Measuring tube for a flow meter, flow meter and method for producing a measuring tube

Description

[0001] The invention relates to a measuring tube for a flowmeter, which flowmeter is designed to determine and / or monitor a process variable of a liquid medium, wherein the liquid medium flows through the measuring tube during the determination and / or monitoring of the process variable. The invention further relates to a flowmeter and a method for producing a measuring tube, in particular a measuring tube of a flowmeter.

[0002] In automation technology, especially in process automation technology, flowmeters are often used as field devices for determining and / or monitoring process variables. Field devices essentially refer to all devices that are used close to the process and provide or process-relevant information. Flowmeters typically have a measuring tube through which a fluid, especially a liquid, medium flows during the determination and / or monitoring of the process variable. The process variable can be determined using the flowmeter.

[0003] A variety of flowmeters are known from the state of the art and described in corresponding publications. These operate according to different measuring principles, including, for example, ultrasonic, thermal, magnetic-inductive, vortex, and Coriolis flowmeters. Typically, at least the flow rate of the medium flowing through the measuring tube is determined. Furthermore, flowmeters can also be used to determine other process variables, such as viscosity and / or density in the case of a Coriolis flowmeter. Such a Coriolis flowmeter with a single measuring tube is described, for example, in DE 10220827 A1 and EP 1502085 B1.

[0004] The measuring tube must be designed to ensure sufficient chemical resistance to the medium flowing through it. Therefore, for highly acidic media, a measuring tube made of tantalum is often used, as tantalum exhibits high acid resistance. This is due to the formation of a protective oxide layer on the surface of tantalum. At the same time, tantalum is a material with high mechanical strength. Documents DE 10 2009 030904 A1 and JP S52 159653 U show prior art measuring tubes made of tantalum for a flowmeter, comprising an insert in an opening of the measuring tube.

[0005] However, so-called hydrogen embrittlement poses a challenge for long-term mechanical resistance in tantalum. This refers to the change in brittleness caused by the penetration and deposition of hydrogen into a material. Hydrogen embrittlement occurs when atomic hydrogen forms on the surface of a material and diffuses into the material faster than it can combine at the surface to form non-diffusible H2 molecules. This results in hydrogen-induced cracking or brittle fracture in the material.

[0006] In the case of a measuring tube made of tantalum that is in contact with an acidic medium, hydrogen embrittlement represents a particularly great challenge, especially in the case of small material thicknesses, e.g., small diameters and / or wall thicknesses of the measuring tube. Investigations by the applicant show that in this case, almost all failures of measuring tubes can be traced back to cracks and / or brittle fracture induced by hydrogen embrittlement.

[0007] Various approaches to preventing or at least reducing hydrogen embrittlement in tantalum have been described in the literature. One possibility is to couple tantalum with a more noble metal. This can reduce the binding of hydrogen ions (H+) to tantalum. This approach is discussed, for example, in the article "A Method for Prevention of Hydrogen Embrittlement of Tantalum in Aqueous Media," CORROSION 1961;17(8):379t-385t. It is observed that, to prevent hydrogen embrittlement, it is sufficient to create a comparatively small contact area between tantalum and the more noble material. This effect is independent of whether pure tantalum or, as in practice, tantalum with an oxide layer is present; see, for example, "Platinum implantation into tantalum for protection against hydrogen embrittlement during corrosion," Nuclear Instruments and Methods in Physics Research Section B, 2012, 272: 441-445.

[0008] The invention is therefore based on the object of designing a measuring tube made of tantalum as simply as possible and as mechanically resistant.

[0009] The problem is solved by a measuring tube for a flow meter and a method for producing a measuring tube for a flow meter.

[0010] With regard to the measuring tube, the object is achieved by a measuring tube for a flow meter, which flow meter is designed for determining and / or monitoring a process variable of a liquid medium, wherein the liquid medium flows through the measuring tube during the determination and / or monitoring of the process variable, comprising: A substantially cylindrical measuring tube section through which a liquid medium can flow, wherein the cylindrical measuring tube section consists of tantalum at least on the inner wall and the cylindrical measuring tube section has two opposite end openings for the inflow and outflow of the medium; At least one insert which consists substantially of a precious metal, wherein the insert is inserted into one of the two end openings such that the inner wall consisting of tantalum is in direct contact with the precious metal.

[0011] According to the invention, an insert is provided which is inserted into the cylindrical measuring tube section from the end opening of the cylindrical measuring tube section. At least at one of the openings (i.e., at the first opening for inflow or the second opening for outflow), there is direct contact between tantalum and the more precious material or the precious metal. This direct contact creates an electrically conductive connection between the two materials, so that they are electrochemically coupled.

[0012] For the purposes of this application, the term "tantalum" encompasses both the pure element and the pure element with an alloy of less than 10% by weight, as well as tantalum (with or without an alloy) with a thin oxide layer applied thereon. As already mentioned above, an oxide layer is always present in practice.

[0013] The major advantage of the solution with the precious metal insert is that it makes it very easy to bring the tantalum into contact with the precious metal. For example, the insert can also be retrofitted to an existing measuring tube. To prevent hydrogen embrittlement, it is sufficient if the contact between the tantalum and the precious metal is limited to one area of ​​the inner wall.

[0014] In one embodiment of the measuring tube, the first opening and the second opening each expand to form a circular end face for connecting the measuring tube to a process connection. The circular end faces are formed, for example, by discs welded to the cylindrical measuring tube section. They therefore form, for example, part of a pipe flange, with which the measuring tube can be connected to a flange of a process connection at the first opening and at the second opening, respectively.

[0015] "Consists essentially of a precious metal" in the context of the invention means that the insert is made of a pure precious metal or a precious metal alloy, with the non-precious metal content being less than 20% by weight. Of course, the insert can also comprise an alloy of two different precious metals.

[0016] In one embodiment of the measuring tube, the insert consists of a sleeve-shaped section and a circular disc-shaped section.

[0017] In one embodiment of the measuring tube, the sleeve-shaped portion of the insert protrudes from the opening into the cylindrical measuring tube portion, and the inner wall of the cylindrical measuring tube portion is in direct contact with the outer wall of the sleeve-shaped portion, with the annular disc-shaped portion being pressed onto the annular end face of the opening. By pressing the annular disc-shaped portion onto the annular end face, the insert is secured to the cylindrical measuring tube portion or the opening.

[0018] In one embodiment of the measuring tube, the insert is made in one piece and is formed from a sheet metal by means of tensile and compressive forming, in particular flanging.

[0019] In the context of this application, "one-piece" means that the insert is made of one piece and as such can be inserted into the cylindrical measuring tube section.

[0020] In particular, the insert is formed, i.e. flanged, using a flaring tool.

[0021] The insert is formed, for example, from a circular sheet, e.g., a disk with a bore, or a sleeve-shaped sheet. The material thickness of the circular sheet or sleeve-shaped sheet is, for example, less than 1 mm, especially less than 0.6 mm.

[0022] In one embodiment of the measuring tube, the precious metal of the insert is selected from the group of platinum metals.

[0023] The platinum group metals comprise the following metals: ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). Platinum is the most precious metal.

[0024] In one embodiment of the measuring tube, the precious metal of the insert has a Vickers hardness (VH) that is less than or equal to 200 VH.

[0025] Such soft precious metals are very easy to form, making tensile-compression forming, for example, as simple as possible. Furthermore, pressing a "soft" insert onto the cylindrical measuring tube section makes it particularly easy to achieve a mechanically stable and fluid-tight connection between the insert and the cylindrical measuring tube section. The hardness of the cylindrical measuring tube section is influenced, for example, by the alloying and / or a final treatment, such as work hardening, such as forging.

[0026] In one embodiment of the measuring tube, the insert is mechanically pressed onto the opening, whereby a mechanical, in particular non-detachable, connection exists between the cylindrical measuring tube section and the insert.

[0027] The insert is pressed into the cylindrical measuring tube section using a rolling tool, for example. The rolling tool comprises, for example, a rotating conical mandrel, which uses rollers to press the insert against the inner wall of the cylindrical measuring tube section with an outward-directed force.

[0028] In one embodiment of the measuring tube, the mechanical connection between the cylindrical measuring tube section and the insert is designed as a liquid-tight connection.

[0029] The mechanical connection is therefore free of additional sealing elements and is designed to be liquid-tight against a liquid medium even without them. The liquid medium flows through the measuring tube during intended use, and the aforementioned process variable (flow, viscosity, and / or density) for the medium can be determined using the flowmeter.

[0030] In one embodiment of the measuring tube, the cylindrical measuring tube section has a wall thickness of less than 1 mm and an outer diameter of less than 25 mm. The problem of hydrogen embrittlement occurs primarily in such thin or small measuring tubes.

[0031] In one embodiment of the measuring tube, the sleeve-shaped section of the insert, which protrudes into the cylindrical measuring tube section, is at least 0.1 times, in particular at least 0.15 times, the outer diameter of the cylindrical measuring tube section in its longitudinal direction. This ensures that the electrically conductive contact between the insert with the precious metal and the cylindrical measuring tube section is present over a sufficiently large contact area.

[0032] In one embodiment of the measuring tube, it has two inserts, with one insert inserted into each of the two end openings. All of the aforementioned embodiments mentioned in connection with the at least one insert are, mutatis mutandis, of course also embodiments of the second insert, when using a first insert at the first end opening and a second insert at the second end opening.

[0033] The invention further relates to a flow measuring device which is designed to determine and / or monitor a process variable of a fluid medium, with a measuring tube according to the invention, which measuring tube is inserted into a housing of the flow measuring device.

[0034] For example, it is one of the measuring devices mentioned above, such as a Coriolis flowmeter. For details on the functionality of a Coriolis flowmeter, please refer again to the state of the art mentioned above.

[0035] With regard to the method, the object is achieved by a method for producing a measuring tube, comprising the steps: Providing a substantially cylindrical measuring tube section through which a liquid medium can flow and which consists of tantalum at least on one inner wall, wherein the cylindrical measuring tube section has two opposite end openings for the inflow and outflow of the medium, wherein the first opening and the second opening each widen to form a circular end face for connecting the measuring tube to a process connection; providing at least one insert, which insert consists essentially of a precious metal; inserting the insert into one of the two end openings such that the inner wall consisting of tantalum is in direct contact with the precious metal; pressing the insert onto the opening such that a mechanically strong, in particular non-detachable, connection is formed between the insert and the substantially cylindrical measuring tube section.

[0036] In one embodiment of the procedure, this includes the step: Tensile pressure forming, in particular flanging, of a sleeve or a circular disc made of a precious metal, whereby the insert with the sleeve-shaped section and the circular disc-shaped section is formed, and wherein the tensile compression forming takes place before the insert is inserted into the cylindrical measuring tube section.

[0037] The insert is therefore provided by forming a single piece of sheet metal in a forming process.

[0038] In one embodiment of the method, the insert is mechanically pressed into the cylindrical measuring tube section by means of a rolling tool, whereby the mechanically strong, in particular non-detachable and liquid-tight, connection between the insert and the cylindrical measuring tube section is created.

[0039] In one embodiment of the method, an adhesive, in particular an epoxy adhesive, is applied to the insert before inserting the insert into the cylindrical measuring tube section, so that the insert is fastened to the cylindrical measuring tube section by means of the adhesive before the mechanically strong, pressed connection is created, and wherein the adhesive is applied to the insert at most in certain areas, so that the adhesive is present at most in an area between the circular disc-shaped section of the insert and the circular end face of the opening.

[0040] In particular, a maximum of 20% of the contact area between the insert and the cylindrical measuring tube section should be covered with adhesive.

[0041] The invention and further advantageous embodiments are explained in more detail below using exemplary embodiments. Identical parts are provided with the same reference numerals throughout the figures; where clarity requires it or it otherwise seems appropriate, previously mentioned reference numerals have been omitted in subsequent figures.

[0042] They show: Fig. 1 : An exploded view of an embodiment of a measuring tube according to the invention; Fig. 2a,2b : Perspective views of an embodiment of a measuring tube according to the invention in a housing of a flow meter; Fig. 3a-e : Perspective views of embodiments of the use of a measuring tube according to the invention; Fig. 4 : A perspective view during the manufacture of a measuring tube according to the invention.

[0043] Fig 1 shows the components of an embodiment of a measuring tube 1 according to the invention. This comprises a cylindrical measuring tube section 2, which consists of tantalum (or tantalum with a thin oxide layer) at least on its inner wall, with two openings 21, 22 for the inflow and outflow of a liquid medium. The openings 21, 22 widen to form annular end faces SF for connecting the measuring tube 1 to a process connection, for example, by means of a pipe flange.

[0044] 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 of less than 1 mm. For such small and thin measuring tubes 1, brittle fracture induced by hydrogen embrittlement has proven problematic in the applicant's investigations.

[0045] According to the invention, this is counteracted by an insert 3 being inserted into at least one of the openings 21, 22, wherein in the exemplary embodiment shown here an insert 3, 3a is inserted into each of the two openings 21, 22. The insert 3 is essentially made of a precious metal EM and projects into the area of ​​the cylindrical measuring tube section 2, which area is flowed through by a medium during measuring operation. The outer wall AW of the insert 3 is therefore in direct contact with the inner wall IW, ie the tantalum, of the cylindrical measuring tube section 2, which is exposed to a medium that may be acidic and thus promotes hydrogen embrittlement.

[0046] The projection is achieved by the insert 3 having a sleeve-shaped section 31 and a circular disc-shaped section 32. The sleeve-shaped section 31 is, in its longitudinal direction LR, at least 0.1 times the outer diameter AD of the cylindrical measuring tube section 2 or of the measuring tube 1.

[0047] The measuring tube 1 with the cylindrical measuring tube section 2 and the at least one insert 3 can be installed or is installed in a housing 5 of a flow measuring device 4, see Fig. 2a, 2b .

[0048] The great advantage of the insert 3 according to the invention is that the insert 3 can be easily inserted into the cylindrical measuring tube section 2 from the opening 21 leading to the pipe flange. This makes it very easy to couple the tantalum on the inner wall IW of the measuring tube 1 with the precious metal EM for a flow meter 4 by inserting the insert 3 into the cylindrical measuring tube section 2, thereby reducing hydrogen embrittlement.

[0049] In particular, even for an existing measuring tube already installed in a housing 5 of a flow measuring device 4 without an insert according to the invention, its mechanical long-term stability can be increased by subsequently inserting an insert, see Fig. 2b .

[0050] The preferred option is Fig. 1 The shape of the insert 3 shown can only be obtained by forming, for example by flanging. This is shown in Fig. 3a bis 3e shown in more detail. The insert 3, consisting of the sleeve-shaped section 31 and the circular disk-shaped section 32, is obtained, for example, in a first embodiment by a sheet 8 made of a precious metal EM, here platinum, in the shape of a circular (ring) disk 10 ( Fig. 3a ) using a flaring tool 7, see Fig. 3b , is formed. The tensile or compressive forming during flanging is Fig. 3c shown form of insert 3 is obtained.

[0051] Alternatively, the sheet 8 can of course also be provided in the form of a sleeve 9 as the starting shape, Fig. 3d to form the insert 3 with the circular disc-shaped section 32 and the sleeve-shaped section 31 from the sleeve 9, see Fig. 3e .

[0052] In any case, the use of a thin sheet 8 in a standard shape such as a circular disk 10 or a sleeve 9 as the starting shape in the production of the insert 3 provides a very cost-effective solution for producing an insert 3 of a measuring tube 1 according to the invention.

[0053] Preferably, a soft sheet 8 with a Vickers hardness of less than or equal to 200 HV is provided in order to obtain the one-piece insert 3 in one forming step. This offers the advantage that the forming process is simple and that hardly any material stresses are introduced into the insert 3.

[0054] On the other hand, with such a soft platinum sheet 8, a liquid-tight connection between the insert 3 and the cylindrical measuring tube section 2 can be obtained very easily, see Fig. 4 .

[0055] To mechanically connect the cylindrical measuring tube section 2 to the insert 3, the following steps are carried out: Inserting the insert 3 into the cylindrical measuring tube section 2; subsequently pressing a rolling tool 6 from the opening 21 onto the insert 3, thereby establishing a mechanical and fluid-tight connection between the insert 3 and the cylindrical measuring tube section 2.

[0056] By establishing the mechanical connection, the tantalum of the inner wall IW of the cylindrical measuring tube section 2 and the platinum of the outer wall AW of the insert 3 are in direct electrically conductive contact. The sealing of the liquid-tight connection is achieved via the rolling point and the pressing of the soft Pt insert 3, without the need for an additional sealing element between the insert 3 and the cylindrical measuring tube section 2 to seal their connection.

[0057] To improve the placement and fixation of the insert 3 in the cylindrical measuring tube section 2, an adhesive 11, for example an epoxy adhesive such as a 2K epoxy adhesive, can be used if necessary. The adhesive is applied to the insert 3 before inserting the insert 3 and pressing the insert 3 onto the cylindrical measuring tube section 2.

[0058] The adhesive 11 is applied in particular at most in regions to the circular disc-shaped section 32 of the insert 3, for example only on an edge area adjacent to the outer diameter, see Fig. 4 . For example, the adhesive occupies less than 20% of a contact surface between the insert 3 and the cylindrical measuring tube section 2. This results in a liquid-tight, pressed mechanical connection during the subsequent pressing, in which sufficient tantalum is in direct contact with platinum to reduce hydrogen embrittlement. Preferably, at least 1% of the inner wall IW made of tantalum, which is otherwise in contact with the medium, is brought into contact with the precious metal EM (here: platinum). When using two inserts 3, 3a at both openings 21, 22 for inflow and outflow, in the case shown here, a contact surface between tantalum and platinum of at least 180mm^2 is available. Bezugszeichen und Symbole

[0059] 1Measuring tube 2Cylindrical measuring tube section 21First opening 22Second opening 3Insert 3aInsert 31Sleeve-shaped section 32Annular disc-shaped section 4Flow meter 5Housing 6Rolling tool 7Flanging tool 8Sheet 9Sleeve 10Circular disc 11Adhesive IWInner wall EMEPrecious metal SFSent face AWOuter wall VHVickers hardness WDWall thickness ADOuter diameter LRLongitudinal direction

Claims

1. A measuring tube (1) for a flowmeter, said flowmeter being configured to determine and / or monitor a process variable of a liquid medium, wherein the liquid medium flows through the measuring tube (1) when the process variable is being determined and / or monitored, the measuring tube (1) having: A mostly cylindrical measuring tube section (2), which can conduct a liquid medium, wherein the cylindrical measuring tube section (2) is made from tantalum (Ta) at least at the inner wall (IW), and the cylindrical measuring tube section (2) has two opposite, end-side openings (21, 22) to allow the medium to flow in and out; At least one insert (3), which consists of a pure precious metal (EM) or an alloy of at least one precious metal (EM), wherein the non-precious metal makes up less than 20 percent by weight, wherein the precious metal of the insert (3) is selected from the group of platinum metals and is, in particular, platinum; wherein the insert (3) is inserted into one of the two end-side openings (21; 22) in such a way that the inner wall made from tantalum is in direct contact with the precious metal wherein the insert (3) is mechanically pressed onto the opening (21; 22) and this creates a mechanical, permanent, and liquid-tight connection between the cylindrical measuring tube section (2) and the insert (3).

2. The measuring tube (1) as claimed in claim 1, wherein the first opening (21) and the second opening (22) each widen into an annular end face (SF), for connecting the measuring tube (1) to a process connection.

3. The measuring tube (1) as claimed in at least one of the preceding claims, wherein the insert (3) consists of a sleeve-shaped section (31) and a section in the shape of an annular disk (32).

4. The measuring tube (1) as claimed in at least one of the preceding claims, wherein the sleeve-shaped section (31) of the insert (3) protrudes from the opening (21; 22) into the cylindrical measuring tube section (2), and the inner wall (IW) of the cylindrical measuring tube section (2) is in direct contact with the outer wall (AW) of the sleeve-shaped section (31), and wherein the section (32) in the shape of an annular disk is pressed onto the annular end face (SF) of the opening (21; 22).

5. The measuring tube (1) as claimed in at least one of the preceding claims, wherein the insert (3) is one-part and is formed from a single sheet (8) by means of combined tensile and compressive forming, in particular flanging.

6. The measuring tube (1) as claimed in at least one of the preceding claims, wherein the precious metal of the insert (3) has a Vickers hardness which is less than or equal to 200 HV.

7. The measuring tube (1) as claimed in at least one of the preceding claims, wherein the cylindrical measuring tube section (2) has a wall thickness (WO) less than 1 mm and an external diameter (AD) less than 25 mm.

8. The measuring tube (1) as claimed in at least one of the preceding claims, wherein the sleeve-shaped section (31) of the insert (3), said sleeve-shaped section protruding into the cylindrical measuring tube section (2), is equal to at least 0.1 times, in particular at least 0.15 times, an external diameter (AD) of the cylindrical measuring tube section (2) in its longitudinal direction (LR).

9. The measuring tube (1) as claimed in at least one of the preceding claims, having two inserts (3, 3a), wherein one insert (3; 3a) is inserted into each of the two end-side openings (21, 22).

10. A flowmeter (4), which is configured to determine and / or monitor a process variable of a fluid medium, with a measuring tube (1) as claimed in at least one of the preceding claims, which is inserted into a housing (5) of the flowmeter (4).

11. A method for manufacturing a measuring tube (1), comprising the steps: Providing a mostly cylindrical measuring tube section (2), which can conduct a liquid medium and is made from tantalum at least at an inner wall (IW), wherein the cylindrical measuring tube section (2) has two opposite, end-side openings (21, 22) to allow the medium to flow in and out, wherein the first opening (21) and the second opening (22) each widen into an annular end face (SF), for connecting the measuring tube (1) to a process connection; Providing at least one insert (3), said insert (3) being made from a pure precious metal (EM) or an alloy of at least one precious metal (EM), wherein the non-precious metal makes up less than 20 percent by weight, wherein the precious metal of the insert (3) is selected from the group of platinum metals and is, in particular, platinum; Inserting the insert (3) into one of the two end-side openings (21; 22) in such a way that the inner wall (IW) made from tantalum is in direct contact with the precious metal (EM); Pressing the insert (3) onto the opening (21; 22) in such a way that a mechanically strong, permanent, and liquid-tight connection is formed between the insert (3) and the mostly cylindrical measuring tube section (2).

12. The method as claimed in claim 11, comprising the step: Combined tensile and compressive forming, in particular flanging, of a sleeve (9) or a circular disk (10) made mostly from a precious metal (EM), thereby forming the insert (3) with the sleeve-shaped section (31) and the section (32) in the shape of an annular disk, and wherein combined tensile and compressive forming takes place before the insert (3) is inserted into the cylindrical measuring tube section (2).

13. The method for manufacturing a measuring tube as claimed in claim 11 or 12, wherein the insert (3) is mechanically pressed into the cylindrical measuring tube section (2) using a rolling tool (6), thereby creating the mechanically strong, in particular permanent and liquid-tight, connection between the insert (3) and the cylindrical measuring tube section (2).

14. The method as claimed in claim 13, wherein an adhesive (11), in particular an epoxy adhesive (11), is applied to the insert (3) before the insert (3) is inserted into the cylindrical measuring tube section (2) so that the insert (3) is attached to the cylindrical measuring tube section (2) using the adhesive (11) before the mechanically strong, pressed connection is created, and wherein the adhesive (11) is applied to areas of the insert (3), and wherein, in particular, the adhesive (11) is present at most on an area between the section (32) of the insert (3) in the shape of an annular disk and the annular end face (SF) of the opening (21; 22).