Homeowners and property managers take pains to make their dwellings and offices both pleasant and comfortable. Painting, plastering, spackling, re-furnishing and landscaping are all in the service of that aim. Likewise, expelling musty, moldy and decaying odors is also a priority to improve livability. Yet deodorising, while an enhancement to living quarters and workspace, can be dangerous for those in the energy sector or where natural gas is utilized. Smells are indicators, even warnings, to people who process, transport, use and store natural gas — an ordinarily odorless emanation. To that end, natural gas supplies must be subject to odorisation, i.e. the insertion of an odor, to make it noticeable if it leaks.

What Makes Odorless Gas Dangerous?

There is a reason you might have a carbon monoxide (CO) detector in your house. You can not smell it in the air. If present and not discerned, this gas can supplant oxygen in the red blood cells, harming tissues and even causing death. Similarly, radon (Rn) can invade the lining of the lungs and eventually bring on lung cancer. Both these gases require mitigation if present indoors. Natural gas, an increasingly widespread source of energy, is comprised primarily of methane (CH4); also present are alkanes, carbon dioxide (CO2), helium (He) and hydrogen sulfide (H2S). When combusted, nitrogen oxide and CO are released along with CH4. These compounds can be detrimental to both physical and mental health.

When all is said and done, long-term exposure to natural gas is hazardous to your health. This is why invisible, odorless gases, while easy on the olfactory glands, pose a grave threat to human well-being. The key, then, to working safely with the efficient energy source of natural gas, is to make it detectable. Odorisation has proved to be effective in this regard but chemical engineers and other experts advise cautious precision. There are varying approaches to this procedure and one size does not necessarily fit all.

Passing the Smell Test

Anything that is once free of detectable smell and invisible to the eye problematic in any setting much less in a facility where natural gas is flowing or stored. The seminal event that awakened people to the need for odor injection was a 1937 explosion of natural gas in a Texas school. The casualties included 200 dead pupils. Over the years, federal safety standards were tightened and today odorising is mandatory in the United States. Specifically, the law requires that flammable natural gases must be discernible at one-fifth of the lower explosive limit by anyone with a functioning sense of smell.

A word of explanation: natural gas has a combustion range between five and fifteen percent. This means that any air mixture with less than five percent or more than fifteen percent of natural gas present has no potential for ignition. Only within that range lies the danger of combustion. So, the federal rule means that odor must be present when the amount of natural gas is one percent of the mixture, i.e. one-fifth of the lower limit, five percent.

What Creates the Odor?

A chemical compound, most often mercaptan (CH3SH), serves as the odorant — the agent that produces a recognizable aroma signaling the presence of natural gas. Also known as methanethiol, methyl mercaptan is a rancid-smelling gas associated with, among other things, animal brains, blood and fecal matter, as well as flatulence and halitosis (bad breath). It can also be found in compost heaps, land fills, marshes and coal tar. Needless to say, CH3SH is an attention getter. Itself highly flammable, it makes the whereabouts of natural gas dangers unmistakable.

How, then, does the odorant pervade the natural gas. There are two different systems that accomplish this task. The first is one of chemical injection. This method is more common in large-scale operations, transmission pipelines for example. Either a pump or drip apparatus injects the liquid odorant into where the gas is flowing. The flow is monitored so the injection can be adjusted according to volume. The equipment used to do this is represented by one of three devices: a drip system; a pneumatic or electrical pump; or, less common, a Bourdan tube set off by a differential-pressure transmitter.

Other than chemical injection, the alternative is chemical vaporization, best suited to low-flow operations. A wick-type odorizer is installed on the gas line close to the meter. It is a bottle into which a cotton wick end, soaked in odorant, is placed. The other end of the wick reaches into the gas stream, allowing the odorant to travel the length of the wick and evaporate into the gaseous flow. the other type of vaporization is called bypass odorization. True to its name, this system diverts some of the gas stream to a vessel in which liquid odorant sits, after which the odorized gas returns to the line.

In Summary

Natural gas, unseen and unsmelled, is nevertheless a flammable energy source. Difficulty in detecting it can lead to tragic leaks and accidents. Fortunately, natural gas can be and is odorized by means of injection and vaporization using odorants like mercaptan. While the aroma is obnoxious, people know something is amiss when they smell it.



AirFacts Magazine