Hydrogen Sulfide (H2S)

Some oil and gas wells contain significant quantities of sulfur compounds in the form of mercaptans. These compounds can release hydrogen sulfide (H₂S) — a toxic, flammable and corrosive gas. The limited space available on a platform or ship means that an H₂S leak could quickly impact the people on board.

Although H₂S is very hazardous, its properties are well understood, as are the means used to remove it from the produced natural gas stream.

Table 1 summarizes the effects of H₂S at various concentration levels (the effects can vary considerably from person to person).

Table 1
Health Effects of Hydrogen Sulfide (Typical)

Concentration (ppm)	Potential Effect
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.
50	Mild eye and throat irritation after one-hour exposure.
100	Deadens sense of smell in 3 to 15 minutes and may cause coughing, and burning of the eyes and respiratory tract.
300 to 600	Central and peripheral nervous system effects, i.e., tremors, weakness, numbness of extremities, unconsciousness and convulsions. Dizziness, collapse, and unconsciousness after half to one hour.
600 to 1000	Rapid unconsciousness, death if first aid not promptly administered.
>1000	Immediate unconsciousness. Death in three to five minutes.

Despite its characteristic odor, sense of smell cannot be relied upon to detect the presence of H₂S because the gas rapidly deadens the sense of smell by paralyzing the olfactory nerve at concentrations greater than 100 ppm. Also, H₂S is sometimes associated with other materials such as bunker fuel that have odors that could be strong enough to mask the H₂S.

H₂S is naturally present in some hydrocarbon reservoirs and therefore may be present in toxic concentrations in piping and processing equipment associated with oil and gas production. It also occurs in wastewater, asphalt, bunker fuel, molten sulfur and as a product of the decomposition of sulfur-containing organic matter. Any of these may release H₂S when agitated, heated, or depressurized. Typical operational activities that may expose personnel to H₂S include sampling, draining sewers, gauging tanks, and opening lines and equipment.

H₂S is approximately 19% more dense than air at atmospheric conditions. Therefore 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. However, H₂S mixed with natural gas may form a lighter-than-air mixture. Therefore, the oft-repeated statement that H₂S is ‘heavier than air’ should be used with care, particularly when concentrations of the gas are low (say less than 100 ppm).

H₂S is flammable with a wide range of flammability (4.3 — 45.5% by volume in air). When burned, H₂S forms sulfur dioxide (SO₂), which is a colorless, highly toxic and very pungent gas.

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.

Hydrogen sulfide can cause corrosion of stainless steels such as 316 and 410 stainless in the form of sulfide stress cracking. Copper alloys corrode rapidly in H₂S service. Upper limit values have been developed by the National Association of Corrosion Engineers (NACE MR-0175 NACE 2008). 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.

Detecting Hydrogen Sulfide

Hydrogen sulfide levels should be monitored in any work area that may reasonably be expected to exceed an atmospheric concentration of 5 ppm. Monitoring devices can be either fixed or portable.

Fixed Monitoring Systems

Fixed monitoring systems, which continuously measure the concentration of H₂S in an atmosphere, should be installed in enclosed facilities such as buildings that contain process equipment handling gases or fluids containing H₂S. Inadequately vented areas should also have fixed monitors. (When H₂S concentrations exceeding 10 ppm can occur then ventilation is considered to be inadequate.) Fixed monitoring systems may also be installed in other locations where the possibility exists for a significant release of H₂S. Examples include the seals of pumps, compressors and turbines.

The set point for fixed monitors should always be less than 10 ppm; a value of 5 ppm is often used.

Fixed monitoring systems should be connected to emergency power supplies to ensure that the systems continue to operate during power failures.

Portable Detectors and H₂S Clips

H₂S detectors or clips are commonly used wherever H₂S may be present. They attach to a shirt pocket or belt. They typically have an alarm value set in the 5-10 ppm range. Portable detectors are generally used to test spaces for the presence of H₂S before conducting work in the area and to search out release sources. A disadvantage with this type of equipment is that it cannot be used to detect H₂S at more than arm’s length and requires personnel to be in close proximity to H₂S before an alarm occurs. (They can, however, be used with an extendable wand and hose to test an atmosphere without requiring a person to enter the area.)

Portable H₂S detectors are often combined with oxygen and combustible gas sensing equipment to serve as multiple gas detectors.

Alarms

Alarms provide personnel with audible warning or notice to evacuate, shelter-in-place or to don respiratory protective equipment. Flashing lights are useful in high noise areas where conventional alarms are difficult to hear.

Alarms indicating an H₂S emergency should be easily distinguishable from other alarms at locations with multiple sirens, horns or lights. Alarms should be well distributed to ensure that all personnel receive early warning of elevated concentrations of H₂S. Warning lights at sensor locations are useful in areas with multiple, fixed sensors for indicating which monitor has detected a high concentration of H₂S.

Operations and Maintenance

Workers should always remain upwind from the source of H₂S gas. Laboratory operations involving H₂S gas should be conducted under a properly functioning laboratory hood or with local exhaust ventilation placed at the source of emission.

Emergency Response

At locations where the work atmosphere concentration of H₂S could exceed 10 ppm, personal breathing equipment must be provided for essential personnel. Essential personnel are those required to provide proper and prudent safe operations activities and those required to effect control of a hazardous condition.

The personal breathing equipment should be strategically located so that it is quickly and easily available. These locations should be clearly marked within the work area and at least one apparatus more than the normal complement of essential personnel should be provided. Breathing equipment should be provided in pairs to facilitate the use of the buddy system.

Wind direction indicators such as windsocks, streamers, flags, or other suitable can be used for indicating wind direction at the facility. These should be readily visible to personnel on or approaching the facility so that the safest route of entry or evacuation can be determined.

Flags may be used to alert personnel to the risk of dangerous levels of H₂S. An example of color codes is:

Green - Normal
Yellow: Moderate Danger (10 - 50 ppm)
Red: Extreme Danger (>50 ppm)