By John Constance, Professional Engineer, P.E.
The Engineers Collaborative
Many production operations generate combustible dusts that are highly flammable and explosible under the certain conditions. A combustible dust is defined as any finely divided solid material, 420 microns or less in diameter, that presents a fire or explosion hazard when dispersed and ignited in air or other gaseous oxidizer. Plastic, agricultural, food, pharmaceutical, carbonaceous and metal are some of the dusts that can be explosible.
Combustion occurs when dust and air mix together in the proper quantities in the presence of an ignition source. When combustion takes place in a confined space, an explosion occurs accompanied by an increase in pressure inside the confined space. If the confined space is strong enough, the explosion will be contained.
The National Fire Protection Association (NFPA) has issued a number of publications related to the prevention of industrial dust explosions. These standards and guides should be reviewed in detail if your dust control system handles combustible dust. Some of these standards have been made part of state safety codes and should be incorporated in your dust control system specifications and design. To obtain copies of NFPA related publications, contact NFPA at 1-800-344-3555.
The National Fire Protection Association Standards NFPA 654
The purpose of NFPA 654, is the prevention of fire and dust explosions in the chemical, dye, pharmaceutical and plastics industries and to prescribe reasonable requirements for safety to life and property from fire and explosion and to minimize the resulting damage should a fire or explosion occur. Some highlights from this standard are:
- A continuous industrial exhaust system shall be installed for processes where combustible dust is liberated in normal operations.
- The industrial exhaust system shall incorporate a dust collector. Industrial exhaust system components including the ductwork and dust collector must be so constructed such that dust does not leak out of the system components when the system is shut down.
- The dust control system shall comply with the requirements of NFPA 91, Standard for Exhaust Systems for Air Conveying of Materials.
- Dust collectors for industrial dust control shall be located outside of buildings. Dust collectors may be located inside of buildings if they are located near an outside wall, are vented to the outside through straight reinforced ducts not exceeding 10 feet in length, and have explosion vents designed according to information in NFPA 68, Venting of Deflagrations. Some think that installing an explosion vent on a dust collector prevents an explosion. This is not the case. The vent relieves the pressure of an explosion. Dust collectors can be installed safely inside buildings only under one of the following conditions:
* The dust collector is protected by an explosion suppression system meeting the requirements of NFPA 69, Explosion Prevention Systems.
* The dust collector has an explosion relief vent meeting the requirements of NFPA 68, Venting of Deflagrations, and the vent is properly ducted in accordance with NFPA 68 through a nearby outside wall.
Choosing Methods of Dust Control, Dust Collection, and Dust Explosion Protection
It is not normally practical to build a dust collector strong enough to fully withstand the maximum pressure of a dust explosion. Other methods of protection are usually taken. Suppression or venting or a combination of both can be used to minimize the safety hazard and property damage caused by a dust explosion in the dust collector.
The most important factor in determining the best method of dust explosion protection is a proper analysis of the dust. Samples of the dust should be tested by a qualified lab to determine dust explosion severity and the minimum ignition concentration for explosion. Tests following ASTM E1226, Standard Test Method for Pressure and Rate of Pressure Rise for Combustible Dusts, will provide details about your dust's explosible characteristics. It is important to note that the sample tested must be a sample of collected dust and not a sample of your powdered product. The explosion characteristics of the two are usually different with the collected dust being more explosible because of smaller particle size.
You should also realize that changes in powdered product composition, particle size or moisture content, and, as a result, the collected dust may affect dust explosion severity and the minimum concentration for explosion. Don't use someone else's test data. Analyze your own test information to set correct specifications. Test the collected dust, not the product powder, periodically after installation to confirm that safe conditions continue to exist. If conditions have changed, the explosion suppression system and/or explosion venting may need to be upgraded.
Dust Explosion Suppression
NFPA 69, Explosion Prevention Systems, 1992 Edition, defines dust explosion suppression as the technique of detecting and arresting combustion in a confined space while the combustion is in its incipient stage, thus preventing the development of pressures that could result in an explosion. Explosion suppression systems will be successful in cases where the suppressant can be effectively distributed.
The design of every dust explosion suppression system must be thoroughly analyzed with respect to the equipment to be protected, dust characteristics, type and location of detectors, suppressant chemistry and the installation and operation of the dust explosion control system and the related process. Although some explosion suppression systems are more expensive to install and to maintain as compared to explosion venting, suppression systems may be the only choice when venting cannot be properly installed.
There are three dust explosion suppression system manufacturers in the United States who can advise you on applying dust explosion suppression systems to your dust collector as part of your industrial dust control system. Your specification must provide enough details so that the suppression system manufacturer can select the proper system configuration.
Dust Explosion Venting
NFPA 68, Venting of Deflagrations, applies to equipment or enclosures needing to withstand more than 1.5 psig pressure. Most dust collectors need additional reinforcement for that capability. The maximum pressure that will be reached during an explosion will always be greater than the pressure at which the vent device releases. NFPA 68 calls for a pressure differential of at least 50 lbs./ft2 or 0.35 psi between the vent release pressure and the resistive pressure of the dust collector (enclosure). This NFPA guide lists the following basic principles that are common to the venting of deflagrations. You should become familiar with these principles so that you can correctly specify the conditions the dust collector and explosion vent must satisfy.
- The vent design must be sufficient to prevent deflagration pressure inside the dust collector from exceeding two-thirds of the ultimate strength of the weakest part of the dust collector, which must not fail. This criterion does anticipate that the dust collector may deform. So do expect some downtime with the dust control system after an explosion.
- Dust vent explosion operation must not be affected by snow, ice, sticky materials or similar interferences.
- Dust explosion vent closures must have a low mass per unit area to reduce opening time. NFPA recommends a maximum total mass divided by the area of the vent opening of 2.5 lbs./ft2.
- Dust explosion vent closures should not become projectiles as a result of their operation. The closure should be properly restrained without affecting its function.
- Vent closures must not be affected by the process conditions which it protects nor by conditions on the non-process side.
- Explosion vent closures must release at overpressures close to their design release pressures. Magnetic or spring-loaded closures will satisfy this criterion when properly designed.
- Explosion vent closures must reliably withstand fluctuating pressure differentials that are below the design release pressure.
- Dust explosion vent closures must be inspected and properly maintained in order to ensure dependable operation. In some cases, this may mean replacing the vent closure at suitable time intervals.
- The supporting structure for the dust collector must be strong enough to withstand any reaction forces developed as a result of operation of the dust explosion vent.
- Industrial exhaust system ductwork connected to the dust collector may also require explosion venting.
Dust Explosion Vent Ducts
Dust collectors that are vented for dust explosions should be installed in an outdoor location with vents directed safely away from persons and property. When there is no alternative to locating a dust collector inside a building, vent ducts should be installed to safely direct the vented flames, gases and debris from the dust collector to the outside of the building.
You must be aware of the fact that adding a vent duct to a dust collector will change the conditions that the dust collector will be exposed to during an explosion. The use of explosion vent ducts will significantly increase the pressure in the dust collector during venting. The vent duct must have a cross-section at least as great as that of the vent itself. A vent duct with a cross-section larger than that of the vent will result in a smaller increase in the maximum pressure produced during venting. NFPA 68 includes a graph showing the increase in overpressure (within the dust collector) due to the use of vent ducts as a function of straight duct length.
Dust explosion vent ducts should be kept under 3 meters in length and as straight as possible. Any changes in vent duct direction increases the overpressure developed during venting. In all cases, the vent duct must be made as strong as the dust collector. The vent duct configuration must be submitted to the dust collector manufacturer with the dust collector specification for proper design.
Dust Control Explosion Prevention System Inspection and Maintenance
Inspection and maintenance of suppression and venting systems should be done in accordance with the manufacturer's recommendations and NFPA standards.
For dust explosion suppression systems, NFPA 69 states that suppression systems shall be thoroughly inspected and tested at 3-month intervals by personnel trained by the system's manufacturer. In the event of suppression system operation, all components shall be inspected, replacement parts installed if necessary, and the system tested prior to restoration to full operating condition. See NFPA 69 for more details.
For dust explosion vents, NFPA 68 calls for visual verification that the vent closure is in place and able to function as intended. This is done by ensuring that the vent closure is properly installed, that it has not operated or been tampered with, and that there is no condition that might hinder its operation. Maintenance includes preventive and remedial actions taken to ensure proper operation of the vent closure. See NFPA 68 for more details.
An important activity often neglected is the periodic sampling of the collected dust for an explosibility determination. If the process has changed so that the particle size or shape of the collected dust has changed, dust explosibility may be affected. If the chemistry of the processed product has changed, dust explosibility may again be affected. If the collected dust shows an increase in explosibility above the level for which the installed explosion suppression or venting system was designed, immediate action must be taken to correct the deviation from the design condition.
1. National Fire Protection Association (NFPA) Standard Nos. 68, 69, 91, 654.