Issue link: https://www.e-digitaleditions.com/i/1344205
22 / March 2021 powderbulk.com Modifications to the existing silo vent filter by way of adding an exhaust fan, increasing the filtration area, or doing both are some actions to consider. If there's enough headroom under the silo, another approach is to install a vent hopper with a controlled feed and integral dust filter above the rotary airlock and eliminate the vent line going to the silo's top, as shown in Figure 5. A third option is to consider using a venting eductor to over- come the potential back pressure, as shown in Figure 6. The venting eductor takes the leakage air from the rotary airlock and conveys it to another dust collector in the vicin- ity or to the existing silo's top. In summary, while actual ques- tions and recommendations can provide a wealth of information to people involved in the operation and maintenance of pneumatic conveying systems, don't forget the vast database of past papers, arti- cles, and webinars that Powder and Bulk Engineering (PBE) has available to gain more experience and infor- mation on specific issues. PBE's annual directory of equipment suppliers, engineering companies, In further discussions, you advised that the existing pressure conveying system under the silo uses a rotary airlock as the line charger and is vented to the top of the silo above. In the original sys- tem, the silo had a vent filter with no fan to handle the material trans- fer from bulk bags in a dilute-phase pressure system. With the silo mod- ification, the silo's cylinder height was extended, the roof was raised accordingly, no change was made to the vent filter, and the same fill line was used to connect to the self- unloading truck station, which was a bit farther away than the bulk bag unloading station. What is likely happening is the self-unloading truck system is delivering more actual airflow than the original blower package and is experiencing the exact condition as described in the reply to the leak- ing sugar dust question previously answered. When the silo reaches the point of being under a slightly pos- itive pressure, that puts a constraint on the vent system from the airlock below, which in turn creates havoc with the pressure transfer system. larger unit or installing longer bags in the existing unit, which may rob you of some storage silo volume. Q: We had a small silo that we filled by emptying bulk bags into a small pressure pneumatic system and blowing into that silo. This silo had another small pressure conveying system under- neath that conveyed to our weigh hoppers. We made modifications to increase the silo's height and fill it with self-unloading trucks because we wanted to reduce the labor involved. We now have a problem with the original pres- sure conveying system under the silo, which we did nothing to as part of the silo volume and loading modifications. Did we overlook something? A: Well, saying, "We now have a problem with the original pressure conveying system under the silo, which we did nothing to," means you made no mechanical changes to the pressure conveying system equipment, which I can understand. But you likely have made an unin- tended consequential change to that existing pneumatic conveying system under the modified silo. FIGURE 5 A vent hopper can be installed above a rotary airlock. Vent filter Material inlet Inner baffle Airlock surge hopper Rotation Rotation Pressurized air pocket (vented) FIGURE 6 A venting eductor can be used to relieve the air leakage from the rotary airlock and overcome back pressure by venting to a nearby dust collector or silo top. Solids feed hopper Rotary valve rotation Conveying air Suction Solids Motive air (50-80 psi) Discharge Courtesy of Fox Valve