TUBE SHEET PROTECTION IN A PROCESS GAS WASTE HEAT BOILER

Abstract

The invention relates to a process gas waste heat boiler (PGWHB) of the single-pass and straight-tube type and having thin flexible tube sheets being provided with a weld overlay or cladding, particularly process gas waste heat boilers for the recovery of heat from a process gas, in which the process gas is synthesis gas exiting a steam reforming unit.

Claims

1. Process gas waste heat boiler (PGWHB) comprising: a shell within which a plurality of straight tubes are arranged, each of the straight tubes being connected at its one end with an inlet tube sheet and with an outlet tube sheet at its opposite end; in which the inlet tube sheet and the outlet tube sheet are provided with a plurality of holes adapted to cooperate with each of said straight tubes for respectively receiving and exiting a process gas, such as a synthesis gas, wherein each of the holes in the inlet tube sheet defines an inlet opening and each of the holes in the outlet tube sheet defines an outlet opening, wherein at least the inlet and outlet tube sheets are provided with a weld overlay or cladding at least at said inlet and outlet opening of each of the holes, and wherein the inlet and outlet tube sheets are thin flexible tube sheets made of low alloy steel and having a thickness including said weld overlay or cladding of 20-70 mm.

2. Process gas waste heat boiler according to claim 1, wherein the thin flexible tube sheet has a thickness of 15-65 mm, for instance 20, 25, 30, 35, 40, 45, 50, 55, 60 mm.

3. Process gas waste heat boiler according to claim 1, wherein the weld overlay or cladding is of thickness 4-10 mm in a metal dusting resistant material selected from: alloy 601, alloy 690, alloy 602 CA, alloy 625, alloy 693 or alloy 699 XA.

4. Process gas waste heat boiler according to claim 1, which is also provided with a tube to tube sheet welding.

5. Process gas waste heat boiler according to claim 4, wherein the inlet part of the tube is welded to the shell side of the inlet tube sheet.

6. Process gas waste heat boiler according to claim 1, wherein each straight tube is provided with an internal liner, such as an internal protective sleeve, which is arranged along the internal surface of each straight tube and extends towards said inlet or outlet tube sheet at a distance from said inlet or outlet opening, so that an end of the internal liner extends to respectively, said one end or said opposite end of straight tube.

7. Process gas waste heat boiler according to claim 6, wherein said internal liner is mechanically connected to said internal surface of each straight tube, such as by expansion by hydraulic pressure and/or by rolling.

8. Process gas waste heat boiler according to claim 1, wherein said plurality of straight tubes are arranged in one or more compartments, which thereby define a plurality of inlet and outer tube sheets.

9. Method for providing a welding of straight tubes to the inlet and outlet tube sheets of the PGWHB of claim 1, comprising: i) providing a weld overlay or cladding such as explosion cladding at least at the inlet and outlet tube sheet of the PGWHB; ii) providing the plurality of holes in the inlet tube sheet and outlet tube sheet; iii) introducing the plurality of straight tubes into the holes for connecting each of the straight tubes at one end with the inlet tube sheet and at the opposite end with the outlet tube sheet; and iv) providing a tube to tube sheet welding at the one end of a straight tube with the inlet tube sheet, and at the opposite end of the straight tube with the outlet tube sheet.

10. Method according to claim 9, wherein step i) is conducted prior to step ii), i.e. prior to e.g. drilling of holes in the tube sheets.

11. Method according to claim 9, wherein the tube to tube sheet welding is provided as multiple pass strength welds optionally with staggered runs, suitably as fully automatic multiple pass strength welds with staggered runs for the inlet tube sheet and the outlet tube sheet.

12. Method according to claim 9, wherein the thin flexible tube sheets are verified by means of finite element analysis in accordance with ASME Section VIII, Division 2.

13. Method according to claim 9, wherein the method is conducted in the PGWHB fabrication step.

14. Method for protecting the PGWHB of claim 1, in particular the inlet and outlet tube sheets thereof, the method comprising: after long-term operation i.e. after at least one year of operation, plugging at least one of said straight tubes by welding to said weld overlay or cladding.

15. Process for cooling a process gas exiting a reforming unit, said reforming unit being any of: autothermal reformer (ATR), secondary reformer, steam methane reformer (SMR) including e-SMR, or convection reformer; said process gas being a synthesis gas comprising carbon oxides and hydrogen, wherein the process comprises cooling the synthesis gas by passing it through the process gas waste heat boiler (PGWHB) according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] FIG. 1 shows a process gas waste heat boiler of the straight tube type arranged horizontally and having an inlet and outlet tube sheets in several compartments. The cross-section A-A is shown in the right-hand side of the figure.

[0070] FIG. 2 shows a view of weld overlay or cladding applied on the inlet tube sheet of the process gas waste heat boiler of FIG. 1.

[0071] FIG. 3 shows a close-up view of weld overlay or cladding on an inlet opening of a straight tube in the inlet tube sheet of FIG. 2.

[0072] FIG. 4 shows a close-up view of weld overlay or cladding on an outlet opening of a straight tube in the outlet tube sheet of FIG. 1.

[0073] FIG. 5 shows a close-up view of plugging to the weld overlay or cladding in the inlet and outlet tube sheet of FIGS. 3 and 4.

DETAILED DESCRIPTION

[0074] FIG. 1 shows a PGWHB 10 of the single-pass and straight-tube type, having thin flexible tube sheets, and arranged horizontally. The plurality of straight tubes is arranged in one or more compartments (10, 10), which thereby define a plurality of inlet and outer tube sheets, as e.g. also shown in the figure. Within shell 12 a plurality of straight tubes 14 are arranged, as also depicted in the cross-section A-A on the right-hand side of the PGWHB-although not visible therein, it is understood that there is a plurality of tubes 14 corresponding to the cross-section A-A-.

[0075] FIG. 2 shows a cross-section of the inlet tube sheet 16 of the PGWHB 10, this being a thin flexible tube sheet of thickness e.g. 35 mm and being provided with a plurality of holes, each defining an inlet opening 18, 18. The inlet tube sheet 16 is provided with a weld overlay or cladding 20, 20, 20 e.g. 6 mm thick, at least at the inlet openings 18, 18. The weld overlay or cladding 20 extends away from the inlet opening along the surface of the inlet tube sheet 16.

[0076] FIG. 3 shows a close-up view of weld overlay or cladding on inlet opening 18 of the inlet tube sheet 16 of FIG. 2. As illustrated, one of the plurality of straight tubes 14, here depicted as straight tube 22, is connected at one end to inlet tube sheet 16 by means of a tube to tube sheet welding 24, and further a weld overlay or cladding 20, 20 is provided on the inlet tube sheet 16 at the inlet opening 18, as depicted in the figure. The tube 22 is provided with an internal liner 22, such as an internal protective sleeve, which is arranged along the internal surface of the tube and extends towards the inlet tube sheet 16 at a distance from said inlet opening 18, so that the end 22 of the internal liner 22 extends to the one end of straight tube 22. For instance, an end of the internal liner 22 is provided at a distance from the inlet opening 18 corresponding to the thickness of the inlet tube sheet 16 including the weld overlay or cladding 20, 20. The liner 22 further extends and contacts the tube to tube sheet weld 24. Accordingly, the end of the internal liner is not welded to said respective weld overlay or cladding 20, 20. Rather than being welded to the weld overlay or cladding, the internal liner 22 is mechanically connected to said internal surface of each straight tube 22, for instance by expansion via hydraulic pressure and/or by rolling. The inlet part of the tube is suitably welded 24 to the shell side of the inlet tube sheet at the point of connection as illustrated herein, thereby providing a space to accommodate a thermal protection element such a ceramic insert of the tube sheet and the tube to tube sheet weld. The tube to tube sheet weld 24 is thereby located at a position where it is cooled the most by the boiling water on the shell side and which circulates at about 300 C. (the process gas running inside the tube 22 is e.g. about 100020 C., as it receives a hot synthesis gas from an upstream reforming unit).

[0077] FIG. 4 shows a close-up view of weld overlay or cladding on the outlet tube sheet 30 of the PGWHB, the tube sheet also being a thin flexible tube sheet of thickness e.g. 35 mm and being provided with a plurality of holes, each defining an inlet opening, here illustrated as a single inlet opening 26. The straight tube 22 is connected at this opposite end to outlet tube sheet 30 by means of a tube to tube sheet welding 24, and further, a weld overlay or cladding 28, e.g. 6 mm thick weld overlay, is provided on the outlet tube sheet 30 at the inlet opening 26, as depicted in the figure.

[0078] FIG. 5 shows the straight tube 22 with both inlet 16 and outlet 30 tube sheets, as already described in connection with FIGS. 3 and 4. In case of a leak in the PGWHB 10 after years of service, the tubes are plugged by plugs 32, 32 at the tube sheets 16, 30. In order to carry out this plugging in a long-term reliable manner, plugging by welding is required. The plugging is conducted by welding to the weld overlay or cladding 20, 20 of the inlet tube sheet 16, and/or to the weld overlay or cladding 28 of the outlet tube sheet 30, as illustrated by the corresponding black-areas at the point of contact of for instance plug 32 and weld overlay or cladding 28, thereby avoiding Post Weld Heat Treatment (PWHT) or the use of mechanical plugging, such as the so-called pop-a-plug.