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Alan Wallace of Inlec Engineering in Perth shares his understanding
of this much abused method of maintaining weatherproofing on Exd equipment.
We are frequently asked by clients if they can add layers of grease bearing textile tapes such as Denso around the edge of flamepaths to improve weather protection. Reading the European and Australian Standards we find that the current state of play regarding Denso tape on Exd equipment is as follows:
Testing was actioned years ago in England by the UK Safety in Mines Research Establishment with one layer of Denso tape around the gap of a flamepath of test apparatus with a 25mm (1") overlap. It was found that for IIA gas the affect on the MESG (Maximum Experiment Safe Gap) was minimal; for IIB it reduced the required gap by about 50%; and for IIC by about 80%. This reduction in required gap for IIB and IIC gases caused by the tape means that the flameproof equipment we buy and safely use without tape could be unsafe if used with tape because the equipments flamepath gap may exceed the smaller gap required due to the tape. The user can NOT compensate for this as the gap range is fixed by the manufacturers certified design documents.
When the products of combustion escape they are cooled by three mechanisms:
It is the third mechanism that is effected by the tape (this is also why you are not allowed to put solid objects within 40mm of a flamepath). For IIC the escaping
gas is very hot. Based on the test results it obviously relies heavily on the entrainment factor to cool the gas quickly before it can act as an ignition source to the outside atmosphere.STANDARDS My understanding is that the old BS stated that Denso or equivalent tape could be used if the gas was classified as group IIA, but only with IIB if the manufacturer was consulted. What normally happened for IIB was that if the manufacturers max gap per the certified drawings was less than 50% of that permitted by the design standards the use of tape was OK. Tape was not allowed on IIC due to the significant gap reduction required (80%) on what is already a very small gap compared to IIA/B.
AS2380.2 has a similar clause, but unfortunately is poorly written and in my opinion is of limited benefit to users. The AS allows tape on IIA EQUIPMENT only. There is no mention of number of wraps. If you follow the AS you'll never use tape as all Ex d equipment is either IIB or IIC certified regardless of the gas it is used with. I have never seen equipment certified only as IIA.
My understanding of the one wrap requirement is that the tape will jettison upon internal explosion, whereas two or more wraps might not. Another possible reason is the empirical data used to justify the use of tape used only one layer with a 1" overlap so the use of two or more layers is not proven.CONCLUSION One wrap on Denso tape with a small overlap is a suitable method for additional weather protection for SOME Ex d equipment. If the Australian Standards are followed, Denso can only be used on IIA equipment. This precludes Denso with IIB and IIC certified equipment. Based on the history of the testing and European Standard, it is my opinion that it is the gas group of the gas in the area that is important, not the gas group that the equipment is certified to. I personally would be comfortable using one layer of Denso with a small overlap on IIA, IIB, or IIC EQUIPMENT, provided that the hazardous area was only IIA. However, each user must ensure that he complies with any Statutory requirements that may apply.
Perth Western Australia