Letter Re: Differences Between Combustible Gases


James, Hugh,

Before someone gets hurt or blows themselves up, here are some more details on the gases in question.

Natural gas is produced primarily from high pressure gas deposits deep within the earth, and to a lesser extent as a byproduct of oil production. Natural gas is what is provided to most homes that are connected to a gas main, served by a gas company. Natural gas is primarily Methane gas, with a formula of CH4, or one Carbon atom with four Hydrogen atoms.

Methane has the disadvantage that it cannot be liquefied by compression. Natural gas must either be stored at high pressure (2000-3000 PSIG / 130 – 200 bar) for vehicle or other portable use. This is called CNG Compressed Natural Gas. Since methane is stored as a gas, the remaining volume in the cylinder can be determined by the pressure.

To store massive amounts of natural gas, or to ship it across continents in ships, the gas can be liquefied by refrigeration, and low pressure. This is called LNG Liquefied Natural Gas. LNG is stored and transported at approximately (−260 °F /−162 °C) degrees and 4 psi / .25 bar maximum. The liquid can then be vaporized and converted back to gas at the final destination. Methane is lighter than air, and most small leaks will dissipate.

Propane and Butane are produced primarily as a byproduct of oil production and separated by distillation. They are commonly referred to by the generic term LPG, Liquefied Petroleum Gas.

Due to the longer molecular structure, the molecules can be rearranged to make compounds such as Iso-butane, which has the same chemical formula as butane but different physical characteristics. For the purpose of this article, I am only discussing normal propane and butane. In the USA, propane and butane are sold separately. In other parts of the world, especially warmer climates, they are sometimes sold as a mixture.

Propane has a formula of C3H8, or three carbon atoms with eight hydrogen atoms, and is the most commonly available LPG, which is used to fill the BBQ tanks, the little propane tanks for propane torches, propane camping appliances, and lanterns, primarily in the USA. (Most of the rest of the world uses butane, including the small portable stoves that use the hairspray style cans of gas.)

Propane is also stored in the large tanks outside of the houses, of those that are not connected to the natural gas “grid”.

Propane is stored in these pressure vessels as a liquid, at a pressure of 120 psig / 8.3 bar at room temperature. Withdrawing propane from cylinder too quickly will cause the liquid to cool off, and the pressure drop. Condensation will frequently form on the outside of the cylinder as well. The top of the condensation on the cylinder is a good indication of the level in the cylinder. Since the gas is stored as a liquid, a pressure reading cannot be used to determine the remaining gas volume. One has to weigh the cylinder, or use the condensation method to determine the remaining volume. Large storage tanks are equipped with a liquid level gauge. Propane has a boiling point of approximately -44 degrees. As long as the temperature is above -44 degrees, gas will come out of the cylinder, although the pressure drops as the temperature decreases.

Butane has a formula of C4H10, or four carbon atoms with 10 hydrogen atoms. Butane is used primarily in low pressure applications, like filling lighters and for use in the portable stoves using hairspray style gas cylinders. Butane is used for most camping appliances in the rest of the world, outside of the USA, but it does not work well in colder climates. Bulk butane is stored in the same style pressure vessels as propane, and the valves use the same style threads. Butane is stored in these pressure vessels as a liquid, at a pressure of 29 psig / 2.0 bar at room temperature. This low pressure is one reason why butane is used in lighters. The small volume of gas and low pressure makes lighters relatively safe. Withdrawing butane from cylinder too quickly will cause the liquid to cool off and the pressure drop. Condensation will frequently form on the outside of the cylinder as well. The top of the condensation on the cylinder is a good indication of the level in the cylinder. Butane has a boiling point of approximately -32F/0C degrees. This relatively high boiling point makes it unsuitable for use in cold climates. Trying to get butane out of a cylinder during a snow storm is not going to happen, unless the cylinder is heated. General gas items and trivia:

All three gases will form explosive mixtures when mixed at the correct ratio with air.

Both butane and propane are heavier than air and will accumulate in low points and cellars. Severe and fatal building explosions have occurred due to gas accumulations in basements.

Always respect the enormous power of these gases and the energy stored in them.

All three gases are odorless and have an odorant added to be able to detect leaks.

Newer style LPG containers have special valves that contain an OPD– overfill protection device– in the valve to prevent overfilling of a cylinder. LPG containers can only be filled to 80%, to allow expansion of the liquid as the cylinder warms up.

Larger, non-disposable LPG tanks use a left hand thread at the valve connection. This is marked by a small groove in the hex nut. Newer style valves use a large hand tightened connection instead of the older connection.

LPG cylinders have a safety pressure relief built into the valve, or tank, to allow the pressure to be released in case the cylinder is involved in a fire. In a large fire, these relief valves are usually too small, and the containers will explode into a spectacular fireball, sending metal fragments all over the place.

Shooting at a propane tank will make a hole and will not create a spectacular, Armageddon-size explosion, like in the movies; although there may be a spectacular fireball if the gas ignites.

Most Airsoft guns use propane, butane, isobutane, or a mixture thereof as propellant. Always use the correct gas, or you may damage your gun.

Lower pressure butane can be used in most propane rated equipment but not the other way around. Due to the difference in pressures, the gases cannot be interchanged without knowing the pressure rating of the equipment. LPG requires special hoses to handle it. The gas can attack and dissolve hoses that are not formulated for LPG.

Household appliances are usually manufactured for natural gas, but most can be converted to propane by changing the gas orifices at the burners and adjusting the regulator pressure. In areas where there is no natural gas, appliances are usually ready for propane. All appliances have a label clearly showing what gas they are initially set up for.

Gases have their energy content rated in BTU, which is also what appliances are rated in. This makes it easy to determine how much gas you will need. For example, propane contains 21622 BTU per pound, or 91502 BTU per gallon (US). If your stove has a 16000 BTU burner and you plan on cooking for four hours, you will require 4 X 16000 BTU = 64000 BTU to cook your dinner (possibly a large turkey?). This would be 2.6 pounds or 0.7 gallons. These calculations are important if you use propane for heating and don’t want to run out of gas during a winter storm. A 250 gallon home tank is usually filled with 200 gallons propane, which equals 18.3 million BTU. This would operate a 125000 BTU furnace for 146 hours. If the furnace runs four hours per day, this would last 36 days.

Due to the low boiling point, propane can cause frost bite if allowed to get on the skin as liquid.

None of the three gases mentioned are poisonous, but they will cause death by asphyxiation (displacing air with gas). The generic formula for normal hydrocarbon gases is C(n) H(2n+2) where the number in the parenthesis represents the number of atoms. There are always (twice as many, plus 2) H atoms for every C atom.

The four gases are Methane, Ethane, Propane, and Butane. Pentane and larger molecules are liquids and no longer gases.

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