Combustible Dusts

(Aluminum, Magnesium, Niobium, Tantalum, Titanium, Zirconium)

These dusts are highly combustible and present a very significant explosion hazard. There are some stringent fire codes dealing with these dusts which draw their regulations, for the most part, from NFPA 484, Standard for Combustible Metals.

Unfortunately, most end-users are not aware of these standards or safe methods of dust collection for these dusts. Worse, the dust collection industry is very negligent in guiding these people with proper and safe applications engineering. This document is an attempt to provide some of this valuable information.

First of all, we strongly encourage readers to obtain a copy of NFPA 484 and comply with it. There are far too many stipulations which go beyond the scope of this document. A copy of excerpts pertaining to each type of dust can be requested from Quality Air Management.

We will analyze the use of different dust collection methods to these dusts:

1. Dry Dust Collectors; include baghouse (both mechanical cleaning and pulse-jet self-cleaning), cartridge or pleated filter collectors, disposable media filters, electrostatic precipitators, cyclones.
2. Wet Dust Collectors; there are many styles of wet collectors available. The pro’s and con’s of each type is beyond the scope of this document.

In all cases the blower for drawing the dust-laden air into the collector shall be located on the clean air side of the collector. The dust producing equipment and dust collector must be

Mixing of Metals is not permitted, unless the entire system is disassembled and thoroughly cleaned prior to and after its use. A placard must indicate, for example, “Aluminum Metal Only - fire or explosion can result with other metals”.

Wet collectors are designed specifically to be used for all these dusts. These collectors are designed for collection of metal dust only, not for powder, smoke or fumes. The use of additional dry filter medium either downstream or combined with a wet collector is not permitted. Contact QAM technical support for a safe method to handle these contaminants which the wet collector can not handle. The cleaned air can be recycled to the work area if the collector is efficient enough to ensure safety of personnel. A provision for an unimpeded vent, when the machine is shut down, must be provided. Magnesium dust requires a powered positive venting of the sludge tank at all times during shutdown of the collector.

Dry collectors are allowable for aluminum, niobium  dusts, but are prohibited for all the other dusts. They must be located outside buildings. Filter media must dissipate static electric charge (be aware that grounded conductive media gives a false sense of security). You must avoid accumulation or condensation of water at all costs which could cause a hydrogen gas explosion. Explosion vents must be provided. Recycling of air into the building is prohibited.

Mechanical shaker style collectors; are highly susceptible to static electricity charges, and explosions.

Baghouse collectors; there are conventional designs, sold by 95% of dust collector suppliers, and new advanced technology designs.
Conventional; Due to the inefficient clean systems, only 10-20% of the filter media ever gets clean. This allows dust to accumulate in the collector beyond what is permissible by NFPA 484.
Advanced technology (i.e. Ultra-Flow); These are designed to clean 100% of the media on a regular basis. The cleaning frequency can be set to maintain a cleanliness that meets NFPA standards.

Electrostatic Precipitators are prohibited because they filter the air by applying an electrostatic charge across the air stream. That is a source of ignition and the dust will accumulate in the unit and coat the collection plates. This is a prescription of a very large and very load BOOM (explosion).

Cyclones; high efficiency models can be used for these dusts but must be located outside the building. Explosion vents are permitted. Recycling of air into the building is prohibited.

Read more on ... Wet dust collectors

Wet Collector Underperformance

Equipment:
C5-2500, orifice scrubber style wet dust collector, rated for 2500 CFM, purchased to handle explosive aluminum dust particles.

Problem:
The dust was going right through the collector and packing into the fan / outlet compartment. Very little dust was collected in the dust collector sump.

Investigation and observations:
We requested pictures and system layout drawings (sketches were actually provided). From these we observed that the client did not describe the application accurately at the time of purchase.
1. The dust was produced from a spray coating operation. Therefore, it was fine powder type aluminum dust. Wet collectors are designed for metal dust 5 microns and larger, as generated from grinding and cutting operations.
2. The inlet was connected to a properly sized 8” duct but was over 20 feet long with three elbows. These units are designed for maximum 10 feet of duct directly to the collection point.
3. The client also decided to exhaust the discharge of the collector to the outdoors with another 20-25 feet of duct, and three more elbows. These collectors are designed for an open, unrestricted discharge on the top of the unit.
The result of this was a questionable capability of collecting the powder type dust. The biggest problem was that the collector performance was choked by far too much resistance to airflow in the installation. By doing this the air entered the collector with far too little volume to cause the necessary turbulent energy in the “omega” style baffles. The necessary wetting action of the dust particles was not taking place and filtering action was non-existent.

Solution:
1. We asked the client to place some of the collected dust in a closed jar with water. Then shake it and let it sit for a few minutes. If the dust settles, it can be collected. If it doesn’t, a different dust collection solution must be found.
2. A wet dust collector is very particular about the airflow through it. You need to be in a range of +/_ 10% of the rated CFM for that model dust collector. In this case, the minimum flow that could be tolerated is 2250 CFM. Conversely, with more than flow than the 10%, water gets drawn up too much and discharges out the unit. We recommended bringing the collector closer to the application and remove the duct on the outlet. Alternatively, add a booster fan to overcome the restriction.

Other Considerations:
A. If the excess resitriction is minor (within the 10% range) but dust is discharging at the top, You would add more water to the collector, in small increments, until the dust/water stops coming out the top. The added water compensates for the higher restriction. Then reset the float control to maintain this new water level.
B. In some cases the discharge is required to be exhausted by code. An example is beryllium. In such a case, do not attach a duct on the outlet. Instead build a capture hood over the top of the wet collector outlet, approximately 4-6 above the top, and duct that to the outside. Install a fan to give you 5-10% more air flow than what is running through the collector, to ensure no contaminents escape back into the room.
C. If you oversized the wet collector, Do not restrict the flow with a damper more than the 10% tolerance. Install a bleed-in on the inlet duct with an adjustable shut-off damper. Open the damper to the point where the collector performs properly. This is often a scenario when you size the collector for multiple collection points but don’t have them all installed until later.

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