Issue link: https://www.e-digitaleditions.com/i/1344205
20 / March 2021 powderbulk.com a dense-phase conveying mode, but your original system design likely was based on a dilute-phase con- veying mode. The solution may be as simple as increasing the gas vol- ume to reduce the material-to-gas ratio or may be complex enough to look at some conveying line changes that introduce supplemen- tal gas along the conveying line. Q: Our conveying line elbows are frequent points of wear, requiring shutdowns to replace the elbows. What options do we have to consider? A: The abrasive nature of a material being conveyed and the velocity profile taking place in the conveying line can combine to be a very aggressive cause of wearing out pipe bends and diverter valves. There are a couple options to consider for mitigating your prob- lem. The first option is to reduce the overall velocity profile in the sys- tem, which is the root cause of the problem. To do this would require analytical modeling of your system to identify if and where the convey- ing line can be stepped in diameter (meaning the pipe diameter is increased) or if just a reduction of • Is the conveying line's inside diameter building up with material? • Is the vent filter at the end of the system operating properly and not experiencing high differential pressures due to a faulty cleaning mechanism or material backing up? Conveying line The conveying line or pipeline is the conduit that connects each piece of equipment together to form a complete system. Q: Our conveying line shakes violently due to broken pipe sup- ports. What caused the supports to break? A: That's a symptom of having too high of a material-to-gas ratio for the type of system installed and relates more to the conveying mode and material type being conveyed. A comprehensive review of the sys- tem design may well be in order. Conveying lines typically oper- ate in one of three different modes: dilute-phase, dense-phase, and two-phase or semi-dense-phase, as shown in Figure 4. The deciding factors for each mode are far beyond the scope of this column, but just know that a conveying line's operating mode is one of the most important aspects of a total system design. The conveying line shaking violently sounds like you're experi- encing some of the charactristics of A: Remember that a PD blower doesn't compress the air flow- ing through it. The back pressure against which your blower needs to operate is the summation of all the individual pressure losses in the conveying line and can include: • Friction loss of the moving gas • Friction loss of the moving material • Energy required to accelerate the material in the gas stream • Pressure drop of valves, elbows, coolers, filters, and other equipment in the air and material's path • Any back pressure at the end of the system In your case, I would start by ensuring that the relief valve is mechanically correct and the relief pressure is correctly set for your specific application. If the relief valve and pressure are okay, then you need to start looking at the list of individual pressure losses to see if something has changed that would've resulted in higher-than- expected pressure drops. Some questions to ask yourself include: • Has the material being con- veyed been changed at all, meaning has it been altered or is a different material type being used? • Have you changed the con- veying line routing to add another pickup or destination point? FIGURE 3 Shown below from left to right is a fan, a PD blower, and a compressor, which are all used for entraining solids in the airflow to convey them to the material destination point. FIGURE 4 Conveying lines operate in one of three kinds of modes (from top to bottom): dilute-phase, dense-phase, and two-phase or semi-dense-phase. Gas flow