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

 

[pennpoint]

It would be prudent to use a non sparking material such as bronze or aluminum for the empeller.
I do not know of a code or regulation that would guide you. Howerver there are many experts at this site, I am sure you'll get the anwers you seek.

Pennpoint

 

[MixerDude]

Additional information: The impeller operates in a draft tube. The fluid can contain stray metal parts that can currently get trapped between the prop and the tube. The concern is that a trapped part could generate enough heat to ignite the rail car if the prop isn't fully submerged in the waste fuel.

 

[DLANDISSR]

Mixerdude:
How would a trapped part generate heat?

Some suggestions:

The tank could be inerted with N2. The elimination of oxygen would mean that the mixture could not ignite at any temperature or with any ignition source.

Low Explosion Limit (Gas sensors) could be installed to measure the vapor concentration and a control system programed to shut off the mixer if an explosive concentration is sensed.

If the impeller is say aluminum and the tube is aluminum any stray metal which would get in the tube would not spark on the tube or impeller. Even with non sparking mixer components two pieces of sparking metal could be brought together during mixing and cause a spark.

Can you predict the size of stray metal which would be in the mix? If so you could put a screen on the end of the tube to keep out stray metal.

A thermocouple could be put in the bearings and in the tube to sense heat build up.

Have you considered static electricity build up. If all other precautions, etc. are implemented static electricity could produce an ignition source.

The best most reliable system would be to eliminate ignition sources,(non sparking components, grounding, etc.) and inerting with N2 so that if any ignition source is present there would be not oxygen to support combustion.

Im glad you are giving this serious consideration because of the inherent dangers in handling flammable and explosive liquid mixtures.

 

[pennpoint]

Mixerdude:
Been thinking about this mixer thingigy.
Like dlandissr's post; I think you can put a N2 cap on the tank to reduce the explosive potential. Adding screens to the mixer/agitator would help keep out the larger sized scrap thus allowinig smaller material to freely flow through the mixer. Periodically reverse the agitator/mixer to force free the larger material which is sucked against the inlet screen. This continual forward/reverse rotation will aid in clearing the screens and help the fluid/solid suspension and add to the overall agitation.
Another thought is to continuously vent the vapors off during this mixing/agitating process and condense them at a remote location. Steam, compressed air/nitorgen gas, ejectors or a liquid ring vacuum pump could be used. Mechanical refrigeration or liquid nitrogen or CO2 could be used for the condensing medium. You might want remote mount the mechanical refrigeration package however.
The vent system could be a closed loop system, the stripped gas could be returned beck to the tank to continue to drive off vapor to the condenser.
I would find it hard to believe that you can do this process today without AQMD (Air Quality) people on your backs. VOC's are a big no-no!


I hope I might have turned ON a light.

Good Luck

pennpoint

 

[ccw]

Mixerdude:

I agree with the above suggestions. You are not supposed to be grinding up broom sticks, rocks, etc., just suspending the sludge (light fines). Use screens and the reverse flow process, use the non spark propellers, and use the blanket gas or the off gas sweep concentrator. All good suggestions. Go slower to minimize releasing volatile organic carbon based materials. Fuel/Air ratio monitors in the head space are also a good idea. If VOC's are a real nuisance, you might sip off the cover liquid and desludge with an emoluent/cleanser liquid.

The tank desludge jobs are always a challenge, particularly with the limited access, hot dog tanks, even more so those with internal structure.

How are they going to pump the mixed slurry out of the tank?

 

[MixerDude]

Pumping out these tanks and leaving them fairly clean is a service that our customer offers to thier's. The fact that our aggitator design does such a good job at suspending all of the solids in these "hotdog" tanks is why they want to use it. They agitate the tank for a while then begin drawing off the mixtures. There is a grinder in the line that chops up any and all solids. There are extremely tight controls on the emissions coming out of the tank and each unloading is closely monitored with a "sniffer". They don't want the expense of blanketing the tank or drawing off the vapor during the process. The tanks come in the rail yard and they don't want them sitting in the sun all day. They quickly crane the mixer into the tank, mix it, draw it off and the car is on its way. Our first effort in this redesign is to identify whether there is a code or certification (such as NEC and UL for electrical components) that applies to mechanical components in an explosive environment. Our customer wants this mixer to be Class I Division 1 but since this is an NEC designation I am reluctant to limit ourselves to it. I have yet to be able to find a regulatory definition for mechanical components (maybe I don't really want one) and I want to get a clearer scope on what this project will entail before I go back to the customer with a date when they can get thier agitator back. I appreciate all the help so far.