Liquefied Petroleum Gas- its Characteristics and Safety Requirements


Liquefied Petroleum Gas- its Characteristics and Safety Requirements

 Liquefied petroleum gas is  a gas used in steel plants as a fuel gas for heating in various furnaces and in flame cutting machines of continuous casting machines. It is popularly known by its  abbreviation or short form which is LPG. LPG is also used for oxy-LPG gas cutting and welding. Sometimes it is used for carburization of steel, flame heating, flame gouging, flame hardening, flame cleaning, and flame straightening.

Liquid petroleum gases were discovered in 1912 when Dr. Walter Snelling, an American scientist, realized that these gases could be changed into liquids and stored under moderate pressure. From 1912 and 1920, LP gas uses were developed. The first LPG cook stove was made in 1912, and the first LPG  fueled car was developed in 1913. The LPG industry began sometime shortly before World War 1. At that time, a problem in the natural gas distribution process cropped up. Gradually facilities were built to cool and compress natural gas, and to separate the gases that could be turned into liquids (including propane and butane). LPG was sold commercially by 1920.

Like all fossil fuels, LPG is a non renewable source of energy. It is extracted from crude oil and natural gas. It is a safe, clean burning, reliable, high calorific value fuel. The main composition of LPG are hydrocarbons containing three or four carbon atoms. The normal components of LPG thus, are propane (C3H8) and butane (C4H10) (Fig 1). Small concentrations of other hydrocarbons may also be present. Depending on the source of the LPG and how it has been produced, components other than  hydrocarbons may also be present. CAS number of LPG gas is 68476-85-7  while its UN number is 1075. CAS number for propane is 74-98-6  while CAS number for butane is 106-97-8.

Components of LPG

Fig 1 Components of LPG

LPG is a gas at atmospheric pressure and normal ambient temperatures, but it can be liquefied when moderate pressure is applied or when the temperature is sufficiently reduced. It can be easily condensed, packaged, stored and utilized, which makes it an ideal energy source for a wide range of applications.

LPG is a colourless liquid which readily evaporates into a colourless and odourless gas. Normally foul smelling ethyl mercaptan is added as an odouriser to LPG so that leaks can be easily detected. During leakage, the vaporization of liquid cools the atmosphere and condenses the water vapour contained in them to form a whitish fog which may make it possible to see an escape of LPG

When mixed with air, the gas can burn or explode when it meets a source of ignition. It is heavier than air, so it tends to sink towards the ground. LPG can flow for long distances along the ground, and can collect in drains, gullies and cellars.

Normally, LPG is stored in liquid form under pressure in a steel container, cylinder or tank. It  is stored in pressurized vessels to keep it liquefied. Bulk storages are usually done in the tanks which can be installed above or below the ground. They are strong and not easily damaged, but liquid or gas leaks can occur from valves and pipe connections.

When use of LPG is started then some of the pressure in the container is released. Some of the liquid LPG then boils to produce vapour. Heat is needed to convert the liquid to vapour (known as the latent heat of vaporization). As the liquid boils, it draws the heat energy from its surroundings. This explains why containers feel cold to touch and why, if there is a heavy off-take, water or ice may appear on the container. When the use of LPG is stopped then the pressure returns to the equilibrium value for the surrounding temperature. The pressure of the LPG in the container varies with the surrounding temperature. It is also much higher than is needed by the appliances that use it. However it need to be controlled to ensure a steady supply at constant pressure. This is done by a regulator, which limits the pressure to suit the appliance that is being fuelled.

Specific gravity of LPG in liquid form at 15 deg C ranges from is 0.51 to 0.58 (water = 1). Specific gravity of LPG in gaseous form is 1.52 to 2.01 (air = 1). LPG is heavier than air and thus will flow along floors and tend to settle at ground level/ low lying places, and accumulate in depressions.

Boiling point of LPG is in the range of – 42 deg C to 0 deg C. The physical state of LPG is gas at 15 deg C and 1 kg/sq cm pressure.

Vapour pressure of LPG at 40 deg C is 5.3 kg/sq cm to 15.6 kg/sq cm. The pressure inside a LPG storage vessel is equal to the vapour pressure corresponding to the temperature of LPG in the storage vessel. The vapour pressure is dependent on temperature as well as on the ratio of mixture of hydrocarbons. At liquid full condition any further expansion of the liquid, the storage vessel pressure rises by around 14 to 15 kg/sq cm for each deg C. This clearly explains the hazardous situation that could arise due to overfilling of storage vessels.

Solubility of LPG in water at 20 deg C is less than 200 ppm (parts per million). LPG is soluble in organic solvents like alcohol.

LPG has a flash point of – 104.4 deg C.  Auto ignition temperature for propane is 46.1 deg C and that for butane is 405 deg C. Hence LPG does not ignite on its own at normal temperature. LPG is highly flammable with a lower explosive limit (LEL) of 1.9 % and upper explosive limit (UEL) of 9.5 %. This explosive range is considerably narrower than other common gaseous fuels. This gives an indication of the presence of hazard from LPG vapour accumulated in low lying area in the eventuality of the leakage or spillage. It has explosion sensitivity to static electricity.

Entrapped air in the vapour is hazardous in an unpurged storage vessel during pumping/filling-in operation. In view of this it is not advisable to use air pressure to unload LPG cargoes or tankers.

LPG can be compressed at a ratio of 1:250. The pressure at which LPG becomes liquid, called its vapour pressure likewise varies depending on composition and temperature (for example, it is around 2.2 kg/sq cm for pure butane at 20  deg C, and around 22.4 kg/sq cm for pure propane at 55 deg C).

It is a safe fuel and it ignites only within the specified LPG – air ratio of 2 % to 9 %. It has a high calorific value of around 11,000 kcal/kg which results in high efficiency heat output. The combustion products are CO and CO2.

The combustion reaction of LPG increases the volume of products in addition to the generation of heat. LPG requires up to 50 times its own volume of air for complete combustion . Thus it is essential that adequate ventilation is provided when LPG is burnt in enclosed spaces otherwise asphyxiation due to depletion of oxygen apart from the formation of CO2 can occur.

LPG is a pure, clean energy source which provides even and controllable heat. This makes it the ideal heat and power source for a wide range of industrial uses. Since LPG is almost free from sulphur, it can be used in sensitive situations such as chemical processes.

LPG, even though slightly toxic, is not poisonous in vapour phase. However it can suffocate when in large concentrations due to the fact that it displaces oxygen. In view of this the vapour possesses mild anaesthetic properties.

If the LPG concentration in air is greater than 10 %, it causes dizziness in few minutes. 1 % concentration of LPG in air gives the same symptoms in 10 minutes. High concentration causes asphyxiation. LPG liquid causes frostbite (cold burns) on the skin.

In case of inhalement, the affected person is to be removed to the fresh air area. In case large quantity is inhaled then artificial resuscitation is to be provided.

In case of LPG exposure to skin, the wetted clothes are to be removed and the affected area is to be washed with plenty of water.

LPG is highly inflammable and must therefore be stored away from sources of ignition and in a well ventilated area.

A very dangerous hazard exist when exposed to heat or flame. Fire extinguishing media are CO2 , dry chemical powder and water spray. During fire the containers  are to be kept cool by spraying water if they are exposed to fire or heat. If not cooled sufficiently, containers will explode in fire. If fire is big, the surrounding areas are to be kept cool by spraying water.

Contact with oxidizers is to be avoided. Olefinic impurities may lead to narcotic effect or it may act as a simple asphyxiant.

In case of leakage, steps are to be taken shut off leaks if it is without risk. Everyone in the vicinity is to be warned that the air mixture is explosive. In case of waste gas, the gas is allowed to burn under control.

LPG causes natural rubber and some plastics to deteriorate. This is why only hoses and other equipment specifically designed for LPG are to be used.

Precautions

It is to be ensured that LPG tanks are located in a safe place and have all the necessary safety  devices to protect the tanks, pipe work and any LPG appliances attached to them. It is necessary to look after carefully the LPG installation. In particular,  the following precautions are necessary.

  • To keep rubbish and anything combustible well away from the tanks, and keep weeds and grass in the vicinity cut down. Chlorate-based weed killer is not to be used since it can be a fire hazard.
  • To keep the security fencing provided for the tanks in good condition, and to keep the gate(s) locked shut. If there is no fencing, it is necessary to keep the tank valve cover locked.
  • To ensure that there are adequate arrangements for inspection and maintenance of the tank and its equipment.
  • All the records relating to the LPG installation are to be kept at a place so that they are readily available for reference.
  • No person is to work on the installation unless he is suitably trained and competent to do the work.
  • The operating instructions and the emergency actions are to be carefully read, and it is to be ensured that everyone who needs to, knows them, follows them carefully.
  • To keep people not involved with the installation well away from it.
  • To keep vehicles well away from the installation.
  • No modifications are to be carried out which may affect the safety of the installation, such as altering walls or erecting sheds or fences, or installing electrical equipment near the tank. Statutory separation distances are to be maintained.
  • The tank is not to be painted by a different colour than the original since this may increase the amount of heat it absorbs from the sun.
  • No trees or shrubs are to be planted near underground tanks. Also no poles or posts are to be put since it might damage the tank.
  • There must not be any smoking and there must not be any electrical equipment, vehicles, bonfires, barbecues or other sources of ignition near the installation. It is to be ensured that the ‘no smoking’ signs and any other notices are maintained.
  • It is to be ensured that all the above ground pipe work from the tank to the point of use is protected against accidental damage, and is properly supported. For underground piping, it is to be ensured that the route it takes is known, and putting anything in the ground which may damage the pipe work is to be avoided.
  • Any equipment failure or damage is to be immediately reported and is to be promptly attended only by the authorized persons.