Flare knockout drums just might be the most complex corrosion system yet designed by man.  When installed new, the drum is made of plate which has been butt welded, ground smooth and flush (NACE SP0178 Class A), and fully annealed so that the atomic composition and metallic grain structure of the plate and weldment are all the same.  From a corrosion standpoint, this annealing step is critical since differences in atomic distribution and differences in metallic microstructure can give rise to local anodes and cathodes which then initiate a corrosion cell.  Thus as installed, the corrosion rate of the entire drum interior should be uniform if only one knew what fluids were present.

In operation, a refiner not only struggles to predict exactly what and how much of a corrosive fluid may be present in the drum but dwell time, frequency, concentration, and temperature are all difficult to estimate as well.  This inscrutable system is further complicated by inevitable field repairs during outages.  Unless these spot welds are fully post weld heat treated, the atomic distribution and metallic microstructure of the field plate, heat affected zone, and weldment will all differ from each other and almost certainly give rise to accelerated corrosion through local anodes and cathodes.  To further complicate matters even in the case of autogenous welds, the corrosion resistance of a weld can be dramatically decreased by a multitude of factors not typically considered such as weld cooling rates, energy input, preheat, metal thickness, weld bead size, reheating effects, and even the flux composition.¹ 

Work done during a forced outage or planned turnaround is often not properly post weld heat treated due to a shortage of time.  Belzona coatings are an ideal solution since they isolate the electrolyte from the galvanic junction and thus eliminate the possibility of corrosion.  Essentially twenty mils of a suitably specified and installed coating can eliminate all the concerns discussed above.  Frontier Refining selected Belzona® 1391 (Ceramic HT) for their flare knock out drum liner hoping to get five years of usable life from the coating.  At the five year inspection, the coating was in good enough condition that they scrubbed the planned re-coat and ran it another five years before refreshing the coating.  The metallic substrate after abrasive blast cleaning and removal of the then failing Belzona® 1391 (Ceramic HT) is shown in Figure 1 below.  Note that no significant wall loss was experienced during the usable life of the coating which was nominally ten years in this instance.  Whether it is just the welds or the lower section of the drum, Belzona can prevent corrosion in flare knockout drums at a cost significantly less than either replacement or spot welding and save days or weeks of lost production which vastly outweigh the cost of the entire coating project.

1.  Garverick, Linda. "Designing to Minimize Corrosion." Corrosion in the Petrochemical Industry . Materials Park, OH: ASM International, 1994. pp. 113-160. 1994.

Chris Lucas 
clucas@rumfordgroup.com
937-435-4650
RumfordGroup.com