Do Ex d barrier glands prevent flame propagation or gas migration?

Yes, Ex d barrier glands have the benefit of preventing flame propagation and gas migration down the cable.

Flame Propagation: The most common use of an Ex d barrier gland is to prevent flame propagation. This occurs when an explosion takes place in an Ex d enclosure, the high heat and pressure can allow flames to propagate down the interstices or gaps in the cables. Dangerous flame propagation down the cores of a cable can be reduced by ensuring good compact cable construction, with extruded bedding and fillers that are non-hygroscopic. In addition to good cable construction, cable length can also play a role, increased lengths of cable can help to minimize the potential for flame transmission through the cable. If there is any doubt as to the compactness or length of the cable to prevent flame propagation, a barrier gland or other suitably rated sealing device must be used.

Gas Migration: One problem that exists in hazardous areas is the potential for a cable to become a transmission path for gases and vapour. Gas and vapour can migrate through the interstices between individual cores of a cable. This can be a big problem if the cable leads to a non-hazardous area or between different zones.

One of the keys ways to reduce the likelihood of gas migrating down cables is to examine the cable construction. Cables that exhibit the following features have a higher likelihood of gas, vapour or liquid migration.

1. Cables with construction that is not substantially compact,
2. Cables with wrapped sheaths and bedding,
3. Cables with non-filled interstices,
4. Cables with fillers that are hygroscopic fillers (e.g. fibre fillers). Hygroscopicity refers the tendency of a material to absorb moisture.

Note: Any bedding or cable sheath should be extruded. Meaning the wire is run through a die with the plastic tube hot-extruded around the wire as a cylinder

The next element that needs to be looked at it is the enclosure the cable is entering, and the potential pressure that can exist within the enclosure. Increased internal enclosure pressures can lead to a pressure differential existing at both ends of the cable which will increase the likelihood of gas being forced down the cable.

Enclosures can have an internal pressure created by a process referred to as ‘pumping action’. This occurs when an enclosure is subjected to changes in temperature giving rise to an expansion and contraction of the gases inside the enclosures as the temperature changes. It should be noted that this effect can be significantly reduced by fitting a suitably certified breather to the enclosure.

The other way an enclosure or transmitter can become pressurised is a failure of the seals that separate the transmitter from the process gas. Good equipment selection and design should ensure that at risk equipment is installed and selected so that process fluid containment is reliably separated from the electrical equipment (e.g. by use of a primary seal for the main process interface and a secondary seal internal to the equipment in case of primary seal failure). If this is not done, the equipment should be vented (via a suitably explosion protected vent, drain or breather) and/or the wiring system shall be sealed to prevent the transmission of any fluid.

Taking into consideration the risk factors listed above the use of as special sealing joint, or a cable gland incorporating a seal around the individual conductors (Ex d Barrier gland) may be required to reduce gas migration. It is important to note that a barrier gland may not prevent all gas migration. The application of a barrier gland may only mitigate the rate of vapour transmission and additional attenuation measures may be necessary. The barrier gland does not always provide an effective seal because most modern barrier glands are fitted with a metal setting compound chamber. There can be a gap around this compound chamber and the main body of the gland that can provide a path for gas flow. In certain circumstance a flameproof conduit seal may be required to provide a more effective seal.

 

“This following information represents and opinion only of a general nature, the specifics of each individual situation must be taken into account with reference to the relevant Legislation, Codes of practice and Australian standards. Professional advice should be sort if there is any doubt.”

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