Dates: Nov 3, 2023
The main purpose of a Quantitative Risk Assessment (QRA) is to evaluate the risk levels of a process due to potential Loss of Containment scenarios (LOCs). Moreover, the analysis of detailed QRA results is the basis for more specific studies for facility and critical equipment siting and also domino effects analysis due to thermal radiation of fires.
Here we illustrate with a case study how a risk-based QRA can provide valuable information for blast loading characterization at a given location of interest. The analysis takes into account both the identification of the total number of explosion outcomes that impact a given structure under analysis; and the individual frequency, overpressure, positive phase duration, and associated impulse of each identified outcome.
Simplified Quantitative Risk Assessment (QRA) Flowchart
Results from QRA development following worldwide risk-based criteria are the basis for emergency and land use planning, providing the foundation for risk reduction decision-making. However, specific analysis of explosion outcomes allows for more dedicated evaluation for complying with specific criteria for human vulnerability of occupied buildings and estimating the damage level of buildings, structures, and key critical safety equipment.
Example of QRA Results for Individual Risk Contours and FN Curve
Below is a detailed risk-based facility siting study based on blast exceedance analysis. Our approach is valuable for evaluating the location of occupied buildings, safety critical equipment, and any other key structures of interest. Process Safety Office® SuperChems™ software from ioMosaic was used to prepare the Exceedance Curves, Pressure-Impulse Diagrams, and FN Curves shown.
A risk-based approach requires the identification of a hazard level (e.g., overpressure value) that will not be exceeded at a given frequency threshold. OECs can then be developed for a given location of interest:
Thereafter, the cumulative frequency of all outcomes producing a given level of overpressure is calculated, and the OEC is plotted, showing the cumulative frequency versus the overpressure. Using a given cumulative frequency criteria (e.g., 1.00E-04 yr-1 based on API Standard RP-752 2009), it is possible to determine which building/structure complies with the criteria and which requires further evaluation.
Three criteria have been considered for relating building occupant vulnerability (BOV; probability of fatality) and blast loading phenomena:
CCPS Human Vulnerability vs. Overpressure
According to the CCPS Guidelines for Evaluating Process Plant Buildings for External Explosions and Fire (1996), the probability of fatality is evaluated from experimental data that relates vulnerability as a function of building type (i.e., six (6) different building classes from A to F), and overpressure.
CIA Human Vulnerability vs. Overpressure
According to the Chemical Industry Association (CIA) Guidance for the Location and Design of Occupied Buildings On Chemical Manufacturing Sites (1998) similar to criteria established by the CCPS, the probability of fatality is evaluated from experimental data that relates to vulnerability as a function of building type (i.e., four (4) different building classes from 1 to 4), and overpressure.
DOD Human Vulnerability vs. Percentage of Building Damage Level
According to the US Department Of Defense (DOD) Approved Methods and Algorithms for DOD Risk-Based Explosives Siting Rev. 4 (2009), the probability of fatality is evaluated as a function of building type and provides detailed Pressure-Impulse (P-I) Diagrams per each building class.
Taking into account the number of occupants per building (NOB), including the associated presence factor, and the estimated probability of fatality BOV, the probability of fatality for a certain number of occupants (PN) can be estimated by applying criteria illustrated in the equation below. An FN Curve for each occupied building can be generated by cumulating individual frequencies of all explosion outcomes as a function of the number of fatalities.
Criteria for Estimating PN Equation
NOB: Number of building occupants
N: Number of fatalities
BOV: Building Occupant Vulnerability; Probably of Fatality
PN: Probability of fatality for N occupants
Overpressure Exceedance Curve
Building Damage Evaluation Pressure Impulse Diagram
Building Damage Exceedance Curve
Number of Fatalities Exceedance Curve for Occupied Building 01 (FN Curve)
A QRA is an invaluable method for making informed risk-based process safety planning decisions, as well as being fundamental to any facility siting decision-making. ioMosaic can help you reduce incident frequency and consequences of potential accidents. Call us today at 1.844.ioMosaic or send us a note. We'd love to hear from you.