Standards for fired heater design: An assessment based on computational modelling

Abstract

Fired heaters are used all over the World for refinery processes. Many of them are designed according to American Petroleum Institute (API) Standard 560. Among these, some have suffered significant tubes deformation and other defects related to heat loads in high tube walls. This observation motivated the present investigation into the design parameters used by API standard 560. Key design parameters are related to non-uniformity factors, which represent wall heat flux distribution over the length and circumference of a tube. Since sufficiently detailed measurements in operating units are almost impossible, numerical flow simulations were used instead. 3D simulations of combustion and heat transfer in the radiant chamber of a vertical cylindrical fired heater were carried out. The results are reported in the form used by furnace designers, i.e. in terms of circumferential and longitudinal heat flux factor. Additionally, this paper reports 2D parametric analyses of the effect of commonly encountered types of tube deformations on heat flux uniformity. In this manner, the research presented here provides important information on the effect, which tubes deformations may have on heat flux non-uniformity over the lifespan of a furnace. In conclusion, we found that standard 1D design calculations of fired heaters are significantly underestimating real heat flux variability.