FAQs: A Practical Discussion of NFPA 484 and the Use of Wet Dust Collectors*

  1. Does the code apply to me?
  2. Who enforces the code?
  3. What does this code say in plain language?
  4. What are the basic dangers?
  5. What is the most common practice in handling combustible dust?
  6. How do I test my dust?

1. Does the code apply to me?

If you process or finish metals that can create explosive dusts, (Aluminum, Magnesium, Niobium, Tantalum, Titanium, or Zirconium), one way or another NFPA 484 Standard for Combustible Metals applies to you. If you are in a US city, then this code is part of the fire code and is a part of the building code. If your city has adopted the International Building code, it also refers back to the NFPA code. OSHA and your own fire and liability insurance carrier may also have an interest in your compliance to this code.

2. Who enforces the code?

The most common enforcement comes through the Fire Marshal. This is simply because on the local level, the Fire Marshal is charged with the job of understanding the code and how to apply it. However, the authority having jurisdiction (AHJ) can be anyone who is a public safety officer or inspector. Fire, health, or labor workers are all able to enforce the code if you are in their city or jurisdiction. The AHJ may also interpret the code when that is necessary. OSHA is getting more involved with the issue of compliance to the code as the concerns for worker safety increase.

3. What does this code say in plain language?

NFPA 484 is the standard for all combustible metals. Most of the code applies to a small part of the industry that actually manufactures metal powders. However, 484 Chapter 6 addresses Aluminum, and Section 6.3 is dedicated to processing and finishing operations.

Here are some of the "Dos and Don'ts" relating to dust collection systems and collector design.

  • Do connect all dust producing equipment to hoods that capture and transport the dust.
  • Do not mix the buffing-polishing with the grinding.
  • Do keep your duct velocity at 4500 feet per minute.
  • Do not allow the minimum explosible concentration (MEC) of the dust to occur. (This is a problem with vacuum cleaners.)
  • Do use metal, not plastic, duct and keep the duct system simple and smooth, with seams downstream and as straight as you can.
  • Do not install any dead spots or ports for future use.
  • Do bond and electrically ground all the machines, the duct, and the dust collector itself.
  • Do not mix any other metals with Aluminum in a dry collector or in any ducting.

Wet Dust Collectors -- Minimum Design Features:

  • The blower must be on the clean side.
  • The collector cannot have any un-vented pockets (hydrogen
  • The efficiency must be high if you plan to recirculate the air.
  • The wet unit should be as close to the dust generation as possible.
  • No after-filters are allowed (for Aluminum).
  • Mix the sludge with dry clay (kitty litter) to make sure it is safe, i.e., to prevent it from overheating and giving off hydrogen.

Dry Dust Collectors -- Special Instructions:

  • The collector must be located outside the building.
  • The collector must have explosive venting, barriers and warning signs regarding using the collector for Aluminum only and a warning about the contents being explosive dust.
  • Portable collectors must be limited to one pound of dust.
  • Do not mix any other metals with Aluminum in a dry collector or in any ducting.
  • The filter media must be grounded.
  • The blower must be on the clean side.
  • No electrostatic collectors are permitted.
  • Water condensation must be prevented.
  • There must be no accumulation of dust anywhere in the collector except for inside the dust receptacle.

(For added comments on safety considerations for Dry Dust Collectors, see the Annex A. below.)

4. What are the basic dangers?

The dangers involve the risk of fires and explosions. Here are a few real world examples:

  • A dry downdraft table used for Aluminum finishing is set on fire when a employee sharpens a knife on lunch break.
  • A young man is burned badly when he uses a bench grinder normally used for Steel to grind away aluminum rivets.
  • While sanding Aluminum parts, a worker is badly injured when a small collector with a Steel blower wheel on the dirty side of the filter explodes.

Other examples of potential dangers include:

  • Aluminum and Steel together burn very very hot and are used in the military to melt away Steel parts in the field. Mixing the two metals can be very dangerous.
  • Steel parts or Steel grating in the workplace where Aluminum or Magnesium grinding occurs provide a high risk of sparks.
  • Common shop vacuum cleaners can be very dangerous due to the likelihood of creating an explosive fuel/air mixture in the vacuum hose and in the canister. If a static charge spark occurs, the risk of explosion is high.

5. What is the most common practice in handling combustible dust?

Wet unit sales indicate a move in the industry to the wet dust collector for finishing applications for almost all Aluminum, Magnesium or Titanium alloys. Recent improvements in the design of the wet unit provide even better reliability and a lower cost of operation. Wet dust collectors offer the kind of peace of mind that busy plant, safety and maintenance managers desire.

6. How do I test my dust?

Testing is not necessary in most cases. The dust deflagration index, or the K value, is known to be in the highest category for these metals. Metal dusts that are produced in a grinding and finishing operations tend to vary widely in particle size and in chemistry (the alloy and degree of oxidation) that affect volatility. Most all of the operations are below the size that is considered flammable (420 microns) and the size that wet collectors are able to filter at high efficiency (5 microns). Most all mechanical (not thermal) dust-generating operations produce particles above 10 microns. If a wet collector is used, it is the best available technology for safety with all of the dust variables.

Excerpt: Additional discussion on the use of Dry Dust Collectors for combustible dusts:

A. A high-efficiency cyclone-type collector presents less hazard than a bag- or media-type collector and, except for extremely fine powders, will usually operate with fairly high collection efficiency. Where cyclones are used, the exhaust fan discharges to atmosphere away from other operations. It should be recognized that there will be some instances in which a centrifugal-type collector can be followed by a fabric- or bag-type, or media-type collector or by a scrubber-type collector where particulate emissions are kept at a low level. The hazards of each collector should be recognized and protected against. In each instance, the fan will be the last element downstream in the system. Because of the extreme hazard involved with a bag- or mediatype collector, consideration should be given to a multiple-series cyclone with a liquid final stage.

Industry experience has clearly demonstrated that an eventual explosion can be expected where a bag- or media-type collector is used to collect aluminum fines. Seldom, if ever, can the source of ignition be positively identified. In those unusual instances when it becomes necessary to collect very small fines for a specific commercial product, it is customary for the producer to employ a bag- or mediatype collector. With the knowledge that strong explosive potential is present, the producer will locate the bag- or media-type collector a safe distance from buildings and personnel.

If a bag- or media-type collector is used, the shaking system or dust removal system can be such as to minimize sparking due to frictional contact or impact. Pneumatic- or pulse-type cleaning is more desirable because no mechanical moving parts are involved in the dusty atmosphere. If the bags are provided with grounding wires, they can be positively grounded through a low-resistance path to ground. Where bags are used, it is customary that the baghouse be protected by an alarm to indicate excessive pressure drop across the bags. An excess air temperature alarm is also frequently employed. A bag- or media-type collector is customarily located at least 15 m (50 ft) from any other building or operation. It is not customary to permit personnel to be within 15 m (50 ft) of the collector during operation or when shaking bags. Explosion vents are usually built into the system, as described in NFPA 68, Guide for Venting of Deflagrations. Care is customarily exercised in locating the vents because of the possibility of blast damage to personnel or adjacent structures.

*Note: This is an informal industry discussion of NFPA 484 for the purpose of providing a practical understanding of its requirements and the helpfulness of wet dust collectors in meeting safety concerns. To confirm code requirements and enforcement for your facility, please refer questions to your local Fire Marshal and/or contact OSHA or the NFPA. A copy of NFPA 484 may be purchased through the NFPA or viewed as a read-only online version by submitting your profile information at: