Centrifugally-cast tube and related method and apparatus for making same

1. Field of the Invention

The present invention relates to furnace equipment for use in petrochemical plants, and more particularly, to improved centrifugally-cast tubes for use in such equipment and to a method and apparatus of making such tubes.

2. Description of the Related Art

It is well known that there are two basic types of furnaces used in petrochemical plants, one being “steam cracker” furnaces, and the other being “steam reformer” furnaces. Steam cracker furnaces are mainly used to make ethylene, and steam reformer furnaces are mainly used to make hydrogen. Both types of furnaces include a number of tubes, generally arranged vertically, that form a continuous flow path, or coil, through the furnace. The flow path or coil includes an inlet and an outlet. In both types of furnaces, a mixture of a hydrocarbon feedstock and steam are fed into the inlet and passed through the tubes. The tubes are exposed to extreme heat generated by burners within the furnace. As the feedstock/steam mixture is passed through the tubes at high temperatures the mixture is gradually broken down such that the resulting product exiting the outlet is ethylene in the case of a steam cracker furnace and hydrogen in the case of a steam reformer furnace.

The petrochemical industry has in the past recognized at least three desirable features in a steam cracker or steam reformer furnace. First, it is important to maximize the heat transfer rate from the furnace burners through the walls of the tubes and into the mixture of hydrocarbons and steam in order to increase the efficiency of the furnace. Second, it is important to make furnace tubes from materials that are resistant to what is known in the metallurgical arts as “creep”. Third, it is important to make furnace tubes so as to be resistant to corrosion, carburization and metal dusting.

With regard to the second important feature, “creep” is basically the gradual elongation of a metal when placed under stress and subjected to high temperatures. Various creep-resistant alloys are known to those of skill in the art. Two main methods have developed within the industry of making furnace tubes with creep-resistant alloys, one being to extrude the tube, and the other being to centrifugally cast the tube. A centrifugally-cast tube is one formed by pouring an alloy in liquid form into a tubular mold that is rotating at a high speed. The alloy is allowed to cool so as to form the centrifugally-cast tube. The internal bore of the tube is then mechanically-machined by boring to achieve the desired inner diameter, resulting in a cylindrical tube having a circular cross section with a generally constant inner and outer diameter. The industry has discovered, however, that centrifugally-cast tubes exhibit superior creep properties in comparison to extruded tubes. In particular, upon inspecting cross-sections of extruded and centrifugally-cast tubes, the industry has discovered that extruded tubes have a very fine grain metallurgical structure, whereas centrifugally-cast tubes have much larger, and columnar, grains. Further, extruded tubes have a lower carbon content when compared to the carbon content of centrifugally-cast tubes. The larger, columnar grains and higher carbon content are what give the centrifugally-cast tubes superior creep properties in comparison to the fine grain microstructure and lower carbon content of extruded tubes.

One approach to achieving two of the above-identified objectives is disclosed in U.S. Pat. No. 6,250,340 (“the \'340 patent”). In particular, the \'340 patent discloses a method of modifying a centrifugally-cast tube by adding a series of longitudinally-disposed fins and valleys in the typically-circular internal bore of the tube. In this manner, the internal surface area of the tube is increased, thereby increasing the heat-transfer rate therethrough. As such, the \'340 patent results in a tube that is resistant to creep (since it is centrifugally cast from a creep-resistant alloy) and has an increased heat transfer rate (by virtue of its modified internal profile). A key drawback to the tube disclosed in the \'340 patent, however, is that it is not resistant to corrosion, carburization or metal dusting. This is because the tube in the \'340 patent is made using an electrochemical machining (ECM) method, as opposed to a mechanical machining process (e.g., the boring process traditionally used to provide the desired diameter in a centrifugally-cast tube). As is known in the art, use of the ECM method results in an electropolished surface and does not provide adequate deformation and/or orientation of the subsurface or material lattice of the inner surface of the tube. In this regard, it is well known that an electropolished surface is not resistant to corrosion, carburization or metal dusting. See, e.g., MATERIALS AND CORROSION, Carburization, Metal Dusting and Carbon Deposition, ISSN 0947-5117, Vol. 49, No. 4/5, April/May 1998, pp. 221-225 and 328-335. These articles compare the effect of machining or any other surface deformation (e.g., grinding, blasting, peening, honing, etc.) to electropolishing and clearly show the advantage of conventional machining over electropolishing on resistance to carburization and metal dusting. An additional drawback to the ECM process is that it results in a tube having an interior surface with an inferior surface roughness and dimensional accuracy when compared to the interior surface that has been prepared by mechanical machining. A still further drawback to the ECM process is that it is more expensive relative to the cost of mechanical machining.

As such, there remains a need in the art for a centrifugally-cast tube, and method and apparatus of making same, that (1) has an increased heat-transfer rate, (2) is resistant to creep, (3) is resistant to corrosion, carburization and metal dusting, (4) has a desirable surface roughness and dimensional accuracy, and (5) is cost-efficient. The present invention has been developed to overcome the foregoing deficiencies and meet the above-described needs.

In one aspect, the present invention may be a centrifugally-cast tube comprising: a tubular body made from a creep-resistant alloy, the body having an exterior surface and an interior surface, the interior surface including a plurality of bosses and a plurality of grooves, and the plurality of bosses and grooves being mechanically machined into the interior surface. Another feature of this aspect of the invention may be that the plurality of bosses and grooves are defined by a plurality of intersecting concave and convex radii. Another feature of this aspect of the invention may be that the plurality of bosses and grooves are formed by a broaching process. Another feature of this aspect of the invention may be that the interior surface is resistant to carburization and metal dusting. Another feature of this aspect of the invention may be that the surface roughness and dimensional accuracy of the interior surface is superior to that of a centrifugally-cast tube wherein the plurality of bosses and grooves are formed by a electrochemical machining process. Another feature of this aspect of the invention may be that the plurality of bosses and grooves form a profile that is at least 5% longer than a circumference of a smallest circle that encompasses the entire resulting profile. Another feature of this aspect of the invention may be that the interior surface has a surface roughness of less than 125 RMS (CLA).

In another aspect, the present invention may include a method of improving a centrifugally-cast tube comprising: passing a first cutting tool having a plurality of first cutting inserts through a bore of the tube so as to mechanically remove a first quantity of material from the bore and to commence the formation of a plurality of grooves and bosses; passing a second cutting tool having at plurality of second cutting inserts through the bore so as to mechanically remove a second quantity of material from the bore and to continue the formation of the plurality of grooves and bosses, the plurality of second cutting inserts having dimensions different than corresponding dimensions of the plurality of first cutting inserts; and continuing to pass additional cutting tools having a plurality of additional cutting inserts through the bore so as to continue to mechanically remove additional quantities of material from the bore until a desired profile of grooves and bosses is achieved, each set of additional cutting tools having dimensions different than corresponding dimensions of the cutting inserts employed in the immediately preceding pass. Another feature of this aspect of the invention may be that the dimensions of each subsequent set of cutting tools are larger than the corresponding dimensions of the cutting tools employed in the immediately preceding pass. Another feature of this aspect of the invention may be that the dimensions incrementally increase at a rate of between 0.05 mm and 0.1 mm per pass.

In another aspect, the present invention may include a method of improving a centrifugally-cast tube comprising forming a plurality of grooves and bosses in an interior surface of the tube by mechanically deforming the interior surface. Another feature of this aspect of the invention may be that the plurality of grooves and bosses are gradually formed by passing a series of cutting tools having incrementally-differing dimensions over the interior surface.