Abu Dhabi National Oil Company
HEALTH, SAFETY, ENVIRONMENTAL MANAGEMENT
MANUAL OF CODES OF PRACTICE & TECHNICAL GUIDANCE NOTES (TGN)
VOLUME 4: SAFETY & RISK MANAGEMENT
CODE OF PRACTICE ON SAFETY & RISK MANAGEMENT
MANAGEMENT OF HYDROGEN SULPHIDE (H2S)
ADNOC-COPV4-10
HSE Management – Manual of Codes of Practice & TGN
Volume 4: Safety & Risk Management
Version 2,
April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 2 of 77
Document Control Data
Revision
No
Revision
Date
Page or
Section
Reason For Revision
1
April 2014
Various
Code of Practice Review and Update. Also consolidating
contents of COPV4-11 with V4-10.
Copyright
The Copyright and all other rights of a like nature in this document are vested in Abu Dhabi National Oil Company
(ADNOC), Abu Dhabi, United Arab Emirates. This document is issued as part of the Manual of HSE Codes of Practice
(the “Manual”) and as guidance to ADNOC, ADNOC Group Companies and independent operators engaged in the Abu
Dhabi oil & gas industries. Any of these parties may give Copies of the entire Manual or selected parts thereof to their
contractors implementing HSE standards in order to qualify for award of contracts or for the execution of awarded
contracts. Such Copies must carry a statement that they are reproduced by permission of ADNOC, and an explanatory
note on the manner in which the Manual is to be used.
Disclaimer
No liability whatsoever in contract, tort or otherwise is accepted by ADNOC or any of its Group Companies, their
respective shareholders, directors, officers and employees whether or not involved in the preparation of the Manual for
any consequences whatsoever resulting directly or indirectly from reliance on or from the use of the Manual or for any
error or omission therein even if such error or omission is caused by a failure to exercise reasonable care.
All administrative queries must be directed to the Manual of HSE Codes of Practice Administrator in:
Health, Safety & Environment Division
Abu Dhabi National Oil Company,
P.O. Box : 898, Abu Dhabi,
United Arab Emirates.
Telephone : (9712) 6023782
Fax: (9712) 6668089
Internet site: www.adnoc.com
E-mail: hse@adnoc.ae
HSE Management – Manual of Codes of Practice & TGN
Volume 4: Safety & Risk Management
Version 2,
April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 3 of 77
TABLE OF CONTENTS
I.
Purpose
6
II.
Definitions & Glossary of Terms
6
III.
Existing Legislation
10
1
Introduction
11
2
Hydrogen Sulphide Zone Classification
13
2.1
Classification of Hydrogen Sulphide Areas
13
2.2
Definition and Size of Red, Amber and Yellow Zones
16
2.3
Location Of Release Sources
17
2.4
Access Control to Classified Areas
17
2.5
Planning Of Hydrogen Sulphide Zoning studies
19
2.6
Philosophy Of Risk Reduction
19
2.7
Determination Of Red, Amber, Yellow and Green Zones
20
Sizing of Red, Amber and Yellow Zones
Determination Of Emergency Planning Zones
Time To Protect
Software For Dispersion Calculations
20
20
20
21
2.7.1
2.7.2
2.7.3
2.7.4
2.8
Working In Red Zones
21
2.9
Working In Amber Zones
22
2.10
Working In Yellow Zones
23
2.11
Major Maintenance
23
3
Site Selection
24
4
Design Practices
25
4.1
Layout and Design
25
4.1.1
4.1.2
4.2
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.2.6
4.2.7
4.2.8
4.2.9
4.2.10
4.2.11
4.2.12
4.3
4.3.1
4.3.2
Plant Design
Plant Layout
Design Measures For Accident Prevention
Choice of Materials
Selection of Pipe Wall Thickness
Protection of Pipelines and Flowlines
Minimization of Pipework Failure
Control of Accidental Releases
Pipeline Leak Detection
Well Systems During Drilling, Testing and Workover
Equipment Isolation
Cold Venting and Flare Flame-Out
Sumps, Drains and API Separators
Pig Launchers and Receivers
Sewers and Waste Water Treatment
Minimization of Long Term Hydrogen Sulphide Emissions
Emission Reduction
Valve Seals
25
27
28
28
29
29
30
32
32
33
34
34
35
35
36
36
36
37
4.3.3
4.3.4
4.3.5
4.3.6
4.3.7
4.3.8
4.3.9
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
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Pump Seals
Centrifugal Compressor Seals
Venting
Sulphur Pits
Sulphur Flakers
Drilling
Disposal of Ventilation Air
37
37
37
38
38
38
38
4.4
Back Fitting
39
4.5
Exposure Monitoring Of Workers
39
5
Detection
40
5.1
Personal Alarms
40
5.2
Fixed Detectors
40
5.3
Detector response times
41
5.4
Hydrogen Sulphide Alarms
41
5.5
Hydrogen Sulphide Exposure Monitoring
41
5.6
Detector Selection And Placement
42
5.6.1
5.6.2
5.6.3
5.6.4
Selection Of Gas Detectors
Alarm Levels
Upgrade Of Existing Fixed Detection Systems (Back Fitting)
Fixed detection near and around wells
42
43
43
43
6
Control
44
6.1
Permit to Work
44
6.2
Activities Requiring Breathing Apparatus
44
6.3
Surveys
45
6.4
Start Up
45
6.5
Worker Competency
45
7
Mitigation
46
8
Evacuation, Recovery & Rescue
47
8.1
Emergency Plans and Procedures
47
8.2
Escape Routes in Yellow, Amber and Red Zones
48
8.3
Evacuation
48
8.4
Escape Routes And Assembly Areas
49
8.5
Minute Ventilation Rate
49
9
Eduction & Training
50
9.1
Training for All Personnel
50
9.2
Hydrogen Sulphide Competency
51
9.3
Working with Air Lines
52
9.4
Training of Emergency Response Teams
52
9.5
Hydrogen Sulphide Trainers
52
9.6
Training Methodology & Content
53
9.7
Common / General precautions of Reducing Risk
53
HSE Management – Manual of Codes of Practice & TGN
Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
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9.8
Untrained Personnel
54
10
Personal Protective Equipment
55
10.1
General Requirements
55
10.2
Emergency Escape Masks
55
10.3
Self-Contained Breathing Apparatus
55
10.4
Air-line Fed Breathing Apparatus
56
10.5
Fit Testing
57
10.6
Facial Hair
57
10.7
Eye-Glasses (Prescription Glasses)
58
11
Competency
59
12
Toxic Gas Refuges
60
12.1
Design of toxic gas refuges
61
12.2
Offshore installations
62
13
Emergency Planning Zones
63
14
Assessment of the Impact of Hydrogen Sulphide
64
15
Enforcement
65
17
References
66
18
Appendices
69
Appendix 1
: Properties of Hydrogen Sulphide
70
Appendix 2
: Exposure Calculations
73
Appendix 3
: Dispersion Calculations
74
Appendix 4
: Calculation of Zone Size & Detector Placement
76
I.
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 6 of 77
PURPOSE
This Code of Practice addresses the requirements for safe handling and working with
hydrogen sulphide. Its purpose document is to ensure that risks to people from hydrogen
sulphide as a result of Group Company operations are prevented, controlled and mitigated
so that the risk to people is as low as reasonably practicable (ALARP).
II.
DEFINITIONS & GLOSSARY OF TERMS
Accident
An event or chain of events which has caused fatality, injury,
illness and/or damage (loss) to assets, the environment, company
reputation or third parties.
ADNOC
Abu Dhabi National Oil Company.
AEGL
Acute exposure guideline levels are published by the US
Environmental Protection Agency to assist in Emergency
Planning.
ALARP
As Low As Reasonably Practicable. A risk level between the
upper, unacceptable, limit and the lower, tolerable, limit. Risks in
this region must be reduced as far as possible consistent with
there being a practical method (i.e. demonstrated to work properly
in actual industrial use) and at a cost which is reasonable when
compared to the risk reduction achieved.
Buddy System
A person assigned to assist someone who is working in a
hazardous activity such as working in a hydrogen sulphide area
whose duties include remaining alert to hazards, giving of alarms,
keeping rescue lines clear, cross-checking that the correct
procedures are being followed and similar activities. The buddy
must be protected to the same degree as the person he is
assisting.
Bump Test
A short exposure of a detector to a test gas which demonstrates
that the sensor and alarm are operational, without carrying out a
calibration
CAPP
Canadian Association of Petroleum Producers.
Cascade system
Facility to allow self-contained breathing apparatus to tie-in into an
air supply manifold to allow extended work in a Red Zone, or in a
confined space.
Contractor
Any person or company employed under contract (irrespective of
period of contract or employment).
Competence
The ability to perform a particular job in compliance with
performance standards.
Will usually require specific and
documented blend of skills, training and experience.
DTL
Dangerous Toxic Load – see SLOD, SLOT.
EF
Emission Factor, used to estimate the rate of chronic hydrogen
sulphide emission from equipment. It is the average rate of
release from a given equipment type averaged over the whole
plant including both leaking and non-leaking equipment.
Emergency Escape
Mask
Breathing apparatus which allows escape from a toxic gas
classified area in an emergency, regarded in this CoP as a
positive pressure, self-contained breathing air type
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
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Equilibrium Vapour
It is hydrogen sulphide gas which presents the main risk to
people. Liquids containing hydrogen sulphide will release vapour
containing the gas. The equilibrium vapour is the vapour
associated with the liquid in which the concentration of hydrogen
sulphide has reached equilibrium.
ESA
European Sealing Association
Hazard
Potential source of harm. Source, situation, or act with a potential
to cause harm. Note: in the context of international standards, the
potential harm may relate to human injury, damage to the
environment, damage to property, damage to reputation, or a
combination of these.
HAZOP
Hazard And Operability Study; a formal hazard identification
technique.
HSE Management
System
The company structure, responsibilities, practices, procedures,
processes and resources for implementing health, safety and
environmental management. HSEMS, ADNOC-CoPV1-09 [Ref:
14]
HSEIA
Health, Safety and Environmental Impact Assessment – A
systematic process identifying HSE impacts. A demonstration
required for ADNOC Group Company sites to demonstrate that
health, safety and environmental issues have been adequately
dealt with.
HSECES
HSE Critical Equipment And Systems – Part of an installation and
such of its structures, plant equipment and systems (including
computer programmes) or any part thereof, failure of which could
cause or contribute substantially to; or a purpose which is to limit
the effect of a major accident or any accident with severe or
catastrophic consequences as defined in the ADNOC Group
Code of Practice Guideline on HSE Risk Management [Ref: 18]
and Code of Practice V6-01 [Ref: 10]
HVAC
Heating, Ventilation and Air Conditioning.
IEC
International Electro-technical Commission
Incident
An event or chain of events which has caused or could have
caused fatality, injury, illness and/or damage (loss) to assets, the
environment, company reputation or third parties.
Injury
Physical harm or damage to a person resulting from traumatic
contact between the body of the person and an outside agency, or
from exposure to environmental factors.
ISO
International Standards Organisation
Lethal Service
Equipment is in lethal service whenever failure can lead to such
high hydrogen sulphide concentrations in the vicinity that
personnel may have insufficient time to don an emergency
escape mask before being overcome.
LC 50
Concentrations of the chemical in air that kills 50% of the test
animals during the observation period
LF
Leakage Fraction – the fraction of any one equipment type leaking
at any one time
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
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MVR
Minute Ventilation Rate – the quantity of air a person consumes in
one minute, used for setting the capacity of emergency escape
masks and self-contained BA and fro the design of air-line
systems and Safe Havens.
NACE
National Association of Corrosion Engineers
NIOSH
National Institute of Occupational Safety and Health
Personal Hydrogen
Sulphide Detector
Device which must be worn by all personnel entering hydrogen
sulphide classified areas, which will alarm if the hydrogen
sulphide concentration reaches the STEL.
PPE
Personal protective equipment
Probit
A statistical measure which relates probability of fatality or injury
to time and concentration
QRA
Quantified Risk Analysis
RBI
Risk Based Inspection – an approach to determining appropriate
methods, locations and frequencies of inspection so as to
minimise risk without incurring excessive inspection costs or
interruptions of production.
Risk
The measure of the likelihood of occurrence of an undesirable
event and of the potential adverse consequence, which this event
may have upon people, assets, the environment, or economic
measures and reputation of the company.
Safe Haven
Protected location designed to allow people to shelter in the event
of an accident, sometimes referred to as a Temporary Refuge
Shelter. Protection is provided against toxic gas and smoke
ingress, fire and explosion blast, in some cases also explosion
projectiles.
SCBA
Self-Contained Breathing Apparatus, regarded in this CoP as
positive pressure type. Breathing apparatus that can be used
without other support, so that the user is mobile. Often supplied
with a cascade mode that allows tie-in to a remote air supply via a
supply manifold.
SCC
Stress Corrosion Cracking
SIL
Safety integrity level, a measure of the degree of risk reduction
provided by a safety measure. Defined in standard IEC 61508
[Ref 17].
SLOD
Significant Likelihood of Death – DTL at which there is a
probability of fatality.
SLOT
Specified Level Of Toxicity – a level which is considered to be the
limit of lethality for a gas.
SOHIC
Stress Orientated Hydrogen Induced Cracking
SRU
Sulphur Recovery Unit – a gas processing unit whose purpose is
to dispose of hydrogen sulphide and/or other sulphur containing
species by recovering the sulphur as elemental sulphur.
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STEL
Short term exposure limit – a 15-minute TWA exposure that must
not be exceeded at any time during the work day, even if the 8-hr
TWA is within the WEL.
The STEL is the concentration to which it is believed that workers
can be exposed continuously for a short period of time without
suffering from (1) irritation, (2) chronic or irreversible tissue
damage, (3) dose-rate-dependent toxic effects, or (4) narcosis of
sufficient degree to increase the likelihood of accident, injury,
impaired self-rescue, or materially reduced work efficiency.
Compliance with the STEL will not necessarily protect against
these effects if the daily WEL is exceeded.
The exposure above TWA up the STEL should be less than 15
minutes, should occur no more than 4 times per day and there
should be at least 60 minutes between successive exposures in
this range.
Toxic Gas Warning
Level
Gas concentration threshold at which an alarm device must give a
warning and must serve to bring personnel to the muster point
Toxic gas refuge
(TGR)
Refer Safe Haven.
TWA
Time-weighted average exposure. The TWA concentration for a
conventional 8-hr workday, to which it is believed that nearly all
workers may be repeatedly exposed, day after day, over a
working lifetime, without adverse health effects.
UK HSE
United Kingdom Health And Safety Executive.
USEPA
United States Environmental Protection Agency.
VOC
Volatile Organic Compounds.
WEL
Workplace Exposure Limit – Refers to airborne concentrations of
chemical substances, and represents conditions under which it is
believed that nearly all workers may be repeatedly exposed, day
after day, over a working lifetime, without adverse health effects.
The ADNOC recommendation for the TWA WEL is 5ppm over
eight hours.
Yellow Zone
Area where hydrogen sulphide hazard is Medium-Low
III.
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
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EXISTING LEGISLATION

Federal Law No. 8, 1980, Regulation of Labour Relations and Order Issued in
Implementation Thereof.
The following ADNOC Codes of Practice are generally relevant to the safe handling and
working with hydrogen sulphide:

CoP V1-02: Health, Safety and Environmental Impact (HSEIA) Requirements [Ref.
1];

CoP V3-01: Framework of Occupational Health Risk Management [Ref. 62];

CoP V4-01: Framework of Occupational Safety Risk Management [Ref. 63];

CoP V4-04: Personnel Protective Equipment [Ref. 64]

CoP V4- 05: Non-routine Operations [Ref. 65];

CoP V5-02: Crisis and Emergency Management [Ref. 2].

CoP V5-03: Qualitative and Quantitative Risk Assessment [Ref. 7]
Group Companies must ensure that their activities comply with all relevant Federal
and Abu Dhabi laws and regulations at all times, including any that may be
introduced after the publication of this Code of Practice.
1
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
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INTRODUCTION
Hydrogen sulphide is a colourless gas commonly associated with ADNOC Group Company
oil and gas operations. It is highly toxic causing fatality after as little as ten seconds’
exposure to high concentrations. It has been responsible for the death of many people
worldwide, including many cases where people have been overcome attempting to rescue
colleagues.
This Code of Practice has been developed following a number of fatal accidents in Abu
Dhabi oil and gas industries involving hydrogen sulphide. It incorporates the conclusions of
the ADNOC Group Company Workshop – H2S – A Silent Killer.
This Code of Practice provides regulations to allow ADNOC Group Companies to adopt a
consistent and safe approach to handling and working with hydrogen sulphide. It applies to
all ADNOC Group Company facilities and operations with the potential for exposure of
people to concentrations of hydrogen sulphide above the Workplace Exposure Limit or for
acute exposure which could be lethal or injurious.
This COP makes a clear distinction between 8-hr Time Weighted Average [TWA] and 15min Short Term Exposure Limit values which are intended for worker protection against
chronic health effects, and accidental exposures to higher concentrations, which can cause
injury or death. The former require monitoring and release reduction engineering at the
work site, whilst the latter also require alarms to alert personnel to evacuate the workplace
using appropriate PPE.
This COP applies to all ADNOC Group Company operations and activities where there is a
hydrogen sulphide hazard to people. It includes, but is not limited to, onshore plant,
pipelines, offshore platforms, and drilling and servicing of wells.
The requirements of this COP must be taken into account for all stages of the facility lifecycle whenever a hydrogen sulphide hazard exists. They also apply to ADNOC Group
company operations where potential personnel exposure to hydrogen sulphide is of an
intermittent nature such as when a vessel where there is normally no hydrogen sulphide
hazard is working alongside an offshore platform handling hydrocarbons containing
hydrogen sulphide.
This Code of Practice outlines:

Zone Classification to delineate areas within a site where specific precautions
against hydrogen sulphide hazards must be adopted. (Section 2);

Site Selection covering requirements to prevent personnel exposure by appropriate
site selection (Section 3);

Design Practises for preventing accidents involving hydrogen sulphide for the
range of plant operated by ADNOC group companies are described in Section 4.
These are not just applicable to new design, but also to existing installations.
Guidance has been given on back-fitting of equipment, for the cases where there
are significant practical difficulties in bringing older plant up to the standards in this
COP

Detection of dangerous levels of hydrogen sulphide, including requirements for
personal hydrogen sulphide detectors and for fixed detection systems and alarm
settings (Section 5);

Control of activities that can potentially lead to a release of hydrogen sulphide.
This includes such diverse operations as confined space entries and boarding an
offshore installation where hydrogen sulphide may be present.
It covers
requirements for permit to work systems, activities for which breathing apparatus is
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Volume 4: Safety & Risk Management
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April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 12 of 77
specifically required, surveys for occupational health compliance and start-up
controls (Section 6);

Mitigation of the effects of hydrogen sulphide releases, including risks arising from
fires involving hydrogen sulphide which can generate sulphur dioxide, a highly toxic
combustion product (Section 7);

Evacuation Recovery and Rescue requirements in the event of a hydrogen
sulphide release, including the development of emergency response plans and
testing through exercises (Section 8).

Education and Training requirements to ensure that all personnel on site including
contractors, temporary workers and visitors are aware of the hydrogen sulphide
hazards to which they can be exposed, are able to carry out their work with due
attention to those hazards and know what to do in the event of an emergency
involving hydrogen sulphide. It lists training requirements for different personnel
groups including hydrogen sulphide trainers. (Section 9);

Personal Protective Equipment requirements and usage covering the different
types of PPE and including facial hair requirements and fit testing (Section 10);

Competency requirements including use of hydrogen sulphide competency
certificates (Section 11);

Toxic Gas Refuges and their use in an emergency, including the requirement to
determine if installation of a TGR would reduce risks to as low as reasonably
practicable (Section 12);

Emergency Planning Zone requirements (Section 13); and

Assessment Of Hydrogen Sulphide impacts using QRA and other techniques
for site selection, development of emergency plans and for analysis of
potential risk reduction measures (Section 14); and

Enforcement arrangements detailing Group Company authority and responsibility
(Section 15);
Appendices are provided to give technical guidance in fulfilling the requirements of this
CoP, as follows:

Appendix 1 lists the toxic properties of hydrogen sulphide. These are the
recommended values that should be used in the majority of cases.

Appendix 2 contains guidance on the quantitative assessment of acute exposure
and likelihood of injury or fatality

Appendix 3 contains guidance on performing dispersion calculations.

Appendix 4 contains practical guidance on assessing the size of Red, Amber and
Yellow hazard zones.
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
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2
HYDROGEN SULPHIDE ZONE CLASSIFICATION
2.1
Classification of Hydrogen Sulphide Areas
Locations where a hydrogen sulphide hazard exists must be classified according to the
potential threat from hydrogen sulphide based on the precautions needed to allow people a
good chance to escape in the event of an accidental release of hydrogen sulphide:
Four Zones: Green, Yellow, Amber and Red are defined, each with its own specific PPE
requirement (or no requirement). The criteria for defining the extent of each zone is
logically derived from the PPE requirements using the Time To Protect principle (see
Section 2.8.3) together with the ADNOC Risk Matrix and The ALARP Principle presented
in the ADNOC Code of Practice on HSE Risk Management, CoP V5-06 [Ref: 18]. The
results are believed to be consistent with those used elsewhere in the industry.
Zone
Red
Description
Areas where potential exists for exposure to such high concentrations of
hydrogen sulphide that fatality may occur due to relatively short
exposure (minutes). Definition of the size of the red zone is given in
Section 2.2.
Note that the definition of a Red Zone is dynamic, i.e. an area which is a
Red Zone when equipment is operating normally can become an Amber
or a Yellow Zone or even unclassified due to isolation and de-pressuring
and purging of the toxic gas inventory. This may be a practical
requirement to allow inspection or other operations, which would
otherwise require breathing apparatus.
Toxic LSIR
Level
1E-3 per year [refer to the modelling considerations in Appendix 4]
Entry
Requirements
 Carry EEBA when entering the red zone for inspections not
interfering with equipment. In case of maintenance or intervention
activities in the RED zone, airline or SCBA set should be worn at all
times if the equipment/pipeline is not purged/decontaminated. New
plants need to be designed for minimum work in the red zone. Task
Risk Assessment or JSA shall clearly specify the SCBA / air-line
requirements.
 Personal hydrogen sulphide detector to be worn at all times (see
Section 5.1).
 Access control or personnel tracking.
 Authorised and trained personnel only.
 Hydrogen sulphide competency certificate required for entry to Red
Zone.
Risk
High
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Zone
Amber
Risk
Medium-High
Description
Areas outside the Red Zone where potential exists for exposure which
may be injurious to personnel after relatively short exposure (minutes).
See sections 2.2, 2.3 and 2.8 for the approach to calculating the extent
of the Amber Zone.
Group companies may choose to define the entire process area of a
plant or well area as an Amber Zone, for convenience in controlling
access and checking that persons entering are properly equipped and
trained.
Toxic LSIR
Level
1E-4 per year [refer to the modelling considerations in Appendix 4]
Entry
Requirements
 EEBA carried while visiting for inspections and not interfering with
equipment and personal detector (worn). At all times those involved
in performing activities requiring opening of process components
which contain, or may contain hydrogen sulphide must have the
airline breathing apparatus or SCBA set donned and in use
 Hydrogen sulphide competency certificate required for entry
Zone
Yellow
Description
Areas where the risks of injurious or fatal concentrations of hydrogen
sulphide are low enough for personnel to reach defined locations before
Emergency Escape Masks are Stored and don the mask before being
overcome.
Group companies may choose to make Yellow Zones, Amber Zones
where this will simplify the practical administration of access controls
and the provision of PPE.
Toxic LSIR
Level
1E-5 per year [refer to the modelling considerations in Appendix 4]
Entry
Requirements
 EEBA available at strategic points and Personnel entering zone
must be aware of PPE locations
 Personal detector (worn)
 Hydrogen Sulphide competency certificate required for entry.
Zone
Green
Description
Areas outside the Yellow Zone where personnel would be able to safely
evacuate in the event of an accidental release without the need for
PPE, but where exposure above the limit is possible.
Toxic LSIR
Level
1E-6 per year [refer to the modelling considerations in Appendix 4]
Entry
Requirements
 No restriction (but refer to Section 6)
Risk
Risk
Medium-Low
Low
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Zone
Unclassified
Hazard
Description
Exposure to hydrogen sulphide is not credible
Entry
Requirements
 No restriction
None
Group Companies must classify all parts of all facilities, including laboratories, offshore
platforms and remote locations where hydrogen sulphide may be handled according to the
above scheme and must control access as detailed below. Sub-sea pipelines do not need
to be considered, except at terminations on platforms and shore. Cross-country pipelines
only need to be considered at valve stations, pigging stations, compressor stations and the
like. The classification system must be applied to both new and existing facilities. Some
advice on modifications which may be necessary on existing facilities to allow practical
operation with the zoning restrictions is given in Section 4.
It is the responsibility of the Group Company to ensure that valid calculations are performed
for the sizing of Red, Amber and Yellow zones, regardless of whether the calculations are
performed in-house or by a third party consultant.
Any change to a facility which results in a change in status of a location to a more
hazardous zone, e.g. the introduction of sour fluids into equipment that normally handles
lower hazard fluids, cannot be made until reclassification of the relevant area has been
made.
The above classification represents a mandatory minimum requirement. Areas may be
classified as Red, Amber or Yellow Zones even when they are technically a less hazardous
zone to simplify the application of controls and the usage of PPE. For example, an entire
facility can be defined as a Yellow Zone; even where one area might technically be a Green
Zone.t is acknowledged that some operations may be impractical in Red Zones. In such
cases Group Companies must either modify the facilities to avoid either the impractical
operation, or the Red Zone. This might include temporarily removing the Red Zone by
isolation and de-pressuring of toxic inventories. It is emphasised that the definition of the
Red Zone is such that personnel may not have time to don an Escape Set before being
overcome. Carrying an escape set as a means of protection in a Red Zone is therefore not
adequate by definition (See Section 2.2).
The existence of the zones and associated restrictions are due to hydrogen sulphide being
present and confined within equipment or pipework (i.e. during normal operation), a release
of sour material does not suddenly introduce a Red Zone.
2.2
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Definition and Size of Red, Amber and Yellow Zones
The purpose of defining hydrogen sulphide classified areas is to ensure that safety
measures taken provide a level of safety such that the risk level is acceptable if ALARP.
One principle used for determining the presence and amount/size of hydrogen sulphide is
that of “time to protect”. Special protection is required in an area where there is insufficient
time to self-evacuate or to don emergency breathing apparatus in the case of a hydrogen
sulphide release. Such a principle, if applied to the largest accidents, such as pipe ruptures,
leads to very large red zones. A probabilistic approach is therefore preferred, in which
classified areas are defined in terms of the level of risk of there being insufficient time to
don emergency protective equipment is in the acceptable or lower part of tolerability region.
Red Zone
A red zone is considered to exist if there is a significant risk of hydrogen sulphide release
which could be rapidly lethal. It is considered the limit at which personnel carrying
emergency escape masks will have time to don the mask before being overcome. It
comprises a 45 seconds time to don the mask together with a 30 seconds alert period.
Note that isolation and de-pressuring of the toxic gas inventory could remove the Red Zone
(as defined above) and this may be a prerequisite for inspection, maintenance or other
operations where entry wearing Breathing Apparatus is impractical.
Amber Zone
An Amber Zone is considered to exist where persons affected would have time to don PPE
carried ready for use, but would not have time to reach the nearest PPE location.
Yellow Zone
A Yellow Zone is considered to exist if there is a possibility of hydrogen sulphide release
which could be rapidly injurious.
Note that in this case the location specific individual risk must be evaluated for unprotected
personnel, i.e. without PPE. The presence of PPE within the Yellow Zone is effectively an
ALARP measure to reduce the risk within the Yellow Zone.
Green Zone
A Green Zone is considered to exist where hydrogen sulphide concentrations above the 8hr TWA limit are foreseeable, but the area is not in an otherwise classified zone.
Group Companies may extend Red, Amber or Yellow Zone beyond that calculated to
simplify entry or other arrangements if required.
The full range of potential release sizes (hole sizes) must be considered when sizing red,
Amber and yellow zones. See Appendix 2 for recommendations regarding exposure times.
Notes on the type of models which can be used for red, Amber and yellow zone sizing
calculations can be found in Section 2.8.4 and Appendix 3.
2.3
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Location Of Release Sources
Selecting the potential location of a release is extremely important when determining the
extent of Red, Amber and Yellow Zones. Typically in QRAs and similar studies, a number
of representative release scenarios are analysed, each of which is based on a leak from a
particular inventory.
In the case of Red, Amber and Yellow Zone determination, the leak location must be
considered to be any point on the equipment that could leak and give rise to the
representative release scenario. This is essentially similar to the definition used when
determining area classification for flammable materials. In other words each representative
scenario does not have a singular potential origin (as is often assumed in many QRA
methodologies), but must be considered as potentially arising from any point of the relevant
equipment.
2.4
Access Control to Classified Areas
Entry to Red Zone areas must be controlled and secure to prevent unauthorised persons
accidentally wandering into the area. Access to Red Zones must be limited to personnel
holding a valid certificate for hydrogen sulphide competency as detailed in Section 9.2. A
system must be in place for personnel to register in/out when entering or leaving Red
Zones, or a wireless personnel tracking system used, so that it is possible to establish who
is present in the event of an emergency. Personnel entering Red Zones must have a
personal portable hydrogen sulphide detector which will alert them if the hydrogen sulphide
level rises to the Toxic gas warning level so that predetermined action can be taken.
In the Red Zone breathing apparatus must be worn and in use at all times. This requires
appropriate design of the plant facilities (see Section 4). The types of breathing apparatus
which are suitable are discussed in Section 10. When carrying out work involving opening
of equipment containing hydrogen sulphide, positive pressure breathing apparatus must
also be worn and in use.
Where registration of entry /exit to a Red Zone is not reasonably practicable, for example
where entry into a Red Zone is by helicopter landing on a platform which is a Red Zone in
entirety, then Group Companies may establish an alternative system that fulfils the intent of
the registration system: controlling entry to authorised persons only, determining who is
present in the event of an emergency and ensuring PPE is provided. Many offshore
personnel tracking systems will already fulfil this requirement. It is noted that landing a
conventional helicopter in a Red Zone may be impractical because of the probable
requirement for the pilot to be wearing suitable breathing apparatus. (The Red Zone will
extend into the helicopter when a door is opened.)
There must always be at least two independent escape routes out of a Red or Amber Zone.
In order to simplify access controls, one of the escape routes can be designated for
emergency exit only, and entrance to the Red or Amber Zone via that route prohibited. The
escape/entry routes must be decided based on the prevailing wind direction and other
relevant site specific considerations.
All personnel entering Amber Zones must have a valid hydrogen sulphide competency
certificate. Also they must have all necessary PPE to facilitate escape in an emergency as
detailed in Sections 8, 10 and 13. Personnel entering Amber Zones must have a personal
portable hydrogen sulphide detector which will alert them if the hydrogen sulphide level
rises to the Toxic gas warning level, and that the emergency escape mask must be donned
immediately on alarm.
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All personnel entering Yellow Zones must have a valid hydrogen sulphide competency
certificate. They must also be both familiar with the location of PPE provided to allow
escape in the event of a toxic gas release, and the type of PPE provided. The number of
personnel in a Yellow Zone must not exceed the PPE provision at any time. In other words
the number of emergency escape masks available at the nearest PPE location must be
greater than or equal to the number of personnel for whom that is the nearest location. In
practice this will typically limit the number of personnel in a single yellow zone to the
number of emergency escape masks provided at each station. In practice it may often be
simpler to extend the Amber Zone to encompass the Yellow Zone so as to avoid excess
provision of escape masks, which as safety equipment must also be subject to audit and
verification.
The demarcation between Green and Yellow Zones must be clearly established by the use
of floor markings and pictographic warning signs. (If the boundary of the yellow zone is
defined as the process area security gate, markings may be made just at the gate(s)).
For access to sour gas fields or wells, the access point must be along the access road(s),
at the edge of the field if the entire field is regarded as a yellow zone, or at the road
approach to the individual well or well cluster. Warning signage must be placed at this
location. If the access point is not manned, telephone numbers to the field control centre
must be displayed on a sign, and procedures must require that persons approaching the
well or entering the field a) have proper PPE and b) inform the control centre.
The Red and Amber Zones must be clearly delimited and entry restriction should be
applied. Generally it is sufficient to provide a clear marking by a perimeter chain on which
hydrogen sulphide hazard signs with PPE requirements are hung.
There must be at least two access/escape routes leading out of the Red and Amber Zones
from all work locations.
The recommended method of recording entry and exit from the Red and Amber Zones is
using a swipe card system and / or wireless personnel tracking system which will allow
automatic monitoring and recording of personnel numbers and which will assist in muster
verification in the event of an emergency, but T-Card systems are considered acceptable.
For some older plants, reporting to the control room by radio on entry and exit may be the
only practical solution. Of all the possible practices, the current best is to provide RFID
(Radio Frequency Identification) or GPS remote locator systems for employees so that their
location is known at all times, and so that emergency search and rescue teams do not need
to search, only to rescue.
The onus is on Group Companies to use an appropriate system which achieves the two
objectives required: a means of making sure only authorized personnel enter Red and
Amber Zones and so that Emergency Responders will know precisely how many people
there are in each Zone in the event of an emergency.
2.5
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Planning Of Hydrogen Sulphide Zoning studies
The size of the Red, Amber and Yellow zones and the area impacted is probably best
evaluated in conjunction with the project / facility QRA, which often forms part of the HSEIA.
It is recommended that Group Companies modify the scope of QRAs to include evaluation
of Red, Amber and Yellow Zones. Red, Amber and Yellow zones should also be evaluated
for simultaneous activities such as Unit Maintenance while an adjacent unit is in production,
well interventions beside a producing well, etc.
It is important that zoning is carried out at an early stage in the project, so that any practical
restrictions, which can be extremely severe in Red and Amber Zones, can be taken into
account in equipment design and selection. The design should, as far as practicable,
minimize the need for personnel to enter a Red or Amber Zone. In particular the need for
routine intervention in a Red Zone should be avoided.
It is important that the zone restrictions are mentioned in enquiry documents for vendor
packages that will be located in Red, Amber or Yellow zones. It is also important that Zone
restrictions are taken into account when designing Health, Safety and Environment Critical
Equipment and Systems (HSECES), especially when considering the practicality of
inspection and testing, which are required to ensure that the element meets its designated
performance standard (safety integrity level). In general, wherever practicable, HSECEs,
which require regular inspections and testing, should be located outside Red Zones.
Similarly firefighting systems (e.g. fire monitors) located in a Red Zone must be considered
for remote operation.
As the emergency zones (EPZ, EAZ) need to be communicated with Stakeholders, these
must be evaluated at early stages on the facility.
The impact of zones on the engineering design and, specifically on the design and
operation of HSECESs must be taken into account when developing the HSEIA.
2.6
Philosophy Of Risk Reduction
During project life cycle, philosophy of risk reduction requires the following demonstration:

Elimination of Red Zones as far as reasonably practicable

Elimination / Reduction of personnel exposure to Red Zones as far as reasonably
practicable

Reduction in the size of Red and Amber Zones

Elimination of overlap of Red and Amber Zones in cases where maintenance is
required on one unit, when the other unit is operational.

Limiting the size of Yellow Zones within the facility boundary
If the Red or Amber zones extend beyond the plant boundary, the risk is deemed
unacceptable and risk reduction is required.
During operations, the philosophy of risk reduction requires the following demonstration:

Manual of Permitted Operations within Red and Amber Zones should be updated;

Demonstration of ALARP is required when simultaneous Activities will be
undertaken within Red and Amber Zones. This will require developing risk
assessments pertaining to the works that are going to be performed and

Reducing or maintaining the initial estimates of Red, Amber and Yellow Zones. This
should take into account the aging patterns of the facilities especially when
producing the updated HSEIAs.
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2.7
Determination Of Red, Amber, Yellow and Green Zones
2.7.1
Sizing of Red, Amber and Yellow Zones
In order to determine the size of a zone, a risk assessment should be performed for
hydrogen sulphide releases only, and which takes into account:

The frequency of releases.

The geometry of the release, in particular jets from a pressurized release, which can
travel past a person without having an effect.

The wind direction, wind speed, and atmospheric stability.

Any impingement of escaping gas jets on the ground or on other equipment.

Delayed ignitions should not be considered while determining Red, Amber and
Yellow Zones.
Note that no exposure factor is included in this calculation because many of the failures
which result in the releases are latent failures, triggered by the presence of persons, such
as corrosion failures where deposits are broken loose during maintenance. In other words
only LSIR should be considered for determining the extent of the Yellow Zone and not IRPA
(Individual Risk Per Annum).
2.7.2
Determination of Emergency Zones
For Emergency Planning, two zones are defined as mentioned below:
Emergency Planning Zone (EPZ) corresponds to the zone where any member of the public,
animal pens or non essential personnel at large has to be evacuated to a safe area prior to
the start of the operations. Only operations related personnel (essential) are allowed to be
present in the area. Working personnel from simultaneous operations in this zone shall be
treated as operations workforce for the purpose of H2S training and emergency evacuation.
This zone is outside the plant or industrial zone boundaries and could be exposed to a
concentration corresponding to AEGL-3 (10 min) i.e., 76 ppm.
Emergency Awareness Zone (EAZ) corresponds to the zone where the public at large
should be informed about the consequences of a toxic release (H2S or SO2). No
evacuation planning is required for this zone. This zone goes beyond the EPZ to the extent
where the H2S concentration will reach AEGL-2 (1 hour) i.e., 27 ppm as a maximum
2.7.3
Time To Protect
The time to protect is a key concept in optimizing emergency response arrangements in the
event of a hydrogen sulphide release. Indeed it is the rationale behind selecting the various
zone classifications and the associated access rules.
In order to use this principle, the time to don emergency escape mask or SCBA sets needs
to be known. The following results have been found in tests:

Donning an SCBA – 45 seconds (75 seconds, including alarm and response time
[Ref: 57].

Donning an emergency escape mask, with some fumbling – 20 seconds (completed
pre-checks and kept ready for use).

Donning an emergency escape mask, after some practice – 15 seconds

Donning an emergency escape mask, hood type – no data found.
To these times must be added the time to realize that there is a release – This is nearly
instantaneous close to a medium or high pressure gas jet because of the noise and
visibility, but about 20 seconds for a personal alarm.
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The hydrogen sulphide concentration where an exposure for 75 seconds has a 1% fatality
probability is 1120ppm according to the UK Health and Safety Executive (See Appendix 3).
The corresponding concentration for a 45 second exposure is 1400ppm.
For new plants, special measures which have been selected include minimum in service
maintenance (by providing redundant equipment), minimum use of flanges (“all welded”
construction), and segregation of red zone equipment from other equipment types. On
existing plant, techniques which can be used are remote isolation and venting of affected
equipment (MOVs), and use of replacement maintenance rather than repair in place.
The use of risk assessment in this way could potentially be very onerous. Firstly, very high
quality assessment is required in order to ensure that detailed aspects of the hazard are not
missed (secondary aspects such as gas jet impingement could impose a very high risk.
Secondly, it is likely that the zoning will need to be updated in the case of plant changes, at
times when risk analysis consultants are not readily available. Thirdly, the approach could
involve a lot of analysis work. For these reasons, best practice will be to provide a very
streamlined approach to the analysis, with a good selection of worked examples provided.
This approach to the definition of the Amber Zone implies that anyone entering the zone
must be trained to don the emergency escape mask rapidly, and must be ready to don it
immediately on receiving a gas alarm. This means also that escape masks must be worn
on the belt or similar and readily accessible, or in the case of an SCBA, must be worn, even
though not in use.
2.7.4
Software For Dispersion Calculations
When sizing Red, Amber or Yellow Zones and at other times when performing hydrogen
sulphide dispersion calculations, Group companies must use suitable models to give the
accuracy sort. Consult Appendix 3 for general guidance or consult the model developer.
2.8
Working In Red Zones
All personnel entering a red zone must carry a personal H2S monitor and must wear
breathing apparatus at all times. New plants must be designed for minimum work in the red
zone, and operations at existing plants must be reviewed to minimize working in Red Zones
as far as practicable.
At all times those involved in performing activities requiring opening of process components
which contain, or may contain hydrogen sulphide must have the airline breathing apparatus
or SCBA set donned and in use, regardless of the zone classification of the area in which
the operation occurs.
For some existing plants, where the requirement for using of breathing apparatus for all
work in Red Zones may be impractical, or may introduce other hazards such as those of
heat stroke, the following is recommended:
1.
Move any operator rooms, e.g. those for sour gas compressors, outside the red
zone, or make the operator room a toxic gas refuge;
2.
Use airline fed breathing apparatus for all prolonged work involving opening of
process equipment, flange opening etc. Use airline fed breathing apparatus or
SCBA for sampling from equipment known to contain hydrogen sulphide or for
which there is a significant possibility that there is hydrogen sulphide. (This is a
mandatory requirement in any case);
3.
Avoid as far as possible any work such as painting, or civil works maintenance until
the process plant is depressurised. This is reasonably practical if the plant is
designed for this approach, e.g. has good segregation distances and is designed for
minimum onsite work;
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4.
Avoid as far as possible the need for routine work in the Red Zone for example: use
transmitters rather than gauges, and automatic sampling rather than manual sample
taking; and
5.
Provide a means of constant communication fixed plant telephone or hand held
radio) with someone located outside the Red Zone, in addition to use of the buddy
system.
Operating companies must review whether it is really necessary to carry out the tasks in the
Red Zone while the plant is operating.
2.9
Working In Amber Zones
All personnel entering an Amber zone must wear a personal H2S monitor, and, as a
minimum, all must have an EEBA set close to hand whenever they are in the Amber zone.
At all times those involved in performing activities requiring opening of process components
which contain, or may contain hydrogen sulphide must have an airline breathing apparatus
or SCBA set donned and in use.
For plants where large numbers of personnel are expected to work in Amber zones, a
temporary toxic refuge should be made available during the entire span of the activity.
For existing plants the following must be considered:
1.
Moving any operator rooms, e.g. those for sour gas compressors, outside the
Amber zone, or making the operator room a toxic gas refuge.
2.
Use airline fed breathing apparatus for all prolonged work involving opening of
process equipment, flange opening etc.
3.
Avoid as far as possible any work such as painting, civil works maintenance until the
process plant is depressurised. This is reasonably practical if the plant is designed
for this approach, e.g. has good segregation distances and is designed for minimum
onsite work.
4.
Avoid as far as possible the need for routine work in the Amber zone, for example,
use transmitters rather than gauges, and automatic sampling rather than manual
sampling.
5.
Provide a means of constant communication fixed plant telephone or hand held
radio) with someone located outside the Amber Zone, in addition to use of the
buddy system.
Not actually carrying breathing apparatus while working in the Amber Zone will still be a
deviation.
2.10
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Working In Yellow Zones
Anyone entering a Yellow Zone must be aware of the location of the location of strategically
placed EEBA sets and must wear a H2S Monitor.
While the risk of exposure to dangerous concentrations of hydrogen sulphide is very much
lower when working on equipment in the yellow zone, there can still be a significant risk of
toxic injury and illness during some operations. Opening of hydrogen sulphide containing
equipment should therefore only be undertaken by persons wearing air line set or SCBA set
and a portable hydrogen sulphide detector used for monitoring.
For sampling SCBA must be worn and the area must be cleared to a safe distance while
opening and isolating and until the equipment is purged and while de-spading and restoring
to service. The area must be marked off and signed as for Red and Amber Zones during
these activities. The size of the restricted area should be determined during the Job Safety
Assessment (JSA) or Task Risk Assessment (TRA) based on dispersion analysis.
In effect, during the activities, the area around the equipment should be subjected to the
same rules of working as for a Red and Amber Zones.
2.11
Major Maintenance
Areas may be declassified as hydrogen sulphide classified areas e.g. to improve access
during maintenance provided that:

The equipment has been depressurized, positively isolated, degassed and
ventilated, and tested for the presence of hydrogen sulphide;

It has been determined that hydrogen sulphide cannot be generated within
equipment or piping e.g. by chemical reaction such as acid on sulphide scale; and

The area is not overlapped by a hydrogen sulphide classified area arising from other
equipment.
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SITE SELECTION
New facilities (defined for the purposes of this CoP as facilities where front end engineering
design has not commenced at first issue of this CoP) must be sited sufficiently far from
public locations, including housing and roads so as not to exceed either the individual risk
or the societal risk as stated in the Code of Practice V5-01 – Risk Assessment And Control
of Major Accident Hazards [Ref: 11]. This applies even where the new facility is to be
constructed on an existing group company site.
For onshore sites, worker accommodation, such as construction camps, and places where
off-duty workers may be present must be sited so as not to exceed, even temporarily, either
the individual risk requirement or the societal risk as stated in the ADNOC Code of Practice
V5-01 – Risk Assessment And Control of Major Accident Hazards [Ref: 11]. The risk from
all relevant facilities, new, existing, and under development (E.g. well testing) must be
considered.
Compliance with these requirements must be demonstrated using QRA/dispersion
calculations in the HSEIA, and must take into account the anticipated duration of exposure.
For further details refer to Section 14.
For offshore facilities, including relevant vessels, accommodation, muster points, lifeboat
embarkation points must be sited to reduce the risk to all personnel to as low as reasonably
practicable taking into account Sections 8, 9 and 12 of this Code of Practice and as stated
in the ADNOC Code of Practice V1-02 on HSEIAs [Ref: 1].
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DESIGN PRACTICES
These following sections describe three groups of design practices, which can reduce the
risk of personnel exposure to hydrogen sulphide.
4.1

Section 4.1 describes those design practices, including layout development, which
serve to reduce the potential impact of hydrogen sulphide classified areas on plant
operability;

Section 4.2 describes design measures whose intended function is to reduce risk
from accidental releases; and

Section 4.3 describes design practices whose intended function is to reduce
personnel exposure to continuous or episodic releases.
Layout and Design
This section applies to all new facilities (defined for the purposes of this CoP as facilities
where front end engineering design has not commenced at first issue of this CoP) and,
where reasonably practicable, to modifications to existing facilities.
4.1.1
Plant Design
Facilities with Yellow, Amber or Red Zones must be designed and laid out to:

Minimise the size of Red Zones;

Allow simple segregation of the site into Red, Amber and Yellow Zones;

Plants must be designed for minimum intervention requirements within the Red
Zone(s), so that operator and maintenance team visits to the red zone are
minimised.

Plants must be designed to facilitate airline working in the Red Zone

As far as practicable working at height or on elevated platforms in the Red Zone
must be avoided by design.

Avoid venting of fluid containing > 5ppm hydrogen sulphide direct to atmosphere for
any reason. Such gases must be routed to a suitable acid gas recovery system or
flare;

Wherever the OPCO’s carryout cold venting of gases, they have to obtain special
permission by demonstrating ALARP

Where breaking of containment is required to take equipment out of service for
maintenance or other reason, the level of isolation from hazardous fluids must be a
minimum of provable double-block and bleed for temporary isolations lasting less
than a shift and full positive isolation for longer isolations. Note that short term
isolation is required to allow swinging of a spectacle plate or inserting a spade to
achieve positive isolation. Refer to [Ref 1] for further details. Ensure that such
equipment can be fully purged prior to opening and that facilities are present to
allow control of any residual pyrophoric hazards;

Selection of metallic materials of construction must conform to ISO 15156 or better.
(If better, the improvement must be documented by test results following ISO or
NACE International standards). Deviations or changes must be subject to a
management of change review process;
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
Flares and associated sterile areas must be designed to avoid exposure of any
personnel to hydrogen sulphide beyond 15-min STEL limits in the event of flame
out. Flares handling material containing hydrogen sulphide must be designed to
avoid exposure of personnel to sulphur dioxide combustion products beyond the 15min STEL limit.

Avoid placing equipment in confined spaces, pits, or low lying areas where
hydrogen sulphide could build up and where entry may be required from time to time
e.g. for maintenance, sampling etc. Where it is not reasonably practicable to avoid
such arrangements, facilities must be provided to avoid the need for man entry into
the confined space on a regular basis.

Provide a fixed hydrogen sulphide gas detection system in accordance with Section
5;

Primary access to all elevated platforms where planned access (i.e. access to carry
out scheduled activities including maintenance) or escape under air may be required
must be provided by staircase as opposed to vertical ladder. Secondary escape
may be provided by vertical ladder where the provision of a secondary staircase is
not reasonably practicable. In such cases the vertical ladder must be designed to
allow safe and efficient escape wearing self-contained breathing apparatus;

Fail safe down hole safety valves must be provided on all new wells where the well
fluid contains more than 0500 ppm hydrogen sulphide;

Devices whose primary purpose is to detect, control or mitigate hydrogen sulphide
hazards must be defined as HSE Critical Equipment or Systems (HSECES’s) so
that maintenance and testing can be properly controlled and verified. For further
details see the ADNOC Code of Practice V6-01: Verification of Technical Integrity
[Ref: 10];

Demonstrate that the sectionalisation of plant by ESD valves and the time required
for emergency blowdown of plant sections is such as to reduce risks to people to as
low as reasonably practicable;

Demonstrate that the sectionalisation of pipelines reduces risks to people to as low
as reasonably practicable.

Office, canteen and accommodation areas should preferably lie outside the yellow
zone. This should be achieved by design of well, process and safety systems.
Where this proves impossible, one or more rapidly accessible toxic gas refuges
must be provided.
Compliance with these design and layout points must be demonstrated in the HSEIA.
All the above must be considered as potential measures to reduce risks to ALARP levels
for existing facilities. For existing plant, where back fitting of some measures may be
difficult, the quantified ALARP principle must be applied quantitatively to determine
reasonable levels of practicability.
Precautions to be taken for facilities abandonment, well abandonment must also be
considered in the HSEIA based on ALARP principles.
In the field of accidental exposure, almost all of the Technical Guidelines are very clear. For
example, in selection of piping, the selection of the best possible alloy, and the highest
practical corrosion allowance, will give the lowest risk. However, use of the absolute best
Technical Guideline is not always justified.
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This may be looked at in terms of cost/benefit, or in another way, that money can be used
more effectively to improve safety by focusing on the highest risks, and those which are
most easily reduced. To support this, the ALARP principle and associated risk matrix
described in the ADNOC Code of Practice CoP V5-06 [Ref: 18] should be used, wherever
the cost effectiveness of a measure is in doubt. Measures must always be implemented to
reduce risk when the risk is in the HIGH, unacceptable region of the risk matrix.
For continuous and intermittent exposures however, risk based approaches are in their
infancy and have not yet reached the status of industry best practice. In this case the
necessary criterion is to avoid personnel receiving a dose beyond the 8-hr TWA and 15-min
STEL limits. Therefore design measures, including modification of existing plant (back
fitting), should be selected in order to optimize the use of the operational controls needed
and where practicable to avoid the need for routine use of breathing apparatus and other
personal protective equipment (PPE).
4.1.2
Plant Layout
New plant should be laid out to achieve the following:

Minimization of the extent and number of red and yellow zones;

Control rooms are not only important to the safe running of a facility, but are also a
location where many people can be present and are commonly a command and
control centre for emergency response They should therefore be located as far as
practicable from red and yellow zones, consistent with the ability to manage any
incidents arising.;

Where the level of hydrogen sulphide hazard is such that it is not reasonably
practicable to fully protect the control room by distance from all foreseeable
hydrogen sulphide hazards, then further protection should be added such as sealing
the building and provision of a dedicated air supply sized for the potential duration of
the emergency; and

Areas of high manning such as workshops and canteens and, especially,
accommodation should be located as far practicable from red, Amber and yellow
zones. The safety of such locations should be verified by calculation. Moving of
such facilities to an alternate more remote location is preferable to providing
additional protection to the building.

Control rooms, operator rooms and muster points should as far as possible be
outside hydrogen sulphide classified areas, and in any case outside the red zone.

Risk can to some extent be reduced to some extent by placing operator rooms,
control rooms, offices, workshops and accommodation on the upwind side of
facilities with respect to the prevailing wind. The effect is not large (up to a factor 2.5
in the Abu Dhabi region).

More importantly than wind direction, escape routes and temporary gas refuges
must be selected for the possibility for rapid and safe evacuation.

HVAC air intakes should be well away from potential hydrogen sulphide releases
The level of protection afforded by location should be verified by calculation.
Best practice in modern plant design is to completely segregate the sections of the plant
containing hydrogen sulphide from those where hydrogen sulphide content is minimal, and,
where hydrogen sulphide is present, to design these sections for minimum intervention.
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Measures which have been used in the past to achieve minimum intervention are:

Provision of dual control valves, so that control can be diverted to a back-up valve if
failure occurs.

Design to avoid the use of pumps, using pressure driven liquid transfer, where
possible.

Provision of transmitters for remote reading of equipment, rather than transmitters.

Avoiding the use of local instruments and level gauges, using high reliability
instruments or redundant instruments where a single traditional instrument would be
provided along with a level gauge or sight glass in earlier design practice.

Using primarily welded construction rather than flanges wherever possible.

Complete avoidance of screwed fittings on hydrogen sulphide or hydrocarbon
containing piping

Planning integrity inspections so that these can be carried out during major turnrounds, when the plant is depressurised and preferably emptied of gas.

Design which allows major maintenance when the plant is shut down for major turnrounds and emptied of hydrogen sulphide gas.
All these measures must be considered and implemented unless demonstrated as not
reasonably practicable.
4.2
Design Measures For Accident Prevention
4.2.1
Choice of Materials
Hydrogen sulphide is corrosive to steel, especially when present with free water. The
sulphur combines with iron, freeing hydrogen. Hydrogen induced cracking (HIC) occurs
when hydrogen diffuses into the steel and collects at inclusions or defects. Pressure can
become very high and cracks and blistering can occur. Harder steels and areas such as unheat treated welds are more susceptible to such cracking.
Stress corrosion cracking (SCC) occurs on passivated materials such as stainless steels
subjected to tensile stress. Micro cracks open up slightly, and because there is little oxygen
in the cracks the passivating oxide layer is not formed, corrosion can occur. Classic SCC is
then caused by chlorides (or other halides), but hydrogen sulphide can accelerate the
corrosion. The combination of sour salt water is particularly corrosive.
Stress oriented hydrogen induced cracking (SOHIC) occurs where there is a high stress
concentration in sour service equipment. High stress fields allow hydrogen generated by
H2S dissociation to accumulate without the need for inclusions in the steel. Stress from the
hydrogen joins the local tensile stress, causing cracking. Cracks can join up along high
stress lines, leading to stepwise cracking.
These forms of failure can be prevented or reduced to low levels by proper selection of
materials. NACE-MR0175-2003 [Ref: 19] has provided a basis for material selection.
Current best openly published guidance is ISO 15156 Standard [Ref: 20] which combines
information from the NACE standard and from European Development. Some oil company
guides, provide supplements to these standards, providing more streamlined procedures,
and advice on application. Of these, the guidance by CAPP is openly published, as is the
NORSOK guidance for sour gas with CO2 [Ref: 21 and 22].
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In Abu Dhabi, it is important to take account of external corrosion due to sanding. Pipes and
pipelines which are partly covered in sand tend to collect moisture at the 6 o’clock position
due to night time condensation of water vapour, the sand often contains calcium sulphate
and together these cause acid corrosion. The phenomenon can be prevented in some
cases by pipe support design and in some cases by avoiding “wind shadows” such as long
fences. The most effective solution through, is periodic inspection, sand clearance and
sand replacement.
External corrosion must be prevented as well as internal corrosion. An important measure
in Abu Dhabi is to ensure that onshore buried pipelines are installed above the water table.
Selection of coating type of pipelines is important. Failure rates corresponding to different
coating types are given in [Ref: 22].
Elastomers, seals and other non-metallic components also represent an important area
affected by hydrogen sulphide. Special types, suitable for the service should be used.
NORSOK gives guidance on qualification of these materials [Refs: 76 & 77].
QRA and ALARP analysis have become an important part of the design approach for
potentially hazardous plant, but the current methodologies used rarely allow selection of
special materials to be taken into account in determining frequencies of releases. The
methods of structural safety, as used in risk based inspection (RBI) methods do allow such
account to be taken however [Refs. 31 & 32] and software is available (e.g. from API) to
allow such accounting to be made effectively.
4.2.2
Selection of Pipe Wall Thickness
Increasing pipe wall thickness by selection of a higher schedule or class than that required
for service is a standard risk reduction approach. Increasing thickness reduces stress,
provides an increased corrosion allowances, and makes pipelines more robust against third
party interference.
The technique is generally only used where there is special risk, such as in pipeline
sections near villages or through towns. The technique has been introduced as a standard
in French regulations [Ref: 23]. Risk assessment techniques for a selection of pipeline
protection measures are described in [Refs: 24 &: 25].
4.2.3
Protection of Pipelines and Flowlines
Third party interference is one of the major causes of pipeline failure. Risk to pipelines can
be reduced by various techniques. Techniques used in Abu Dhabi are deep burial, use of
designated and fenced rights of way or easements, use of mounding to signal pipe location
and to discourage excavation and driving over pipes, and use of plastic marker strips at 30
cm above pipeline depth.
Many of these techniques can be selected as standard, for example use of dedicated rights
of way, and mounding of the pipelines. Some introduce additional costs, and many have
large effect or limited effect, depending on circumstances. An example is that of selection of
burial depth, which may have large impact in an industrial area, but little effect in the desert.
Risk assessment can be a useful technique in selecting appropriate depth [Refs: 26 & 27]
and in evaluating similar measures.
Sectioning the pipeline can be an important measure to limit the consequences of pipeline
failure, and is of particular relevance for sour gas pipelines. The advantages of sectioning
though should be weighed (by risk calculation) against the additional possibilities of leaks
from block valves and flanges. It may be optimal to provide block valves only where
pipelines pass close to inhabited areas.
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An important issue on shore in Abu Dhabi is avoidance of pipeline bridging due to sand
drift. Flow line and some gathering lines are susceptible to this. Resulting static stresses
can exacerbate SOHIC, and can also lead to wind induced vibration fatigue.
Design measures can include avoidance of sand drift areas, organization of flow lines in
engineered corridors and good support design [Ref. 28]. There is no absolute preventive
measure for bridging due to sand movement however and periodic inspection will be
required. Maximum bridging length criteria for pipes must be calculated, for design where
bridging is likely (sand dune areas) and remediation applied where bridging occurs.
For most cases in Abu Dhabi, the most common context of third party interference is during
construction of or maintenance on neighbouring pipelines. For these activities, a thorough
job safety analysis should be performed where all concerned parties are represented.
Similarly for subsea pipelines, factors which must be addressed to prevent accidents as far
as reasonably practicable include:

Burial versus sea bed placement;

Use of dropped object protection mats;

Providing exclusion zones at platform riser locations and in the areas near
platforms, to reduce risk from dropped objects, anchor drag and trawling risks;

Design to avoid bridging due to sea bed scouring and to limit current induced
vibration fatigue; Use of subsea isolation valve which can close to shorten the
duration of a leak.

Use of dropped object protection mats should be considered for subsea pipelines on
a zone by zone basis, and particularly in the zone near risers.
Analysis of these issues is described in [Refs: 29 & 30].
4.2.4
Minimization of Pipework Failure
The following pipework failure mechanisms have been identified in audit, near-miss and
accident reports:

External dripping water from condensation at road overpasses and conduits causing
heavy external corrosion;

External corrosion by pipes touching soil or sand;

Missing or ineffective pipe supports leading to overstress or vibration;

Pipes “crawling” due to alternating (diurnal) heating and cooling, resulting in pipes
resting on, or even curling around each other, creating possible points for galvanic
corrosion.

Cavitation downstream of control valves;

Vibration and slugging at vertical two phase flow lines; and

Condensate pooling in gas lines, followed by condensate pick up, resulting in
vibration or hammer.

Pitting corrosion especially in dead legs and at the internal 6’o clock position in
pipes such as those on infrequently used (spare) pump piping, pigging piping and
some manifold sections
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All such mechanisms should be taken account when designing facilities, including
modifications, and also in carrying out inspection and corrosion management activities.
Failure frequencies used in quantified risk analysis (QRA) and ALARP analyses can be
adjusted to take into account such factors using structural reliability methods used in risk
based inspection (RBI) as, for example, in API RP 581 [Refs: 31 & 32]. Note that existing
standard RBI techniques may need to be extended by reference to research reports to
cover issues such as those listed above.
Pipe supports should be designed to prevent external corrosion. Best practice is to provide
pipe shoes. On smaller pipes, and when back fitting is needed, use of neoprene rubber
support pads provides good corrosion reduction, though they still allow (possibly salt) water
collection at the contact point. In all cases, steel pipe on steel support contact should be
avoided.
Pipe leaks into low lying parts of plants, such as bunded areas, sumps and lagoons should
be considered for potential for release of sour gas, or of oil with a potential to release
hydrogen sulphide
Section 4.1 of this CoP specifies minimum isolation requirements for maintenance
activities, including isolation valves and blind flanges.
Provision of these valves and
associated equipment will result in additional leak points. There is therefore a trade-off
between providing more equipment for safer isolation, but with a corresponding increase in
the expected leak frequency.
Where isolation from a live system cannot be achieved to the standard required, then the
system cannot be opened and alternative arrangements must be made, which might
require shutdown and purging of large sections of plant. This can result in potentially large
expense and an unnecessary temptation to cut corners.
Group Companies should therefore define the isolation requirements for the full range of
maintenance activities and ensure that the isolation requirements can be met for each
operation to the standard required by the CoP without unreasonable levels of shutdown.
Isolation would typically be provided for:

All equipment where opening is expected once per six months or more frequently;

All spared equipment; and

Where the shutdown of additional systems or units would require purging of large
volumes or result in isolations a long distance from the system being worked on e.g.
to allow maintenance of a gas compressor whilst an associated oil production
separator remains in operation.
4.2.5
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Control of Accidental Releases
There are a number of measures which can be applied to reduce the total quantity of gas
released following breach of containment:
1.
2.
3.
4.
4.2.6
Battery limit ESD Valves should be provided on main incoming lines, at the inlet to
sour gas absorbers and in some cases at the inlet to SRU incinerators (to be decided
by ALARP analysis). The objective is to prevent upstream units from “feeding the
release”. These ESD functions can be designed to SIL2 or SIL3 standard, with SIL 3
being best practice. The actual choice of SIL level may be made according to IEC
61508, if there is difficulty in achieving a SIL 3 performance. (Note that if ESD valves
are to be closed on the basis of gas detection signals, only SIL 2 rated detectors are
currently available. Redundant systems will therefore be needed to reach a SIL 3
performance. This applies equally to fully automatic systems and to manually
activated ESD systems).
Inventory isolation valves can be fitted on the entry and exit lines of separators,
columns, surge and feed vessels, and especially at amine regenerator overhead
lines. These have the effect of limiting release via piping leaks and ruptures to the
inventory of the piping itself. SIL3 implementations of inventory isolation ESD systems
are usually considered best practice at present, but SIL 2 may be justified on the
basis of ALARP analysis.
High Capacity blowdown systems to flare may be provided so that small and medium
size releases can be depressurised quickly. Current systems are usually designed for
15 minutes for pressure reduction to 50% of working pressure or 7 barg. Such
systems do not provide much reduction in immediate risk because most persons
should have evacuated or been rescued within 15 minutes, but, depressuring will
reduce the time during which sheltering is required. Note that large and rupture
releases on process plant will often depressurize themselves much more rapidly than
a blowdown system could. This does not apply to large inventory (long) pipelines
however. Combinations of sectioning valves and blow down venting or flaring may
need to be chosen.
Pipelines may be constructed with sectioning ESD valves. The need for these should
be determined by QRA, which should also take into account the risk arising from
failure of the section valves, blockage, and the risk arising from maintenance and
repair of the valves [Ref. 33].
Pipeline Leak Detection
Leak detection is essential for long sour gas pipelines which threaten large numbers of
persons. Leak detection in gas pipelines is technically difficult. Pressure loss sensors work
only for very large leaks and pipe ruptures, and do not work well on manifolded pipelines.
Model based and statistical compensated mass balance detectors have much better
capability. Detection of losses of 0.3% of total flow has been claimed for liquid lines, and
1% for gas pipelines. This still represents a very significant release for most sour gas
pipelines, so releases of these sizes should be considered in risk assessments when there
are large numbers of persons located in a hazard zone with no credit for gas detection.
There are three API standards for model based leak detection [Ref. 34, 35 and 36]:

API 1130
Computational pipelines modelling for liquid pipelines;

API 1155
Evaluation methodology for software based leak detection systems;

API 1149
Pipeline variable uncertainties and their effect on leak detectability.
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Acoustic detectors can be used to detect liquid leaks as low as a few ml. per second, and
sensitivities as low as 10 - 20 ml/second have been claimed for gases [Refs: 37 & 38].
These successes are dependent on the detector being close to the leak (50 to 100m) so
that the approach is only practical as a portable inspection device or close to an inhabited
location. Fibre-optic distributed acoustic detection devices are available on the market.
For small gas leaks from high pressure pipelines, temperature reduction due to Joule
Thompson cooling can be detected using fibre-optic cable [Refs: 39, 40 & 41]. Sensitivities
down to 0.1 deg C can be achieved with reasonable signal repeater spacing. Placement of
the cable needs to be considered carefully, to ensure that temperature reduction is actually
sensed.
4.2.7
Well Systems During Drilling, Testing and Workover
The prime safety measures during drilling and workover are good control of mud weight,
and blow out preventers. The design of the blow out prevention should be suitable for the
levels of hydrogen sulphide expected, and should take into account the possibility of acid
gas break through to sweet gas or oil formations. The possibility of underground blow out,
including blow out from injected gas, should be considered. In assessing these issues, high
quality risk analyses should be used, which provide analysis on a part by part basis [Ref.
78, 84, 85, 86 & 87] where standardized drilling and workover arrangements are used. The
analyses may be generic, but should nevertheless be detailed. Where generic analyses are
used, a specific section giving gas dispersion characteristics for each well or group of
similar wells should be provided, and the well/drilling design chosen should be reviewed
accordingly. The analyses should take into account any low points or dune formations
which could collect or channel the gas.
Wells which blow out sour gas should normally be ignited, with a delay to ignition of at most
30 minutes. Suitable ignition apparatus (remote spark igniters, flare pistol etc.) should be
provided at a location suitable for ignition of all foreseeable blowouts. Whilst ignition of the
blowout will often reduce the hazard from hydrogen sulphide, there are a number of issues
which must be addressed when responding to a blowout in this way:

Combustion products (smoke) will contain significant concentrations of toxic sulphur
dioxide. All persons will need to stay well away from the smoke plume unless using
a BA set. Suitable toxic gas detectors will be required to verify affected areas. The
potential impact of the smoke plume on the public as well as the work force must be
considered;

Generally toxic combustion products will rise and disperse, but this may not always
be the case, for example of the application of firewater which cools the smoke.
Water spray from ground monitors can be used to enhance the dispersion of the
smoke plume; and

Depending on the nature of the blowout it can happen that some hydrogen sulphide
is emitted separately from the main plume and is therefore not combusted.
Similarly, liquids emitted with the blowout can continue to release quantities of
hydrogen sulphide. These are further reasons why the use of BA is necessary
when responding to blowouts and for the use of detectors in defining hazard zones.
Potential for Hydrogen Sulphide release during well testing and intervention operations
should be considered. The sources of such releases must be identified and managed.
Dispersion calculations should be performed from all possible sources of H2S and SO2.
Under no circumstances should H2S be vented to atmosphere. Detailed Risk Assessments
should be performed for all activities undertaken during drilling, testing, workover and other
well intervention operations. The management of these controls should be part of the
HSEIA report.
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Sour gas wells in operation should be provided with high integrity ESD systems including
wing valve, master valve and down hole safety valve closure. The system should be built to
SIL2 or SIL3 as required by IEC 61508 [Ref: 17] taking into account the environment of the
well and proximity of local populations. Good practice also includes provision of gas
detectors, located in such a way that the probability of detection is high.
4.2.8
Equipment Isolation
Many of the accidents associated with Hydrogen Sulphide occur during maintenance, when
fitting or removing spades, or by inadequate isolation. Good isolation during maintenance is
an essential measure for hydrogen sulphide release prevention [Ref: 42].
A single isolation valve is not regarded as adequate for safety, when a vessel or pipe
section is to be opened. For general maintenance operations there must be a positive
isolation in the form of a blind flange or spade, or a removable spool piece.
For some operations such as filter cleaning, frequent removal and replacement of spades
can increase risk. A double block and bleed valve arrangement may be provided for such
situations.
For lethal service, double block and bleed should be supported by a pressure indicator
between the block valves. Care should be taken in the design of this indicator, since it must
withstand full process pressure, but must be able to detect low pressures arising from valve
leakage.
Bleeds from double block and bleed isolation must be routed to a suitable location, with no
possibility of back flow of gas, as could arise for example by routing the bleed to a
pressurized flare header.
Double block and bleed using valves remote from the section to be isolated is not
acceptable, being too prone to error.
These solutions should be provided by design, and should be identified in design reviews
such as HAZOPs. Difficult situations can arise in practical work, for example when leaks
occur at sections of pipe where there is only one block valve between the location and a
large hydrogen sulphide inventory. In this case the best overall risk reduction may be to
close the single isolation valve, and make any repair etc. using air line breathing air supply,
or SCBA equipped with full face mask, if the work is very short, as the second level of
protection. In this case care must be taken to ensure that the air supply is adequate, that
the person(s) carrying out the work are supported by a “buddy” who also has an air line or
SCBA air supply, and that there are no other persons in the potential hazard zone.
4.2.9
Cold Venting and Flare Flame-Out
Cold venting occurs when vents or safety valves release directly to atmosphere or where
sour gas is sent to flare (from vents and from pressure safety valves) and there is a
possibility of the flare flame-out. (for various reasons, including nitrogen release into the
flare, carbon dioxide relieving from CO2 rich sour gas, low gas flow, high wind speeds and
flammable liquid in the flare).
When the flare is extinguished, the gas will rise if it is light (methane rich) or fall to the
ground if CO2, propane, butane or H2S concentrations are elevated, or if the gas is cold.
Automatic flare igniters should be used for preventing prolonged cold venting (sometimes
called cold flaring). The reliability of these igniters should be studied and adequate
redundancy should be provided to bring risks to ALARP.
Relief valves venting sour gas to atmosphere must be avoided by design. Reliefs shall be
routed to a suitable flare or an acid gas recovery system.
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It should be noted that standard calculations used for design of vents are based on simple
models, which ignore secondary aspects of gas dispersion such as vent downwash,
building downwash, and stripping of the vent jet by the wind. Although these effects do not
generally affect the main features of vent flow, and the possibilities of gas ignition, they can
give significant problems with gas smell and low level gas exposures.
The effects can make it difficult to meet 8-hr TWA and 15-min STEL criteria, unless they
are taken into account in vent design (see Appendix 2).
4.2.10 Sumps, Drains and API Separators
Sumps and drains are a frequent area for accidents. Mostly, the accidents can be
prevented by confined space entry restrictions. There are cases however where hydrogen
sulphide can be emitted from sumps and drains. The most important ones are admission of
rich amine solution to drains, admission of hot water (steam condensate) to drains, and gas
release into closed drains due to level control failure.
Amine solutions which are drained down from process equipment should be collected, not
released to drains. This may be done by providing a closed sump, from which solution can
be returned to storage, or sent to disposal, or by draining to a portable vessel.
Drainage of hot water to drains needs to be controlled procedurally (e.g. under PTW). It is
particularly likely when draining steam condensate during steaming out for maintenance.
For this reason it is good practice to ensure that drains and sewers which could contain
amine (or caustic solutions used for sweetening) are flushed prior to major maintenance.
Where the possibility of toxic gas release to drains exists as a result of level control failure,
appropriately SIL rated shutdowns must be provided. Also, the area around sumps and
drains may need to be made into a Red Zone.
4.2.11 Pig Launchers and Receivers
Pig launchers and receivers are equipment frequently opened, sometimes several times
per year. They are a source of a significant number of incidents.
Important safety measures for pig launchers and receivers are:

A good signalling system so that the location of the pig in the launcher/receiver is
known.

A receiver or launcher venting system for depressurisation which is routed to a
suitable location but close enough for the depressurisation to be heard

Provide a pressure gauge which is sensitive enough to be able to indicate pressures
below 1 barg, but robust enough to withstand the pipeline pressure.

Provide a mechanically interlocking door, which cannot be opened under pressure.

Provide water at the location for wetting down any iron sulphide scale.

Provide a metal container with lid for scale disposal
For sour service some additional measures are needed:

Provide two block valves on the incoming line to the launcher and similarly two block
valves on the incoming receiver line, to be closed at all times when the pig trap is
open;

If there is a chance of back pressuring the receiver from the discharge side, provide
two block valves;

Pigging on sour service lines should be carried out with all engaged personnel using
BA sets;
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
Provide documented task risk assessment covering the operation including H2S
risks; and

Provide a detailed documented method statement.
4.2.12 Sewers and Waste Water Treatment
Around the world, the largest cause of H2S fatalities is not process plant but sewers,
sewage treatment and animal waste plant Spills of sour water can also lead to the emission
of large quantities of hydrogen sulphide. Whilst this is typically a lesser hazard for ADNOC
companies compared to hydrogen sulphide in process fluids, it still needs to be taken into
account in safety assessment and design.
For sanitary sewers, hydrogen sulphide is generated by biological decomposition of wastes,
and the main method of reducing hazard is to ensure good drainage and short hold up time.
[Ref: 43] gives a good guide to design practice and [Ref: 44] gives calculation methods.
Waste Water treatment plants handling sour water should be designed for safety like any
other sour service process plant, and must be provided with toxic gas detectors and alarms
whenever a significant hazard to people exists from hydrogen sulphide.
Beyond these design issues, the most important safety measure connected with sewers is
to regard any entry to a sewer, sump or pit as a confined space entry requiring gas testing
before entry, and monitoring during presence in the confined space, as well as appropriate
PPE.
4.3
Minimization of Long Term Hydrogen Sulphide Emissions
4.3.1
Emission Reduction
The principle of emission reduction for hydrogen sulphide is to keep the gas inside the
equipment. This means sealing all openings. The European Sealing Association has given
statistics concerning the sources of volatile organic compound (VOC) releases as well as
best practice [Ref: 45]. Although hydrogen sulphide has some other potential sources, the
majority of releases resemble those for VOCs.
For new plants, it is highly desirable that chronic hydrogen sulphide emissions are
calculated prior to operation. Emissions can be calculated on the basis of emission factors
(EF) which give the average rate of release from specific equipment types. The total fugitive
emission rate depends not just on the leak rate, but also on the fraction of equipment which
are leaking. The US Environmental Protection Agency has given tables both of leaking
fraction (LF – the fraction of a particular type of equipment found to be leaking at any one
time) and EFs [Refs: 46 to 55].
For actual exposure protection close to a source for employee protection, the expected
release rates (ER) should be calculated as:
ER = EF/LF
Appropriate dispersion calculations can then be used to determine at what distance WEL
and STEL concentrations will occur. Then:

Exposure time in these zones should be at most a fraction of the time to exceed the
WEL and STEL;

Release reduction measures should be implemented; or

Operating procedures should contain warnings and require PPE.
4.3.2
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Valve Seals
Valve seals are intended to prevent leakage along the movable stem of the valve. Use of
low emission seals can give release rates 1,000 times smaller than simple traditional seals
(mineral fibre rings). Best practice in packing selection should be used [Ref: 45]. In the
most hazardous cases, the use of bellow-sealed valves should be considered.
4.3.3
Pump Seals
Pumps handling fluids with significant hydrogen sulphide (defined as greater than 500ppm
by volume in the vapour phase when flashed to atmospheric pressure) should be a
minimum of double-seal design with a seal fluid or should be of seal-less (fully closed or
“canned”) construction. Seal fluid system should be designed to minimize hydrogen
sulphide build up in the seal fluid and seal fluid reservoirs, and should take operator and
maintenance personnel safety into account. Seal fluid systems should be designed to
alarm following failure of one seal to allow intervention before a second seal fails.
For small pumps handling fluids with lower hydrogen sulphide concentration, simpler
designs may be selected, provided that:
a)
b)
During design, emission limits can be calculated to be less than the STEL in the area
around the pump, at 0.5m from the seal.
During operation, emission limits can be measured, under a range of operating and
wind conditions, to be less than the STEL at 0.5m from the seal.
The ESA (European Sealing Association) best available technology guide [Ref: 45] may be
used in support of design to achieve these levels.
All pumps handling liquids with 500ppm or more of hydrogen sulphide in the equilibrium
vapour should be provided with drains piped up to a closed drain system for maintenance
purposes.
4.3.4
Centrifugal Compressor Seals
For centrifugal compressors, gas lubricated primary seals, with a secondary seal to retain
the gas, coupled with gas flow and pressure monitoring and with safe disposal of the seal
gas should be used.
Alternative designs may be acceptable provided that the manufacturer/vendor can
demonstrate a performance close to zero release, as defined in [Ref: 48].
Reciprocating compressor seals compressor shaft seals should have a high integrity
primary seal, and flushing gas. For lethal service compressors, best practice is to have a
spacer chamber with primary and secondary seals and inert gas flushing of the spacer
chamber.
4.3.5
Venting
Planned venting of gas containing hydrogen sulphide is forbidden. Gas containing
hydrogen sulphide must be routed to a suitable flare with a sufficiently high stack height to
ensure that concentrations of SO2 and potentially un-burnt H2S at ground level cannot
exceed the STEL or WEL under adverse wind conditions, taking into account downwash
due to unstable atmospheric conditions and to buildings, tanks and process equipment.
The type of calculation used for dispersion assessment needs to be selected carefully.
Many calculation methods provide average concentrations only, not peak concentrations,
and many do not take phenomena such as inversion into account. See Appendix 2 for
further details on performing this type of calculation).
4.3.6
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Sulphur Pits
Sulphur pits are designed for degassing the sulphur and are agitated. Sulphur pits will
therefore contain free hydrogen sulphide gas. Good practice is to ventilate (extract) with air
and pass the air to an incinerator or thermal oxidizer [Ref. 56].
Liquid sulphur degassing down to 5ppm hydrogen sulphide is regarded as good practice.
To achieve this, several proprietary processes are available, including sulphur spraying and
the use of stripping and columns.
4.3.7
Sulphur Flakers
In a flaker, liquid sulphur is passed between cooled rollers. The sulphur cools and freezes
to form bright yellow flakes. In the process part of the residual hydrogen sulphide is
released. This is particularly the case if the sulphur is “off spec” due to failures in the
degassing process.
To keep hydrogen sulphide emissions low, sulphur flakers are ventilated to a safe location,
usually to a high vent because of the low hydrogen sulphide emission rate. New flakers
should be completely enclosed to minimize emissions.
4.3.8
Drilling
Mud rooms on platforms offshore drill rigs and mud shakers on onshore rigs are a typical
source of H2S exposure during the period where the drill is approaching or is in sour oil or
gas producing locations. Care should be taken when such formations are known to exist, or
are suspected [Refs: 15 & 16].
Adequate ventilation must be provided, and operators and others approaching the area
should be provided with exposure monitors as well as personal escape devices. Closed
areas such as mud rooms must be provided with fixed gas alarms.
4.3.9
Disposal of Ventilation Air
When ventilation is used to reduce local hydrogen sulphide emissions, it can be difficult to
dispose of the resulting contaminated air. Disposal to flare may be undesirable because a)
introducing air to a flare system is dangerous and b) there is often a significant back
pressure from the flare. Disposal via dilution and stack emission may be permissible at very
low concentrations.
There is a wide range of more appropriate solutions for disposal including the use of
thermal oxidizers/ scrubbers (usually sodium hydroxide based), absorption on iron sponge
and absorption on alkali treated activated charcoal. For large quantities of acid gas, well
injection is often used, in some cases with the dual intent of disposal and oil production
stimulation.
Choice depends on the concentration, total release quantity and target concentration.
Design of such systems requires that the performance can be calculated and also that
dispersion calculations can be made to determine effectiveness.
The design goal of these measures should be to ensure that WEL and STEL levels are not
exceeded during normal operation and maintenance activities.
4.4
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Back Fitting
The design guidance given above is current best practice for new designs. It may be
difficult or very expensive to apply retrospectively to existing designs. Where there are such
difficulties, the ALARP principle maybe used to determine whether designs measures
should be taken. If an ALARP analysis shows a good benefit/cost ratio, even when taking
back fitting costs into account, the approach should be taken. If not, then appropriate and
effective PPE should be used for protection of employees for long term or chronic releases.
Some modification may be required to allow practical application of the hydrogen sulphide
zone classification system. For example, it may be necessary to introduce additional
means of depressuring or acid gas disposal facilities to allow depressurization of plant to
effectively remove a Red Zone during maintenance, for example.
For accidental releases where the severity is in the Severe or Catastrophic category as per
the ADNOC Risk Assessment Matrix [Ref: 1], changes must be made.
4.5
Exposure Monitoring Of Workers
Most of the currently used personal H2S and SO2 monitors have facilities to download time
stamped concentration profiles. These downloads must be maintained against each person
with the following information:

Name of person

Serial number and make of the H2S and SO2 monitors assigned to the person

Date / time stamped record of concentrations

Justification for exceeding WEL or STEL values
Workers Exposure Monitoring reports must be maintained by Group Companies as part of
their H2S Management Dossier.
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5
DETECTION
5.1
Personal Alarms
Personnel, including temporary workers and visitors entering Yellow, Amber or Red zones
must be provided with personal hydrogen sulphide detectors which will alarm in the event of
exposure to hydrogen sulphide concentrations at or exceeding the toxic gas warning level
of 10ppm.
Personal hydrogen sulphide detectors must have the following minimum specification:

Cannot be switched off either deliberately or accidentally; and

Alarm at the toxic gas warning level in such a way as to be recognisable under all
foreseeable conditions including in high noise areas.
A system must be in place to ensure that personal hydrogen sulphide detectors are
properly maintained. Devices must be tested and calibrated as per the manufacturer’s
recommendations or more frequently or whenever the sensor is exposed to high
concentrations of H2S, if this is determined necessary for reliable operation. Bump testing
is permissible for routine testing on a frequent basis, but actual calibration testing must also
be made at an interval in accordance with manufacturer’s recommendations.
Some personal detectors are able to provide TWA total dose and STEL event recording.
Some have a resolution down to 0.1ppm, making them useful as chronic exposure
monitoring device, in addition to providing accidental exposure warning.
5.2
Fixed Detectors
All sites with Yellow or Red zones must evaluate their requirement for a fixed hydrogen
sulphide detection system in accordance with the performance needed to meet ADNOC
risk Criteria and accordingly provide, operate and maintain the system which will reduce
risks to ALARP level.
Group companies must define the purpose of the fixed detection system, e.g. to warn of a
toxic gas release passing from a plant area to a non-plant area, or to detect toxic gas
passing beyond the boundary fence. The fixed detector system must then be designed so
that it fulfils the intended purpose, with reliability sufficient to meet the risk and ALARP
criteria.
Alarms from fixed detectors must be set at the toxic gas warning level of 10ppm for warning
and at the toxic gas emergency level of 15ppm for immediate evacuation.
The detectors must be designed and placed so as to minimize the frequency of nuisance
alarms and to provide a high probability of detection in an actual emergency.
Hydrogen sulphide alarms must be operable during the entire period of plant operation or
where this cannot be done as for example during steaming out; the area must be
evacuated up to a predetermined safe distance, or to a distance determined to be safe by
the use of portable hydrogen sulphide detectors. The possibility of change in wind speed,
wind direction, and of release size must be taken into account.
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For plants in remote areas with minimum manning and for remote wells and manifolds,
fixed gas detection networks must be designed to ensure detection and alarming sufficient
to:

Guarantee detection and alarming for releases which threaten persons in the any
local operator room, with reliability sufficient to meet the risk and ALARP criteria.

Guarantee detection and alarming of releases which could threaten persons
approaching the area along approved routes with a reliability sufficient to meet the
risk and ALARP criteria.

Guarantee alarming at the central control room and emergency centre for any
releases which could be a threat to the public, with a reliability sufficient to meet the
risk and ALARP criteria.
For work teams in the field, provision of fixed detection systems will often be impractical.
For these cases, detection must be provided in the form of mobile detectors and personal
alarm monitors. The detection needs to be backed up by means of a portable wind sock (for
example attached to a small mast or vehicle) because the origin of the release will often be
unknown to the work team, and may be remote.
5.3
Detector response times
The response time for alarm systems is an important parameter. Many of the worst
releases can be over in a few minutes. Typical response times for electrochemical and
MOS hydrogen sulphide detectors are 30 seconds up to 2 minutes.
Some newer MOS sensors based on newer technology have response times as fast as 10
seconds, and optical point detectors can have response times down to 1 sec. The response
time for open path detectors depends on the path length, but can be as short as 1 sec for
paths of a few meters, and 10 sec for paths up to 100 meters.
Since emergency shutdown and evacuation alarm times depend very much on detection
times, companies must take response times into account when selecting detectors.
Response time should be minimized, consistent with reliability and reasonable (ALARP
level) cost.
5.4
Hydrogen Sulphide Alarms
Confirmed releases of hydrogen sulphide must be alarmed with a continuous high-pitch
tone and flashing beacon audible / visible at all relevant locations.
Alarms must be compliant with EN ISO 7731 and EN ISO 11428 [Refs: 12 & 13]. The
audible alarm signal must be a continuous high pitch siren tone and a flashing blue light.
5.5
Hydrogen Sulphide Exposure Monitoring
Hydrogen sulphide surveys are required to ensure that persons are not exposed to
continuing or intermittent injurious levels of hydrogen sulphide
Initial scoping studies for finding potential areas of hydrogen sulphide exposure can
conveniently be carried out using hand held electronic monitors, these are available with
sensitivities in the ppb region, but for 8-hr TWA measurement purposes, instruments which
measure down to the 0.1 ppm level are more suitable, and more readily available as
standard industrial equipment. Instruments with a probe are often more convenient for
identifying the source of small releases from valves or rotating equipment.
Once the hydrogen sulphide survey has been defined, the survey can be carried out using
dosimetry badges, absorption tubes, or personal recording monitors.
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5.6
Detector Selection And Placement
5.6.1
Selection Of Gas Detectors
There are four main types of hydrogen sulphide gas detectors currently available:

Electrochemical

Solid state gas detection

Point optical gas detectors

Open path optical detectors (tuned diode laser type)
Electrochemical detectors have known reliability problems in arid environments, due to
dehydration. Solid state gas detectors have known reliability problems in humid climates,
and also can become insensitive unless exposed to hydrogen sulphide on a regular basis,
typically every 6 months (see manufacturer recommendation). Optical detectors can fail
due to dirt on the lens. Open path optical detectors can fail due to vibration or obstruction of
the light path, as well as dirt on the lenses and reflector (if used).
Some electrochemical and solid state detectors are especially unreliable [e.g. Ref: 58].
Detectors of these types should only be used in Abu Dhabi climates if there is good
operational experience of the specific model, or a closely related model from the same
manufacturer, for a climate similar to either Abu Dhabi desert environment for the inland
sites, or a climate similar to Abu Dhabi maritime climate for coastal and offshore
applications.
Optical detectors provide the advantage that they will alarm on loss of infra-red beam signal
or other failure detection, and are effectively fail safe. The detectors can fail due to dust
settlement on lenses, but in this case will lose a good deal of the signal, and most types in
this case automatically raise an alarm.
Electrochemical and MOS type detectors are generally only fail safe with respect failures in
the electronic circuitry, not in the detector element.
Support structures for open path detectors should be rigid and robust, and decoupled from
any vibration sources such as compressors or pipework subject to two-phase (slug) flow.
Open path detectors have a disadvantage that it is generally not possible to identify the
source of emission. A combination of point detectors and open path detectors will therefore
generally provide the most effective design.
Hydrocarbon gas detectors can be used as surrogates for, or supplements to, dedicated
hydrogen sulphide detectors, provided that the concentration of hydrogen sulphide within
the gas is less than 500ppm. If concentrations of hydrogen sulphide in the equipment are
higher than this, concentrations in the release may be well above hazard limits, even
though the flammable gas is below the lower explosion limit.
The response time for alarm systems is an important parameter. Many of the worst
releases can be over in a few minutes. Typical response times for electrochemical and
MOS hydrogen sulphide detectors are 30 seconds up to 2 minutes. Some newer MOS
sensors (NT MOS) based on newer technology have response times as fast as 10
seconds, and infra-red point detectors can have response times down to 1 sec. The
response time for open path detectors depends on the path length, but can be as short as 1
sec for paths of a few meters, and 10 sec for paths up to 100 meters. As can be seen, this
CoP favours optical and the rapid MOS detectors.
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Since emergency shutdown and evacuation times depend very much on detection times,
companies must take response times into account when selecting detectors. Response
time should be minimized, consistent with reliability and reasonable (ALARP level) cost.
Fire & Gas mapping is the best tool to determine the locations and reliability of the
Detectors. The type of detectors will be determined based on the function they have to
perform in accordance with international standards.
5.6.2
Alarm Levels
Alarm levels for hydrogen sulphide should be consistent so that operators and emergency
response teams know how to interpret the alarm. Values of 10ppm for an initial (warning)
alarm and 15ppm for an action alarm are adopted. The action on alarm must be to don the
emergency escape set or start to use the BA set and to evacuate to a safe haven or muster
point.
Confirmed releases of hydrogen sulphide above the 15ppm level must be alarmed with a
continuous sweeping high pitch alarm and flashing beacon.
Alarms must be compliant with standard EN ISO 7731 and EN ISO 11428 [Refs. 60 & 61].
These require audible alarm levels at least 15 dB above background, and for emergency
evacuation, should be continuous sweeping tone, and beacons should be blue. The
standards require placement of sirens and beacons in locations where they can be, seen
and heard.
5.6.3
Upgrade Of Existing Fixed Detection Systems (Back Fitting)
Most existing plants already have extensive existing gas detection systems. These may
have a full coverage analysis available. Where coverage has not been analysed, the
opportunity should be taken to provide such an assessment, for example if there is a
concern about inadequacy, following a near miss, or when there is a change to the design
of the plant or when preparing the 5 yearly HSEIA report update.
When back fitting detection systems:
5.6.4

The purposes of the detection system in protecting people should be considered,
and

Any upgrading proposed for the network should meet the ALARP criterion.
Fixed detection near and around wells
Fixed toxic gas detection shall be provided for oil and gas well locations based on the
following criteria, as a minimum

H2S gas concentration in the gas well / reservoir stream >500 ppm

H2S gas concentration in the oil well / reservoir stream >1000 ppm
The above criterion shall be applied to the all the new well locations. For the existing well
locations the OPCOs shall conduct a risk assessment to determine the requirement for
providing fixed gas detection. While carrying out the risk assessment, the OPCOs shall
consider the environmental sensitivities of the location, animal pens, farms, public roads,
railway, social/public settlements, simultaneous operations etc. around the well location.
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6
CONTROL
6.1
Permit to Work
All facilities where a hydrogen sulphide hazard exists must operate a Permit-To-Work
system. Permits-To-Work in Yellow and Red zones must not be issued without pre-work
site inspection, which must include as a minimum: determination of potential for release, a
task risk assessment (TRA), correct isolation, appropriate controls/PPE, buddy system and
requirements for continuing detection.
All lockouts, interlock removal, including the movement of locked valves and similar
operations undertaken in Yellow and Red Zones must be controlled by permit and verified
using the buddy system.
6.2
Activities Requiring Breathing Apparatus
Air-line fed positive pressure supplied air breathing apparatus must be used for all
prolonged operations where there is a risk that personnel may be exposed to hydrogen
sulphide concentrations above the STEL as a result of equipment failure, human error
during the operation or deliberate venting. This includes all of the following operations
whenever such a hydrogen sulphide risk could exist:

Breaking of containment, including swinging spectacles or inserting spades;

Taking samples, including product quality activities where the possibility of
dangerous levels of hydrogen sulphide exist;

Confined space entry; and

Local venting of equipment, including instruments during calibration or testing,
where personnel are so close to the vent that they could be exposed to
concentrations above the STEL.

Drilling activities on rig floor, flaring, BOP, Mud tanks, shale-shakers etc. wherever
the H2S levels exceed or potentially exceed the STEL.
Positive pressure breathing apparatus must be used for these activities. Self-contained
breathing apparatus (SCBA) may be used when the task involved is very short, for example
sample taking. Generally air-line breathing apparatus is preferred.
Note that these requirements apply also when working in a Red Zone where wearing
breathing apparatus is an entry requirement and may influence the type of breathing
apparatus to be used for the Red Zone entry, which must also take into account, for
example, the size of the red zone; in very large red zones personnel can exhaust their
SCBA whilst walking from the entry point to the work location.
Design to avoid continuing exposure to hydrogen sulphide must be preferred to the use of
breathing apparatus wherever this is practicable.
Further details of breathing apparatus requirements are given in Section 10.
6.3
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Surveys
Where not already carried out, Group Companies must carry out surveys of operations
such as sampling and venting to determine the need for breathing apparatus and any
limitations on the time personnel can be present to avoid exceeding the allowable timeweighted average dose. These surveys must take into account hydrogen sulphide
concentration arising from unavoidable emissions during the operation together with
background emissions from such sources as leaking valve stems or local venting.
Where it is necessary for personnel to wear breathing apparatus to avoid exceeding the
STEL or the 8 hour TWA concentration, Group Companies must repair or replace
equipment, modify the operation to be carried out, or take other appropriate steps to avoid
the need for breathing apparatus wherever this is reasonably practicable.
6.4
Start Up
Group Companies must ensure that there is independent verification of readiness before
introduction or reintroduction of substances containing greater than 500 ppm hydrogen
sulphide in the vapour phase after let down to atmospheric pressure. This applies both to
initial commissioning and start-up and to start-up following a shutdown where breaking of
containment, introduction of lockouts or overrides, or other activities have occurred which, if
not properly reinstated, could compromise the ability to prevent, control or mitigate
hydrogen sulphide hazards.
In this context, an independent verifier must be familiar with the plant, but must not be part
of or report to the individual or team who are responsible for commissioning, starting up or
reinstating the plant.
The scope of the verifier must include checking of all relevant items such as:
6.5

Correct position of isolations, including spades;

Reinstatement of overrides; and

Successful completion of leak testing where relevant
Worker Competency
All personnel involved with operations involving materials containing hydrogen sulphide
must be competent to perform the activities required of them. Personnel working in Green,
Yellow or Red Zones must have the level of competency for those Zones; training
requirements are given in Section 9, whilst specific competency requirements are given in
Section 11.
7
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MITIGATION
Where evolution of hydrogen sulphide from an accidental spill of liquid with the potential to
harm people is foreseeable, then the following must be provided unless demonstrated to be
not reasonably practicable:

Secondary containment; and

A means to limit the evolution of hydrogen sulphide, such as blanketing with foam.
Hydrogen sulphide is a reactive gas which is susceptible to explosion when mixed with air.
Appropriate provisions for explosion prevention and consequence mitigation must be taken,
such as rapid ESD, and explosion protection measures.
ADNOC group companies storing solid sulphur must determine the possibility of hydrogen
sulphide being released from solid sulphur, in the case of both properly specified product
and off spec production. If injurious concentrations can be generated, appropriate
ventilation must be provided and also appropriate personal protective equipment.
Accidental release of gas in the rig floor is diluted by using bug blowers/fans.
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8
EVACUATION, RECOVERY & RESCUE
8.1
Emergency Plans and Procedures
The general requirements for the development and testing of emergency plans and
procedures at group Company facilities are given in the ADNOC CoP V5.02 Crisis and
Emergency Management [Ref. 2].
There must be a scenario specific emergency response plan for each type of scenario in
which persons can be affected by hydrogen sulphide. In order to make a scenario specific
emergency plan:
1.
2.
3.
4.
5.
All the possibilities for release should be identified, e.g. from the COMAH report,
which must include practical experience from the plant operations.
The extent of any release (plume size) should be determined for normal and
“reasonable worst case” release sizes (e.g. 25 mm for process plants and
representative open hole conditions for drilling etc.) and wind and atmospheric
stability conditions. Cases in which safety measures work and in which they fail
(escalation factors) should be considered, with corresponding contingency actions.
A few scenarios should be selected from the overall set, preferably the largest cases
of each type of accident. These are the base accidents to be used in emergency
planning.
The generic emergency plan should be applied to base accidents.
Timings should be determined for each base accident scenario.
The resources required for emergency response, particularly for rescue, should be
quantified.
Calculations for the emergency plan should take into account the inventory of gas available
for release, and the corresponding release duration, to avoid gross overestimation of the
emergency response needs. When making the calculation, the possibility for failure of
emergency shutdown valves, and the resulting time needed for manual shutdown, should
be taken into account.
Ensure that the medical centre and the emergency response centre are not in the hydrogen
sulphide plume (cloud path), or alternatively, that these locations are designed as a high
integrity toxic gas safe haven.
Assembly points must be sited at safe distances, as determined in the emergency plan.
The onsite assembly points should be outside the Yellow zone for the base scenarios. If an
assembly point is designated, even though it can be within the cloud path inside the AEGL2
limit for some more extreme scenarios, it should be adequately protected and provided with
detection and alarm, or assembly point wardens should be equipped with hand held or
personal hydrogen sulphide detectors.
Onsite emergency plans must be developed for the Risk-Based zones (Red, Amber and
Yellow zones). Offsite emergency plans must be developed for EPZ and EAZ.
Where a hydrogen sulphide hazard exists, the emergency plans and procedures must take
into account the nature and extent of those hazards, especially:

Any need to alert the public or third parties of a hydrogen sulphide hazard beyond
the boundary fence;

The control of visitors, temporary workers and contractors, including contractors
under the supervision of a third party, any of whom may have limited knowledge of
Arabic or English;

The need for trained and suitably equipped rescue teams to retrieve personnel
overcome by hydrogen sulphide;
8.2
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
The existence and nature of Green, Yellow and Red zones and how access
between them is controlled;

The possible requirement for a toxic gas refuge where personnel can seek shelter
whilst a gas cloud disperses (see Section 12);

Any special requirements for personnel on air-lines in the event of either a nearby
hydrogen sulphide release or other type of emergency elsewhere on the facility; and

The time required to don emergency escape mask or breathing apparatus.
Escape Routes in Yellow, Amber and Red Zones
All escape routes on facilities where a hydrogen sulphide hazard exists must be passable
by personnel wearing self-contained breathing apparatus. Escape routes must be identified
and marked (see Ref: 2 – ADNOC CoP V5-02). For fixed installations a means of
emergency communication to the control room/emergency centre must be provided.
8.3
Evacuation
Self-evacuation is one of the most effective protective measures in response to hydrogen
sulphide release. However, evacuation itself can be dangerous if not done properly:

Smell is not a reliable basis for evacuation. Evacuation should occur when there is a
hydrogen sulphide alarm;

All alarms must be treated as real until proven false, and the appropriate response
must be initiated (alarm sirens and beacons based on fixed alarms will generally
only be activated on confirmed gas release);

Small local releases should lead to local evacuation. Persons should retreat to a
distance at which the personal hydrogen sulphide detector ceases to register;

All alarms must be investigated by qualified personnel and conclusions reported. If
the release is a nuisance release the alarm may be reset, but the person
investigating should test the area with a hand held indicating instrument to attempt
to determine the source;

Failed personal detectors should be replaced promptly;

Evacuation can be hazardous, especially if large numbers of persons are present,
e.g. due to falling and trampling;

It is a great help in securing safe evacuation, if the alarms are “staged”, so that a
local alarm leads to local evacuation, a plant unit alarm leads to evacuation of a
specific plant unit etc. This allows most large scale evacuations to be avoided. A
single instrument should never be the basis for a full site or plant evacuation unless
it can be demonstrated to be highly reliable in the field;

Running away from a gas plume is appropriate provided that you can see the plume
taking in consideration that gas is colourless and plume will be invisible in many
cases [Ref. 66]. When they cannot see the plume persons should walk briskly, to
avoid getting out of breath and increasing breathing rate, which increases toxic
dose;

Wind direction should be indicated by wind socks, at least one of which should be
visible from every outdoor location. No person in any location should have to move
more than 5 meters to see a wind sock.
Evacuation routes must be wide enough to take the evacuation traffic without bunching or
crowding, and certainly to avoid locking the crowd. For large groups of persons e.g. during
major maintenance activities or during large construction projects, simulation should be
used to determine whether evacuation is possible.
8.4
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Escape Routes And Assembly Areas
Each site must establish contour maps of areas where H2S effects can arise, giving the
maximum possible concentration from releases and the maximum possible dangerous toxic
load. Escape routes must be marked and appropriate signs installed. Escape routes,
assembly areas, and wind socks must be clearly identified on weather resistant maps in all
H2S classified areas.
Assembly areas must be sufficiently far from the potential release sources to provide a
good level of protection for the most probable release cases such that personnel are not
exposed to doses which can either lead to lethal effect or irreversible health effect, and
there should be alternative assembly areas or safe havens to provide for the worst cases.
At least 2 assembly areas are required at the established safety distance, taking into
account the possibility of different wind directions. The escape/entry routes must be
decided based on the prevailing wind direction and/or other relevant locational
considerations.
All escape routes must be maintained free of obstruction. Where obstruction is
unavoidable, e.g. due to specific maintenance works; alternative routes must be identified
as part of the Job Safety Analysis / Risk Assessment. The alternative escape route must be
marked and signed.
8.5
Minute Ventilation Rate
Air supply and replacement air cylinders will be needed for search and rescue long duration
emergencies. The amount of air required needs to be considered during emergency
planning.
The amount of air breathed by a person is measured as the minute ventilation rate. The
standard minute ventilation rate for an adult is most commonly quoted as 5 to 8 l/minute
when resting, but this value can rise up to 19 when moving and up to 30 when exercising
(bicycling).
Values of >69, >95, and >100 have been reported for fire fighters (presumably fit) on a
tread mill in full fire gear, [Refs: 81, 82, and 83]. The minute ventilation rate (MVR) used for
determining the capacity of emergency escape mask and self-contained BA and for the
design of air-line breathing air supplies and for safe havens is by standard 35 l/ minute.
Companies must consider extreme cases when determining the volume needs and cylinder
refilling and reserve capacity needs, particularly for rescue.
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9
EDUCTION & TRAINING
9.1
Training for All Personnel
All personnel, including temporary workers who will not always be accompanied, entering a
site that has a hydrogen sulphide hazard must be trained to understand:

Hazards of hydrogen sulphide;

The meaning of the zone designation;

The requirements for entering a Yellow or Red Zone;

The understanding of warning signs related to hydrogen sulphide;

Recognising and reporting a hydrogen sulphide leak;

Use of personal hydrogen sulphide detectors;

The meaning and types of alarms, both general and from personal hydrogen
sulphide detectors;

Actions to be taken in the event of an emergency;

Escape routes and importance of moving crosswind; and

What to do if a colleague is overcome by toxic gas.
Accompanied visitors must receive a briefing on the hazards of hydrogen sulphide, the use
of emergency escape masks and escape procedure.
This training must be site specific. This is in addition to other training that may be required
for personnel to work safely.
Personnel must undergo refresher training on a minimum yearly basis. Training records
must be maintained by the Group Company and audited annually.
Visitors who are accompanied by a Group Company Employee at all times must be
appraised of:

Hazards of hydrogen sulphide;

Use of personal hydrogen sulphide detectors

The meaning and types of alarms, both general and from personal hydrogen
sulphide detectors; and

What to do in an emergency involving hydrogen sulphide.
Personnel who work at more than one site where hydrogen sulphide hazards exist, but
whose duties are such that they will never enter Hydrogen Sulphide classified zones need
only receive instruction in site specific issues as part of the formal site induction training
and do not need to repeat the full hydrogen sulphide training at each new site. Such
personnel must never enter hydrogen sulphide classified zones.
9.2
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Hydrogen Sulphide Competency
Valid hydrogen sulphide competency certificates are required for all personnel:

Entering Red , Amber or Yellow Zones; and

Working with breathing apparatus as a result of hydrogen sulphide hazards.
This is in addition to other entry requirements listed in Sections 2.2 and 2.5.
Group Company HSE departments can, at their discretion, issue such a certificate to
anyone graduating from a hydrogen sulphide competency course that includes the following
elements in addition to the topics noted in Section 9.1:

Requirements for and reasons behind facial hair restrictions;

Requirements for entering Yellow Zones

Requirements for entering / leaving Red Zones including register in / out procedure
and interface with the Permit-To-Work system;

Operational controls applicable for Red Zones (see Section 6);

Understand the buddy system and the duties of a buddy (remain alert, give alarm,
keep any rescue lines clear);

Consequences of not adhering to procedures;

Use of PPE required by operational controls, which must include both self-contained
breathing apparatus, emergency escape mask and air-line fed breathing apparatus;

Recognition of alarms and response;

Use of PPE in an emergency;
Before graduation and issue of a Hydrogen Sulphide Competency Certificate, personnel
attending a hydrogen sulphide competency course must:

Demonstrate understanding of the course topics;

Demonstrate the ability to correctly don an emergency escape mask within 20
seconds and SCBA in 45 seconds.

Demonstrate correct use of breathing apparatus; and

Demonstrate correct operation and use of air-line fed breathing apparatus.
The time period of 20 seconds to don an emergency escape mask (pre-checked and ready
for use condition), is based on tests carried out which indicate that this can readily be
achieved.
Hydrogen Sulphide Competency Certificates must be valid for 12 months, after which
refresher training is required.
Group Company HSE departments must audit third party hydrogen sulphide competency
training providers before beginning to issue certificates and at least once per 12 months
thereafter to ensure that training standards are maintained.
9.3
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Working with Air Lines
Personnel who will be working under air line supply must be tested for fitness. They must
also demonstrate competency in:
9.4

Understanding of air line supplies

Understanding the importance of keeping lines free and untangled

Understanding the importance of having a person standing by to assist in case of
difficulties

The ability to check connections and air supply

The ability to change over connections safely

Emergency air supply in case of air line failure

The procedure for escape
Training of Emergency Response Teams
Personnel in teams who will be responding to hydrogen sulphide emergencies must be
trained in:

Methods of rescuing personnel overcome by hydrogen sulphide;

The use of equipment they may be using for emergency response;

How to make the area safe e.g. performing isolations;

Means of communication; and

Treatment for personnel exposed to hydrogen sulphide.
Personnel must be able to demonstrate their competency in these subjects in a practical
test before being allowed to take up their duties in an emergency response team.
This is in addition to any other training which may be required for them to fulfil their
function.
9.5
Hydrogen Sulphide Trainers
All trainers providing hydrogen sulphide training for emergency response teams, hydrogen
sulphide competency or for personnel working on a hydrogen sulphide site must be able to
demonstrate working experience on sites where a hydrogen sulphide hazard exists and
with the various forms of breathing apparatus and other PPE in which they will be training
others.
The competency of the hydrogen sulphide trainers must be subject to audit before a Group
Company can issue a Hydrogen Sulphide Competency Certificate to graduates of that
trainer. Trainer qualification must be in accordance with ANSI/ASSE Z390.1-2006,
Accepted Practices for Hydrogen Sulphide (H2S) Training Programs.
Trainers must have formal qualification to train at the level they will be working e.g. rescue
qualification for trainers who will be training in rescue, and should also have site specific
knowledge, particularly knowing the location and type of potential release sources, escape
and rescue procedures, and escape routes.
9.6
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Training Methodology & Content
In recent years, some general principles regarding H2S Training have been identified:

Various levels of H2S training must be provided to all personnel including
contractors and other stakeholders (e.g. security services), as appropriate for the
tasks to be carried out;

Training should be given in a language which the trainee understands;

Training material should be highly pictorial, preferably with photographs of actual
locations and photographs or real or simulated accidents. Where possible, video
should be used;

Training should be largely site specific, so that trainees can recognise the situation
and the hazardous locations. Generic training should be avoided;

Assessment of the trainee after the training session should be performed and
sanctioned by a certificate on successful results; and

Every trainee should be able to practice putting on the appropriate level of PPE, and
should demonstrate competency in putting on PPE.
Immersive and participative training is much more effective than lectures. On site walk
through of evacuation procedures is essential.
9.7
Common / General precautions of Reducing Risk
There are a few simple rules which can be taught, and which significantly reduce risk,
particularly for persons working in a Red, or Amber Zone, or going to an area to investigate
a leak:

Before going to the H2S hazard zone, check your personal H2S detector (press the
test button), and check your emergency escape mask or BA set.

Before entering a Red Zone, or approaching a possible leak, pause for 30 seconds
and check your personal detector.

Before climbing a ladder or ascending stairs to a working platform, check your
personal detector.

When going to the roof of a sour oil or sour water tank, check the wind direction.
Also, when reaching the top of the ladder or stair, pause for 30 seconds and check
your personal detector.

If you smell hydrogen sulphide, or see a gas release, check your personal detector
immediately. The release may be worse than you think, it may get worse, or the
wind may change.

Generally, it is not a good idea to rely on fixed toxic gas detectors if you are
approaching to within a few metres of a potential release source; your personal
detector is a better indicator of localised releases.

When entering a Yellow Zone, it is essential to know where emergency escape
masks are stored and to be familiar with the routes from the work location to the
nearest escape mask store. Therefore:

When planning work in Yellow Zones give consideration to the location of the
nearest emergency escape masks to the work location and the route to be followed;
and

The location of emergency escape mask stores and routes thereto must be included
in any Task Risk Assessment for work in a Yellow Zone.
9.8
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Untrained Personnel
Untrained personnel should not be allowed in hydrogen sulphide classified areas.
untrained persons need to enter an area, they must first be trained.
If
For accompanied visitors briefing on the hazards of hydrogen sulphide and the escape
procedures and training on the use of emergency escape masks are required if the visitors
are to enter the Yellow Zone as well as briefing on evacuation and assembly. Red Zone
entry by visitors should generally be avoided, but exceptions can arise, for example a
specialist needing to inspect equipment. For these persons full training in the escape
equipment and procedures for the specific site is required.
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10
PERSONAL PROTECTIVE EQUIPMENT
10.1
General Requirements
The sections below detail the minimum mandatory requirements for personal protective
equipment.
Other restrictions may also apply, such as the ability to work safely in an area that may
contain flammable gas.
All PPE, including personal detectors, for use in hydrogen sulphide classified areas must be
tested and calibrated as per manufacturer’s recommendation, or as per company
experience, whichever gives the greater protection.
Where relevant a system must be in place to ensure that the shelf-life of consumables is
not exceeded, and this system must be subject to audit.
10.2
Emergency Escape Masks
Hydrogen sulphide at emergency levels can cause difficulty in vision and short term
blindness. Escape sets must be of full-face positive pressure or positive pressure hood type
with an air supply of at least 10 minutes.
The protection offered by a given respirator is contingent upon (1) the respirator user
adhering to complete program requirements (such as the ones required by OSHA in
29CFR1910.134) [Ref: 80], (2) the use of NIOSH-certified respirators in their approved
configuration, and (3) individual fit testing to rule out those respirators that cannot achieve
a good fit on individual workers’.
Hydrogen sulphide can cause severe irritation of the eyes and temporary blindness. For
this reason, hood type escape masks are preferred. Goggles may be of some use, but only
if they are put on before exposure to H2S because the goggles themselves can fill with gas.
Full face mask types or hood types with positive pressure air supply are freed from gas as
air is passed from the storage bottle to the mask.
10.3
Self-Contained Breathing Apparatus
Self-contained breathing apparatus (SCBA) must only be used for short tasks such as
sampling, emergency response, for rescue activities and for entering and leaving Red
zones. Where work activities longer than a few minutes require breathing apparatus, units
must be capable of transfer to an air-line fed mode (cascade system) which must be used
during the work. This is because of the finite capacity of self-contained apparatus
compared to the extended capacity when air-line fed.
Sufficient SCBA sets of appropriate capacity must be ready and available at all times to
allow response to identified emergencies, in addition to those sets in use by personnel
working in Red Zones, or engaged in activities which require breathing apparatus. The
number, location and condition of the sets must be subject to yearly audit. The
appropriateness of these arrangements must be verified by drills or exercises as often as
necessary to ensure that SCBA sets will be readily available in an emergency.
10.4
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Air-line Fed Breathing Apparatus
Air-line fed positive pressure breathing apparatus must be used where there is a risk of
hydrogen sulphide above the STEL outside Red zones. Where this is impractical, work may
be carried out using positive pressure escape sets which are worn and ready to be quickly
donned, provided that this can be demonstrated to be acceptable on an ALARP basis.
Air line fed positive pressure breathing apparatus is required to be used in all cases when
equipment is being opened, if the equipment contains, or has a reasonable chance of
containing, hydrogen sulphide. (Use of SCBA is allowed for very short operations, such as
taking of a single sample).
Filter masks are not considered to provide adequate protection for use.
Air must be provided in compressed air bottles and cascaded to the breathing apparatus
through a manifold. There must be an alarm set to warn of low air supply pressure and it
must be monitored at all times whilst the system is in use.
Air line fed breathing apparatus must include a personal air reserve cylinder which can be
used for escape in the case of air line supply failure.
The bottles must be refilled by a dedicated compressor located at a location free of
hydrogen sulphide or other contaminant, and must not have cross connection to any other
air or gas system.
Air for piped breathable air supplies must be taken from a safe place. Requirements are as
follows:

The breathable air system must be completely separate from any other air, gas or
nitrogen piping;

There must be an alarm for low air supply pressure.

Often the air will be supplied from trolley mounted cylinders. If this is the case, the
compressor for cylinder filling must be located remotely at a definitely safe place.
The compressor must be dedicated to cylinder filling.

The air supply must be provided with a low pressure alarm audible to those using
the air.

No cross connections to gas or nitrogen piping, or to process air supplies, must ever
be installed;

The air intake to compressors must be taken from well above ground level, at least
2m;

The air intake must be remote from the discharge of diesel or petrol engine
exhausts, including from portable compressor drives if these are used;

A multi gas alarm must be provided at the air intake, and the air intake shutdown
following detection of contamination. The capacity of the air reservoir must be
sufficient to allow the people under air to safely stop work and to evacuate to a safe
location;

If a portable compressor is used, there must be an accumulator and the compressor
must be continuously monitored to allow timely intervention in the event of failure or
an emergency;
Breathable air supplies should be tested at least once every six months, or just prior to use
if they are used only occasionally. Testing must include functioning through the actual air
supply points, pressure, and possible contamination.
The requirements of this section apply to both new and existing facilities.
10.5
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Fit Testing
All personnel who are required to use respiratory protection for hydrogen sulphide
atmospheres, must pass a fit test for all face mask types they may be using. The fit test
must:

Be carried out by a competent person; and

Quantitatively measure the fit factor (the ratio of substance concentrations outside
the face mask, to those inside), which must be greater than 2000:1 to register a
pass [Refs: 3, 79];
A further fit test must be repeated and passed whenever:

There is any change to the person which might compromise the integrity of the face
mask such as from, facial hair, glasses, significant dental work or dentures, weight
change, or facial scarring;

There is any change to the mask style, size, material or manufacturer; or

Deemed necessary by the Group Company to ensure employee safety.
To be competent, the fit test operator must have adequate knowledge and have received
adequate instruction and training in the following areas:
10.6

Applicable regulations

The Purpose and Applicability of Fit Testing

Procedures for Selection of Adequate and Suitable RPE

Fit Factors and Protection Factors

Examination Procedures for RPE, Identification of Maintenance Problems

Correct Donning Procedures and Pre-Use Fit Checks

Qualitative and Quantitative Face-Fit Testing

Sampling from the Face piece

Diagnostic Checks on Equipment

Correct Fit Testing Procedures, Purpose of the Fit Testing Exercises

Problem Solving

Capabilities and Limitations of the Fit Testing Equipment

Interpretation of Results
Facial Hair
ADNOC companies must decide whether to allow personnel to have significant facial hair. If
so, they must provide appropriate breathing or escape apparatus
Many types of respiratory protection, including many emergency escape masks, are
incompatible with significant facial hair. If these types of breathing apparatus are used,
personnel who may need to use respiratory protection in the course of their work or for
escape in an emergency must not have facial hair that prevents tight-fitting of the mask
provided.
Compliance with the company facial hair requirement, where applicable for all such
employees, and contractors must be verified when the person first arrives on site and as
often as necessary to ensure continued compliance. No person can be allowed to carry out
duties or enter areas that require use of respiratory protection, which cannot pass the fit
test specified in Section 10.5.
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At the Group Company’s option, personnel with facial hair may be given alternative
respiratory protection which is compatible with their level of facial hair, provided the
apparatus is suitable for the conditions under which it is to be used. However, the fit test
specified in Section 10.5 must be carried out and passed before they can be allowed to
enter zones or carry out activities that either require or potentially require use of the
respiratory protection. Persons who will be working regularly in red zones, such as
operators and maintenance personnel must not have facial hair which could cause
inadequate sealing of breathing apparatus face-pieces or hoods.
For accompanied visitors, if they have significant facial hair, appropriate (hood type)
emergency escape masks must be provided.
10.7
Eye-Glasses (Prescription Glasses)
Wearing eye-glasses can be incompatible with some forms of respiratory protection.
Personnel who may need to wear respiratory protection and who normally wear glasses,
either must be provided with suitable respiratory protection which is compatible with
wearing eye-glasses, or must be given suitable respiratory protection containing
prescription lenses.
If neither of these is practicable, such personnel must not carry out activities which may
require the use of respiratory protection, including for emergency escape.
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COMPETENCY
Group companies must verify that all personnel, including contractors, and temporary
workers as well as employees, requiring Hydrogen Sulphide Competency Certificates have
a valid certificate before they can be allowed to commence those duties for which the
certificate is required. Expired certificates must be confiscated and destroyed.
Correct certification must be verified on commencement of duties and at yearly intervals to
confirm that refresher training has been carried out.
Where verification cannot be obtained then retraining will be necessary before a new
certificate can be issued.
Specific competency requirements must be demonstrated at the completion of training for:

Hydrogen sulphide competency (Section 9.2);

Emergency response team training (Section 9.4); and

Hydrogen sulphide trainers (Section 9.5)

Fit testing (Section 10.5)
Refer to the specific parts of sections 9 and 10 for further details. Anyone unable to provide
a practical demonstration of their competency must not be allowed to perform the
respective roles.
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TOXIC GAS REFUGES
Toxic gas refuges are protected locations where people can take shelter during a toxic gas
release. Sites handling hydrogen sulphide and not already having designated safe havens
must evaluate whether the installation of a TGR is a reasonably practicable risk reduction
measure.
Some potential releases of hydrogen sulphide can travel many kilometres before dispersing
to safe levels. In such cases one or more TGR may be required remote from the potential
releases of hydrogen sulphide and for the benefit of third party populations. This possibility
must be included in the evaluation.
Safe havens and refuges can be designed to protect personnel against a variety of
hazards, for example fire and explosion as well as toxic gas. In this section guidance is
given on the design of toxic gas refuges to protect people from toxic gas. In any particular
instance, a true safe haven will require additional design features to protect people from
other hazards. Such features are not considered here.
Where a full safe haven is provided to cover a particular area, which also meets the toxic
gas refuge requirements, a separate toxic gas refuge should not be provided in the same
area. Provision of multiple refuges of different types complicates the emergency muster
count, and could lead to persons sheltering in a toxic gas safe haven when they are in fact
threatened by a fire or explosion.
Wherever there is a significant risk of toxic gas ingress to a building where people are
normally present, shut down of the HVAC system on gas detection should take place, as
this is always a reasonably practicable measure.
Releases from large gas inventories such as long pipelines of from wells may continue for
considerable periods (days or weeks), even when the rate of release is large. Toxic gas
refuges are not intended for protection over long periods, because it is very difficult to make
buildings so leak tight that there is no in-leakage, and in any case there will be some gas
ingress as people enter. For this reason, where there is a significant level of risk of
prolonged toxic gas release, companies should develop a plan for safe evacuation of
personnel from the refuge.
Ordinary toxic gas refuges may be created from existing buildings by the following
measures:

The building should be reasonably airtight, to the level usually required for
implementation of HVAC systems e.g. tight metal frame windows, non-opening;

Building entrances should be designed with double sets of doors and located as far
as possible away from sources of hydrogen sulphide. A mechanism should be in
place to prevent both airlock doors being open simultaneously or to alarm to warn of
this condition;

Doors should be closed all the time except when in use, or be automatic closing, for
example by magnetic hold open, released on gas detection. Attention should be
paid to avoiding hazards with automatically closing doors such as trapped fingers,
e.g. doors should be slow closing (but not too slow);

A hazard which has been noted on some installations is the use of excessively
heavy blast doors, which in themselves constitute a crushing hazard. These should
be avoided.

The HVAC should automatically shut off on detection of gas at the air intakes, and
dampers should close in the air intakes. Reliability of detection should be
determined as part of the design, and should comply with the SIL requirement for
protection;
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
Provide a mechanism to allow manual initiation of HVAC shutdown from within the
refuge; and

If air intakes are at roof level, they may be well above the level of a gas cloud. Any
automatic closing of doors should therefore either be activated from the fire and gas
system, or on signal from a local gas detector.
In addition to these measures, a high integrity toxic gas safe haven should have positive air
pressure. For smaller buildings, positive pressure can be provided by means of stored air
(at high pressure). For large buildings, a breathable air supply taken from a safe place (e.g.
from two alternative intakes, at opposite sides of the plant), or filters or scrubbers in the air
intakes, may be used. Positive air pressure supplies must be tested at an interval which
ensures high reliability (low probability of failure on demand). This is typically every 6
months. Any air intake for stored air supplies must have a multi-gas detector and must shut
off automatically on gas detection at their intake.
In all cases, toxic gas refuges should be assessed for rate of gas ingress, and
corresponding maximum safe duration of use.
Where long duration releases are possible, for example a sour well blowout, it may not be
practicable to design a toxic gas refuges to protect for the full duration of the release.
Where such cases are relevant it will be necessary to make arrangements to safely
evacuate the toxic gas refuges. This particularly applies to offshore installations where the
ability to locate air intakes away from hydrogen sulphide sources is limited, and where the
presence of other hazards may force evacuation of the toxic gas refuges whilst an external
toxic atmosphere exists.
For offshore toxic gas refuges which can be affected by fire, explosion and smoke as well
as toxic gas, the design should follow industry standard for toxic gas refuges or temporary
refugee standards, since the guidance given above may not be adequate because of fire
and explosion risks. The same requirements will generally apply also to control rooms and
operator rooms close to, or within, process plant.
On offshore installations, the ability to safely leave the TGR / Safe Haven and evacuate the
installation, for example following ignition of a sour gas release, must be considered as part
of the TGR/Safe Haven evaluation and specification.
An example of a typical TGR would be a pressurised building equipped with airlock doors
which close automatically on either detection of hydrogen sulphide approaching the building
or detection in the ventilation inlets.
12.1
Design of toxic gas refuges
Toxic gas refuges must be designed for purpose. In many cases where there is a possibility
of hydrogen sulphide release, the release will either be at a small rate, or will be short lived,
because the hydrogen sulphide inventory in equipment is limited. In other cases, such as
close to sour gas or sour oil wells, or close to gas pipelines, gas releases may last for hours
or days.
For short lived toxic gas releases, an effective toxic gas refuge can be made by:

Providing toxic gas detectors in the air intake, and arranging automatic shut off of
the air conditioning and ventilation

Providing dampers in the air intakes which are automatically closed in the case of
toxic gas detection

Eliminating small single room ventilation

Providing split unit air conditioning for individual rooms where centralised air
conditioning is impractical
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
Sealing/locking closed all windows

Providing two sets of doors so that the door inter-space functions as an air lock /
weather lock.

Keeping the doors closed, or providing automatic door closing on toxic gas detection

Checking the ceiling and floor for possible routes of gas ingress, and sealing any
leaks
For places where there is a potential for long duration toxic gas releases, all of the above
steps must be taken, and in addition, the following steps are to be taken:
12.2

Check all potential ingress, by testing refuge leak tightness, and seal any leaks.

Provide a positive pressure safe air supply, either by providing bottled compressed
air, or providing a filtered air intake of sufficient capacity.

Providing an evacuation or rescue strategy for the case of refuge engulfment for an
extended period.
Offshore installations
Offshore installations will require a temporary safe refuge (TSR) or Safe haven. The
measures described above in Section 12.1 will be needed. However, for offshore
installations, protection against fire and explosion will generally be needed also. Specific
provisions are outside the scope of this CoP
For small platforms such as well head platforms, provision of a toxic gas refuge can be
extremely difficult, or practically impossible. The risks to personnel on these platforms must
be reduced to acceptable or ALARP levels by other means, such as reducing exposure and
providing good emergency shutdown. Provision of rapid escape to attending boats, with
boats preferably moored upwind (where possible) are additional means. Risk to boat crews
from hydrogen sulphide must be considered in QRA’s. In difficult cases, some of the
techniques described in section 12.1 can be applied to service boats.
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EMERGENCY PLANNING ZONES
The extent of hydrogen sulphide hazard zones for emergency planning purposes both offsite and on-site must be determined based on distances to the Acute Exposure Guideline
Levels (AEGL) as published by the US EPA [Ref: 4] taking into account the anticipated
duration of personnel exposure.
The key AEGLs are: AEGL-1, AEGL-2 and AEGL-3
AEGL-1
This is the is the airborne concentration (expressed as parts per million (ppm) or mg/m3 of a
substance at or above which it is predicted that the general population, including
“susceptible” but excluding “hyper-susceptible” individuals, could experience notable
discomfort. Airborne concentrations below AEGL-1 represent exposure levels that could
produce mild odour, taste, or other sensory irritations.
AEGL-2
This is the airborne concentration (expressed as ppm or mg/m3) of a substance at or above
which it is predicted that the general population, including “susceptible” but excluding
“hyper-susceptible” individuals, could experience irreversible or other serious, long-lasting
effects or impaired ability to escape. Airborne concentrations below the AEGL-2 but at or
above AEGL-1 represent exposure that may cause notable discomfort.
AEGL-3
This is the airborne concentration (expressed as ppm or mg/m3) of a substance at or above
which it is predicted that the general population, including “susceptible” but excluding
“hyper-susceptible” individuals, could experience life-threatening effects or death. Airborne
concentrations below the AEGL-3 but at or above AEGL-2 represent exposure that may
cause irreversible or other serious, long-lasting effects or impaired ability to escape.
Examples of susceptible persons are old persons and small children. The AEGL values will
therefore be conservative for the healthy working population.
The current published AEGL values are given in Appendix 1. In order to determine the size
of the emergency planning zones, gas release and dispersion calculations are needed to
determine the duration length, width and concentration of hydrogen sulphide plumes.
These calculations must take into account the inventory and corresponding duration of
possible releases, and the reasonable worst cases of wind speed (1.5m/s) and atmospheric
stability (stability category F).
Calculations must be made using a computer program which has been validated for the
purpose, for the conditions, and for the type of releases which could occur.
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ASSESSMENT OF THE IMPACT OF HYDROGEN SULPHIDE
When people are exposed to hydrogen sulphide the impact will depend on both the
concentration of the gas in air and the duration of the exposure. Any calculations of the
potential impact of hydrogen sulphide releases must take into account both these factors.
Therefore, when QRA type calculations are undertaken to assess the hazards of hydrogen
sulphide, including optimising protective features or in performing an ALARP analysis, the
impact on humans must be assessed using a toxic load of the form:
TL = ∫Cn.dt
Where:
TL =
toxic load
C=
concentration of hydrogen sulphide in air, which may vary as a function of time;
t=
time
n=
toxicological exponent
Units are usually ppm for concentration and minutes for exposure time. Units of mg/m3 and
seconds are also given in some sources. Units for the toxic load will depend on the units
used for concentration and time. It is important to check the units in use for any calculation.
Values for n and the method of determining the impact of specific toxic loads published by
the UK HSE [Ref: 5] or TNO [Ref: 6] are recommended.
Dangerous Toxic Loads (DTL) [Ref: 68] are specified in terms of SLOD [Ref: 68] (significant
likelihood of death) and SLOT [Ref: 68] (specified level of toxicity) values which have been
given by UK HSE.
Probit, SLOT and SLOD values must be used for calculation of toxic release in QRA.
The duration of exposure must be selected based on the expected worst case for the
population for whom exposure is foreseeable. Where relevant and technically justifiable a
dynamic time-varying concentration can be used, including allowing for protection inside
non-pressurised, buildings.
15
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ENFORCEMENT
It is the responsibility of senior management to have oversight of compliance with this COP
Implementation of the requirements of this COP must commence immediately on its
issuance to the extent reasonably practical.
Personnel who do not adhere to the requirements of this COP are not only putting their own
lives at risk, but also those of their colleagues, emergency service and others working on
and off site. Offenders must be subject to disciplinary action up to and including removal
from site. Personnel removed from any ADNOC Group Company facility for violation of this
CoP must be prohibited from entering all ADNOC Group Company facilities.
Organisations or groups carrying out design work, contractors or individual personnel who
repeatedly fail to meet the requirements of this Code of Practice must not be considered for
future projects, contracts or employment across the ADNOC Group of companies.
Strict compliance with the requirements of this Code of Practice must be monitored. Audits
of operational sites where a hydrogen sulphide hazard exists must be carried out as often
as necessary to ensure continued compliance and at a maximum interval of 12 months.
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Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 67 of 77
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Volume 4: Safety & Risk Management
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Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
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Signals 2008.
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Hoot T G, Meroney R N, and Peterka J A: Wind Tunnel Tests of Negatively Buoyant
Plumes; Fluid Dynamics and Diffusion Laboratory, Colorado State University;
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APPENDICES
Appendix 1
: Properties of Hydrogen Sulphide
Appendix 2
: Exposure Calculations
Appendix 3
: Dispersion Calculations
Appendix 4
: Calculation of Zone Sized & Detector Placement
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Document No. ADNOC-COPV4-10
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Appendix 1
: Properties of Hydrogen Sulphide
Hydrogen sulphide is a colourless flammable gas, which burns with a pale-blue flame forming sulphur
dioxide and water vapour. It has a very offensive odour similar to that of rotten eggs, which can be detected
at concentrations between 0.0002 to 0.3 ppm. The chemical and physical properties are as follows:
Molecular Formula
H2S
Molecular Weight
34.08
UN Number
1053
CAS Number
7783-06-4
Specific Gravity
1.19 [air = 1.0]
Boiling Point
-60.33 oC
Explosive Limits, by
volume in air
46% upper
4.3% lower
Auto-Ignition
Temperature
260 oC
Solubility @ 20 oC
0.5 gm H2S in 100 ml
water. Soluble in water.
Aqueous solutions of
hydrogen sulphide are
not stable.
NFPA 704 Code
4
4
0
GHS / CLP
Classification
Signal Word
Hazard statements
Danger - Extremely
Flammable Gas
Danger Fatal if inhaled
Very toxic to aquatic life.
Very flammable - F+
Very Toxic – T+
Dangerous for the
Environment
CHIP
Classifications
Risk Phrases
R12
Extremely flammable
S16
Keep away from sources of
ignition
R26
Very toxic by inhalation
S36
Wear suitable protective
clothing
R50
Very toxic to aquatic
organisms
S38
In case of insufficient
ventilation, wear suitable
respiratory equipment.
S45
In case of accident or if you
feel unwell, seek medical
advice immediately
Safety
Phrases
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Health effects based on the three most detailed studies/reports (ACGIH [Ref: 69], UK Health Protection
Agency [Ref: 8] and Skrtic [Ref: 9]) are summarised below:
Concentration, ppm
Exposure Patterns
Reported Effects
0.0057
Chronic/community
Eye and nasal symptoms, coughs, headaches and/or
migraines
0.003 – 0.02
Acute
Detectable odour
0.01
Chronic/community
Neurophysiological abnormalities
0.1 – 1
Not reported
Abnormal balance with closed eyes, delayed verbal
recall, impaired colour discrimination, decreased grip
strength
0.2
Not reported
Detectable odour
0.25 – 0.30
Chronic
Nuisance due to odour
1–5
Not reported
Abnormal balance with open and closed eyes, delayed
verbal recall, impaired colour discrimination,
decreased grip strength, abnormal simple and choice
reaction time, abnormal digit symbol and trailmaking
2–8
Chronic/community
Malaise, irritability, headaches, insomnia, nausea,
throat irritation, shortness of breath, eye irritation,
diarrhoea, and weight loss
10
Short term [10-mins]
Eye irritation, chemical changes in blood and muscle
tissue
>30
Chronic
Fatigue, paralysis of olfactory systems
50
Not reported
Eye and respiratory irritation
50 - 100
Chronic
Eye irritation ranging from painful conjunctivitis,
sensitivity to light, tearing, clouding of vision to
permanent scarring of the cornea
150 - 200
Not reported
Olfactory nerve paralysis
200
Not reported
Respiratory and other mucous membrane irritations
250
Not reported
Damage to organs and nervous system, depression of
cellular metabolism
Chronic
Possible pulmonary oedema
320 – 530
Not reported
Pulmonary oedema with risk of death
500
Short term [30 mins]
Systemic symptoms
500 – 1000
Acute
Stimulation of respiratory system leading to rapid
breathing, followed by cessation of breathing
750
Acute
Unconsciousness, death
1000
Acute
Collapse, respiratory paralysis followed by death
750 – 1000
Acute
Abrupt physical collapse, with the possibility of
recovery if victim is removed from area; if not fatal
respiratory paralysis
1000 – 2000
Not reported
Immediate collapse with respiratory paralysis
5000
Acute
Death
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Published toxicological data parameters for hydrogen sulphide is summarised below. It is recommended that
Group Companies consult reliable source in each case for further information before use.
Description
Value
Reference
10 ppm
Ref Sec 5.1
Workplace Exposure Limit [8-hr TWA]
5 ppm
Ref. 8
Workplace Exposure Limit [15-min STEL]
10 ppm
Workplace monitoring limit ( Personal H2S Detector)
Occupational Health Use ( Chronic effects)
QRA Use
UK-HSE
n, Toxic Load Exponent [note 1]
4
Probit A Parameter
-30.8
Probit B Parameter
SLOT Value
SLOD Value
Ref. 6
1.16
2x
1012
ppm4.min
1.5 x 1013 ppm4.min
ADNOC Probit
n, Toxic Load Exponent [note 1]
4.6
Probit A Parameter
-27.8
Probit B Parameter
1.16
ADNOC
Proprietary
Emergency Planning Use
AEGL 1 (10 minutes) – no effect
0.75 ppm
AEGL 2 (10 minutes) – disabling
41 ppm
AEGL 3 (10 minutes) – lethal
76 ppm
AEGL 1 (30 minutes) – no effect
0.51 ppm
AEGL 2 (30 minutes) – disabling
35 ppm
AEGL 3 (30 minutes) – lethal
59 ppm
AEGL 1 (1 hour) – no effect
0.51 ppm
AEGL 2 (1 hour) – disabling
27 ppm
AEGL 3 (1 hour) – lethal
50 ppm
Ref. 5
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April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 73 of 77
Appendix 2
: Exposure Calculations
All calculations of impact of accidental releases on people must use the toxic load (dose) defined as
TL = ∫Cn.dt
Where:
TL
C
t
n
=
=
=
=
toxic load
concentration of hydrogen sulphide in air
time
toxicological exponent
The integral is over the period of exposure.
The probability of fatality as a function of toxic load in the form of a probit defined by:
Probit = A + B ln TL
Where A and B are specific parameters derived from toxicological experiment.
Values for n, A and B associated with the recommended ADNOC Probit are given in Appendix 1.
A probit is a normally distributed function with mean of 5.0 and standard deviation of 1.0, so a probit of 5.0
corresponds to a 50% chance of fatality and a probit of 2.67 corresponds to a 1% chance of fatality. It is
related to, but not quite the same as the probit function used in statistics. The following table can be used to
translate between probits and probability of fatality:
Probit Values As A Function Of Percent Probability Of Fatality
0
1
2
3
4
5
6
7
8
9
0
-
2.67
2.95
3.12
3.25
3.36
3.45
3.52
3.59
3.66
10
3.72
3.77
3.82
3.87
3.92
3.96
4.01
4.05
4.08
4.12
20
4.16
4.19
4.23
4.26
4.29
4.33
4.36
4.39
4.42
4.45
30
4.48
4.50
4.53
4.56
4.59
4.61
4.64
4.67
4.69
4.72
40
4.75
4.77
4.80
4.82
4.85
4.87
4.90
4.92
4.95
4.97
50
5.00
5.03
5.05
5.08
5.10
5.13
5.15
5.18
5.20
5.23
60
5.25
5.28
5.31
5.33
5.36
5.39
5.41
5.44
5.47
5.50
70
5.52
5.55
5.58
5.61
5.64
5.67
5.71
5.74
5.77
5.81
80
5.84
5.88
5.92
5.95
5.99
6.04
6.08
6.13
6.18
6.23
90
6.28
6,34
6.41
6.48
6.55
6.64
6.75
6.88
7.05
7.33
It is emphasised that it is the dynamic concentration to which the person is exposed that is important. So, for
example, a person in a non-pressurised building may be exposed to a lower concentration than a person
outdoors at a similar location. This can be taken into account in QRA type calculations and when selecting
remedial measures. However, be aware that once the release is isolated, the concentration in the open air
will drop faster than in the building. Personnel who remain in the building can end up with a higher
probability of fatality than those who remain outside. In such cases it is important that procedures and
training regarding personnel action following the dispersion of the release are reflected in the calculations as
well as procedures and training covering the initial threat and response.
HSE Management – Manual of Codes of Practice & TGN
Volume 4: Safety & Risk Management
Version 2,
April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 74 of 77
Appendix 3
: Dispersion Calculations
Dispersion calculations are important at several points in the design and operations for hydrogen sulphide
safety management, notably:




In the QRA, for determining the necessary levels of protection.
For determining the necessary segregation and separation for new plants.
For determining the size of Red and Yellow zones.
As a basis for design of the gas detection networks.
At present, dispersion calculations are not recommended for the assessment of chronic exposures. Group
Companies should carry out surveys instead to establish actual measurements in the field. However,
emission and dispersion calculations will be useful at the early stages of design of new plant, in order to
avoid problems before plans are fixed too rigidly. When dispersion calculations are used in this way they
should be used cautiously, and with reference to good design practice.
For pressurised releases, the initial part of the release will take the form of a cone shaped momentum jet.
Once the velocity of gas in the jet has fallen to close to that of the wind the release transforms to a billowing
plume. The plume may be light, as for a natural gas which is largely methane, or neutrally buoyant or heavy,
as for a gas or vapour which contains heavier hydrocarbons, large amounts of carbon dioxide or hydrogen
sulphide, or which is very cold.
Light gas may ascend in a plume, but dispersion can carry the gas down to ground level once again,
depending on atmospheric conditions. Heavy gas will tend to fall to the ground, and spread under its own
weight, as well as dispersing due to wind turbulence.
Models for dispersion outside the plant are well developed, and there are many models, both commercial
and free, which provide reasonably accurate calculations for jet dispersion, light plume dispersion, and heavy
plume dispersion. Programs which are available from public domain sources, and which have been
validated, are available from Ref. 57 to 60. Users should not expect accuracies to better than a factor 2 on
concentrations and 30% on plume lengths from any programs because the underlying experiments used to
develop models do not provide data with variations less than this. A special pitfall is that some programs
calculate plume widths incorrectly, giving plumes which are too wide, and correspondingly too short. This can
be very misleading, especially in determining detector coverage and in carrying out calculations to support
emergency planning. Only calculations made with models which have been validated both according to
plume length and width for the relevant types, sizes and durations of release should be used.
For dispersion inside a plant, effects such as impingement of jets on equipment and on the ground become
important, and turbulence around equipment become important. Also, down-draughts and recirculation
behind vents, buildings, tanks and process equipment and wind stripping of gas from jets can be important in
determining gas concentrations. These are not taken into account by simple standard models.
For elevated or vertically directed jet releases of heavy gas, the models developed by Ooms, or by Hoot,
Meroney, and Peturka [Ref. 70 - 75] provide a well validated approach.
For neutral or buoyant elevated releases, the regulatory model AERMOD goes some way to treating downdraught, but can underestimate ground level concentrations by a large factor (up to 5 in some validation
studies). For cases where the simpler programs suggest that there may be a problem in achieving working
environment goals, it is strongly recommended that computational fluid dynamics approaches be used, for
example in setting vent heights.
Most of the simple models assume dispersion in a free flat field and do not take into account cross wind
dispersion or jet impingement. The effect of this on risk assessment and red zone sizing is generally
conservative i.e. the actual plumes are shorter (though also wider) than the free field calculations indicate.
For determining gas detector locations, models which do not take impingement and cross wind dispersion
should be avoided, to ensure that there is no problem of over optimistic detector network design. It is
possible, for example, for some detectors never to detect gas because gas jets are blown away by the wind
before reaching them.
HSE Management – Manual of Codes of Practice & TGN
Volume 4: Safety & Risk Management
Version 2,
April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 75 of 77
Some features of process plant topology and location lead to dispersion patterns where the simple free field
models are not conservative. Examples are:




Releases within bunded areas
Releases which pass into pipe trenches
Releases in varying terrain, such as dunes
Releases in enclosures, or into areas which are partially enclosed, such as between separators
Where these effects are important for safety, specialised models developed for varying terrain, or
computational fluid dynamics models, are recommended.
HSE Management – Manual of Codes of Practice & TGN
Volume 4: Safety & Risk Management
Version 2,
April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 76 of 77
Appendix 4
: Calculation of Zone Size & Detector Placement
Calculation of risk from hydrogen releases should always take into account exposure time. In most cases of
large releases in process plant, and in cases of pipeline releases where the pipeline is sectioned, exposures
will be short, and may be of the order of minutes or even seconds. Calculations which assume continuing
large releases give calculated plume sizes which are excessive for most process plant, sometimes
increasing hazard zone sizes by a factor of 10 to 20. Models used should be those which take limited
inventories, emergency shutdown, and self-evacuation into account. Cases in which shutdown works and in
which shutdown fails must be included in risk calculations. Similarly, cases where self-evacuation fails due to
error or failure of PPE should be taken into account.
Most dispersion calculations carried out for QRA purposes under current practice are based on the concept
of isolatable section. This is often defined as the section of plant between two isolation valves. The isolatable
section concept is a simplification, which makes it easier and quicker to perform QRAs. It implies though that
there will be just one location for release. Since an isolatable section may extend to the limit of a plant unit,
typically up to 30 m., the concept itself introduces an inaccuracy in location of the release which may be
critical in determining risk levels within the red zone, in determining red zone size and in determining gas
detector coverage. For most purposes, including in-plant QRA, the release locations for a given scenario
should be the most likely release locations, and there should be sufficient locations calculated to allow
representative coverage of the plant at an accuracy of 1 to 2 m. Such accuracy is not required for
calculations intended for land use planning, or for public emergency response planning. However, as stated
in the CoP, the size of the Zone must be based on the release being from any potential location within the
isolatable section.
For example, for an amine unit, the amine pumps, each vessel, each column and the heat exchangers will
generally need to be considered as release locations. The overhead pipes may need to be considered,
particularly the regenerator overhead, to take account of the possibility of gas plumes passing over
detectors.
The concept of isolatable sections should generally be retained for calculating release inventories, but not for
specifying release location. There will generally be many significant release locations for each isolatable
section.
Equipment within the plant will in most cases obstruct jet releases, and such obstruction will affect plume
widths, as described in the previous section. The plume widths will be wider, and correspondingly shorter.
The effect can be quite significant inside process plant areas, increasing risk for operators and maintenance
workers, while reducing risk outside the plant.
It may not be practical to take into account the many different plant obstructions etc. in a full QRA, but the
effect should be considered when assessing or interpreting results, and when arriving at conclusions. For
detector system design there will generally be significant benefit in making such calculations of obstructed
dispersion, since the number of sensors needed is reduced as the plumes are widened.
The following table should be used when carrying out dispersion calculations. Note that some accidental
releases will be orientated vertically and may result in relatively lower risk to personnel than horizontally
orientated releases.
HSE Management – Manual of Codes of Practice & TGN
Volume 4: Safety & Risk Management
Version 2,
April 2014
Code of Practice on Management of Hydrogen Sulphide (H2S)
Document No. ADNOC-COPV4-10
Page 77 of 77
Zone
Averaging
Time (sec)
Directional Release
Scenarios To Consider
Basis
Red
18.75
Directional Release
based on probability
(refer OGP)
All sections considering
escalation factors
Risk (based on ADNOC
Probit)
Amber
18.75
Directional Release
based on probability
(refer OGP)
All sections considering
escalation factors
Risk (based on ADNOC
Probit)
Yellow
18.75
Directional Release
based on probability
(refer OGP)
All sections considering
escalation factors
Risk (based on ADNOC
Probit)
EPZ
600
Horizontal Release
Worst Case
representative scenarios
Consequence (based on
dispersion analysis)
EAZ
600
Horizontal Release
Worst Case
representative scenarios
Consequence (based on
dispersion analysis)
PRACTICAL ISSUES IN ZONE SIZING
The possibility of flare flame-out and the possibility of unintended cold venting of hydrogen sulphide
containing vapours must be considered when sizing Red, Amber and Yellow zones.
All-welded pipes running through an area need not be considered when sizing Red, Amber or Yellow zones
provided such pipes are protected from foreseeable dropped objects and are within their original design life
or fitness for service requalification life with respect to corrosion.
Similarly sub-sea pipelines do not need to be considered in defining Red, Amber and Yellow zones except
where there are special hazards, such as at risers or at boat access areas. Cross country pipelines are not
expected to carry lethal quantities of H2S and only need to be considered at valve stations, pigging stations,
compressor, pumping and similar stations.
When defining Red, Amber or Yellow Zone boundaries, the boundary distance as calculated may pass
through equipment. In this case the boundary distance should be extended, so that an easily recognisable
boundary can be identified, which will be workable during operation.
In most cases it will be desirable to use in plant roadways and access ways as Red, Amber or Yellow Zone
boundaries. Where calculated Red, Amber or Yellow Zone boundaries are close to a roadway or access way,
the distances should normally be extended, so that the Zone boundary coincides with a natural boundary.