Abstract: To compensate the insufficiency in conventional monitoring and emergency measures for leaks in spherical tanks for liquefied petroleum gas (LPG), and to monitor and identify minor leaks on flange faces of spherical tank in timely manner, an on-line monitoring system based on detection of minor leaks on flange face is proposed. This paper introduces design objectives, principles, fabrication of the core element - sealing sleeve, and techniques available to determine failure of the sealing sleeve. By using one or more flange face minor leak on-line monitoring systems, it is possible to determine accurate position of minor leak. Since monitoring operations can be performed in initial stages of LPG leaks, relevant operators may take necessary maintenance operations in timely manner to eliminate possibility of more severe leaks. By minimizing potential risks of relevant incidents, the system may prevent major incidents from happening.