LPG safety in industry
Modern industrial processes at times involve the handling, storage, or transport of gases and vapours that can form gas air mixtures that can burn or explode under certain circumstances. The safe handling of such gases and vapours therefore calls for the adoption of suitable measures to minimise the risk of ignition of such potentially hazardous atmospheres.
Liquified petroleum gas (LPG) is one of these flammable gases and needs to be handled with due care to ensure safety of plant and personnel. Industry professionals should therefore gain a degree of familiarity with the basic concepts of hazardous area protection to be able to better address the challenges the safe handling of this gas presents.
Basic concepts
So what is a hazardous area? A hazardous area can be said to be a location in which the probability of the formation of a potentially explosive atmosphere due to operational or other conditions exists. Potentially explosive atmospheres are mixtures of air and flammable gases, dust under atmospheric conditions that allow combustion to propagate after ignition has been initiated. It may be pertinent at this point to mention that all hazardous areas do not present the same level of ignition risk. A system classification on the basis of the risk of ignition is therefore in order.
Hazardous area classification
Hazardous areas are classified on the basis of the probability of the occurrence of a potentially explosive atmosphere and how long such an atmosphere can be expected to last. Accordingly there are three zones, commonly associated with industrial processes into which hazardous areas can be classified. Hazardous areas are also called Classified areas.
Zone 0 – Zone 0 includes areas, in which an explosive gas atmosphere is expected to be present continuously or over long periods. These areas are typically the insides of tanks, reaction vessels, and equipment like vapourisers etc.
Zone 1- Zone 1 covers areas in which an explosive gas atmosphere is expected to be present occasionally under normal operation, such as areas surrounding Zone 0 areas, areas surrounding drains, discharge equipment etc.
Zone 2 – Zone 2 covers areas, in which an explosive gas atmosphere is not expected to occur normally and in the event of its occurrence lasts only for a short duration. Typical Zone 2 areas are areas surrounding flanged gaskets, areas surrounding Zone 0 or Zone 1.
It needs to be clarified, however that for combustion to take place two other elements – the presence of oxygen/ air and a source of energy are required. Given these three elements, the so called combustion triangle, combustion can take place. However, this requires the presence of the air gas mix in quantities between the Upper and Lower Explosive Limits of the gas and the energy source supplies and simultaneously sufficient energy to raise a small portion of the mixture to ignition temperature which can then trigger a self sustaining explosion.
| Gas | LEL % Volume | UEL % Volume | Ignition Temp | Group |
| Ethylene | 2.7 | 36 | 450 C | IIB |
| Ethane | 3 | 12.5 | 472 C | IIA |
| Acetaldehyde | 4 | 60 | 175 C | IIB |
| Hydrogen | 4 | 75 | 520 C | IIC |
Lower & Upper Explosive Limits
Not all flammable gas/ air mixtures will burn or explode. The concentration of the gas determines the flammability of the mixture, which is different for different gases. The lower limit of this concentration called the Lower Explosive Limit (LEL) is the lowest concentration of the gas in air that can cause an explosion, while the Upper Explosive Limit is the maximum concentration of the gas in air beyond which no explosion can take place.
Some common gases and their flammability limits are tabulated below:
What are the common sources of energy that can cause ignition?
The following sources can provide the energy that can cause an explosion:
a) Flames
b) Sparks
c) Hot surfaces
Flames
Flame temperatures range between 1000 C to 2000 C and are the most potent source of energy as they can supply energy at a high rate making them very effective sources of ignition.
Sparks
Sparks are produced during the normal operation of many electrical equipment such as push buttons, contactors, etc that can supply enough energy to the surrounding explosive atmosphere to raise the gases in the immediate vicinity to their ignition temperature, which can then cause a self-propagating flame through the whole volume of the gas/air mixture.
Hot Surfaces
Electrical equipment also can supply the energy required for ignition from the temperature that their surfaces can reach during normal operation or abnormal operation. When these temperatures exceed the ignition temperatures of the surrounding flammable gases an explosion can occur. Other electrical components of the system can also become potential sources of ignition in the event of insulation failures e.g. in transformers, impedance coils, solenoids and in the basic wiring system.
The normal/faulty operation of electrical equipment, electrical systems etc can be said to constitute a potential hazard in classified areas and special measures have to be therefore adopted to ensure the safe running of electrical system, equipment in such areas.
LPG safety challenges
Liquefied Petroleum Gas (LPG) is basically commercial butane, commercial propane or a mixture of both and is supplied in pressurised cylinders. LPG cylinders need to be marked with design pressures and temperatures and other information as specified under relevant standards and users must ensure that they have all necessary information to operate the LPG cylinders, systems etc safely.
Under normal temperatures and pressures LPG is a gas but is supplied as a liquid under pressure. When the pressure is reduced for example with the opening of the control valve, the liquid starts to boil and gas is release. One litre of the liquid provides as much as 250 litres of gas. LPG is a colourless liquid and has a weight roughly half the same volume of water, however propane and butane vapours are heavier than air. Propane is at least 1 -1/2 times heavier than air and butane is twice as heavy. Propane therefore tends to flow along the ground, and can reach a considerable distance. It can also collect in cellars, drains, excavations and other low-lying places.
The vapour can remain for some time when the air is relatively still, but when ignition occurs at a remote point the resulting flame can reach back to the source of the leak.Small quantities of LPG can lead to large volumes of hazardous gas/air mixtures given that 2 per cent of the vapour in air can form a flammable mixture. An ignition of the mixture in a confined space can cause an explosion.
Storage
LPG cylinders in industrial installations should be at ground level, with the storage area should be readily accessible to allow prompt removal of cylinders in case of emergency. Storage areas should preferably be located in the open air and fenced off by means of a protective fence at least 2 metres high. Such storage spaces should have exits via gates that open outwards and that are not self-locking.
The cylinders should have an arrangement that can provide shelter against exposure to extremes of weather. The storage area floor should be paved or compacted level, with suitable hard standing for the delivery and dispatch of cylinders. The storage area should be located away from any flammable material and it should be cleared of all weeds, rubbish etc.
These should also be away from drains or cellars and should have permanent notices, prominently displayed, pointing out the hazardous nature of the stored material. Further, the area should also have notices prohibiting smoking and naked lights.
Handling
The risk of accidental leakage of gas can be minimised and consequently the major hazard- fire through the adoption of good handling, loading and unloading practices. Damaged or leaking valves can be a source of danger, while a base ring out of shape cannot be hold a cylinder in an upright position.
Transportation
During transportation, cylinders, full or empty should be in upright position, with valves at the top and proper means should be adopted to prevent movement. Cylinders should have proper ventilation and an open vehicle should be used where possible. Suitable fire fighting equipment should be ready at hand and the vehicle driver should be trained in matters such as emergency procedures.
Leak detection
Regular inspection should be carried out on cylinders, valves, connections, piping and hoses. Leaks can be detected by
- Characteristic smell
- Sound of escaping gas
- Condensation around the leak spot
- Bubbles from the suspected leak spot after bushing it with soapy water
In the event it is not possible to stop leakage, the cylinder should, immediately be removed to an open space. The space should not have people or buildings in the vicinity, and it should also not have any drains, culverts or hollows. Further, these should be free from any potential source of ignition in the space. The cylinder should be placed with the leak uppermost, marked with warning notices and the supplier notified urgently.
Precautions against fire risk
The storage area should be provided with a sufficient number of fire extinguishers, preferably of the dry powder type, which should be placed at easily accessible locations in the storage area and properly maintained. Automatic alarm systems if installed at the storage area should be properly maintained and regularly checked.
Emergency procedures
In the event of an emergency the priority should be to keep people and personnel from harm, and the following actions should be taken:
- Call the fire brigade, and police if required. Emergency services personnel should be given the details and location of all LPG stocks.
- Apart from those personnel would be actually involved in emergency operations other persons should be evacuated from the danger area. The emergency services personnel should be informed about any persons trapped.