Class II Group G Explosive Dust

[BigMotorGuy]

I have a grinding booth air filtration system where we grind steel welds along with cured epoxy resin over steel for the electric motor industry. The dust created from grinding the epoxy recently was tested to have an Explosion Severity of 1.5 and a Kst of 160 which puts it into the Class II Group G explosive dust catagory. The operation of grinding (both the steel as well as the epoxy on the steel) obviously creates large amounts of sparks. The air filters are cartridge type Donaldson-Torit units which have fire resistant filters as well as we added Gaylord type inlet baffles to keep sparks out of the filter cabinets themselves.

My question is, does all the electricals (lights/switches, outlets,etc.) in the work area need to be intrinsically safe even though there are huge amounts of sparks being created by the normal operation? Does the fact that there is typically constant air flow ( which basically means there are not stagnant dust clouds) negate the need for the area to be listed as Class I versus Class II hazardous area? I have little experience with this type of system and the manufacturer will not help, so any feedback would be appreciated!

 

[DRWeig]

Hi BigMotorGuy,

Classifying hazardous areas is just about impossible without seeing the area in question.

Best bet is to get a local engineer who's experienced with hazardous-dust environments to have a look-see.

Sorry no easy answer on that one! Your constant barrage of sparks doesn't eliminate the hazard, the sparks might just be at a lower temperature or energy than what's requiered to set off the blast.

All that said, my gut feeling is that you'll not have too much trouble staying safe.

Please be sure to hit us back and let us know how it works out for you!

Good on ya,

Goober Dave

 

[BigMotorGuy]

Thanks Goober Dave,

The more I look into this the more I think you are correct in getting a consultant to look it over. The interesting thing is that this operation is typical for pretty much all motor manufacturing, but everyone I talked to (including the epoxy supplier) has never tried testing the dust created during normal sanding of the epoxy. However, the lab test results show it is quite an effective explosive......but once the lab data comes in, there is not turning back!

I will follow up on my direction.......

 

[DRWeig]

Yah, that was surprising to me. Epoxy isn't conductive, so normally we don't even consider it to be hazardous in dust form -- almost as inert electrically as fiberglass.

When you're mixing metal filings in there, though, I can see how you might get something to blow.

As for dust-ignitionproof stuff, somebody will have to observe and see where the dust can settle and where it can't. You might be able to minimize exposure with some simple curtain work...

Let us know what you find!

Goober Dave

 

[rbalex]

The answer to the original root question, "...does all the electricals (lights/switches, outlets,etc.) in the work area need to be intrinsically safe even though there are huge amounts of sparks being created by the normal operation?," is no. Intrinsically Safe methods are not a requirement; in fact, you would be hard put to find intrinsically safe products for the various equipment you mentioned, especially light fixures (Luminares).

This doesn't some mean anti-ignition techniques aren't warranted.

The reason the epoxy dust is Group G is that it is still carbonaceous, but doesn't have residual volitile materials. Epoxy is listed as Group G in NFPA 499[2008], Table 4.5.2. It has a fairly high "Cloud Ignition" temperature (540C) which is why the sparks are not likely to ignite it. Each spark fragment has too low an energy density.

The effect of the continuous air flow may be questionable. Positively, it may keep the system cooler; negatively, it keeps the particles in suspension.

The first thing to consider is if it is actually "dust" in the first place; i.e., is an epoxy particle less than 420 microns or less. If not, it would more likely fall into the Class III categorey.

The other major consideration is if a "dust layer" forms routinely or if the "dust cloud" is readily visible. That affects determination of the Division. See NFPA 499 for typical examples. This also describes the location's "Division" envelopes which may be much smaller than you might expect and much of your equipment may be located outside the Classified envelope.

All that said,in Class II the "T-rating" of equipment is generally more relevant than anything else and any "T-Rating" will do since the highest (T1) is 450C maximum. Beyond that, "dust ignitionproof" and "dusttight" would probably be sufficient.

Last, but not least, the industry's experience is also a relevant basis for determination of CLassification. See NFPA 499, Chapter 5, especially Section 5.5.8.

In summary, I doubt you have a problem but Goober Dave is still right that getting a knowledgeble person to review it is worth the trouble.

 

[yasky]

The 1st thing you want to do is get the area properly classified by a professional. Once the area has been defined, you can then determine what type of equipment you need and where. For example, if a certain area is deemed a Class II, Division 2 environment, then you know all of your equipment within that area needs to be certified for Class II, Div. 2, Group E/F/G (whichever applies), which can be non-incendive, hermetically sealed, dusttight, etc. or any Div. 1 protection method. Once your equipment is all set, don't forget about doing the same for your conduit, as defined in the NEC for hazardous locations. Go from there. Good luck and be safe.

 

[BigMotorGuy]

I appreciate all the input. I am in process of getting a pro in to help in evaluating the set up. Will post updates.........