H2S is a colorless,
flammable gas that can be naturally present in most fossil fuels. It occurs as
a byproduct of the production and processing of oil and natural gas. We often
characterize our crude oil and gas fields as “sweet” or “sour” depending on the
concentration of H2S. H2S can also be produced during
metal refining, pulp and paper production, mining, and sewage and wastewater
treatment.
Who is at Risk?
Anyone who works or lives around:
•
Drilling rigs, Pipelines, Refineries,Gas plants
Gas-oil separation
plants (GOSPs)
Sewers and wastewater
treatment operations
What are the Hazards of H2S?
Toxic, Invisible, Heavier than air, Flammable and explosive,
Corrosive and reactive
1. Toxic
H2S is toxic and can affect
several different systems in the body. When inhaled it goes from the lungs into
the bloodstream immediately, where it can rapidly paralyze the respiratory
center in the brain and cause the lungs to cease to function, with death from
asphyxiation (suffocation) following in minutes.
Concentration and Effects
|
0.10 ppm
|
Foul, rotten egg smells noticeable.
|
10 ppm
|
TLV-TWA; eye irritation; use SCBA.
|
15 ppm
|
Short-term exposure limit (STEL) for 15 minutes.
|
50 ppm
|
Maximum peak exposure (MPE).
|
100 ppm
|
IDLH;
severe eye irritation, coughing, loss of sense of
Smell in
1 to 5 minutes.
|
250 ppm
|
Pulmonary edema; gastrointestinal disturbance.
|
500 ppm
|
Dizziness; loss of consciousness within 30 minutes.
|
1,000 ppm
|
Death within minutes.
|
Above
Table shows the toxic effects of the gas at different concentrations. The
permissible exposure limit (PEL) for H2S at 10 ppm. This is the maximum H2S
concentration a person is allowed to be exposed to over an average 8-hour day
during a typical 40-hour workweek, also called the threshold limit value-time
weighted average (TLV-TWA). When H2S concentration is suspected to be at 10
ppm, appropriate respiratory protection must be used. The short-term exposure
limit (STEL) for H2S is 15 ppm for 15 minutes. Note that at 100 ppm, H2S is
considered immediately dangerous to life and health (IDLH) and no work is
allowed. Appropriate respiratory protection is a full-face respirator connected
to either an auxiliary air supply or self-contained breathing apparatus (SCBA).
2. Invisible
H2S gas cannot be seen. H2S is
colorless at room temperature so it cannot be detected with the naked eye. Low
concentrations of the gas — even as low as 5 parts per billion — have a foul,
rotten egg smell, but higher concentrations can quickly deaden your sense of
smell — your nose becomes overpowered by the gas and stops sensing it
completely. Never rely on the sense of smell to detect the presence of H2S.
3. Heavier than air
H2S is heavier than air. With
poor ventilation, it tends to accumulate in low lying areas around drilling rig
platforms, pipeline trenches, sewers, sumps and excavations. In confined
spaces, H2S settles at the bottom, forming a deadly gas layer.
4. Flammable and explosive
H2S forms explosive mixtures
with air and can ignite. H2S is extremely flammable and explosive over a wide
range of concentrations in air. Its lower explosive limit (LEL) is 4.3% and its
upper explosive limit (UEL) 46%.
When H2S burns, it produces
sulfur dioxide (SO2), which is a colorless and toxic gas that can severely
irritate the eyes, nose, throat and respiratory system.
5. Corrosive and reactive
When H2S
is combined with moisture in the air, it forms a corrosive acid called sulfuric
acid that irritates the eyes and mucous membranes. It can also corrode metal,
causing dangerous leaks in pipes, valves, tanks and other metal equipment. When
H2S reacts with steel in pipes and other equipment it forms iron sulfide, which
is pyrophoric — meaning it can ignite when dry and exposed to air.
Controlling
the Hazards of H2S
We take all the reasonable
precautions necessary to protect everyone from harmful exposure to H2S. The
three basic approaches to controlling hazards are:
• Engineering
controls
• Administrative
controls
• Personal
protective equipment (PPE)
1. Engineering controls
4 Safe
and effective design, construction, operation and maintenance of drilling, work
over, process, transportation and storage equipment.
4 Ventilation
(using exhaust fans, air pumps).
4 Flaring
systems (safely burn remaining gas).
Engineering controls offer the
first line of defence against H2S hazards. Engineering controls or efforts to
keep H2S away from people are by far the most effective controls for hazardous
exposure to H2S. Some examples include safe design, construction and maintenance
of drilling, work over, process, transportation and storage equipment.
Additional engineering controls for H2S during routine equipment maintenance
and process upsets include ventilation and gas flaring.
Ventilation systems are
engineered to provide either local exhaust or dilution ventilation. Mechanical
ventilation is a widely used engineering control for diluting H2S
concentrations to safe levels. The idea behind dilution ventilation is to
provide the means to reduce concentrations of H2S below the threshold levels
for both toxicity and the lower flammable/explosive limits to simply avoid
these types of hazards. Local exhaust is just what the term suggests — using
ventilation for the remaining unwanted gases or fumes and safely transporting
them somewhere elsewhere so they won’t pose a hazard. Proper ventilation
prevents the accumulation of hazardous levels of H2S. Common forms of
ventilation are natural winds, powered air-blower systems and powered
air-exhaust systems.
Gas flare systems are
engineered to burn any H2S gas that may be released through the process of
venting pressure relief systems to mitigate otherwise serious personnel
exposures. However, there is still risk from exposure to sulfur dioxide (SO2),
produced when burning H2S.
2. Administrative controls
4 Assigning
workers to work in areas with no potential for H2S exposure or limiting their
work time in certain environments.
4 Substituting
with less hazardous materials.
4 Implementing
procedures to safely handle substances.
Administrative
controls are next in the control hierarchy. Administrative controls include
provisions, such as administratively preventing exposures by transferring
employees to other work locations (i.e., areas without potential H2S exposure),
limiting the time they work in certain environments (their maximum peak
exposure time over an 8-hour shift), substituting less hazardous materials for
more dangerous ones (to completely avoid the potential exposure), and
establishing and implementing procedures for safely dealing with substances
that are not reasonably controlled with engineering controls. Because of H2S’s
high toxicity, using administrative controls to simply limit the length of
exposure is generally not as effective.
Examples
of procedures to be implemented include: Well control procedures to avoid
blowouts or manage a kick; and procedures for detecting, monitoring and
controlling potential personnel exposure to H2S while working in confined
spaces and in low-lying (below grade) areas or equipment in sour service.
When
persons are required to work inside confined spaces, such as excavations, tanks
or vessels, and where concentrations are 10 ppm or greater, the appropriate
respiratory protection must be used, and a standby man, fully trained in rescue
methods and properly equipped to carry out rescue, must be present.
H2S detection
4 Fixed
detection systems
4 Portable
monitors
4 Personal
monitors
The
only reliable way to detect the presence of H2S is to measure its volume in air
using either fixed and/or portable calibrated detectors. Fixed detectors are
installed in areas where H2S may be present. They consist of a detector
(sensing head), an indicator/beacon and an audible alarm. When the H2S levels
exceed the preset limits, normally 10 ppm, an alarm sounds in the control room
and locally in the vicinity of the detector.
To be
effective, the detectors must be regularly checked, maintained and calibrated
as per Standard.
3. Personal protective equipment (PPE)
4 Self-contained
breathing apparatus (SCBA)
4 Supplied-air
breathing apparatus (SABA)
When
detection and monitoring systems indicate the presence of H2S at levels between
10 ppm to 100 ppm, while testing for H2S, and if there is an indication of
equipment failure or product leak, personal protective equipment (PPE) must be
used. PPE can also provide certain specific hazard protection when engineering
and administrative controls are not feasible or in conjunction with these other
control methods.
Because
H2S is extremely toxic the first priority is to provide respiratory protection.
When exposure to H2S is possible and only up to 100 ppm level, those in the
area must wear one of two basic types of respiratory protective equipment
appropriate for use around the gas. All work must be stopped at H2S levels
above 100 ppm and all workers leave the affected area.
•
A self-contained breathing apparatus (SCBA)
supplies compressed air from a cylinder worn on the user’s back to a sealed
face piece. It provides air for up to 30 minutes.
•
A supplied-air breathing apparatus (SABA)
supplies air through a hose from cylinders or a compressor in a location remote
from the user. Although lighter than the SCBA, it restricts movement. A
5-minute escape bottle must also be worn in case of a problem with the main air
supply.
Chemical gloves, coveralls and
a face shield must always be worn when handling H2S to protect the eyes and
exposed skin. An eyewash fountain and emergency shower must be provided and
used as necessary.
Supplied Air Breathing Apparatus
•
Be fitted and receive
training in its proper use.
•
Learn how to inspect and
maintain the device.
•
Check the cylinders to
make sure they are full.
•
Be trained in emergency
rescue.
Before you use respiratory protection equipment, you must:
•
Be fitted for the
specific device and trained in its proper use (i.e., how to put on the cylinder
and harness, open and check the air supply, and adjust the mask for an airtight
seal).
•
Know how to inspect and
maintain the device. Check for cracks in the face seal, broken head straps and
cracked or clogged hoses.
•
Check the cylinders to
ensure that they are full and ready for use.
Be trained in emergency rescue if it’s part of your assigned
responsibility.
Emergency Response Plans
Every operation where H2S exposure may be
possible must have an emergency response plan in place for H2S
release. Each facility has its own unique emergency plan depending on the
nature of its operations. Should an uncontrolled H2S release occur,
follow your department’s emergency response plan. In general, take these steps:
1.
Stop work immediately
and warn your co-workers (sound the alarm).
2.
Look for H2S
beacon to identify the location of the leak.
3.
Check wind direction. If
you are near or downhill from the affected area, put on an SCBA immediately, if
available.
4.
Evacuate to an area
upwind if the release is downwind or crosswind if the release is upwind; and
move to higher ground.
5.
Notify appropriate
personnel/control room, providing all pertinent information (i.e., location and
number of injured persons).
6.
If you are attempting a
rescue, protect yourself first by wearing the appropriate respiratory
protection.
7.
Move victim upwind or
crosswind to a safe area.
8.
If properly trained,
apply Basic Life Support (BLS) techniques.
Call for medical assistance if someone has been overcome or
strongly affected by H2S.
Summary comments:
•
Understand the hazards
of H2S.
•
Receive proper training
in the use of SCBA and SABA.
•
Follow safe work
practices.
Familiarize yourself with emergency response plans
