“A mighty flame follows a little spark.” Dante Alighieri
Sparks can cause fires. Good news if you want fire. Bad news if fires, or worse, explosions, are something you want to avoid. So, it should come as no surprise that the National Fire Protection Association takes a dim view of sparks. The Dust Hazard Analysis (DHA) checklist included in Annex F of the most recent edition of NFPA 61 explicitly asks “Are hammer mills and other processing equipment equipped with spark detection?” (Checklist Item 10.1) A quick reading of the combustible dust standards suggests that spark detection is more than just a good idea.
NFPA and Spark Detection
There are several NFPA standards that address dust hazards. These include
- NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities (2017)
- NFPA 69, Standard on Explosion Prevention Systems (2014)
- NFPA 484, Standards for Combustible Metals (2015)
- NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (2017)
- NFPA 652, Standard on the Fundamentals of Combustible Dust (2016)
- NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids (2017)
- NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities (2017)
Many of them, specifically NFPA 61 (§8.9.6), NFPA 652 (§8.10.6), and NFPA 664 (§9.1.2), address spark detection with this requirement: “Where provided, spark/ember detection and extinguishing systems shall be designed, installed, and maintained in accordance with [relevant standards].” NFPA 654 (§10.2.4) also addresses spark detection and goes on to stipulate the responses to spark detection that are either required or allowed.
Interestingly, except for the checklist reference in NFPA 61, which is in an Annex and not the main body of the standard, there is nothing in any of the NFPA standards that come right out and say that spark detection is a requirement. They all take spark detection for granted, however. The many references make it easy to conclude that spark detection is a valuable weapon in the arsenal against dust explosions.
Spark detection is a good idea, but is it the law?
OSHA and Spark Detection
Unlike highly hazardous chemicals, OSHA does not have a specific regulation that addresses combustible dust. Instead, OSHA relies on the housekeeping regulation, 29 CFR 1910.22(a)(1), which states, “All places of employment, passageways, storerooms, service rooms, and walking-working surfaces are kept in a clean, orderly, and sanitary condition,” and for the grain handling industry, on the grain handling facilities standard, 29 CFR 1910.272, Grain Handling Facilities. Neither regulation mentions spark detection. Yet OSHA routinely cites employers for failure to provide spark detection systems. This includes
- New England Wood Pellet, LLC, Jaffrey, NH 17-Apr-2012
OSHA cited them under the general duty clause for the absence of spark detection devices in conveyors, bucket elevators, and ductwork.
- New England Wood Pellet, LLC, Deposit, NY 13-Mar-2013
OSHA cited them under the general duty clause for the lack of spark detection systems in the wood pellet processing system.
- Inferno Wood Pellet, Inc., East Providence, RI. 03-Mar-2014
OSHA cited them under the general duty clause for a lack of spark detection systems.
- US GreenFiber LLC, Wilkes-Barre, PA. 28-Apr-2016
OSHA cited them under the general duty clause for failing to protect air-moving devices and hammer mills with spark/fire detection systems.
- RWS Manufacturing Inc., Queensbury, NY. 11-Jul-2016
OSHA cited them under the general duty clause for an inoperable Flamex spark/ember detection system.
When there is no regulation, OSHA can rely on the General Duty Clause of the Occupational Safety and Health Act, 5(a)(1): “Each employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.”
The mention of spark detection in the NFPA standards and the availability of spark detection devices means that sparks are a recognized hazard and that there is a feasible mean to address them.
How Spark Detection Works
There are two spark detection technologies in use today: light detection and heat detection. Heat detection sensors detect black-body radiation. They detect bodies that are hotter than a threshold temperature. The further the hot particle is from the sensor, the less likely the sensor is to detect it.
Light detection sensors are far more common. They detect infrared radiation. Depending on the wavelength range the sensor detects, they either work only in the dark, which is suitable for closed piping and equipment, or they can work in daylight as well, which is suitable for conveyor systems.
The choice of a spark detection system will depend on the type of process and operating conditions (some sensors do not perform well in hot processes). It will also depend on the details of the installation. Vendor guidance is essential to a successful installation.
Detection is Not Enough
Any safety system must have three subsystems to be of any use: detection, decision, and action. Detection of a problem without the ability to decide to do something about it is useless. Worse, the decision to respond to a problem without the means to respond is hopeless. So, spark detection by itself is both useless and hopeless. There has to be a means to decide and to act.
Most spark detection systems are packages that include both the sensors to detect sparks and the logic solver that takes that detection and converts it into a signal to act. That leaves response.
In many safety systems, the response is an alarm that prompts an operator to respond. In the case of sparks, however, an operator response cannot be fast enough. Even if the operator’s hand remained constantly hovering over the big red button and the operator’s eyes were exclusively focused on the screen monitoring spark detection, they wouldn’t respond fast enough. So, a rapid, automated response is necessary for spark detection to be of any use.
Typically, one of two actions respond to the detection of sparks. One is to extinguish the sparks as soon as they are detected. The other is to isolate or divert the material that may have been ignited by the spark.
Water is the most commonly used extinguishing medium. It is not a deluge system, but a system that atomizes the water as it sprays in immediately downstream of the spark detectors. When water is unacceptable, either because of incompatibility or product quality issues, CO2 or an inert gas is sometimes used.
The other common approach is to use a high-speed abort gates. These are very large damper systems that immediately direct the flow through a duct away from the process, typically dumping the steam on the ground. They not only direct the spark-bearing flow away from the process, they isolate the process from the section where sparks are being generated and detected. When installed downstream of the spark detector, they isolate the downstream process. To isolate the upstream process, a second abort gate must also be installed upstream of the spark detector.
Spark Detection is Going to Be More Common
With the new requirements of the NFPA standards for DHAs, more and more facilities are going to encounter the question, “Are hammer mills and other processing equipment equipped with spark detection?”. This, coupled with OSHA’s aggressive enforcement of spark detection requirements under the general duty clause, means that more and more facilities are going conclude that they need spark detection and response.
Call to Action
Facilities that use processing equipment with an ability to generate sparks need to consider spark detection as one of the tools in their safety toolbox. The explosion pentagon consists of fuel (combustible dust), oxidizer (air), ignition source, confinement, and dispersion. In systems where combustible solids are processed, the process equipment has four of the five sides of pentagon as a given: fuel, oxidizer, confinement, and dispersion. The only control is an obsessive interest in controlling ignition sources. In process equipment where sparks are likely, such as hammer mills, spark detection and response may be essential.