An introduction to fouling in fired heaters — Part 2: exterior fouling
An important fouling mechanism on the fireside is corrosion fouling, especially for oil fired equipment.
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The bad actors are compounds of sulphur, vanadium, and sodium. In fired process heaters the main problem is hot fuel ash corrosion that occurs when firing heavy fuel oil.
Hot fuel ash corrosion
Hot fuel ash corrosion is an accelerated form of corrosion in which molten sulphate salts form a film that destroys the normal protective oxide layer. Problems occur with fuel oil that contain sodium and/or vanadium and sulphur (0.6-3.6 wt%). Salts like Na2SO4 form at high temperature and condense on the tubes, causing rapid metal consumption at about 550°C. Reducing conditions exacerbates fuel-ash corrosion. The presence of carbon monoxide and/or unburned carbon and hydrogen sulphide promote the formation of metallic sulphides. Iron sulphide, for example, is less protective than iron oxide. Sulphides tend to be less protective because they are porous and less firmly attached to the steel.
Corrosion rates can be exceedingly high. In boilers, where this type of corrosion is more common, carbon steel wastage rates of about 1/2 inch per year (failure in less than 2,000 hours of operation are documented by the NBBI.
High temperature oxidation
High temperature oxidation is a corrosion process mostly associated with CS or low alloy tubes. These tube materials oxidise at temperatures within creep/rupture limits. Reasons for overheating include flame impingement, overfiring of the heater, or incorrect selection of tube metallurgy for the service. The resulting oxidation layer (“scale”) results in tube wall thinning and can reduce the local absorbed heat flux by half.
- The lower emissivity of the scale reduces the radiant absorbed heat flux
- The thermal conductivity of the scale is of the same order of magnitude as coke and therefore has a comparable effect on overall thermal resistance
- Problems compound exponentially if the oxide layer loses contact with the tube and further reduces any heat transfer by convection, conduction, or radiation. The loss of radiant section efficiency can be substantial, which is particularly an issue in heaters where the primary process is only in the radiant section
Each cycle of scale formation and removal reduces the tube wall thickness until the tube is too thin to contain the fluid pressure and failure occurs. This issue can be detected several ways:
- Visual tube inspection during outage. When inspecting the radiant tubes, metal oxides are sometimes confused with combustion deposits. Since oxide scales are magnetic, they can be distinguished from combustion deposits with a magnet
- IR thermography when the heater is in operation. IR can be used to compose a tube temperature map, where areas of high temperature can be the result of internal/external fouling, an oxidation layer or a combination of these. Areas with oxidation typically are found to have sharp edges revealing that layers have grown and spalled off, leaving a step change in thickness. Areas with internal coking tend not to have sharp transitions
- CFD can model the temperature patterns inside the heater and demonstrate high local fluxes resulting in high metal and film temperature
- Skin thermocouples can detect a fouling problem if they are installed in the correct location. Even then, it is difficult to differentiate between scaling, flame impingement, and internal fouling. Scaling can be a very local problem depending on the cause of high temperatures
Mitigation / prevention
High temperature oxidation can be prevented by reducing hot spots on the coils. Flame interactions that result in impingement and hot spots on tubes can be prevented by changing burner design / layout inside the firebox or with XRG’s latest patented technology, Xceed.
Tube metallurgy can be upgraded to higher chrome and nickel alloys to better withstand high temperature oxidation. While a chromium-rich oxide layer mainly accounts for the corrosion resistance of stainless steels, the addition of nickel improves creep resistance of austenitic steels. Nickel also promotes stability of the protective oxide film and reduces spalling during thermal cycling.
While cleaning tubes during outages is easiest and most effective, severe scaling and fouling may have to be removed during furnace operation. Specialised companies developed methods to clean radiant tubes, as shown in Figure 11. Efficacy is a function of the ability to get a lance within a couple feet of the area to be cleaned. It should not be used where ceramic fibre is the refractory behind the tubes. In addition to online chemical blasting, walnut shell blasting can remove loose scale from the tubes. Neither method is effective at removing scale tightly adhered to the tubes.
Ceramic coatings can be applied to the exterior surface of tubes to delay onset and reduce oxidation rate. For example, Cetek Ceramic Coatings provide a durable, protective, thin-film layer on the surfaces of process tubes which resist oxidation and corrosion of the metal and maintains the tube thermal conductivity coefficient and emissivity close to new tube conditions. Having uniform tube OD surface conditions also makes interpretation of IR thermography easier.
To maximise the heat transfer in the convection bank, tubes are fitted with radial fins up to 1 inch high, spaced at maximum 5 fins per inch. The dense spacing makes the fins great trapping sites for refractory fibre, ash from oil combustion, dust, and sand. Other potential sources for convection fouling are:
- Deposits of silica that volatilised from radiant section walls. This is typically limited to applications with ceramic fibre walls and wall-fired burners.
- Tubes with fin tips that experience high oxidation rates.
- Operating with tube skin temperatures below the sulphuric acid dew point. This creates a damp surface that can catch any fibres or particulate.
Even with a small layer of deposits, heat transfer efficiency is severely reduced.
Convection section fouling manifests itself as high convection pressure drop due to the accumulation of fouling on the fins. The heater can become draft limited when the convection section pressure drop exceeds the draft capacity of the stack, resulting in low firebox oxygen. It also lowers the process crossover temperature and increases the flue gas stack temperature due to the loss of heat transfer in the convection section.
The risks of excessive convection fouling are:
- Low heater efficiency, high fuel consumption
- Running the burners out of oxygen
- Accumulation of unburned hydrocarbons in the firebox
- Increased flue gas temperature which can overheat tube supports, fins, and downstream components not rated for high temperatures
- Heater throughput becomes limited
- Exceeding stack temperature limit
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