Abstract
In this paper, reliability underperformance of welded lipseal is analyzed. A fillet weld located between the channel cover and lipseal on a hydrocracker second stage feed/effluent exchanger developed leaks. The reactor effluent fluid comprises hydrogen, naphtha and diesel above 1,900psig. Before the leaks occurred, there were multiple start-up and shutdown cycles. These cycles created sufficient stress to drive cracking though the fillet weld of the seal. During the failure investigation, some deviations in weld location and size were identified. The stress from subsequent thermal cycles was high enough to exceed the weld strength.
The lip height created a difficult angle for the welder to work. This resulted in less metal being applied in some areas and variation in weld placement. The flange and gasket area were too restricted to use a fillet gauge. Liquid penetrant inspection and hydrotest were not robust enough barriers to prevent the leaks.
Finite element-based fatigue analyses following industry standard API 579-1/ASME FFS-1 was performed. It found that a minimum weld size above what was found in the failure inspection was required to sustain observed fatigue cycles. The finite element model was calibrated using results of an infrared inspection.
Recommendations following investigation include design and fabrication modifications to minimize occurrences of similar events. The importance of considering potential severe cyclic service scenarios in the design is highlighted. Similar seal weld leaks have been reported in the industry.
