Catalytic Reformer Heaters (CRU)
Addressing Oxidation and Carburization Issues on the External Surfaces of Fired Heater Radiant Tubes
High emissivity coating technologies, used in process heaters in the refining, petrochemical and chemical processing industries for over 40 years, increase process efficiency and extend operating life of radiant tubes.
These benefits lead to fuel savings and/or the elimination of unit limitations to allow production increases and emissions reduction; all with a rapid payback and large return on investment.
Radiant Tube Oxidation Issues in Catalytic Reformer Heaters
Radiant tubes in fired heater applications in oil refineries are typically steel alloy, ASTM A335 P22, P5, or P9 which contain 2.25%, 5% and 9% Cr respectively. Radiant tubes oxidize at operating temperatures and scale will grow continuously on the surface, often reaching 2mm in thickness in higher temperature / high heat flux units.
The layers of oxide scale are highly insulating and represent a significant barrier to conductive heat transfer to the process fluids. During fired heater operation, in order to overcome the insulating effects of the oxide and scale layers, it is necessary to increase the firing rate, to maintain the heat flux through to the process feed inside the tubes.
Tube Scale – Dependent Heater Bottlenecks
Besides the extra cost implications of using more fuel/energy, there are dire consequences on the fired heater operation. Excess firing leads to an increased bridgewall temperature and stack temperature. As firing rate increases, so does the rate of oxidation. The situation eventually reaches a point where operational limits, established by equipment licensors to ensure safe and reliable operation are reached. As a result, an unplanned shutdown is needed to clean the tubes, or the production rate is cut. Neither situation is satisfactory, and both result in significant lost revenues and profits from the sale of finished products.
Radiant Tube Scale
In addition to productivity issues, the oxidation process naturally results in metal loss from the external surfaces of the tubes, leading to tube wall thinning. This can result in the need for the premature replacement of tubes, leading to extra capital cost and expensive time lost during planned shutdowns.
Benefits of High Emissivity Coatings
Cetek’s high emissivity ceramic coating, applied to clean new radiant tubes or cleaned old tubes, will prevent oxidation of the tube surface and maximize the conductive heat transfer property. The result is an ability to run the process at design condition, with no penalty from excessive fuel use. Alternatively, and more interestingly, perhaps, it is possible to fire the unit harder and obtain significant increases in throughput and productivity.
Fired Heater Radiant Tube Carburization Issues
Carburization is the formation of metal carbides in a material as a result of exposure to a carbon containing atmosphere. In some cases, materials are intentionally carburized to impart a high surface hardness and wear resistance. However, in service, carburization can result in the loss of mechanical properties over time in addition to wastage of the material. The corrosion process referred to as “Metal Dusting” is a carburization process which results in surface attack and the formation of a powdery residue. Carburizing environments are commonly found in process piping and furnace tubes that contain an excess of carbon-containing species which includes hydrocarbon gases or carbon monoxide.
In refinery fired heater applications, it is not common to find carburization of external tube surfaces. Typically, excess oxygen levels and flue gas circulation ensure that combustion of gaseous fuels is complete and residual carbon or carbon monoxide levels are negligible, or non-existent.
Where it is the practice to operate fired heaters at low excess oxygen levels, closer to stoichiometric conditions, it has been found that external tube surfaces do exhibit effects of carburization, leading to tube wall thinning and the need for premature replacement.
Cetek Turn-Key Coating Application
Cetek of Integrated Global Services (IGS), is the market leader in the design and application of Ceramic Coatings for process tubes and refractory. Cetek’s high emissivity ceramic coatings are applied to radiant tube external surfaces for an effective barrier to carbon atoms in the flue gases/combustion products. The effect is to stop carburization for the life of the ceramic coating, significantly extending the useful life of radiant tubes in these applications.
In between shutdowns, IGS offers Hot-Tek maintenance, cleaning and repair solutions, applied online without the need to shut down the fired heater.