Gas explosion question

Hi For all those who read this question, you might be interested in the answer I found.


An Ex e enclosure is used in Zones 1 and 2 where explosive gas is unlikeley.  In this case a gas leak had occured, which made the explosive mixture permanent and when an electrical fault occured it ignited.  I inspected a similar enclosure and found the same gas, so shut this one down.  I will now make a regular inspection with a gas detector for all similar installations.


Note that for a location where gas is likeley and a source of ignition is present and Ex d enclosure should be used.


Stephen

My next logical question is, can anyone recommend a good hand held gas detector?

Hi Stephen,

I will answer your second question first. There are a number of good makers of hand held gas detectors. There are probably some less good ones but I have only had dealings with the better ones (i.e. the ones who are after EU Directive certification for their equipment). The best way to judge is probably to check what certification the detectors have. An alternative would be to find out which detectors are used by the gas board or their successors (I am assuming that it is methane and not some more abstruse gas leakage you are looking at, though the gas detector manufacturers can cope with any gas you may encounter as may be necessary).

The other issue is the Ex e enclosure. The comment that "where gas is likely and a source of ignition is present and Ex d enclosure should be used" is, strictly speaking, correct but is misleading. Zone 0 is where there is gas present 'permanently of for long periods'; Zone 1 is where gas may be present under normal operating conditions (but not expected as a rule), and Zone 2 is where gas will only be present exceptionally under abnormal conditions for short periods. The durations normally quoted to quantify this are: Zone 2 < 10 hours per year and Zone 1 < 1000 hours per year (and by elimination Zone 0 > 1000 hours per year).

Both Ex e and Ex d are suitable for both Zone 2 and Zone 1, but neither are suitable for Zone 0. The Ex e protection eliminates by design the source of ignition. Therefore if there is a source of ignition present which can't be eliminated, Ex e is not an option and Ex d is the one to use, hence the comment "where gas is likely and a source of ignition is present and Ex d enclosure should be used".

To go back to your original question ("Am I right in thinking that there should not normally be an explosive mixture in an Ex e box ie this is a rare event?  If so is the presence of gas in itself a fault to be rectified?") I would say that an explosive mixture in an Ex e box is a distinct possibility (1000 hours per year works out at about 40 days, so about 10% of the time) but the fact that it has been ignited indicates something went wrong. I can think of four options, though there may be more.

1. A manufacturing fault.

2. An installation fault.

3. A maintenance fault.

4. A separate but adjacent ignition source so that the Ex e enclosure was not itself at fault but just appears to have been.

There is a possibility of an 'operational' fault as Ex e equipment needs care in the setting of overload protection to prevent a fault on the circuit causing excessive temperatures leading to ignition but I would include that under 2 and 3 above.

The biggest problem with Ex equipment is badly informed intervention. If you have any doubts about your own knowledge, please look for help.

"A little learning is a dangerous thing", Alexander Pope, An essay on Criticism : 1709


Alasdair Anderson

Thanks that's very helpful.  I have found that there was a gas leak into the box, this is under repair now.  As for the source of ignition, it's hard to tell, because there's not much left after the explosion.  I did find a loose connection to a repair I had just done, so I'm guessin that's a possible source of a spark.  It's very concerning that this leak had been going for about six months undetected.  This shows the importance of routing maintanance.


Stephen

Stephen,

Agreed. I think the loose connection is a definite suspect. It could result in a spark, or alternatively could give rise to elevated temperatures causing ignition. If there had been a leak going on for six months but you had just done maintenance it is very suggestive.

Alasdair

I always thought that comp-ex equipment installed where infamable/explosive gases were present should not be hemetically sealed but allow gases to enter the accessory/switch and then contain any explosions releasing depressureized cooled exhaust gases caused by sparking of switching contacts.


Legh

Legh,

What you have just described is an Ex d enclosure where the enclosure can 'breathe' through the flame path gap. If there is ignition the flame passes and cools through the flame path such that when it reaches the outside atmosphere the temperature is insufficient to ignite the surrounding vapour/gas. If I had an Ex p enclosure (pressurised) I wouldn't expect it to allow gases to enter the enclosure, nor with Ex m (encapsulated) or Ex o (oil filled) protection arrangements.

Alasdair