Is velcro use acceptable in explosive environments? 3

[zappedagain]

A colleague wants to use Velcro to attach cables to a robot arm in a paint booth that is classified as Class I Division 1. The only threads I see about this are from NASA in the 1970s. I see some people make anti-static Velcro straps for ESD controlled environments. I'm sure they work, but are they needed?
Thanks,

Z

 

[ScottyUK]

If velcro provides sufficient accuracy of location for the sensor then a magnetic mount should be equally or more capable given a little thought. The neodymium magnets which have become commonplace in recent years can produce surprisingly large forces from a small magnet.

 

[zappedagain]

Scotty - the sensor is bolted in place. The Velcro is only to support the cabling along the exterior of the robot arm.

I like the magnetic cable mount idea. As long as the robot arm isn't aluminum! This may take this thread off topic, but does anyone know why aluminum is acceptable in a Class I Div 1 environment in the USA (i.e. UL regs?) but not in the EU (i.e. IEC regs)? There must be some history there why the two standards diverged.

Z

 

[ScottyUK]

Aluminium isn't prohibited for ATEX Zone 1 applications which is fairly close to Class 1 Div 1 in the US. There are plenty of cast aluminium Ex 'd' enclosures from European manufacturers alongside similar designs in cast iron or in cast stainless steel for those with very deep pockets. With that said, I don't personally like it as an enclosure material because of the endearing habit of stainless steel screws to cold-weld into the enclosure body, but that applies to aluminium in both hazardous and non-hazardous applications. Some folks don't like aluminium because of a perceived / theoretical risk of rust and aluminium producing thermite although I don't know of any instance where this has actually occured outside of laboratory conditions (anyone?).

 

[Skogsgurra]

Beat the horse some more?
I was involved in a discussion about EDM in bearings some time ago. This was in a refinery and everyone was thinking Ex with different numbers and letters attached. So it wasn't easy to focus on what was going on in the innards of the bearings and I nearly left the room when one of the more influential guys said that the motor's paint seemed to be too thick.

It took some time to get him off that track.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Influential = loud voice, not large brain?

How did you avoid laughing?

 

[Skogsgurra]

I did. I even told him not to be ridiculous.

He was very confused and told me that thick paint could get charged when brushed against. Probably true.

The problem was the ground grid. It hadn't been checked since it was buried in 1973. Sulphuric acid, salt water.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zappedagain]

Scotty, I'm referring to IEC 60079-0 Clause 8.1.2 for Group II (explosive gas) for EPL Ga - aluminum content in the enclosure needs to be less than 10%. UL s60079-0 copies the same text, but both my Intrinsic Safety expert and my UL contact dismissed Aluminum enclosures as acceptable. I couldn't get details from either of them why this Clause is not applicable in the USA.

Why isn't aluminum prohibited in ATEX Zone 1? Isn't IEC 60079-0 a requirement for ATEX Zone 1?

Or is 60079-0 still showing 'the way we used to do it' and people have realized that the aluminum/steel 'thermite' spark is so improbable that it is rarely a concern? Perhaps it is a requirement only for those extreme cases?

Thanks,

Z

 

[ScottyUK]

Clause 8.1.2 has three separate requirements for light metal content of enclosures:

• for Zone 0
10 % in total of aluminium, magnesium, titanium and zirconium, or
7,5 % in total of magnesium, titanium and zirconium;

• for Zone 1
7,5 % magnesium;

• for Zone 2
no requirements except for fans, fanhoods, and ventilating screens which shall comply with the requirements for Zone 1.

ATEX Zone 0 is an environment where an explosive atmosphere is frequently or continuously present, so I guess the even the minute risk of a thermite reaction between iron and aluminium is unacceptable. Zone 1 is an environment where an explosve atmosphere is likely to occur occasionally in normal operation, so the risk of an incendive spark and an combustible atmosphere being simultaneously present is consderably lower.

There is a further line in Clause 8.1.2 which allows use of light metals in Zone 0 subject to the user determining that there is very little risk of an incendive spark due to impact or friction, which gives some idea as to the origin of the requirement.

 

[moltenmetal]

Wow- I'm sorry, but those enclosure rules are absolutely NUTS. Regulating the content of potentially sparking components like fans I totally understand- but an enclosure?! Thank God I don't need to build equipment under IEC rules!

 

[ScottyUK]

We Europeans don't blow up our oil refineries as often as you guys do in North America.

 

[moltenmetal]

Sure. When the refinery ignites, the source of ignition is definitely the aluminum enclosures spontaneously combusting, rather than the FIRED EQUIPMENT located inside the refinery!

 

[ScottyUK]

Probably not - just suggesting that our excessively cautious standards are fairly good at preventing a bad day at the office. On the other hand I could well imagine that the ban on light metals is a result of some event long ago, although I have no idea what it was or when it might have taken place.

 

[ScottyUK]

You got me looking for the origin of this requirement, and it seems to have its origins in the coal mining industry rather than in petrochemicals or oil & gas.

Not so long ago the UK had a massive coal industry - although you would barely know it now - and there have been more than a few explosions in British coal mines which were attributed to aluminium and rusty steel coming in to contact. Some of the first accidents where the link was recognised occured at Horden Colliery in 1953 and at Glyncorrwg Colliery in 1954. Here's the link to the investigation into the Horden accident:

http://www.dmm.org.uk/reports/9399-10.htm

 

[moltenmetal]

The result of over-extension of a good rule. Avoiding sparking combinations of metals in frictional contact is one thing, and banning "light metals" for services like enclosures is entirely another!

 

[zappedagain]

So maybe it is an over-extension that was dropped so long ago that my Intrinsic Safety experts no longer know why or how it was dropped.

"We know where were goin'. But we don't know where we've been". That Talking Heads lyric is making so much more sense these days!

Z

 

[MikeHalloran]

ScottyUK's link was educational.

It was also scary, because nominally minor items like 'enclosures', whatever you choose that word to mean, are almost never designed or selected under scrutiny of any engineer, much less one who would be aware of the special risks of explosive atmospheres.


Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

At least part of Canada has a blanket ban on light metals in underground coal mining applications.

http://web.cim.org/ugcoal/img/factsheetmodFeb2006.pdf

 

[moltenmetal]

Generally if you have a zone 0 or even a zone 1 area on your plant and you need to have people working in that area on a frequent basis, you've already got a huge potential problem. Very difficult to avoid that in a mine, of course! The material you make your enclosures out of is just the very bottom of the list of the problems you're up against to make that kind of an arrangement safe.

The flammables in a plant environment aren't likely to be as benign as methane from a health/personnel chemical exposure perspective for one thing. And it's tough to imagine an enclosure being a source of ignition, irrespective of what metal it's made of, unless personnel are working in or around it.

The usual situation is that zone 0 or zone 1 are selected as a means of being extra diligent and protective (i.e. for CYA reasons). As an example, gasoline dispensing outdoors. The filling stations are all Div 1 (don't know if it's zone 0 or zone 1 in this case because I have nothing to do with designing filling stations) for the first 3 feet above grade and any space below grade, because the vapours are denser than air. However, vehicles with non-Div 1 electrics and burning hot brakes and exhaust manifolds and emission control catalysts are being operated with in that zone- with all those parts being within 3' of grade.

 

[ScottyUK]

When your plant inventory is tens of thousands of tonnes of LPG and several hundred thousand tonnes of oil an incendive spark has the potential to level the whole district.

The more risks I can eliminate the better.

 

[moltenmetal]

...but if your refinery ALSO has fired equipment, and miles of hot pipe which is covered by gas permeable insulation (i.e. real insulation rather than some perfect, imaginary insulation), aren't you already done for in the case of a major flammables release? An aggressive avoidance of even marginal potential sources of ignition like aluminum enclosures is merely making the vapour cloud larger before it ignites, isn't it?

I totally understand your point of view- I'm just pointing out the limitations of focusing on ignition source elimination as a means of protection against fire, in an environment which has hard process needs for sources of ignition such as hot pipe and fired equipment, to be present within the plant boundaries. This seems to be rather easily forgotten or overlooked when setting out a design for fire risk mitigation. Area classification is often used out of context because it is an easily understood set of rules, rather than because it is an effective risk mitigation means. As a consequence, other means of risk mitigation are sometimes ignored or overlooked. I've seen clients who wanted a plant designed for class 1 zone 2 but then wanted the electric heaters "exempted" because they must operate above 80% of the autoignition temperature for the process to work...it's just plain crazy.