Fired heaters materials

J. Use of fired heaters the presence of boilers or furnaces, heated by the combustion of fuels, increases the probability of ignition should a leak of flammable material occur from a process unit. The risk involved will depend on the siting of the fired equipment and the flash point of the process material. The factor to apply is determined with reference to Figure 6 in the Dow Guide. 

Electrical Substation Water treatment plant Cooling tower Air compressors Parking lot Main water pumps Warehouses that contain non-hazardous, nonexplosive, and nonflammable materials Fired heaters All ignition sources... 

The most logical place to begin is to size the furnaces and direct-fired heaters. Often the chemistry of the process has dictated the conditions, but frequently they can be modified in order to conserve energy usage. The burning of waste materials should also be considered as a means of both disposing of unwanted by-products and reducing fuel requirements. For each of these units, the amount of fuel needed per pound of product should be determined. 

The procedure begins with a material factor that is a function only of the type of chemical or chemicals used. This factor is adjusted for general and special process hazards. These adjustments or penalties are based on conditions such as storage above the flash or boiling point, endo- or exothermic reactions, and fired heaters. Credits for various safety systems and procedures are used for estimating the consequences of the hazard, after the fire and explosion index has been determined. 

Fired equipment will have penalties, if the material in process unit near the air intake of fired heater could be released above its boiling point, if the material is a combustible dust, or if the material could be released above its boiling point. The penalty depends on the distance from the possible leak source and it varies between 0.1 and 1.0. For instance the distance of 15 m gives the penalties 0.27 (above the flash point) and 0.60 (above the boiling point). Any situation involving a material processed below its flash point receives no penalty. 

It was noticed that the order of process items in the layout spacing recommendations is almost identical. The furnaces and fired heaters are on the top of the list (see Table 18). The next group is formed by compressors and high hazard reactors. Air coolers, ordinary reactors and high hazard pumps appear next. After that come towers, process drums, heat exchangers and pumps. The last and safest group is formed of equipment handling nonflammable and nontoxic materials. 

Fired heaters are extensively used in the oil and gas industry to process the raw materials into usable products in a variety of processes. Fuel gas is normally used to fire the units which heat process fluids. Control of the burner system is critical in order to avoid firebox explosions and uncontrolled heater fires due to malfunctions and deterioration of the heat transfer tubes. Microprocessor computers are used to manage and control the burner system. 

Process furnace or direct-fired heater Design type, absorbed heat duty, pressure, tube material, capacity... 

If intentional chemistry is performed, chemical reactivity hazards can be expected to exist at your facility. An exception is intentional, essentially complete combustion with air, such as the burning of propane in a gas-fired heater. The burning of ordinary flammable and combustible materials has been excluded from our definition of chemical reactivity hazards and is adequately treated elsewhere. If the answer to Question 5 is YES, then the rest of this section on intentional chemistry need not be considered further, and you should proceed to Question 2 at the beginning of this chapter. 

Solid materials are often dried or heated using combustion gas exhaust from a fired heater as the material is conveyed through a hot combustion gas zone. Solids handling dryers may take a number of forms, e.g., a rotary kiln. Losses involving dryers usually involve internal fires or explosions. 

Figure 8.2 is based on direct-fired heaters, both shop and field-erected, with carbon steel tubes, and includes all direct materials, shop and field labor, and subcontractor overhead and profit. 

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. 

Novatec has developed two systems which can be applied to drying fillers. One is an indirect gas fired heater which can be used with any drier to improve the process economy. About 80% of the heating cost can be saved by the use of these heaters. Novatec also offers a portable drying/conveying system which conveys particulate materials through the drier and delivers them directly to the next process step. 

For fired heaters subject to creep problems, make sure that the tube metal temperature was considered in materials selection, hi the absence of better information, assume the fireside temperature is 100°F (38°C) higher than the process temperature. (If tube-side fouling is anticipated [e.g., coke formation], assume the tube metal temperature is 150°F [85°C] higher than the process temperature.) If necessary, make a note on the template to ensure that creep is accommodated during design of heater tubes, in accordance with API 530 [23]. 

The manufacturing process for fabricating rod-shaped heaters is similar to those used in planar technology, because the active heater pattern is typically printed on an alumina tape that is wound around a pre-fired alumina rod or tube. The alumina allows sintering at low 02 pressure and therefore inexpensive tungsten is the typical heater material. However, leaks and the residual porosity of the materials lead to a continuous increase in electrical resistance, caused by slow tungsten oxidization. 

Decomposition. As column pressure rises, so does column temperature. When the materials distilled are heat-sensitive, decomposition or thermal cracking of the liquid may set in. Decomposition is likely to yield gaseous products which behave as noncondensables. Decomposition will therefore increase the relief requirement and may reduce the credit that can be allowed for cooling. The rate of decomposition may be particularly high upon a heat input controller failure. This problem is most severe when the reboiler is a fired heater or one that has a high temperature difference. 

An unreliable facility will experience increased losses, most of which occur during shutdown and restart for reasons such as flaring of waste gases, recycling of off-spec products, and reduced reactor selectivities. Not only do such situations such as these result in a loss of materials, they also create an increase in energy consumption because some equipment items will have to be cooled down, and then reheated and because items such as pumps, compressors, and fired heaters have to keep operating, even though they are on total recycle. 

---