Becht Engineering Performs Crack Flaw Analysis of Coker Bottom Flange
During a routine inspection of a Delayed Coker Bottom Flange, a ½” deep, 360-degree crack was found by a refinery client the location of the crack is shown in Figure 1. At the time of discovery, the flange was set to be in service until the next turnaround, so a crack-like flaw analysis was performed to evaluate the flange integrity.
Becht Engineering was contacted to perform a detailed analysis. Once the client provided Becht Engineering with all requested material data, Becht conducted a detailed investigation and analysis and presented the client with a report with conclusions and recommendations.
Becht performed a variety of unique methodological analysis during for this project, some of which included:
- Modeling the Coker Bottom flange, and the connecting flange, gaskets and bolts using axisymmetric elements in ABAQUS finite element software
- Applying a PWHT temperature of 1275oF to the Coker Bottom Flange to determine the residual stress.
- Analyzing the model for bolt pre-load, internal operating pressure, and thermal transient effects from the heat up and quench cycles to determine the stresses on the model
- Using the calculated stresses to evaluate the existing crack to determine the crack is Fit for Service (FFS) using the procedures in API 579-1/ASME FFS-1 Fitness–For-Service standard.
Becht also conducted a Transient Heat Transfer Analysis, a Stress Analysis, and a Crack Stability and Crack Propagation Analysis for the client. After a thorough analysis for the client, Becht provided the following conclusions:
- The existing 360-degree 0.5” deep crack was stable and fit for service.
- Becht found that it would take approximately 8 years for the crack to grow to a depth of 2.5” and become unstable.
- Becht Engineering recommended monitoring the crack after one year to determine the crack depth. Based on the crack growth, future monitoring could be modified if necessary.
- Becht found it unlikely that the crack would propagate into the bolt holes since the axial load on the bolt hole/threads from the bolts should reduce the axial stresses and hence preventing the crack from propagating.
Becht found that if the crack propagates into the bolt holes, the crack should stop once it reaches the bolt hole.
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