Hydrogen Sulfide gas is a toxic gas found in oil and gas reservoirs. It is often referred to as sour gas in the oil and gas industry. Materials that carry sour oil or gas need to have special properties to increase their resistance to hydrogen sulfide stress cracking.
The following are the properties and characteristics of sour gas.
- Though hydrogen sulfide smells like rotten eggs at low concentrations it destroys your sense of smell at high concentrations.
- It is colorless
- It is highly flammable with lower explosive limit of 4.3% and higher explosive limit of 46%.
- It is heavier than air and tends to settle in low confined spaces.
- It is extremely corrosive gas.
Effects and Symptons of H2S gas
Below 10 ppm the rotten egg odour is moderate.
Between 10-100ppm coughing and eye irritation, headache symptons are evident.
At 100-300ppm you will have severe throat pain, difficulty in breathing and loss of smell.
Above 500ppm the gas can prove fatal causing death in few minutes to seconds.
An exposure limit of 10ppm for upto 8 hours a day and 5 days per week is considered acceptable with no long term ill effects and is defined as long term exposure limit (LTEL). However some countries and operators define lower limit of upto 5ppm as LTEL. A concentration of 15ppm is generally taken as short term exposure limit (STEL). STEL allows you to work for 15 minutes at a stretch with an hour gap between the next exposure and a maximum of four exposures per day.
Safety with H2S
H2S detectors are installed in all sour oil and gas installations that will sound an alarm and/or shutdown valves to prevent further dispersion of gas.
In the event of gas leak, if the source of leakage is known it is best to move upwind away from the source of leak. If the source of leakage is not known the best option is to move crosswind with respect to the wind direction.
For equipment and piping in H2S service, dissimilar welds are not allowed. The reason is that these dissimilar welds will always contain high hardness zones, which are susceptible to sulfide stress cracking. Therefore only flanges are recommended for change in material e.g. from carbon steel to stainless steel.