Processing, handling and storing of combustible dusts within enclosed equipment (i.e., mixers, blenders, mills, separators, cutting tools, holding bins, dust collectors, cyclones, etc.) can easily create an explosive atmosphere that could be ignited by tramp metal, static electricity, burning/hot dust particles or some other means. The resulting explosion could lead to a large property loss and subsequent interruption to production that could be devastating to your business. This does not take into account the possibility of subsequent dust explosions if there is poor housekeeping and fugitive dust accumulations in the building housing the subject equipment, which could be catastrophic.
For the purposes of this article, we are going to specifically address the equipment explosion hazard. The fire protection engineering industry treats all equipment that handles combustible dusts as having a dust explosion hazard. The first recommendation is locate all such equipment outdoors. If this is not practical, then it is recommended to eliminate the explosion hazard or to mitigate the explosion hazard.
Elimination of an equipment explosion hazard through inerting or liquid misting is not easily accomplished in many cases and explosion hazard mitigation features are then recommended for the equipment/system design. These typically involve the use of explosion venting, suppression, containment, or vacuum operation. In many situations involving a complex system/arrangement (i.e., storage, handling and conveyance system with several bins, vessels and lengths of ductwork), a combination of these may be required for best protection. The most common methods of equipment explosion hazard mitigation are venting and suppression.
Explosion venting is a passive safety approach and is usually the first choice. When dealing with a vessel, it may be easiest to locate it next to an exterior wall and vent the explosion to the outdoors via a short vent duct; however, this may not be practical for existing installations surrounded by other important process and manufacturing equipment. An alternative would be to vent the explosion to the surrounding area indoors through an explosion quench pipe, again if practical. Vents should be sized based on the properties of the dust being handled/collected and the design strength of the equipment/vessel involved. Vent sizes and properties should be designed in accordance with NFPA 68, Standard on Explosion Protection by Deflagration Venting, or FM Global Standards (using by FM Global DustCalc software if you are an insured of FM Global).
It is recommended to provide explosion suppression for high valued equipment, equipment that exposes high valued processes, or equipment that has frequent explosions, when explosion venting, pressure containment, or inerting is impractical or cannot be provided. It should be noted that explosion suppression systems are generally less reliable than explosion venting or explosion resistant construction.
Explosion suppression relies on detecting the start of an explosion and delivering an extinguishing agent as quickly as possible (within milliseconds) to quench the explosion and reduce the maximum explosion pressure to a substantially lower amount. The lower pressure is called the reduced explosion pressure (or suppressed pressure), which must be lower than the vessel design strength for an explosion to be successfully suppressed.
The first step in the design of an effective explosion suppression system for a specific application or piece of equipment is to quantify the potential explosion hazard. The minimum required information is as follows:
- Dust explosibility parameters (burning velocity, auto-ignition temperature, maximum explosion pressure and the maximum rate of pressure rise)
- The vessel geometry and volume
- The maximum pressure that the vessel is designed to withstand (To prevent permanent equipment damage, the pressure experienced during a suppressed deflagration (Pred) should not exceed two-thirds of the equipment yield strength (stress). The suppression system alone can produce pressures of 0.13-0.2 bar (2 - 3 psi), which may exceed the design strength of some equipment such as dust collectors. This must be considered in the design process.)
- Process parameters such as pressure and temperature
- Process conditions - in particular the amount of turbulence
As recommended by FM Global Property Loss Prevention Data Sheet 7-17, Explosion Protection Systems, use caution when installing explosion suppression systems on enclosures larger than those used in the verification testing of the suppression systems. It is technically possible to suppress explosions in vessels with volumes of up to 1,000 cu. m (35,300 cu. ft). In practice, dust explosions have been successfully suppressed in vessel sizes of 0.2 cu. m (7.1 cu. ft) up to 250 cu. m (8,825 cu. ft). The appropriate system should be selected and designed as noted above.
In addition, do not install explosion suppression systems where ST3 dusts (Kst > 300; e.g., aluminum) are used unless proven by full-scale tests. In general, ST3 dusts cannot be effectively suppressed due to their extremely fast rate of pressure rise.
An explosion suppression system is typically comprised of explosion detectors, a suppressant delivery system, and a control and monitoring unit.
Detectors - to be effective, the detector chosen must be capable of recognizing the existence of an explosion very early after ignition. There are three types of detectors: thermoelectric, optical and pressure.
- For dust explosion hazards, use pressure detectors; thermoelectric sensors (activated by direct heat transfer from hot gases) and optical detectors should not be used since they only work effectively if located close to the heat source.
- Pressure detectors continuously measure pressure and monitor the rate of pressure rise and threshold pressure.
- To minimize false trips of the explosion suppression system, it is recommended to position two detectors in two planes (i.e., cross-zoned). This is especially important for pressure detectors.
- In general, locate pressure detectors a maximum distance of 20 ft. from the suspected ignition source(s).
- Wire detectors actuating explosion suppression systems to a Class A initiating device circuit, as defined by NFPA.
Suppressant Material - the suitability of a particular extinguishing agent for a given application should be determined by explosion suppression tests.
Control/Monitoring Systems - control systems detect changes in the explosion sensor output or mechanical condition; they determine whether a hazard exists, and activate the suppressors accordingly.
- The control system should be interlocked to shut down the equipment involved (safely) upon detector activation and should prevent the process equipment from restarting without re-arming the suppression system.
- Monitor the electrical system of the detector and suppressor activation circuits at a constantly attended location. Any component failure should sound an alarm and automatically shut down the process.
- Provide a standby battery that engages automatically when the electrical power fails.
- Install the control system in a safe, dust-free area or within enclosures approved for explosive atmospheres.
- De-activate the control system, shut down the process and purge any combustible dusts before entering the vessel or performing any activity that could accidentally trip the suppression system.
Explosion Isolation Systems:
Explosion suppression is usually ineffective in interconnected vessels; hence, explosion isolation systems should be provided between two pieces of connected equipment (e.g., connected via process piping or ductwork) to prevent fire or explosion propagation whenever one or both of the interconnected pieces is protected using an explosion suppression system.
Explosion isolation is a means of preventing flame front and ignition (primarily through the use of mechanical valves or chemical suppressants) from being conveyed past a predefined point (i.e., to other process equipment through ducting or piping). The detection and control functions are identical to explosion suppression. Examples of explosion isolation are as follows:
Chemical Explosion Blocking Systems:
These systems are typically used with explosion suppression systems. A chemical blocking system should be activated by optical or pressure detectors at the same time as an explosion suppression system. These systems are generally activated using the same control equipment as the explosion suppression system, but they can also be installed/activated separately.
In general, gaseous clean agents and dry chemical powders (based on ammonium phosphate) are used as suppressants. As with explosion suppression systems, the vessel or ductwork should be designed to withstand the expected local pressure that would result from the blocking system.
Discharge duration, quantity of suppressant discharged, location of 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.
A flame arrestor is a device that prevents the transmission of a flame through a flammable gas/air mixture by quenching the flame on the surfaces of a series of small passages (or heat sinks) through which the flame must pass. The emerging gases are sufficiently cooled to prevent re-ignition. The arrestor must be placed in the flame path between the source of ignition and the system to be protected.
Deflagration and detonation arrestors should be used in accordance with their approval/listing. These types of arrestors are not interchangeable (i.e., do not use in-line arrestors as end-of-line and vice versa) and must be used in the proper arrangement. The vendor or manufacturer should supply the necessary details regarding the specific arrestor.
Inspect arrestors at least annually, and after each incident in which they have functioned.
Flame Front Diverters:
The flame front diverter (also known as explosion diverters, back-blast dampers, back-flash interrupters or backflow preventers) incorporates the need to vent deflagration pressures with the need to direct the flame front in such a way that it does not ignite material in the process downstream. The main advantages are low initial cost and maintenance costs.
Flame front diverters that are either commercially manufactured and distributed, or fabricated in-house according to design guidelines from VDI (Germany) are acceptable (see FM Global Property Loss Prevention Data Sheet 7-76, Prevention and Mitigation of Combustible Dust Explosions and Fires, for guidelines). Using prefabricated rupture disks for the rupture membrane will eliminate the need to test or calculate the rupture pressure.
Do not use explosion diverters in air streams that have a significant loading of abrasive dust. Such dust would eventually erode through the pressure relieving diverter cover.
Fast-Acting or Rapid-Action Valve:
An explosion isolation valve such as a fast-acting 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 primary advantage of this isolation method is the certainty of preventing flame propagation to other equipment or processes.
The distance between the valve and the explosion detection device should be far enough to allow the valve to fully close before the arrival of the flame front. The valve should be activated simultaneously with the explosion suppression system (by using pressure detection).
System Advantages and Disadvantages:
Some advantages of explosion suppression systems are:
- They stop an explosion before the developing pressure can damage the process equipment
- They control any ensuing fire and reduce flame front propagation to other process equipment
- They do not vent flame or other material, therefore are useful when toxic and other hazardous materials are being handled, equipment is located indoors, or venting exposes personnel to discharge of pressure and combustion products
- They are maintained in an active condition with continuous electrical supervision of components
Some disadvantages of explosion suppression systems are:
- Design and installation of systems is expensive
- Refilling and resetting of the system after a discharge is expensive
- Maintenance requirements (recommended quarterly inspections/tests and regular visual inspections of detector ports and discharge pipes) are much more involved than for conventional venting systems
- Use in low strength enclosures or equipment is limited because suppressed pressure or even system discharge pressure alone may exceed vessel strength. Vessels therefore may need to be constructed or reinforced to withstand the increased pressure resulting from a suppressed explosion and from the discharge of the suppression system itself.
If you would like further information regarding explosion protection systems for combustible dusts, please contact Risk Logic Inc.
NFPA 68, Standard on Explosion Protection by Deflagration Venting
NFPA 69, Standard on Explosion Prevention Systems
FM Global Property Loss Prevention Data Sheet 7-17, Explosion Protection Systems
FM Global Property Loss Prevention Data Sheet 7-76, Prevention and Mitigation of Combustible Dust Explosions and Fires