Natural Gas

Natural gas has the chemical formula of CH4. Natural gas, also known as processed natural gas, is a dry gas which means it has all the heavier hydrocarbons removed and stored in underground storage tanks for use. Natural gas has no color, no taste, no odor (your distributor adds mercaptan for safety which gives a rotten egg smell), is non-toxic (although you may die from insufficient oxygen if there is too much natural gas in the air), and is transparent.

The specific gravity of natural gas is .55, although some go by .6. This means a weight of a volume of a vapor gas weighs less than an equal volume of air. This is why when you have a natural gas leak, the vapor will rise towards the ceiling since the gas weighs less than air.

The ignition temperature for natural gas is between 1200°F and 1300°F, and the limits of flammability are 4% to 15%. This means if you have below 4% natural gas in the air, combustion cannot happen. Likewise, if you have above 15% gas in the air combustion cannot occur either. Opening a window during a gas leak can be dangerous due to this.

The products of incomplete combustion are carbon monoxide and aldehydes.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and are likely accompanied by carbon monoxide.

Carbon monoxide (CO) is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can be combustible.

The products of complete combustion for natural gas are heat, water vapor, and carbon dioxide.

The calorific value of natural gas is 1000 which may vary from location to location. This means 1 ft³ of natural gas gives off 1000 BTUh.

The flame speed for natural gas is 12’’/second.

Natural gas has a boiling point of -260°F and an expansion ratio of 1:600 when boiling from a liquid to vapor.

Propane

Propane has the chemical formula of C3H8 and is a wet gas containing the heavier hydrocarbons. Propane has no color, no taste, no odor (your distributor adds mercaptan for safety which gives a rotten egg smell), is non-toxic (although you may die from insufficient oxygen if there is too much natural gas in the air), and is transparent.

Propane as a vapor has a specific gravity of 1.52. The specific gravity of a gas is the weight of a volume of a gas compared to an equal weight of a volume of air. This means propane as a vapor is heavier than air and during a leak will build up closer to the ground.

Propane as a liquid has a specific gravity of .51, which is less dense than water. The specific gravity of a liquid is the weight of the liquid, propane, compared to the weight of an equal volume of water.

The ignition temperature of propane is lower than natural gas. It takes about 920°F to ignite.

The limits of flammability for propane are 2.4% to 9.5%. So less than 2.4% or over 9.5% of propane in the air will not be enough for combustion to take place.

The products of incomplete combustion are carbon monoxide and aldehydes.

Aldehydes are irritating to the eyes, throat, and nose. They smell terrible. Aldehydes taste somewhat metallic and are likely accompanied by carbon monoxide.

Carbon monoxide (CO) is the most harmful product of incomplete combustion. It’s colorless, odorless, tasteless, and toxic. In large quantities, it can be combustible.

The products of complete combustion are heat, water vapor, and carbon dioxide.

The calorific value of propane is 2500. This means 1 ft³ of propane gives off 2500 Btuh.

The flame speed for propane is 11’’/second.

Propane has a boiling point of -44°F and an expansion ratio of 1:270 when converting from a liquid to vapor.

What this means on a real service call

Specific gravity is the number that tells you where a leaking gas will go. Natural gas, with a specific gravity around 0.55 to 0.6, is lighter than air, so it rises toward a ceiling. Propane vapor, at 1.52, is heavier than air, so it sinks and can collect in low spots, basements, pits, and crawl spaces. That difference changes where you look for a leak, where you place a detector, and why a propane leak in a low area is especially dangerous.

The other numbers on this page matter for the same reason. Natural gas and propane are not interchangeable. They have different calorific values, about 1,000 Btu per cubic foot for natural gas and about 2,500 Btu per cubic foot for propane on this page, different flame speeds, and different ignition temperatures. An appliance set up for one fuel will not burn the other correctly without the proper conversion, orifices, and regulator settings for that appliance. Never assume a conversion is just a matter of changing the gas supply.

Numbers and measurements a technician actually sees

On a gas job, the readings you check most often are manifold pressure and supply pressure, measured with a manometer in inches of water column, and the flame appearance. A lazy, yellow, lifting, or floating flame is a clue, not a diagnosis on its own. Pair what you see with a combustion check where you are equipped to do one, and with the appliance rating plate. Also remember the expansion ratios given above: natural gas expands about 1 to 600 and propane about 1 to 270 when changing from liquid to vapor. A small amount of liquid therefore makes a very large volume of vapor, which is why liquid propane handling needs care and why a leak must never be treated as minor.

A common mistake

A common mistake is to rely on smell alone to rule out a leak. Natural gas in its natural state has no odor; the rotten-egg smell is added for safety, and it can fade, be masked, or go unnoticed, especially if you have been around the odor for a while. Propane also sinks, so a check at standing height can miss gas collecting near the floor. Use a proper gas detector and a leak-check solution or method approved for the job, and check high for natural gas and low for propane.

A practical checking step

If you suspect a gas leak, stop, do not operate electrical switches, phones, or ignition sources in the area, ventilate only if it can be done safely, and leave the area if the smell is strong or a detector alarms. From a safe place, follow your gas utility’s emergency instructions and the site’s emergency procedure. Do not relight a pilot or restart an appliance until the leak has been found and repaired by a qualified person and the system has been tested. For a routine check, confirm the fuel type on the rating plate, check supply and manifold pressures against that plate, look at flame pattern, and check for soot, which can point to incomplete combustion.

Related posts

For what happens when that fuel burns, see About Combustion, Incomplete Combustion, and Combustion Air Supply. For how pressure and volume relate, see Boyle’s Law.