Hydrogen Sulphide is one of the most dangerous gases that can be encountered in the workplace.
H₂S is produced naturally by decaying organic matter and is released from sewage sludge, liquid manure, sulphur hot springs, and natural gas.
It is a by-product of many industrial processes including petroleum refining, tanning, mining, wood pulp processing, rayon manufacturing, sugar-beet processing, and hot-asphalt paving.
H₂S can cause death, even when exposed to relatively low concentrations. Fatal hydrogen sulphide poisoning usually occurs in confined spaces where concentrations are not dissipated by wind.
Therefore Hydrogen Sulphide or H₂S training is a requirement of workers in many different industries, including but not limited to; wastewater treatment, pulp and paper, natural gas, mining, and the petroleum industry.
H₂S is produced naturally by decaying organic matter and is released from sewage sludge, liquid manure, sulphur hot springs, and natural gas.
It is a by-product of many industrial processes including petroleum refining, tanning, mining, wood pulp processing, rayon manufacturing, sugar-beet processing, and hot-asphalt paving.
H₂S can cause death, even when exposed to relatively low concentrations. Fatal hydrogen sulphide poisoning usually occurs in confined spaces where concentrations are not dissipated by wind.
The main characteristics of H₂S are that it is:
Invisible: Hydrogen sulphide is a colourless gas.
Odorous: H₂S is very pungent, with a strong characteristic odour of rotten eggs. However, smell cannot be relied upon as a dependable indicator of H₂S presence. While H₂S may be smelled easily by some people at small concentrations, continuous exposure to even low levels of H₂S quickly deadens the sense of smell. Exposure to high levels of the gas can deaden the sense of smell instantly.
Dense: H₂S is heavier than air and may travel along the ground. It collects in low-lying and enclosed, poorly ventilated areas such as basements, manholes, sewer lines, under- ground telephone vaults and manure pits.
Toxic: H₂S is extremely toxic when inhaled. With toxicity similar to carbon monoxide, which prevents cellular respiration, exposure to Hydrogen Sulphide can cause death within a very short space of time.
Flammable: Hydrogen sulphide is highly flammable and burns with a blue flame, producing a toxic gas called sulphur dioxide.
Explosive: H₂S has a lower explosive limit (LEL) of 4.3% and an upper explosive limit (UEL) of 46%. H₂S may explode if heated. If gaseous hydrogen sulphide is put into contact with concentrated nitric acid, it explodes. As noted, when H₂S burns, sulphur oxides are formed. These vapours are heavier than air and may travel to an ignition source and flashback.
Corrosive: H₂S is an aggressive gas, provoking acid corrosion, which in this case is called hydrogen sulphide corrosion. It can cause hydrogen embrittlement, cracking of components and processing equipment.
Soluble: Hydrogen sulphide is soluble in water, crude oil and petroleum fractions. Dissolving in water, H₂S forms a weak acid, which can cause pitting in the presence of oxygen or carbon dioxide.
Hydrogen Sulphide exposure incident case studies
Over the years there have sadly been many workplace disasters caused by H₂S. Disasters which might have been avoided had appropriate safety precautions been taken. As we explore some case studies, consider how H₂S exposure might have been avoided:
Sludge Silo
Three workers died in an unconfined interior room at the same time that a truck dumped several tons of sludge from water purification stations into the adjoining silo. The hydrogen sulphide gas that had accumulated inside the silo spilled into the interior of the room due to a ‘splashing effect’ caused by the impact of the dumped sludge.
Gas Well Blowout
A natural-gas well blowout in South-West China got out of control. Its high concentration of deadly hydrogen sulphide created a 25m death zone, killing 243 people, mostly villagers, in their sleep or as they tried to flee the toxic fumes.
In the following days, some 4,000 people were hospitalized and more than 60,000 displaced. This occurred when toxic gases escaped from a filled well, whilst a drilling team were working on a 400 metre deep well.
Ship Repair
During repair work activity on a ship in France, the engineer carrying out the work suffered from severe lung oedema (a build-up of fluid), due to a leak of H₂S. The oil from the cargo heating system had been drained into an oil tank in the engine room. The hydrogen sulphide was released into the air when the tank started to leak into the engine.
Offshore Vessels
A vessel loaded slops from a North Sea installation. As outlined in the analysis report, the crew believed the slops to be “grey-black muddy water”, free from H₂S. However, upon arrival for discharge ashore, it was discovered that the slops were contaminated and had a ‘moderate’ H₂S content. Further, it subsequently emerged that this was the fourth such instance in ten days. There were health risks to the crew and contractors.
Pump Replacement
In the USA, two workers were replacing a water transfer pump and hydrogen sulphide was released when a clamp was removed. The two workers died.
Wildfires
A lightning strike caused a fire which ignited a natural gas line. The leak was isolated by the gas crew and the residual gas burnt off. However, when the crew were repairing the line, the smell of ‘rotten eggs’ became apparent to some of the workers. Some crew members started suffering symptoms related to H₂S exposure (eye irritation, headaches, etc.). This triggered the workers to move away to higher grounds to recover. The lessons learnt from the investigation found that the workers had inadequate H₂S equipment training, and an insufficient amount of gas monitors.
Paper Mill
Two fatalities occurred, with other workers sustaining serious injuries, as a result of a hydrogen sulphide gas leak at a paper mill. The two men had been working on top of a tank checking valves when they were apparently overcome by fumes and went into cardiac arrest. The fumes are thought to have come from pulp which was unused while annual maintenance was carried out.
Subsequent incident investigation in each of these cases found that the resulting illness and fatalities could have been avoided if adequate safety procedures had been in place. Don’t become a statistic. Know how to work safely around H₂S or wherever there is the potential for H₂S exposure.
Wildfires
A lightning strike caused a fire which ignited a natural gas line. The leak was isolated by the gas crew and the residual gas burnt off. However, when the crew were repairing the line, the smell of ‘rotten eggs’ became apparent to some of the workers. Some crew members started suffering symptoms related to H₂S exposure (eye irritation, headaches, etc.). This triggered the workers to move away to higher grounds to recover. The lessons learnt from the investigation found that the workers had inadequate H₂S equipment training, and an insufficient amount of gas monitors.
Paper Mill
Two fatalities occurred, with other workers sustaining serious injuries, as a result of a hydrogen sulphide gas leak at a paper mill. The two men had been working on top of a tank checking valves when they were apparently overcome by fumes and went into cardiac arrest. The fumes are thought to have come from pulp which was unused while annual maintenance was carried out.
Subsequent incident investigation in each of these cases found that the resulting illness and fatalities could have been avoided if adequate safety procedures had been in place. Don’t become a statistic. Know how to work safely around H₂S or wherever there is the potential for H₂S exposure.