Tubesheet protection in process gas waste heat boilers

Abstract

The present invention relates to a process gas waste heat boiler (PGWHB) having thin flexible tube sheets with welded overlay or cladding, of single pass and straight tube type, in particular a process gas waste heat boiler for recovering heat from a process gas, said process gas being synthesis gas leaving a steam reforming unit.

Description

[Technical field]

[0001] The present invention relates to the field of waste heat boilers, in particular to the protection of tubesheets in single pass and straight tube type process gas waste heat boilers with thin flexible tubesheets, in particular for recovering heat from a process gas, said process gas being synthesis gas leaving a steam reforming unit, e.g. an autothermal reformer. [Background technology]

[0002] Process gas waste heat boilers (PGWHBs) are an important piece of equipment that cools the hot synthesis gas that leaves a reforming unit, e.g. an autothermal reformer (ATR), a secondary reformer, a steam methane reformer (SMR) including an electrically heated reformer (e-SMR), a convection reformer including the so-called HTCR (Haldor Topsoe Convection Reformer) and the so-called HTER (Haldor Topsoe Exchange Reformer), or any other type of reforming unit that is typically used to generate synthesis gas for downstream synthesis processes, e.g. for methanol synthesis, ammonia synthesis and hydrogen production. The heat from the process gas, i.e. the synthesis gas leaving the ATR for example, is utilized to generate high quality steam at high pressure for the process and to drive the turbine. The synthesis gas, as is well known in the art, is a hot gas, e.g. 1000-1100 °C, containing carbon oxides and hydrogen when it leaves the ATR, typically at a pressure in the range of 20-70 bar.

[0003] Process gas waste heat boilers (PGWHBs), hereafter also referred to as waste heat boilers for simplicity, are most commonly manufactured as tube-and-shell heat exchangers with thin tubesheets fabricated from low alloy steel. In many applications, these tubesheets are supported by the restraining action of the tubes, and the tubesheets must be flexible to accommodate the differential mechanical and thermal expansion of the tubes and shell, respectively; therefore, PGWHBs are usually referred to as having thin flexible tubesheets.

[0004] In case of leakage (which is a common failure mechanism in waste heat boilers after many years of operation), the tubes must be plugged at the tubesheet. To ensure this plugging in the long term, plugging by welding is necessary. Due to the presence of hydrogen in the process gases, subsequent post-weld heat treatment (PWHT) is mandatory, which is both difficult, expensive and dangerous in terms of the integrity of the whole unit, i.e. the integrity of the PGWHB. Alternatively temper bead welding can be performed, which does not require post-weld heat treatment, but this technique is complex to master and the results are not always predictable and reliable.

[0005] Thus, traditionally, the approach has involved: - Providing PWHT of the entire waste heat boiler. However, as mentioned above, this is difficult, costly and dangerous in terms of the integrity of the entire unit; - provision of mechanical plugging, e.g. the so-called "pop-a-plug", by a third party, i.e. an external supplier. However, this option is only effective if the leak is in the tube. This solution is difficult or practically impossible to apply if the fault is located in the weld between the tube and the tubesheet; - accepting high hardness of the plug weld, which for example entails lower ductility and therefore a higher risk of hydrogen induced cracking at the tube-to-tubesheet weld, and therefore making said weld more susceptible to subsequent leakage;

[0006] Furthermore, the flexible thin tubesheets in the PGWHB are typically refractory lined on the hot side. This refractory lining may crack or separate after extended operation and after such separation, the tubesheet is directly exposed to the process gases which may cause very fast degradation of the tubesheet due to metal dusting corrosion. Metal dusting, as is well known in the art, is a catastrophic form of corrosion that occurs on metallic materials exposed to gases with high carbon activity. Such separation of the refractory may occur undetected and thus cause loss of equipment integrity without any warning.

[0007] US2013199462A1 (Patent Document 1) discloses a steam generator heater tube repair sleeve and a method for repairing the heater tubes of steam generators, in particular those used in nuclear power plants. Said repair sleeve is used to repair damaged weld seams, for example instead of the installation of welded plugs or so-called rewelding. The cited document teaches the installation of a repair sleeve at each damaged tube end. The lower end of the repair sleeve is thus welded to the cladding of the tube plate. Moreover, it is known and implicit that this type of WHB for use in nuclear power plants comprises a thick tube plate, i.e. a thick tube sheet, and heater tubes emerging from said tube plate, as shown in FIG. 1 of the cited document.

[0008] EP 3355022 A1 discloses a synthesis gas loop waste heat boiler (WHB) preferably comprising U-shaped, i.e. U-shaped tubes, which is a pressure device installed in an ammonia production plant immediately downstream of an ammonia converter (ammonia synthesis reactor), typically for operation above 100 bar. The waste heat boiler comprises a tube sheet which is protected against a specific corrosion phenomenon called nitriding. The tube sheet is thick (up to 500 mm) and protected at both ends by an internal protective sleeve welded to each weld overlay, allowing removal and re-installation without post-weld heat treatment. It will be understood that a WHB with U-shaped tubes implies that the WHB comprises a thick tube sheet, since the tube sheet can support itself. This is in contrast to thin flexible tube sheets which cannot support themselves and are therefore not suitable for use in waste heat boilers with U-shaped tubes. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] US2013199462A1 [Patent Document 2] EP3355022A1 Summary of the Invention [Problem to be solved by the invention]

[0010] A general object of the present invention is to be able to extend the life of a single pass straight tube PGWHB having a thin flexible tubesheet.

[0011] More specifically, the object of the present invention is to provide protection for thin flexible tubesheets already in the fabrication step of such PGWHBs, which allows easy plugging by welding without subsequent PWHT.

[0012] Another object of the present invention is to provide a simple method to protect PGWHBs with thin flexible tubesheets of single pass straight tube type against metal dusting corrosion of the tubesheets in case of failure of the refractory lining. [Means for solving the problem]

[0013] These and other objectives are met by the present invention.

[0014] Accordingly, in a first aspect, the present invention provides a process gas waste heat boiler (PGWHB) comprising: a shell within which are disposed a plurality of straight tubes, each of the straight tubes being connected at one end to an inlet tubesheet and at an opposite end to an outlet tubesheet; the inlet tubesheet and the outlet tubesheet comprising a plurality of holes adapted to receive and emit, respectively, a process gas, e.g., synthesis gas, in cooperation with each of the straight tubes; each of the holes in the entry tubesheet defines an entry opening, and each of the holes in the exit tubesheet defines an exit opening; the inlet and outlet tubesheets include a weld overlay or cladding at least at the inlet and outlet openings of each of the holes; and the inlet and outlet tubesheets are thin flexible tubesheets made of low alloy steel and having a thickness of 20 to 70 mm including the weld overlay or cladding. The process gas waste heat boiler (PGWHB).

[0015] This allows plugging by welding, which is easily feasible, without the need for post-weld heat treatment. In case of leakage or failure mechanism of the PGWHB after many years of operation, it becomes possible to plug one or more of the straight tubes (hereinafter also referred to as "tubes") without resorting to post-weld heat treatment, thus providing the equipment owner or plant operator with a significant advantage in terms of reduced plant production downtime, costs and risks. Thus, the tubes can be plugged by welding with an uncomplicated, i.e. simple, welding procedure.

[0016] In one embodiment, the thickness, including the weld overlay or cladding, is 25, 30, 35, 40, 45, 50, 55, 60 or 65 mm. In a particular embodiment, the thickness is between 25 and 35 mm, for example 35 mm.

[0017] As is known in the art, the term "thin flexible tubesheet" refers to a tubesheet that is not self-supporting, for example a thin flexible tubesheet is a stayed surface as described in Section VIII Div. 2-Section 4.9 of the ASME Code.

[0018] As is well known in the art, and for purposes of this application, the term "weld overlay" refers to a welding process for depositing one or more phases of metal having specific properties over a base metal to improve desirable properties not inherent in the base metal or to restore the original dimensions of a component. Typically, a wear-resistant material is applied over the base metal, often also referred to as the base metal.

[0019] As is well known in the art, and for purposes of this application, the term "cladding" refers to the joining of dissimilar metals. Cladding includes explosive cladding, which is also a well-known process, and more specifically, is a solid-state welding process that uses a precision explosion to join two dissimilar metals while retaining the mechanical, electrical and corrosion properties of both.

[0020] In one embodiment, the weld overlay or cladding is 4 to 10 mm thick.

[0021] In one embodiment, the thin flexible tubesheet has a thickness of 15 to 65 mm, for example 20, 25, 30, 35, 40, 45, 50, 55, 60 mm.

[0022] For example, the thin flexible sheet may be 30-45mm and the overlay or cladding may have a thickness of 4-10mm, for example 5, 6, 7, 8, 9mm, or for example, the thin flexible sheet may be 30mm thick and the overlay or cladding may be 5mm thick, thus providing said thickness, including welded overlay or cladding, of 35mm.

[0023] While weld overlays on thick tubesheets, for example in U-tube waste heat boilers, are known, the application of an overlay on a thin flexible tubesheet is not contemplated at all, at least due to the direct and concomitant negative impact on the stiffness and thus the structural integrity of the PGWHB; whereas in a thick tubesheet, for example 100-500 mm thick, or more often 300-700 mm thick, for example 400, 500, 600 mm thick, the weld overlay applied thereto represents only 3-4% or less of the thickness of the tubesheet, in the thin flexible tubesheet of the PGWHB according to the invention, the weld overlay represents a significant proportion of the thickness of the tubesheet. For example, in a weld overlay 6-10 mm thick and a tubesheet 35 mm thick, the weld overlay represents about 15-20% of the thickness (for example 6 / (35+6)×100 about 15%). With regard to the thick tubesheet, the weld overlay of a smaller thickness compared to the tubesheet is ignored when the structural integrity of the tubesheet is analyzed. However, for thin flexible tubesheets this is not possible, since the thickness of the weld overlay constitutes a significant portion of the total tubesheet thickness. As a result, the introduction of a weld overlay or cladding onto the tubesheet essentially reduces the flexibility of the tubesheet, thus forcing that non-trivial considerations and analytical techniques must be applied to ensure that the PGWHB is certifiable with respect to international standards. A thick tubesheet thickness minimizes the effect from the weld overlay or cladding on the overall stiffness of the tubesheet, but for U-shaped tube configurations it is not desirable here, since the tubesheet temperature increases and there is an increased risk of approaching the metal temperature limit for metal dusting (see also further below). Furthermore, a thick tubesheet would result in excessively high tubesheet stresses and / or excessively high stresses in the tube-to-tubesheet joints.

[0024] It will be understood that the thickness of the tubesheet is the thickness measured at the perforated portion of the tubesheet, i.e. at the portion of the tubesheet (entry tubesheet or exit tubesheet) with said holes defining the entry and exit openings of the entry and exit tubesheets, respectively. For example, in accompanying FIG. 2, the thickness is measured at the portion of the metal sheet 16 having the entry openings 18, 18' and the corresponding weld overlays 20', 20''. This corresponds to the portion of the tubesheet having the thinnest thickness.

[0025] It will also be understood that in the feature "said entry and exit tubesheets comprise a weld overlay or cladding at least at said entry and exit openings of each of said holes," the term "at least" means that the weld overlay or cladding may extend away from the entry opening along a surface of the entry or exit tubesheet, such as weld overlay 20 in attached Figure 2.

[0026] As mentioned above, the flexible thin tubesheets in the PGWHB are typically refractory lined on the hot side. This refractory lining may crack or separate after extended operation, and after such separation, may expose the tubesheet directly to the process gases and cause very fast degradation of the tubesheet due to metal dusting corrosion. Such separation of the refractory may occur undetected and thus cause loss of equipment integrity, without any warning.

[0027] More specifically, PGWHBs can be attacked by catastrophic metal dusting if metal temperatures exceed 400-450°C, depending on the gas composition. The tubesheet of a PGWHB is typically protected by a refractory layer to keep temperatures at acceptable levels; however, if the refractory becomes damaged over time, temperatures can increase and cause metal dusting corrosion, ultimately resulting in costly PGWHB replacements and significant downtime for our clients.

[0028] Thus, in an embodiment according to the first aspect of the invention, the weld overlay or cladding is made of a metal dusting resistant material selected from alloy 601, alloy 690, alloy 602CA, alloy 625, alloy 693 or alloy 699XA and is of a thickness of 4 to 10 mm.

[0029] As used herein, alloy 601 is a material having the following composition (by weight): Ni 58-63%, Cr 21-25%, Al 1-1.7%, Mn max 1%.

[0030] As used herein, alloy 690 is a material having the following composition (by weight): Ni > 58, Cr 27-31, Fe 7-11, Cu 0.5, C 0.05, Si 0.5, Mn 0.5, S 0.015.

[0031] As used herein, alloy 602CA is a material having the following composition (by weight): Cr 26.0, Ni (balance), C 0.25, Al 2.40, Ti 0.20, Y 0.12, Zr 0.10, Mn 0.15, Si 0.50, Cu 0.10, Fe 11.00 (all weight percentages are maximum values).

[0032] As used herein, alloy 625 is a material having the following composition (by weight): Cr 20.0-23.0, Ni minimum 58.0, C 0.10, Al 0.40, Ti 0.40, Mn 0.50, Si 0.50, Fe 5.0, P 0.015, S 0.015, Nb+Ta 3.15-4.15, Mo 8.0-10.0 (all weight percentages are maximum values).

[0033] As used herein, alloy 693 is a material having the following composition (by weight): Ni (balance), Cr 27.0-31.0, Fe 2.5-6.0, Al 2.5-4.0, Nb 0.5-2.5, Mn 1.0, Ti 1.0, Cu 0.5 Si 0.5, C 0.15, S 0.01.

[0034] As used herein, alloy 699XA is a material having the following composition (by weight): Ni (balance), Cr 26.0-30.0, Fe 2.5, Al 1.9-3.0, Nb 0.5, Mn 0.5, Ti 0.6, Cu 0.5, Si 0.5, C 0.1, S 0.01, Zr 0.1, N 0.05, P 0.02, B 0.008.

[0035] Thereby, a weld overlay or cladding made of a high nickel type material, such as alloy 601, alloy 690 or alloy 602CA (which are metal dusting resistant materials), is applied over the thin flexible tubesheet through the weld overlay or cladding, so that in addition to facilitating plugging by welding without a subsequent PWHT, the tubesheet simultaneously has the ability to resist cracking or separation of refractories and the adverse consequences of direct process gas exposure, thus providing additional protection of the tubesheet from such aggressive forms of corrosion, metal dusting, while at the same time maintaining the required safety and structural integrity of the PGWHB and allowing the flow of synthesis gas along the tubesheet surface.

[0036] In another embodiment according to the first aspect of the invention, the PGWHB also includes tube-to-tubesheet welds, whereby the end of each straight tube is welded to each tubesheet. In a particular embodiment, the inlet portion of the tube is welded to the shell side of the inlet tubesheet, thus providing space to accommodate the thermal protection element, e.g., the ceramic insert of the tubesheet, and the tube-to-tubesheet weld. This places the tube-to-tubesheet weld in a position that is most cooled by the boiling water on the shell side. For example, the process gas flowing in the tube is about 1000°C, and the boiling water is about 300°C. It will be appreciated that this is at the inlet portion of the tube. In addition, this configuration makes it possible to avoid gaps in the inlet tubesheet that may result in undesirable corrosion, e.g., crevice corrosion. For example, as shown in Figures 3-5, the opposite end of the tube is welded to the outlet tubesheet closer to the process gas side, when thermal protection of the outlet tubesheet is not required due to the lower temperature of the process gas at this end of the tube.

[0037] For the purposes of this application, the term "preferably" in relation to any embodiment of the present invention means "optionally", i.e., any embodiment.

[0038] In one embodiment, each straight tube comprises an internal liner, e.g. an internal protective sleeve, which is arranged along the inner surface of each straight tube and extends away from the inlet or outlet opening towards the inlet or outlet tubesheet, such that an end of the internal liner extends to the one or the opposite end of the straight tube, respectively. Optionally, the liner extends further to contact the weld between the tube and the tubesheet. For example, as shown in the attached FIG. 3, one end of the internal liner is provided away from the inlet opening corresponding to the thickness of the inlet tubesheet including the weld overlay or cladding. Thus, said end of the internal liner is not welded to the respective weld overlay or cladding provided on the inlet and outlet tubesheets. In a particular embodiment, the internal liner is mechanically connected to the inner surface of each straight tube, and thus to the inside of the tube, i.e. by mechanically established contact, e.g. by hydraulic stretching and / or rolling. This allows for easier replacement of the internal liner, since no welding is required.

[0039] Thereby, an inner liner can be provided more simply compared to, for example, the above-mentioned EP 3355022 A1 (wherein an inner protective sleeve is welded at both ends to the respective weld overlay). Thus, in the present invention, each straight tube does not have an inner protective sleeve arranged along the inner surface of each straight tube, the ends of said sleeve being welded to the respective weld overlay. Moreover, in said EP 3355022 A1, the protection (protective sleeve) is provided to prevent corrosion by nitriding of the tubesheet and the welds between the tube and the tubesheet, whereas in the present invention, the inner liner is provided to protect the tubes themselves, and not to protect the tubesheet from metal dusting. It will be understood that EP 3355022 A1 concerns a waste heat boiler of an ammonia synthesis loop, the waste heat boiler comprising a thick tubesheet. The boiler is located downstream of an ammonia converter of the ammonia synthesis loop. In the present invention, the weld overlay or cladding is provided on the thin flexible tubesheet of the front-end process gas waste heat boiler, thus, for example, upstream of the ammonia converter. Thus, the inner liners, such as protective sleeves, are used for entirely different purposes in the two types of boilers.

[0040] In one embodiment, the PGWHB is positioned horizontally, for example as shown in FIG. 1 of the accompanying drawings.

[0041] In one embodiment, the straight tubes are arranged in one or more compartments, which thereby define a plurality of inlet and outlet tubesheets, as also shown, for example, in accompanying Figure 1. The two compartments allow for the use of smaller tubes in the subsequent compartment, thereby increasing the average heat transfer coefficient of the boiler. This reduces the overall required heat transfer area.

[0042] In a second aspect, the present invention relates to a method for providing welding of straight tubes to an entry tubesheet and an exit tubesheet of a PGWHB of the first aspect of the invention, the method comprising, preferably in the following order: i) providing a welded overlay or cladding, e.g., detonation cladding, on at least the inlet and outlet tubesheets of the PGWHB; ii) providing a plurality of holes in the inlet and outlet tubesheets, for example by drilling; iii) introducing, e.g., inserting, a plurality of straight tubes into said holes to connect each of the straight tubes to an entry tubesheet at one end and an exit tubesheet at an opposite end; and iv) providing an entry tubesheet at said one end of the straight tube and an exit tubesheet at said opposite end of the straight tube, and a tube-to-tubesheet weld.

[0043] In one embodiment according to the second aspect of the invention, step i) is performed before step ii), i.e., before drilling holes in the tubesheet, thereby avoiding excessive distortion of the tubesheet due to the application of a weld overlay or cladding.

[0044] In one embodiment according to the second aspect of the invention, the welded overlay cladding is applied at least two times, for example 3, 4, 5 or 6 times, preferably according to a predetermined sequence.

[0045] In an embodiment according to the second aspect of the invention, the tube-to-tubesheet welds are provided as multi-pass load-bearing welds, optionally with staggered runs, preferably as fully automatic multi-pass load-bearing welds with staggered runs for the inlet and outlet tubesheets. This provides space for introducing sufficient thermal protection to the tubesheets by installing insulation such as ceramic inserts as described above, and also ensures that no gaps form between the tubes and the tubesheets, even after long operating times, thus minimizing the risk of e.g. crevice corrosion. The tube-to-tubesheet welds are for example performed by gas tungsten arc welding (GTAW) or other conventional techniques.

[0046] In an embodiment according to the second aspect of the present invention, the thin flexible tubesheet is qualified by finite element analysis in accordance with ASME Section VIII, Division 2.

[0047] At the design stage, PGWHBs with thin flexible tubesheets are analyzed by finite element analysis (FEA) in order to be certified according to international pressure vessel standards, e.g., ASME pressure vessel standards. The tubesheet must be able to accept displacements on the order of 1-5 mm without being overstressed and without exerting unacceptable stresses on the components adjacent to the shell joint, e.g., tubes and tubesheet. As already mentioned, the introduction of a weld overlay or cladding on the tubesheet, having a thickness close to or nearly the same size as the tubesheet, as in the present invention, essentially transforms the tubesheet from a "monometallic" plate to a bimetallic plate, so that non-trivial considerations and analytical techniques must be applied to the latter to ensure that the PGWHB is certifiable with respect to international standards. Such techniques include splitting the stresses in the weld overlay from the stresses in the base material and performing individual evaluations of these two layers. Such evaluations can be performed only by finite element analysis, which is usually a significant deterrent to the use of weld overlays or cladding on thin flexible tubesheets.

[0048] In an embodiment according to the second aspect of the invention, the method is carried out at the PGWHB fabrication stage, i.e. outside the location where the PGWHB will be installed for operation.

[0049] Thus, the implementation of the present invention can be carried out during the construction of a new PGWHB, so that already at the outset it is possible to provide a boiler (PGWHB) that is robust against potential undetected internal refractory failures, as well as ensuring that the plugging of the tubes can be carried out in an uncomplicated and timely manner after many years of operation. Essentially, this means that the typical life expectancy of a PGWHB is significantly increased.

[0050] In a third aspect, the present invention is a method for protecting a PGWHB according to the first aspect of the present invention, in particular its inlet and outlet tubesheets, comprising plugging at least one of said straight tubes by welding to said weld overlay or cladding after an extended period of operation, i.e. after at least one year of operation, for example after 5 or 10 years of operation.

[0051] According to a third aspect of the invention, the method does not include a subsequent post-weld heat treatment (PWHT).

[0052] This allows plugging of the tubes by welding with an uncomplicated welding procedure, avoiding mechanical plugging by solutions supplied by third parties such as "pop-a-plug", which are only effective if the leak is in the tube, and which are difficult or impossible to implement if the fault is located, for example, in the weld between the tube and the tubesheet.

[0053] It will be understood that the term "plugging" as used herein means blocking tube openings with plugs to prevent flow through the tubes. The plugs need only withstand pressure from the shell side. Such plugs must be installed in the inlet tubesheet and in corresponding holes in the outlet tubesheet, for example as shown in attached FIG. 5.

[0054] In a fourth aspect, the present invention is a method for cooling a process gas leaving a reforming unit, said reforming unit being either an autothermal reformer (ATR), a secondary reformer, a steam methane reformer (SMR) including e-SMR, or a convective reformer, and said process gas is a synthesis gas comprising carbon oxides and hydrogen, said method comprising cooling the synthesis gas by passing it through a process gas waste heat boiler (PGWHB) according to the first aspect of the present invention.

[0055] The syngas flows through the tubes of the PGWHB while the boiler feed water (BFW) flows through the shell side, thus generating steam while the syngas is cooled and fed to downstream processing units. For example, the syngas leaves the ATR at 1000-1100°C and pressures ranging from 20-70 bar.

[0056] Any of the embodiments and associated benefits of the first aspect of the invention may be used together with embodiments according to the second or third or fourth aspects of the invention. [Brief description of the drawings]

[0057] Figure 1 shows a straight tube process gas waste heat boiler with inlet and outlet tubesheets in several compartments, arranged horizontally. Cross section A-A' is shown on the right side of the figure.

[0058] FIG. 2 shows a diagram of a weld overlay or cladding applied to the inlet tubesheet of the process gas waste heat boiler of FIG.

[0059] FIG. 3 shows an enlarged view of the weld overlay or cladding at the inlet opening of the straight tube in the inlet tubesheet of FIG.

[0060] FIG. 4 shows an enlarged view of the weld overlay or cladding at the exit opening of the straight tube in the exit tubesheet of FIG.

[0061] FIG. 5 shows an enlarged view of the plugging to weld overlay or cladding at the entry and exit tubesheets of FIGS. 3 and 4.

[0062] Detailed Description FIG. 1 shows a single-pass and straight tube type PGWHB 10 with thin flexible tubesheets arranged horizontally. A number of straight tubes are arranged in one or more compartments (10', 10'') which thereby define a number of inlet and outlet tubesheets, as also shown in the figure. A number of straight tubes 14 are arranged in the shell 12, as also shown in cross section A-A' on the right side of the PGWHB, and it is understood that there are a number of tubes 14 corresponding to cross section A-A', although not visible therein.

[0063] 2 shows a cross section of the entry tubesheet 16 of the PGWHB 10, which is a thin flexible tubesheet, e.g., 35 mm thick, with a plurality of holes each defining an entry opening 18, 18'. The entry tubesheet 16 is provided with a weld overlay or cladding 20, 20', 20'', e.g., 6 mm thick, at least at the entry openings 18, 18'. The weld overlay or cladding 20 extends along the surface of the entry tubesheet 16 away from the entry openings.

[0064] FIG. 3 shows an enlarged view of the weld overlay or cladding at the entry opening 18' of the entry tubesheet 16 of FIG. 2. As shown, one of the straight tubes 14 (here represented as straight tube 22) is connected at one end to the entry tubesheet 16 by a tube-to-tubesheet weld 24, and further, a weld overlay or cladding 20', 20'' is provided on the entry tubesheet 16 at the entry opening 18' as shown. The tube 22 includes an internal liner 22', e.g., an internal protective sleeve, disposed along an inner surface of the tube and extending toward the inner tubesheet 16 at a distance from the entry opening 18' such that an end 22'' of the internal liner 22' extends to said one end of the straight tube 22. For example, one end of the internal liner 22' is provided at a distance from the entry opening 18' corresponding to the thickness of the entry tubesheet 16 including the weld overlay or cladding 20', 20''. The liner 22' extends further to contact the tube-to-tubesheet weld 24. Thus, this end of the inner liner is not welded to the respective weld overlay or cladding 20', 20''. Rather than being welded to the weld overlay or cladding, the inner liner 22' is mechanically connected to the inner surface of each straight tube 22, e.g., by hydraulic stretching and / or rolling. As shown here, the inlet portion of the tube is preferably welded (24) at a connection point to the shell side of the inlet tubesheet, thereby providing space to accommodate thermal protection elements, e.g., ceramic inserts in the tubesheet and the tube-to-tubesheet weld. This places the tube-to-tubesheet weld 24 in a position that is most cooled by boiling water circulating on the shell side at about 300° C. (the process gas flowing inside the tubes 22 is, e.g., about 1000° C., since it receives hot synthesis gas from the upstream reforming unit).

[0065] 4 shows an enlarged view of the weld overlay or cladding on the exit tubesheet 30 of the PGWHB, which is also a thin flexible tubesheet, e.g., 35 mm thick, with a plurality of holes each defining an entry opening, here represented as a single entry opening 26. A straight tube 22 is connected at its opposite end to the exit tubesheet 30 by a tube-to-tubesheet weld 24', and a weld overlay or cladding 28, e.g., a 6 mm thick weld overlay, is provided on the exit tubesheet 30 at the entry opening 26, as shown in the figure.

[0066] FIG. 5 shows the straight tube 22 with both the inlet tubesheet 16 and the outlet tubesheet 30, as already described in connection with FIGS. 3 and 4. In case of leakage in the PGWHB 10 after many years of operation, the tubes are plugged by plugs 32, 32′ at the tubesheets 16, 30. To ensure this plugging in the long term, plugging by welding is necessary. The plugging is performed by welding the inlet tubesheet 16 to the weld overlay or cladding 20′, 20″ and / or the outlet tubesheet 30 to the weld overlay or cladding 28, as shown for example by the corresponding black areas at the contact points of the plugs 32′ and the weld overlay or cladding 28, thereby avoiding the use of post-weld heat treatment (PWHT) or mechanical plugging, for example the so-called pop-a-plug.

Claims

1. A process gas waste heat boiler (PGWHB) (10), comprising: a shell (12) having a plurality of straight tubes (14, 22) disposed therein, each of the straight tubes (22) connected at one end to an inlet tubesheet (16) and at an opposite end to an outlet tubesheet (30); the inlet tubesheet (16) and the outlet tubesheet (30) having a plurality of holes adapted to cooperate with each of the straight tubes (22) to receive and discharge a process gas, e.g., synthesis gas, respectively; Each of the holes in the entry tubesheet (16) defines an entry opening (18, 18') and each of the holes in the exit tubesheet (30) defines an exit opening (26); at least said inlet and outlet tubesheets (16, 30) are provided with a weld overlay or cladding (20, 20', 20'', 28) at least at said inlet and outlet openings (18, 18', 26) of each of said holes; and said inlet and outlet tubesheets (16, 30) are thin flexible tubesheets made of low alloy steel and having a thickness of 20 to 70 mm including said weld overlay or cladding. The process gas waste heat boiler (PGWHB) (10).

2. 2. The process gas waste heat boiler (10) of claim 1, wherein said thin flexible tubesheet has a thickness of 15 to 65 mm, for example 20, 25, 30, 35, 40, 45, 50, 55, 60 mm.

3. 3. The process gas waste heat boiler (10) according to claim 1 or 2, wherein the welded overlay or cladding (20, 20', 20'', 28) is a metal dusting resistant material selected from alloy 601, alloy 690, alloy 602CA, alloy 625, alloy 693 or alloy 699XA, with a thickness of 4 to 10 mm.

4. A process gas waste heat boiler (10) according to any one of claims 1 to 3, also comprising tube-to-tubesheet welds (24, 24').

5. 5. The process gas waste heat boiler (10) of claim 4, wherein the inlet portions of the tubes (22) are welded to the shell side of the inlet tubesheet (16).

6. 6. A process gas waste heat boiler (10) according to any one of claims 1 to 5, wherein each straight tube (22) is provided with an internal liner (22'), e.g. an internal protective sleeve, the internal liner (22') being arranged along an internal surface of each straight tube (22) and extending away from the inlet or outlet openings (18, 18', 26) towards the inlet or outlet tubesheet (16) or (30) such that an end (22'') of the internal liner (22') extends to the one end or the opposite end of the straight tube (22), respectively.

7. 7. The process gas waste heat boiler (10) according to claim 6, wherein the inner liner (22') is mechanically connected to the inner surface of each straight tube (22), for example by hydraulic stretching and / or rolling.

8. 8. The process gas waste heat boiler (10) according to any one of claims 1 to 7, wherein the plurality of straight tubes (14, 22) are arranged within one or more compartments (10', 10'') thereby defining a plurality of inlet and outlet tubesheets.

9. A method for providing welding of straight tubes to entry and exit tubesheets of a PGWHB according to any one of claims 1 to 8, comprising: i) providing a weld overlay or cladding, e.g., detonation cladding, on at least the inlet and outlet tubesheets of the PGWHB; ii) providing a plurality of holes in the inlet and outlet tubesheets; iii) introducing a plurality of straight tubes into said holes to connect each of the straight tubes to an entry tubesheet at one end and an exit tubesheet at an opposite end; and iv) providing an entry tubesheet at said one end of the straight tube and an exit tubesheet at said opposite end of the straight tube, and a tube-to-tubesheet weld.

10. 10. The method of claim 9, wherein step i) is performed before step ii), i.e., before drilling holes in the tubesheet.

11. 11. The method according to claim 9 or 10, wherein the tube-to-tubesheet welds are provided as multi-pass, load-bearing welds, optionally with staggered runs, preferably as fully automatic, multi-pass, load-bearing welds with staggered runs for the entry and exit tubesheets.

12. The method of any one of claims 9 to 11, wherein said thin flexible tubesheet is qualified by finite element analysis in accordance with ASME Section VIII, Division 2.

13. The method according to any one of claims 9 to 12, carried out in a PGWHB fabrication step.

14. A method for protecting a PGWHB according to any one of claims 1 to 8, in particular its inlet and outlet tubesheets, comprising plugging at least one of the straight tubes by welding to the weld overlay or cladding after a long period of operation, i.e. after at least one year of operation.

15. 9. A method for cooling a process gas exiting a reforming unit, the reforming unit being either an autothermal reformer (ATR), a secondary reformer, a steam methane reformer (SMR) including an e-SMR, or a convective reformer, the process gas being a synthesis gas comprising carbon oxides and hydrogen, the method comprising cooling the synthesis gas by passing it through a process gas waste heat boiler (PGWHB) according to any one of claims 1 to 8.

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