Bucket elevators are one of the most common pieces of equipment in any grain handling or processing facility. They are the most efficient and economical means of elevating and conveying bulk products such as raw grain. They are an essential part of every mill’s flow sheet.
Unfortunately, bucket elevators account for a high percentage of primary explosions in grain processing facilities, approximately 48% between 1986 and 2005, according to data compiled by Robert Schoeff, professor emeritus, Kansas State University.
Dust buildup is a major hazard every processing facility must deal with. Combustible dust testing gives a complete picture of potential hazards. It analyzes many variables of dust, such as:
• Kst value – This is a measure of explosive power relative to other dusts. ST1 or weak explosions are rated at 0-199 bar-meters per second (bar-m/s). ST2 or strong explosions are rated at 200-299 bar-m/s. ST3 or very strong explosions are rated 300 bar-m/s and greater.
This article is based on a presentation by Jim Peters, regional sales manager and grain dust application specialist, Boss Products, LLC, Schertz, TX (210-664-4200), Sept. 16 as part of the IAOM All-District Virtual Conference & Expo. IAOM members who registered for the conference can view the presentation using their provided unique link.
Most grain dusts fall under the ST1 category, but a few reach ST2.
• Pmax – This is the expected maximum pressure for a dust in a confined vessel without explosion venting.
• Pred – This is the expected maximum pressure for a dust in a confined vessel with explosion venting.
Bucket elevators contain all five elements needed to ignite an explosion: fuel, oxygen, ignition source, confinement, and dispersion. Little can be done to remove fuel, oxygen, and dispersion from the normal operation of a bucket elevator. That leaves mills to focus on the areas of ignition and confinement, which are addressed in National Fire Protection Association (NFPA) 61.
The standard requires a dust hazard analysis (DHA) to be performed by January 2022 on equipment such as bucket elevators. Performing a DHA will help mills determine the measures that are and are not needed to operate safely. A standard DHA form is available in NFPA 61 Annex F.
According to NFPA 61, to eliminate ignition sources:
• Each leg shall be provided with a speed sensor device that will cut off the power to the drive motor and actuate an alarm in the event the leg belt slows to 80% (220.127.116.11.5).
• Inside legs shall have monitors at head, tail, and knee pulley bearings that indicate high bearing temperature or vibration detection (18.104.22.168.12.1).
• Inside legs shall have monitors for head, tail, knee pulley alignment, and belt alignment (22.214.171.124.12.2).
• Abnormal conditions shall acuate a visual or an audible and visual alarm requiring corrective action (126.96.36.199.12.3).
• All garners, bins, or other receptacles into which material is spouted directly from legs, and which are not designed with automatic overflow systems, shall be equipped either with devices to shut down equipment or with high-level indicating devices with visual or audible alarms (188.8.131.52.13).
Fortunately, hazard monitoring systems like the one pictured on p. 35 provide all of these functions.
Legs handling raw grain should be located outside if possible. If they must be inside, they should follow one of the following guidelines.
• The leg is located within 10 feet of an outside wall and vented to the outside with ducting strong enough to withstand the pressure of an explosion.
• The leg is vented per NFPA 68, which allows for the operator to choose the venting requirements from a table that considers the Kst value of the dust and design strength of the casing based on the Pred. This might allow for no ventilation panels, if the design of the casing is strong enough and the Kst value is below 100 bar-m/s.
• The leg is protected by an explosion suppression system per NFPA 69. Explosion suppression systems of this type are active systems requiring pressure sensors located at critical points in the head and boot sections. These, in turn, activate cannisters placed in all spouting and exhaust ducting to isolate the flame front. Suppression is performed by completely flooding the vessel with dry chemical. In the case of a single casing, the entire leg is considered one vessel and must have suppression for its entirety. For double casings, the leg casings are considered to be ducts and are isolated similarly to spouting or exhaust ducting.
If you know the Kst value and Pred of your application, you can determine your venting requirements per NFPA 68 by using the chart on p. 36. If you do not know the required information, you will and need an explosion suppression system per NFPA 69.
For newly constructed legs handling raw grain outdoors:
• Explosion vent panels shall be spaced at a maximum of 20 feet along the length of the casing.
• For personnel safety, the first vent at ground level is to be placed between eight and 12 feet.
• The first panel should be placed between one and four feet above the roofline where a leg penetrates a rooftop.
• Vent panels should be located in pairs on the sides of the casing and should be sized to be a minimum of 2/3 of the cross section of the leg casing each for a total of 4/3 of the cross section total.
• Single vent panels on the face of the casing are acceptable, if the side panels would present a hazard to work spaces, such as platforms or ladders, or if there would be interference of proper function due to any structure. The sizing of the single vent should be a minimum of 4/3 of the cross-section of the casing.
• The head section should have a vent panel located in the top surface of the section or on both sides with a deflector to divert the explosion upward and away from personnel on the maintenance platform. The vent size should be five square feet per 100 cubic feet of head section volume. Vents shall deploy when an internal pressure of 3.5 kPa (0.5 – 1 psi) occurs.
• Portions of the leg that are below grade and that pass through ground-level structures do not require venting.
• NFPA 61 gives an option of using a flame-arresting and particulate retention vent system (flameless explosion vent) in accordance with NFPA 68.
Tucker Scharfenberg, managing editor