What is liquified petroleum gas?
Liquified petroleum gas (LPG) is a fuel providing energy that can be found in our everyday life as it is used in many household appliances for cooking, heating, and hot water.
It is called liquefied gas because it is easily transformed into a liquid.
LPG needs only low pressure or refrigeration to change it into liquid from its gaseous state.
As a gas, LPG expands to 270 times its volume as a liquid. Therefore, it’s only logical that LPG is stored and transferred as a liquid, under pressure, in a gas bottle (e.g. propane tanks). LPG turns back into gas vapor when you release some of the pressure in the gas bottle by turning on your gas appliance.
Almost all of the uses for LPG involve the use of the gas vapor, not the liquefied gas. The LPG gas is ignited and burned to provide heat energy for various applications.
In this extensive guide, we are going to explore what is LPG and how does it work.
What is liquefied petroleum gas used for?
To begin with, LPG is used on gas stoves, cooktops, ovens, as heaters, fireplaces, and gas hot water systems are all popular in-home gas appliances.
There are also clothes dryers that run on petrol. BBQs, patio heaters, rooftop fireplaces, pizza ovens, and fire pits are all types of outdoor gas appliances.
Thousands of different commercial and industrial applications exist. LPG-fueled hot air balloons, Zamboni devices for ice rinks, and use as a propellant gas in various aerosol items are only a handful of the more uncommon applications.
Composition & properties of liquefied petroleum gas
LPG composition is primarily propane, butane, isobutane, butylenes, propylene and mixtures of these gases.
LPG is composed of liquid or gas (vapor), depending on pressure and LPG gas temperature.
Natural Gas Liquids (NGL) have the same LPG composition plus a few more gases not normally included in LPG. The full NGL list of components includes: ethane, ethene, butylenes, propylene, propene, isobutene, butadiene, pentane, pentene and pentanes plus, as well as propane, butane and isobutane.
Volume in gas/liquid state
LPG expansion is 270 times the volume of gas to the volume of liquid. So, 1L of liquid LPG (propane) expands to equal 270L of gaseous LPG. As there are 1000L in a cubic meter (m3), 1L of liquid LPG expands to 0.27m3.
What’s the boiling temperature (point) of LPG?
Water boils at 100°C or 212°F, becoming a gas (steam). In contrast, LPG (propane) boils at -42°C or –44°F, becoming gas vapor. LPG stays liquid because it is under pressure in a gas cylinder.
What’s the ignition temperature of LPG?
The propane ignition temperature in air (ignition temperature of propane gas) is when it reaches a temperature between 470°C – 550°C (878°F – 1020°F). At this temperature, the propane will ignite without the need for a flame, spark or other ignition sources.
What’s the flame temperature of LPG?
Propane flame temperature is 1967°C (3573ºF).
What’s the dew point of LPG?
The dew point for LPG-propane is the temperature at which the gas transforms into a liquid state, a process known as liquefaction.
What is liquefaction?
Liquefaction is the method of transforming LPG vapor to LPG liquid, and it is dependent on the temperature and pressure of the vapor. The higher the temperature of the vapor, the higher the pressure needed to convert the vapor to liquid.
For propane vapor at 20°C must be pressurized to about 836 kPa to see it liquefy, and at 50°C, about 1713 kPa pressure is required. The lower the temperature, the easier it is to liquefy the vapor. For n-Butane vapor at 20°C must be pressurized to about 115 kPa to see it liquefy, and at 50°C, about 510 kPa pressure is required.
The liquefaction conditions for Propane and Butane mixtures are also affected by the structure of the mixture, as well as the temperature and pressure of the vapors.
What’s the energy content of LPG?
The energy content of LPG is around 25MJ per litre. The LPG energy value of one gallon of propane is 91,547 BTU (60°F). In comparison, 25MJ equals 6.9kWh.
A summary of LPG properties can be found in the table below.
|LPG – Propane Boiling Point||-42°C or -44°F|
|LPG Melting – Freezing Point||-188°C or -306.4°F|
|Specific Gravity of Liquid LPG-Propane||0.495 (25°C)|
|LPG Density Propane Gaseous Density||1.898 kg/m3 (15°C) or 0.1162 lb/ft3|
|Energy Content of LPG||25 MJ/L or 91,547 BTU/Gal (60°F)|
|LPG Gaseous Expansion||1 L (liquid) = 0.27 M3 (gas)|
|Propane Flame Temperature||1967ºC or 3573ºF|
|Propane Ignition Temperature in Air||470°C – 550°C (878°F – 1020°F)|
|Limits of Flammability||2.15% to 9.6% LPG/air|
|Propane Auto Ignition Temperature||470°C or 878 °F|
|Molecular Weight||44.097 kg/kmole|
Table 1 – Properties of liquefied petroleum gas (LPG)
Where does liquefied petroleum gas come from?
Drilling wells, processing, transport, packaging, vaporization, gas regulators, and the gas appliances themselves are all part of the LPG supply chain. LPG is extracted from oil and gas wells.
LPG is usually present in mixtures of other hydrocarbons, such as crude oil and natural gas. Natural gas production and petroleum extraction are used to make LPG.
Transporting liquefied petroleum gas (LPG)
When LPG is stored under a moderate amount of pressure in gas tubes, cylinders, reservoirs, and wider LPG storage vessels, it remains as either a gas (vapour) or a liquid.
Since gaseous LPG has a volume 270 times that of liquid LPG, it is nearly always shipped in its liquid form. Cars, rail, tanker trucks, intermodal containers, cylinder trucks, pipelines, and municipal gas reticulation facilities are all options for transporting LPG (propane).
The majority of homeowners get their LPG from swap cylinders or tanker deliveries into a huge in-ground tank. When it comes to BBQs, most people carry their depleted petrol bottles to a store to be refilled or swapped out.
Storing liquefied petroleum gas (LPG)
At comparatively low pressures, LPG is compressed into a liquid and contained in specially designed gas bottles, cylinders, or tanks. LPG is typically contained in steel or composite containers as a liquid, varying in size from small BBQ gas bottles to larger gas cylinders and LPG storage tanks.
Storing LPG in gas bottles
The typical BBQ gas bottle sizes can be found below:
Gas Bottle Sizes
4kg LPG Gas Bottle
9kg Gas Bottle
|Diameter||240–265 mm||309–335 mm|
|Height||345–450 mm||450–525 mm|
|Capacity||7.25 L||16.66 L|
|Tare (empty) Weight||4.5–6.6 kg||7.8–10.4 kg|
|Full Weight||8.2–10.3 kg||16.3–19.0 kg|
|Energy||181 MJ||417 MJ|
Table 2 – Typical sizes of BBQ gas bottles where LPG can be stored
Did you know?
A 9kg gas bottle can last around 7 to 8 days if you cook a meal every day on a 4 burner BBQ grill for one hour. When cooking with a small 2 burner portable BBQ on high, a 9kg gas bottle (patio gas) would last about 29 hours. On lower settings even with just one burner switched on, it can last much longer.
Storing LPG in gas cylinders
The typical gas cylinder sizes can be found below:
|Propane Gas Cylinder Sizes||45kg
Propane Gas Cylinder
Propane Gas Cylinder
Propane Gas Cylinder
|Diameter||375 mm||510 mm||765 mm|
|Height||1250 mm||1380 mm||1460 mm|
|Tare (empty) Weight||33 kg||65 kg||140 kg|
|Full Weight||78 kg||155 kg||350 kg|
|Energy||2205 MJ||4410 MJ||10,290 MJ|
Table 3 – Typical sizes of gas cylinders where LPG can be stored
The water capacity of a gas cylinder is a metric that most customers are uninterested in. Engineers make use of it.
What’s the water capacity of a gas cylinder?
The water capacity of gas cylinder sizes is the cylinder capacity of LPG gas cylinder sizes if they were filled with water, as the name implies. For cylinder size, a reasonable exchange rate to consider is that 1 kilogram is roughly equivalent to 1.96 liters of LPG.
|Water Capacity of Gas Cylinder|
|Gas Cylinder Sizes||Water Capacity|
Table 4 – Water capacity of a gas cylinder
Storing LPG in gas propane tanks
The typical gas tank sizes can be found below:
|LPG Tank Sizes||0.5 tonne
Table 5 – Typical sizes of gas propane tanks where LPG can be stored
The LPG tank sizes (propane tank sizes) depicted are approximate. The size of an LPG gas tank can vary.
What happens inside the LPG gas bottle?
Below, we will examine in detail what really happens inside a liquefied petroleum gas container:
- when it’s idle
- during operation
How much pressure is in an LPG gas bottle?
The average force per unit of area applied by the LPG gas on the internal walls of the LPG gas container is referred to as LPG gas pressure. As a result, the LPG pressure within the cylinder will remain constant from the time the cylinder is full to the time the last of the liquid LPG is vaporized. When the remaining LPG vapour is used, the LPG pressure will drop.
LPG Pressure Units
PSIG, kPa, and bar are the three most common units of measurement for LPG gas cylinder pressure (LPG gas bottle pressure). LPG gas pressure is measured in pounds per square inch gauge (PSIG). PSIA stands for pounds per square inch absolute pressure of LPG gas.
1 atm = 14.7 psia = 0 psig = 101.325 kPa is a metric pressure unit that is part of the International System of Units (SI).
How much pressure is in a propane tank?
BBQ gas bottles have the same LPG gas cylinder-bottle pressure as big gas bottles, such as a 20lb propane tank or a 9kg gas bottle. The pressure in the LPG gas cylinder-bottle depends entirely on the temperature.
A 20lb propane tank has 172 PSIG of pressure at 100°F. Similarly, a 9kg gas tank has 1183kPa at 38°C.
Larger gas bottles, such as a 100-pound propane tank or a 45-kilogram propane bottle, have the same LPG cylinder-bottle pressure as small gas bottles. The pressure in the LPG gas cylinder-bottle depends entirely on the temperature. A 100-pound propane tank has 172 PSIG of pressure at 100°F. Similarly, at 38°C, a 45kg gas tank has 1183kPa.
Pressure inside LPG container at idle
LPG is liquid in a cylinder with a pressure-sensitive area at the top where it transforms into LPG vapor. As seen in the figure below, the LPG gas vapor pressure is kept at the top of the container, while the liquid LPG is kept at the bottom.
LPG cylinders are usually filled to 80% capacity, which means 80% liquid and 20% vapor. When you use the gas, the percentage changes, with the liquid LPG declining.
At idle, the pressure in the LPG cylinder will range from 0 bar at -43oC to 24.8 bar at 70oC. As a result, most pressure relief valves are set at about 25 bar, which is around the maximum pressure that an LPG cylinder can reach.
LPG is both a liquid and a vapor within the cylinder (gas). Propane is gaseous at -42°C (-43.6°F) and at normal temperature and pressure. Propane is a vapor under pressure or at lower temperatures. Propane is odorless, colorless, and has the appearance of water.
The LPG gas cylinder-bottle pressure and temperature relation are shown in the table below.
Table 6 – Pressure & temperature relation of LPG in a container
Pressure during operation (turning on gas appliances)
Every time one of your gas appliances is turned on, the LPG in your gas bottles begins to boil, increasing LPG gas pressure.
If you could see through the steel, you’d find that it looks just like boiling water.
At 100°C, water boils and turns into a gas (steam). On the other hand, LPG boils at -42°C and transforms into gas vapor. The conversion of a liquid to a vapor (gas) is known as vaporization. Since it is under LPG pressure in a gas cylinder, LPG remains liquid. It has a similar appearance to water as a liquid. In its natural state, it is colorless and odorless.
Pressure during operation (turning off gas appliances)
When you turn off the gas appliance, the vaporization process is terminated, and the reverse process starts. The method of converting LPG vapor to LPG liquid is known as liquefaction, and it is based on the vapor’s temperature and LPG pressure.
The higher the vapor’s temperature, the higher the LPG vapor pressure required to turn it into liquid.
For Propane vapor at 20°C must be pressurized to about 836 kPa to see it liquefy, and at 50°C, about 1713 kPa pressure is required. The lower the temperature, the easier it is to liquefy the vapor. For n-Butane vapor at 20°C must be pressurized to about 115 kPa to see it liquefy, and at 50°C, about 510 kPa pressure is required.
The liquefaction conditions for propane and butane mixtures are also affected by the structure of the mixture, as well as the temperature and pressure of the vapors.
LPG safety tips and precautions
Cylinders, tanks and bottles containing liquefied petroleum gas are generally safe to use provided they are used correctly. Below there are some safety tips so you can take the necessary precautions when using LPG. Please make sure to always follow the manufacturers’ safety guidelines.
Max pressure rating of a propane tank
The propane tank’s maximum pressure level is the propane tank’s design LPG pressure limit. While propane tanks are engineered to withstand much higher pressures than typical running LPG pressures, it is still critical not to surpass the propane tank’s maximum pressure rating.
The typical propane tank would probably burst only with a propane tank pressure rating over 6895 kPa or 1,000 PSIG.
That means the propane tank’s maximum pressure rating is approximately 5 times the usual operating pressure of an LPG gas cylinder-bottle. The maker and the cylinder itself will determine this. LPG pressure release valves are built into the main valve of propane tanks. The pressure release valve for LPG is normally set at 2585 kPa (375 PSIG).
As a result, the propane tank maximum pressure will never exceed this so the valve would open to allow any gas to escape, reducing the LPG pressure within the tank and never reaching the maximum pressure rating limit. Propane tank pressure varies with temperature.
LPG pressure regulator
To minimize and sustain a stable working LPG gas pressure, an LPG-propane pressure regulator is used.
Pressure controls for natural gas and LPG-propane appliances run at varying pressures, at 1.1 kPa and 2.75 kPa, respectively.
The real LPG operating pressure in LPG gas appliances is significantly lower than the 2.75 kPa pressure in LPG gas cylinders (LPG gas bottle pressure).
Can an LPG gas cylinder explode?
LPG cylinder explosions, according to Hollywood and the internet, are a frequent phenomenon.
Well, that’s not accurate at all.
In fact, explosions are incredibly unlikely, and deliberately causing an LPG cylinder to explode is extremely difficult. The majority of gas explosions occur as gas leaks into a small room, such as a kitchen. This isn’t as possible for LPG as it is with natural gas piped in. Since gas bottles are often kept outdoors, the gas bottle is rarely included in such an incident.
Liquefied petroleum gas leakage
LPG is an odorless gas in its natural state. The distinctive propane odor that many people associate with LPG is simply added as a safety precaution. Leaking propane gas could concentrate without being detected if it didn’t have a scent.
Direct contact with LPG
Avoid immediate contact at all times, since liquid LPG is cold enough to inflict serious cold burns on exposed skin.
Measuring the pressure in LPG cylinders
ESCP-MIS1 can be used for pressure measurement of LPG gas bottles.
Continue reading: What other pressure sensor types are available?