Atmosphere Maintenance Tips

Working Safely with Furnace Atmospheres

Guest Article contributed by P. F. Stratton, The BOC Group, writing from the UK

Many furnaces use an atmosphere to protect the work during heat treatment.  Sometimes that atmosphere is flammable and some of the components can be toxic. This article highlights some of the dangers associated with the use of atmospheres in furnaces and explains some of the principles behind the European standard, EN 746 : 1997 Part 3 and the American NFPA 86 standard. However, the article should not be seen as a substitute for thoroughly reading the whole of these documents.

All the component gases used in furnace atmospheres, whether generated from the partial combustion of fuel gases or mixed from the pure constituents, are asphyxiants. Some are toxic and some are flammable as can be seen in Table 1.

Table 1. Heat-treating atmosphere constituent gases

*Liquid at room temperature, but the vapour is an asphyxiant

Asphyxiation risks

All gases except oxygen are asphyxiants. Reducing the oxygen level in the air to 20% should not result in any problems but 18% oxygen is potentially dangerous and 13% oxygen will result in death.  However, the conditions that can cause reduced oxygen levels must be controlled by carrying out risk assessments of the tasks to ensure that the risks are minimised. The dangers can be minimised by ensuring adequate ventilation/extraction, using an oxygen meter before entering any confined spaces, particularly pits, and ventilating and breaking pipework to furnaces before entering.

Burning risks

Risk of burning is always a potential hazard with flammable atmospheres.  The risk can be minimised by ensuring controlled atmosphere burning at exits and using the correct personal protective equipment. The risk can of course be eliminated by using a non-flammable atmosphere.

Explosion risks

All flammable gases can be used safely provided they are stored and handled correctly. However, mixtures with oxygen can explode if conditions are correct.   To minimise the risk, atmosphere gases should be burnt in a controlled manner at the furnace exits and steps taken to ensure uncontrolled leaks are eliminated.  High areas where flammable gas could collect should be well ventilated and uncontrolled sources of ignition eliminated.

To ensure that a flammable mixture cannot form during atmosphere introduction or removal it is safest to purge with an inert gas.  The flow of gas must be maintained so that air cannot leak into the furnace and an automatic inert gas purge should be initiated should it do so. Again the best solution is to use a non-flammable atmosphere if possible.

Toxicity risks

Table 1 shows that many of the component gases used in furnace atmospheres are toxic. While some, such as carbon dioxide, are less toxic than carbon monoxide or ammonia there is still a risk.  That risk should be minimised by ensuring that the atmosphere gases are burnt and/or vented from the furnace in a controlled way to a safe location. Uncontrolled leaks should be eliminated.  The risk can be eliminated only by using an atmosphere with no toxic constituents. such as nitrogen/hydrogen.

The following is a summary of conclusions and best practices from these universal safety standards:

• All atmospheres containing a flammable component are potentially flammable.
• When the furnace temperature is above 760^oC (1400^oF), a flammable atmosphere gas may be introduced or removed, either by the "burn in/burn out" method or the "purge in/purge out" method using an inert gas.
• Indeterminate atmospheres need to be treated as flammable atmospheres with the following consideration: where one special atmosphere is replaced with an atmosphere (e.g., flammable with non-flammable) that can cause the atmosphere to become indeterminate at some stage, burn in or burn out procedures should not be used. Inert gas purge procedures alone need to be used for introduction and removal of special processing indeterminate atmosphere.
• When the furnace temperature is below 760^oC (1400^oF), a flammable atmosphere gas must be introduced only by the "purge in/purge out" method using an inert gas.
• Flammable gases must not be introduced into a furnace until a source of ignition is present at all exits.  The atmosphere flow rate must be sufficient to maintain a stable burn-off under all conditions.
• Sufficient flow of atmosphere gas must be ensured at all times to maintain positive furnace pressure and prevent air getting in.  There must be visual indication of the atmosphere flow rate, and an alarm to signal abnormal flow conditions.
• A sufficient inert gas purge, when required, must be available automatically should any potentially dangerous situation occur.  The flow of this purge must be monitored in the same way as the atmosphere gas flow. A manually initiated purge flow must also be available.
• The system design systems must ensure that sufficient purge gas is always available from either liquid nitrogen or nitrogen generators.
• The automatic safety purge must be initiated in the event of power failure, loss of carrier gas flow, or if the furnace temperature falls below 760^oC (1400^oF). In addition, safety shut-off valves must be provided such that all addition gases and flammable carrier gases are cut off when the purge starts and require manual intervention to re-establish their flow when all safety conditions have been satisfied.
• Any potentially dangerous situation should result in an audible and visual alarm.
• Easily accessible isolation valves must be provided at a safe distance from the furnace.
• Flammable liquids must not be introduced into furnace zones where the temperature is less than 760^oC (1400^oF)
• Pipelines need to be constructed to a standard suitable for the gas or liquid to be carried.

Further Reading

A more comprehensive introduction to the subject of atmosphere safety can be found on BOC’s specialist heat treating web site at www.catweb.boc.com together with a collection of other safety documents.

For further information please contact Tony Palermo (tony.palermo@boc.com or 908-771-1215).  BOC Gases is one of the world's leading suppliers of industrial gases and related application technologies and services for industry and commercial markets.

The latest design, materials and equipment specifications should be obtained from the company before any reliance is placed on the enclosed since changes may occur due to product improvement.