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topic 44536

Fire safety of Shot-blasting machines



A discussion started in 2007 but continuing through 2018

(2007)

Q. Sirs,

I'm a production supervisor in a die-casting company in the outskirts of Chennai, Tamil Nadu. After the die-casting process and fettling process, we do shot blasting on our parts to get good surface finish. We use S110 and S170 Steel and Aluminium shots in the ratio of 3:1. We have 6 shot-blasting machines.

Recently, there was a fire accident in our plant due to the dust of the shot blasting machine. This is the second time to happen in the last 4 months. Much of our property was damaged. Luckily no one was hurt or affected.

Please suggest ways to prevent such fires in shot-blasting machines. Also tell about the maintenance and safety related features that we should follow, to avoid such an incident in the future.

Expecting more suggestions from you, sirs.

Thanks.

Rajkumar
Production supervisor - Chennai, Tamil Nadu, India


simultaneous (2007)

A. Fine zinc dust is quite combustible if there is a hot enough heat source to get it started. The airborne dust can actually form an explosive mixture with the right ignition source.
For some reason, I think that the addition of aluminum dust makes it even worse. I am thinking of model rocket motors use aluminum dust as one component.
I would vacuum the dust out daily with a compressed air vacuum as an electric motor could be the source of ignition what was explosive.
Next, I would have several different people look for the source of ignition in the blast cabinet. It could be friction, but I would look for electrical first.

James Watts
- Navarre, Florida


(2007)

A. Good afternoon:

I did a quick search of USA's OSHA website, but couldn't find any information on this subject.

If you can somehow separate the iron (Fe) and aluminum (Al) in your process, that should make for a safer work environment. There's a good possibility your creating the conditions for a thermite reaction between Fe oxide and aluminum powder. When thermites ignite, the fire is very intense.

If you can't separate Fe and Al, I only have a few ideas. If the fire started in the equipment or ventilation system, maybe you will have to clean this out more frequently. Might it be feasible to safely introduce some nitrogen into your blasting equipment while it is in operation to act as an inert gas? Is there faulty electrical wiring, or another source of ignition? Has the equipment manufacturer offered any suggestions?

If the fire started where you have accumulated the waste dust products, make sure that you are using proper containers for this waste and keep them stored in a safe place, away from sources of ignition and moisture.

I hope someone else with more experience in thermite powders answers your post, because I'm just taking an educated guess. Good luck.

Steve Bizub
- St Louis, Missouri


(2007)

A. Rajkumar, If I understand your working mix of abrasives,you are using steel shot with Aluminum shot in a 3:1 ratio? How well does this abrasive mix presently clean the parts? Using this mix will yield Both FeO and Al dusts and present a fire hazard.

One approach would be to use another abrasive, stainless steel cut wire shot. I have experience where it lasts 20x longer than S70 or S110 and even longer than Al shot, with out the FeO or Al dust issues. The primary dust will be that of the substrate. Its worth a try in one machine.

Could you respond as to the material of the die cast parts? Are they Zinc? Another type of abrasive that may also work on your die cast parts is Zinc cut wire shot. This would eliminate the FeO dust and aluminum dust if the substrate is Zinc.
Some equipment changes to reduce dust fires may be a simple drop out chamber in the dust collector plenum. This would allow dust particles to fall out for easy daily removal.

Tim Deakin
North Tonawanda, New York



August 18, 2008

A. We used to have shot blasters to clean our welded steel cabinets prior to painting. We used iron shot on mild steel parts. This dust is also very flammable We installed lime injectors on the system that continuously injected a small amount of powdered lime into the air stream. This helped but didn't cure the problem.

Tom Ward
manufacturing transformers - Jefferson City, Missouri


Filter Dust Collectors: Design and Application
from Abe Books

or

September 6, 2016

A. Hello Rajkumar,

Please Add Mineral Abrasive Powder to your shot blasting Machine 0.5 to 1 liter/day/Machine.
This can make your shot blasting machine performance good and can make dust non flammable. We have tried this at many places and results are very good and dust does not catch fire. Also, addition of this powder cleans your machine as well. It has tendency to suck all the dust and remove all the oil, grease and dust on shots and increase the life of coating if any after shot blasting.
You can add this powder in all types of Metal Abrasive.

Regards,

Deepak Singh
ABRASIVE BUSINESS - Pune, Maharashtra



January 25, 2018

Q. Deepak Singh,
What Mineral Abrasive Powder are you suggesting to add to the shot blasting machine to make the dust non flammable?
Regards,

Brian DeLucenay
- Fort Recovery, Ohio, USA


April 12, 2018

A. My company is a distributor of shot blast equipment as well as dust collection equipment. I also own another company that builds wet dust collectors for combustible dust applications. That is what we are talking about here... combustible dust. Aluminum, titanium, magnesium, zinc, etc are all combustible dusts. When they are mixed with other metals (especially carbon steel), they become highly combustible. Since shot blast machines often use steel shot on aluminum parts, dust collector fires are very common.

While I have never heard of mineral dust being added to the abrasive (it can contaminate the substrate of the parts being blasted), mineral dust (limestone) is used to help prevent dust collector fires. It is injected into the air stream between the process machine and the dust collector. It coats the filters to help prevent combustion as well as absorbing oils from the dust. It is not a fail-safe, but it helps. A complete prevention system is necessary to completely mitigate fires and explosions:
> Limestone adders
> Spark arrestors
> Deflagration vents (prevent fire/explosion from traveling back to process machinery)
> Powered slide gate on exhaust
Smoke/heat/spark detectors attached to a control panel to shut off blower, pulse board and close exhaust slide gate to shut off fire air supply

Wet dust collectors will perform the same function as the above, however they do not work well in applications that have a great deal of dust that is smaller than 5 micron. Unfortunately, blasting applications always generate large volumes of dust smaller than 5 micron.

Lance Nord
- Pomona, California



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