Nearly all dusts are combustible, except those known to be inert and having a Kst value of zero (like limestone, fume silica and rock dust). If your company is questioning the combustible potential of a dust they are handling, it is strongly recommended to have it tested by a third party company. In conjunction with combustible dust, screw conveyors are mentioned in various National Fire Protection Association (NFPA) standards and codes, but details for their design are – for the most part – lacking.
From a design standpoint, a screw conveyor attached to a dust collector hopper on an explosive dust application becomes a part of that dust collector. If venting or suppression is employed then the screw conveyor must meet the same design criteria as the baghouse. That is, the screw conveyor must meet Pred criteria of the maximum pressure allowed with venting or suppression. This means the flange connection(s) must be tight and not allow for dust leakage. Additionally, any ports or inspection openings need to be latched firmly to avoid opening during an explosion or suppressant discharge. Pressure testing of this assembled conveyor is not required by NFPA. Extended conveyors, or those that extend beyond just the dust collector discharge flange, need to comply with NFPA’s “Covered Conveyors” requirements.
Based on the above, Sly recommends the following guidelines when designing screw conveyors for use under a baghouse hopper with explosive dust.
RECOMMENDED DESIGN GUIDELINES FOR BAGHOUSE SCREW CONVEYORS:
1. Trough thickness - A trough thickness of 10 gauge minimum shall be used. This thickness is a practical, accepted standard appropriate for a strong conveyor.
2. Connection flanges - Flanges can be angle iron construction or formed from the trough material, (assuming trough material is at least 10 gauge). Keep in mind angle iron construction adds significant cost to assembly and is not always appropriate for conveyor design.
3. Bearings - Where possible, bearings should be located external to the screw conveyor. That is, you should eliminate internal hangar bearings where possible. Bearings have the potential to rub together during conveying, which can cause sparks and set fire to combustible dust.
Shorter conveyors will not require bearings at all. If hangar bearings are required due to the conveyor length of 20 feet or more, they should be constructed to minimize the potential for heat generation or sparks by using non-metallic bearing material (e.g. no hard iron bearings).
4. Size - Screw conveyors should be sized to maintain a trough loading less than 30% under maximum loading conditions when hangar bearings are required.
5. Shafts and flights - Shafts and flights should be designed to provide a maximum deflection of ¼” under operating conditions to minimize the potential for sparks due to flight/trough contact.
6. Discharge - The discharge on the screw conveyor should be isolated from downstream equipment to prevent the passage of combustion products, burning gases, or pressure waves. This can be achieved with a rotary airlock that should be designed to comply with the appropriate NFPA requirements for isolation valves.
7. Interlock - Upon the sensing of a deflagration or explosion, the screw conveyor and discharge equipment should be stopped immediately. An interlock is required that will send a signal back to the PLC or DCS to disengage the motor starters for the discharge equipment. This is usually done through a temperature or pressure sensor and transmitter.
8. Grounding - To minimize the potential for static-generated sparks, all equipment should be properly grounded at all flange connections. This includes any covers on extended conveyors.
9. Packing material - A packing gland is a section of pipe that is larger than the shaft and is filled with non-combustible material that prevents the internal dust from blowing out of the trough end where it could ignite, or, if already burning, could trigger a secondary explosion. Packing material used in end flange packing glands should be non-combustible (aramid) where possible. If end bearings are required they should provide maximum sealing against dust intrusion. This can be best accomplished with equipment such as multiple lantern ring seals.
10. Drive motor - NFPA, NEC, and other safety groups have set up certain “area designations” to describe the chemical make-up of areas within a plant. If the area has explosive gas or dust, then the area classification notes that and all electrical devices in that area must meet certain design criteria so that they do not create a combustion/explosion risk to the materials (dust, gas, vapor, etc.) that may be in that area. The screw conveyor drive motor should be designed for the area classification where it is located.
11. Extended screw conveyors - Extended screw conveyors, or conveyors that extend past the collector discharge opening, must have covers that are secured in such a way that they will not release dust. This means they are typically bolted shut. They must also be designed to withstand the design Pred for the vented dust collector it is attached to, and be designed in a way that the screw conveyor loading does not exceed the maximum throughput of the rotary airlock it is connected to.
12. Zero speed switches - Screw conveyors should be provided with zero-speed switches in explosion proof enclosures and/or motor overload switches to shut down the screw conveyor in case of plug or broken screw shaft.
When screw conveyors are used in applications involving combustible materials proper design and precautions are necessary to ensure that the conveyor does not create a potential risk to the system. Following defined engineering procedures will provide this insurance.
For questions or concerns, or to order a customized screw conveyor of your own, give Sly a call at 866-721-3545 and a representative will assist you.