Liquified petroleum gas (when mixed with other hydrocarbons)
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Propane was first distilled by American chemist Dr. Walter Snelling in 1910, with homemade equipment. Dr. Snelling was concerned that consumers were paying for more gasoline by volume than they were actually able to bring home, and discovered that gasoline contained a number of volatile hydrocarbons which quickly boil off, mainly propane and butane. Within two years, propane was being sold as a separate fuel, and by 1920 it was extremely popular. Rather than being distilled from gasoline, however, it is collected as a byproduct when other petrochemicals are produced.
Today what people call "propane" is generally liquified petroleum gas. LP gas is actually a mixture of only about 90% propane with the remainder being made up mostly of butane with small amounts of other light hydrocarbons. Most people use the terms interchangeably. LP gas is a very popular fuel for two reasons. First, it has a high energy density. Second, unlike methane (or natural gas), propane has a very low vapor pressure. This means that under moderate pressure, around 140 psi at room temperature, propane can be compressed into a liquid state, which reduces its volume by a factor of 270 for convenient storage. Although heating oil has a higher energy density, because LP gas is pressurized it can be piped to various locations in a house for use in not only home heating, but also the clothes dryer, range, oven, water heater, gas grill, gas fireplace, and other appliances. It is a popular camping fuel for its low weight and can also be used to run internal combustion engines for forklifts and generators.
Propane is delivered to homes by a truck carrying a pressurized tank. In some areas the delivery crew will regularly come by to check the storage tank to see if it needs a refill, but in other areas this keep fill service is not available and the owner will need to request refills as needed. The truck uses a strong, thick hose which clamps securely to the propane tank and drains the truck from the bottom to fill the storage tank from the top. This ensures the liquid is transferred. A flowmeter keeps track of the volume of propane transferred for billing purposes. The tank should be refilled before there is a danger of running dry — if it runs dry it will have to be tested for leaks, for a fee, before refilling. 20% is generally a good refill level, with 5% being an absolute minimum, because the truck might not be able to deliver immediately. Bulk tank level is monitored by a gauge on top of the tank which reports the percent full based on a magnetic float inside the tank which floats on top of the liquid propane.
A second way to monitor the level of a propane tank, whether bulk or portable, is with hot water. Hot water poured over the side of a propane tank will heat up the top portion of the tank because the gas does not conduct heat very well. However the liquid in the bottom portion of the tank will conduct heat away from the side of the tank and feel cool to the touch, so the level where the tank feels warm is the top of the liquid level of the tank. This is especially useful for portable tanks which generally do not have gauges.
Propane is piped to where it is needed from the top of the tank, ensuring the gas is released into the line and not the liquid. 140 psi is far too much pressure to actually use, however, so a pressure regulator is used to bring the pressure down to manageable levels. A bulk tank will often have a 10 psi regulator (color code: red) to bring the propane to the home. When the line actually enters the home, there are two options. In most applications, a 1/2 psi (11" water column, color code: brown) regulator will be used. In rare cases, a 5 psi regulator (color code: blue) will bring the propane into the home, with additional 1/2 psi regulators located inside the home to feed the appliances. In either case, the appliances will generally run on 1/2 psi. Portable cylinders will not generally have a regulator attached to them, rather the regulator will be on the hose leading to the appliance.
Liquified Gas Storage
Propane is sold as a pressurized liquid in refillable steel containers of various sizes, from small bottles the size of a softball used for cooking stoves while hiking, to 20 pound cylinders for camping or small trailers, to 500 gallon bulk storage tanks for home heating and cooking. Cylinders are filled according to how much liquid propane is in them, whatever free space remains is taken up by gaseous propane. Smaller, portable tanks are filled by the liquid weight, while larger, permenant tanks by liquid volume. When buying propane, the customer often has the option of having his original portable cylinder refilled, or merely swapping it for a full bottle on the spot. The bottle swap is faster, but the cylinders typically traded this way are dirty and unattractive (but never unsafe, or they wouldn't be resold).
Newer portable propane tanks are equipped with a float called an Overfill Prevention Device (OPD), which automatically shuts off the fill valve when the liquid level gets too high. These cylinders are marked by a characteristic three-sided valve for easy identification. The OPD is intended as a safety device, not a notice that the cylinder is full, so the cylinder should still be filled by weight.
Since propane becomes a liquid at 140 psi (at room temperature), any container with a mix of liquid and gaseous propane will be pressurized to 140 psi, regardless of the ratio of liquid to gas. This is similar to having ice in a glass of water: If there is a mix of ice and water, the water will be exactly at its freezing point regardless of the ratio of ice to water. The temperature can only drop below freezing if all the water freezes, or above freezing if all the ice melts. The pressure in the tank, likewise, is purely a function of temperature so long as there is a mix of liquid and gaseous propane. Propane is normally liquid at -42° C (-43° F), so this is its point of minimum pressure. From here, the pressure increases along an increasingly sloping curve until 97° C (206° F), its critical point, the temperature above which it can no longer be liquid no matter how much pressure is applied.
It's interesting to note that because the pressure in the tank is always around 140 psi, regardless of how much propane is in it, storage cylinders of all sizes follow pretty much the same basic design. They are all cylinders with half-spherical end caps, which is the second most efficient design for a pressure vessel, surpassed only by a spherical tank which would be bulkier. Since propane is not corrosive or otherwise reactive, they are all made of regular steel with welded seams, and painted on the outside (usually a light color to limit heat absorption). The valves will need to be protected, because they are the weak point of the tank. Bulk tanks have a hinged dome to cover them, and portable tanks have a guard around the top which doubles as a carrying handle.
Because the pressure fluctuates so much with temperature, a propane tank should not be filled above 80% with liquid. This allows some space to expand with temperature changes. Try not to overfill a propane tank in cold weather, as the liquid level will rise and the pressure will increase when the tank warms up. If a propane tank becomes too hot and exceeds a safe pressure, a safety valve will pop open to vent propane to the atmosphere. If this happens, the best thing to do is spray water over the tank to cool it down. But be careful not to spray the safety valve! The venting propane is depressurizing rapidly, so, like a CO2 fire extinguisher, the gasses being released are very cold and could freeze the water over the safety valve, blocking the vent and stopping up the pressure release, allowing the tank to build up dangerous levels of pressure. Although the propane tank will be rated to safely handle much higher pressures than that which triggers the safety valve, it does have its limits. The venting gasses are cold enough to cause frostbite to unprotected skin, so anyone working with propane, connecting or disconnected hoses or valves, including refilling procedures, should wear appropriate gloves and eye protection.
If a propane cylinder is caught in a fire, it will rapidly increase in pressure until the safety valve pops open. If everything is working correctly, the safety valve will be able to vent pressure fast enough that the propane tank will be in no danger of a rupture. However, unsecured cylinders could have so much pressure built up in a very large fire that the cylinder will actually take off like a rocket under the force of the venting gas. This could cause a lot of damage to anything it hits, and could easily kill a person. Properly secured cylinders shouldn't move, but the venting propane could catch fire. If this happens, the best thing to do is let the venting gas burn. Attempting to put out the venting gas will likely freeze up the safety valve and prevent the gas from escaping, and then the tank could rupture. Instead, fight the rest of the fire and try to cool down the cylinder itself. When the safety valve closes, there won't be any more venting gas to burn.
Because propane is so flammable, it is important to keep open flame, sparks, and cigarettes away from storage tanks, especially when filling the tank or making an attachment. Although there is practically no chance of causing the propane tank to explode, there is still a fire hazard. Since propane gas is heavier than air, it does not dissipate quickly, and leaks can collect in invisible, highly flammable pools. Adequate ventilation should always be provided when working with propane indoors. There are two ways of detecting a propane leak. First, although propane itself is odorless, ethyl mercaptan is added to the propane (and also natural gas) to give it a detectable odor. Second, when the attachments have been made, it is a good idea to rub the joints down with soapy water. If there is a leak, bubbles will form around the joint when the line is pressurized. Teflon tape rated for gas use or pipe dope should always be used to help create a tight seal when connecting permanent pipe fittings.
Some people strongly recommend pipe dope over Teflon tape for sealing pipe for gas use, because Teflon tape is prone to shredding between the pipe threads. If a shredded piece gets swept downstream by the gas, it could clog vents and valves. Pipe dope does not have this problem. If you do use Teflon tape, it is a good idea not to wrap the last two or three threads at the end of the pipe, to minimize the chance that a piece could fall into the pipe. Appliances that are intended to be attached and removed often, such as camp stoves and gas grills, will have an O-ring on the hose connector which ensures a tight seal without using tape or dope.
When burned with an appropriate supply of oxygen (a 5-to-1 ratio), propane will burn very cleanly, forming only water vapor and carbon dioxide. If starved of oxygen, it could form other molecules, including carbon monoxide. A quick visual check to make sure it is burning cleanly is to inspect the color of the flame — blue flame is burning clean, yellow flame is burning dirty. Usually turning the gas down will help, because less oxygen is needed to handle a lower gas flow. Modern propane burning appliances intended to be used indoors have oxygen sensors which shut off the flow of gas if there isn't enough oxygen present for complete combustion. As a back up measure, it is a good idea to purchase a carbon monoxide detector.
When used properly, propane is safe. Propane gas is not toxic except in very large doses, but it is an asphyxiant. Although high concentrations could displace enough oxygen to cause suffocation, the real dangers of propane are the fire hazard and frostbite from rapid depressurization. Both of these hazards are very minor if a few basic precautions are taken. Natural gas is more popular, due mostly to its existing underground distribution system and lower cost. The underground piping ensures a near-endless supply of natural gas while propane must be stored on site in bulk tanks. However, propane is extremely popular in less urban areas which do not have access to the distribution system, and also for portable applications like forklift truck fuel and RV heating and cooking. Propane, along with natural gas, gasoline, and other materials derived from crude oil, won't last forever though. The world's supply of oil is limited and will run out some day, and the price of propane fluctuates along with the price of oil. But until we run out, propane will be keeping us warm and our food hot safely, conveniently, and efficiently.