City and natural gas

METHAN, BUTAN, PROPAN, LPG - The explosive DANGER

Urban and natural gas, what is that actually?

Let's start with town gas, which no longer exists. It was produced from coal gasification and contained a fairly high proportion of toxic carbon monoxide. City gas existed until about the end of the 1970s, in West Berlin until the mid-1990s. It was gradually converted to natural gas, which was not quite so toxic. To do this, the incinerators had to be rebuilt, and different seals and valves were needed. However, the name "town gas" is still present in the population, which is why we still call our sensor for combustible gases the town and natural gas sensor (SE).

Natural gas, the gas that our municipal utilities and gas suppliers provide us with today for heating, hot water and cooking, is a naturally occurring gas that is mainly a byproduct of oil production, but also comes from pure natural gas fields that do not supply oil.

The main component of natural gas is the highly flammable gas methane with up to 90% by volume. Other substances besides butane and propane are various traces of sulfur compounds, ethane, CO2, noble gases, nitrogen and water vapor.

Once extracted, natural gas is purified of toxic and unusable substances such as water, hydrogen sulfide and carbon dioxide and fed into our gas supply system, not without first again adding the sulfur compounds thioether or alkanethiol to give the gas its typical odor, which we perceive quite naturally as the smell of gas. Without these additives, natural gas would have no odor at all. Every combustible gas that is sold has to be mixed with these substances to create an odor.

So we already wear the best gas sensor in the middle of our face: our nose. Fortunately, our nose is not always located exactly where gas could escape unintentionally.

At the various connections of our gas pipeline, at the transfer point, at the gas tap, at the meter and directly at the heating, stove or boiler. Here, mostly in the so-called heating and utility rooms (HWR), but also in the kitchen directly at the gas stove, the "city and natural gas warning detectors" from Elektrotechnik Schabus come into play.

They immediately determine whether gas is leaking and warn of a line defect with a loud piercing sound and, if necessary, switch off a connected solenoid shut-off valve so that no further gas can flow in.

Since natural gas consists mostly of methane, which is very light, it is lighter than air and immediately volatilizes upward as it escapes. So a GX-SE sensor must be placed at the top of the room to immediately detect the gas. But not at the very top, but about 30 cm below the ceiling, because there is the so-called dead space in the corners. Air that is in the corners and edges on the ceiling cannot escape and displaces the gas.

Gas from cylinders (butane/propane) is heavier than air, so the sensor is placed 15-30 cm above the ground.

At what point does escaping gas become dangerous?

There is the term "lower explosion limit", it is abbreviated LEL and given in percent. A gas-air mixture only becomes explosive when 100% is reached. In this context, it is important to know that it is not the pure amount of gas that has escaped that is decisive, as is the case with CO, which is easily expressed in ppm, but that other variables always play a role. Whether it is the temperature, the humidity or the oxygen content, because every combustion necessarily requires oxygen, otherwise nothing burns.

If the humidity is higher, there is less oxygen present, and if the temperature is higher, there are also fewer particles in the room that can react with each other. These three variables are taken into account by our SE sensors and converted into a voltage that is detected by the warning devices.

Now, one could warn immediately if even one molecule is detected or more realistically at e.g. only 3% LEL, but no customer would accept such behavior in the long run. Up to 5% LEL, a supposed false alarm occurs more frequently than one might associate with a gas line defect.

The sensors could do that, but who wants a warning when open paint and varnish cans outgas or someone walks past the sensor with freshly painted nails or freshly applied perfume? Solvents, among many other household substances, are very similar to the hydrocarbons in town and natural gas and are detected just as well by the sensors.

Some of the many DIN standards dealing with the detection of natural gas in residential areas recommend a warning at the latest when the 20% LEL limit is reached.

Since our sensors detect liquid gas (LPG with a high percentage of butane and propane) as well, we agreed on an early warning level of 12% LEL.

Always in time so that it does not become dangerous, but sufficiently tolerant to avoid frequent false alarms. And of course within the norm.