Properties of hydrogen sulfide

Every control in a hydrogen sulfide program follows from a handful of physical and chemical properties. The gas is heavier than air, so it settles. It dissolves in water and oil, so it travels with liquids and comes out when they are agitated, heated or depressurized. It is flammable, so a release can become a fire or explosion. It burns to sulfur dioxide, another toxic gas. It reacts with iron to form deposits that can catch fire on their own. And it attacks steel, making equipment brittle. This lesson explains each property and what it means on the job.

The basic numbers

Heavier than air: where it goes

With a relative vapor density of about 1.19, hydrogen sulfide is only about a fifth heavier than air. That is enough for a pure or concentrated release to sink and flow downhill like an invisible liquid, collecting in pits, trenches, sumps, valve boxes, basements, ditches, low ground and the bottoms of tanks and vessels. On a calm night, gas from a leak can drift along a creek valley and pool in a low spot far from the source.

The heavier-than-air rule has limits. Once hydrogen sulfide is diluted in air at a few hundred ppm, the mixture is almost exactly as dense as air, and wind, warm air rising and ventilation move it around freely. A hot gas leaving a process can rise before it cools. Gas released under pressure jets in whatever direction the leak points. So the practical rule is: expect hydrogen sulfide to concentrate in low and enclosed places, but never assume high places are safe. Detectors should be placed and carried at the level where people breathe and where gas is likely to collect.

Converting between ppm and mg/m³

Gas concentrations in air are usually measured in parts per million by volume (ppm). Some limits and laboratory results are given in milligrams per cubic meter. The conversion depends on the molar mass of the gas and the volume one mole of gas occupies, which is about 24.45 liters at 25 °C and normal atmospheric pressure.

For hydrogen sulfide, with a molar mass of 34.08 g/mol, 1 ppm is about 1.39 mg/m³. So 10 ppm is about 14 mg/m³, and 100 ppm is about 139 mg/m³.

M = 34.08      # g/mol for hydrogen sulfide
MOLAR_VOL = 24.45  # L per mole at 25 C and 101.3 kPa

def ppm_to_mg(ppm):
    return ppm * M / MOLAR_VOL

def mg_to_ppm(mg):
    return mg * MOLAR_VOL / M

for ppm in (1, 5, 10, 20, 100):
    print(f"{ppm:4d} ppm = {ppm_to_mg(ppm):6.1f} mg/m3")
print(f"14 mg/m3 = {mg_to_ppm(14):.1f} ppm")

   1 ppm =    1.4 mg/m3
   5 ppm =    7.0 mg/m3
  10 ppm =   13.9 mg/m3
  20 ppm =   27.9 mg/m3
 100 ppm =  139.4 mg/m3
14 mg/m3 = 10.0 ppm

Percentages, ppm and mole fraction

In oil and gas, the hydrogen sulfide content of a gas stream is usually given as a mole percent or mole fraction, because the gas is mostly hydrocarbon. One percent by volume equals 10,000 ppm. A gas stream that is 2 percent hydrogen sulfide therefore contains 20,000 ppm, two hundred times the concentration that is immediately dangerous to life. Even small leaks of such gas need large dilution before the air is safe.

# How much dilution is needed before sour gas reaches a given level in air.
def dilution_needed(mole_percent, target_ppm):
    source_ppm = mole_percent * 10_000
    return source_ppm / target_ppm

for pct in (0.1, 1, 5, 20):
    print(f"{pct:5.1f} % H2S = {pct*10_000:9,.0f} ppm; "
          f"needs {dilution_needed(pct, 100):7,.0f}x dilution to reach 100 ppm, "
          f"{dilution_needed(pct, 10):8,.0f}x to reach 10 ppm")

  0.1 % H2S =     1,000 ppm; needs      10x dilution to reach 100 ppm,      100x to reach 10 ppm
  1.0 % H2S =    10,000 ppm; needs     100x dilution to reach 100 ppm,    1,000x to reach 10 ppm
  5.0 % H2S =    50,000 ppm; needs     500x dilution to reach 100 ppm,    5,000x to reach 10 ppm
 20.0 % H2S =   200,000 ppm; needs   2,000x dilution to reach 100 ppm,   20,000x to reach 10 ppm

The numbers make the point. A leak of gas containing 20 percent hydrogen sulfide must be diluted twenty thousand times before it falls to 10 ppm. That is why sour facilities are designed with emergency shutdown valves, flare systems and emergency planning zones, rather than relying on dilution alone.

Dissolved hydrogen sulfide

Hydrogen sulfide dissolves readily in water, crude oil and many other liquids. The amount dissolved depends on pressure, temperature and how much gas is above the liquid. When pressure drops, temperature rises or the liquid is agitated, dissolved gas comes out. That is why so many hydrogen sulfide exposures happen when workers open a hatch on a tank, gauge a tank by hand, dump a sample, transfer liquids between trucks and tanks, clean out sludge or open a line that has been under pressure.

Produced water and crude oil from sour wells can hold enough dissolved gas to create a lethal atmosphere at an open tank hatch, even when the gas phase of the well looks low in hydrogen sulfide. In sewers, hydrogen sulfide dissolved in sewage comes out where flow becomes turbulent: at drops, pump discharges, siphons and where force mains empty into gravity sewers. Lesson 3 describes these release points in more detail.

Flammability and fire

Hydrogen sulfide is flammable between about 4 and 45 percent in air. Those concentrations are tens of thousands of ppm, far above the levels that kill, so in most toxic exposures the gas is not in its flammable range. In a large release, however, the cloud near the source can be flammable, and an ignition source such as a running engine, a heater or a spark from tools can cause a flash fire or explosion. Its autoignition temperature of about 260 °C means a hot exhaust manifold or a steam line can ignite a rich mixture without a flame.

Gas detectors measure flammability separately as a percentage of the lower explosive limit (% LEL), usually calibrated to methane. Lessons 6 and 7 explain why a four-gas detector reads hydrogen sulfide in ppm on one sensor and flammability in % LEL on another, and why both readings matter.

Burning makes sulfur dioxide

When hydrogen sulfide burns, in a flare, an incinerator or an accidental fire, it forms sulfur dioxide and water:

Sulfur dioxide is a colorless gas with a sharp, choking smell. It is strongly irritating to the eyes and airways and can trigger severe asthma attacks; the ACGIH has set a short-term limit of 0.25 ppm, and OSHA's older limit is 5 ppm. Igniting a large sour gas release is sometimes the right decision, because sulfur dioxide is much less acutely toxic than hydrogen sulfide and rises with the hot plume, but ignition is a decision for trained responders following an emergency response plan, not for workers on site.

Iron sulfide: a fire waiting to happen

Hydrogen sulfide reacts with iron and steel to form iron sulfide, a black scale that coats the inside of pipes, tanks, vessels and filters in sour service. When this scale is exposed to air, for example when a vessel is opened for cleaning or a filter is removed, it can oxidize rapidly and heat up enough to glow and ignite nearby hydrocarbons. This is called pyrophoric iron sulfide. Fires and explosions in tanks and vessels during maintenance have been traced to it.

The usual controls are to keep iron sulfide deposits wet until they can be removed and disposed of properly, to use chemical treatments that neutralize the deposits before opening equipment, and to keep removed scale in water-filled containers. Workers cleaning sour equipment should treat black scale as a fire hazard and a possible source of trapped hydrogen sulfide.

Corrosion and cracking

Hydrogen sulfide also damages steel. In wet sour service, hydrogen atoms released by corrosion enter the metal and can cause a brittle failure called sulfide stress cracking, along with related forms of hydrogen damage. A pipe, fitting, valve or bolt that would be fine in clean service can crack suddenly under load in sour service. Equipment for sour service is therefore made from materials that meet recognized standards for resisting this damage, most widely the international standard published as NACE MR0175 and ISO 15156.

For workers, the practical messages are to use only equipment rated for sour service on sour systems, never to substitute an ordinary fitting or hose because it fits, and to report leaks, weeping joints and corrosion promptly. The details are a matter for engineers and are touched on again in HSE-332.