Spark Detection: Plant’s First Line Of Defense
The National Fire Protection Association defines combustible dust as “A finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations.” (NFPA 654, the Standard for the Prevention of Fire and Dust Explosions, 2013)
The key to preventing a catastrophic event is to install effective prevention technology for detecting all sparks and embers in the incipient stage in the process, and extinguishing or diverting sparks before they ignite the transported material and dust.
Spark detection and extinguishing systems include detectors, control consoles and countermeasures like extinguishment or diverter gates. If a spark occurs, the detector signals the control console, which records the event and triggers programmed countermeasures and interlocks, all within milliseconds. Typically, the control console activates atomized extinguishment for a programmed time to extinguish the hazard without affecting production.
Two general categories of spark detectors are sensors that detect spark energy in the visible and invisible near infrared range, and those that detect heat radiation, called black-body or hot particle detectors.
Standard spark detectors are preferred for most applications to detect visible spark energy in the near infrared (IR) range and are effective for identifying ignition sources in the incipient stage. They detect the infrared energy emitted by a spark, and can detect sparks and embers through material flow. A visible spark in the near infrared range can also be detected at much greater distance than heat.
Black-body heat detectors do not detect sparks or embers until they reach a minimum temperature in close proximity to the sensor. Heat radiation becomes harder to detect the farther the detector is from the source. Systems based only on heat detectors rely upon the theory that only particles above a certain temperature are dangerous and may miss sparks that when combined with proper conditions for combustion farther downstream may still result in a fire or explosion. Because of this, the NFPA specifies in its Standard 664, paragraph A.18.104.22.168: “The spark extinguishing system should activate every time a single spark is detected.” Industry expert, Vahid Ebadat of Chilworth Technology Inc., a firm that investigates explosions, concurs, saying, “…the ‘bottom-line’ response to this question would be a suggestion to consider the above-quoted guidance from NFPA 664, and detect and extinguish every single spark” (see http://pbs.canon-experts.com/2011/08/).
Transitions and low-pressure pneumatic conveying systems with ambient air temperatures use standard IR spark detectors, flush mounted on opposite sides of the duct to detect sparks and embers through the material stream. For dryers or high-pressure conveying, IR spark detectors should be used containing features like the GreCon FM 3/8 that uses stainless steel clad fiber-optic cables to connect to the duct cross-sectional viewing area. Using fiber-optic cables protects the sensor electronics from the radiant heat of the transport or conveyor from a material dryer or other heat source.
Transitions where ambient light is present require a black-body radiation detector similar to the GreCon DLD 1/8 day light sensor. Black-body detectors are best suited for applications where there is ambient light present, such as drop chute transitions onto or from belt conveyors with detectors viewing through the cascading material on opposite sides of the chute. Multiple detectors versus one single detector provide redundancy and greatly reduce the material masking effect.
Never mount a detector with a lens protruding into the material flow. Depending on the type and size of material, this exposes the lens to abrasion that wears through the lens. These conditions affect the sensor’s ability to function properly and make the system unreliable over the long-term.
A better way to achieve the required visibility, while reducing the exposure of the sensors to wear and tear, is to use sensors with flat lenses and mount them flush on opposite sides of the transport duct where the material flow helps keep the lenses clean. Using IR detectors on either side of a duct has the benefit of ensuring redundant detection from different viewing angles throughout the cross-sectional area of the duct or transition.
A large biomass processing plant will use spark detectors in key locations. The wood pellet mill shown in the accompanying schematic has detectors at the output of the dryer, the hammermill, pellet press and cooler, as well as the conveying systems between each production process, and all dust collection systems. Control systems include atomized water extinguishment systems and fire dumps to remove burning materials. Other countermeasures can be interlocked, including deluge, abort gates, equipment shutdown or programmable logic controller actions.
Advanced multimicroprocessor control consoles can monitor and raise alarms on various hazardous conditions. Other instruments can detect smoke, rate of heat rise, combustion gas and flames. These advanced control consoles can also trigger multiple combinations of countermeasures from multiple detection zones and be set up for complex special configurations, if required.
When evaluating systems, industry professionals recognize Factory Mutual Approved equipment for the extensive testing that ensures it will reliably deliver on its promise. FM Approval certifies compliance with recognized standards.
Awareness is the key to fire and explosion prevention, and understanding spark detection technologies is the key to protecting material transport systems.
Author: Jeffrey C. Nichols
Managing Partner, Industrial Fire Prevention LLC