Anti-vibration type temperature sensing component

Abstract

The thermal probe assembly includes a temperature-sensitive element having an electrical characteristic that varies with temperature. A plurality of leads are operatively coupled to the temperature-sensitive element. The temperature-sensitive element is disposed within a sheath and spaced apart from a distal end of the sheath by a distance selected to provide vibration resistance to the temperature-sensitive element.

Description

Technical Field

[0001] The present invention relates to the field of process industries, and more particularly to a thermal probe assembly, a field device, and a method of measuring the temperature of a process fluid located in a vibrating environment. Background Art

[0002] Process industries employ process variable transmitters to monitor process variables that are associated with substances such as solids, slurries, liquids, vapors, and gases in chemical plants, pulp mills, oil refineries, pharmaceutical plants, food plants, and other fluid process plants. Process variables include: pressure, temperature, flow rate, liquid level, turbidity, density, concentration, chemical composition, and other properties.

[0003] Temperature sensors are used in various process variable transmitters to provide temperature indications related to the process fluid. Although there are various types of temperature sensors, in cases where improved measurement accuracy and precision are required, resistance temperature devices (RTDs) are commonly used. RTDs typically employ: windings of temperature-sensitive wires around a central support, or patterned circuits of temperature-sensitive metals deposited on or otherwise placed on a non-conductive substrate (e.g., ceramic). The temperature-sensitive metal can be any metal having a conductivity that varies with temperature. Suitable examples include nickel and platinum. RTDs can be made in various sizes, but as the RTDs become larger, the cost typically increases. Thus, small RTDs offer the important advantages of higher measurement accuracy and precision and lower cost.

[0004] Although RTDs are used in a variety of applications, one particular application is the thermal probe. In a thermal probe, the RTD is placed within a sheath or conduit that protects the RTD from shock and provides direct contact with the medium. In some devices, the thermal probe is then inserted into a larger sensing structure such as a thermowell or a flow sensor. These devices are typically exposed to process fluid flow and are subject to high temperatures and vibrations. In a thermowell, the end of the sheath of the thermal probe contacts the bottom surface of the thermowell. This contact is advantageous because it not only ensures effective thermal contact, but this contact also helps to mechanically anchor the sheath within the thermowell, which provides vibration resistance. Summary of the Invention

[0005] A thermal probe assembly includes a temperature-sensitive element having an electrical property that varies with temperature. A plurality of leads are operably coupled to the temperature-sensitive element. The temperature-sensitive element is disposed within a sheath and is spaced apart from the distal end of the sheath by a distance selected to provide vibration resistance to the temperature-sensitive element. Brief Description of the Drawings

[0006] Figure 1 is a schematic cross-sectional view of a known RTD probe assembly.

[0007] Figure 2A and Figure 2B are a cross-sectional view and an exploded perspective view, respectively, of a standard RTD probe assembly integrally formed into a thermocouple sheath.

[0008] Figure 3A and Figure 3B are a perspective view and an exploded view, respectively, of a standard RTD probe assembly integrally formed into a flow measurement device.

[0009] Figure 4A and Figure 4B are a schematic cross-sectional view of an RTD probe assembly according to the prior art in a flow measurement application ( Figure 4A ) and a schematic cross-sectional view of an RTD probe assembly according to an embodiment of the present invention ( Figure 4B ).

[0010] FIG. 5A and Figure 5B compare a known RTD probe design (FIG. 5A) with an RTD probe design according to an embodiment of the present invention ( Figure 5B ).

[0011] Figure 6 is a comparison of the vibration analysis of a conventional RTD probe assembly with the vibration analysis of an extended-length RTD according to an embodiment of the present invention.

[0012] Figure 7 is a comparison of the vibration analysis of a conventional RTD probe assembly with a sheath length of 17.2 inches with the vibration analysis of an RTD probe assembly with an extended sheath length according to an embodiment of the present invention.

[0013] Figure 8 is a schematic diagram of an RTD probe assembly showing the position with the least relative deformation.

[0014] Figure 9 is a graph showing the output amplitude of the vibration analysis versus the vibration frequency.

[0015] Figure 10A-1 and Figure 10A-2 are schematic diagrams of a spring assembly that can be applied to the outer diameter of an RTD probe assembly according to another embodiment of the present invention.

[0016] Figure 10B-1 and Figure 10B-2 are schematic diagrams of another embodiment of a structure that can be applied to an RTD probe assembly according to another embodiment of the present invention. DETAILED DESCRIPTION

[0017] RTD components are used in a variety of applications. Such applications include use in thermocouple wells to measure the temperature of a process fluid or other substance flowing in a process conduit, or to measure the temperature of a process fluid or other substance placed in a storage container or tank. Another common application for an RTD probe assembly is process fluid flow measurement. Since fluid properties can be affected by the temperature of the process fluid, temperature measurement of the process fluid is important during process fluid flow measurement. An example of a process fluid flow measurement system that can also sense the process fluid temperature is sold under the trade name Rosemount 3051SFA-Annubar ® Flow meter (Rosemount 3051SFA-Annubar ® flow meter). For an Annubar averaging pitot tube primary element, the RTD element is typically positioned generally in the center of the process fluid conduit, where the Annubar primary element extends through the process fluid conduit. Typically, the RTD is specified within the Annubar primary element to position the RTD element at the middle one-third portion of the pipe inner diameter without support at the distal end. It can be appreciated that since the process fluid flows past the Annubar flow element, the Annubar flow element is subject to vibration. In some cases, the vibration experienced by the RTD element within the primary flow element can damage or even destroy the RTD element.

[0018] Embodiments of the present invention generally result from a careful study of the problem of vibration on the RTD element, and although the embodiments are particularly applicable to RTD assemblies located in process fluid flow devices, embodiments of the present invention are also applicable to any device or application in which an RTD probe assembly is subject to vibration.

[0019] Figure 1 is a schematic cross-sectional view of an RTD probe assembly. The RTD probe assembly 100 includes an RTD element 102 that is located within a protective sheath 104 and near the distal end 106 of the sheath 104. The sheath 104 is mechanically coupled to a mount 108, which in some cases is threaded so that the mount 108 can be attached in a threaded connection to other structures, such as a thermocouple well. A plurality of conductors 110, 112, 114, and 116 are electrically coupled to the RTD element 102 to permit external connection to the RTD element 102.

[0020] The RTD element 102 is formed by a winding or loop of a material (e.g., metal) having a conductivity that varies with temperature. Examples of such metals include platinum and nickel. As Figure 1As shown, the RTD probe assembly 100 has four conductors 110, 112, 114, and 116. This allows for a high-precision four-wire measurement technique to measure the RTD element 102. However, if such precision is not required, embodiments can be implemented with fewer than four wires. The RTD element 102 is shown positioned very close to the distal end 106 of the sheath 104. Typically, especially in thermocouple sheaths, the RTD element is designed to measure the temperature of the distal end of the thermocouple sheath. For cases where there is material between the RTD element and the end of the sheath and / or the end of the thermocouple sheath, this additional material will create additional time for heat to flow through the intervening structure so that it can be measured by the RTD element. Therefore, the distance 118 is typically minimized to reduce the time constant of the thermal system.

[0021] Figure 2A is a schematic cross-sectional view of the RTD probe assembly 100 installed within the thermocouple sheath 120. As shown, the sheath 104 is inserted within the bore 122 of the thermocouple sheath 120. Additionally, the distal end 106 is located near the distal end 124 of the thermocouple sheath 120. The threaded adapter 126 receives the RTD probe assembly 100 in a threaded connection and then includes an external threaded portion 128 that is received in a threaded connection within the mounting portion 130 of the thermocouple sheath 120. This forms a highly robust structure that allows the RTD probe assembly 100 to be thermally exposed to the process fluid or other material while still mechanically and chemically protecting the RTD probe assembly 100 from such exposure.

[0022] Figure 2B is Figure 2A an exploded view of the same assembly shown. It can be seen that the sheath 104 passes through the threaded mount 132.

[0023] Figure 3A is a schematic diagram of an RTD assembly integrated into a process fluid flow measurement system. The process fluid flow measurement system includes a primary element 150 having a mounting flange 152 and a flow measurement probe 154. The mounting flange 152 is configured to mount the primary element 150 to a process fluid flow conduit via standard pipe flanges, etc. The measurement probe 154 extends into the process fluid flow conduit and includes an orifice 155 that provides a pitot-static tube that provides excellent measurement accuracy over a wide flow range. When the flow measurement probe 154 is disposed in the process flow, the edge 156 of the measurement probe 154 can generate Von Karman vortices that cause vibrations in the flow measurement probe. The RTD probe assembly 100 (shown in Figure 3B is disposed within the flow measurement probe 154 to allow the transmitter 158 to provide an indication of the process fluid temperature.

[0024] Figure 3B Shows a standard RTD probe assembly 100 disposed within the thermal probe port 160 of a primary element 150. The conductors of the RTD assembly are coupled to electronics or a terminal block within the housing 162, which are typically configured to measure the resistance of the RTD element and provide an indication of the process fluid temperature.

[0025] Figure 4A and Figure 4B Compare a known RTD probe design with an RTD probe design according to an embodiment of the present invention. As Figure 4A shown, the known design has an RTD element 102 disposed near the distal end 106 of the sheath 104. It has been found that when the RTD element is at or near the end of the sheath, the RTD element is more vulnerable to damage or destruction by vibration. Correspondingly, as Figure 4B shown, although the RTD element 102 is still located at generally the same axial position within the flow conduit 170, the end 206 of the sheath 204 is spaced apart from the RTD element 102. In this way, the vibration present along the sheath 204 is spaced from the position of the RTD element 102 in a manner related to the wavelength of the vibration. The amount of the sheath extension can vary according to various embodiments of the present invention. The sheath extension can be set according to the diameter of the sheath. For example, the extension can be specified as at least 3 sheath diameters in terms of distance. And a preferred distance in terms of distance can be 6 diameters. In other embodiments, the sheath extension can be selected based on the natural frequency of the structure in which the thermal probe will be used. For example, a primary flow element having a resonant frequency of 300 Hz may require an extension of 1.5 inches. Finally, the sheath extension can simply be set as the distance from the RTD element. In one embodiment, the distance is greater than 0.5 inches. In a preferred embodiment, the sheath extension is approximately 1.5 inches.

[0026] Figure 5A and Figure 5B Compare a known RTD probe assembly design (Figure 5A) with an RTD assembly according to an embodiment described herein ( Figure 5B ). As shown in Figure 5A, the RTD element 102 is generally disposed within approximately half an inch (12.7 mm) of the distal end 106 of the sheath 104. In contrast, as Figure 5BAs shown, the RTD element 102 is significantly spaced from the end 206 of the sheath 204. This extended length of the sheath 204 is shown by reference numeral 208. This embodiment is generally contrary to the normal design of an RTD probe, in which the sensing element is placed as close as possible to the distal end to ensure a reduced time constant for the RTD assembly. This relatively simple but counterintuitive change can be facilitated by a detailed understanding of the different vibration modes of the RTD probe assembly.

[0027] Figure 6 FIG. 4 is a schematic diagram of a vibration analysis comparing a known RTD probe assembly having a length of 17.2 inches with an RTD probe assembly having an extended length of 18.7 inches according to an embodiment of the present invention. In each case, the individual orders of harmonic vibration are shown for each design. By extending the sheath length, the RTD element is positioned at the location of minimum deformation when the RTD vibrates at its resonant frequency, thereby achieving improved vibration resistance. ANSYS is used to simulate the deformation of the RTD sheath with the RTD NPT side fixed. The first-order relative deformation trend of the sheath is shown in FIGS. 5 and 6. From the first-order relative deformation trend to the fifth-order relative deformation trend, the minimum deformation is located at positions from 1 inch to 5 inches. The higher the frequency, the closer the associated deformation location is to the sheath end. When the frequency exceeds 200 Hz, the minimum associated deformation occurs at a position approximately 1.5 inches from the end of the RTD assembly. Figure 6 and Figure 7 FIGS. 5 and 6 show the first-order relative deformation trend of the sheath. From the first-order relative deformation trend to the fifth-order relative deformation trend, the minimum deformation is located at positions from 1 inch to 5 inches. The higher the frequency, the closer the associated deformation location is to the sheath end. When the frequency exceeds 200 Hz, the minimum associated deformation occurs at a position approximately 1.5 inches from the end of the RTD assembly.

[0028] Figure 7 FIG. 7 is a schematic diagram of a vibration analysis comparing a conventional RTD probe assembly (RTD sheath length of 17.2 inches) with an RTD probe assembly having an extended length according to an embodiment of the present invention (RTD sheath length of 21.2 inches).

[0029] Based on the simulation results, various prototypes with different extension lengths were fabricated. For example, for the IEC-7700 vibration test, a conventional RTD assembly was provided with a sheath end extended by 1.5 inches, while another prototype was provided with a sheath end extended by 4 inches. The initial resonance search was set from 10 Hz to 500 Hz, with the frequency sweep as shown in FIG. 8. Figure 9As shown. Two peaks occur at approximately 294 Hz and 364 Hz. The durability condition scan frequency was set from 285 Hz to 305 Hz, with a durability duration of 40 hours, and all RTDs read normally after the first run of the vibration test. Then the durability scan frequency was changed to 360 Hz to 380 Hz, and some failures began to occur. The traditional RTD (without extension) failed the fastest, and the longest extension (4 inches) failed subsequently. However, the prototype with a 1.5-inch extension did not fail in the durability test. The vibration test results indicate that any sheath extension is better than no sheath extension, but a specific sheath extension based on the wavelength of the resonant frequency can produce the best results. Therefore, it is believed that embodiments of the present invention can provide minimal or reduced deformation at the RTD element location by increasing only the length of the RTD sheath without changing the position of the sensor element, thereby improving the vibration resistance of the RTD probe assembly. Relatively few additional changes to the manufacture of the RTD probe assembly can be made, and this improvement can be provided at a minimal increased cost. Although embodiments of the present invention are particularly useful for the pitot tube differential pressure primary element as shown in FIG. 3, embodiments of the present invention are useful for any RTD probe assembly exposed to any vibration.

[0030] Although the embodiments described herein generally improve the vibration resistance of the RTD assembly by extending the sheath of the RTD assembly, it has also been found that additional improvements can be provided by placing a biasing element around the outer diameter of the sheath. This can help reduce the degree to which the sheath rattles or otherwise vibrates within the thermocouple well or other measurement assembly.

[0031] Figure 10A-1 is a perspective view of a formed spring that can be applied to the outer diameter of an RTD sheath according to an embodiment of the present invention. The spring 300 has a profile such that the ends 302, 304 have a relatively small diameter in order to attach the ends to the outer diameter of the RTD sheath 104 or otherwise be biased against the outer diameter of the RTD sheath 104 (as Figure 10A-2 shown). Additionally, the spring 300 is formed such that the spring 300 includes a larger diameter portion 306 that is configured to engage an inner diameter, such as the inner diameter of the hole 122 of a thermocouple well (as Figure 2A shown) or the inner diameter of other measurement structures. In this way, the spring 300 helps reduce the degree to which the sheath 104 rattles or otherwise vibrates within the measurement structure.

[0032] According to another embodiment of the present invention, other suitable elements can be applied to the outer diameter of the sheath 104 to reduce the degree to which the sheath 104 is allowed to move within the measurement structure. As Figure 10B-1As shown, the spring barrel 350 includes openings 352, 354 sized to fit around the outer diameter of the sheath 104. Additionally, slots 356 allow the spring barrel 350 to deform to permit the ends 352, 354 to fit around the outer diameter of the sheath 104. Further, the spring barrel 350 is shown having longitudinal slats 358 that bend outwardly to engage an inner surface, such as the inner diameter of the bore 122 of the thermocouple well or other suitable measurement structure. In this way, the spring barrel 350 can act similar to the spring 300 (as Figure 10A-1 and Figure 10A-2 shown) to reduce the extent to which the RTD sheath can move within the thermocouple well or measurement structure. The spring 300 or spring barrel 350 can be secured to the sheath by clamping or other means to mitigate relative movement between the RTD element 105 and the thermocouple well.

[0033] Although the invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

1. A thermal probe assembly, comprising: A temperature-sensitive element having electrical characteristics that vary with temperature; A plurality of leads operatively coupled to the temperature-sensitive element; A sheath; And Wherein the temperature-sensitive element is disposed within the sheath and spaced apart from a distal end of the sheath by a distance selected to provide vibration resistance to the temperature-sensitive element.

2. The thermal probe assembly according to claim 1, wherein, The temperature-sensitive element is an RTD element.

3. The thermal probe assembly according to claim 2, wherein, The RTD element is spaced 1.5 inches from the distal end of the sheath.

4. The thermal probe assembly according to claim 2, wherein, The RTD element is spaced more than 1.5 inches from the distal end of the sheath.

5. The thermal probe assembly according to claim 4, wherein, The RTD element is spaced 4.0 inches from the distal end of the sheath.

6. A field device, comprising: A thermal probe assembly, the thermal probe assembly comprising: An RTD element having a resistance that varies with temperature; A plurality of leads operatively coupled to the RTD element; A sheath; Wherein the RTD element is disposed within the sheath and spaced apart from a distal end; and A process element configured to be inserted into a process fluid, the process element having a bore configured to receive the thermal probe assembly, the process element having a natural vibration frequency; and Wherein the RTD element is spaced from the distal end of the sheath by a distance that positions the RTD element within the process element at a location having a reduced vibration amplitude at the natural vibration frequency.

7. The field device according to claim 6, wherein, The process element is a thermocouple well.

8. The on-site device according to claim 6, wherein, The process element is a primary flow element.

9. The field device according to claim 8, wherein, The RTD element is spaced 1.5 inches from the distal end of the sheath.

10. The field device according to claim 9, wherein, The primary flow element generates von Karman vortices in the process fluid.

11. The field device according to claim 6, wherein, The RTD element is spaced from the distal end of the sheath by a distance that positions the RTD element within the process element at a location having a reduced vibration amplitude at the natural vibration frequency.

12. The on-site device according to claim 6, wherein, The RTD element is spaced from the distal end of the sheath by a distance that positions the RTD element within the process element at a location having a reduced vibration amplitude at a harmonic of the natural vibration frequency.

13. The field device according to claim 6, further comprising a mechanical spacer positioned around the sheath and configured to bias the sheath away from an inner diameter of the bore.

14. The field device according to claim 13, wherein, The mechanical spacer is a formed spring.

15. The field device according to claim 14, wherein, The formed spring has a pair of ends configured to engage an outer diameter of the sheath, and the formed spring has an intermediate portion configured to engage an inner diameter of the bore.

16. The field device according to claim 13, wherein, The mechanical spacer is a spring cartridge having a plurality of longitudinal slats.

17. The field device according to claim 16, wherein, The spring cartridge includes longitudinal slots extending from a first end to a second end.

18. A method of measuring the temperature of a process fluid located in a vibrating environment, the method comprising: Providing an RTD element having a resistance that varies with temperature; Positioning the RTD element within a sheath having a distal end; Provide a process element in the process fluid, the process element having a resonant frequency; Spaced the RTD element from the distal end within the sheath by a distance selected to position the RTD element at a location having a reduced amplitude of vibration at the resonant frequency.

19. The method according to claim 18, wherein, Spacing the RTD element from the distal end is performed by mounting a sheath extension to the sheath.

20. The method of claim 18, further comprising using the measured process fluid temperature to provide a process fluid flow measurement result.

21. The method of claim 18, further comprising providing an indication of the process fluid temperature.

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