H2S is one of the leading safety related concerns in the petroleum industry in Alberta. In addition, it is prevalent in pulp and paper mills, barges, sewer systems, land fills, waste water treatment facilities and pig farms. As a result of its toxic and flammable characteristics, emphasis has been placed on safety procedures when working around this naturally occurring gas. Its main route of entry is by inhalation which can lead to pulmonary edema, conjunctivitis of the eyes, respiratory problems, and if in enough concentration to a worker’s death.
Hydrogen sulfide gas is colourless in nature and has a repulsive rotten-egg smell at smaller concentrations. People become exposed to it when they inhale it, which can cause respiratory and cardiac failure. Due to its corrosive properties, is also damaging to mucous membranes. It can form sulfuric acid on contact with sinus cavities and eyes . causing severe burns to human tissue . H2S is a secondary byproduct of many industrial processes but it is also generated when an organic matter decomposes. Because it is a little denser than air it is particularly dangerous in confined spaces or low-lying areas. Examples of potential areas where it might accumulate are under the sub structure of a drilling rig or in the mud tanks. In plants, it can depressurize from valves, seals, unions, thief hatches, sample valves, pipes or in high pressure buildings such as compressors, dehydrators, separators or treater units. In temperatures of 260oC (500F) or even lower, hydrogen sulfide can exhibit explosive reactions. Its flammable range is approximately 42%, with a lower explosive limit of only 4.3% when mixed with air. Static electricity and flammable vapours can result in ignition, so it is important to use intrinsically safe equipment when working near potential areas for the gas.
It is commonly called sewer gas , stink damp or sour gas or by its other spelling “hydrogen sulphide”. Burning it can produce an even more explosive, toxic and corrosive gas, sulphur dioxide, which can be seen above flare stacks. Both gases are serious environmental concerns and are closely monitored by environmental agencies.
It is a Deadly Poisonous
Hydrogen sulfide is extremely poisonous to carbon based life forms. When you breathe it in through your lungs, it enters into your bloodstream. In order for your body to protect itself, it strives to break the gas down very quickly into a non-harmful compound. Poisoning of the blood begins when the rate at which the gas is absorbed becomes higher than the rate at which it is removed from the blood. It is a nerve gas, classified as a chemical axphiant, causing respiratory failure in low does.
If you are acutely exposed to large concentrations of H2s, it can lead to a sudden coma and or kill you by causing your respiratory system to fail. As it accumulates in your blood, it quickly paralyses the brain, thereby inhibiting the functioning of the lungs. If not arrested in good time, acute poisoning can cause death.
Sub-acute or lower-level exposure may cause dizziness, loss of balance, diarrhea, headaches and other similar health problems. If you’re exposed repeatedly to very small quantity of the gas, it may lead to fatigue, slow down your pulse rate, make you unable to sleep, make you lose weight, give you eye infections or initiate eruptions on your skin. Tests should be carried out if the presence of hydrogen sulfide is suspected.
Who are those at risk?
Danger areas are confined and enclosed spaces which do not have proper ventilation.leading to a rapid buildup of H2s gas. A restricted or confined space may be difficult to enter or escape from and is partially or totally enclosed.This may render immediate escape from the gas to be difficult, should it accumulate in a baffle, basement, root cellar or cellar of a rig. A few examples of such places are tunnels, pits, ovens, grain elevators, manholes, tanks, and open-topped spaces which are more than 4 feet deep, thereby making it difficult to conduct a rescue or to escape from when overcome by the gas. It is also troubling that it can dissolve in fluids such as petroleum crude oil and condensate. Merely agitating it at a shale shaker, depressurizing it by opening up a pipe or heating it can cause it to rise.
H2s is measured in ppm (parts per million). In Alberta, the current Occupational Exposure Limit for Hydrogen Sulfide is 10 ppm, but in many parts of the world, they allow no exposure in an 8 hour work day! Short term and Ceiling exposure limits in Alberta are presently 15ppm. Its immediately dangerous to life and health limit (IDLH) is considered to be 100 ppm. 1% = 10000 ppm, which is a different scale and can confuse workers.
The exposure limits in several Canadian jurisdictions are similar to the Threshold Limit Values which is recommended by the American Conference of Governmental Industrial Hygienists.
The 10 ppm recommended limit is not a guarantee of worker safety and is not advised for use as a guideline for demarcating dangerous and safe H2s concentrations. Because there are large differences in the way individuals may be susceptible, some workers are likely to suffer at concentrations below or at the threshold.
It is best to eliminate the hazard by engineering controls, followed by administrative controls and proper personal protective gear. Self contained and Supplied Air breathing apparatus are the recommended respirators as they seal the face and protect the eyes from the gas. They must have positive pressure and a 30 minute minimum capacity to meet the protective requirements for this IDLH gas.
At low concentrations, H2s gas has a distinct smell of a rotten egg s. Workers therefore wrongly believe that if they do not smell it they are not exposed to it. Smell is an unreliable warning sign of H2s and cannot be relied upon as a means of testing the gas.
A sweet smell may be noticed at when its concentration is high, but at much higher concentrations, H2s can “paralyze” your sense of smell, leading to loss of your ability to smell. Some workers cannot smell hydrogen sulfide because of congenital problem. That explains why we should always monitor the air by using equipment which are designed for the detection of the gas. It can overwhelm the olfactory senses and numb the ability to detect it.
After exposures,some workers who may experience unnatural reflexes like being unable to sleep, feeling dizzy and lose their appetite and this can last for several months or even years. Even if acute poisoning does not lead to death, it may cause permanent symptoms like memory loss, depression and facial muscles paralysis.
How We Can Protect Workers ?
1. Ventilate and Monitor your Workplace.
The first step is monitoring the workplace air and controlling H2s in such a way that no worker will be exposed to any level above 10 parts per million. Ventilation should be carried out with a fresh air blower in all spaces where poisonous gases are detected. The ventilation system should be spark-proof and should be inspected at regular intervals. You should close, empty, lock and tag all areas where the gas emanates from.
You should permanently install modern electronic gas monitors that can easily detect low levels of H2s in important locations close to the ground. Devices like these should possess sound alarms which can be set to alert workers any time 10 parts per million level is hit.
Workers should clip portable monitors onto their belts and carry them into enclosed spaces as supplements or when they find the fixed ones unsuitable. The coloured lights and audible alarm on the monitors should give warning signs to the workers and should be capable of being used continuously for over 8 hours without needing to recharge their batteries. They need to be calibrated and functioned regularly to be reliable.
2. Prevention and First-Aid Equipment
Specific work procedures and permit system should be posted on all entries of enclosed/confined spaces . The permit system should give written authority for admitting any worker into the area, provide a list of all likely hazards and the safety equipment needed to safeguard workers and clearly declare that the workers involved have undergone proper current training in confined entry procedures. First aid, safety and rescue instrument, together with proper instructions, should be easily accessible.
Before you enter a confined or an unknown area, you should carry out sampling with a remote monitor on a wand connected to a toxic gas meter. You should make sure the monitor reach the lowest point in the space and this should be done throughout your work schedule.
A worker should be restricted from entering a confined space until at least one person is placed on standby outside in constant contact with him and another third person should be provided to watch over the operation. Workers should always use gas monitors that are calibrated for this usage and have good sensors and power capacity.
In the case of an emergency where an unconscious worker has be pulled out of a confined space, a proper rescue system, including a hoist, harness and a lifeline should be worn. It is also advised that a horn of an aerosol type which can easily be blasted should an emergency occurs should also be worn. Rescue workers too should be equipped with appropriate safety equipment that will be capable of supplying air while conducting the retrieval. A typical response strategy includes:
a) leave the area going crosswind
b) sound alarm and notify workers of the danger
c) Assess the situation
d) Don breathing apparatus and shut down the source
e) perform rescue
f) conduct revival skills
g) Recontact medical aid
3. Respirators and their Problems
An appropriate breathing apparatus such as that approved by NIOSH (the U.S. National Institute for Occupational Safety and Health) or C.S.A (Canadian Standards Association) must be worn before going into an enclosed space. Appropriate supplied-air respirator or self-contained breathing apparatus are recommended by NIOSH for hydrogen sulfide levels of 10 to 100 parts per million or any supplied-air respirator with a full facepiece for levels up to 250 ppm.
When in dangerous atmosphere, workers are advised to wear a self-contained breathing apparatus. Cartridge respirators or air purifying filters will not stop smaller gas molecules from passing through the mask. They also have a limited lifetime and are advised not to be used when the atmosphere is life-threatening. .
Suitable respirators with proper fit tests must constantly be used. They should also be made easily accessible, cleaned and maintained on regular basis.
4. Keeping Records
At every location where H2S may be present, air monitoring should be carried out and a survey record or log should be kept. Near miss or accident reports should be submitted and medical testing be performed.
5. Training/Education Program
Workers should be educated and made to undergo a comprehensive training program which includes knowledge of exposure hazards; how to recognize and deal with the symptoms; every emergency procedure such as evacuation, first aid, and interpretation of alarms; location of monitors and respirators training. This training should be given before workers start the job and should be repeated every 3 months. Workers should receive training certification before they are allowed to enter confined areas. Certified ENFORM training is available in Calgary Alberta from Allstar Enviro Safety – H2S Alive Enform
6. Workers Medical examination.
Yearly and before starting a job, a worker should be given medical examination. Close examination should be made on their eyes, respiratory and nervous systems. Checkups should be done to see if workers can use respirators, listen to and see warning signs.