Table 14C.2 Road and Railway Population within the Assessment Area
|
Location |
Road Population in Vicinity of SCISTW
during Day Time |
|
|
73.0 |
|
|
9.9 |
|
|
37.5 |
|
|
3.3 |
|
|
32.4 |
|
|
198.1 |
|
|
15.6 |
|
Sham |
10.1 |
|
KCRC West Rail |
78.9 |
14C.40
The raw data of the site survey and detailed calculations of the road
population are presented in Appendix
14C.3.
14C.41
A site survey was conducted to gather data for the estimation of marine population
in vicinity of SCISTW. The site survey
data and observation, as well as the detailed estimation of marine population
are presented in Appendix 14C.4. Based on the site observation,
most of the marine vessels were stationed in three clusters near SCISTW, and
about 10% of marine vessels were mobile and travelling at the sea area near the
SCISTW. The mobile marine population was
assumed to be evenly distributed at the sea area near the SCISTW. The estimated population of the population
clusters and mobile marine population are presented in Table 14C.3. The locations of the marine population
clusters are shown in Figure 14C.6.
Table 14C.3 Estimated Marine Population in vicinity of SCISTW
Meteorological Data
14C.42
Meteorological data are required for consequence modelling and risk
calculation. Consequence modelling
(dispersion modelling) requires wind speed and stability class to determine the
degree of turbulent mixing potential whereas risk calculation requires
wind-rose frequencies for each combination of wind speed and stability class.
14C.43
Meteorological data at Ching Pak House (Tsing Yi) weather station, which
is closest to SCISTW, of year 2005 was obtained from which wind speed,
stability class, weather class and wind direction were available. This data represents the weather conditions
for the whole year in 2005 and has already taken into account of seasonal
variations, and is therefore considered applicable for the assessment.
14C.44
Table 14C.4a shows the
wind speed-stability frequencies.
Table 14C.4a Stability Category-Wind Speed Frequencies at Ching Pak House Weather Station (day-time)
|
|
Stability Class |
|
|||||||
|
Wind Speed (m/s) |
A |
B |
C |
D |
E |
F |
G |
Total (%) |
|
|
0.0-1.7 |
0.71 |
5.68 |
0.02 |
3.33 |
0.02 |
4.41 |
0 |
14.18 |
|
|
1.8-3.2 |
0.32 |
8.26 |
5.46 |
5.73 |
3.36 |
1.99 |
0 |
25.11 |
|
|
3.3-5.2 |
0 |
5.71 |
15.57 |
14.06 |
2.21 |
0 |
0 |
37.56 |
|
|
5.3-8.2 |
0 |
0 |
3.56 |
16.92 |
0 |
0 |
0 |
20.48 |
|
|
Over 8.3 |
0 |
0 |
0.39 |
2.28 |
0 |
0 |
0 |
2.67 |
|
|
All (%) |
1.03 |
19.66 |
25 |
42.33 |
5.59 |
6.39 |
0 |
100 |
|
14C.45
From the data presented in Table 14C.4a, 6
combinations (2B, 1D, 4C, 7D, 1F and 3E) of wind speed and stability class were
chosen as the representative daytime meteorological conditions. These combinations are considered adequate to
reflect the full range of observed variations in these quantities. It is not necessary and efficient to consider
every combination observed. The
principle is to group these combinations into representative weather classes,
which together cover all conditions observed.
14C.46
Once the weather classes have been selected, frequencies for each wind
direction for each weather class can then be determined. These frequency distributions are given in Table 14C.4b for the daytime
meteorological conditions.
Table 14C.4b Weather Class-Wind Direction Frequencies at Ching Pak House Weather Station (day-time)
|
|
Weather Class |
|
|||||
|
Direction |
2B |
1D |
4C |
7D |
1F |
3E |
Total |
|
0 – 30 |
5.18 |
0.55 |
7.56 |
3.65 |
0.46 |
0.84 |
18.24 |
|
30 – 60 |
0.66 |
0.39 |
0.16 |
0.02 |
0.27 |
0.14 |
1.64 |
|
60 – 90 |
1.35 |
0.14 |
1.14 |
0.39 |
0.34 |
0.09 |
3.45 |
|
90 – 120 |
1.32 |
0.34 |
1.62 |
0.48 |
0.16 |
0.25 |
4.18 |
|
120 – 150 |
1.71 |
0.30 |
3.11 |
0.34 |
0.68 |
1.05 |
7.19 |
|
150 – 180 |
1.62 |
0.34 |
1.53 |
0.73 |
0.78 |
0.59 |
5.59 |
|
180 – 210 |
0.71 |
0.21 |
1.23 |
0.21 |
0.18 |
0.30 |
2.83 |
|
210 – 240 |
1.12 |
0.14 |
4.43 |
1.76 |
0.30 |
1.21 |
8.95 |
|
240 – 270 |
0.84 |
0.16 |
3.93 |
2.47 |
0.37 |
0.91 |
8.68 |
|
270 – 300 |
0.96 |
0.21 |
4.45 |
4.59 |
0.09 |
0.68 |
10.98 |
|
300 – 330 |
2.72 |
0.34 |
8.68 |
6.78 |
0.50 |
1.16 |
20.18 |
|
330 – 360 |
2.49 |
0.25 |
2.99 |
1.74 |
0.30 |
0.32 |
8.08 |
|
All |
20.68 |
3.36 |
40.82 |
23.15 |
4.43 |
7.56 |
100.00 |
Overview
14C.47
The hazard identification stage consists of three tasks to identify
hazardous scenarios associated with the sodium hypochlorite and sodium bisulphite
related activities in disinfection facilities operations, as follows:
·
Incident Review
·
Hazard and Operability (HAZOP)
Study
·
Review of Previous Relevant
Studies
14C.48
The risk of the identified hazardous scenarios was quantitatively
assessed in the later stage of the hazard to life assessment.
Incident Review
14C.49
Under the HATS ADF EIA Study, incident review for the advance
disinfection facilities operation was conducted. Since the chemicals and activities associated
with the disinfection facilities under HATS 2A are same to those of advance
disinfection facilities, the results of the conducted incident review are
applicable to the current risk assessment.
14C.50
Forty-six relevant incidents were identified in the conducted incident
review. A summary of these incidents is presented in Appendix 14C.7.
It should be noted that no fatality was reported in all identified
incidents.
14C.51
The following scenarios were identified from the incident review
exercise:
·
Mixing of incompatible
chemicals
·
Spillage of sodium hypochlorite
from road tanker
·
Spillage of sodium hypochlorite
from delivery pipe
·
Spillage of sodium bisulphite
from delivery pipe
·
Spillage of sodium hypochlorite
from storage tank
14C.52
With respect to mixing of incompatible chemicals, 21 incidents have been
identified. The identified causes of
such incidents of mixing of incompatible chemicals are as follows:
·
Error in unloading operation
·
Spillage of incompatible
chemicals in different tanks
·
Spillage of incompatible
chemicals in two different pipelines
·
Spillage of chemical from valve
at soft drinks maker
14C.53
In conclusion, the incidents identified can be grouped into two types,
which will be further assessed in this hazard assessment.
·
spillage of chemicals
·
accidental mixing between
incompatible chemicals and release of toxic gas
HAZOP Study
14C.54
A Hazard and Operability (HAZOP) Study has been conducted under the HATS
ADF EIA Study to systematically identify potential hazardous scenarios
associated with sodium hypochlorite or sodium bisulphite related activities for
the advance disinfection facilities operation.
This has also identified corresponding precautionary or mitigation
measures to avoid the occurrence of the hazardous scenarios. Since the operation of the disinfection
facilities under HATS 2A are same to the advance disinfection facilities, the
identified scenarios by the conducted HAZOP Study are applicable to this risk
assessment. In addition, another HAZOP
Study has been conducted under the current HATS Stage 2A EIA Study in January
2008 to systematically identify the potential hazards scenarios associated with
the construction and operation of the proposed new plant facilities of the
current EIA study.
14C.55
The identified scenarios by the HAZOP Study are as follows:
·
Spillage of sodium hypochlorite
from delivery barge
·
Spillage of sodium hypochlorite
from loading hose
·
Spillage of sodium bisulphite
from loading hose
·
Spillage of sodium hypochlorite
from road tanker
·
Spillage of sodium bisulphite
from road tanker
·
Spillage of sodium hypochlorite
from delivery pipelines
·
Spillage of sodium bisulphite
from delivery pipelines
·
Spillage of sodium hypochlorite
from storage tank
·
Spillage of sodium bisulphite
from storage tank
·
Mixing of incompatible
chemicals
14C.56
Again, the incidents identified can be grouped into two types: (1)
spillage of chemicals; and (2) accidental mixing of incompatible chemicals.
Review of Previous Relevant Studies
14C.57
Previous relevant studies involving risk assessment for the operations
associated with sodium hypochlorite or sodium bisulphite were reviewed to
identify hazardous scenarios.
14C.58
It was found that quantitative risk assessment for the operations
associated with sodium hypochlorite or sodium bisulphite is sparse. It may be because these two chemicals are not
particularly hazardous, which may be reflected by the observation that no
fatality was noted in the incidents associated with these chemicals. The following previous studies involving
evaluation of risk due to sodium hypochlorite / sodium bisulphite were
identified and reviewed:
·
Local Study
-
Construction of an
International Theme Park in Penny’s Bay of North Lantau together with its
Essential Associated Infrastructures – Environmental Impact Assessment,
completed in 2000 (Study A)
-
Provision of Disinfection Facilities
at Stonecutters Island Sewage Treatment Works – Investigation, completed in
2007 (Study B)
·
Overseas Study
-
Croton Water Treatment Plant –
Final Supplemental Environmental Impact Statement (Study C)[3],
a study in the
-
Yamba Sewerage Augmentation –
Environmental Impact Statement[4]
(Study D), a study in
14C.59
The hazard assessment methodology adopted and hazardous scenarios
evaluated in the abovementioned studies are described below.
·
Study A - Hazardous scenarios of sodium
hypochlorite spillage as well as mixing of sodium hypochlorite with acids were
identified in the study. Only the risk
associated with the latter scenario was considered and assessed quantitatively.
·
Study B – Hazardous scenarios of
chemicals (sodium hypochlorite and sodium bisulphite) spillage and mixing of
incompatible chemicals were identified in the study.
·
Study C - Sodium hypochlorite was
proposed to be used as one of the chemicals in the water treatment
process. No quantitative risk assessment
for chemicals was conducted in the Study.
The Study only mentioned mitigation measures, including installation of
containment and storing incompatible chemicals in separate areas, would be
implemented and concluded that no potentially significant adverse impacts are
anticipated to occur from the transport, storage, or usage of the chemicals.
·
Study D - Sodium hypochlorite was
proposed to be used in the sewage treatment plant for recycled water
reuse. The Study did not quantitatively
assess the risk due to sodium hypochlorite.
Rather, the Study proposed mitigation measures including installation of
bunds and warning signs, worker training and use of personal protective
equipment.
14C.60
From the review of previous studies, two scenarios are noted:
·
Spillage of chemicals
·
Chemical to be used mixes with
incompatible chemicals
Results of Hazard Identification
14C.61
The identified hazardous scenarios from incident review, HAZOP Study,
and review of previous relevant studies are listed as follows:
·
Spillage of chemicals
-
Spillage of sodium hypochlorite
from delivery barge (hazardous scenario 1)
-
Spillage of sodium hypochlorite
from loading hose (hazardous scenario 2)
-
Spillage of sodium bisulphite
from loading hose (hazardous scenario 3)
-
Spillage of sodium hypochlorite
from road tanker (hazardous scenario 4)
-
Spillage of sodium bisulphite
from road tanker (hazardous scenario 5)
-
Spillage of sodium hypochlorite
from delivery pipelines (hazardous scenario 6)
-
Spillage of sodium bisulphite
from delivery pipelines (hazardous scenario 7)
-
Spillage of sodium hypochlorite
from storage tank (hazardous scenario 8)
-
Spillage of sodium bisulphite
from storage tank (hazardous scenario 9)
·
Mixing of incompatible chemicals on-site
(hazardous scenario 10), caused by:
-
Error in chemical unloading
operations
-
Spillage of incompatible
chemicals in different tanks
-
Spillage of incompatible
chemicals in two different pipelines
-
Spillage of chemical from valve
at soft drinks maker
14C.62
In light of the identified hazards, a range of comprehensive chemical
safety measures has been developed, as summarised in Sections 14C.63 to 14C.88.
These hazardous scenarios are further assessed in Sections 14C.89 to 14C.182, with the application of the safety
measures in mind.
Proposed
Safety Measures for Chemicals-related Operations
14C.63
A package of safety measures was identified under the HATS ADF EIA Study
to minimize the risk due to chemicals-related operation. The applicable safety measures in that
package will also be adopted in the current Project. The safety measures can be categorized into
several groups: chemical supply contract arrangement, and design measures,
procedures and safety measures.
Special Chemical Supply Contract Arrangement
14C.64
A separate supply contract will be awarded for each of the three
chemicals (sodium hypochlorite, sodium bisulphite and ferric chloride
solutions). In each supply contract, the
chemical supplier will be required to provide dedicated transport specifically
used for delivering the chemical to be supplied, and the road tankers will need
to be registered with SCISTW. In
addition, the supply contract for sodium hypochlorite will specify that the
delivery barge provided will be dedicated for delivering sodium hypochlorite
directly and exclusively from the supplier’s production plant to SCISTW during
the contract period. The delivery barge will not be allowed to provide other
services, such as carrying other chemical or carrying chemicals to other
facilities other than SCISTW.
Design Measures
Separation of Chemical Storage Areas
14C.65
As proposed in the HATS ADF EIA Study, locate sodium hypochlorite and
ferric chloride tank farms in separate areas of SCISTW, which is about
14C.66
The sodium bisulphite tank farm will be located outside the SCISTW main
compound (Figure 14C.2 refers).
Protection and Separation of Chemical
Delivery Pipelines
14C.67
Each chemical delivery pipeline will be installed in designated and
separate service duct or pipe trench to provide additional protection. The routes of the pipe trenches for sodium
hypochlorite pipelines and the service duct that ferric chloride pipelines are
laid are shown in Figure 14C.8.
Although it can be seen in Figure 14C.8 that there is an intersection
between the routes of the service duct and the pipe trench, the two set of
pipelines for ferric chloride and sodium hypochlorite will be installed in
separate service duct / pipe trench. Figure 14C.9A
shows the cross section of the service duct accommodating the ferric chloride
delivery pipelines and the pipe trench where sodium hypochlorite delivery
pipelines are installed. It can seen
that the service duct and pipe trench will be located at different level below
ground.
14C.68
Also, it was proposed to install a vibration sensing system at the
ferric chloride and sodium hypochlorite tank farm to enable shut down of the
chemical pumping system (at storage tank or sodium hypochlorite delivery barge)
whenever excessive vibrations are detected, in order to minimize the amount of
chemical that can escape in the event of pipeline failure due to excessive
vibrations.
14C.69
A section of the pipe trench accommodating sodium hypochlorite feeding
pipelines was proposed to be wrapped by heavy-duty impervious membrane to
provide an additional barrier to migration of the leaked sodium hypochlorite
solution. Furthermore, a road kerb was
proposed to be constructed near the section of hypochlorite pipeline near the
barge unloading facility and a U-channel is proposed at the foot of the
concrete barrier to facilitate surface drainage into the sea.
14C.70
For the sodium bisulphite delivery pipelines, they will be installed in
the dechlorination plant located more than 400m away from pipelines for sodium
hypochlorite and ferric chloride. There
is no intersection between the routes of the sodium bisulphite delivery pipelines
and the pipeline of the other two chemicals.
Dedicated Chemical Delivery Route
14C.71
Specific road tanker transport route will be assigned to each
chemical. Such arrangement provides the most
direct route for the road tanker travelling to the intended tank farm.
14C.72
Provide road signs on service road indicating the route to specific
chemical storage area.
Security of Chemical Loading Points
14C.73
Chemical delivery staff will need to register with SCISTW staff upon
entering the site. Loading points for
ferric chloride, sodium hypochlorite and sodium bisulphite will be secured by
locks and the keys will be kept by SCISTW staff. The chemical unloading operation cannot start
without presence of SCISTW staff to open the locks.
Unique Colour and Size of Pipelines and
Hose Coupling for Different Chemicals
14C.74
Delivery pipelines for different chemicals will be in different colours
and different sizes.
14C.75
There will be specific hose connection design for each chemical, the
type, size and colour of coupling will be specific for each chemical – for
loading point at SCISTW, the coupling size, type and colour of the loading
point of each tank farm will be unique (different from the other two tank
farms). For the loading hose, the
supplier will be required to carry the loading hose specifically used for the
delivery to SCISTW in each chemical delivery.
The loading hose should be clearly coloured and labelled with the name
of chemical to be delivered, with its coupling matches the size, type and
colour of the one at the tank farm loading point. This measure will be included in the chemical
supply contract.
14C.76
There are many specialty couplings available by different manufacturers,
not only those couplings are available in a large range of sizes, but they also
offer different fittings which would make coupling not of the same type could
not be fitted or would render inoperable when forced together. Examples of different types of hose coupling
are presented in Figure 14C.10.
The locking arms or the switch of the coupling can only be fastened or
turned when two pieces of matched (i.e. same type and same size) hose coupling
are put together, in order to open the seal of both pieces of coupling. By adopting this mechanism, chemical
unloading can only proceed when two pieces of matched hose coupling are put
together.
Operation Procedures and Safety Measures
14C.77
Clear Labelling of Chemicals-related Equipment:
·
Provide clear and sufficient
signage / labels to indicate the identity (i.e. for which chemical) of each
tank farm and associated equipment including pipelines, loading points and
loading hoses
14C.78
Ensuring Quality of Chemical Supplier:
·
Only appoint chemical suppliers
with satisfactory quality system.
Suppliers equipped with good quality system are less prone to errors in
chemical delivery operations
·
Request the chemical supplier
to employ an independent checker to audit the quality and safety management
system of the supplier, in order to ensure the standard of the quality and
safety management system. This measure
will be included in the chemical supply contract
·
The chemical supplied to SCISTW
can only be produced in designated chemical production plants and delivered
directly from designated locations. This
requirement will help ensure the quality of the chemical supplied and reduce
the likelihood of errors in chemical delivery operation. This measure will be included in the chemical
supply contract
Procedural Control of Chemical
Unloading Operation
14C.79
Develop clear procedural controls for barge / road tanker filling and
unloading operation. Procedural controls
should include: chemical supplier staff cannot start chemical unloading without
authorization and presence of SCISTW staff, checking of the delivery
documentation prior to unloading process etc.
14C.80
SCISTW staff will be present at the tank area to receive the barge /
road tanker, check barge / road tanker labels, check the transport documents
carried by the barge crew / road tanker driver, check type, size and colour of
coupling and hose coupler, conduct chemical analysis to check the identity of
delivered chemical and only then authorize the driver to unload the content.
14C.81
Chemical supplier needs to fax or electronically transmit copies of
delivery bills-of-lading information and barge crew / road tanker driver
identification to SCISTW prior to delivery barge / road tanker arriving
on-site. Such information will be in
compliance with the supplier’s independently accredited quality assurance
system (to ISO:9000 or equivalent), to ensure that the right person driving the
right tanker / barge to the right tank farm and further avoid occurrence of
delivery operation error. This measure
will be included in the chemical supply contract.
14C.82
Conduct chemical analysis to confirm the right chemical is
delivered. The analysis needs to be
conducted by SCISTW staff or independent checker before the chemical is
authorized to be unloaded to the tank farm.
HKOLAS accredited analysis methods are currently used in the laboratory
in SCISTW to analyze the identity and concentration of ferric chloride
(GB4482-93 Section 5.1) and sodium hypochlorite (ASTM D2022-89 cl. 6 to
9). Accredited analysis method for sodium
bisulphite will be adopted in SCISTW for testing of delivered chemical in the
future.
14C.83
If the coupling of hose connected to the barge / road tanker is found to
be unmatched with the coupling of loading point of tank farm, chemical
unloading operation must not proceed and the situation must be reported to the
SCISTW management for follow-up actions.
14C.84
Chain-of-custody documentation system will be used to ensure both the
supplier (factory) and SCISTW staffs have checked the chemical identity and the
consistency of the chemical analysis result.
Such measure will ensure that the identity of the delivered chemical has
been checked twice by two different parties before it is unloaded to the
storage tank. This measure will be
included in the chemical supply contract.
Measures to Identify and Stop the
Chemical Unloading in Error Events
14C.85
Error in chemical unloading operation could lead to mixing of
incompatible chemicals and toxic gas could be produced in such event. The abovementioned safety measures were
identified to avoid the occurrence of such event. Further, in order to identify and stop such
unlikely event, chlorine gas detectors will be installed near the tank vent for
each ferric chloride and sodium hypochlorite storage tank, whereas sulphur
dioxide gas detectors will be installed near the tank vent for each ferric
chloride and sodium bisulphite storage tank.
When chlorine gas detectors detect a chlorine gas concentration of 3ppm
or higher, or sulphur dioxide gas detectors detect a sulphur dioxide gas concentration
of 15ppm or higher, alarm will annunciated at the tank farm area and in the
central control centre.
14C.86
Emergency shutdown valve will be installed on the inlet of the tank
farms, to stop chemical unloading to the storage tank when the valve is closed. Closure of the emergency shutdown valve can
be automatically initiated by the activation of the alarm in the tank farm area
or central control centre. Also, as
chemical supplier staff and SCISTW operator will be present throughout the
chemical unloading operation, they can stop the chemical unloading operation by
turning off the pump for pumping the chemical to the storage tank when they
notice the activation of the alarm or other abnormal conditions (e.g. emission
of gas from the tank vent). In such case,
it is expected that the wrong chemical unloading operation can be stopped
within 3 minute in most cases.
14C.87
In addition, CCTV system will be installed to monitor the situation at
the chemical tank farm. In case the
rapid stoppage (within 3 minute) of wrong chemical unloading operation fails,
SCISTW operator at the central control centre will be notified of the incident
by the CCTV image, and apply appropriate action (e.g. close the emergency
shutdown valve) within a short period time, probably 10 minutes.
Special Arrangement of SCISTW Public Event
14C.88
Public evens might sometimes be held in SCISTW which allow access of
public to the plant facilities. As a
precautionary measure, chemical delivery operation will be suspended on days of
SCISTW public event. Also, public
members visiting the SCISTW will be guided by DSD staff and will not be allowed
to visit the area near the chemical storage locations in SCISTW.
Assessment of Chemical Spillage Hazards (Scenarios 1 to 9)
Introduction
14C.89
As mentioned above, sodium hypochlorite and sodium bisulphite are not
acutely toxic, flammable, or explosive substances. It is acknowledged that there is the
potential for off-site population to contact and/or ingest the spilled
chemicals in these chemical spillage scenarios.
However, there will be no lethal effects on off-site population due to
contact with and ingestion of these chemicals.
Further elaboration is given below.
Dermal Contact
14C.90
Under the IMDG Code, both sodium hypochlorite (12%) and sodium bisulphite
are assigned to be Class 8 dangerous goods (corrosive substances) with packing
group III. Packing group III is assigned
to substances that cause skin tissue damage within an observation period of up
to 14 days starting after an exposure time of more than 60 minutes but not more
than 4 hours. As indicated in the
assigned packing group, sodium hypochlorite (12%) and sodium bisulphite will
not cause chemical burn to skin unless there is a prolonged exposure (i.e. more
than 1 hour) to the chemical.
14C.91
Individuals will not be exposed to chemicals for a prolonged period
unless they are unable to escape from an area holding considerable depth of
chemicals (e.g. chemical storage tank or bund area filled with spilled
chemicals). Such structures that are able
to contain chemicals are present within the site boundary but not identified
outside the site boundary.
14C.92
For Scenarios 2 (hose connecting delivery pipe and road tanker) to 9,
the spilled chemical might spread on land and form a liquid pool, which might extend
out of the site boundary. However, the
depth of the liquid pool extended out of the site boundary will not be
considerable because no structure, which could contain the spilled liquid, is
identified outside the site boundary.
Since the spilled chemical pool will not have considerable depth,
off-site population will not have large area of skin exposed to the spilled
chemical for a prolonged period.
Therefore, there will be no off-site fatality in these scenarios due to
dermal contact of the spilled chemicals.
14C.93
For Scenarios 1 and 2 (hose connecting the delivery barge and delivery
pipe), the spilled sodium hypochlorite will be largely diluted by seawater
immediately and the corrosivity of the spilled chemical will be greatly reduced
due to the dilution. Therefore, these
chemical spillage scenarios will not cause off-site fatality. Note that the sea area near the delivery
barge berthing location is not an area for swimming activities or other
water-based recreational activities.
Ingestion
Sodium Hypochlorite
14C.94
Literatures have been reviewed to understand the clinical effects of
acute exposure of sodium hypochlorite via ingestion. It is revealed that the vast majority of
patients ingesting sodium hypochlorite developed no symptoms or only minor
gastrointestinal irritation. Corrosive
damage to the gastrointestinal tract is unusual following ingestion of the
chemical unless a large quantity of the chemical is ingested. It should be noted that no description of
“lethal impact due to ingestion” was stated in literatures reviewed.
14C.95
The above findings are consistent with the hazardous properties
described in the IMDG Code, a widely used document and the “risk phrases” for
dangerous substances defined in the EU Commission Directive (2001/59/EC). In the IMDG Code, sodium hypochlorite is only
classified as a corrosive substance (packing group III, indicating slight
corrosivity) but not considered as a toxic substance. The risk phrase “R22 – harmful if swallowed”
(rather than R25 – toxic if swallowed or R28 – very toxic if swallowed) is
assigned to sodium hypochlorite. Such
descriptions do not indicate lethal effect due to ingestion of the chemical
concerned.
Sodium Bisulphite
14C.96
In addition, literatures were reviewed to understand the clinical effects
of acute exposure of sodium bisulphite via ingestion. It is revealed that ingestion of sodium
bisulphite will cause swelling of the tongue, angioedema, difficulty in
swallowing. Ocular injury may occur from
exposure to high concentrations. Acute
ingestions of 3.5mg/kg produce vomiting in most individuals. Larger doses may result in gastric
irritation, abdominal pain, and gastric haemorrhage. Again, it should be noted that no description
of “lethal impact due to ingestion” was stated in literature reviewed.
14C.97
The above findings are consistent with the hazardous properties
described in the IMDG Code, a widely used document and the “risk phrases” for
dangerous substances defined in the EU Commission Directive. In the IMDG Code, sodium bisulphite is only
classified as a corrosive substance (packing group III, indicating slight
corrosivity) but not considered as a toxic substance. The risk phrase “R22 – harmful if swallowed”
(rather than R25 – toxic if swallowed or R28 – very toxic if swallowed) is assigned
to sodium bisulphite. Such descriptions
do not indicate lethal effect due to ingestion of the chemical concerned.
Conclusion
14C.98
In conclusion, an assessment of the consequence of the chemical spillage
scenarios (hazardous scenarios 1 to 9) has found that while there is the
potential for off-site population to expose to the spilled chemical, there will
be no lethal effects on off-site population due to contact with or ingestion of
these chemicals. Hence, the chemical
spillage scenarios will not cause off-site fatality under the circumstances of
the current Project.
Assessment of Incompatible Chemical Mixing
Hazards (Scenario 10)
Overview
14C.99
There would be the potential for off-site fatality when mixing of
incompatible chemicals occur on-site leading to generation of toxic gas that
eventually migrates off-site. There are four identified causes for the scenario
of mixing of incompatible chemicals:
·
Cause 1 - Error in chemical
unloading operations
·
Cause 2 – Simultaneous release
of incompatible chemicals from their respective storage tanks
·
Cause 3 – Simultaneous release
of incompatible chemicals from their respective conveyance pipelines
·
Cause 4 – Simultaneous release
of incompatible chemicals from their storage tanks and pipelines
14C.100
These four causes are illustrated in Exhibit 14C.1.
Exhibit 14C.1 Diagrammatic Illustration of Pathways for Toxic Gas Formation due to Mixing of Incompatible Chemicals
Cause 1 - Error in Unloading Operations
14C.101
There will be separate supply contracts each for ferric chloride, sodium
hypochlorite, and sodium bisulphite. For
ferric chloride and sodium bisulphite, these two chemicals will only be
delivered by road tanker, as there is no barge-unloading facility for the
ferric chloride or sodium bisulphite tank farm.
14C.102
For sodium hypochlorite, barge-unloading facilities will be provided so that
the chemical can be delivered by a specially designed delivery barge, unloaded
to the sodium hypochlorite storage tank via separate dedicated pipelines. Road tanker unloading facilities will also be
provided at the sodium hypochlorite tank farm, so that the chemical can be
delivered by road tanker in the event that conditions do not allow barge
delivery.
14C.103
As sodium hypochlorite will be mainly delivered by barge, special
conditions will be included in the supply contract for sodium
hypochlorite. In the contract, the use
of the delivery barge provided will be restricted for delivering sodium
hypochlorite directly and exclusively from the supplier’s production plant to
SCISTW during the contract period. The
delivery barge will not be allowed to provide other services, such as carrying
other chemical or carrying chemicals to other facilities other than
SCISTW. In addition, SCISTW staff will
only accept the chemical delivered by the specially designed delivery barge from
the chemical supplier. Under such circumstances,
the delivery barge provided by the sodium hypochlorite supplier will only carry
sodium hypochlorite to SCISTW.
14C.104
As mentioned above, no barge-unloading facility will be provided for
ferric chloride and sodium bisulphite tank farm. Therefore, there is no physical pipe
connection from the barge unloading facilities to ferric chloride and sodium
bisulphite tank farm so it is impossible to unload sodium hypochlorite from
delivery barge to ferric chloride or sodium bisulphite tank. As a result, sodium hypochlorite carried by
the specially designed delivery barge will only be unloaded to the sodium
hypochlorite tank farm via the dedicated pipeline but not ferric chloride and
sodium bisulphite tank farms. Hence,
sodium hypochlorite will not mix with ferric chloride or sodium bisulphite in
the unloading operation by delivery barge.
Therefore, the cause “error in unloading operation” is not applicable to
barge delivery operation.
14C.105
Nevertheless, error in unloading operations via road tanker delivery of chemicals
could be possible in the current Project.
The frequency of occurrence of this event is further assessed in Sections 14C.183 to 14C.211.
Cause 2 - Simultaneous Release Incompatible Chemicals from Storage Tanks
Release Mode
14C.106
The incompatible chemical mixing scenario under the cause “release of
incompatible chemicals from their respective storage tanks” can only occur if:
·
Both the ferric chloride and
sodium hypochlorite solutions are somehow released, at the same period of time,
into unintended areas of SCISTW from their respective storage; and
·
Then, the two substances are
allowed to migrate to a location(s) where they come into contact with each
other
14C.107
Unintended release of either chemical from their respective storage
tanks could be caused by failure of the storage tank because of cracks,
punctures, rupture, etc. These in turn
could be caused by intrinsic factors (e.g., weathering of tank material) or
external impacts (e.g., a crashing airplane, earthquakes, malicious attacks, etc).
14C.108
The following paragraphs outline the existing/proposed engineering
control measures in place/to be implemented to prevent the ferric chloride and
sodium hypochlorite solutions from coming into contact with each other, in the
event of simultaneous failure of the storage tanks leading to unplanned release
of the chemicals.
14C.109
Firstly, it should be noted that ferric chloride and sodium hypochlorite
are stored under ambient atmospheric pressure conditions, which are not
pressurised. Therefore, in the event of
leakage of chemical from the storage tank, the content will be released from a
hole in the tank discharged as a liquid stream under the force of gravity.
14C.110
In the event of storage tank rupture, the stored chemical will be
released in a manner similar to a cylindrical column of liquid free to collapse
under gravity (i.e. released liquid would fall vertically into the bund). Owing to the vertical momentum of the
released liquid, some liquid may splash over the bund wall and spread to other
areas of SCISTW.
Existing Ferric Chloride Storage
Facility
14C.111
There are six existing ferric chloride solution storage tanks (each
about 8m diameter, 13m tall). These are
located in a tank farm on the northeastern corner of the SCISTW, as shown in Figure 14C.7.
The figure presents a proposed layout plan of SCISTW after HATS Stage 2A
construction works are completed, which shows the layout of the existing
facilities, the proposed sodium hypochlorite storage tanks (located at the
southern side of the site) and other structures for HATS Stage 2A.
14C.112
To the north of the ferric chloride tank farm is the sea. At its farthest point, the tank farm is
approximately 30m from the seawall cope line.
The waterfront pavement is graded towards the sea to facilitate surface
drainage. Stormwater drains are also
present to intercept surface runoff.
14C.113
Immediately behind the tank farm on the south (landward) side is the
14C.114
Any leaked or spilled ferric chloride solution will be caught initially
inside the bund wall. Depending on the
rate of release, two scenarios may arise:
·
Scenario 1 - If the rate of the
unintended release is low, the ferric chloride solution can be pumped out
before it overtops the containment. The
pumped-out liquid can then be disposed of as chemical waste safely and transported
out of the SCISTW. This way, the
released ferric chloride will not enter into the vicinity of the chlorination
plant located on the other (southern) side of the site.
·
Scenario 2 - If the rate of
release is high or violent (e.g., due to rupture of several or all storage
tanks at the same time), the ferric chloride solution could overtop the bunded
containment before it can be pumped away or it somehow could escape the
confines of the bunded containment. In
this case, there is a potential for the escaped solution to migrate farther
than the immediate vicinity of the tank farm.
14C.115
However, even under Scenario 2, given the substantial size and form of
the Chemical Dosing Building relative to the storage tanks, it is providing a solid,
effective barrier to prevent any spilled or leaked chemical from flowing
towards the south side of the SCISTW site (where the chlorination plant is
located). Moreover, the new pumping
station and associated works will serve as additional barrier to stop spilled
ferric chloride flowing towards southward.
14C.116
Further, the site surface gradient will direct the spilled ferric
chloride solution towards the sea before it can reach the road separating the
sedimentation tank and the sludge treatment facilities. In addition, the surface drains located
southwest of the ferric chloride tank farm will intercept any spilled ferric
chloride that find its way in the uphill direction and will discharge the
ferric chloride solution into the sea through the stormwater outfall.
14C.117
In view of the above mitigating factors or engineering controls, it is
concluded that any ferric chloride solution released unintentionally from the
tank farm would not flow to the southeastern part of SCISTW, where the sodium
hypochlorite tank farm will be located.
Sodium Hypochlorite Tanks (to be
constructed in HATS ADF)
14C.118
The sodium hypochlorite tank farm will be located at the southeastern
part of SCISTW, as shown in Figure 14C.7.
In the tank farm, there will be six sodium hypochlorite storage tanks
(each about 8m in diameter, 12.5m tall) and these tanks are surrounded by bund
walls with 2m high.
14C.119
As mentioned previously, located between the two tank farms will be the
existing
Multi-barrier Approach to Avoid Mixing
of Ferric Chloride and Sodium Hypochlorite Solutions
14C.120
Since the spilled chemicals will spread by flowing along the ground
surface, the scenario of ferric chloride mixes with sodium hypochlorite (in the
event of simultaneous tank failure in both tank farms) can be avoided by the
following design features, which serve as a “multi-barrier system”, along the
separation area between the tank farms:
·
Surface gradient to direct the
spilled chemicals to flow in opposite direction
·
The HATS Stage 2A structures as
shown in Figure 14C.7
to block the spilled sodium hypochlorite from flowing towards the ferric
chloride tank
14C.121
The objective of the above mitigation measures is to prevent the spilled
sodium hypochlorite from flowing to the northeastern corner of SCISTW, to avoid
the pools of the two spilled chemicals from contacting with each other.
14C.122
At the same time, the areas where the two pools of spilled chemicals
will be contained would be served by separate drainage systems with different
discharge points to prevent mixing of spilled chemicals in the drainage system.
14C.123
Specific design measures will be included in HATS ADF:
·
Design Feature – Containment
Structure
-
A bund wall of 2m tall will be
built around the sodium hypochlorite tank farm to contain spillage of the
chemical (up to 110% of the capacity of a single tank), which will retain the
spilled sodium hypochlorite within the bund wall in the event of storage tank
leakage.
·
Design Feature – Surface
Gradient
-
To enhance further the
effectiveness of the screening structure, the surface for the land reserved for
the future flocculation tanks and rapid mixing tanks will be designed to grade
towards the sodium hypochlorite tank farm.
This arrangement could reduce some momentum of the spilled chemical
flowing northeast when it travels uphill along the 70m long surface.
·
Design Feature – Separate
Drainage System
-
To avoid mixing of spilled chemicals
within the drainage systems, the drainage channels within the containment made
of screening structure will be connected to drainage system different from
those serving the area around the ferric chloride tank farm. The two drainage systems will discharge via
different outfall to different sea area.
14C.124
Figure 14C.12 presents the cross section from
the ferric chloride tank to the sodium hypochlorite tank farm in 2013/14
(completion of HATS Stage 2A works). The
cross sections show a series of structures and engineering measures that avoid
mixing of spilled sodium hypochlorite and ferric chloride.
14C.125
In view of the above mitigating factors or engineering controls, it is
concluded that any sodium hypochlorite solution released unintentionally from
the tank farm will not flow to the northern part of SCISTW, where the ferric
chloride tank farm is located.
14C.126
A sodium hypochlorite day tank with capacity of about
Conclusion
14C.127
The above discussion has demonstrated that in the events of low chemical
release, such as tank leakages and single-tank rupture, the provision of the
bund wall, the large separation distance and also the physical barriers between
the two chemical farms will be more than adequate to prevent the spilled
incompatible chemicals from mixing.
14C.128
Even in the extreme scenario when all storage tanks in the two tank
farms collapse simultaneously (which by the way is considered an extremely
remote scenario), the design features can prevent the spilled ferric chloride
solution from flowing to the south-eastern part of SCISTW and prevent the
spilled sodium hypochlorite from flowing to the north-eastern corner of SCISTW.
14C.129
Therefore, ferric chloride and sodium hypochlorite will not mix with
each other, under any situations at SCISTW involving unintended release of the
two chemicals from their storage tanks.
Therefore, the occurrence frequency of incompatible chemicals mixing due
to the cause “simultaneous release incompatible chemicals from storage tanks”
is zero.
Cause 3 - Simultaneous Release Incompatible Chemicals from Conveyance Pipelines
Release Mode
14C.130
The incompatible chemical mixing scenario under the cause “simultaneous
release of incompatible chemicals from conveyance pipelines” can only occur if:
·
Both the ferric chloride and sodium
hypochlorite solutions are somehow released, at the same period of time, into
unintended areas of SCISTW from their respective conveyance pipelines; and
Then, the two substances are allowed to migrate to a location(s)
where they come into contact with each other
14C.131
Unintended release of either chemical from their respective pipelines
could be caused by failure of the pipeline because of cracks, punctures,
rupture, etc. These in turn could be
caused by intrinsic factors (e.g., weathering or degradation of pipe material)
or external impacts (e.g., construction of the new plant facilities near to the
pipelines, earthquakes, malicious attacks, etc).
14C.132
The following paragraphs outline the existing/proposed engineering
control measures in place/to be implemented to prevent the ferric chloride and
sodium hypochlorite solutions from coming into contact with each other, in the
event of simultaneous failure of the pipelines leading to unplanned release of
the chemicals.
Existing Ferric Chloride Pipeline
14C.133
As shown in Figure 14C.8, the alignment of the existing ferric chloride delivery pipeline runs
from the ferric chloride tank farm to a reception point at the upstream end of
the CEPT plant. As shown in Figure 14C.9A, the ferric chloride pipeline is
protected by an external sleeve pipeline in a pipe-in-pipe configuration,
inside a buried watertight reinforced-concrete service-duct. Alongside the duty pipeline is a duplicate
(standby) pipeline that is kept empty.
14C.134
The service-duct cross-section has internal height and width of 2.2m and
3.0m, respectively, and provides for easy access by personnel for regular
maintenance and inspections. The
service-duct is buried with a minimum cover of 0.55m below ground surface. The thickness of both the wall and the roof
slab is 0.3m, and the ground slab is 0.6m thick. The service-duct structure is supported on
piles.
I. Release Mitigation Measures
14C.135
The ferric chloride solution is pumped from the tank farm to the CEPT
plant. Two measures are in place to
minimise the amount of chemical release from the pipeline in case of pipe leaks
or bursts. These are:
· While there are six storage tanks (five-duty and one-standby) in the tank farm, only one tank will be connected to the delivery pipeline at any one time. This is achieved by controlling the valves at the bottom of each storage tank.
· In addition, differentiate pipe pressure is continuously monitored by plant operators, who will shut down the pumps immediately if irregular pressure drops occur (indicating faults such as pipe leaks or bursts).
14C.136
In addition, it was proposed to install a vibration sensing system[6] at the
ferric chloride tank farm to enable shut down of the chemical pumping system
whenever excessive vibrations are detected.
The objective of the vibration sensing system is to isolate the ferric
chloride delivery pipeline from the upstream storage tanks in case of excessive
vibrations, so that the amount of chemical that can escape in the event of
failure of the pipeline due to excessive vibrations is minimised.
I.
Release during Normal Operations
14C.137
Owing to the above multi-barrier arrangement, release of ferric chloride
solution into the environment is impossible under normal operating
conditions. In this scenario, the
concern is on intrinsic factors such as weakening or failure of pipe material
due to aging, leading to leakage.
Alternatively, pipe bursting could occur at undetected weak points of
the pressurised pipeline. For this to
happen, however, the condition of the pipeline will have to deteriorate
significantly without being noticed, which is unlikely given regular inspection
and maintenance. In any case, there are
two further barriers that will prevent release of the chemical into the
environment:
· Firstly, any ferric chloride solution released from the duty pipeline due to leakage or a pipe burst will be contained by the sleeve pipeline.
·
Secondly, even if the sleeve
pipe fails, the pressure monitoring procedure would have worked to isolate the
pipeline from the upstream storage tank.
If the isolation could be effected immediately, then the amount of
chemical release would be limited to the volume of the pipeline (i.e., about
II. Release during Extreme Scenario
14C.138
In the extreme scenario, a catastrophic external impact on the chemical
pipeline is assumed to occur at SCISTW.
In this context, as the pipeline is buried underground, the external
impact may be represented by large vibrations caused by
an earthquake affecting
14C.139
In this case, the duty pipeline could burst and sleeve pipeline could
crack simultaneously, but the pressure and vibration monitoring procedures
would have worked to contain the chemical release to
14C.140
The reinforced concrete service-duct is a strong piled-structure. Therefore, it could only fail in the sense
that its watertightness might be compromised due to cracks induced by excessive
vibrations, and that it may suffer some minor structural damages, but the
service-duct should physically remain intact.
This means that the service-duct would still act a barrier to the
immediate release of the chemical solution to the environment.
14C.141
While the cracks so opened up in the reinforced concrete slab by the
vibrations would allow liquid to seep through under the force of gravity,
several mitigating factors would stop the spread of the chemical beyond the
immediate vicinity:
· Firstly, the liquid chemical in the large confines of the service-duct would be spread along the length (several hundred metres) of the service-duct. This means that the liquid chemical will not create a sizable hydraulic head that can rapidly drive the liquid through the cracks under the force of gravity only.
·
Secondly, the concrete slabs
are extremely thick (
· Thirdly, unsaturated concrete could absorb some of the liquid into its own material matrix, thereby reducing the amount of ferric chloride that could pass through.
· Fourthly, any liquid chemical that eventually migrates through the reinforced-concrete box duct, which will be relatively small in quantity, will be quickly soaked up by the vast underlying soil matrix.
14C.142
Overall, it is concluded that no ferric chloride solution can migrate
beyond the immediate alignment of the box duct even in the extreme scenario.
Sodium Hypochlorite Pipelines (to be constructed under HATS ADF)
14C.143
The alignments of the sodium hypochlorite pipelines to be constructed
under HATS ADF are shown in Figure 14C.8.
The first section (or feed section) starts at the barge unloading point
and travels in a buried reinforced-concrete pipe trench to the sodium
hypochlorite tank farm. This section of
pipeline (
14C.144
From the tank farm, the second section (or dosing section) of the
pipeline (
14C.145
The hypochlorite pipe trench is of watertight reinforced-concrete
construction. Alongside the duty
pipeline in the same trench will be a standby pipeline. Figures 14C.9A and 14C.9B show the typical cross-sections
of the feed and dosing sections, respectively.
14C.146
Similarly, a multi-barrier approach to containment of any leaks or pipe
bursts was proposed in HATS ADF:
· The duty pipeline (for both feed and dosing sections) will be protected by a sleeve pipeline, and both will be contained in a covered concrete trench
· The pipeline sections will be regularly inspected and maintained, such that any deterioration in pipe material condition will be repaired or rectified before it becomes critical
· A leak (pressure) monitoring procedure will be implemented during unloading of the hypochlorite solution from the barge
· A vibration sensing system will be installed at the hypochlorite tank farm to raise alarm for shut down of the pumping system in the event of excessive vibrations
·
Further, a section of the
hypochlorite feed pipe-trench will be
wrapped by heavy-duty impervious membrane[8]
to provide an added barrier to migration of hypochlorite solution in the
unlikely event of failure of all the other barriers. As shown in Figure 14C.8, this section has a length
of about
· Also shown in Figure 14C.8, the hypochlorite pipe trench will cross over the ferric chloride service-duct at a point near the middle of the southern site boundary. At this cross over location, the impervious membrane will be protected by a geo-textile sheet (de-bonding layer) on both sides to isolate it from the concrete surface. Details of the arrangement are shown in Figure 14C.9C.
Feed Section (from Barge Unloading Point to Storage Tank Farm)
14C.147
With respect to the sodium hypochlorite feeding pipe trench (section between
the barge unloading facility and hypochlorite storage tank), its average
cross-section area is
14C.148
The crossover section is the point where the hypochlorite pipeline will
be closest to the ferric chloride pipeline.
Therefore, in addition to the multi-barriers (i.e., sleeve pipe,
watertight reinforced-concrete pipe trench, impervious membrane, feed pressure
monitoring, vibration monitoring, etc), it is proposed to grade (drain) the
pipe trench to prevent accumulation of any spilled or leaked hypochlorite
solution inside the feed pipe trench.
The intention is to eliminate the possibility of hypochlorite migrating
to the ferric chloride pipeline located below under any circumstances. The proposed cross-over arrangement is
illustrated in Figure 14C.9C, and described below:
· The hypochlorite pipe trench normally runs flush with ground surface except at where it crosses over the ferric chloride pipeline service-duct. At this point, the former will rise above the ground surface to cross over the latter. Thereafter, the hypochlorite pipe trench will sink back to below ground surface and runs towards the hypochlorite tank farm
· To prevent accumulation of any spilled or leaked hypochlorite solution at this point (which is the highest point of the feed pipe trench), the feed pipe trench will be drained in two directions from this point:
- Moving upstream from this point, the pipe trench will be graded to fall towards the north and northeast so that any liquid inside this section of the pipe trench will be discharged into the sea via an opening at the upstream end of the pipe trench through the existing seawall near the Barge Unloading Facility.
-
Moving downstream from this
14C.149
Under normal chemical unloading conditions, any leakage or pipe burst
(due to undetected weak points in the pipe material) causing a pressure drop
will be identified by the operators, who will quickly shut down the pumps to
isolate the pipeline from the delivery barge.
Assuming that the pump rate is
14C.150
It should be noted that as the chemical is fed from the top into the
storage tanks, no hypochlorite solution could escape back into the feed pipe
under any circumstances.
14C.151
Again, for the purposes of this assessment, the extreme scenario would
be for excessive vibrations from an earthquake affecting
14C.152
However, in any case, the unloading operation will cease within a short
period after the catastrophic impact strikes, as the vibration detection system
will raise an alarm to cause the operators to stop the pump at the barge. Allowing for 10 minutes for reaction time,
the maximum quantity of chemical release into the pipe trench would be
equivalent to perhaps no more than 2.2 pipe-volumes of chemical (
14C.153
As mentioned previously, while the cracks so opened up in the reinforced
concrete slab by the vibrations would allow liquid to seep through under the
force of gravity, several mitigating factors would stop the spread of the
chemical beyond the immediate vicinity:
· Firstly, as mentioned above, the gradient in the pipe trench will quickly drain the liquid rather than allowing it to accumulate at any point in the pipe trench. This means that the condition for formation of a sizable hydraulic head that can rapidly drive the liquid through the cracks will not occur.
·
Secondly, the reinforced
concrete slab of
· Thirdly, unsaturated concrete could absorb some of the liquid into its own material matrix, thereby reducing the amount of ferric chloride that could pass through.
· Fourthly, any liquid chemical that eventually migrates through the reinforced-concrete pipe trench, which will be relatively small in quantity, will be stopped by the heavy-duty impervious HDPE membrane. The membrane will probably deform under stress, but owing to its high tensile strength, it will not tear, break, or puncture in the circumstance.
14C.154 Overall, it is concluded that no sodium hypochlorite solution can migrate beyond the immediate alignment of the concrete pipe trench even in the extreme scenario. An assessment of the potential for the sodium hypochlorite solution released from the feed pipeline to mix with the ferric chloride solution simultaneously released from the ferric chloride is summarised in Section 14C.157.
Dosing Section (from Hypochlorite Tank Farm to Day Tank)
14C.155
The cross sectional area of the sodium hypochlorite dosing pipe trench
(from storage tank to day tank) is about
14C.156
Under normal conditions, any leakage or pipe burst (due to undetected
weak points in the pipe material) can be quickly identified by the pressure
monitoring procedure and rectified by shut down of the upstream valve
connecting it to the storage tank. As
mentioned previously, if instantaneous isolation is achieved, the volume of
chemical that can be released cannot be more than the capacity of the duty
dosing pipeline, which is about p x 0.0752 x 332 =
14C.157
In case the alarm system was to fail or the operators were unable to
shut down the system for whatever reasons, a whole tank load of sodium hypochlorite
could be drained. However, even in this
case, mixing of the two chemicals will not be possible, as there is significant
separation (
14C.158
In conclusion, in the extreme scenario of an earthquake affecting Hong
Kong, the vibration sensing system, the reinforced concrete pipe trench, and
the underlying soil matrix will work together to stop chemical migration beyond
the immediate vicinity of the pipe trench alignment. In any case, as mentioned earlier, the
presence of a massive barrier (in the form of the existing sedimentation tanks)
will prevent any released hypochlorite solution from reaching the ferric
chloride pipeline located some
Assessment of Possibility of Mixing of Incompatible Chemicals
14C.159
For mixing of sodium hypochlorite and ferric chlorides solutions to
occur, they must be released from their respective pipelines at the same time
due to a rare catastrophic impact (e.g., a significant earthquake), and then be
allowed to migrate to a point(s) where they meet.
· For the feed section of the hypochlorite pipeline, the most critical section for mixing is where the two pipelines run parallel to each other (as shown in Figure 14C.8). Here, the extreme case will be an earthquake (occurring at the same time as unloading of the hypochlorite solution) leading to excessive vibrations that simultaneously damage the two pipelines and associated ducting/trenches. However, as discussed above, mixing of the two chemicals will still not be possible due to the multi-barriers, in particular the functioning of the heavy-duty impervious membrane (which might deform but will not tear or fail) which will totally contain the hypochlorite solution in this extreme circumstance.
·
For the dosing section of the
hypochlorite pipeline, it is both remote and separated from the ferric chloride
pipeline. As shown in Figure
14C.8, the two pipelines
are located on opposite sides of the existing sedimentation tanks (which may be
regarded as a massive physical barrier) with a separation of about
· Therefore, mixing of the two chemicals is not possible in any circumstances.
14C.161
In conclusion, ferric chloride and sodium hypochlorite will not mix with
each other, under any situations at SCISTW involving unintended release of the
two chemicals from their respective pipelines.
A range of mitigation measures has been proposed, including measures to
drain the released hypochlorite solution rapidly to the sea or an underground
sump pit even under the extreme scenario.
Therefore, the occurrence frequency of incompatible chemicals mixing due
to the cause “simultaneous release incompatible chemicals from conveyance
pipelines” is zero.
Cause 4 - Simultaneous Failure of Storage Tanks and Pipelines
14C.162
This scenario is a combination of Causes 2 and 3, and refers to mixing
due to the simultaneous failure of pipeline and storage tank. It can
only occur if:
· Both the ferric chloride and sodium hypochlorite solutions are somehow released, at the same period of time, into unintended areas of SCISTW from their respective conveyance pipeline and storage tank; and
· Then, the two substances are allowed to migrate to a location(s) where they come into contact with each other
14C.163
For this scenario, two cases are considered:
· Case 1 – Simultaneous Failure of a Ferric Chloride Tank and a section of the Sodium Hypochlorite Pipeline
· Case 2 – Simultaneous Failure of a Hypochlorite Tank and a section of the Ferric Chloride Pipeline
Case 1
14C.164
The Ferric Chloride Tank Farm is located on the eastern edge of the
SCISTW site adjacent to the sea, as shown in Figure
14C.8.
As mentioned earlier, failure of any ferric chloride storage tank will
lead to a release of the chemical. As
the tank farm is surrounded by a
14C.165
In the event of tank rupture, however, some of the released chemical
solution could overtop the bund wall. As
the tank farm is shielded on the western side by the massive
14C.166
Therefore, in the extreme scenario of a seismic event occurring during
unloading of sodium hypochlorite solution and simultaneous failure of the feed
hypochlorite pipeline/pipe-trench and one or more ferric chloride storage
tanks, there is a potential for the two chemicals to mix with each other.
14C.167
To prevent the mixing of incompatible chemicals from occurring in the
extreme scenario, the following precautionary or mitigation measures are
proposed (Figure 14C.8
and 14C.9D refers):
·
Increase the height to 3.0m of a (
·
Construct a raised road kerb with height of
· Construct a drain at the foot of the kerb to collect and drain any spilled ferric chloride solution to the sea.
14C.168
The following calculations explain how the abovementioned precautionary
mitigation measures could avoid released ferric chloride solution from storage
tank spreading to the barge unloading facility.
14C.169
Thyer et al. (2002) give the following correlation for the bund
overtopping fraction Q over vertical bund walls:
· Q = 0.044 – 0.229 ln(h/H) – 0.116 ln(r/H)
·
Where Q = bund overtopping
fraction; h = bund wall height (which is 3.0m in this case); H = tank liquid
height (which is 8.61m), and r = distance from the centre of the Tank A to the
bund wall (which is
14C.170
This gives Q = 0.044 – 0.229 ln(3.0/8.61) – 0.116 ln(5/8.61), that is Q
= 0.348. In other words, 34.8% of the
liquid in the tank could overtop the bund wall.
Therefore, the volume of ferric chloride solution that could overtop the
bund wall is = 433m3 x 0.348 = 150.7m3.
14C.171
Considering a circular liquid
pool is formed by the 150.7m3 of ferric chloride solution with
radius of
14C.172
In conclusion, with the above
proposed precautionary mitigation measure, it will be impossible for the two
chemicals to mix in any circumstances at this location.
Case 2
14C.173
As shown in Figure
14C.8, the Hypochlorite
Tank farm is located on the western side of the SCISTW site. The nearest section of the ferric chloride
pipeline is located about
Conclusion
14C.174
In conclusion, with the proposed
precautionary design features (i.e., For Case 1, measures listed in Section
Operation and Construction of the New Plant Facilities
14C.175 A comprehensive systematic brainstorming HAZOP workshop was conducted in January 2008 for the operation and construction of the new plant facilities. Hazards associated with the operation of the new plant facilities were discussed. Further, detailed procedures for the construction and associated hazards, as well as the implementation of safety and precautionary measures for construction activities has been discussed and agreed in the HAZOP. Details of hazards identified are given in the hazard register and meeting minutes attached in Appendix 14C.8.
Operation of New Plant Facilities
14C.176 The proposed influent pumping station, CEPT tanks and sludge treatment facilities are the extensions of the existing SCISTW facilities. Also, there are no hazardous chemicals or dangerous goods involved in the proposed odour control facilities and the proposed effluent tunnel. As such, there is no risk due to the new plant facilities that would cause hazardous interaction with the disinfection chemicals (i.e. sodium hypochlorite and sodium bisulphite).
14C.177 The proposed electrical substation (132kV / 11kV) is located next to the proposed dechlorination facilities (Figure 14C.7). A typical substation of CLP consists of four 132kV / 11kV transformers, relevant switchboxes and panels. It would not impose any hazards to the adjacent facilities under normal operation but there would be risk of explosion escalated from short circuit fire. However, it should be noted that even if explosion of the electrical substation does occur, incompatible mixing is impossible, as the substation would be located more than 200m away from the sodium hypochlorite or ferric chloride pipelines or storage tank. Only spillage of sodium bisulphite is anticipated. No toxic gas generation and offsite fatality associated with the disinfection facilities due to the operation of the new plant facilities would result. Therefore, the occurrence frequency of incompatible chemicals mixing caused by the electrical substation operation is zero.
Construction of New Plant Facilities
14C.178 Construction method of the new plant facilities, associated hazards and precautionary measures have been specified in the HAZOP held in January 2008. Details are given in the hazard register and meeting minutes attached in Appendix 14C.8.
14C.179 The deep effluent tunnel will be constructed using Tunnel Boring Machine or drill and blast method. The location of the proposed effluent tunnel is near to the hypochlorite dosing pipelines but remote and far away from the ferric chloride pipelines (Figure 14C.8). The ferric chloride pipelines is located at the opposite side of the existing sedimentation tanks (which is regarded as a massive physical barrier) with a separation of about 160m. It is therefore impossible for ferric chloride and sodium hypochlorite released from their respective pipelines to reach each other even if there were simultaneous failures of these pipelines caused by vibration or settlement arising from the drill and blast construction of the deep effluent tunnel.
14C.180 Other new plant facilities will be constructed using conventional methods including excavation and piling. Given that the proposed influent pumping station and proposed sludge holding tanks are located close to the sodium hypochlorite and ferric chloride pipelines, more thorough discussion were given in the HAZOP. Excavation with diaphragm wall techniques would be applied for influent pumping stations construction while bore piling instead of percussion piling would be used for the sludge treatment facilities construction so to keep the vibration to a minimum. It was also proposed to maximise the distance between the excavation works and the pipelines. Since the effect of diaphragm wall techniques and bore pile are anticipated to be no worse than the effect of the external events like earthquake and aircraft crash, incompatible substance come into contact with each other to cause incompatible mixing is impossible with the existing / proposed engineering control measures applied as stated in Section 14C.120 to 14C.126. Nevertheless, other general precautionary measures would further be implemented to reduce the probability of damaging the storage tanks and pipelines. The proposed mitigation measures during construction have been categorized and tabulated in Table 14C.5.
Table 14C.5 Mitigation Measures during construction associated with SCISTW
|
Ref No. |
Major Construction
Activity |
Anticipated Mitigation
Measures[12] |
|
- |
General |
Ø
Set up monitoring system
for vibration and settlement control Ÿ
Employ vibration detectors and settlement markers Ÿ
Develop action plan(s) for situations where vibration or settlement
level is found to exceed the set limits
(vibration limit would be in the order of 10 mm/s; settlement limit: would be
in the order of 25mm) Ø Strict Traffic
Management Ÿ
Designated delivery route and off-loading area
for delivery trucks Ø Close supervision and monitoring by safety officers Ÿ
If there is any construction work within 2m of
the pipelines, an immediate inspection to the pipeline section and the
impervious membrane wrapping should
be conducted to ensure no damage to the integrity of the pipeline and the
membrane Ÿ
Report any
damage of the disinfection facilities to operators for remedial actions Ø Provide
indication / signs for sodium hypochlorite pipeline and associated impervious
membrane wrapping, as well as the ferric chloride pipelines Ø Any damage
during construction work to the sodium hypochlorite/ ferric chloride pipelines/tanks would
be immediately repaired to its original safety function Ø Regular
checking of chemical delivery pipelines Ø Provide a physical barrier between the
sodium hypochlorite tanks and the ferric chloride tanks during the
construction stage before the new above ground structures for HATS Stage |
|
1 |
Excavation works |
Damage of chemical
storage / delivery facilities Ø
General precautionary measures to be implemented Ø
Close liaison with plant operators should be
maintained at all times to minimize impacts during chemical deliveries and
blasting operations Ø
Excavation running close or parallel to sodium hypochlorite
delivery pipelines and associated impervious membrane wrapping under road /
pavement shall be avoided as far as possible |
|
2 |
Piling works |
Ø
General precautionary measures to be implemented Ø
Use bore piles instead of percussion piles in order
to keep vibration to a minimum Ø Maximise the
distance between piling and delivery pipelines, as well as the associated
impervious membrane wrapping. Monitor vibration resulted from construction
works to ensure the velocity and amplitude of vibration limit will not be
exceeded |
|
3 |
Installation of electrical and mechanical equipments |
Ø
General precautionary measures to be implemented Ø
Designated routes for delivery trucks Ø
Designation of off-loading area Ø
Effective traffic management Ø Conduct hazard assessment
and obtain Hot Work Permit before starting welding / hot works |
|
4 |
Mobilising and usage of construction
equipment (e.g. drill rig, backhoe, bulldozer, dump truck, site vehicle etc) |
Construction plant
overturns / crashes Ø
Control of truck transportation route Ø
Stability of the equipment should be checked and
certified Ø
Location of any large-scale or high-elevated
equipment should be agreed with SCISTW operator before delivery Mitigation measures
associated with crane operation Ø Close
inspection and supervision to ensure proper operation of construction plants |
|
5 |
Welding / Hot works |
Ø
To confine hot works in designated
areas. Ø
Area within the boundaries of chemical
storage facilities are subject to hazardous area control Ø Conduct hazard assessment and obtain Hot Work Permit before starting welding / hot works |
|
6 |
Shafts construction |
Ø
General precautionary measures to be
implemented (e.g. probing) Ø
Provide lateral support to excavation Ø
No explosives to be stored on-site or
used close to the chemical storage compounds Ø
Maximise the distance between the tunnel
shaft and delivery pipelines Ø
To adopt a blasting vibration limit in order to minimise the impact upon
storage tanks and pipelines during blasting operations |
14C.181 As stated above, the proposed precautionary measures would eliminate the risk of damage to the tanks,, pipelines and the impervious membrane wrapping. Furthermore, the existing/proposed engineering control measures stated in Section 14C.120 to 14C.126 would also prevent two incompatible substances come into contact with each other to cause incompatible mixing. No toxic gas generation and offsite fatality associated with the construction of the new plant facilities would occur. Therefore, the occurrence frequency of incompatible chemicals mixing caused by the construction of new plant facilities is zero.
Frequency Estimation
Frequency Estimation based on Incident Review Data
14C.182 As mentioned above, the chemical spillage scenarios will not cause off-site fatality under the circumstances of the current Project. Therefore, the frequency of such scenarios to cause off-site fatality is zero.
14C.183 The occurrence frequency of “mixing of incompatible chemicals on-site” may be determined based on an incident review. By using the incident review results, the occurrence frequency of the scenario of ‘mixing of incompatible chemicals on-site” for the Project can be determined by the following equation:
Occurrence frequency = A / B Equation 1
Where,
A = No. of incompatible chemicals mixing incidents in wastewater disinfection facilities (or similar facilities, e.g. wastewater recycling plant) identified in incident review
B = The population of wastewater disinfection facilities (in terms of facility-year) located in the region during the period that the incident review has covered
14C.184
A comprehensive review of
recorded international incidents involving mixing of either sodium hypochlorite
or sodium bisulphite with an incompatible chemical has been conducted, as
summarized in Appendix
· This showed that mixing did occur at leisure centres (swimming pools) and some industrial facilities, because of unloading errors or spillage/leakage from storage tanks, but none of these incidents caused fatality.
· Further, the review did not find any incompatible chemicals mixing incidents in wastewater disinfection facilities (i.e. A = 0). This may be explained by the generally stringent safety measures (particularly in handling chemicals) adopted in wastewater disinfection facilities.
14C.185 For the estimation of parameter “B”, since the incident review has covered the incidents occurring in Europe and the USA for the period from 1970 to 2005, the population of wastewater disinfection facilities (that use hypochlorite) located in Europe and the USA for the above period was determined. Based on the results of survey on disinfection practice in coastal cities conducted under this Study, 57 wastewater treatment works (that use sodium hypochlorite for disinfection) were identified in 10 cities with a total population of 37,355,000. That is, a wastewater disinfection facility using sodium hypochlorite would serve a population of about 655,351 people.
14C.186
The human population at 1990
was taken to represent the population for the period from 1970 to 2005. The population of Europe and the
14C.187
From the above data, the
population (in terms of facility-year) of the wastewater disinfection facility
in Europe and the
14C.188 None of the identified incompatible chemicals mixing incidents occurred in wastewater disinfection facilities. This shows that such mixing incident has not occurred in the experience history of wastewater disinfection facilities. Where an incident that could occur (but has not occurred) it is normal to base the expected number of incidents on a 50% chance of it having (or not having) occurred, leading to an estimate of 0.7 incidents based on a Poisson distribution. The probability of not having seen an incident as a function of the expected number of incidents based on a Poisson distribution is shown below.
14C.189
Generally, wastewater
disinfection facilities are operated by experienced and trained personnel in an
organized and careful manner. Design and
operational measures for wastewater disinfection facilities are mature and well
developed. Therefore, assuming a 50%
chance of occurrence of mixing of incompatible chemicals, per the general
Poisson model, leading to an estimate of 0.7 incidents, may be overly
pessimistic for wastewater disinfection facilities. For a less pessimistic but yet cautious
estimate, we propose to apply a factor of “
14C.190 Therefore, the generic occurrence frequency of incompatible chemicals mixing incident in wastewater disinfection facilities is estimated to be = 0.35 / 51,765 = 6.76 x 10-6 per year.
Adjustment of Generic Occurrence Frequency based on Project-specific Circumstances
14C.191
Next, the generic occurrence
frequency of 6.76 x 10-6 per year for incompatible chemicals mixing
incident in wastewater disinfection facilities needs to be adjusted to take
into account the specific circumstances of the Stage
14C.192 In considering what might be a reasonable adjustment factor, the following aspects may be relevant:
·
Precautionary measures to avoid
incompatible chemicals mixing in Stage
·
Number of transportation modes
for chemical delivery to Stage
·
Number of incompatible
chemicals to be handled at Stage
·
Number of chemicals delivery to
be made to Stage
Precautionary Measures to Avoid Incompatible Chemicals Mixing
14C.193
As mentioned above, a comprehensive package of
precautionary measures, including special chemical supply contract arrangement,
design measures as well as operation procedures and safety measures, has been
developed for HATS ADF and Stage
14C.194 Referring to the surveyed wastewater disinfection plants, these included facilities that have been operating for a long period. For example, some of the wastewater disinfection facilities using sodium hypochlorite were commissioned in the 1970s or earlier years.
14C.195
Therefore, while chemical hazard control
measures are routinely adopted in wastewater disinfection facilities, it is
likely that the level and sophistication of the precautionary measures adopted
in the proposed Stage
14C.196
This suggests that, with respect to precautionary
measures for avoiding unloading error, the proposed Stage
Number of Transportation Mode for Chemical Delivery
14C.197
In HATS Stage
14C.198
For wastewater disinfection plants in general, they could involve one or
more modes of chemical delivery.
Obviously, the minimum would be “one”, either by land or by sea. On this basis, and considering the unique
chemical delivery arrangement for the Stage
Number of Incompatible Chemicals to be Handled
14C.199
Three incompatible chemicals
(sodium hypochlorite, sodium bisulphite, and ferric chloride) will be handled
in SCISTW in Stage
14C.200
It is not clear from the
incident review or survey the average number of incompatible chemicals that are
handled in wastewater disinfection plants in general. Obviously, the minimum would be “two”, but
for those chlorination plants employing dechlorination, the number could be
“three”, as in the case of the Stage
14C.201
On this basis, and considering the unique
chemical delivery arrangement for the Stage
Number of Chemical Delivery to be Made
14C.202
The Stage
14C.203
Owing to the larger scale of the Stage
14C.204
However, for the Stage
14C.205
In view of this barge delivery arrangement, the number of land delivery
of sodium hypochlorite solution to the Stage
Conclusion
14C.206 The above assessment suggests that:
·
The Stage
· On the number of transportation mode and number of incompatible chemicals, no adjustment to the generic frequency is proposed.
·
The larger scale/capacity of the Stage
The first and last point would act
to counter-balance the adjustment factor to be applied to the Stage
14C.207
Nevertheless, as a conservative approach, we
propose to apply an upward adjustment factor of 8 to the generic
frequency. This means that the occurrence frequency of
incompatible chemicals mixing incident of Stage
= 6.76 x 10-6 per year x 8
= 5.408 x 10-5 per year.
Reference to Estimated Frequency in other Relevant Project
14C.208 Reference is drawn to the approved EIA Study Report for the Hong Kong International Theme Park (HKITP), which provides relevant information on the occurrence frequency of incompatible chemical mixing in its on-site attractions water disinfection facility (also using sodium hypochlorite solution). In this case, the frequency of occurrence of mixing sodium hypochlorite with an incompatible chemical has been estimated at 3 x 10-6 per year.
14C.209
In the HKITP operation, it was estimated that
the operation of the attractions water disinfection
facility would involve about 312 chemical deliveries per year. For the Stage
14C.210
In comparing the occurrence frequency and conditions of both HATS Stage
Consequence Analysis and Risk Summation
Determination of Toxic Gas Generation Rate
14C.211
In HATS Stage
· Sodium hypochlorite is unloaded into ferric chloride tank, generates chlorine gas – event 1
· Ferric chloride is unloaded into sodium hypochlorite tank, generates chlorine gas – event 2
· Sodium bisulphite is unloaded into ferric chloride tank, generates sulphur dioxide gas – event 3
· Ferric chloride is unloaded into sodium bisulphite tank, generates sulphur dioxide gas – event 4
· Sodium hypochlorite is unloaded into sodium bisulphite tank, generates heat (but no toxic gas would be generated) – event 5
· Sodium bisulphite is unloaded into sodium hypochlorite tank, generates heat (but no toxic gas would be generated) – event 6
14C.212 The rate of toxic gas generation in events 1 to 4 is estimated by the following steps:
Determination of Toxic Gas Generation Reaction
14C.213 When ferric chloride solution is accidentally mixed with sodium hypochlorite / sodium bisulphite, the hydrochloric acid in the ferric chloride solution will react with sodium hypochlorite / sodium bisulphite to form chlorine / sulphur dioxide gas. The chemical reactions and the molecular weight of the reactants are written as follows:
· Sodium hypochlorite reacts with hydrochloric acid to give chlorine gas
NaOCl + HCl à NaOH + Cl2
74.45 36.45 40.00 70.91
· Sodium bisulphite reacts with hydrochloric acid to give sulphur dioxide gas
NaHSO3 + HCl à H2O + NaCl + SO2
104.06 36.45 18.02 35.00 64.1
Determination of Density and Concentration of Involved Chemicals
14C.214 According to the Material Safety Data Sheet, the density and concentration of the involved chemicals are as follows:
·
Sodium hypochlorite – density:
·
Sodium bisulphite – density:
·
Ferric chloride – density:
14C.215 Combining the unloading rate of the chemical with the abovementioned data, the toxic gas generation rate of the incompatible chemicals mixing events can be calculated. The toxic gas generation rate is estimated based on the conservative assumption that there would be perfect mixing and reaction between the incompatible chemicals. In the real situation, chemical is loaded from high level of storage tank (i.e. above the liquid level of chemical present in the tank) and therefore the loaded chemical and the chemical present in the tank will not perfectly mix with each other. Hence, the toxic gas generation rates estimated below are at the conservative side.
14C.216 The calculation of toxic gas generation rate for events 1 to 4 is shown below:
I. Event 1
14C.217
The sodium hypochlorite unloading rate by the road tanker is assumed to
be
II. Event 2
14C.218
The ferric chloride unloading rate by the road tanker is assumed to be
III. Event 3
14C.219
The sodium bisulphite unloading rate by the road tanker is assumed to be
IV. Event 4
14C.220
The ferric chloride unloading rate by the road tanker is assumed to be
14C.221 The sodium hypochlorite, sodium bisulphite and ferric chloride storage tanks are atmospheric tanks equipped with vent pipe for venting of built up air / gas inside the tanks. When the air / gas pressure inside the tanks is higher than the atmospheric pressure outside, the built up air / gas will be released to the outside atmosphere through the vent pipe. Since the toxic gas will be continuously generated during the wrong chemical unloading operation, it is reasonable to assume that the toxic gas release rate (from storage tank to the atmosphere) is equal to the estimated toxic gas generation rate.
14C.222 The toxic gas generated will be released through the vent pipe at the top of the storage tank. Therefore, the toxic gas will be released at an elevation level similar to the top of the storage tank.
V. Events 5 and 6
14C.223 No toxic gas will be generated in Events 5 and 6.
14C.224
Table
Table
|
Event No. |
Incompatible Chemicals
Mixing Event |
Toxic Gas Generation
Rate |
Toxic Gas Release
Location |
Toxic Gas Release Height |
|
1 |
Sodium hypochlorite unloaded into ferric chloride tank |
Chlorine, |
Ferric chloride tank farm |
|
|
2 |
Ferric chloride unloaded into sodium hypochlorite tank |
Chlorine, |
Sodium hypochlorite tank farm |
|
|
3 |
Sodium bisulphite unloaded into ferric chloride tank |
Sulphur dioxide, |
Ferric chloride tank farm |
|
|
4 |
Ferric chloride unloaded into sodium bisulphite tank |
Sulphur dioxide, |
Sodium bisulphite tank farm |
|
|
5 |
Sodium hypochlorite unloaded into sodium bisulphite tank |
No toxic gas generated |
||
|
6 |
Sodium bisulphite unloaded into sodium hypochlorite tank |
No toxic gas generated |
||
Consequence and Risk Summation Modelling
14C.225
Risk Software SAFETI micro was
used as the tool to execute the consequence analysis and risk summation. SAFETI
micro is a consequence and risk summation model, which can handle heavy gas
(such as chlorine) dispersion and has been used in previous risk assessment
studies in
Frequency of the Incompatible Chemicals Mixing Events
14C.226
As
presented above, the occurrence frequency of incompatible chemicals mixing
incident of Stage
14C.227 The occurrence frequency of incompatible chemicals mixing incident, which is due to the chemical unloading error, is estimated to be 5.408 x 10-5 per year. There would be 1,602 chemical unloading operations made per year and therefore each unloading operation would contribute to an occurrence frequency of 3.38 x 10-8 per year (= 5.408 x 10-5 / 1,602).
14C.228 As an example, if chemical unloading operation error occurs in a sodium hypochlorite delivery operation, the delivered chemical is not unloaded into the sodium hypochlorite tank (the intended storage tank) and would be unloaded into either sodium bisulphite or ferric chloride storage tank. Since dedicated road tanker transport route will be assigned to each chemical, the probable cause of the unloading operation error would be the road tanker driver enters the SCISTW site at the wrong entrance. It is considered that the probability of the road tanker driver enters the wrong entrance for ferric chloride tank farm would be the same to that for entering the wrong entrance for sodium bisulphite tank farm. Hence, in case of chemical unloading operation error, the probability of delivered sodium hypochlorite being mistakenly unloaded into the ferric chloride tank would be equal to that being mistakenly unloaded into the sodium bisulphite tank, which is equal to 0.5.
14C.229
The
situation is similar to chemical unloading error in sodium bisulphite and
ferric chloride delivery operation.
Based on the above, the occurrence frequency of each incompatible
chemicals mixing event can be estimated as shown in Table
Table
|
Event No. |
Event Description |
Estimated Frequency |
|
1 |
Sodium hypochlorite unloaded into ferric chloride tank |
= 3.38 x 10-8 per year (freq. of error of an unloading operation) x 510 (no. of NaOCl unloading operation) x 0.5 (prob. of NaOCl being unloaded into ferric chloride tank, given error occurs) = 8.619 x 10-6 per year |
|
2 |
Ferric chloride unloaded into sodium hypochlorite tank |
= 3.38 x 10-8 per year x 780 (no. of FeCl3 unloading operation) x 0.5 (prob. of FeCl3 being unloaded into sodium hypochlorite tank, given error occurs) = 1.318 x 10-5 per year |
|
3 |
Sodium bisulphite unloaded into ferric chloride tank |
= 3.38 x 10-8 per year x 312 (no. of NaHSO3 unloading operation) x 0.5 (prob. of NaHSO3 being unloaded into ferric chloride tank, given error occurs) = 5.273 x 10-6 per year |
|
4 |
Ferric chloride unloaded into sodium bisulphite tank |
= 3.38 x 10-8 per year x 780 (no. of FeCl3 unloading operation) x 0.5 (prob. of FeCl3 being unloaded into sodium bisulphite tank, given error occurs) = 1.318 x 10-5 per year |
|
5 |
Sodium hypochlorite unloaded into sodium bisulphite tank |
= 3.38 x 10-8 per year x 510 (no. of NaOCl unloading operation) x 0.5 (prob. of NaOCl being unloaded into sodium bisulphite tank, given error occurs) = 8.619 x 10-6 per year |
|
6 |
Sodium bisulphite unloaded into sodium hypochlorite tank |
= 3.38 x 10-8 per year x 312 (no. of NaHSO3 unloading operation) x 0.5 (prob. of NaHSO3 being unloaded into sodium hypochlorite tank, given error occurs) = 5.273 x 10-6 per year |
|
|
Total |
5.408 x 10-5 per year |
14C.230 Wrong chemical unloading operation would be detected by the installed toxic gas detectors (and alarm will be annunciated) and stopped rapidly by automatic closure of emergency shutdown valve upon alarm activation. Also, as chemical supplier staff and SCISTW operator will be present throughout the chemical unloading operation, they can stop the chemical unloading operation by turning off the pump for pumping the chemical to the storage tank when they notice the activation of the alarm or other abnormal conditions. With the gas detector activated automatic stoppage system, toxic gas emission could be stopped almost immediately and the staff on-site provides a backup mechanism to stop the wrong chemical unloading operation. The time needed for the rapid stoppage of wrong chemical unloading operation is assumed to be 3 minutes. This assumption of the time needed for isolation by such stoppage system (gas detector automatic stoppage system, with manual stoppage as backup) is consistent with previous risk assessment study (Meinhardt Infrastructure and Environment Limited, 2007) considering similar emergency shutdown systems. Events 1b, 2b, 3b and 4b are developed to represent the above scenario.
14C.231 It is specified that the gas detector activated automatic stoppage system shall meet the Safety Integrity Level (SIL) 2. According to International Electrotechnical Commission (1997), a system meeting SIL 2 shall have a failure probability (on demand) of 0.001 to 0.01. Taking into account the extreme case of the automatic stoppage system failure, a failure probability of 0.01 (higher bound failure probability for SIL 2) is taken for the rapid stoppage of unloading operation.
14C.232
In the case of automatic unloading operation
rapid stoppage failure, the chemical unloading operation can be stopped by the
SCISTW operator by turning off the pump for pumping the chemical to the storage
tank. Nevertheless, it is conservatively
assumed that the wrong chemical unloading operation does not stop until all
chemical in the road tanker is unloaded.
Such scenario is represented by Events
Table
|
Event No. |
Release Material |
Release Rate |
Occurrence Frequency |
Release Location |
Toxic gas Release Duration |
Rapid Stoppage of Wrong Unloading Operation? |
|
|
Cl2 |
|
8.619 x 10-8 Per year |
FeCl3 tank farm |
1700sa |
No |
|
1b |
Cl2 |
|
8.533 x 10-6 Per year |
FeCl3 tank farm |
180sb |
Yes |
|
|
Cl2 |
|
1.318 x 10-7 Per year |
NaOCl tank farm |
1700sa |
No |
|
2b |
Cl2 |
|
1.305 x 10-5 per year |
NaOCl tank farm |
180sb |
Yes |
|
|
SO2 |
|
5.273 x 10-8 per year |
FeCl3 tank farm |
1700sa |
No |
|
3b |
SO2 |
|
5.220 x 10-6 per year |
FeCl3 tank farm |
180sb |
Yes |
|
|
SO2 |
|
1.318 x 10-7 per year |
NaHSO3 tank farm |
1700sa |
No |
|
4b |
SO2 |
|
1.305 x 10-5 per year |
NaHSO3 tank farm |
180sb |
Yes |
|
5 |
Event not modelled as no toxic gas is generated |
|||||
|
6 |
||||||
Note: a The release duration is the time for the chemical
delivery road tanker to unload all the chemical delivered, where the duration
is = capacity of road tanker / chemical unloading rate =
b The wrong unloading operation is rapidly
stopped within 3 minutes (180s)
Evaluation of Toxic Impact
14C.233 In SAFETI micro, the probability of death due to toxic gas impact at a point is calculated by the “probit equation”, Pr = a + ln L, where “Pr” is the probit value, “a” is probit equation constant and “L” is the toxic load. The Probit value can be transformed to probability of fatality. For example, probit value of 6.28, 5.00, 3.12 and 2.67 corresponds to fatality probability of 90%, 50%, 3% and 1% respectively.
14C.234 Toxic load “L” is calculated by the following equation, where the toxic load at a particular point is dependent to the toxic gas concentration-time history over the point:
T
L = ò Cn dt,
0
where “T” = duration of toxic gas exposure
“C” = concentration of toxic gas
“n” = probit equation constant
14C.235 The probit equation constants applied for chlorine and sulphur dioxide gas are listed as follows, which are consistent with the probit constants recommended by the Dutch Government (TNO, 1992):
|
|
a |
n |
|
Chlorine |
-14.3 |
2.3 |
|
|
-19.2 |
2.4 |
14C.236 The maximum duration of toxic gas exposure by population for each event is taken as 10 minutes. Even in the cases of failure to stop the wrong chemical unloading operation rapidly (toxic release duration = 1700s), the population exposed to the released toxic gas shall be able to take action to escape from the toxic gas cloud rather than remains at the original location and exposes to the toxic gas for the whole toxic gas release period. Therefore, the maximum duration of toxic gas exposure in these cases is limited to 10 minutes. This assumption is consistent with the previous risk assessment studies for local water treatment works (ERM, 2001).
14C.237 According to the User Manual of SAFETI micro, in the case of toxic gas continuous release, the duration of the toxic cloud’s passage at various locations is equal to the duration of the toxic gas release. Therefore, for Events 1b, 2b, 3b and 4b that rapid stoppage of wrong chemical unloading operation is successful, the toxic gas exposure duration is same as the toxic gas release duration (3 minutes).
Model Parameters
14C.238 The relevant SAFETI parameters are presented in Appendix 14.5. By adopting the parameters, probability of fatality for a person indoor is 10% of that for a person remains at outdoor environment.
14C.239
By extracting the modelling results from SAFETI micro, the estimated fatality probability at various distances (at
Table
|
Hazardous Event |
Distance to Receive 3%
Fatality Probability |
Distance to Receive 1% Fatality Probability |
|
Event |
tank farm |
|
|
Event 1b |
tank farm |
tank farm |
|
Event |
|
tank farm |
|
Event 2b |
|
tank farm |
|
Event |
tank farm |
|
|
Event 3b |
tank farm |
tank farm |
|
Event |
|
|
|
Event 4b |
Fatality probability at |
|
Population Input
14C.240
Since the outdoor population / indoor fraction of the population at
locations listed in Table
14C.241 After the population data, meteorological data and failure case was input, SAFETI micro combined these input data and characterized the risk levels in terms of individual risk (presented by individual risk contours) and societal risk (presented by FN curves and Potential of Loss of Life).
Evaluation of Hazard to Life Impact
Introduction
14C.242 Individual risk is a measure of the risk to a chosen individual at a particular location. As such, this is evaluated by summing the contributions to that risk across a spectrum of incidents that could occur at a particular location.
14C.243 Societal risk is a measure of the overall impact of an activity upon the surrounding community. As such, the likelihoods and consequences of the range of incidents postulated for that particular activity are combined to create a cumulative picture of the spectrum of the possible consequences and their frequencies. This is usually presented as an FN curve and the acceptability of the results can be judged against the societal risk criterion under the risk guidelines.
14C.244 The hazard distance calculated by SAFETI micro was compared with the assessment results in previous hazard assessment study for local water treatment works. It was found that modelling results by SAFETI micro are comparable to the previous assessment under weather class with neutral stability. For weather class with stable condition, the modelling results of SAFETI micro were found to be more conservative than that of the previous hazard assessment study. Details of the comparison of estimated hazard distance are presented in Appendix 14.6.
Assessment Results – Individual Risk
14C.245 The associated individual risk levels are shown in Figure 14C.14. The risk levels are based on a 100%-occupancy, which can be referred from the user manual of SAFETI micro. Two risk contours of individual risk level of 1x10-7, and 1x10-8 per year are shown. As no off-site location would experience an individual risk level of greater than 1x10-5 per year, it can be concluded that the level of individual risk associated with the Project should be acceptable when compared to the individual risk guideline stipulated in Annex 4 of the EIAO TM.
Assessment Results – Societal Risk
14C.246
Table
Table
|
|
||
|
Event No. |
Fatality of 1 or more |
Fatality of 10 or more |
|
7.03E-08 |
1.18E-09 |
|
|
1b |
3.40E-06 |
0 |
|
1.27E-08 |
||
|
2b |
3.64E-07 |
0 |
|
3.74E-08 |
0 |
|
|
3b |
9.10E-07 |
0 |
|
4b |
1.70E-06 |
0 |
Note: Cumulative frequency
for fatality of 1 or more of all events = 6.53 x 10-6 per year
14C.247 The FN curves showing the societal risk level with fractional fatalities rounded up to 1 are shown in Figure 14C.15. As shown in the figure, the societal risk associated with the Project was estimated to be in the “acceptable” region. Therefore, it can be concluded that the level of societal risk associated with the Project should be acceptable when compared to the societal risk guideline stipulated in Annex 4 of the EIAO TM.
Conclusion
14C.248 Hazard to life impact associated with the proposed disinfection facilities at SCISTW was quantitatively assessed. The risk of identified hazardous scenarios involving sodium hypochlorite and sodium bisulphite was quantitatively assessed, with consideration of identified precautionary measures / operation procedures that minimize the risks associated with the chemicals related operations.
14C.249
The
individual risk and societal risk associated with the chemicals related
operations were found to be acceptable in accordance with the risk guidelines
stipulated in the Annex 4 of the EIAO TM.
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