Explosion Proofing Mechanical Mixer in Waste Fuel Recovery

[MixerDude]

My company has just been asked to supply our mixer for agitating sludge in a waste fuel rail car for offloading. The tank may contain any combination of very volatile fuels including Benzene, Acetone, Toluene, diesel, and more generic fuels as well as waste from paint shops which could include Turpentine, Alcohol, Mineral Spirits, etc. The customer confirmed that the volatile fuels can go right to vapor and fill the air void in the top of the tank above the liquid. There is no way to keep the more combustible fuels suspended or dissolved in the rest of the liquid in the tank. They have grit, sand, pebbles, broom handles, rags, etc. in the mix and our mixer will help them pump out any and all solids.

Our mixer consists of a 10hp motor driving a shaft and metal square pitched prop spinning at 1750rpm in a metal draft tube. The shaft is supported top and bottom by bearings so there is potential for metal to metal contact and heat buildup.

The mixer is to be Class I Div 1 but this NEC code only applies to electical components. Is there is code that would apply to mechanical equipment in an explosive environment? Bill

 

[StoneCold]

MixerDude
Do you have any experience with this mixer? Does it actually work running at 1750 rpms? Most mixers that I have experience with run the agitator blades around 125rmps. Granted they are overhung loads and not suspended between two bearings. We deal with flamable solvents all the time but we would never try to run the mixer that fast, and put the bearing down in the fluid. It is very probable that the solvent would displace the grease in the bearing and that it would overheat in a matter of a couple of hours. Granted if the tank is inerted it will not explode but purposely putting a hot object in a flamable fluid is not a good idea. Would it not be easier to put strainers on the loading points of the rail cars and catch the rags and brooms there?

 

[MixerDude]

We have a lot of experiene with this mixer and the customer actually has already been using it for this application. We just recently found out they were operating it in an explosive environment and insisted that they return it to us for retrofitting. It works better than anything else they have ever used and are anxious to get it back in service. The lower bearing is a teflon bushing and there is a "slinger" just oavove it on the shaft to minimize any griy from getting inside it. It runs at 1750 and works great for this application. The rail cars come in to the customers facility from all over the country and they have no control over what ends up in the tanks. In order to satisfy my company that this mixer is safe for use in this environment I am trying to identify an industry spec that covers the design of mechanical components in an explosive environment (like NEC or UL for electrical equipment). Any ideas??

 

[wms]

BradStone
Touched on the best method of avoiding your impending catastrophe. Nitrogen or other inert shielding also some sort of mixer mounted strainer would be a good idea. Along with your God-Given common sense you will find some reference in NFPA 36 or 26 with regard to working in solvent laden environment.

 

[rmw]

The NEC Code for electrical devices is not to prevent the electrical device from causing an explosion per se, as much as it is to require the device to contain an explosion, if explosive gasses do enter the device, and any "arkey sparkey" situation in the device has a propensity to torch off the gasses.

Therefore, what you are seeking is not apples to apples with respect to class I div 1, or any other classification.

Some devices are rendered "explosion proof" by stout construction, with small flame propagation paths through shafts, seals, penetrations, etc., and others are rendered "explosion proof" by merely maintaining a positive purge of air or inert gas from inside the device to the outside atmosphere, preventing the explosive gas from entering the device.

rmw

 

[MixerDude]

Thanks for the response. It has been a while since this has been resolved but I am still interested in the subject for future dsafety and sales. We made the decision not to risk being in this plant so we have since taken back the mixer in question because of our safety concerns. 2 months after we did so I heard through the grapevine that there was an explosion at the customer that killed a 19 year old kid as he tried to repair a pump or valve that was next down the line. Obviously this was just a coincidence but apparently the problem was caused because of the uncertainty of the identity of the combined fuels being offloaded. We are still pursuing ideas but it will be a while before we redesign this mixer for this type of service.

 

[Thom2005]

I am watching your thread with keen interest. I also must place a mixer in a Class 1 Div 1. (group C&D)area. I cannot find any clue to a CODE that I must apply to strickly mechanical aspects of the mixer. What we really need is someone like NEC, NFPA to tell us the applicable code or that there is not one apply. I'd be interested if you inquired with them.

The only mechanical feature we apply is anti-static belts on our reducer but it sounds like your mixer is direct drive.? Not sure if anti-static belts are NEC code or just standard practices.

My supply under a pending contract dictates "All equipment including accessories shall be suitable for operation in a class 1 division 1 hazardous environemnt."

My client wants me to certify that my complete mixer is Class 1 Div 1 and I can say only that we meet the NEC electrical requirements.

Our intention is to meet the NEC on the electrics. And unless some "spark proof?" code really exists.. well what do we do? We have contemplated "non-Sparking materials" but with no CODE to guide us, what are our material selections based on?

I have found some assistance surfing under "Explosion Proof Tools". Info on these suppliers web sites has given me some insights but nothing difinitive. For example,
In order for a spark to cause ignition, it must be large enough and live long enough. Regardless, so called "NON-SPARKING" tools can spark.