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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-29
4.9.12 Consequence of Releases Containing Multiple Toxic Chemicals
Consequence results for releases of multi-component toxic chemicals are uncommon but determined by
calculating the consequence area for each of the individual toxic components within the mixture. The overall
toxic consequence area is the largest of the individual toxic areas.
4.9.13 Effects of Mitigation Measures on Toxic Releases
To this point, isolation and detection capabilities have been taken into account in calculating the quantity of
material that may be released during a loss-of-containment event (see Section 4.7.1). However, there may be
additional systems in place, such as water sprays, that can mitigate a release once the material has reached
the atmosphere.
The effectiveness of mitigating systems are accounted for by reducing the release rate and duration for
continuous releases or by reducing the release mass for instantaneous releases. The RBI analyst will need to
provide his or her own reduction factors, based on the effectiveness of their particular spray-system design or
passive mitigation technology.
Where mitigation is a major issue, specialists should be consulted to get an accurate input. As an example, it
is possible to mitigate HF releases with a water spray. However, the fraction of HF that is removed by a water
spray may vary from near 0 % to near 100 % depending on the size of the release, the droplet size, flow rate
and orientation of the spray, and several other variables.
4.9.14 Determination of Final Toxic Consequence Areas
The final toxic consequence is determined as a probability weighted average of the individual toxic calculated
for each release hole size. A consequence area calculation is performed for the personnel injury areas only
since toxic releases do not result in component damage. The probability weighting utilizes the generic
frequencies of the release hole sizes obtained in STEP 2.3. Equation (3.68) is used to calculate the probability
weighted toxic consequence area.
CA
⎛
4
tox
⎜∑
gffn
⋅CAinj,
n ⎟
tox n=
1
f , inj
= ⎜
⎟
⎜ gfftotal
⎟
⎜
⎝
4.9.15 Calculation of Toxic Consequence Areas
⎞
⎟
⎠
(3.68)
a) STEP 9.1—For each release hole size selected in STEP 2.2, calculate the effective duration of the toxic
release using Equation (3.67).
tox
mfrac , in the release material. If
tox
the release fluid is a pure fluid, mfrac = 1.0 . Note that if there is more than one toxic component in the
released fluid mixture, this procedure can be repeated for each toxic component.
b) STEP 9.2—Determine the toxic percentage of the toxic component,
tox
c) STEP 9.3—For each release hole size, calculate the release rate, rate
n
to be used in the toxic analysis using Equation (3.61) and Equation (3.62).
, and release mass, mass ,
d) STEP 9.4—For each release hole size, calculate the toxic consequence area for each of the release hole
sizes.
tox
1) HF Acid and H2S—Calculate CA
inj,
n
using Equation (3.63) for a continuous release or Equation (3.64)
for an instantaneous release. The constants used in these equations are from Table 4.11.
tox
n