the higher I2SI score, have a better situation of inherent safety also application of the inherent safety principles to them has a better impact.
In order to calculate the economic aspects of inherent safety implication compared to conventional safety, the cost indices including conventional safety cost index (CSCI) and inherent safety cost index (ISCI) was also calculated and analyzed.
Results: This study aimed to evaluate the inherent safety status of a process unit and justify the cost of implementing the inherent safety principles and its impact compared to conventional safety. The results showed, due to using a large tower with a capacity of 1137 m3, that the I2SI for the Methanizer tower is lower than other equipment. It is also because of its high process pressure which almost 33.5 brag, which is high pressure from the standard atmosphere and could have severe consequences in the possible accidents. The results of the simulation of worst-case scenario with PHAST software showed that the DI for the Methanizer tower is higher than the other equipment.
On the other hand; DI for LP REFR. Circulation Drum, which has the smallest amount of inventory resulting in has the lowest capacity from other equipment, is less than the rest. Similarly, the PHCI for the Methanizer tower was calculated more than any other equipment, indicating the low inherent safety of this equipment and the need for control measures. Also, the I2SI for Methanizer tower was lower than other equipment and calculated equal to 0.29. For this reason, the DI for the Methanizer tower for mortality of 50% is high as a result the HI obtained high, too (200 and 4, respectively). The same thing has affected the inherent safety of this equipment. The I2SI for the equipment involved in the propane cooling cycle were 1.05, which is higher than other equipment.
Costs of the possible incident that infliction to process and environmental due to the nature of material released, the cost of its environmental clearance and equipment operating conditions were estimated based on the method presented in the I2SI approach. According to this index in an incident case; MP REFR. Circulation Drum could cause the most financial damage to the system, besides the feed drum has minimal damage. The results received from the sensitivity analysis of the Methanizer tower by PHAST software showed if the temperature of this tower to be more negative, also if the operational pressure will approach atmospheric pressure, the inherent safety of the Methanizer will be improved; in the other words, the Methanizer tower will be inherently safer. The reason is that in the case of a leakage scenario, low temperature will be reduced the material flow rate also the severity of its consequences such as type of fire and/or vapor cloud dispersion. Low pressure can also have a similar effect on the outcome of potential scenarios.
The mass flow of the cooling propane could be decreased by increasing the temperature of the flow 22-26. This line acts as heating of the Methanizer tower, thereby reducing its temperature can result in reducing the amount of cold (propane) required. By lowering the pressure of the Methanizer tower, the cooling propane flow rate can be reduced as well, which will improve the operating conditions of the Methanizer tower inherently to enhance its safety and, in other words, the implementation of the “attenuation”. Also, optimizing operating conditions can reduce the volume of vessels, especially the Methanizer tower, thus enforcing the principle of “minimization”.
The results of this study showed that the I2SI for the process studied was 0.41, indicating a low level of the inherent safety status of the process. This is probably because of lack of unawareness of process designers and engineers about the inherent safety principles during the process design and operating it. The results of this study showed that although the design phase of the system life cycle is the best time to implement the inherent safety principles, some of the inherent safety principles could be economically justified to implement in the operational phase.
Based on the results of the study, the inherent safety cost index (ISCI) for the Methanizer tower was higher than the conventional safety cost index (CSCI). These indicators mean that 3.58$ should be spent to maintain a dollar of capital by increasing the inherent safety of the Methanizer Tower, and 3.11$ should be spent to maintain a dollar of capital by enhancing the safety of the Methanizer Tower. In other words, it is more cost-effective to implement conventional safety principles to improve the safety status of this equipment. The reason for this is that the operating conditions of the Methanizer tower are dangerous and inventory is very high. For this reason, the inherent safety of the Methanizer tower is very low and provide the inherent safety principles for this equipment will be costly. However, the difference between the cost of implementing conventional and inherent safety principles is less than 0.5$.
This indicates that if the “attenuation” is applied to the operating conditions of the tower, its will be inherently safer, therefore; its inherent safety cost index will decrease. Also, if the inherent safety principles had been adhered to in the design of the Methanizer tower, the implementation of the inherent safety principles in the operational phase would have been more justified.
Conclusion: The I2SI could use for the assessment of inherent safety status also can be performed at all stages of the system's life cycle, including the operational and design phases. The results showed that attention to the inherent safety principles, especially in the design phase, could increase the safety level without reducing the amount of production in addition to reducing the strategic costs of the process and equipment or other Imposed cost for increasing the safety level of safety.
