Explosion Protection
BCE explosion protection is engineered to meet and exceed your requirements in order to provide the best protection for your staff.
In order for a dust explosion to occur, the following factors must be present:
• A concentration of dust in suspension above its flammable limit.
• A sufficient concentration of oxygen to ignite the fuel.
• A source of energy (e.g. a spark) to ignite the fuel.
• Confinement of the dust/oxygen mixture.
Particle size has a major influence on combustibility. Any oxygen concentration above 13% will support combustion. Additionally, the smaller the particle size, the greater the specific surface and the higher is the likelihood of spontaneous combustion.
Ignition can be initiated from a number of possible sources. For example, there could be friction on collector surfaces from hard impurities in the coal, such as pyrites, metals or rock, or sparks coming off of defective fan blades or dampers. Reaction rate increases with temperature, therefore an explosion is more likely with a high operating temperature.
Standards for Explosion Protection
The National Fire Protection Association (NFPA) issues a wide variety of safety standards pertaining to securing buildings and equipment against damage from fire. The accepted standard in the United States for dust collector explosion venting is NFPA-68, called the "Standard on Explosion Protection by Deflagration Venting." A deflagration is defined as the propagation of combustion at less than the speed of sound in the un-reacted medium, as opposed to a detonation, which exceeds the speed of sound. Dust collector explosions are deflagrations by nature.
This publication was first introduced as a guide in 1954, compiling the best available test data on the fundamentals of explosions, using vent types current at the time. Panels and doors were sized according to "vent ratios," or the vented volume of the enclosure divided by the panel area. Dust types were categorized as Type 1, Type 2 or Type 3, corresponding to ratios of 50:1, 30:1 and 15:1, respectively. These rule-of-thumb ratios were very conservative in some cases, untested in others, and were used to size dust collector explosion vents for many years.
The deficiencies of using such an approach became apparent as issues of insurance and liability came to the fore. Besides the limited accuracy of the rules of thumb, other parameters needed to be better quantified, such as the effects of vent panel inertia (or weight per square foot), enclosure geometry, locations of vents, pressure effects, vent aspect ratios and ducting the vent to an external location. As more testing was done in the US, UK and West Germany, and better methodology was created, NFPA-68 went through a number of major revisions dating from 1974. The earlier revisions incorporated the researches of the German VDI and the Factory Mutual Research Corporation with the object of improving the accuracy of the vent ratio calculation. Subsequently, the vent ratios were abandoned altogether for the much more rigorous approach used today, whose accuracy was improved with each succeeding edition. The latest edition (2007) is a "standard" rather than a "guide" for the first time. As a standard, more power is given to the Authority Having Jurisdiction (e.g. OSHA or an authorized company official). ATEX is the European equivalent with the additional requirement that its use in the EU is legally mandatory.
The following parameters used fir explosion vent sizing are fundamental and deserve discussion:
Kst: The value K is known as the deflagration index, and is the maximum rate of pressure rise attained by combustion of a particular dust in a spherical test vessel of at least 20 liter capacity. This is a specific property of the dust, which can be found in the literature, if known, or by test. Dust Hazard Class 1 has a Kst ≤ 200. Class 2 dusts have 200 < Kst ≤ 300 and Class 3 dusts have Kst > 300. These classes replace the types associated with the vent ratio method.
Pmax: This is defined as the maximum pressure developed for an optimal mixture of a particular dust in a contained deflagration and is a property of the dust. The Pmax for most dusts fall into a range between about 7-10 bar, but this can be much higher for highly reactive metal dusts such as aluminum or magnesium.
Pstat: The static activation pressure of the vent closure, or the pressure at which the closure opens with rising pressure.
Pred: The "reduced pressure" or the maximum pressure developed in a vented enclosure after an explosion.
The relative importance of these factors is seen in the following basic equation for determining the minimum required vent area.
A = 10-4 (1+1.54 Pstat4/3) Kst V3/4 ((Pmax / Pred) -1)1/2
Types of Explosion Vents
At the time when vent ratios was the predominant method of sizing explosion vents, most vent closures on dust collectors were hinged steel doors, usually spring-loaded but sometimes using refrigerator-style latches. Because the high inertia of these doors increased their resistance to opening, a relatively large number of doors were needed in most cases to keep the backpressure (Pred) from exceeding the design pressure of the unit. Many older dust collectors can be seen with these types of explosion doors lining almost the entire perimeter of the dusty air plenum. The 2002 edition of NFPA-68 specifically stated that the movable part of a vent closure should not exceed 2.5 lb/ft², essentially making these older hinged steel doors noncompliant. A new method to account for higher-inertia doors was introduced in the 2007 edition, but the use of these type doors will be at the cost of a significantly higher overall vent area for the same Pred. We recommend one of the following vent closures, which are under 2.5 lb/ft², and have been proven in thousands of installations:
• Rupture Membranes
A rupture membrane is a composite diaphragm, either round or rectangular, which is installed on a flanged duct and secured with a mating frame. The membrane is scored and tears away at the scoring lines when the design burst pressure is reached. For high-vacuum applications, round membranes are bulged and are available up to a full vacuum rating. Flat panels are usually fitted with support bar frames to withstand the normal dust collector negative pressure rating. Rupture membranes come in standard sizes and are stocked at specific burst pressures (Pstat). If none of the standards are suitable, custom burst pressures can be specified at higher cost. Spares should be kept on hand for immediate replacement of burst membranes.
• Doors
Specialty hinged doors are available which are under 2.5 lb/ft² inertia and specifically designed to meet NFPA-68 explosion venting requirements. The major advantage of the door is that no panels need replacement after an explosion; it's ready for service again after being reclosed. The disadvantage is the cost, as it is several times as expensive as the same size rupture panel.
• Flame-arresting Vents
This is a cylindrical device which combines a rupture membrane with a flame-arresting element. This was designed specifically for an indoor dust collector for which it is impractical or impossible to duct an explosion vent to the exterior. In this device, the deflagration is vented through the rupture membrane, after which combustion gases are cooled in the element, with only steam emerging with no flames, while all burned and unburned dust is retained inside the element. This is by far the most expensive option, but is the only one which allows operation of a vented indoor collector without an external duct. It is also useful in minimizing the collector Pred since a long external duct adds significantly to the backpressure, requiring a stronger collector design at higher cost.
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BCE has a full staff of registered Structural Engineers. We design and supply structural supports and access packages for all BCE equipment. In addition, we provide a separate service to perform structural engineering analyses for other equipment. 
