“Business as usual” can be deadly. For instance, the Hoeganaes metal-powders-manufacturing plant in Gallatin, Tenn., continued to operate while the U.S. Chemical Safety and Hazard Investigation Board (CSB) investigated a fatal 2011 accident due to a flash fire from combustible dust; two subsequent incidents within months killed three more workers. Besides issuing a report, the CSB released a video, in which the then-chair of the CSB noted that since its founding in 1998, three of the deadliest accidents it has investigated involved combustible dust explosions.
Previous columns have covered how to determine the hazard (“Get Fired Up About Combustible Dust,” and the effect of particle properties (“Prevent Dust Explosions During Processing”). Now, let’s look at an important way to address dust hazards — choosing the correct electrical area classification (EAC).
There really isn’t much difference between setting up Class 2 (dust) and Class 1 (gases and liquids) zones: identify a source point and draw a circle around it for a Division 1 (always dangerous) area and Division 2 (dangerous when open) area. The radius is 10–20 ft for Div. 1 and 10 ft for Div. 2. Use sources that have the greatest chance of an actual leak: seals, valve stems, sight glasses, threaded connections (which are more likely to leak than flanges and are far more dangerous because of dust falling between threads), hose connections, etc.
An envelope around a source never should be square, except due to a wall or other solid obstruction. EAC drawings begin as plan drawings; always walk-down and, as necessary, correct drawings before using them, whether you’re working with a piping and instrumentation diagram or a plan drawing. With computer-aided-design software, it is best to draw the circle around the source points and then choose the primary source (based on pressure or volume) to trim the Venn circles until the zones are defined.
While dusts and gases/liquids can pose similar threats, the physical characteristics of dust raise particular issues. Solids tend to find a low point but not if fine particles are ejected into air; so, knowing the settling velocity of dry particles is important. Dust can have unique source points: rectangular vents, bellows, shakers, conveyors (screw, bucket, etc.), electrical separators (magnets, precipitators), open containers, mills or grinders, and agglomerators. In addition, dust may settle away from its source. Another unique risk is that purging won’t as easily displace dust as vapor: a dead zone will accumulate dust until it is full, leaving behind a time bomb buried inside equipment.
NFPA-654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, https://bit.ly/3CwfA08, of the National Fire Protection Association, Quincy, Mass., provides several useful drawings for defining a zone. Figure 6.10(a) is for an indoor dust source, unrestricted area, open or semi-enclosed equipment; Class II, Div. 1 is a semi-circle (truncated at the ground) with a radius of 20 ft for Group F or G dusts greater than ⅛-in. thick. Figure 6.10(b) is for open or semi-enclosed equipment for the same dust thickness with a Group E dust; a 30-ft truncated radius for Class 1, Div. 1; there is not a Class II, Div. 2 zone for more dangerous Group E dust. Based on a dust thickness <⅛ in. and for Group F, G dusts, a 10-ft Class II, Div. 2 truncated radius exists around a fully enclosed source; there is no Class I, Div. 1 envelope. Likewise, a fully enclosed indoor source maintained without a visible dust layer is labeled “unclassified.” Figure 6.10(e) shows that a securely enclosed drum or hopper has no envelope whatsoever.
Fully enclosed outdoor sources generally can be unclassified — but be very careful when making that designation. And realize that it doesn’t apply to open or partially enclosed dust sources just because they happen to be outdoors. In this situation, it’s best to consider them as you would indoor sources.
Another common situation is where a source is “walled-off.” As you could guess, classification depends on how secure the wall is and, of course the type of dust: E is more dangerous than F or G. A door opened frequently or is left open extends Class 1, Div. 1. Refer to Figures 6.10(f,g,h).
A good summary is all you need to get started in zone classification.
DIRK WILLARD is a Chemical Processing Contributing Editor. You can email him at [email protected]
