Abstract: The light oil metal tank results in the concavity phenomina because of excess negative pressure, caused by the sudden change of air temperature and bad management, which impose on the tank roof a considerable atmosphere pressure. It affects severely the tank sfety. On the basis of experiences during operation years, the article details the causes which make tank in excess of negative pressure, provides examples from tanks at various locations. An example is a vault tank on ground with 5000 m³, numeric values of excess negative pressures and the inspiration amount which is necessary for ballance of inside pressure and outside pressure of tank. The improvement is suggested pointed at the trouble of mechanical inspiration valves. The operation management are emphasized under the condition of high air temperature and the tank with large free space.