Conditions Influencing Coke Formation



Hotspots in fired heaters can lead to accelerated rates of coking and tube failure. Two examples show how integrating fireside and oil-side calculations can help identify and correct conditions leading to high localised peak flux rates.



High tube metal temperatures (TMTs) and coking in fired heaters continue to reduce run lengths and in some instances cause tube failure. High TMTs are the result of non-uniform heat flux distribution in the firebox and tube-side conditions that lead to accelerated rates of coke formation. Several factors including the number of burners, burner layout, burner flame length, burner to tube spacing, firebox height to width (L/D) and non-ideal flue gas flow patterns influence heat flux distribution. Moreover, tube-side conditions that affect the rate of coke formation such as tube size, oil mass velocity and oil residence time need to be considered. When local conditions generate high TMT, determining the root cause is essential. Otherwise, the problem cannot be fixed. In some instances, it is more cost effective to improve fireside performance. In others, tube-side changes are needed.

Two cases will illustrate how local conditions can lead to high TMT, high rates of coke formation and short run lengths. In each example, revamp modifications increased run length by reducing heat flux variability or improving tube-side operation.

Hotspots exist in all fired heaters'...


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Tags: CDU, VDU