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Is velcro use acceptable in explosive environments?
3

Is velcro use acceptable in explosive environments?

Is velcro use acceptable in explosive environments?

(OP)
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

RE: Is velcro use acceptable in explosive environments?

Seems to me that's more of a question for your own people, i.e., why Velcro as opposed to a cable tie?

TTFN
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RE: Is velcro use acceptable in explosive environments?

Velcro will probably saturate with paint and become very difficult to undo. Not that it has anything with your question to do - just mention what I experienced when I tried to change a sensor on such an arm.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Is velcro use acceptable in explosive environments?

Methinks that the term "anti-static Velcro straps" is referring to the fact that they are anti-static straps, that use Velcro, as opposed to elastic bands that can pinch you or unwieldy buckles like on old watch bands, with the Velcro just making it easier to take on and off. I don't think it has anything to do with the static properties of Velcro.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Is velcro use acceptable in explosive environments?

Some anti-static Velcro straps I've seen have a tracer wire or some other conductive additive to make them conductive to avoid static buildup.

But I'm like Gunnar, I think you'll get paint buildup either way. Maybe not due to electrostatic attraction, but the powder will have plenty of places to accumulate in the fabric of the Velcro.

-tg

http://xternal.me

RE: Is velcro use acceptable in explosive environments?

(OP)
Good point on the paint clogging. I'll suggest the cable tie idea.

Z

RE: Is velcro use acceptable in explosive environments?

Bear in mind that antistatic properties are needed if there are rubbing motions that might generate ESD, so if the Velcro were being released and applied, ESD could be generated, but that would only be in the context of a person doing that, and if they were working on the robot, they should be wearing ESD protection anyway, so the possibility of a serious ESD event would be unlikely. Moreover, given that this is simply to tie down a cable run, the insulation and shielding (there is shielding, hopefully?) would protect the wires from getting the ESD zap.

TTFN
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RE: Is velcro use acceptable in explosive environments?

I'd worry more about a triboelectic zap triggering a fire or explosion than affecting a machine control.

Mike Halloran
Pembroke Pines, FL, USA

RE: Is velcro use acceptable in explosive environments?

(OP)
Mike, you are correct. I'm worried about a spark event not a data corruption event; I am using ESD as a generic term.

Z

RE: Is velcro use acceptable in explosive environments?

Ask the folks who make the Velcro you want to use.

Best to you,

Goober Dave

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RE: Is velcro use acceptable in explosive environments?

Releasing Velcro involves rubbing dielectric on dielectric, many times. I wouldn't trust it in an explosive atmosphere no matter how much carbon black was compounded in.

Mike Halloran
Pembroke Pines, FL, USA

RE: Is velcro use acceptable in explosive environments?

Ask yourself: is someone going to be opening this Velcro WHILE the painting robot is spraying paint? Will there not be a hot work permit or similar maintenance procedure which will require the robot to be locked out before anyone touches it, much less starts ripping off cable tie straps?

There are all sorts of wiring methods, devices etc. which are approved for Div 1 or Div 2 that can be made unsafe by maintenance happening at the wrong time or done incorrectly. From a health and safety perspective alone, it seems to me that you wouldn't want people (probably in supplied air respirators) to be doing maintenance on this thing while it continues to spray paint.

Is the concern that the maintenance will be happening while other, adjacent robots are still operating? Or is there something else in the area making this location Div 1 other than just the paint spray?

Unless you're concerned about the Velcro coming loose due to motion of the robot and that motion causing a static-electric ignition spark, it would seem to me that you're making a mountain out of a molehill.

The point about the Velcro gluing together with overspray is a good one too of course.

RE: Is velcro use acceptable in explosive environments?

(OP)
My concern is the Velcro getting stressed during robot motion while painting (i.e. explosive environment) and loosening. While I'm waiting on a response from the safety engineer for the booth, I'm leaning toward Mike H's viewpoint.

Z

RE: Is velcro use acceptable in explosive environments?

The exposive environment doesn't go away when the robots stop. The place remains loaded with dust.

Mike Halloran
Pembroke Pines, FL, USA

RE: Is velcro use acceptable in explosive environments?

Is it powder (enough electrostatic charge already) or wet paint?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Is velcro use acceptable in explosive environments?

If the place is so loaded with a dust which is explosive enough that you're worried about minor static electric discharge from parts rubbing together, you're suggesting that it's OK to send PEOPLE into that environment to do maintenance?! I have a hard time imagining Velco igniting dust, personally.

RE: Is velcro use acceptable in explosive environments?

ESD events generally require a number of things to occur:

> Some sort of rubbing/contact motion event -- but this requires two dissimilar materials, one that strips electrons and one that accepts electrons. It would seem to me that Velcro pretty much has similar materials on either side of the rubbing event, so both sides are either trying to strip electrons or both trying to accept electrons, resulting in a net nothing

> One side has to be reasonably conductive -- a balloon can generate static charge, but it doesn't really generate discharges because it's not conductive enough to get all the generated charges into a single location where they can then jump to the other side of an arc. You never see carpets as part of a discharge, nor your shoes rubbing the carpet, nor the body of the door itself, only your finger and the door knob do anything, because they're both conductive


In general, if you're concerned about the Velcro loosening, then it's not the right solution, regardless of the ESD possibilities. And if it's on a robot, why does it need Velcro? Are the cables that bad that you need to replace them so often that you want Velcro and not cable ties? My impression is that cables on a robot arm are pretty much permanent installations, unless someone breaks the cable, or the entire arm.

TTFN
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RE: Is velcro use acceptable in explosive environments?

zappedagain,

Sounds like this is about the dust jackets that wrap around the whole robot body, and close around the connectors, too, not just follow a single harness of cables.
Strips of magnetic tape are useful for closing seams and being easy to re-open. You can get it with a self-adhesive backing to stick to the jackets, or sew-on tabs if you prefer. The material is likely to have a high content of ferrite (that is what is magnetized) so I don't think it can retain much ES charge.

Many things release static-electric discharges when they are separated. An example that I noticed last week is the wrapper on band-aids. Peel one in a dark room sometime if you don't believe me.

STF

RE: Is velcro use acceptable in explosive environments?

(OP)
The paint fumes are ventilated from the booth before service personnel enter, so it is a safe enough environment when people enter. Someone else makes that call, not me. Again, I'm concerned about what happens when the robot is moving through the explosive environment. Reproducibility (i.e. installer to installer variation) on the cable mounting is what has me thinking about this.

The user wants to remove the sensor when they clean the robot, so removing the "cable ties" needs to be part of the SOP.

I expect they can use the same thing that they use to hold the dust jackets in place. Unfortunately I'm at least four levels away in the contact chain from the user, so I'm looking for the most complete info that I can give when I hear the middle links talking about Velcro.

Thanks for all the insight.

Z

RE: Is velcro use acceptable in explosive environments?

Well, many tribo-electric effects require a certain amount of speed, so unless the arm is running amuck, it's unlikely that any serious ESD event could occur during actual operations, since it would only be a fairly slow disengagement of the Velcro caused by the cable moving well beyond where it's supposed to go.

TTFN
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Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx

RE: Is velcro use acceptable in explosive environments?

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.

RE: Is velcro use acceptable in explosive environments?

(OP)
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

RE: Is velcro use acceptable in explosive environments?

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?).

RE: Is velcro use acceptable in explosive environments?

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

RE: Is velcro use acceptable in explosive environments?

Influential = loud voice, not large brain?

How did you avoid laughing?

RE: Is velcro use acceptable in explosive environments?

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

RE: Is velcro use acceptable in explosive environments?

(OP)
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

RE: Is velcro use acceptable in explosive environments?

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.

RE: Is velcro use acceptable in explosive environments?

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!

RE: Is velcro use acceptable in explosive environments?

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

RE: Is velcro use acceptable in explosive environments?

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!

RE: Is velcro use acceptable in explosive environments?

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.

RE: Is velcro use acceptable in explosive environments?

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

RE: Is velcro use acceptable in explosive environments?

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!

RE: Is velcro use acceptable in explosive environments?

(OP)
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

RE: Is velcro use acceptable in explosive environments?

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

RE: Is velcro use acceptable in explosive environments?

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.

RE: Is velcro use acceptable in explosive environments?

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. sadeyes

The more risks I can eliminate the better. smile

RE: Is velcro use acceptable in explosive environments?

...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.

RE: Is velcro use acceptable in explosive environments?

I told my wife that if our site ever has a Very Bad Day then our kids will be able to play on the beach every day after school, because the beach will suddenly be at the end of our street. We live 8 miles from the sea... I'm not sure she gets my humour. lol

RE: Is velcro use acceptable in explosive environments?

Yeah, I worked at a plant like that once. We made pharmaceuticals, but one of the intermediates is explosive. The plant has an aggressive safety program, and its own fire brigade, but everybody knows a really bad day will end with a really big smokin' hole in the ground.

Mike Halloran
Pembroke Pines, FL, USA

RE: Is velcro use acceptable in explosive environments?

I had that sort of situation back during the Cold War. I worked across the street from a naval weapons depot that purportedly stored nuclear weapons. Sort of machts nichts, given that we were a defense contractor, so the bomb was either going to hit us or hit across the street. Either way, we were going to be toast. Not even enough time to duck and cover, as if...

TTFN
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RE: Is velcro use acceptable in explosive environments?

My gut feel is that the energy released in a spark generated by a velcro rip is too small to be a problem. The electrostatic charge is a different problem - our requirements (Group I, Zone 0, AusEx/IECEx) restrict the continuous region of non-conductive plastic to be less than 100mm2. Any more than that could store a hazardous amount of energy and needs to be made conductive. So as long as your velcro is small enough, ESD is not a problem. But check your standard - Group II can be a lot more stringent (eg. 525uJ is magic number for methane, but from memory it's as low as 20uJ for ethylene!).

The only way to be sure would be to mechanically rip apart some sample velcro in a chamber with your hazardous dust. We do the equivalent exercise in methane chambers regularly whenever we have a new material/design/arrangement and need to get it Ex approved. If it goes bang, it's a no go.

For those incredulous at the requirements of hazardous area certification, be assured it is real and on-going. Contraband is a serious issue in underground mines in Australia - no aluminium, no watches, no phones, nothing that could release enough energy to ignite methane. It's hard to believe that people could work in such hazardous environments, where the mere presence of aluminium is a concern, but it works. Lots and lots of study goes into setting the limits and manufacturers manage (through quite extreme constraint) to comply. The problem with aluminium in an underground mine is that things are always bumping into other things, including mobile machines into boring old enclosures. Aluminium is known to be a problem, so we do without it.

Which reminds me - another avenue of investigation might be your customer - if they have a site rule that says employees can't wear velcro strapped watches then there's your answer.

RE: Is velcro use acceptable in explosive environments?

(OP)
Thanks. The install is occurring next week, so I expect I'll get some feedback from the safety engineer on site.

Scotty, that's some nice history you dug up there. Kudos to you!

Z

RE: Is velcro use acceptable in explosive environments?

Thank you. smile

RE: Is velcro use acceptable in explosive environments?

It would be well worth a look at existing loom and raceway clips that are available as stock items.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

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