PROTECTIVE TUBE WITH REDUCED SENSITIVITY TO VORTEX INDUCED VIBRATIONS

Abstract

The present disclosure includes a protective tube for insertion into a pipe or vessel containing a medium, a measuring apparatus having such protective tube and to a method for producing the protective tube. The protective tube comprises a tubular member having a bore extending between an upper and lower end of the tubular member, and at least one helical fin formed on at least a section of an outer surface of the tubular member, winding around the outer surface of the tubular member and defining a flow channel along at least a part of the tubular member. An outer surface of the tubular member comprises a surface structure in an area of the at least one flow channel.

Claims

1. A protective tube for insertion into a pipe or vessel containing a medium, the protective tube comprising: a tubular member including an outer surface and a bore extending between a proximal end and a distal end of the tubular member; and at least one helical fin formed on at least a section of the outer surface of the tubular member, winding around the outer surface and thereby defining a flow channel along at least the section of the tubular member, wherein, in an area of the at least one flow channel, the outer surface of the tubular member comprises a surface structure.

2. The protective tube of claim 1, wherein the surface structure comprises a surface corrugation.

3. The protective tube of claim 1, wherein the surface structure comprises a pattern with alternating depressions and elevations.

4. The protective tube of claim 3, wherein, the pattern winds around the outer surface of the tubular member along at least a part of the tubular member.

5. The protective tube of claim 1, wherein the protective tube is a thermowell, and the tubular member is closed at the distal end.

6. The protective tube of claim 1, wherein at least one geometrical parameter of the at least one helical fin is chosen such that it depends on at least one process condition of the medium in the vessel or pipe.

7. A measuring apparatus for determining and/or monitoring a process variable of a medium, the measuring apparatus comprising the protective tube according to claim 1.

8. The measuring apparatus according to claim 7, wherein the protective tube is configured to receive a measuring insert configured to determine and/or monitor a process variable of the medium.

9. The measuring apparatus according to claim 8, wherein the process variable of the medium is the temperature of the medium.

10. A method of manufacturing a protective tube for insertion into a pipe or vessel containing a medium, wherein the protective tube comprises: a tubular member including an outer surface and a bore extending between a proximal end and a distal end of the tubular member; and at least one helical fin formed on at least a section of the outer surface of the tubular member, winding around the outer surface and thereby defining a flow channel along at least the section of the tubular member, the method comprising forming a surface structure on an outer surface of the tubular member in an area of the at least one flow channel.

11. The method of claim 10, further comprising selecting at least one geometrical parameter of the at least one helical fin such that the at least one geometrical parameter depends on at least one process condition of the medium in the vessel or pipe.

12. The method of claim 10, wherein the surface structure is fabricated by removing portions of a wall of the tubular member.

13. The method of claim 12, wherein the surface structure is fabricated by at least one turning or milling process.

14. The method of claim 12, wherein the surface structure is fabricated using at least one cutting tool.

15. The method of claim 14, wherein the at least one cutting tool comprises a cylindric base body with at least two knives arranged on the base body, which are circumferentially distributed across an outer surface of the base body.

16. The method of claim 14, wherein the surface structure is fabricated by at least at times modifying at least one cutting parameter.

17. The method of claim 16, wherein the at least one cutting parameter includes at least one of a draft angle used to position the at least one cutting tool in relation to the tubular member and the cutting tool used.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The described embodiments and other features, advantages and disclosures contained herein, and the manner of attaining them, will become apparent and the present disclosure will be better understood by reference to the following description of various embodiments of the present disclosure taken in junction with the accompanying drawings, wherein:

[0032] FIG. 1 illustrates the origin of vortex shedding for an insertion body exposed to a flowing medium;

[0033] FIG. 2a shows a partial longitudinal cross-section of a thermometer having a state of the art thermowell;

[0034] FIG. 2b shows a radial cross-section of the thermometer of FIG. 2a;

[0035] FIG. 2c shows a side view of the thermometer of FIG. 2a with a process connection;

[0036] FIG. 3 shows a thermowell with a plurality of helical fins according to the state of the art;

[0037] FIGS. 4a-4c show embodiments of the surface structure according to the present disclosure; and

[0038] FIGS. 5a and 5b show an exemplary cutting tool to produce the surface structure according to the present disclosure.

[0039] In the figures, the same elements are always provided with the same reference symbols.

DETAILED DESCRIPTION

[0040] FIG. 1 illustrates the origin of vortex shedding w at a cylindrical, conically tapered protective tube 1 exposed to a flowing medium M in a pipe 2, which is indicated by one of its walls. Downstream of the protective tube 1 in the flow direction v of the medium, a ridge-like pattern develops. Depending on the flow velocity v of the medium M, this can lead to coherent vortex shedding which in turn may cause the protective tube 1 to vibrate.

[0041] The vibrations are mainly due to two forces acting on the protective tube 1, a shear force in the in y-direction and a lifting force in x-direction. The shear force causes oscillations at a frequency f.sub.s, while the lifting force causes oscillates at a frequency of 2f.sub.s. The frequency f.sub.s now depends on the flow velocity v of the medium M, and on various physical or chemical medium properties such as its viscosity and density, as well as on the dimensions of the protective tube 1, such as its diameter and length. The closer the frequency f.sub.s is to the natural frequency of the protective tube 1 and the higher the flow velocity v of the medium M, the greater are the resulting oscillation causing forces.

[0042] As a result of the vibration causing forces, the protective tube 1 can be damaged or even break down completely. This is known as the so-called resonance condition.

[0043] FIGS. 2a-2c exemplarily and without limitation to such embodiment show a state of the art thermometer 3 having a protective tube 1 in the form of a thermowell 4. As can be seen in FIG. 2a, the thermowell 4 comprises a tubular member 5 having a first end section 5a, and a second end section 5b with a closed end. The tubular member 5 further comprises a bore 6 forming a hollow space within the tubular member 5, which is defined by an inner surface s and a predeterminable height h parallel to a longitudinal axis A of the tubular member 5, which bore 6 serves for receiving a measuring insert 7 [not shown] for determining and/or monitoring the process variable, e.g. the temperature of the medium M.

[0044] Further, as illustrated in FIGS. 2c, a fastening unit 8 is provided, which exemplarily is attached to the tubular member 5, here. This fastening unit 8 is a process connection and serves for mounting the thermowell 4 to the pipe 2 [not shown] such that the tubular member 5 at least partially extends into an inner volume of pipe 2 and such that it is at least partially in contact with the flowing medium M.

[0045] The outer surface S the thermowell 4 has an essentially round shape as becomes visible in FIG. 2b. However, such construction can easily lead to undesired vortex induced vibrations of the thermometer 3.

[0046] To overcome the problems associated with coherent vortex shedding, protective tubes 1 with helical fins 9 which are typically arranged on the outer cross-sectional surface S of the protective tube 3 have been suggested. An exemplarily protective tube 3 having three such helical fins 9 is shown in FIG. 3. The helical fins 9 form flow channels 10 along the tubular member 5 and thus reduce VIV of the protective tube 3. Each flow channel 10 is formed by the volume between to adjacent helical fins 9 which proceed around the tubular member 5 along its length axis A.

[0047] The present disclosure additionally provides a surface structure 11 for which three exemplary embodiments are shown in FIGS. 4a-4c.

[0048] FIG. 4a depicts a cross-sectional view of tubular member 5 with helical fins 9 forming flow channels 10 which in turn are provided with surface structure 11 by means of which a further improvement regarding the stability of the tubular member 5, or the protective tube 1 respectively, against vortex induced vibrations is achieved. The surface structure 11, among others, has the advantage of laminarization of the flow profile within the flow channels 10.

[0049] In this 2D view, each a surface contour is visible for each flow channel 10 which is defined by the surface structure 11. Here, the pattern of the surface structure is given by two valleys 13 separated from each other by an elevation 14, the pattern winding around the tubular member 5 in the same way as the helical fins 9, i.e., in the form of helixes. There are no sharp edges in the contour of the surface structure 11 for the embodiment shown. Further, the widths of the two valleys parallel to the longitudinal axis A of the tubular member 5 from each other. However, other embodiments of the protective tube can also comprise a multitude of equally designed valleys 13. Additionally, other embodiments can include surface structures with more valleys 13 and elevations 14, the various valleys 13 and elevations 14 at least partially having same or differing dimensions. The surface structure 11 may also comprise a surface corrugation 12 [not shown here].

[0050] A second exemplary embodiment of the surface structure 11 is shown in FIG. 4b. In this embodiment, the surface structure 11 is build in a symmetric manner around elevation 14. In contrast, for the embodiment of the surface structure 11 shown in FIG. 4c the elevation 14 comprises a sharp edge. In all embodiments shown, the helical fins 9 and surface structure 11 proceed along the entire length 1 parallel to the length axis A of the tubular member 5. However, in other embodiments, also only a given section of the tubular member 5 might be covered by helical fins 9 and/or the surface structure 11.

[0051] It is preferred, if the surface structure 11 is produced while producing the flow channels 10, which in turn are preferably produced by removing parts of the wall S of the tubular member 5, e.g., by means of at least one turning or milling process. That way, the entire production process of the protective tube 1 is simplified, optimized and can be carried out based on a reduced number of manufacturing steps.

[0052] For producing the protective tube 1 a cutting tool 15 can be utilized. FIGS. 5a and 5b depict two exemplary embodiments for a cutting tool 15 suitable to manufacture a protective tube 1 according to the present disclosure. Such cutting tool 15 preferably is applicable in connection with a turning or milling process. In both embodiments shown, the cutting tool 15 comprises a cylindric base body 16 with multiple knives 17 arranged in an end section on the base body 16. For the embodiment shown in FIG. 5a the cutting tool 15 comprises eight knives 17 and in case of the embodiment shown in FIG. 5b three knives 17. In both cases, the knives 17 are circumferentially distributed across an outer surface of the base body 16 as visible in the cross-sectional views on the left side.