Feb-2012

Gases: identifying hazards and mitigating risks

Although critical for the operation of refineries and petrochemical plants, industrial gases can be associated with a spectrum of possible hazards and need to be treated with respect, assessing and controlling the relevant risks.

Stephen Harrison, Linde Industrial Gases

Article Summary

From minor injury at one end of the scale, to potentially more serious damage to health and — although rare – even death, at the other end, safe handling of gases is vital. Effective product stewardship is therefore required to uphold safety levels at such facilities — a holistic approach to product responsibility that involves identifying the possible hazards and evaluating the risks that may stem from a product throughout all stages of its lifecycle. The risk potential is assessed in relation to employees, neighbouring facilities and the environment. The aim is then to avoid or reduce these risks as far as possible.

Physical and health hazards
One of the main hazards when handling gases is toxicity. Examples of gases that fall under this hazard category include carbon monoxide or some of the gases that contain sulphur, such as hydrogen sulphide or carbonyl sulphide, which are commonly produced in refinery operations, especially with increased desulphurisation taking place in refineries as oil companies are forced to tap into a lower quality of crude. Any leakage of these gases potentially creates a significant safety issue.

Another leading hazard on the refinery is the flammability of gases — and with flammability comes the more severe implication of explosion. Flammable gases present in a refinery include hydrogen and any of the hydrocarbons being processed in the plant, such as LPG (liquefied petroleum gas), LNG (liquefied natural gas) and any of the volatile petroleum derivatives. These gases generally only pose a flammability hazard when mixed with air or other oxidants, creating a combustible atmosphere that might be ignited by flames, sparks, hot surfaces, electrostatic energy or other ignition sources.

Asphyxiation is another primary hazard in refineries and petrochemical plants. Asphyxiation occurs where there is a lack of oxygen and it only takes two deep breaths in an atmosphere that excludes oxygen – for instance, where air has been displaced by pure nitrogen — to cause an individual to collapse instantly into unconsciousness. If they continue to inhale the nitrogen while unconscious, death will follow very rapidly. Asphyxiation is often referred to as the silent killer, as there is seldom any warning that these inert gases are present in such a large volume. With these inert gases, there are no warning odours and often the absence of visible vapour clouds.

Oxygen deficiency can occur as a result of an excess of nitrogen, which might perhaps be used intentionally on a refinery for the purpose of enhancing safety. For example, to blanket a flammable atmosphere, or as an initial step to purge a storage vessel that previously contained a flammable gas, to allow welders to subsequently work in the vessel after a second purge with breathable air. In these cases of confined space entry, it is imperative to have a robust work permit system in place, to ensure the atmosphere in the confined space is safe before entering. The entry permit procedure should include confirmation of ventilation, gas monitoring equipment, availability of rescue equipment and a safety guard on duty just outside of the confined space.

Conversely, the other safety issue associated with oxygen is if too much of it is present. During oxygen enrichment, if oxygen levels exceed 23%, combustion procedures that are very familiar are altered. For example, in a normal air atmosphere, a welding flame would burn at a controllable rate, but where there is too much oxygen present, flames unexpectedly burn with a much larger flame.

Oxygen enrichment could be caused by a leak of oxygen, which is often used in large quantities on refineries in many applications, including contemporary desulphurisation processes. The potential for a major leakage is small, but it must be considered as a potential hazard.

The next safety issue is corrosive gases, which are generally also toxic, and have the potential to cause rashes, long-term skin damage and also burn respiratory tissue on contact, with potentially fatal consequences. Gases in this category include ammonia and the hydrogen chloride used on plants for descaling piping and process equipment. These scale deposits are sometimes flushed with gases such as hydrogen chloride, which are acidic and capable of efficiently returning process equipment to its original state, thereby improving its performance.

These potentially hazardous gases are intentionally handled in plants for maintenance purposes or produced as by-products and, providing they remain within the system, there is no threat. However, these gases are aggressive – particularly in combination with high moisture content — and have the potential, over time, to dissolve pipework and create holes from which they and other gases can escape.

A niche category of hazards is radiation. Some instrumentation and level control devices harness radioactive sources for optimal function. For example, a radioactive float can be used to measure the liquid levels. The rare gas isotope Krypton 85 might also be present in some refineries. Although radiation is an uncommon hazard on refineries and petrochemical plants, where radioactive materials are being processed, plant personnel should have a good understanding of the associated risks.

Then there is the element of hazard related to pressure. Many cylinder gases are stored under pressures up to 300 bar. This represents high energy and the hazard therefore relates to a sudden release of pressure that could harm personnel and damage assets, especially if the cylinder its self is projected at a high velocity due to the sudden release of the pressure energy.

Providing gas cylinders are used as instructed and stored in well-ventilated areas and at normal temperatures, there is rarely an incident. However, if these cylinders are placed in direct and intense sunlight, they begin to heat up and the gas inside starts to increase in pressure. With most gas products, this does not pose a problem because the cylinder is designed to withstand this pressure build-up, but with liquefied gas such as LPG the contents begin to vapourise when the cylinder is heated and a leak due to over-pressurisation could occur.