G.2 Process Risk Assessment
305
G.2.2 Step 3: Risk Identification
Table G.2 shows the completed HAZOP study table.
G.2.3 Step 4: Risk Analysis
Consequence Analysis
(a)
c max (x) =
˙
M
π × u × σ y × σ z
=
5.6 × 10 −4 kg
s
π × 1.5
m
s × 18 m × 9 m
= 7.3 × 10
−7 kg
m 3
Multiplying this concentration by one million and dividing by the average
density of air leads to an estimation of the HCN concentration in units of parts
per million:
7.3 × 10 −7 kg HCN/m 3
1.3 kg air/m 3
× 1,000,000 = 0.6 ppm
Considering a short-term exposure limit for HCN of 3.8 ppm (ECHA, 2019b)
and that the estimated concentration at the neighboring site derived from the
turbulent Gaussian model is 0.6 ppm, the effect on people at the neighboring
site can be classified as negligible or potentially as low (minor injury).
(b) Considering the toxicity hazards of HCN and H 2 S, the effects of an HCN
release on operators in the plant are classified as high (injuries with irreversible
damages).
Probability Analysis
(a) Figure G.1 shows a complete version of the fault tree analysis.
(b) The assumption that the events in the fault tree are independent is not correct
since a power failure could cause all electrical equipment to fail. This would
lead to an increase in the probability of HCN release. The events that would
be triggered by a power failure include alarm failure, active safeguard failure,
reflux failure, stirrer failure, etc. In this case, a more thorough analysis should
be carried out using, e.g., a common cause failure analysis (see Appendix E.3).
(c) Table G.3 shows the results of a qualitative probability analysis for the release
of HCN.
305
G.2.2 Step 3: Risk Identification
Table G.2 shows the completed HAZOP study table.
G.2.3 Step 4: Risk Analysis
Consequence Analysis
(a)
c max (x) =
˙
M
π × u × σ y × σ z
=
5.6 × 10 −4 kg
s
π × 1.5
m
s × 18 m × 9 m
= 7.3 × 10
−7 kg
m 3
Multiplying this concentration by one million and dividing by the average
density of air leads to an estimation of the HCN concentration in units of parts
per million:
7.3 × 10 −7 kg HCN/m 3
1.3 kg air/m 3
× 1,000,000 = 0.6 ppm
Considering a short-term exposure limit for HCN of 3.8 ppm (ECHA, 2019b)
and that the estimated concentration at the neighboring site derived from the
turbulent Gaussian model is 0.6 ppm, the effect on people at the neighboring
site can be classified as negligible or potentially as low (minor injury).
(b) Considering the toxicity hazards of HCN and H 2 S, the effects of an HCN
release on operators in the plant are classified as high (injuries with irreversible
damages).
Probability Analysis
(a) Figure G.1 shows a complete version of the fault tree analysis.
(b) The assumption that the events in the fault tree are independent is not correct
since a power failure could cause all electrical equipment to fail. This would
lead to an increase in the probability of HCN release. The events that would
be triggered by a power failure include alarm failure, active safeguard failure,
reflux failure, stirrer failure, etc. In this case, a more thorough analysis should
be carried out using, e.g., a common cause failure analysis (see Appendix E.3).
(c) Table G.3 shows the results of a qualitative probability analysis for the release
of HCN.