Abstract: Large floating roof storage tanks have basically matured after decades of development. But up to now, there is still no standard for the setting principles and calculation method of PVV. In this paper, four kinds of positive and negative pressure working conditions of annular gas phase space are put forward by analyzing the structural characteristics and operation process of floating roof in detail. Through analyzing the damage risk of pressure components under each working condition, the paper arrived at the conclusion that PVV is an indispensable safety accessory of floating roof storage tank. With the help of certain reasoning and assumption, the calculation model of four working conditions is built. Referring to API 2000, the ventilation under various conditions is calculated, and finally the installation number of PVV is determined.

Key words: floating roof, storage tank, edge breathing valve/PVV, necessity, calculation method