how to design screw conveyors on baghouses handling explosive dust
TRANSCRIPT
COMBUSTIBLE DUST AND SCREW CONVEYORS
• Nearly all dusts are combustible, except those known to be insert and having a Kst value of 0
• Screw conveyor attached to a dust collector becomes part of that collector and must meet Pred design criteria
– Must also be able to accommodate explosive dust
• Design details for NFPA compliance are lacking
TROUGH THICKNESS
• Trough thickness of 10 gauge minimum shall be used
• Practical, accepted standard appropriate for a strong conveyor
CONNECTION FLANGES
• Flanges can be iron angle construction or formed from trough material
– Note: angle iron adds significant cost and may not be an appropriate choice
BEARINGS
• Only required when conveyor is 20 feet or longer
• Should be made of non-metallic bearing material to avoid heat generation
• Should be located external to the screw conveyor whenever possible
– Internal bearings can rub together and ignite combustible dust
CONVEYOR SIZE
• Screw conveyors should be sized to maintain a trough loading less than 30% under maximum loading conditions when hangar bearings are required
SHAFTS AND FLIGHTS
• Should be designed to provide a maximum deflection of 1/4” under operating conditions
– Minimizes the potential for sparks due to flight/trough contact
DISCHARGE
• Discharge on the screw conveyor should be isolated from downstream equipment
– Prevent the passage of combustion products, burning gases, or pressure waves
• Use NPFA-approved rotary airlock to achieve this
INTERLOCK
• Install an interlock to detect a deflagration or explosion and send a signal back to the PLC or DCS to stop the screw conveyor immediately
– Usually done through a temperature or pressure sensor and transmitter
GROUNDING
• To minimize the potential for static-generated spark, all equipment should be properly grounded at all flange connections
– Includes any convers on extended conveyors
PACKING MATERIAL
• Use packing glands
– Sections of pipe larger than the shaft and filled with non-combustible material that prevents triggering a secondary explosion
• If end bears are required they should provide maximum sealing against dust intrusion
– Can be accomplished with equipment such as multiple lantern ring seals
DRIVE MOTOR
• As per NFPA, NEC, etc., if a plant area has explosive gas or dust, then all electrical devices in that area must meet certain design criteria to prevent explosions
• Screw conveyor drive motor should be designed according to these classifications
EXTENDED SCREW CONVEYORS
• Extended screw conveyors that extend past the collector discharge opening must have secured covers
– Typically bolted shut
• Must be designed in a way that the screw conveyor loading does not exceed the maximum throughput of the rotary airlock it is connected to
ZERO SPEED SWITCHES
• Zero-speed switches or motor overload switches should be present in explosion proof enclosures
– Shut down the screw conveyor in case of a plug or broken screw shaft
SCREW CONVEYORS AND EXPLOSIVE DUST
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