Great Balls of Fire
Dust clouds can be ignited by the effects of mechanical friction from overheated bearings or motors, sparks from grinding machinery, static electricity, electrical arcing, welding sparks or naked flames. Here’s Part 2 of our guide to understanding, preventing and controlling dust explosions.
BY SIMON FRIDLYAND
How can you create a ball of fire? Where combustible material is conveyed, filtered, milled or sprayed in a fine powdered form, and the dust is confined to an unprotected process volume with it being dispersed, and when an ignition source is present, a deflagration can take place resulting in an immediate overpressure and flame ball.
Should this scenario take place, it will result in deadly consequences, as noted in the previous issue of this magazine (Dust, Dangerous Dust, Sept. 2011, page 29). However, there are methods to prevent such tragedies and some recommendations follow.
An explosion risk assessment will typically recommend that a series of protective measures be implemented, ranging from improvements in housekeeping to an investment in protection equipment.
Applying protection technology requires that the constraints of the process equipment, building facilities, environment and budgets are taken into account in achieving the level of personnel safety required by codes and standards. A Pre-Start Health and Safety Review (PSR) must be done in Ontario, as it is mandated by Section 7 of Reg. 851, under the province’s Occupational Health and Safety Act.
Here are the options available to minimize the impact of a dust explosion:
* Explosion venting
* Process isolation
* Use of pressure vessels
* Explosion suppression.
Explosion venting: The basic principle of venting provides for the rapid opening of a vent of sufficient area to allow unburned dust and explosion products to escape, thus limiting the resulting pressure rise to an acceptable level. The acceptable pressure rise is determined by the requirement that the vessel should not rupture, and in some cases, that it should not deform.
The maximum explosion pressure in a vented vessel is called Reduced Explosion Pressure, or Pred. This is usually designed to be approximately two-thirds of the pressure required to rupture the vessel.
In a given vessel, the Reduced Explosion Pressure will depend upon the size, number and location of the vents, the opening pressure and inertia of the vent cover, the presence of ducts from the vent, the presence of obstructions inside the vessel, and the state of the dust cloud. The explosive characteristics of the dust will also have a bearing on the vent area.
Process isolation: An effective method of explosion protection is to isolate the particular process, such as the unloading of powdered ingredients, into in a separate room that must be constructed with sufficient strength to withstand the effects of an explosion, or a room that has its own explosion relief vents positioned in the wall. Doors and other closures in an isolated room must be capable of withstanding the effects of an explosion.
Equipment isolation: Individual plant equipment, such as silos, filter receivers, etc., should be protected against the effect of an explosion and should be isolated from each other in order to control the transfer of burning and smouldering material; and to avoid an explosion in the first vessel causing re-compression, increased turbulence and a subsequent increase in the rate of pressure rise in the second vessel.
There are three types of equipment isolation equipment.
Chemical explosion blocking systems: These systems are typically used with explosion suppression systems. The duration of the discharge, quantity of the suppressant discharged, location of the discharge point, flame propagation velocity and operating flow rates must all be considered in the design of a blocking system. Processes with high flow rates and/or large primary vessels may not be suitable for chemical explosion blocking systems.
Flame-front diverters: The flame-front diverter incorporates the need to vent deflagration pressures with the need to direct the flame front so that it does not ignite material in the process downstream. Advantages of this equipment are low initial capital and maintenance costs.
Rapid-action valve: An explosion isolation valve such as a rapid-action valve provides a mechanical barrier against the flame front of an explosion. The intent is to isolate the explosion and protect the area beyond the valve. The valve must be activated upon detection of the explosion.
An explosion suppression system or explosion venting is required on the ignition side of the valve because, when the isolation valve closes, the ducting or vessels are subject to over-pressurization. The main advantage of this isolation method is the certainty of preventing flame propagation to other equipment or processes.
Pressure vessels: Where the equipment is relatively small, or hygiene constraints limit the application of other systems, it may be preferable to design the vessel to withstand the pressure, rather than use other protection systems.
Vessel design can be based on either pressure-resistant vessels or pressure-shock-resistant vessels. In both design cases, the pressure rating of the vessel must be able to withstand the maximum pressure rise for the dust concerned.
Pressure-resistant vessels are designed to contain an explosion without rupture or deformation. Pressure-shock-resistant vessels are designed to withstand the maximum explosion pressure without rupture but would be liable to permanent deformation. This approach reduces the capital cost but accepts that following an explosion, the vessel might need substantial repair or replacement.
Explosion suppression: Explosion suppression is an active protection method which relies on sensing the start of an explosion and delivering an extinguishing agent as quickly as possible to quench the explosion and reduce the maximum explosion pressure to a substantially lower level. The lower pressure is called the Pred – the reduced explosion pressure – and must be lower than the vessel design strength for an explosion to be successfully suppressed.
The advantages of explosion suppression systems include the ability to:
• Stop the explosion before the developing pressure can damage the process equipment
• Control any ensuing fire and reduce flame front propagation to other process equipment
• Not vent flame or other material, which is useful when toxic, radioactive or corrosive materials are being handled, equipment is located indoors, or venting exposes personnel to a discharge of pressure and combustion products.
The disadvantages of explosion suppression systems include the following:
• The design and installation of systems are expensive. Also refilling and resetting the system after a discharge is expensive.
• Maintenance requirements are more severe than for conventional venting systems.
Clearly, there’s no simple solution to the important task of reducing the chance of a dangerous dust explosion, or dealing with one after it occurs, but most importantly, the appropriate solution must be matched to the specific conditions of each industrial facility.
Simon Fridlyand, P.Eng., of SAFE Engineering Inc., specializes in industrial health and safety concerns and PSR compliance. For more information, visit www.safeengineering.ca.
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Go online to mromagazine.com to read Explosions and Fires in Dust Collectors, an online exclusive feature by Gary Berwick, P.Eng., of Quality Air Management Corp. He reports that not all combustible dusts will produce explosions. He explains that to produce a conflagration, the dust must have a sufficient surface area ratio to weight to sustain the rapid oxidation to create and sustain an explosion.