Analysis, design and construction procedures of refinery installation similar to other engineering sciences are going to improve and many different parameters such as environmental and maintenance situation may damage the process activities. In this case the vulnerability of refinery equipments that are designed according to the existing codes is very important. Vertical pressure vessels in the refineries are the most important equipments that are used for optimum refining of the crude oil. In the refineries, equipment such as vertical pressure vessels consists of vertical vessel which is surrounded by different pipes, and seated on the concrete frame and foundation. Since the vertical pressure vessels have special geometries and should have resistance against earthquake, therefore special computer calculation and software such as SAP2000 program is used and with the aids of final results, the damage mechanism of the vertical pressure vessels can be predicted. The most important of failure modes of vertical pressure vessels in the past earthquakes are failure due to supporting structures, overturning, incomplete stability, insufficient of earth connections, insufficient of bolts at the connection to the foundation and weakness in foundation and soil beneath it. In order to evaluate the seismic vulnerability of vertical pressure vessels, static analysis, modal analysis and spectrum analysis are used. For determining the real behavior of vertical pressure vessels, seven real vertical pressure vessels in one of refineries (V802, V657, V639, V630, V151, V102 and V101) with their different temperature and pressure is considered. For modeling the body of the vertical pressure vessels, shell element and for modeling the supporting structure, beam element are used. The results obtained from static analysis, modal analysis and site spectrum analysis, denotes that the vertical pressure vessels regarding to displacement, sliding and overturning are in a good conditions, and regarding to the stresses in the bolts and in the foundation, there are some difficulties that different strengthening methods should be used.
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