MANAGING HIGH H 2 S RISK IN OIL & GAS INDUSTRY
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14-06-2009, 08:56 AM


MANAGING HIGH H
2
S RISK IN OIL & GAS INDUSTRY
GIRISH JOSE
Sr. ENGINEER, SAFETY DESIGN
PETROFAC INTERNATIONAL LIMITED
SHARJAH, UAE
Email “ girish.jose@petrofac.com
ABSTRACT
One of the most common hazards associated with
production and processing of oil & gas is the
potential for exposure to Hydrogen sulphide.
Hydrogen sulphide is present in oil & gas reserves
in free state as well as in dissolved phase. Upon
a leak from the processing stream, depending
upon the concentration, the released H2S will
pose toxic risk to the operating people as well as
the nearby public offsite to the facility. Although
very pungent at first, H2S quickly deadens the
sense of smell, so potential victims may be
unaware of its presence until it is too late to
escape. Accidents and incidents involving H2S
can be prevented by good engineering practice,
job planning and the proper use of personal
protective equipments. This paper presents the
key aspects of good engineering practice to be
followed for the design of an oil & gas facility
containing high concentrations of H2S.
The risk to personnel on a facility where H
2
S is
present in the process fluids arises:
¾
During an accidental release
¾
During normal maintenance operations,
eg: sphering, vessel entry, instrumentation
or valve maintenance
¾
During venting from tanks
The level to which personnel could be
subjected depends upon the concentration of
H
2
S in the process fluid and the dispersion and
dilution under local conditions.
All facilities potentially exposed to H
2
S must be
designed to resist the harmful effects of H
2
S at
the anticipated operating temperatures and
pressures.
Properties and Characteristics of H
2
S
H
2
S Characteristics
x Highly toxic, flammable gas
x Colourless
x At low concentrations, it is detectable by its
characteristic rotten-egg odour. At higher
concentrations (above 100 ppm), H
2
S
rapidly paralyses the sense of smell. Odour
shall not therefore be used as a warning
measure.
x Normally present as a gas and not as a
liquid
x Soluble in both water and hydrocarbon
liquids. Pools of water or sludge at the
bottom of a tank may thus contain
concentrations of H
2
S and if agitated or
heated will release the gas
x Pure H
2
S is heavier than air and can collect
in low-lying areas, especially pits and
sumps which are closed-in and have poor
Introduction
Hydrogen Sulphide gas (H
2
S) can be present in
the upstream oil and gas facility. The typical
sources of H
2
S are:
¾
The original reservoir, as a result of the
hydrocarbon source material and the conditions
under which it was converted to oil and gas. If this
is the case then H
2
S will be produced with the
process fluids
¾
Stagnant seawater systems, by the action of
sulphate reducing bacteria (SRB)
¾
The reservoir after prolonged injection of water
with oxygen (seawater, brackish, formation water)
which may result in 'souring' of the fluids within it
due to the action of SRB introduced during the
injection process. Any H
2
S will be subsequently
produced with fluids.Page 19

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 10
ventilation. However, the gas density will depend
on the molecular weight of the gas (Methane rich
gas is lighter but will be heavy when mixed with
higher hydrocarbon)
x It can burn when mixed with air and when an
ignition source is available
x It burns with a blue flame and gives off sulphur
dioxide (SO
2
), which also provides a health
hazard
x It can form pyrophoric iron sulphide on the
internal surfaces of carbon steel equipment
containing H2S. Pyrophoric iron sulphide can
auto-ignite when coming into contact with air.
x It is corrosive in the presence of water
x It causes cracking and embrittlement of metals
under certain conditions but steels of the correct
quality, which are resistant to these forms of
attack can be used.
Physical effects of H
2
S
H2S in the absence of free water can be
considered to be non-corrosive, conversely when
free water is present, especially during abnormal
situations, such as start-up/shut-down, drilling or
circulating out a kick, general corrosion will most
probably take place. If carbon dioxide, oxygen,
chloride ions, elemental sulphur are present
either individually or together then severe
corrosion may take place within a very short
period. Pyrophoric iron sulphide can be formed on
the internal surfaces of carbon steel equipment
containing H2S. Iron oxide present on the internal
surfaces will react with the H2S and form
pyrophoric iron sulphide which, on exposure to
oxygen can auto-ignite.
Although pyrophoric iron sulphide can form and
collect on all internal surfaces of carbon steel
equipment, likely places of accumulations are
Wire line lubricators, Filters, Vessels, Pipelines,
spheres and pig receivers, Vent and flare
systems, Storage tanks etc.
Whilst steps can be taken to reduce the formation
of pyrophoric iron sulphide it must never be
assumed that there can be an absolute
prevention of the reaction.
Therefore any equipment constructed of
carbon steel which is in contact with sour
hydrocarbons should be treated as if it
contained pyrophoric iron sulphide. Great care
must be taken on opening equipment likely to
contain pyrophoric iron sulphide, for its
removal, disposal.
Occupational Exposure Limits
The current ACGIH Threshold Limit Value
(TLV) for H
2
S is 10 ppm Time Weighted
Average (TWA) and 15 ppm Short-Term
Exposure Limit (STEL). The IDLH value for
H
2
S is 300 ppm.
It should be noted that where the source is
rich in H
2
S, gas or liquid, the transition from
low to very high exposure may be
instantaneous.
Sour Service Classification
Generally, the Oil & gas facilities are classified
as ËœSweet facilityâ„¢ if the H
2
S concentration in
the gas phase, resulting from reducing the
process fluid or process gas to atmospheric
pressure, is up to 49ppm. If concentration is
between 50ppm and 499ppm, it is termed as
Ëœlow risk sour facilityâ„¢ and the facility is termed
as ËœHigh risk sour facilityâ„¢ if the concentration
exceeds 500ppm.
Toxic effect of H
2
S
Concentration
Effect
0.12 ppm
Odour
threshold
in
humans
1 ppm
Odour of rotten eggs can
be clearly detected.
10 ppm
Unpleasant
odour.
Possible eye irritation. TLV
for an 8-hour workday, 5
days per week.
15 ppm
STEL averaged over 15
minutes.
20 ppm
Burning sensation in eyes,
irritation of the respiratory
tract
after
one-hour
exposure.Page 20

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 11
Toxic effect of H
2
S
Concentration
Effect
50 ppm
Loss of sense of smell after about 15 minutes exposure. Exposure >1
hour may lead to headache, dizziness and staggering. Pulmonary oedema
(accumulation of fluid on the lungs) following extended exposure. Serious
eye irritation/damage.
100 ppm
Loss of sense of smell after 3 to 15 minutes. Coughing altered respiration,
drowsiness after 15 minutes. Prolonged exposure results in a gradual
increase in these symptoms. 100ppm concentration is considered
immediately dangerous to life or health (IDLH).There is no documented
evidence of lethal effects at concentrations <100ppm, regardless of
exposure time, which implies that 100ppm is the threshold for lethal
effects [31].
200 ppm
Sense of smell lost rapidly. Irreversible pulmonary oedema on exposure
>20 minutes.
500 ppm
Unconsciousness after short exposure, breathing will stop if not treated
quickly. Dizziness, loss of sense of reasoning and balance. Victims need
prompt artificial ventilation and/or cardiopulmonary resuscitation (CPR)
techniques.
700 ppm
Unconscious quickly. Breathing will stop and death will result if not
rescued promptly. Artificial ventilation and/or CPR are needed
immediately.
> 1000 ppm
Unconsciousness at once. Permanent brain damage or death may result.
Rescue promptly and apply artificial ventilation / CPR.
* Source/Ref. API/OSHA/NIOSH
¾
Preventive measures (reducing likelihood of
hazards)
¾
Mitigatory measures and
¾
Recovery or recovery preparedness measures
(reducing the chain of consequences arising from
a top event).
The main aspects to be considered in design
stage are:
(1) Minimising process stream H
2
S levels. At
concept and front end design stages, process
selection should seek inherent safe design
concepts to minimise H
2
S levels, and avoid
generation of process streams with very high
levels of H
2
S.
(2) Ensuring that risks associated with H
2
S are
quantified and recorded.
H
2
S RISK MANAGEMENT SYSTEM
Hazards & Effects Management Process
The H
2
S Risk Management System shall be
applied to the consideration of H
2
S throughout
the design process and facility life. The Hazard
and Effects Management Process (HEMP) for
H2S facilities shall consist of the following four
basic steps:
1.Hazard Identification
2.Risk Assessment Qualitative/Quantitative)
Risk Reduction, Control & Mitigation Recovery
& Emergency preparedness planning.
Basic HSE Design philosophy for Sour
service
The HSE design philosophy for sour facility shall
be developed on three levels of Controls viz.Page 21

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 12
(3) Minimising exposure of operational and
maintenance personnel to H
2
S risk, e.g. by
designing for unattended plant operation and
minimum maintenance requirements.
(4) Ensuring that detailed design minimizes the
risk of H
2
S release.
(5) Ensuring that adequate personnel protection is
provided. This includes consideration of training,
access control, gas detection, personal protective
equipment, and escape.
H
2
S RISK MITIGATION MEASURES
Design considerations to prevent H
2
S release
The following guidelines shall be followed where a
facility is classified as High Risk Sour or Low Risk
Sour, to minimize the probability of H
2
S release.
All design considerations should consider the
potential for the increase of H
2
S concentration in
process streams over the life of the plant and the
probability that the classification may change from
Low Risk to High Risk Sour over the production life
cycle.
Material of Construction
Recommended Materials of Construction for sour
service are CS, LTCS, SS316L, CS + Epoxy lining,
CS clad with 825 Alloy, 625 Alloy, GRP etc.
Materials & Piping department shall be consulted
for selecting any particular material for sour
service application.
Piping
To minimise corrosion, piping should be designed
and installed in such a way that dead ends and
areas of intermittent flow are eliminated.
¾
Double block and bleed isolation shall be
provided for High Risk Sour facilities.
¾
Screwed fittings shall not be used in Sour
Service. Flanges on piping in Sour Service
should be minimised wherever practical, to
reduce the number of potential leak sources. To
avoid flanged connections, welded valves are
recommended.
¾
Butt weld joints shall be used for high pressure
sour
service. Socket welds
are not
recommended.
Small bore connections shall be kept away from
positions vulnerable to damage (review during 3D
model review)
Small bore connection rating shall use higher
flange rating from the systems (eg. closed drains
upto a/g-u/g interface will use ratings same as of
parent system)
Valves preferably with less leak path, ie., and top
entry valves shall be considered.
¾
Efforts shall be made to minimize the number of
valves without affecting the maintenance
requirements (review and assess during P&ID
review, design reviews and HAZOP)
¾
Higher size small bore fittings (preferably 2
size) are recommended for instrumentation
connections. Material testing procedure shall be
established to ensure the quality of fitting.
¾
Drain line should not be less than 2 size.
Consideration shall be given to reduce as much as
possible leak sources without creating future
problems during maintenance (to be reviewed
during design review & HAZOP).
Sampling system
The location of sampling points necessary for
future monitoring of facility classification shall be
determined in consultation with Operations during
design. Samples should be taken from the areas
of process plant expected to contain the highest
concentrations of H2S. Typically 3 sample points
would be expected to cover high, low and
atmospheric pressure levels. Where possible
these sample points have to be combined with
sample points for other purposes.
Details of sample points shall be determined in
consultation and agreement with Client to ensure
compatibility with local sample handling facilities.
Closed system bomb sampling systems shall be
used in High Risk Sour service. Sample return
lines should be routed back into a lower pressure
process stream. Where this cannot be achieved,
the sample return line shall be flared or vented.
Vents and drains Page 22

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 13
¾ Depressurizing of equipment and process lines
in Sour Service should be routed to a flare
system.
¾ Where this is not possible, venting may be
allowed but shall be designed such that
personnel
cannot be exposed to H
2
S
concentrations above 10 ppm. Special attention
should be paid to the vents from Glycol
Regenerator vapour outlets, which may contain
very high levels of H
2
S.
¾
All liquid in sour service shall be piped into a
closed drains system.
¾
Vents and drains, which are for hydro testing,
shall have their outlets blocked-off by blind
flanges.
Equipment Design Considerations
a) Centrifugal compressors
Shaft seals
The following mechanical seal types shall be
considered:
¾
Dry gas seals (most preferred) for design
pressure below 120 bar
¾
Conventional oil film seals for design pressure
above 120 bar
¾
Mechanical seals (restricted by speed/pressure)
used typically in refrigeration systems and small
machines.
¾
Dry gas seals shall be provided with a sweet
buffer gas (e.g. nitrogen).
Lubricating and seal oil system for above 120 bar
system
A combined lubricating and seal oil system may be
used unless the process gas contain levels of H
2
S
greater than 6% (mol), which cannot be
adequately removed by a sour seal oil reclaimer. If
the H
2
S concentration is greater than 6% (mol),
then separate lubrication and seal oil systems
shall be provided. Seal oil collected in the
contaminated seal oil trap shall not be recycled.
Such oil shall be reclaimed (vacuum degassing or
air stripping) before re-circulating or be discarded.
Seal vent
A seal leak detection system shall be installed.
Seal vent gas shall be directed to AP flare. If an
AP flare system does not exist, then local venting
may be allowed.
Reciprocating compressors seal leak detection
system shall be installed. Blanket gas shall be
used for distance piece venting. Vent gas shall be
discharged to AP flare. If an AP flare system does
not exist, then local venting may be allowed.
The compressor vendor shall provide process
engineering flow schemes of lubricating and seal
oil systems, seal gas and flare and drain
connections, which clearly shows how the above
sour service requirements have been incorporated
into the design.
Gas turbines and gas engines
It is not recommended to use Sour gas should as
fuel gas wherever possible. If sour gas is used
then:
¾ Turbine engine materials and pipe work shall
be compatible with the sour fuel.
¾ Duplex filters shall be installed on the fuel gas
inlet to ensure removal of ferrous sulphide (fine
black dust). These filters shall have water-flushing
connections to avoid auto-ignition of pyrophoric
iron when they are opened.
¾ Fuel supply shall be superheated to vendor
specification.
¾ Turbine enclosures shall be ventilated to
ensure a safe working atmosphere.
Stack height shall be determined based on
dispersion of exhaust gas to ensure that ground
level concentrations of SO
2
do not exceed 5 ppm.
Centrifugal and positive displacement pumps
If Sour gas can be released in the event of a seal
leakage, a seal leakage detection device should
be installed. This device may be a seal pot with a
level switch or pressure transmitter with switch,
vented to an AP flare.Page 23

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 14
Instrumentation Design Considerations
¾ Sour gas shall not be used as instrument gas.
Pneumatic instruments in Sour Service shall
use instrument air or sweet gas.
¾ High reliability shall be ensured for all
instrumentation systems for monitoring process
over pressures (HIPPS etc.) through structured
safety reviews like SIL.
¾
Flexible connections shall be minimised,
wherever possible
¾
Instrument tubing shall be replaced with hard
piping in vulnerable locations and bracings shall
be provided for small bore piping.
¾
Instrumentation tapping shall be minimised
using electronic indicators on transmitter loops,
instead of
additional
pressure/temperature
gauges.
Design considerations to minimize effect of
H
2
S release
The following guidelines shall be followed to
minimise the effect of an H
2
S release.
Well area Layout
Well site access roads should, where possible, be
located upwind of prevailing winds to minimise
exposure risk to personnel approaching them.
Facility Layout
In selecting a facility site, consideration should be
given to taking advantage of the prevailing wind
direction, climatic conditions, terrain, transportation
routes, and the proximity of populated or public
areas. Clear entrance and exit routes should be
maintained and confined areas within the facilities
should be avoided. Location, spacing, and height
of flares or vent stacks should be determined
based on acceptable gas dispersion calculations.
Plant layout and spacing
Gas containing H
2
S may be heavier or lighter than
air, and so can accumulate in low or high places.
Areas of restricted ventilation (both high and low),
should be avoided so as to allow sour gas
releases to disperse. Equipment handling sour
fluid shall not be placed inside a totally enclosed
area.
The seals of rotating and reciprocating
equipment which are a potential cause of High
Risk Sour releases shall be given special
consideration in plant layout and spacing to avoid
trapping concentrations of H
2
S and to take fullest
advantage of the natural atmospheric dispersal
effects.
All working locations should be equipped with at
least two escape routes in separate directions.
These shall be located as far apart as possible.
Elevated platforms shall use stairs only so that
there is no hindrance when carrying breather
sets or rescuing H2S affected personnel.
Fences
Fences referred to in this document are all to
stop personnel accidentally entering H
2
S areas.
The H
2
S concentration at the facility fence during
normal operation shall not exceed 10 ppm.
Dispersion calculations shall be performed for all
normal or operational vents.
High Risk and Low Risk Sour wellheads located
in predominantly Ëœsweetâ„¢ fields shall be fenced.
Emergency escape exits
Emergency escape exits in Low Risk Sour and
High Risk Sour facilities shall be provided such
that escape is possible upwind of (or at worse,
perpendicular to) the prevailing wind direction. As
a minimum, escape gates shall be provided on
opposite sides of the facility.
Warning signs
It is important that all equipment containing H
2
S
is clearly marked, with vessels and pipelines
individually identified. This is in addition to area
warning notices which should be in all working
languages.
If
available, Country
specific
regulatory requirements shall be referenced for
color coding of process equipment and pipe work
containing H
2
S in hazardous concentrations. H
2
S
designated areas should be identified by signs at
each point of access.Page 24

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 15
Any areas identified as being of particularly high
risk, requiring special precautions or a higher level
of training also be marked by appropriate signs
and markers. Signs with pictorial content are
preferred to text only. The codes ISO 3864, BS
5499, API RP 49 etc. may be used as Reference
for developing proper safety signage layouts.
Windsocks
Each Sour Service designated facility shall have
sufficient windsocks located so that they may
easily be observed from any position within the
station.
Temporary Refuge
Temporary refuge is a place where control room
personnel will be adequately protected from
relevant hazards following a major incident, and
from where they will have access to the
communications, monitoring and control necessary
to ensure a safety and from where, if necessary,
safe and complete evacuation can be effected.
For High sour facilities, Temporary Refuge is
recommended to be a part of Control room. The
TR facility shall have an integrity against any Fire,
smoke or toxic release for two hours.
All cable entries to the TR shall be provided with
gas tight transit blocks to minimize any penetration
of gas.
On confirmed H
2
S detection, the normal HVAC
system of TR room shall shut down and fresh air
inlets and exhausts shall close through motorized
dampers. Following this, fresh air supply shall be
reinstated through emergency HVAC system
equipped with stand alone oxygen generator/
system.
TR shall be kept under positive pressure to
prevent any H
2
S ingress.
First aid medical facilities shall be available within
TR room.
Assembly Points
The preparation of a comprehensive Emergency
Evacuation Plan, including consideration of
requirements for assembly points, shall be
incorporated in the design of all High Risk and Low
Risk Sour facilities.
H
2
S concentration during the worse credible
accidental hydrocarbon release does not exceed
10 ppm. Where this would result in an assembly
point location at an excessive distance from the
facility (which might create additional problems of
access and communication), a relaxation to a local
concentration no greater than 50 ppm at the
assembly point may be allowed provided that
operations agreement is obtained. Where the
possibility exists for an H
2
S concentration greater
than 10 ppm at an assembly point, local
emergency procedures shall highlight that
personal H
2
S Gas Detectors may continue to
alarm at the assembly point.
Dispersion calculations shall be performed for all
Sour Service facilities to verify that assembly
points are suitably located. Maps showing worst
case H
2
S concentrations contours shall be
included in the Facility HSE Case and be
displayed in the facility Control Room. The maps
should include 50 ppm and 10 ppm contours.
H
2
S Detection
The primary objective of fixed H
2
S detection is to
provide a warning to prevent entry of personnel
into a known hazardous area. A fixed system does
not in itself provide personnel protection and shall
not be considered a substitute for entry
precautions, personal H
2
S detection or personnel
protective equipment. A Fixed H
2
S detection
system should be provided around all sour
process plant facilities.
x
Fixed detectors in open air
There are two approaches that can be applied to
fixed H
2
S detection in open areas:
a)
H
2
S Leak Detection. A sensor is installed
to detect leakage from a single source where the
H
2
S concentration in the fluid is relatively high and
/or there is a relatively high probability of a leak.
This may be applied on equipment for which an
alternative design measure to minimise the
probability and/or quantity of a release, or to direct
it to a safe location, is not readily available.
H
2
S Area Monitoring. Sensors are installed to
detect H
2
S dispersed into the plant area from any
source, and thus also cover less likely sources ofPage 25

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 16
release. This system may be applied in areas
where the installation of individual detectors for
each potential source cannot be justified.
All fixed detectors shall have a range of 0 - 20 ppm,
and shall alarm at 10 ppm.
x
Fixed detectors in enclosed areas
Buildings and enclosures where personnel can
enter during normal operations (except well
cellars), located adjacent to or in High Risk Sour or
Low Risk Sour designated facilities, shall contain
fixed H
2
S detectors set to activate alarms at 10
ppm.
For control rooms and other buildings with central
air conditioning systems, which duct the chilled air
throughout the building, one fixed H
2
S detector
shall be installed in the air conditioning inlet. The
air conditioning unit shall be shutdown on H
2
S
detection. For High Risk Sour facilities, gas tight
dampers should be installed on the inlet to the air
conditioning unit, to close on detection of H
2
S.
For buildings where the air conditioning system
does not duct air throughout the building (such as
standard gathering station control rooms), fixed
H
2
S detectors shall be installed; one inside the
main entrance and others in the principal rooms
(e.g. the control room and the rest room).
x
Alarms
Fixed H
2
S detectors shall alarm audibly and
visually at the:
(1)
Station control room panel.
(2)
Main entrance to the station.
(3)
H
2
S detector location.
(4)
Remote monitoring centre, if telemetry is
installed.
The alarm shall also be audible throughout the
facility under all normal operating conditions.
These detectors should be designated for H
2
S
alarm only, and should not normally be connected
to the plant shutdown, due to the risk of spurious
shutdown. Consideration may be given to a
shutdown action on H
2
S detection for High Risk
Sour facilities, as part of an overall Safeguarding
Philosophy.
The alarm accept button shall be on the station
control room panel. This alarm shall be
distinguishable from the station fire alarm. On
acceptance of alarm at control room, only the
audible alarm in the control room should silence;
the visual indication should remain on.
Environmental Considerations
Emission controls should be set to protect offsite
people from toxic risks and avoid public nuisance.
There are no known health effects associated with
long-term exposure to H
2
S at concentrations at or
below the point where short-term symptoms (for
example eye or respiratory irritation) are observed.
SO
2
is one of the products formed when H
2
S is
burned in the atmosphere and is also formed
when pyrophoric iron sulphide oxidises. It is also
often present in combination with H
2
S. Sulphur
dioxide is a colourless, non-flammable gas (or
liquid) with a strong suffocating odour. It is a
respiratory irritant and causes coughing, an
increase
in
sputum
production
and
bronchoconstriction at low concentrations.
Recommended occupational exposure limits for
sulphur dioxide have been set in order to prevent
these acute symptoms. The ACGIH Threshold
Limit Value based on an 8-hour time weighted
average is 2 ppm. The 15-minute Short Term
Exposure Limit is 5 ppm.
The air quality in respect of
allowable
concentrations of H
2
S varies considerably from
country
to
country.
Allowable
emission
concentrations can be as low as 1ppm H
2
S with a
corresponding air quality of 0.02 ppm H
2
S over a
30 minute period. These levels do not present a
toxic risk although they can result in a pungent
odour. The odour threshold for H
2
S depends upon
the individual and can be as low as 0.02 ppm.
Country regulations shall be referenced while
developing the control strategy for environmental
emissions for H
2
S and SO
2
for a particular project and implimentation.
Conclusion
H2S risks are always present in majority of oil &
gas operations. The key components of effective
management of high H2S risks consists of goodPage 26

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 17
engineering practice to prevent the H2S release &
reduce the effects, operational & administrative
controls, emergency preparedness planning and
training to the personnel.
Reference:
1. API RP 55, Recommended Practices for
Conducting Oil and Gas Producing and Gas
Processing Plant Operations
Involving
Hydrogen Sulphide.
2. API RP 49, Recommended Practices for Safe
Drilling of Wells Containing Hydrogen
Sulphide.
3. NACE MR 01-75, Sulphide Stress Cracking
Resistant Metallic Materials for Oilfield
Equipment.
4. EP 95-0300, SHELL HSE Manual, Overview.
5. EP 95-0317, SHELL HSE Manual, Hydrogen
Sulphide (H
2
S) in Operations.
6. NIOSH, Pocket guide to Chemical Hazards.
7. US EPA, Health assessment document for
Hydrogen Sulphide (EPA/600/8E86/026A)
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