Experimental Investigation of Pressure Drop in Bypass Pneumatic Conveying of Fly Ash

Abstract

Bypass pneumatic conveying systems provide a passive capability to reduce conveying velocity and therefore reduce attrition and abrasion in the process of conveying many fragile and erosive particulate solids. Because of these capabilities, bypass pneumatic conveying systems have been used in coal-fired power stations for removing fly ash for the last couple of decades. In bypass systems, the differential pressure between bypass pipe and main pipe as well as the pipeline pressure drop are two of most significant parameters as differential pressure represents the aeration mechanism within the pipeline while pressure drop is an essential parameter for bypass pneumatic conveying system design. In bypass systems, these two parameters are determined not only by the turbulent mode of the gas solids two-phase flow but also by the bypass configurations. The objective of this study was to experimentally investigate the differential pressure between bypass pipe and main pipe as well as the pressure drop during the bypass pneumatic conveying of fly ash. Pneumatic conveying tests in bypass systems and a conventional pipeline were carried out in this study. The bypass pipeline was a 79 mm diameter main pipe with a 27 mm inner diameter bypass pipe with orifice plate flute arrangement. Fly ash was discharged to the system from the bottom of a positive pressure blow tank. The receiving bin was mounted on load cells for measuring the mass accumulation. In order to monitor real time behavior of the system, pressure transmitters were used to measure the gauge pressure. Differential pressure transmitters were employed in the system for measuring the pressure difference between the bypass pipe and main pipe. Differential pressure results between bypass pipe and main pipe in the process of conveying fly ash showed that the pressure before the orifice plate in the bypass pipe was higher than that in main pipe as a result of orifice plate airflow resistance. Therefore, air came into main pipe and aerated the material continuously. The differential pressure also illustrated that the particulate may go into the bypass pipe as pressure in the bypass pipe after orifice plate is lower than that in main pipe. The pipeline pressure drop results also showed that pressure drop was higher than in the conventional system when using the same operating parameters due to the increase of friction. The influences of bypass configurations on pressure drop of bypass system were also discussed.