Southern African Natural Gas Advisory Service Service

Natural gas is also supplied at low pressure to most premises reducing the risk of explosion and fire even further. There is also a shut off valve installed next to the gas meter enabling instant isolation of a property from the further flow of gas should an internal leak be detected. Once a leak is detected by shutting off the inlet valve and opening the windows the gas will quickly dissipate and remedial work can be quickly carried out to make the situation safe again.

Another historical safety concern with natural gas has been associated with certain older gas heating and hot water appliances. Although they are now illegal in many countries, open flue instantaneous gas fired water heaters that use air from the room for combustion in bathrooms can cause carbon monoxide deaths. In winter months in unheated bathrooms users may block air inlets into the room to keep the bathroom warm when bathing. Without proper air available for efficient combustion of the natural gas or proper ventilation of the bathroom, the water heating appliance emits carbon monoxide which is poisonous that is directly vented into the bathroom.

Today, rarely are such appliances in use and they are banned in most countries. These older appliances have now been replaced with what are referred to as ‘room sealed’ or ‘balanced flue’ appliances. Since all types of combustion require oxygen it is essential that natural gas burning appliances operate with an adequate supply of air. Combustion also produces what are referred to as flue gases or flue emissions that must be vented to outside atmosphere. A diagram of a modern concentric room sealed flue is shown below. They enable air for combustion to be drawn in through the outer tube while the combustion flue gases are discharged to the outside of the dwelling through the inner tube. Such flues are installed through an outside wall with the discharge vane on the exterior face of the wall.  

Safety concerns

Extraction and supply of Natural Gas

Natural gas is a hydrocarbon, a fuel and an inflammable substance.  In view of this and the manner in which it is produced, transported and used there are a number of safety concerns that need to be understood and addressed. Although the safety risks of natural gas are lower than for other fossil fuels and it has an excellent comparative safety record there are certain aspects of this hydrocarbon that need proper regulation and safe practices.     

In terms of the production of natural gas, in some cases when sour gas is extracted from underground reservoirs it contains high levels of hydrogen sulphide (H2S) and this untreated gas is toxic and must be treated accordingly. Extraction of natural gas and oil from underground reservoirs reduces the pressure in these cavities that can lead to subsidence and the sinking of surface ground.  Similar subsidence may arise with underground coal and other mineral mining operations.

When deploying hydraulic fracturing techniques to release natural gas in ‘tight’ shale beds and other geological formations the three major safety issues are protection of groundwater from contamination, dealing with the fracturing fluid and residues on the surface; and, the small risk of seismic tremors induced by the underground drilling and hydraulic or pressurized fracturing of the low porous underground rock formations. Hydraulic fracturing or ‘ hydro-fracking ’ or simply

‘ fracking ‘ as it is often referred to, is a relatively new technique for the production of shale gas. As with all extractive techniques used by the oil, gas, coal and other extractive and mining industries, there are associated environmental and safety risks that have to be mitigated with the correct use of efficient technology and vigilant independent inspection and regulation. With hydraulic fracturing being a relatively new technology it has aroused controversy regarding its safety and environmental impacts.

This technique is continuing to be studied and improved operational and regulatory protocols developed that will ensure that hydraulic fracturing will take its place alongside many other safe energy production methods. In comparison with major nuclear safety incidents, crude oil spills on land and at sea from accidents with offshore production platforms and oil tankers, the pollution from oil refineries and coal and oil fired power stations; and, the safety risks and environmentally damaging residues from coal mining, hydraulic fracturing of shale beds to liberate natural gas involves safety and environmental risks on a greatly reduced comparative scale. See the section on Hydraulic Fracturing in the Features of Natural Gas section via the menu bar above.  

To help detect gas leaks a very small amount of odorant, usually mercaptan, is added to natural gas. Another related compound may also sometimes be used called thiophane. Occasionally an odorant added to natural gas can be detected by instrumentation but not by normal human sense of smell. This olfactory difficulty is referred to as ‘odour masking’ when one odorant overpowers the sensation of another and local natural gas supply utilities regularly test for these potential problems taking remedial measures when necessary.

There have however been some accidents involving LNG land based facilities and three are briefly described below:

- In January 2004 at the Sonatrach LNG Liquefaction Plant at Skikda, Algeria there was a major incident when a steam boiler that was a part of liquefaction trains exploded and led to a further big explosion of gas. The explosion occurred where ethane and propane refrigeration storage was located. In the incident 27 workers were killed, 56 injured and three LNG trains destroyed.

- In October 1979 at the Cove Point LNG facility in Lusby, Maryland, U.S. gas vapours were released that entered an electricity conduit that in turn led to an explosion killing one worker and severely injuring another. Following a full investigation local fire codes were improved.  

- In October1944 at the East Ohio Natural Gas Company in Cleveland, U.S. an LNG tank ruptured spilling LNG into the city sewer system that vaporized turning into gas which exploded and burned. 128 people died in the disaster that could have been avoided. The tank had no retaining wall to catch and LNG spill and the metal used for the tank had low nickel content largely due to metal rationing during World War II. When exposed to the extremely low temperatures from the LNG, the tank became brittle and ruptured.   

Natural Gas safety in consumers premises

Accidents also occur infrequently when natural gas is distributed to customers and used in the consumer’s premises. Explosions caused by natural gas leaks occur from time to time that typically involve internal gas leaks within premises beyond the gas meter, regulator and shut off valve. In these unfortunate cases faulty internal gas pipe work, tampering with gas equipment by unqualified people or theft are the common causes. Natural gas builds up inside the structure and can lead to an explosion that causes extensive damage to the property. Occasionally the gas can gather in sufficient quantities, usually in basements or other closed areas, to cause an explosion seriously destroying property and life. Such tragedies usually become newspaper headlines and are frequently sensationalized by the media. It is worth remembering however that fatalities caused by natural gas are significantly lower than from electricity. There are also many millions of premises in many countries in the world that use natural gas so the individual risk of using this fuel is extremely low.

Liquefied Natural Gas Safety

With liquid natural gas (LNG) when in its liquid state it is not inflammable, will not burn or explode. Before LNG will burn it must first vaporize and then mix with air (oxygen) to form a combustible mixture in the right proportions of between 5 to 15% and then be ignited. If an LNG leak occurs it vaporizes rapidly turning into a gas. Providing the gas does not ignite, associated damage to surroundings is usually minimal.  LNG bulk tankers have sailed hundreds of millions of miles without a shipboard fatality or major safety incident.

In recent years in many countries safety has been improved even further with the replacement of gas meters and control valves located within basements and other positions within the premises to a position on the exterior of the property.  Gas safety regulations also exist in countries with a whole range of building regulations covering the safe installation of gas supplies within premises.

Room sealed or balanced flue natural gas appliances use a concentric flue pipe with an inlet pipe and outlet pipe attached to the back of the gas appliance that vents the appliance through an opening in an outside wall of the premises. Room sealed natural gas appliances can be fitted anywhere within a property on an outside wall and there are a wide range of small wall mounted central heating boilers, air conditioning and cooling equipment, water heaters and space heating appliances now available using this extremely safe flue technology.