Safety Moment #87: Hydrogen Sulfide

Hydrogen Sulfide

Hydrogen Sulfide is a highly toxic chemical compound that is found in a wide variety of oil processing operations. High concentrations of H₂S may be present in crude oil, molten sulfur, tank and pit-bottom sludge and produced water, all of which may release H₂S when agitated, heated, or depressurized. Typical operational activities where personnel may be exposed to H₂S include drawing samples, handling and testing samples, gauging tanks, and when opening lines and equipment. Typical maintenance activities where personnel may be exposed to H₂S include tank cleaning and repair, vessel or sump clean-outs and repair, and well maintenance. These and other similar activities may place workers at a higher risk of exposure to H₂S.

Toxicity

Exposures to H₂S at concentrations as low as 600 parts per million (ppm) can cause death in a matter of minutes due to paralysis of the respiratory system. The gas is colorless and flammable. It is also 19% more dense than air. Therefore any H₂S that leaks is likely to accumulate at a low point. H₂S is soluble in many liquids, including hydrocarbons. However, H₂S mixed with natural gas may form a lighter-than-air mixture. The fact that H₂S is "heavier than air" is a statement that should be used with care, particularly when concentrations of the gas are low (say less than 100 ppm).

Table 5.6 summarizes the effects of H₂S at various concentration levels. (Guidance regarding the management of H₂S offshore is provided in API RP 14C.)

Table 5.6
Health Effects of Hydrogen Sulfide (Typical)

Concentration (ppm)	Potential Effect
<10	Not a health concern.
10 to 20	Eye and respiratory irritation.
20 to 100	Inflammation, corneal blistering and opacity of the eye, loss of the sense of smell, headache, cough and nausea.
100 to 300	Respiratory difficulty.
300 to 600	Central and peripheral nervous system effects, i.e., tremors, weakness, numbness of extremities, unconsciousness and convulsions.
600 to 1000	Rapid unconsciousness, death if first aid not promptly administered.
>1000

(1ppm = 1.4 mg/m³).

H₂S oxidizes rapidly in the body; therefore, there are normally no permanent aftereffects from acute exposure if the victim is rescued promptly and resuscitated before experiencing prolonged oxygen deprivation.

H₂S is not a carcinogen.

The gas is approximately 19% more dense than air. Hence it tends to accumulate in low or enclosed places such as pits, trenches, enclosed well bays and cellars, sumps, the tops of floating roof tanks, buildings, shale shakers and portable containers.

Hydrogen sulfide is easily detected by sense of smell up to values of around 100 ppm. (Most texts state that hydrogen sulfide smells like rotten eggs, but, with modern refrigeration, it is probably more apropos to state that rotten eggs smell like hydrogen sulfide.) Above the value of 100 ppm 'olfactory fatigue' can set in, and a person becomes unable to smell the gas. Therefore, the inability to detect H₂S through the sense of smell does not prove that the gas is not present. Moreover, the ability to detect the gas by smell varies widely among individuals. For these reasons portable H₂S detectors are commonly used. Each person at the site carries one of these devices, which is typically set to alarm in the 5-10 ppm range. 

In addition to the personal alarms, fixed sensors located around the facility will warn of a release. These sensors should send their signal to the control room.

Flammability

H₂S has a wide flammable range (4.3 - 45.5% by volume in air). When burned, H₂S forms sulfur dioxide (SO₂). In an oxygen-deficient atmosphere, iron and steel will react with H₂S to form iron sulfide deposits on the surface of the metal.

Location of Monitors

API RP 14C provides the following guidance for the location of H₂S monitors for offshore installations.

• Atmospheric H₂S concentration is > 50 ppm;
• H₂S concentration in piping is > 100 ppm;
• Enclosed areas as defined by API RP500 where H₂S could reach >50 ppm;
• Poorly ventilated areas;
• Sensors should be no greater than 1 meter above the floor/deck with a grid pattern of at least one detector per 400sf (37 m²) of floor space;
• Sleeping quarters;
• Within 3 meters of applicable equipment:
  • Vessels
  • Compressors (>50HP/38 KW should have two monitors)
  • Pumps
  • Headers
  • Wellheads

If H₂S is detected both visual and audible alarms should be triggered.

Corrosion

Hydrogen sulfide can cause corrosion of stainless steels such as 316 and 410 stainless in the form of sulfide stress cracking. (Other factors, such as pH, chloride concentration and temperature also affect the potential for steel cracking.) Copper alloys corrode rapidly in H₂S service. An industry value that has been developed is NACE MR-01, 2003 from the National Association of Corrosion Engineers. In the gas phase, a stream is sour if the H₂S partial pressure exceeds 0.05 psia. If a single phase liquid is in equilibrium with a gas phase, where the gas phase H₂S partial pressure exceeds 0.05 psia, then that liquid is also considered to be sour. If the liquid is not in equilibrium with the gas phase, then the liquid is considered sour, if this bubble point gas phase H₂S partial pressure exceeds 0.05 psia. The presence of water is not required for a gas and/or liquid to be considered to be sour, nor is there a minimum pressure to avoid designating a gas or liquid as sour.

In an oxygen-deficient atmosphere, iron and steel will react with H₂S to form iron sulfide deposits on the surface of the metal. Some iron sulfides (known as pyrophoric iron sulfide) are unstable and, when exposed to air, will undergo a rapid chemical reaction creating an ignition source that should be considered during equipment shutdowns.