If high pressure air, or a high vacuum, is used for conveying a material, it would generally be recommended that the pipeline should be stepped to a larger bore part way along its length. This is to cater for the expansion of the air that occurs with decrease in pressure, and so prevents excessively high conveying air velocities towards the end of the pipeline.
Figure 5.3 illustrates the case of a high pressure dilute phase conveying system. The minimum value of conveying air velocity that must be maintained is about 15 m/s and 60 m3/s of free air is available to convey the air. The conveying line inlet air pressure is 4 bar gauge. From Figure 5.3 it will be seen that a 125 mm bore pipeline will be required for these conditions, and the resulting conveying line inlet air velocity will be 16.5 m/s. If a single bore pipeline is used, however, the conveying line exit air velocity will be about 81.5 m/s .
A velocity of 81.5 m/s will cause considerable damage to any conveyed material and very serious wear to the plant if the material is only slightly abrasive. By stepping
the pipeline twice, as shown in Figure 5.3, it will be seen that the velocity profile can be kept within reasonably low limits. The stepping of a pipeline to a larger bore would also be recommended for high vacuum conveying systems and high pressure dense phase conveying. The stepping of a pipeline is only dependent upon conveying air pressure and should be undertaken for any length of pipeline.
The stepping of a pipeline is also likely to lead to a significant improvement in performance of the conveying line. In a programme of tests undertaken by the author, fly ash was conveyed at 20 tonne/h through a 115 m long pipeline of 53 mm bore with a conveying line pressure drop of 2 bar. By stepping the pipeline up to 68 mm bore half way along and then to 81 mm towards the end, 40 tonne/h was achieved with the same air flow rate and 2 bar pressure drop .
In the above programme, one pipe was simply pushed inside the larger pipe and welded to make it airtight. For larger bore pipelines it would always be recommended that a tapered expansion section should be used to join pipeline of different bore. By this means the expansion can be achieved in a more controlled manner and should result in slightly better performance.