Abstract: Unloading can hardly be carried out in some unloading racks where the coniditions necessary for top unloading cranes are not available.The paper points out that crane shall be selected reasonably depending on the volatility of the medium to be unloaded, and shall receive siphon test prior to shipment, and that some measures shall be taken to improve crane's leakproofness (sealing ring shall be of self-sealing design so as to compensate its abrasion, or, if "O" ring is used, its compressability shall be guaranteed).For top unloading process, upper header installation is likely to decrease crane's siphon performance, slowing down or even stopping unloading, whereas lower header installation can avoid inter-crane interference, and has lower leakproof requirement, thus being beneficial to successful unloading.In addition, if an air collector with a level indicator and a branch tube running to the vacuum pump is installed on the pipe prior to the pump inlet, the working condition can be greatly improved, vacuity of the header can be increased, and the successful top unloading of any liquid guaranteed.