Coping with risk has always been part of the challenge of human existence through the ages and the level of fear provoked by any specific threat is usually exaggerated by lack of knowledge.
As recently as 2009 the threat of a ‘swine flu’ epidemic resulted in the widespread disruption of international travel until a proper understanding of the virus, the severity of infection it could cause, and its ability to spread, quelled most of the fear.
Even relatively well informed post-modern society is seen to react with fearful vengeance, when any event whether caused through accident, negligence, terrorism, vengeance or simple natural disaster, cannot be attributed to a known cause.
Industrial gases are a class of hazard that is perceived as a threat to people today and a brief recount of past events quickly reminds us why.
Unseen monsters that cause havoc
Gas has been witnessed to strike with dreadful effect in many different ways depending on its condition or character: sometimes it is a silent intruder that smothers its victims by choking out the source of life-sustaining oxygen.
Sometimes it releases stored energy when its contained pressure escapes from a ruptured pipe or container with uncontrolled fury. Sometimes gas molecules are toxic to their victims flesh and bones, burning the surfaces of vulnerable tissue like eyes, nasal passages, and lungs with searing agony.
Many gases are flammable and if a spill or leak should be ignited by a random spark or heat, it can quickly spread to fill spaces, tunnels, buildings, streets or even the surface of the ocean with vast flowing rivers of fire.
Other gases are stored in the form of super-cold liquid and human contact with such cryogens can freeze limbs and extremities into brittle lifelessness. Gases are used in many industrial processes where they operate under massive pressure and temperature and the potential damage from a gas as benign as water vapour can be extreme, if superheated steam escapes in an uncontrolled event.
Gases are also chemicals and if allowed to mix together, certain combinations of gas are violently explosive and powerful enough to burst through any valves or other containment devices.
Why is it that well managed industrial commodities like oxygen, nitrogen and natural gas carry the potential to strike terror?
Sure there have been nasty incidents in the past that in hindsight were preventable, but why does that little three letter word ‘gas’ wield such power?
An examination of how the properties of gases can result in unsafe conditions will perhaps be instructive and lead to an answer.
Classification removes the mystery
Many gases are colourless and odourless despite their other qualities, so unlike forces like fire, volcano or wild beast they are invisible to man – requiring detectors or analysers to determine their presence and concentration.
In confined spaces, underground mines, tunnels, tanks and other industrial vessels, gases that cannot support life may accumulate and suffocate workers that enter unaware of the risk.
A common characteristic of gaseous substances is a relatively high level of molecular energy that is absorbed during the process of vaporisation. This means that gases are not as easily contained as solids or liquids, because their energy state give them the ability to expand and fill any space available, or even to diffuse through many packaging materials.
Storage of gases to facilitate their accumulation, transport or use, always requires a suitable containment and distribution system.
Gases exert pressure in all directions against the boundaries of any container and this pressure rises in proportion to the temperature. The application of heat to any container of gas therefore results in increased pressure that can damage or rupture the container if the pressure exceeds safe limits. It is required that most gas containment systems are protected by pressure relief devices, to allow gas to vent out and therefore prevent critical failure.
Gases are commonly compressed to very high pressure into cylinders forged from high strength steel, as a practical means of storage and transport. Modern industrial gas cylinders are commonly rated to safely contain gases at pressure of 200 bar and a migration to 300 bar specification is in progress in many countries.
Typically the majority of the world’s cylinders only carry only 15% to 20% of their mass of common industrial products like oxygen, nitrogen and welding gases and in the case of hydrogen, even the latest generation as little as 1,5% by mass of hydrogen at 300 bar.
High pressure gas cylinders are designed to offer the highest practical strength-to-mass ratio, but with mass exceeding 60kg, a typical height of 1.5 metres and diameter of only about 24cm, they are notorious for falling over unless secured in the upright position.
Millions of these containers, each with a full mass of 60kg to 100kg are loaded, delivered, collected, offloaded, rolled about, re-filled and stored each day, representing a major risk for personal injury to the service personnel. Portable liquid gas containers and multiple cylinder packs are growing in popularity because of convenience in use, but unless provided with appropriate handling equipment, can increase the opportunity for injury.
Many gases useful in industrial processes like propane, ammonia and carbon dioxide, have the property of returning to the liquid state under relatively little pressure and therefore can be stored as liquids at ambient temperature in low strength containers. Others like oxygen, nitrogen and methane require energy intensive compression and cooling cycles to reach the liquid state that can be maintained under super insulation at temperatures approaching absolute zero.
Any spill or leak of liquefied gas will boil under ambient conditions, while rapidly expanding in volume by up to 800 times.
Possibly the most widely used of all industrial gases, oxygen is an oxidant meaning that it readily reacts chemically with many other substances in an exothermic reaction. Pure oxygen is used under control to accelerate heating and combustion processes, but accidental contact can cause everyday materials like packaging, clothing, grease, and oil to ignite spontaneously and burn ferociously.
The potentially disastrous consequences of introducing the incorrect gas into any industrial process, machine or plant coupled with the fact that gases are generally not visible, requires meticulous effort to label and colour code containers, tanks and pipelines to prevent wrong identification.
Very often the workers handling supplies of gases and plant operators who connect gas cylinders and open control valves do not possess detailed understanding of the thermodynamics or chemistry of the substances they work with, but rely on procedures and standards to prevent disasters from occurring.
Another category of gases that provoke fear with good reason are those that are chemically toxic. Chlorine has been widely used for disinfection and water purification and is highly toxic, while nitrogen and carbon dioxide, both constituents of air, are also toxic when breathed in sufficient concentration.
Gas blending laboratories and hospitals are the prime examples of situations where correct gas identification is a life-and-death issue. The inadvertent mixing of explosive combinations of gas inside a high pressure cylinder can create a bomb with lethally destructive power, even if the quantities present are tiny.
Some gases like hydrogen chloride, while contained in the appropriate cylinder, are stable and can be safely transported and used in a correctly designed plant. If contact with the air or any other source of moisture is allowed, then hydrolysis forms a highly corrosive gas that quite apart from presenting a health hazard, attacks piping, valves, pressure regulators and other equipment – destroying the system’s integrity.
Fuel gases like acetylene and propane are commonly distributed for industrial use where they are combusted using cutting and heating equipment, either with air or with pure oxygen. Safety incidents with these products are rare because safe procedures have been developed over long experience, are well understood and widely practised.
If cylinders of these flammable gases are subjected to shock from a vehicle accident or exposed to fire, then the risk of explosion is greater due to extreme pressure developed of chemical decomposition.
Gases are often soluble in liquids and solids and this leads to another class of hazard. When air is breathed under elevated pressure, such as while diving, the nitrogen component dissolves into the bloodstream and rapid decompression can be fatal. During welding processes hydrogen gas dissolves readily into the heated metal and can cause cracking and brittle failure of structures later.
The industrial gas business has learned to cope with all these risks associated with gases and many more, and it is to their very great credit that the global rate of disabling accident or injury has declined steadily, while the volumes of hazardous gases handled escalates by around 5% each year.
Thousands of people are physically engaged every day around the world in the labour intensive business of manufacturing and supplying multiple grades or blends of gas to industrial operations that depend on these gases for vital processing of their wide diversity of products.