Kia Gharib - Optimum Engineering Solutions
Cracking of welds in the blowdown headers for delayed-coking units are typical failures in refineries around the world. Failures of the blowdown headers can result in significant maintenance costs, as well as loss of production. Contributing factors to failure of blowdown headers may include:
- Blowdown header differential temperature in both circumferential and longitudinal directions along the header pipe due to sequential / staggered operating cycle of the coke drums.
- Non-uniform Inside Diameter (ID) of the header pipe along its length, allowing for pooling of hydrocarbon and vapor condensate.
- Rough finishes of welds at the interior of header pipe joints causing low fatigue life expectancy of the welded joints.
- Substandard weld quality and limited NDE of welds resulting in cracks and eventual welds failure.
Openso was contracted by a major refinery to investigate the cracking issues in the girth welds of the blowdown header piping for a newly installed Coker Unit. Openso utilized Infrared Imaging (IR) and thermocouple temperature readings to determine the actual temperature distribution of some specific predefined locations in the header pipe throughout the operating cycle. The numerical analysis and simulation of the Coker header piping was performed using Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA). CFD was utilized to model the process and determine the fluid flow characteristics inside the header pipe. The actual field thermal readings were utilized to adjust the boundary conditions used in the CFD. The thermal distributions resulting from the CFD were implemented in the FEA to determine the structural behavior of the blowdown header pipe. The circumferential and longitudinal thermal gradients obtained from the CFD were similar to the thermal gradients observed in the field.
Utilizing CFD and FEA in conjunction with the field data readings Openso successfully determined the root cause failure of the girth welds and determined the remaining number of cycles for the girth welds that had not yet failed. The analyses indicated that the thickness transition (non-uniform pipe ID) was the governing/significant factor affecting the life expectancy of the welds in the blowdown header pipe. The transition in thickness caused eccentricity at the transition junction and consequently additional local bending in the blowdown header. The lack of uniform thickness in the blowdown header pipe proved to be the principle cause of pooling of hydrocarbon and vapor condensate.
Further modeling and analysis indicated that uniform thickness eliminated the local bending and minimized the accumulation of fluid in the header pipe resulting in significantly lower stresses, and thus increased life expectancy of the welds. After completion of root cause analysis of the blowdown header, Openso conducted numerical analyses to determine a permanent solution for the blowdown header weld cracking. FEA conducted on the existing installed bands and clamps indicated that the existing installed bands and clamps would not give the header its intended design life if installed at each and every weld. Depending on the location of the welds along the blowdown header, a 6” half pipe band manufactured from a 6” SCH 80 pipe and a custom elliptical band were designed such that when installed at appropriate weld locations, they would provide the blowdown header with at least 20 years of life expectancy.