Pipeline bore

The major influence on material flow rate is that of pipeline bore. If a greater material flow rate is required it can always be achieved by increasing the pipeline bore, generally regardless of the other parameters. In a larger bore pipeline a larger cross-sectional area is available and this usually equates to the capability of conveying more material. 

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The discharge limit of the blow tank used was about 26 tonne/h with the 53 mm bore pipeline and approximately 50tonne/h with the 81 mm bore pipeline. Within this capability of the conveying facility, however, tests were carried out with conveying line pressure drop values of well over two bar and the materials were all capable of being conveyed at solids loading ratios of well over one hundred. A minimum of fifty individual tests were undertaken with every material/pipeline bore combination, in order to draw the various families of curves required. 

Conveying data presented in this form clearly show the capability of pneumatic conveying systems and the inter-relating effects of pressure, material concentration and pipeline bore, as well as air flow rate when designing a system to convey a material at a given flow rate, over a specified distance. Since there is generally a limit on air supply pressure, a compromise has to be made between solids loading ratio and pipeline bore. 

A conveying line inlet air velocity, Ci, will need to be specified for the given material and conveying conditions, and a pipeline bore, d, will also need to be evaluated. By re-arranging the above equations and substituting for constants (including R) and free air conditions it can be shown that ... 

Conveying data for the two materials was obtained from a pipeline 95 m long, of 81 mm bore and included nine long radius 90° bends. The influence of pipeline bore only is investigated, with a material flow rate of 40 tonne/h for the cement and 12 tonne/h for the potassium sulphate considered by way of example. The results are shown in Figure 4.42. 

With a wide range of pipeline bore and air supply pressure combinations capable of achieving a given material flow rate, the obvious question is which bore or air supply... 

Figure 4.42 Influence of pipeline bore on air supply pressure for given parameters.

For the cement the increase in power with increase in pipeline bore can also be explained in terms of velocity profiles, but in this case it is values of conveying line inlet air velocity that are relevant. Since cement is capable of being conveyed in dense phase, the relationship between minimum velocity and solids loading ratio, as shown in Figure 4.41, dictates. In an 81 mm bore pipeline the inlet velocity is only 4.2 m/s, since the solids loading ratio is 109. In the 200 mm bore pipeline the solids loading ratio is reduced to 14 and so the inlet air velocity is 12.0 m/s. 

Since pipeline bore comes in incremental sizes, fine tuning and spare capability need to be considered in terms of reserve pressure available. With first approximation values for pressure and pipeline bore, the available conveying data can be scaled more precisely to take account of differences between pipeline geometries. Conveying air velocities and the solids loading ratio can be evaluated so that differences between air only pressure drop and acceleration pressure drop values can also be taken into account. This is an iterative process, as there are many inter-dependent variables, and so in the initial stages approximations can be made. 

Ideally stepping of the pipeline should be incorporated into the pipeline design process, but this does require an additional level of iteration. In the cases shown in Figure 4.42, lower air supply pressures could have been employed for the smaller bore pipelines to achieve the material flow rates quoted. This would then have resulted in an approximate 30% reduction in power required in both cases in Figure 4.43 for the 80 mm bore pipeline, gradually reducing with increase in pipeline bore, and hence air supply pressure. 

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