Estimating process fluid temperature using improved heat flow sensors

Abstract

The process temperature estimation system (200) includes a mounting assembly (202) configured to mount the process fluid temperature estimation system to the exterior surface (116) of the process fluid conduit (100). A hot end thermocouple (314) is thermally coupled to the exterior surface (116) of the process fluid conduit (100). A resistance temperature detector (RTD) (310) is spaced apart from the hot end thermocouple (314). A measurement circuit (228) is coupled to the hot end thermocouple (314) and configured to detect an electromotive force (emf) of the hot end thermocouple (314) and a resistance of the RTD (310) that vary with temperature to provide sensor temperature information. The controller (222) is coupled to the measurement circuit (228) and is configured to measure a reference temperature based on the resistance of the RTD (310) and generate an estimated process temperature output using heat transfer calculations using the reference temperature, the electromotive force (emf) of the hot end thermocouple (314), and the known thermal conductivity of the process fluid conduit (100).

Description

[Technical field]

[0001] Background technology Many industrial processes transport process fluids through pipes or other conduits. Such process fluids may include liquids, gases, and sometimes entrained solids. Such process fluid streams may be found in any of a variety of industries, including, but not limited to, sanitary food and beverage production, water treatment, high purity pharmaceutical production, chemical processing, the hydrocarbon fuel industry including hydrocarbon extraction and processing, and hydraulic fracturing technologies utilizing abrasive and corrosive slurries.

[0002] It is common to install a temperature sensor in a thermowell, which is then inserted into the process fluid flow through an opening in a conduit. However, this approach is not always practical because the process fluid may be very hot, very corrosive, or both. Furthermore, thermowells must be designed to be at a defined location in the process fluid flow system, typically requiring a threaded port or other robust mechanical attachment / seal in the conduit. Thus, while thermowells are useful for providing accurate process fluid temperatures, they do have a limited number or limitations.

[0003] More recently, process fluid temperatures have been estimated by measuring the external temperature of a process fluid conduit, such as a pipe, and using heat flow calculations. This external approach is considered non-invasive because it does not require openings or ports defined in the conduit. Thus, such a non-invasive approach can be placed at virtually any location along the conduit. Summary of the Invention

[0004] The process temperature estimation system includes a mounting assembly configured to mount the process temperature estimation system to an exterior surface of the process fluid conduit. A hot end thermocouple is thermally coupled to the exterior surface of the process fluid conduit. A resistance temperature detector (RTD) is spaced apart from the hot end thermocouple. A measurement circuit is coupled to the hot end thermocouple and configured to detect an electromotive force (emf) of the hot end thermocouple and a resistance of the RTD that varies with temperature to provide sensor temperature information. A controller is coupled to the measurement circuit and configured to measure a reference temperature based on the resistance of the RTD and generate an estimated process temperature output using a heat transfer calculation using the reference temperature, the electromotive force (emf) of the hot end thermocouple, and a known thermal conductivity of the process fluid conduit. [Brief description of the drawings]

[0005] [Figure 1] 1 is a schematic diagram of a system for estimating a process fluid temperature to which embodiments of the present invention are particularly applicable; [Diagram 2] 1 is a block diagram of a system for estimating a process fluid temperature to which embodiments of the present invention are particularly applicable; [Diagram 3] 1 is a schematic diagram of a sensor capsule for a process fluid temperature estimation system according to the prior art; [Figure 4] FIG. 1 is a schematic diagram of an improved heat flux sensor according to an embodiment of the present invention. [Diagram 5] FIG. 1 is a circuit diagram of an improved heat flux sensor according to an embodiment of the present invention. [Figure 6] 1 is a schematic diagram of a system for estimating a process fluid temperature in accordance with an embodiment of the present invention; [Figure 7] FIG. 2 is a schematic diagram of a system for estimating a process fluid temperature in accordance with another embodiment of the present invention. [Figure 8] FIG. 2 is a schematic diagram of a heat flux sensor according to another embodiment of the present invention. [Figure 9] FIG. 2 is a schematic diagram of an improved heat flux sensor in accordance with another embodiment of the present invention. [Figure 10] FIG. 2 is a schematic diagram of a heat flux sensor according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] Detailed explanation As discussed above, there are many applications where heat flow measurements and non-invasive process fluid temperature estimation provide a better method than using a thermowell to measure process temperature. Some commercially available non-invasive process fluid temperature estimation systems use a temperature measurement of the external conduit surface (i.e., skin) in combination with an internal measurement of the transmitter, such as the temperature of the transmitter terminals in the transmitter housing, and use these two measurements in a heat flow calculation to estimate the internal process fluid temperature within the conduit. However, the commercially available offerings are typically limited to a relatively low temperature range. To extend the temperature range of process fluid temperature estimation systems, thermocouples (which typically have a higher maximum temperature than RTDs) have been used as the process fluid temperature estimation measurement input. Although thermocouples can extend the maximum temperature in estimating process fluid temperature, thermocouple technology has several inherent challenges. One such inherent challenge includes measurement uncertainty.

[0007] FIGURE 1 is a schematic diagram of a process fluid temperature estimation system to which embodiments of the present invention are particularly applicable. The system 200 generally includes a pipe clamping portion 202 configured to clamp around a conduit or pipe 100. The pipe clamping portion 202 may have one or more clamping ears 204 to allow the clamping portion 202 to be positioned and clamped to the pipe 100. The pipe clamping portion 202 may replace one of the clamping ears 204 with a hinge so that the pipe clamping portion 202 may be opened to be positioned on the pipe and then closed and secured by the clamping ears 204. While the clamp illustrated with respect to FIGURE 1 is particularly useful, any suitable mechanical device for securely positioning the system 200 on the exterior surface of a pipe may be used in accordance with embodiments described herein.

[0008] The system 200 includes a heat flow sensor capsule 206 that is biased against the outer diameter 116 of the pipe 100 by a spring 208. The term "capsule" is not intended to imply any structure or shape, and thus can be formed in a variety of shapes, sizes, and configurations. Although a spring 208 is illustrated, one of ordinary skill in the art will appreciate that a variety of techniques can be used to bias the sensor capsule 206 into continuous contact with the outer diameter 116.

[0009] The sensor capsule 206 generally includes one or more temperature sensing elements. The temperature sensing elements in the capsule 206 are electrically connected to a transmitter circuit in the housing 260 that is configured to obtain one or more temperature measurements from the sensor capsule 206 and calculate an estimate of the process fluid temperature (or the inner surface of the pipe) based on the measurements from the sensor capsule 206 and a reference temperature, such as a temperature measured within the housing 260 or a temperature otherwise provided to the circuit in the housing 260. As an example, a basic heat flow calculation can be simplified to the following: T corrected = T skin + (T skin - T reference ) * (R pipe / R sensor )

[0010] In this formula, T skin is the measured temperature of the exterior surface of the pipe 100. T reference is T skin The known thermal impedance (R sensor ) is the second temperature obtained for the position having the reference is typically sensed by a dedicated sensor within the housing 260. However, T referencecan also be sensed or inferred in other ways. For example, a temperature sensor can be placed external to the transmitter and replace the terminal temperature measurement in the heat transfer calculation. This external sensor measures the temperature of the environment surrounding the transmitter. As another example, industrial electronic devices typically have on-board temperature measurement capabilities. This electronic device's temperature measurement can be used in place of the terminal temperature in the heat transfer calculation. As another example, if the thermal conductivity of the system is known and the environmental temperature surrounding the transmitter is fixed or user controlled, then the fixed or user controlled temperature can be used as the reference temperature.

[0011] R pipe is the thermal impedance of the conduit, which can be obtained manually by obtaining the pipe material information, pipe wall thickness, etc. Alternatively, R pipe Parameters related to the process fluid temperature (T) may also be determined during calibration and stored for subsequent use. Thus, using appropriate heat flux calculations such as those described above, the circuitry within the housing 260 can calculate the process fluid temperature (T corrected ) and communicate such indication regarding the process fluid temperature to appropriate devices and / or a control room. In the example shown in FIG. 1, such information may be communicated wirelessly via antenna 212.

[0012] 2 is a block diagram of circuitry within a housing 260 of a process fluid temperature estimation system 200 to which embodiments of the present invention are particularly applicable. The system 200 includes a communication circuit / interface 220 coupled to a controller 222. The communication circuit 220 can be any suitable circuitry capable of communicating information regarding an estimated process fluid temperature to an external device. The communication circuit 220 enables the process fluid temperature estimation system 200 to communicate a process fluid temperature output over a process communication loop or segment. Suitable examples of process communication loop protocols include the 4-20 milliamp protocol, the Highway Addressable Remote Transducer (HART®) protocol, the FOUNDATION® Fieldbus protocol, and the WirelessHART protocol (IEC 62591).

[0013] System 200 also includes a power supply module 224 that provides power to all components of system 200, as indicated by arrow 226. In embodiments in which system 200 is coupled to a wired process communication loop, such as a HART® loop or a FOUNDATION® Fieldbus segment, power supply module 224 may include appropriate circuitry to condition the power received from the loop or segment to operate the various components of system 200. Thus, in such wired process communication loop embodiments, power supply module 224 may provide appropriate power conditioning to enable the entire device to be powered by the loop to which it is coupled. In other embodiments, where wireless communication is used, power supply module 224 may include a power source, such as a battery, and appropriate conditioning circuitry.

[0014] The controller 222 includes any suitable configuration capable of generating an estimate of process fluid temperature based on heat flow using measurements from sensors in the capsule 206 and / or reference temperature measurements. The controller 222 can include or be coupled to a memory 232 that stores instructions that, when executed by the controller 222, cause the controller 222 to perform heat flow calculations, as well as any other functions of the system 200. In one embodiment, the controller 222 is a microprocessor.

[0015] Measurement circuitry 228 is coupled to controller 222 and provides a digital display for measurements obtained from one or more temperature sensors 230. Measurement circuitry 228 may include one or more analog-to-digital converters and / or appropriate multiplexing circuitry for interfacing to the one or more analog-to-digital converters for sensors 230. Additionally, measurement circuitry 228 may include appropriate amplification and / or linearization circuitry as appropriate for the various types of sensors used. As shown in FIG. 2, system 200 may include a local operator interface 234, which may include a display and / or one or more user-operable buttons.

[0016] FIG. 3 is a schematic diagram of a sensor capsule for a process fluid temperature estimation system according to the prior art. In the illustrated sensor capsule, a first thermocouple is formed by the junction of thermocouple wires 250 and 252 with end cap 254. This first thermocouple senses the temperature of end cap 254, which is typically biased into contact with the outer surface of the conduit. As shown, a second thermocouple is formed at the junction of thermocouple wires 250 and 256. This second thermocouple provides a reference temperature measurement. The temperature difference between the first and second thermocouples provides an indication of heat flow. Although prior art dual thermocouple heat flux sensors provide an effective solution for rugged extended range temperature applications, they also have several limitations. One limitation is that special wiring is required for extension cables and unused conductors, which can create ease of wiring issues. Additionally, special marking may be required so the user knows which wire is for the hot end of the sensor and which wire is for a sensor located away from the hot end (e.g., at the cold end). This marking effectively ensures proper wiring at the transmitter so that the transmitter can perform the appropriate calculations. Yet another limitation is that the thermocouple extension wires are made of a slightly different material than the thermocouple wire used in the sensor, making the junctions of such wires more susceptible to temperature gradients external to the sensor. Additionally, thermocouples are relatively difficult to calibrate because the voltage output depends on the temperature difference between the hot and cold junctions of the thermocouple. Users who require sensor calibration for traceability purposes can only have part of the calibration of the measurement system, since the cold junction sensor is typically located within the electronics of the transmitter head. Finally, manufacturing characterization is a relatively complex process when external thermocouple wiring is considered.

[0017] FIG. 4 is a schematic diagram of an improved heat flux sensor according to an embodiment of the present invention. In the illustrated embodiment, a heat flux sensor 300 is provided that utilizes an RTD (resistance temperature detector) as a reference within a sensor capsule 306. As defined herein, an RTD is any device that has a resistance that changes with temperature. Examples include, but are not limited to, thin film resistive temperature devices, wire wound resistive temperature devices, and thermistors. This reference RTD 310 is located away from a hot end 312 of the sensor capsule 306 and uses a thermocouple 314 at the hot end so that the temperature difference between the hot end and the reference point can be determined. In the illustrated embodiment, four standard copper leads 316 are provided back to the end user for ease of installation and connection management. As shown, no special thermocouple extensions are required and the heat flux sensor can be connected to a transmitter as a standard RTD without the need for special connection configurations. The RTD 310 is located within the sensor capsule 306 in an area that protects the RTD 310 from extreme process temperatures. This location can be determined during design by evaluating the thermal conductivity of the capsule's construction (through standard sensor testing, it is believed that if the hot end is exposed to 600 degrees Celsius, the RTD can be placed approximately 3.5 inches from the hot end 312). Note that a thermocouple is formed when two dissimilar metals are joined. As shown in FIG. 4, a thermocouple wire 315 is joined to a copper conductor 317 at a thermocouple 319 located at the RTD 310. For such simplicity, this thermocouple 319 is used to facilitate sensing the temperature difference between the hot end and the RTD 310 using a pair of thermocouples (314, 319) since the signals of the two thermocouples (314, 319) can be combined electrically. However, embodiments can be implemented using a single hot end thermocouple 314 and RTD 310. The thermocouple 319 can be any suitable thermocouple and is wired to one of the common legs of the RTD 310. This is a way to thermally couple the thermocouple 319 and RTD 310 while minimizing cabling back to the transmitter.

[0018] For calibration, either at manufacturing or by the end user, the sensor capsule 306 can be removed from the assembly and simply placed in a thermal calibrator, where the RTD adjustment can be determined using coefficients in the standard Callendar-Van Dusen equation. The thermocouple can then also be evaluated for drift and adjustments made as necessary. This provides traceability to all sensors in the heat flux assembly.

[0019] FIG. 5 is a circuit diagram of an improved heat flux sensor according to an embodiment of the present invention. The thermocouple 314 and RTD 310 measurements can be easily sensed and mathematically extracted from one another to provide two accurate sensor measurements. The RTD resistance measurement can be obtained by passing an excitation current 320 between terminals 2 and 3. The current is shown to pass in one particular direction, but can also pass in the opposite direction. The RTD resistance measurement is then performed by measuring the voltage between terminals 1 and 4 while the excitation current is passed. For thermocouple measurements, the excitation current must be turned off to make an accurate thermocouple electromotive force (emf) measurement. In some instances, any excitation currents can be reversed by subtracting measurements in the direction of each excitation current to nullify lead resistance. In either case, the voltage is measured across terminals 3 and 4 to provide the thermocouple signal.

[0020] Since the wiring of the sensor is completed by the end user and may not be labeled, it is preferable for the location and polarity of the thermocouple to be determined automatically by the transmitter. This can be done in a number of ways. One way is to measure the voltage across all terminals. Terminals 1-2 and 3-4 are meant to be common to the sensor. The thermocouple is placed between one of these common terminals. A measurement across the RTD between terminals 1-4 or 2-3 also helps determine the location. It does not take much of a temperature gradient to identify the thermocouple. If the general ambient temperature at the transmitter is known and the RTD measurement is known, the polarity of the thermocouple can be determined.

[0021] The signals from the thermocouple 314 and RTD 310 can be provided to the measurement circuitry 228 as temperature sensor inputs 230. The controller 222 can then apply the heat flow calculations described above. The process temperature inside (or on the inside surface) of the conduit can be calculated by evaluating the heat transfer through a heat flux sensor located on the conduit, attached either remotely or directly to a temperature transmitter. As described above, the heat flow calculations require knowledge of the thermal properties of the process fluid conduit in order to complete the calculation of its internal conduit temperature.

[0022] 6 is a schematic diagram of a system for estimating a process fluid temperature according to an embodiment of the present invention. As shown in FIG. 6, the sensor capsule 306 includes a thermocouple disposed in the end cap 312 and an RTD reference (T RTD ) 310. The sensor capsule 306 is electrically coupled to a transmitter circuit 330 by an extension cable 332. The extension cable 332 may place the transmitter circuit 330 in a location that may be at a lower temperature than the process fluid conduit. This lower temperature may help protect electrical components within the temperature transmitter 330.

[0023] Figure 7 is a schematic diagram of a system for estimating a process fluid temperature according to another embodiment of the present invention. The system 350 is similar to the system shown in Figure 6, except that the transmitter circuit 330 is attached directly to the process fluid conduit, thus eliminating the need for an extension cable 332 (shown in Figure 6). Thus, a heat flux sensor utilizing two or more sensor measurement points can be interfaced between the measurement circuit 228 as an input to the heat flow calculation. Either a remote or local connection can be used, since the heat transfer between the two elements is all contained within the sensor capsule assembly and does not rely on a heat transfer relationship to the transmitter housing.

[0024] FIG. 8 is a schematic diagram of a heat flux sensor according to another embodiment of the present invention. The heat flux sensor 400 is similar to the heat flux sensor 300 (shown in FIG. 4) except that the heat flux sensor 400 requires only three copper wires. As shown, the wire 402 is common to both the RTD 310 and the thermocouple 314. Additionally, in some embodiments, the RTD 310 can have a location that allows for proper placement such that it is housed within the sensor capsule. In further embodiments, the sensor capsule can include an RFID or NFC chip 404 that includes the characteristic coefficients of the Callender-Van Dusen (CVD) equation and / or performance information of the thermocouple. Although an RFID chip 404 is shown in FIG. 8, it is expressly contemplated that the RFID chip 404 can be used with any of the embodiments described herein.

[0025] 9 is a schematic diagram of an improved heat flux sensor according to another embodiment of the present invention. The heat flux sensor 420 is similar to the heat flux sensor 400 (shown in FIG. 8) except that the heat flux sensor 400 includes a pair of thermocouples 422, 424 coupled to an end cap 454. Providing a pair of thermocouples 422, 424 can simplify or speed up polarity detection of a transmitter coupled to the sensor 420. Additionally, utilizing a pair of thermocouples 422, 424 also provides redundancy in case one of the thermocouples 422, 424 fails. As shown, the RTD reference sensor 310 is still located a similar distance from the end cap 454 compared to the sensor 400 shown in FIG. 8.

[0026] FIG 10 is a schematic diagram of a heat flux sensor according to another embodiment of the present invention. As shown in FIG 10, the heat flux sensor 500 still includes four copper wires passing through a potting 502 at the cold end 504. Two of the wires 506, 508 are coupled to an RTD reference sensor 510. The other two wires 512, 514 are coupled to thermocouple wires 516, 518, respectively. Additionally, as shown in FIG 10, the other two thermocouple wires 520, 522 are coupled to the copper wires 506, 508, respectively, at a junction adjacent to the RTD reference sensor 510 (i.e., the cold junction thermocouple).

[0027] 10 shows that the hot end thermocouple 524 is coupled to the end cap 526 and another thermocouple 528 is placed some distance (X mm) away from the end cap 526 and the thermocouple 524. In one embodiment, the RTD reference 510 of the sensor 500 is a 1000 ohm platinum RTD sensor. Selecting a sensor such as a 1000 ohm platinum RTD sensor helps minimize the effect of lead resistance. The thermocouple placement of the thermocouple 528 can be determined by the resolution of the measurement and the linearity of the heat transfer. Therefore, multiple thermocouple sensors in the sensor 500 can provide a more accurate heat flux measurement.

[0028] There are several variations in the heat flux sensor according to various embodiments of the present invention. In the illustrated case, the location of the RTD is shown in the described embodiment as being relatively centrally located within the sensor capsule assembly. In practice, the location of the RTD reference sensor can be anywhere within the sensor capsule that allows for suitable measurement requirements, including the potted cold end 504. This is also the case for the thermocouple. Additionally, the thermocouple can be a grounded or ungrounded thermocouple, as desired.

Claims

1. 1. A process temperature estimation system, comprising: a mounting assembly configured to mount the process temperature estimation system to an exterior surface of a process fluid conduit; a hot end thermocouple thermally coupled to an exterior surface of the process fluid conduit; a resistance temperature detector (RTD) spaced from the hot end thermocouple; a measurement circuit coupled to the hot end thermocouple and configured to detect the electromotive force (emf) of the hot end thermocouple and the resistance of the RTD as a function of temperature to provide sensor temperature information; and a controller coupled to the measurement circuitry and configured to measure a reference temperature based on a resistance value of the RTD and perform a heat transfer calculation using the reference temperature, an electromotive force (emf) of the hot end thermocouple, and a known thermal conductivity of the process fluid conduit to generate an estimated process temperature output; A process temperature estimation system comprising:

2. 2. The process temperature estimation system of claim 1, further comprising a sensor capsule having an end cap defining a sensor capsule chamber therein, the high temperature end thermocouple being disposed within the sensor capsule chamber proximate the end cap, and the RTD being disposed within the sensor capsule chamber.

3. 3. The process temperature estimation system of claim 2, wherein the sensor capsule is electrically coupled to the measurement circuit by a plurality of copper conductors, and the controller is configured to identify a copper conductor coupled to the hot end thermocouple.

4. The process temperature estimation system of claim 3 , wherein the controller is configured to determine a polarity of the hot end thermocouple.

5. The process temperature estimation system of claim 2 , wherein the sensor capsule is a calibrated sensor capsule.

6. The process temperature estimation system of claim 5 , wherein the calibrated sensor capsule is a traceable calibrated sensor capsule.

7. 6. The process temperature estimation system of claim 5, further comprising an RFID chip containing the Callender-Van Dusen coefficient of the RTD.

8. The process temperature estimation system of claim 7 , wherein the RFID chip also contains information regarding the hot end thermocouple.

9. 3. The process temperature estimation system of claim 2, wherein the sensor capsule is configured to contact a process fluid conduit having a temperature as high as 600 degrees Celsius.

10. The process temperature estimation system of claim 1 , wherein the measurement circuitry and controller are disposed within an electronics housing that is attached to the process fluid conduit.

11. The process temperature estimation system of claim 2 , wherein the sensor capsule is electrically coupled to the measurement circuitry via an extension cable.

12. 2. The process temperature estimation system of claim 1, wherein the RTD is separated from the hot end thermocouple by a known thermal impedance.

13. 1. A sensor capsule for a process temperature estimation system, the sensor capsule comprising: an end cap configured to contact an exterior surface of a process fluid conduit, the end cap having the sensor capsule defining a chamber therein; a hot end thermocouple disposed within a chamber of the sensor capsule, the hot end thermocouple being thermally coupled to the end cap of the sensor capsule; and a resistance temperature detector (RTD) disposed within a chamber of the sensor capsule, the RTD being spaced from the hot end thermocouple; A sensor capsule comprising:

14. The sensor capsule of claim 13 , wherein the RTD is positionable within a chamber of the sensor capsule.

15. The sensor capsule of claim 13 further comprising a cold end thermocouple junction formed proximate to the RTD.

16. 16. The sensor capsule of claim 15, wherein the RTD is coupled to a plurality of conductors, and the cold end thermocouple is electrically coupled to one of the plurality of conductors.

17. The sensor capsule of claim 13 , further comprising a potted end opposite the end of the sensor capsule having the end cap.

18. 20. The sensor capsule of claim 17, further comprising a plurality of copper conductors passing through the potted end.

19. The sensor capsule of claim 13 , further comprising an additional thermocouple disposed within the chamber of the sensor capsule.

20. The sensor capsule of claim 19 , wherein the additional thermocouple is coupled to the end cap.

21. The sensor capsule of claim 19 , wherein the additional thermocouple is disposed within the sensor capsule at a location between the RTD and the hot end thermocouple.

22. The sensor capsule of claim 13 further comprising an RFID chip containing the coefficient of the RTD.

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