Going Round The Bend

Bends provide a pneumatic conveying pipeline with considerable flexibility in routing. In transferring a material from point A to point B the pipeline can be routed horizontally, vertically up and vertically down, all in a single pipeline run if necessary, and so cross roads and railways lines and avoid any obstructions on route. This flexibility, however, does come at a ‘cost’ and can additionally result in specific problems with certain materials. Each bend will add to the overall resistance of the pipeline, and hence to the conveying air pressure required to achieve a given material flow rate, or to the material flow rate possible for a given air supply pressure. If the conveyed material is abrasive an ordinary steel bend could fail in a matter of hours. An abrupt change in direction, such as that caused by short radius bends, will also add to the problem of fines generation with friable materials, and ‘angel hairs’ will be generated in long radius bends with many synthetic materials.

Due to the change in direction, impact of particles against bend walls, and general turbulence, there will be a pressure drop across every bend in any pipeline. The major element of pressure drop associated with a bend, however, is that due to the re-acceleration of the particles back to their terminal velocity after exiting the bend. The situation can best be explained by means of a pressure profile in the region of a bend, such as that in Figure 1.1.

mills_fig1.jpg

Figure 1.1 Pressure Drop Elements and Evaluation for Bends

The pressure drop that might be recorded across the bend itself is quite small, and although this technique might be appropriate fir single phase flows around bends, it is inappropriate for gas-solid flows. The particles leaving the bend will be at a lower velocity than that at entry and so they will have to be re-accelerated. The bend was the cause of the problem but the re-acceleration occurs in the straight length of pipeline following the bend, and so it is here that the associated pressure drop occurs, and not in the bend itself.

If pressure transducers are located along the length of the pipeline a steady pressure gradient will be recorded in the straight pipeline approaching the bend. A similar steady pressure gra-dient will also be recorded in the straight length of pipeline after the bend, but only after suf-ficient distance to allow for the particles to re-accelerate. The total pressure drop that can be attributed to the bend is determined in the way indicated in Figure 1.1. Typical data for wheat flour is presented in Figure 1.2.

Figure 1.2 Pressure Profile in Straight Pipeline Either Side of a Bend

Figure 1.2 Pressure Profile in Straight Pipeline Either Side of a Bend

The wheat flour was conveyed at a solids loading ratio of about thirty in a 53 mm bore pipe-line and the conveying line inlet air velocity was about 16 m/s. The bend had a D/d ratio of about 5:1. The pressure profile indicated by the data points clearly shows the pressure drop due to re-acceleration of the particles that occurs in the straight section of pipeline following the bend. It will be noted that the pressure gradient in the straight section of the pipeline prior to the bend was recorded at about 5.6 mbar/m, and as the pressure drop across the bend was assessed at about 0.13 bar the equivalent length of the bend for the given material and con-veying conditions comes to approximately 23 m.

Conveying conditions are also important and pressure drop will increase with increase in both conveying air velocity and solids loading ratio. Bend geometry is an important factor in terms of pressure drop and with bends having a much smaller radius, or lower D/d ratio, a significant increase in pressure drop can be expected, although articles to the contrary have been written. Is it possible that the conveyed material itself could present yet another variable in the problem? The coefficient of restitution is probably the parameter to consider here. Is it likely, therefore, that for materials having a coefficient of restitution higher than that of wheat flour, the pressure drop across the bend could be lower?

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