Design and application of an active protection system used for large oil tanks

The primary and secondary seal spaces in external floating roof oil tanks are annular oil and gas accumulation spaces which are relatively closed. In the process of oil hanging on the tank wall and oil loading and unloading, a large amount of combustible gas can be generated as the floating roof goes up and down. And the gas may be burnt and even exploded by spark which is resulted from lightning or static electricity. In view of the structural characteristics of large external floating roof tanks, an active safety protection system suitable for large oil tanks was designed by using the closed loop control mode which combines the real-time gas analysis system with the active inerting protection system. This system is mainly composed of control system, monitoring system, nitrogen supply system, pipeline network system and other support systems. And its protection principle is as follows. A set of combustible gas leakage detection and protection system is installed in primary and secondary seal zones of floating roof tanks. When the concentration of combustible gas is above the lower explosive limit, the system will inject inert gas actively into the seal zone to replace the combustible gas. And consequently, the concentration of combustible gas in the seal zone is reduced effectively, and fire or explosion can be avoided. This system is applied in Changqing Oilfield for over three years and its application result is remarkable. It is indicated that this system is stable and reliable.