Corrosion—Fired Heaters

Sulfur is a common contaminate of fuel. For example, pipeline-quality natural gas may contain 20 ppm of hydrogen sulfide and mercaptains. 

Clean refinery fuel gas typically has 100 to 200 ppm of total sulfur. Number 2 furnace oil (at least in the United States) will have perhaps 1000 ppm of sulfur. Heavy industrial fuel oil (No. 6 fuel oil or bunker fuel oil) may contain 6 percent (60,000 ppm) sulfur. About 90 percent of the sulfur in these fuels is converted to sulfur dioxide. However, about 10 percent is converted to sulfur trioxide (SO3). 

As the flue gas cools, it will react with the water formed from the combustion of hydrogen in the fuel to produce sulfuric acid (H2SO4)  

Don t take these temperatures too literally. In practice, the precipitation temperatures for sulfuric acid are higher by 50°F plus. 

The reason is that not all of the flue gas from the convective section of a heater is at a uniform temperature. 

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The petroleum feedstocks that contain sulfur as an impurity are handled under ambient conditions in carbon-steel tanks and pipelines where the corrosion attack by sulfur is less severe. In the desulfurization step, vaporized feedstock is processed at 400°C in the presence of hydrogen sulfide (H2S) and carbonyl sulfide (COS) - both of which are highly corrosive. Stainless Steels (SS) 304, 316 or 321 (for the fired heater) are used as the material of construction for various pieces of process equipment in this section of the plant. Equipment failures occur because of external corrosion and thinning of fired-heater coils and interior deposition of carbon from coking which leads to overheating. Fuel-gas lines, that contain hydrocarbon vapors and H2S, should be constructed of SS 304 and heat traced to avoid condensation88. 

Indirect or direct fired heaters are widely used in the process industries. Heat loss is kept to a minimum by refractory coatings on the furnace wall. Any material in the fuel that is corrosive or forms excess soot has to be avoided. Usually 20-25% excess air is required for fuel oil vs. 5-10% for gaseous fuel, hence the latter is more economic. Energy in the exit flue gas not used to heat water or a product can be recovered by heat exchangers that generate additional steam or preheat the entering air. 

In general, the less heat is applied the greater the cost savings. As heat is applied at production facilities by fuel-gas-fired heaters, any increase in heat is reflected in fuel-gas consumption. The addition of heat also boils lighter hydrocarbon fractions from the crude oil less product at a lower API gravity (defined in the Glossary) results. Addition of heat also accelerates rates of corrosion and increases the likelihood of scale formation on vessel internals, particularly the fire tubes. 

A fired heater is not operated uniformly over the entire run as it eould run light in turndown operation and harder in full capacity and toward the end of run for reaction heaters. To estimate the effects of changing tube wall temperature, corrosion rates, and pressure, a metaUurgic examination can be applied to estimate the remaining life of tubes. Knowing the tube life not only prevents premature tube failure, but also identihes the need for metal upgrade if the operating skin temperature increases over time. 

Tube materials in CRU fired heaters are usually of 2V4Cr 1 Mo specification, although ferritic tubes with higher chromium content, SCrlMo and 9CrlMo are encountered, the last mainly in CCR service. Tube dimension design and corrosion allowance generally follow API RP-530 (various dates). 

An insidious aspect of carburization is its nonuniform nature. Just as for other forms of localized corrosion, it is extremely difficult to predict and model localized carburization damage. As a rule of thumb, carburization problems only occur at temperatures above 815°C, because of unfavorable kinetics at lower temperatures. Carburization is therefore not a common occurrence in most refining operations because of the relatively low tube temperatures of most refinery-fired heaters. 

But if the fins have just become brittle enough to break off by hand, the problem is afterburn. The fins have simply been burned up by secondary ignition. While oxidation is a form of corrosion, we have discussed this problem in our chapter concerning the flue-gas side of fired heaters. (See Chap. 29.)... 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Gas-Fired water heaters are also made more efficient by a variety of designs that increase the recov-ei y efficiency. These can be better flue baffles multiple, smaller-diameter flues submerged combustion chambers and improved combustion chamber geometry. All of these methods increase the heat transfer from the flame and flue gases to the water in the tank. Because natural draft systems rely on the buoyancy of combustion products, there is a limit to the recovery efficiency. If too much heat is removed from the flue gases, the water heater won t vent properly. Another problem, if the flue gases are too cool, is that the water vapor in the combustion products will condense in the venting system. This will lead to corrosion in the chimney and possible safety problems. 

Fresh waters are, in general, less corrosive towards copper than is sea-water, and copper is widely and satisfactorily used for distributing cold and hot waters in domestic and industrial installations . Copper and copper alloys are used for pipes, hot-water cylinders, fire-back boilers, ball floats, ball valves, taps, fittings, heater sheaths, etc. In condensers and heat exchangers using fresh water for cooling, tubes of 70/30 brass or Admiralty brass are usually used, and corrosion is rarely a problem. 

Joints in copper components may be a source of trouble. Copper/zinc brazing alloys may dezincify and consequently give rise to leaks . In some waters, soft solders are preferentially attacked unless in a proper capillary joint. Copper/phosphorus, copper/silver/phosphorus, and silver brazing alloys are normally satisfactory jointing materials. Excessive corrosion of copper is sometimes produced by condensates containing dissolved oxygen and carbon dioxide. Rather severe corrosion sometimes occurs on the fire side of fire-back boilers and on electric heater element sheaths under scales deposited from hard waters . 

ISO 11907-4 1998 Plastics - Smoke generation - Determination of the corrosivity of fire effluents - Part 4 Dynamic decomposition method using a conical radiant heater... 

For corrosion and safety reasons, the condensate recovered from these sources is best not returned to the deaerator for use as boiler feedwater. However, depending on the contaminant, the condensate may be reused for a number of services. Our favorite reuse of such contaminated condensate is as a replacement for velocity steam in the heater-tube passes of a fired furnace. 

The main target of a boiler is to capture the overall energy of the flue. This depends on the total heat exchanger surface, the resistance to corrosion, and the resistance to certain temperatures of raw material, for these reasons, the heat exchanger of a gas- or oil-fired water heater or combi can technically be seen as standard, low temperature, or condensing. 

Figure 22 Schematic diagram of apparatus to determine the corrosivity of fire gases in a static exposure chamber (ISO 1197 2). 1. Ignition source. 2. Test specimen dispenser. 3. Corrosion detector. 4. Thermocouple. 5. Heater.

The critical regions in oil-fired furnaces are the evaporator tubes, steam superheater tubes, air heater, and channel. Evaporator tubes are affected by hot gas corrosion from hydrogen sulfide as the temperature exceeds 280°C. Superheater tubes suffer from sulfate/sulfite corrosion induced by molten sulfates above 620"C. Superheater tube holders undergo vanadic corrosion caused by molten vanadate species in the range of 550 C-600 C. Air heaters are subject to low-temperature corrosion from liquid sulfuric acid at 100°C-140°C. Flue gas channels suffer from acid deposits at the dew point (Balajka 1980). 

Type 321 is similar to 304 but with titanium addition five times the carbon content that reduces carbide precipitation during welding and in 425-815 C service. It has excellent corrosion resistance toward most chemicals and oxidation resistance up to SIS C. Aircraft exhaust manifolds, boiler shells, process equipment, expansion joints, cabin heaters, fire walls, flexible couplings, pressure vessels. 

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