Norton Straw Consultants, the engineering and management consultancy experts, have announced the completion of a detailed integrity study for a major chemical plant.
The operator had detected an incipient flaw in a weld on the outlet manifold of their hydrogen reformer, which operates at extremely high temperatures. It wasn’t clear what the cause of this flaw was or how it would develop during operation. Although future replacement of the component was clearly necessary, analysis was required to determine when.
This outlet manifold weld is subject to complex stresses. Primary loads are imposed by the weight of the header and other components that are attached to it, and these change as the plant enters operation and heats up. Secondary stresses are caused by thermal gradients across the weld where the flaw was detected, both in steady-state and transient stresses during start-up. Detailed analysis was required to calculate these loads before the life of the flaw could be assessed.
Norton Straw began with a Computational Fluid Dynamics (CFD) model of the hot gas flow through the manifold and the air at various temperatures that surrounded it. Both steady-state and start-up conditions were simulated. These temperatures were then mapped onto a detailed Finite Element (FE) model of the manifold that also represented the structural support provided by the rests beneath the manifold and the connecting pipework. The pipework loads were imposed by a counter-weight system that kept them in tension, and it was found that these loads were asymmetric and so also applied a torque around the weld.
Assessment of the resulting stress field identified high-temperature creep as the principal failure mechanism. Norton Straw had to develop material models for the exotic stainless steel from which the manifold was made, and then use these models in a fitness-for-service assessment to the API 579 standard. It was concluded that the component had sufficient remaining operating life to allow production to continue until the next scheduled shut-down, when it could be replaced.
Dr Simon Rees, Norton Straw Projects Director, commented