Issue link: https://www.e-digitaleditions.com/i/1344205
56 / March 2021 powderbulk.com PBE SUPPLIERS' TIPS What maintenance steps should I take to ensure my vacuum pneumatic conveying system runs efficiently? A typical dilute-phase vacuum conveying system is comprised of a feed device, conveying line, receiver, discharge valve, clean-air line, and an air mover. To ensure efficient system operation, you must maintain good control of the material flow and airflow. In addition, the system design must be kept consistent with the material, conveying distance, and conveying rate desired. Feed devices are responsible for ensuring a consistent material flow into and out of a pneumatic conveying sys- tem. Feedpoint options may vary from probe boxes and vacuum wands to rotary valves and screw conveyors. Probe boxes and vacuum wands typically have adjustable air inlets that are mechanically set during commissioning to sustain a consistent material feed into the system. These initial settings should be phys- ically locked if possible or at least recorded. If the material type, flowability, bulk density, or other appli- cation parameters change, the air inlet will need to be adjusted to ensure the vacuum system achieves the proper conveying rate. Rotary valves and screw conveyors deliver mate- rial based on volumetric displacement. Maintain these devices per the manufacturers' recommendations and keep them free of material buildup, which will affect not only the volumetric displacement but also the con- veying rate. Material bulk density changes will likely necessitate a new rotational speed for the valve or screw to ensure the design conveying rate is met. Inspect discharge devices such as rotary valves, double- dump valves, and isolation valves (butterfly valves, knife gates, gravity flapper gates, and others) to ensure flow-inhibiting material buildup isn't present and that the material flow out of the system is adequate. Consistent airflow in a vacuum pneumatic con- veying system is maintained by keeping the system free of air leaks or line restrictions, which can lead to poor system performance or even plugging. Leaks and restrictions can occur in the conveying line, receiving device, clean-air line, and where accessories such as inline filters are located. Keep receivers such as filters or cyclones leak-free by ensuring that door and flange gaskets are seal- ing properly. If a filter is being used, the differential pressure across the unit should be maintained within the manufacturer's recommendations and the media should be cleaned or replaced as needed. Maintain or replace isolation valve seals to keep the valve airtight, and check rotary valve clearances for wear to ensure excessive leakage isn't occurring. If significantly wide clearances are present, the rotary valve will need to be rebuilt or replaced. Air movers, which are typically positive-displace- ment (PD) blowers, normally use an inlet filter that should be kept clean to minimize its pressure drop. If a PD blower has been exposed to material because of a damaged inlet filter, inspection of the blower's lobe clearances may also be required. David Swift, senior technical engineer, Schenck Process, 800-223-2191 I n a conventional vacuum pneumatic conveying sys- tem, materials impact the conveying line's walls or elbow walls at high speed to change direction. As a result, abrasive materials can wear through the elbow wall, fragile materials can break apart, and heat-sensi- tive materials can build up and clog the elbow. These maintenance issues can mean added costs at your plant. One example is when plastic pellets impact and heat the elbow wall and melt, forming streamers, angel hair, and snakeskins. One maintenance option is replacing conventional sweep elbows, ceramic-backed elbows, and plugged tee elbows with deflection elbows that prevent material from impacting the elbow wall. Deflection elbows, also known as vortex elbows, employ a spherical chamber that protrudes partially beyond the desired 90-degree or 45-degree pathway, causing a ball of material suspended in air to rotate in the chamber. Since the ball of material rotates in the same direction as the airstream that propels it, incom- ing material is cushioned by the ball's rotation and is gently deflected around the bend. As a result, elbow wear, material degradation, melting, and plugging problems are virtually eliminated. Charles Williston, national sales manager, HammerTek, 800-505-9665