Abstract: Partial artesian discharge is essential to automatic oil supply under temporary loss of power. To realize artesian discharge, there must be a certain height of liquid level, or an adequate level difference between the liquid stored and the outlet of unloading arm. The level difference depends upon flowrate and resistance in pipes and fittings. Taking as an example the 2×3, 000 m³ inner floating roof tanks and oil unloading system in Fengcun, this paper develops a model for artesian discharge system, giving the calculation formula for level difference between liquid and arm outlet. Minimum operation level can be deduced from the formula. Described are the influence of velocity head on the actual flows with same level and the artesian flows for different liquid levels. Analysis shows that, with the flowmeter having sufficient precision, 79% of the oil stored in a full tank can be artesian, so not only can energy be saved, but normal oil supply be guarranteed under temporary loss of power.