How to Work Safely with H2S
Hydrogen sulfide (H2S) is a gas that can have fatal consequences. Thorough preparation, gas detection, and a well-rehearsed emergency response plan are essential for keeping workers safe from exposure.
Hydrogen sulfide (H2S) is a naturally occurring chemical compound that is familiar to workers in the oil and gas sector, wastewater processors, and operators of certain manufacturing processes. Those who know it also fear it. Not only can it be fatal, but it kills quickly.
Workers who are at risk of encountering H2S require vigorous training to make sure they know how to work safely in its vicinity. Operators take diligent care to determine where the gas may be lurking. And when it’s released, there’s no time to lose - the response needs to be quick and effective.
In this article, we'll go over the basics of detecting H2S and preventing exposure to it.
Locate and Detect H2S
If you work in a sour gas plant, you wouldn't be surprised to find hydrogen sulfide in the environment (in this case, "sour" refers to how much H2S is in the natural gas product). But it also turns up in less obvious places.
It is a byproduct of decomposition, so concentrations of it can be found around manure, compost, and even natural features such as swamps. Sulfate-reducing bacteria, which are sometimes found in tanks, pipelines, and other types of hydrocarbon storage, also produce some H2S as a byproduct of their tiny activities. Since that is undesirable and potentially hazardous, biocide additives or flushes are run down the pipes in an effort to clear them out. I myself was surprised to learn that anything lived in active pipelines to begin with, and so much they have to be actively dealt with!
The concentration of hydrogen sulfide also matters. The Permissible Exposure Limit for H2S is 10 ppm, and no exposure above that level is considered safe even for a short term. Lower exposure is theoretically safe for 8-hour periods - thankfully, because getting every work area to zero is tough. An ambient level in certain areas of a gas plant is normal, but to paraphrase my old supervisor “if you start detecting a level of it, there’s more somewhere else.” In other words, there is a source, and an increasing concentration detected means you’re headed towards it.
So, how do you detect it? There are three ways to identify the presence of H2S on the worksite: odor, air monitors, and hazard assessments.
Hydrogen sulfide has a telltale smell of rotten eggs, but only if it falls in the right concentration range. According to OSHA, some people are able to detect its malodorous presence somewhere in the range of 0.1 to 1.5ppm. At 100ppm, however, H2S causes olfactory paralysis.
In other words, if the concentration is too low, you won't be able to smell it. And if it's too high, you won't be able to smell it either. When the H2S levels are high enough to deactivate your sense of smell, you're already in trouble - at 1,000ppm, OSHA lists "nearly instant death" as a symptom.
Personal Air Monitors
While everyone who works in proximity to H2S should be aware of its odor, you can't rely on smell alone to detect its presence. You have to leave that to more sophisticated instruments. Specifically, personal air monitors that can detect the concentration of specific gases in the air. Any worker entering an area where H2S might be present needs to be equipped with one so they know the air they're breathing is safe - and so they can immediately take action when it's not.
(Get answers to 6 Important Questions About Bump Testing Your Gas Detector)
Well before workers enter a space that may have H2S, a thorough and informed hazard assessment will give you an idea where the gas is likely to be. If any is present, the hope is that the risk can be mitigated before any exposure occurs. The workers' air monitors are a last line of defense – if everything has been done right up until that point, the personal monitor should get only very low or no reading. After all, it is worn on the body, so an alarm means a worker is already exposed.
Understand How H2S Behaves
Knowing how hydrogen sulfide behaves is as important as knowing where to find it.
Hydrogen sulfide is a little heavier than air. In the event of a release, it tends to stick close to the ground or collect in low-lying areas. This is one of the reasons why testing the atmosphere prior to confined space entry is especially important in pipeline country. Even a shallow ditch could be an invisible pool of noxious gas that could put an employee at risk of exposure - though they might smell it on approach and blame it on a coworker.
Confined spaces in a sour gas facility are especially dangerous environments for H2S. Even a small amount that may otherwise disperse quickly in open atmosphere could create a deadly concentration in an isolated vessel or tank. Confined space entry activities typically require someone to sample the atmosphere before anyone is allowed to enter, and continuously monitor it while someone is inside. This way, even if a concentration develops, you should be able to detect it and evacuate before anything serious happens.
It bears mentioning that in addition to its acute toxicity, H2S is extremely flammable. Extra care needs to be taken if the activity inside the confined space has the potential to cause ignition (hot work).
(Learn more about The Dangers of Gas in a Confined Space)
Working safely with hydrogen sulfide is all about preparation. Safety should be in mind at every stage, from the design phase to emergency response planning.
Everything has to be considered well ahead of time, and this planning results in some interesting ground rules for certain work environments. One that plant workers know well is the requirement to report to work clean-shaven, lest you have to use the “loaner razor” to shave before your shift. The reason being: in the event of a release, every worker needs to be able to don respiratory protection to save their lives and potentially assist with the rescue of casualties. A face mask can’t seal over even a day’s beard growth, however. Every worker has to be ready, for every shift.
But a smooth face won’t do any good if workers don’t know how to find and use a self-contained breathing apparatus (SCBA). They have to be fast and effective at donning the equipment, and that means everyone needs training and re-training. SCBAs also have to be tested beforehand to ensure they fit properly and have an adequate supply, then they need to be maintained and charged on a set schedule.
(Learn more in SCBA 101 - Meet the Respirator that Will Save Your Life)
A great deal of work is done up front to lessen the impact of potential emergencies before the worst can happen. The release of a plume of gas from overpressure in a vessel or other failure has the potential to be catastrophic because the cloud lies close to the ground and marauds around like a giant, invisible blob, generally downwind and downhill. Such an event would have huge potential for casualties. Where H2S is processed, emergency drills have to be robust and practiced on the regular, and should include members of the public at risk of exposure. Such a release is not terribly likely, but it comes with such dire consequences that it is a commonly-practiced scenario.
When an imminent release is detected (such as an overpressure event), the strategy in gas plants is to send it up to flare – pipe it up to a tall outlet and set it alight. When H2S burns, it breaks down into sulfur dioxide and water. The former is itself very toxic, with a PEL even lower than H2S itself, but it has the advantageous characteristic of being lighter than air so it goes up to the atmosphere. For obvious reasons, that’s not exactly desirable, it just beats the aforementioned low-lying, toxic, flammable cloud.
H2S Is a Hazard Worth Taking Seriously
In sour gas operations, they don’t shy away from putting the scare in workers about H2S. The training is stern and serious, replete with horror stories (of which there are many) and mandatory for anyone who might come anywhere near the gas. For all the careful preparation, it might come down to the individual worker’s knowledge and skill to save themselves or others from harmful exposure. Emergency response is a crucial, job-specific skill in that industry.