MEASURING TUBE FOR A THROUGHFLOW MEASURING DEVICE, THROUGHFLOW MEASURING DEVICE AND METHOD FOR PRODUCING A MEASURING TUBE

Abstract

A measuring tube for a flow measuring means includes an essentially cylindrical measuring tube section that is composed of tantalum on at least the interior surface. At least one insert composed essentially of a noble metal is inserted into one of two end openings such that the interior surface of tantalum is in direct contact with the noble metal. In this way, hydrogen embrittlement of the measuring tube is very easily reduced. Further disclosed is a flow measuring device having a measuring tube of the invention as well as a method for producing such a measuring tube.

Claims

1-17. (canceled)

18. A measuring tube for a flow measuring device that is embodied for determining and/or monitoring a process variable of a liquid medium, wherein the liquid medium flows through the measuring tube in the determining and/or monitoring of the process variable, the measuring tube comprising: a cylindrical measuring tube section through which a liquid medium can flow and which is composed of tantalum at least on an interior surface, wherein the cylindrical measuring tube section includes two oppositely lying end openings for inflow and outflow of the medium; and at least one insert composed of a noble metal, wherein the at least one insert is inserted into one of the two end openings such that an interior surface of tantalum of the cylindrical measuring tube section is in direct contact with the noble metal.

19. The measuring tube as claimed in claim 18, wherein the first opening and the second opening each widen to a circularly shaped end face for connecting the measuring tube with a process connection.

20. The measuring tube as claimed in claim 19, wherein the at least one insert includes a sleeve-shaped section and a circular flange-shaped section.

21. The measuring tube as claimed in claim 20, wherein the sleeve shaped section of the at least one insert extends from the end opening into the cylindrical measuring tube section and the interior surface of the cylindrical measuring tube section is in direct contact with an exterior surface of the sleeve-shaped section, and wherein the circular flange shaped section bears on the circularly shaped end face of the opening.

22. The measuring tube as claimed in claim 19, wherein the insert is one piece and is formed by a tension compression forming of a piece of sheet metal.

23. The measuring tube as claimed in claim 18, wherein the noble metal of the insert is selected from a group of platinum metals.

24. The measuring tube as claimed in claim 18, wherein the noble metal of the insert has a Vickers hardness that is less than or equal to 200 VH.

25. The measuring tube as claimed in claim 18, wherein the insert is pressed mechanically against the opening which results in a mechanical, non-releasable connection between the cylindrical measuring tube section and the insert.

26. The measuring tube as claimed in claim 25, wherein the mechanical connection between the cylindrical measuring tube section and the at least one insert is embodied as a liquid sealed connection.

27. The measuring tube as claimed in claim 18, wherein the cylindrical measuring tube section has a wall thickness less than 1 mm and an outer diameter less than 25 mm.

28. The measuring tube as claimed in claim 18, wherein the sleeve-shaped section of the insert extending into the cylindrical measuring tube section amounts in its longitudinal direction to at least 0.1 times an outer diameter of the cylindrical measuring tube section.

29. The measuring tube as claimed in claim 18, wherein two inserts are provided, one for each of the two end openings.

30. A flow measuring device embodied for determining and/or monitoring a process variable of a fluid medium, comprising: a measuring tube, wherein the liquid medium flows through the measuring tube in the determining and/or monitoring of the process variable, wherein the measuring tube includes: a cylindrical measuring tube section through which a liquid medium can flow and which is composed of tantalum at least on an interior surface, wherein the cylindrical measuring tube section includes two oppositely lying end openings for inflow and outflow of the medium; and at least one insert composed of a noble metal, wherein the at least one insert is inserted into one of the two end openings such that an interior surface of tantalum of the cylindrical measuring tube section is in direct contact with the noble metal; and a housing, wherein the measuring tube is applied in the housing of the flow measuring device.

31. A method for producing a measuring tube, comprising: providing a cylindrical measuring tube section through which a liquid medium can flow, wherein the cylindrical measuring tube section is composed of tantalum on at least an interior surface, wherein the cylindrical measuring tube section includes a first and a second oppositely lying end openings for inflow and outflow of the medium, wherein the first end opening and the second end opening each widen to a circularly shaped end face for connection of the measuring tube with a process connection; providing at least one insert which is composed of a noble metal; inserting the at least one insert into one of the two end openings such that the interior surface of tantalum is in direct contact with the noble metal; pressing the at least one insert against the opening in such a manner that a mechanically strong, non-releasable connection is formed between the at least one insert and the cylindrical measuring tube section.

32. The method as claimed in claim 31, further comprising: tension compression forming a sleeve or a circular washer of a noble metal whereby the at least one insert with the sleeve-shaped section and the circular flange-shaped section is formed, wherein the tension compression forming occurs before the inserting of the insert into the cylindrical measuring tube section.

33. The method as claimed in claim 32, wherein the at least one insert is mechanically pressed into the cylindrical measuring tube section by a rolling tool whereby a mechanically strong, non-releasable, and liquid sealed connection between the at least one insert and the cylindrical measuring tube section is produced.

34. The method as claimed in claim 33, wherein, before inserting the at least one insert into the cylindrical measuring tube section, an adhesive is applied on the at least one insert such that the at least one insert is secured on the cylindrical measuring tube section by the adhesive before the production of the mechanically strong, pressed connection, and wherein the adhesive is applied on the insert in certain regions, and wherein the adhesive is present at most on a region between the circular flange shaped section of the insert and the circularly shaped end face of the opening.

Description

[0049] The figures of the drawing show as follows:

[0050] FIG. 1 an exploded view of an embodiment of a measuring tube of the invention;

[0051] FIGS. 2a,2b perspective views of an embodiment of a measuring tube of the invention in a housing of a flow measuring device;

[0052] FIGS. 3a-e perspective views of embodiments of the insert of a measuring tube of the invention;

[0053] FIG. 4 a perspective view illustrating production of a measuring tube of the invention.

[0054] FIG. 1 shows the parts of an embodiment of a measuring tube 1 of the invention. This includes a cylindrical measuring tube section 2, which is composed of tantalum (or tantalum with a thin oxide layer) at least on its interior surface IS and which has two openings 21, 22 for the inflow and outflow of a liquid medium. The openings 21, 22 widen to circularly shaped end faces EF, for connecting the measuring tube 1 to a process connection, for instance, by means of a pipe flange.

[0055] The cylindrical measuring tube section 2 of the measuring tube 1 has an outer diameter OD of less than 25 mm and a wall thickness less than 1 mm. For sufficiently small and thin measuring tubes 1, investigations of the applicant have shown hydrogen embrittlement induced brittle fracture to be a problem.

[0056] According to the invention, this is counteracted by inserting an insert 3 into at least one of the openings 21, 22. In the illustrated example of an embodiment, an insert 3,3a is inserted in each openings 21, 22. For the sake of convenience, since insert 3a is the same as insert 3, only insert 3 will be described. Insert 3 is composed essentially of a noble metal NM and protrudes into the region of the cylindrical measuring tube section 2 that, in measurement operation, is flowed through by medium. The exterior surface ES of the insert 3 is, consequently, in direct contact with the interior surface IS, thus the tantalum of the cylindrical measuring tube section 2, which is exposed to an, in given cases, acidic and, therewith, hydrogen embrittlement promoting medium.

[0057] The protruding-in is accomplished in practice by the fact that the insert 3 has a sleeve shaped section 31 and a flange shaped section 32. The sleeve-shaped section 31 amounts in its longitudinal direction LD to at least the 0.1 times the outer diameter OD of the cylindrical measuring tube section 2 and of the measuring tube 1.

[0058] Measuring tube 1 with the cylindrical measuring tube section 2 and the at least one insert 3 is installable, or installed (see FIGS. 2a and 2b), in a housing 5 of a flow measuring device 4.

[0059] A significant advantage of the insert 3 of the invention is that it can be inserted from the opening 21 leading to the pipe flange into the cylindrical measuring tube section 2 without problem. In this way, it is very easy to protect a flow measuring device 4 by inserting the insert 3 into the cylindrical measuring tube section 2 to couple the tantalum on the interior surface IS of the measuring tube 1 with the noble metal NM and, as a result, to reduce the hydrogen embrittlement.

[0060] Especially, an already existing measuring tube installed in a housing 5 of a flow measuring device 4 without the insert of the invention can be retrofitted with an insert of the invention, in order to increase its longterm mechanical durability; see FIG. 2b.

[0061] Preferably, the shape of the insert 3 shown in FIG. 1 is obtained simply by a deformation process, for example, a flanging. FIGS. 3a to 3e show this in greater detail. The insert 3, composed of the sleeve shaped section 31 and the circular flange shaped section 32, is, for example, in a first embodiment, obtained by forming a piece of sheet metal 8 of a noble metal NM, in such case, platinum, in the form of a circular washer 10 (FIG. 3a) by means of a flanging tool 7, see FIG. 3b. The shape of the insert 3 shown in FIG. 3c is obtained by the tension compression forming occurring in the flanging.

[0062] Alternatively, of course, the piece of sheet metal 8 can also be provided in the form of a sleeve 9 as starting shape (FIG. 3d), in order to form from the sleeve 9 insert 3 with the circular flange shaped section 32 and the sleeve shaped section 31. See FIG. 3e.

[0063] In either case, a very cost effective solution for producing an insert 3 of a measuring tube 1 of the invention is provided by using a thin piece of sheet metal 8 in a standard form, such as a circular washer 10 or a sleeve 9 as starting shape for production of the insert 3.

[0064] Preferably, a soft piece of sheet metal 8 having a Vickers hardness of less than or equal to 200 HV is provided, in order to obtain therefrom the one piece insert 3 in a deformation process. This offers, on the one hand, the advantage that the deformation is embodied simply, and only small stresses are introduced into the material in the insert 3.

[0065] On the other hand, using such a soft platinum piece of sheet metal 8, also a liquid sealed connection between insert 3 and the cylindrical measuring tube section 2 can be obtained very easily; see FIG. 4.

[0066] For mechanical connecting of the cylindrical measuring tube section 2 with the insert 3, steps are performed as follows: [0067] inserting the insert 3 into the cylindrical measuring tube section 2; [0068] pressing a rolling tool 6 introduced from the opening 21 against the insert 3, whereby a mechanical and liquid sealed connection between the insert 3 and the cylindrical measuring tube section 2 is produced.

[0069] Because of the resulting mechanical connection, the tantalum of the interior surface IS of the cylindrical measuring tube section 2 and the platinum of the exterior surface ES of the insert 3 are directly in electrically conductive contact with one another. The sealing of the liquid tight connection occurs via the rolling and the resulting pressing of the soft Pt-insert 3, without a supplemental sealing element between the insert 3 and the cylindrical measuring tube section 2 being necessary.

[0070] For improved placement and securement of the insert 3 in the cylindrical measuring tube section 2, in given cases, an adhesive 11, for example, an epoxy adhesive, such as, for instance, a 2K epoxy adhesive, can be used. The adhesive is applied on the insert 3 before the inserting of the insert 3 and the pressing of the insert 3 against the cylindrical measuring tube section 2.

[0071] The adhesive 11 is especially applied at most in certain regions on the flange shaped section 32 of the insert 3, for example, only on an edge region adjoining the outer diameter; see FIG. 4. The adhesive takes up, for example, less than 20% of a contact area between the insert 3 and the cylindrical measuring tube section 2. In this way, upon the subsequent pressing, a liquid sealed, pressed mechanical connection is obtained, in the case of which tantalum is sufficiently in direct contact with platinum, in order to reduce the hydrogen embrittlement. Preferably, at least 1% of the interior surface IS of tantalum otherwise in contact with the medium is placed in contact with the noble metal NM (platinum in this example). In the case of application of two inserts 3,3a, one for each of the two openings 21, 22 for the inflow and outflow, at least a contact area of 180 mm{circumflex over ( )}2 is present between the tantalum and the platinum of the example.

REFERENCE CHARACTERS AND SYMBOLS

[0072] 1 measuring tube [0073] 2 cylindrical measuring tube section [0074] 21 first opening [0075] 22 second opening [0076] 3 insert [0077] 3a insert [0078] 31 sleeve shaped section [0079] 32 flange shaped section [0080] 4 flow measuring device [0081] 5 housing [0082] 6 rolling tool [0083] 7 flanging tool [0084] 8 piece of sheet metal [0085] 9 sleeve [0086] 10 circular washer [0087] 11 adhesive [0088] IS interior surface [0089] NM noble metal [0090] EF end face [0091] ES exterior surface [0092] VH Vickers hardness [0093] WT wall thickness [0094] OD outer diameter [0095] LD longitudinal direction