One of the protection concepts to allow electrical equipment to be used in explosive atmospheres is often referred to as “flameproof” or “ex d”. The pressures generated by hot gasses within a flameproof enclosure during an explosion are vented safely into the surrounding atmosphere though the flamepath. But what is a flamepath and where is it used?
A flamepath is a small gap of defined width formed between the flange of an ex d enclosure and its lid. If a flammable gas mixture enters the enclosure and is subsequently ignited, then the pressure inside the enclosure rises rapidly. The flamepath cools the burning gas mixture, preventing it from leaving the enclosure and igniting any gas mixture surrounding the enclosure.
What are the requirements?
Variables that may affect a flamepath’s operational performance include its width and gap size, the gas type, as well as enclosure volume. The roughness of the flamepath surface can also influence how well it works, having a maximum value of 6.3µm allowed in IEC 60079-1.
- The dimensions required for a flamepath differ depending on the gas mixture being used. Gas types are sub-divided into Group I (least flammable) IIA, IIB and then IIC (the most flammable).
- The size of the flamepath gap must be smaller as the enclosure gets larger. One reason is that in a small enclosure, the length of the flamepath is a higher percentage of the volume of the enclosure compared to a larger enclosure. If the enclosure has a larger volume, there is more gas mixture to ignite and expand before it reaches the flamepath, which could mean a higher pressure is exerted.
- Surface roughness can be critical. When measuring the gap between two surfaces, what is measured is the gap between the peaks of the surface, which is never exactly smooth when considered on a microscopic level. Surface roughness is defined as the distance between the peaks and valleys of the surface, so the real maximum gap between two surfaces can be defined as the “valley to valley” distance. The actual flameproof gap is as much as the measured gap plus 2x the surface roughness. For this reason, the maximum surface roughness of the flamepath has been defined so that the overall effective flamepath gap is controlled.
Flamepath maintenance
The flamepath must operate correctly over the intended life of the product. To ensure this happens, the flamepath must be protected and periodically inspected. The inspection is both a visual assessment for mechanical damage or corrosion, and a measurement check to make sure that the gap has remained correct. Traditional cast iron enclosures are susceptible to corrosion, which increases the surface roughness and flamepath gap, which in turn render the product unsafe.
Leading manufacturer and distributor CCG has developed a specialised metallic corrosion-resistant treatment during the manufacture of its ex d enclosures. Further protection can be provided using a non-setting grease. Some manufacturers in the industry have tried to overcome corrosion by using non-metallic materials for the manufacture of their ex d junction boxes, but non-metallic polymers are softer than metallic boxes, so flamepath surface is at a higher risk of mechanical damage.
When selecting flameproof enclosures, it is important to know and understand any limitations or restrictions that may apply to the ex d housing. While CCG’s flameproof products have no such limitations, some non-metallic flameproof enclosures do not permit the storing or fitting of energy-storing electronics, cells, batteries or switchgear components to the enclosure, which ironically is the reason most ex d enclosures are bought and installed by companies.
When there is the slightest chance of sparks in an environment where flammable matter is present, an ounce of prevention is better than a pound of cure, so make sure that the flamepath is safe and that the flameproof enclosure is going to maintain its integrity no matter what is thrown in its path.
Enquiries: www.ccgcablegland.co.za
