Sparking risks in explosive gas atmospheres

Introduction

Sparks caused by metal to metal contact can be an fire or explosion hazard in areas such as mines or process plant where flammable gasses or vapours are present. Some metal contact may be inevitable in equipment such as lift cages, safety harnesses, door closures, ratchets and gears, forklift truck forks etc.
Stainless steels are sometimes considered as alternatives to harder carbon steels.

Sparking tendency of stainless steels

The sparking tendency of austenitic 304 (1.4301) type was investigated by the Health and Safety Laboratory of the Health and Safety Executive at Buxton, UK, in 1995.
They compared 304 (1.4301) to mild steel in potentially explosive atmospheres containing hydrogen, ethylene, propane and methane using a friction 'rubbing' apparatus. A variety of wheel speeds and loads were used.
It was concluded that there was little difference between the steel types in terms of their tendency to ignite these gases.
This work shows that this commonly used 'soft' austenitic stainless steel type cannot be regarded as 'spark free'. These steels should not be used where there could be a risk of ignition to explosive gases or gas mixtures from frictional contact.

This work was restricted to the 304 (1.4301) type. Whilst it may be reasonable to assume that other austenitic types would react similarly if tested in the same way, it would be unwise to assume that the harder martensitic or precipitation hardening types would have the same sparking characteristics. Even more care should be taking where martensitic or precipitation hardening types are considered for applications in explosive gas mixtures.
It is difficult to predict how the duplex types would compare but they may be more like the austenitics than the martensitics, considering their hardnesses.

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