API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-19
3) flash fire—25 % of the area within the lower flammability limits (LFLs) of the cloud when ignited.
b) Personnel injury criteria:
1) explosion overpressure—34.5 kPa (5 psig);
2) thermal radiation—12.6 kW/m 2 [4000 Btu/(hr-ft 2 )] (jet fire, fireball, and pool fire);
3) flash fire—the LFL limits of the cloud when ignited.
The predicted results using the above threshold limits were intended to produce a relative risk ranking, which,
while being considered to be reasonably accurate, are not the highest levels of consequence that could be
estimated for a given accident sequence. As are most effects data, the component damage and personnel
injury criteria listed above are subject to intensive scientific debate, and values other than those used in this
methodology could be suggested.
4.8.3 Adjustment of Consequence Areas to Account for Mitigation Systems
4.8.3.1 Evaluating Post-leak Mitigation of Consequence
Evaluating post-leak response is an important step in consequence analysis. In this step, the various mitigation
systems in place are evaluated for their effectiveness in limiting the consequence areas. Toxic releases are
typically characterized as a prolonged buildup, then reduction, in cloud concentration, with accumulated
exposure throughout. Flammable events are more often releases that are either ignited quickly or the material
is quickly dispersed below its LFL. For these reasons, different approaches are necessary for evaluating the
post-leak response based on the type of consequence. Mitigation systems and their effect on flammable
release events are presented in this section.
4.8.3.2 Effects of Mitigation Measures on Flammable Consequence Magnitudes
The adjustments to the magnitude of the consequence for flammable releases based on unit mitigation
systems are provided in Table 4.10. These values are based on engineering judgment, using experience in
evaluating mitigation measures in quantitative risk analyses. The consequence area reduction factor, fact
mit ,
to account for the presence of mitigation systems is provided in Table 4.10.
4.8.4 Adjustment of Consequence Areas for Energy Efficiencies
Comparison of calculated consequence with those of actual historical releases indicates that there is a need
to correct large instantaneous releases for energy efficiency. This correction is made for instantaneous events
exceeding a release mass of 4,536 kg (10,000 lb) by dividing the calculated consequence areas by the
adjustment factor, eneff , given by Equation (3.18).
n
[ ]
eneff = 4⋅log C ⋅mass
− 15
(3.18)
n 10 4 A n
Note that the adjustment defined by Equation (3.18) is not applied to continuous releases.
4.8.5 Blending of Results Based on Release Type
The Level 1 consequence area calculations yield significantly different results, depending on whether the
continuous area equations are used or the instantaneous area equations are used. The blending factor is
determined as follows based on the release type.