Patent Description:
It is common to place a temperature sensor within a thermowell, which is then inserted into the process fluid flow through an aperture in the conduit. However, this approach may not always be practical in that the process fluid may have a very high temperature, be very corrosive, or both. Additionally, thermowells generally require a threaded port or other robust mechanical mount/seal in the conduit and thus, must be designed into the process fluid flow system at a defined location. Accordingly, thermowells, while useful for providing accurate process fluid temperatures, have a number or limitations.

More recently, process fluid temperature has been estimated by measuring an external temperature of a process fluid conduit, such as a pipe, and employing a heat flow calculation. This external approach is considered non-invasive because it does not require any aperture or port to be defined in the conduit. Accordingly, such non-intrusive approaches can be deployed at virtually any location along the conduit. An example of such an approach is disclosed in <CIT>, which describes a mounting assembly configured to mount the process fluid temperature estimation system to an external surface of a process fluid conduit, a sensor capsule having an end configured to contact the external surface of the process fluid conduit to form an interface, the sensor capsule having at least one temperature sensitive element disposed therein, measurement circuitry coupled to the sensor capsule and configured to detect an electrical characteristic of the at least one temperature sensitive element that varies with temperature and provide at least process fluid skin temperature information, a controller coupled to the measurement circuitry, the controller being configured to: obtain the process fluid conduit skin temperature information from the measurement circuit; obtain reference temperature information; employ a heat transfer calculation with the process fluid conduit skin temperature information and reference temperature information to generate an estimated process fluid temperature output.

As recent advances have improved the accuracy of process fluid temperature estimation using non-invasive techniques, new sources of error have been identified. Addressing and correcting these new sources of error will improve the accuracy of non-invasive process fluid temperature estimation.

The present invention provides a process fluid temperature estimation system and a method of providing non-invasive process fluid temperature estimation relative to a process fluid conduit having a curved surface according to the independent claims. Further embodiments of the invention may be realized according to the corresponding dependent claims.

Embodiments described herein generally leverage identification and appreciation of additional sources of error in non-invasive process fluid temperature estimation in order to provide solutions with increased accuracy. Two sources of error have been identified and various embodiments described herein can provide solutions to one or both sources.

A first source of error is variability in the heat flow path from a first temperature measurement point to a second temperature measurement point. Often the second temperature measurement point is located within an electronics housing of the process fluid estimation system itself and thus the heat flow from the conduit surface sensor (i.e. the sensor measuring a skin temperature of a process fluid conduit) to the reference temperature sensor (e.g., located within the electronics housing) must be tightly controlled. This means that the system must always be mounted the same distance from the process fluid conduit outer surface. This requirement can eliminate some potential applications for non-invasive process fluid temperature estimation, such as remote mount and high-temperature applications. Additionally, the primary heat path from the process fluid conduit to the electronics housing is generally through a sensor sheath. This makes the measurement highly susceptible to changing ambient conditions and can require a user to install insulation around the sensor. This can limit the accuracy of the system and require additional costs and labor for the end user.

A second source of error is generally believed to be caused by a relatively flat end of a sensor capsule contacting a curved process fluid conduit such as a pipe. This air gap between the flat surface and the curved process fluid conduit varies depending on the curvature of the process fluid conduit. As can be appreciated, in order to provide a solution that is usable with a wide range of diameters, addressing this source of error is important as well.

<FIG> is a diagrammatic view of a heat flow measurement system with which embodiments of the present invention are particularly applicable. System <NUM> generally includes a pipe clamp portion <NUM> that is configured to clamp around conduit or pipe <NUM>. Pipe clamp <NUM> may have one or more clamp ears <NUM> in order to allow the clamp portion <NUM> to be positioned and clamped to pipe <NUM>. Pipe clamp <NUM> may replace one of clamp ears <NUM> with a hinge portion such that pipe clamp <NUM> can be opened to be positioned on a pipe and then closed and secured by clamp ear <NUM>. While the clamp illustrated with respect to <FIG> is particularly useful, any suitable mechanical arrangement for securely positioning system <NUM> about an exterior surface of a pipe can be used in accordance with embodiments described herein.

System <NUM> includes heat flow sensor capsule <NUM> that is forced against external diameter <NUM> of pipe <NUM> by spring <NUM>. The term "capsule" is not intended to imply any particular structure or shape and can thus be formed in a variety of shapes, sizes and configurations, all having a relatively flat lower surface. While spring <NUM> is illustrated, those skilled in the art will appreciate that various techniques can be used to force sensor capsule <NUM> into contact with external diameter <NUM> of conduit <NUM>. Sensor capsule <NUM> generally includes one or more temperature sensitive elements, such as resistance temperature devices (RTDs). Sensors within capsule <NUM> are electrically connected to transmitter circuitry within housing <NUM>, which is configured to obtain one or more temperature measurements from sensor capsule <NUM> and calculate an estimate of the process fluid temperature based on the measurements from sensor capsule <NUM>, and a reference temperature, such as a temperature measured within housing <NUM> or one of the sensors of capsule <NUM>, or otherwise provided to circuitry within housing <NUM>.

In one example, the basic heat flow calculation can be simplified into: <MAT>.

In this equation, Tskin is the measured temperature of the external surface of the conduit. Additionally, Treference is a second temperature obtained relative to a location having a fixed thermal impedance (Rsensor) from the temperature sensor that measures Tskin. Rpipe is the thermal impedance of the conduit and can be obtained manually by obtaining pipe material information, pipe wall thickness information. Additionally, or alternately, a parameter related to Rpipe can be determined during a calibration and stored for subsequent use. Accordingly, using a suitable heat flow calculation, such as that described above, circuitry within housing <NUM> is able to calculate an estimate for the process fluid temperature (Tcorrected) and convey an indication regarding such process fluid temperature to suitable devices and/or a control room. In the example illustrated in <FIG>, such information may be conveyed wirelessly via antenna <NUM>.

<FIG> is a block diagram of circuitry within housing <NUM> of heat flow measurement system <NUM>, with which embodiments of the present invention are particularly applicable. System <NUM> includes communication circuitry <NUM> coupled to controller <NUM>. Communication circuitry <NUM> can be any suitable circuitry that is able to convey information regarding the estimated process fluid temperature. Communication circuitry <NUM> allows heat flow measurement system <NUM> to communicate the process fluid temperature output over a process communication loop or segment. Suitable examples of process communication loop protocols include the <NUM>-<NUM> milliamp protocol, Highway Addressable Remote Transducer (HART®) protocol, FOUNDATION™ Fieldbus Protocol, and the WirelessHART protocol (IEC <NUM>).

Heat flow measurement system <NUM> also includes power supply module <NUM> that provides power to all components of system <NUM> as indicated by arrow <NUM>. In embodiments where heat flow measurement system <NUM> is coupled to a wired process communication loop, such as a HART® loop, or a FOUNDATION™ Fieldbus segment, power module <NUM> may include suitable circuitry to condition power received from the loop or segment to operate the various components of system <NUM>. In such a wired process communication loop embodiments, power supply module <NUM> may provide suitable power conditioning to allow the entire device <NUM> to be powered by the loop to which it is coupled. In other embodiments, when wireless process communication is used, power supply module <NUM> may include a source of power, such as a battery and suitable conditioning circuitry.

Controller <NUM> includes any suitable arrangement that is able to generate a heat-flow based process fluid temperature estimate using measurements from sensor(s) within capsule <NUM> and an additional reference temperature, such as a terminal temperature within housing <NUM> or a temperature measurement from a second temperature sensor disposed within capsule <NUM>. The reference temperature, in some applications, may be known or estimated for a well-controller process or ambient environment (e.g. the system is located within a climate controlled facility). In one example, controller <NUM> is a microprocessor. Controller <NUM> is communicatively coupled to communication circuitry <NUM>.

Measurement circuitry <NUM> is coupled to controller <NUM> and provides digital indications with respect to measurements obtained from one or more temperature sensors <NUM>. Measurement circuitry <NUM> can include one or more analog-to-digital converters and/or suitable multiplexing circuitry to interface the one or more analog-to-digital converters to sensors <NUM>. Additionally, measurement circuitry <NUM> can include suitable amplification and/or linearization circuitry as may be appropriate for the various types of temperature sensors employed.

<FIG> are diagrammatic views of sensor capsule <NUM> as it contacts different process fluid conduits <NUM>, <NUM>, respectively. In order to illustrate the variation in air gap caused by different curvatures, two process fluid conduits <NUM> and <NUM> are shown. Sensor capsule <NUM> is shown containing an RTD element <NUM> therein. Sensor capsule <NUM> has a relatively flat lower surface <NUM> that contacts the process fluid conduit. As shown, when flat surface <NUM> contacts large-diameter process fluid conduit <NUM>, a relatively small air gap <NUM> is formed between process fluid conduit <NUM> and flat surface <NUM>. However, as shown in <FIG>, when a smaller-diameter process fluid conduit <NUM> is used, the higher curvature of the process fluid conduit creates a larger air gap <NUM> between flat surface <NUM> and the smaller-diameter process fluid conduit <NUM>. The thermal conductivity between the two interfaces contains a small interface air gap that makes up a significant part of the required correction. The corrected temperature estimation is provided by the equation set forth below; <MAT>.

In the above equation, the air gap thermal resistance is included in the Rother parameter.

<FIG> is a chart illustrating error induced by conduit geometry as a function of conduit diameter. As can be seen, for smaller diameters, the error induced by the geometry can climb significantly. In accordance with one embodiment of the present invention, controller <NUM> (shown in <FIG>) is provided with an indication of pipe diameter upon which the process fluid temperature estimation system will be used. The pipe diameter is then used to access an error mapping or compensation curve in order to identify an appropriate parameter that models the air gap thermal resistance for the particular conduit being used. A default value can be set that is appropriate for a wide range of diameters, such as setting a default pipe diameter as <NUM> or <NUM> inches. However, an end user can specify a smaller pipe diameter during ordering and acquire a system already having the specified pipe diameter for the compensation curve. Alternately, the pipe diameter can be communicated to controller <NUM> via process communication using communication circuitry <NUM> or entered manually via a user interface (not shown). Using such a compensation curve or lookup table, the non-invasive process fluid temperature estimation system can automatically correct for the geometry differences between the sensor capsule and the surface of the process fluid conduit.

As the pipe diameter decreases, the air gap increases exponentially requiring more correction for small diameter conduits. The compensation curve provides a thermal resistance parameter (Rother) based on pipe diameter that is typically configured by the user. The compensation calculation, set forth above, then adjusts the correction ratio appropriately to provide a more accurate output. Additional configuration options that can be communicated to controller <NUM> or entered via user interface can indicate if the sensor is mounted perpendicular to a selected geometry, such as a pipe, flat surface, etc..

<FIG> is a diagrammatic view illustrating various thermal resistance parameters in the sensor capsule/process fluid conduit interface. The diagram shown in <FIG> is significantly zoomed relative to that shown in <FIG>. The small interface air gap (Rair) is also included. Even though this appears small, Rair is the largest contributor to thermal impedance. In one example, values for the various parameters are set forth in the table below.

As set forth above, providing an Rother parameter that is changeable based on the diameter of the process fluid conduit significantly improves the accuracy of the process fluid temperature estimation calculation set forth above. Further, improved accuracy can still be provided if changes in the end cap material (e.g. something other than silver) is provided as long as the map of compensation curve provided to controller <NUM> includes an indication of thermal conductivity and length for the selected end cap. The same can be done for variations in thermal grease length and composition. Additionally, it is expressly contemplated that if a diameter is used that is not expressly matched to a given Rair parameter, embodiments described herein can interpolate between the two nearest data points.

Another source of error in process fluid temperature estimation using non-invasive techniques is the potential variability in heat flow from the process fluid conduit skin temperature measurement to a reference temperature measurement. In accordance with another embodiment described herein, this heat flow variability is substantially minimized or at least controlled by providing two temperature sensors disposed within the same sensor capsule and spaced from one another along a heat flow path.

<FIG> is a diagrammatic view of an improved sensor capsule employing a staggered bore process fluid temperature estimation system in accordance with one embodiment of the present invention. As used herein, "staggered bore" means that two bores have a different distance from the external surface of the process fluid conduit and that they also have an offset in at least one other dimension (e.g. along an axis of the conduit, and/or in a direction perpendicular to the axis of the conduit). Sensor capsule <NUM> employs a machined tip <NUM> into which multiple sensing elements <NUM> and <NUM> are mounted. Sensing elements <NUM> and <NUM> are preferably RTD's and are connected to wires running through the length of sensor capsule <NUM> to be coupled to measurement circuitry <NUM> (shown in <FIG>). Sensing elements <NUM>, <NUM> are held in place within sensor capsule <NUM> using either mechanical or chemical (i.e. bonding) techniques. The cap can then be welded onto the tube to seal the tip. The machined tip provides precise and consistent sensor spacing between elements <NUM> and <NUM>. These machined parts are relatively easy to inspect and improve consistency of assembly by allowing the sensors inserted in them to bottom out. This eliminates potential problems of inconsistent element mounting within the sensor capsule since error can arise when the spacing between the elements and the surface directly affects the measurements. Additionally, or alternatively, portions of the sensor capsule may be 3D printed in order to facilitate production of more complicated features, such as square holes.

The materials used for tip <NUM> can vary significantly. Materials with high thermal conductivity, such as copper and silver can be used to improve heat transfer but choosing a material with intentionally low thermal conductivity could allow the spacing between elements <NUM> and <NUM> to be much smaller and thus to reduce the overall cost. Tip <NUM> could also be varied to match the pipe and clamp material in order to eliminate galvanic corrosion concerns. As the material of a component of the sensor capsule is varied, the size and thermal conductivity of the component can be stored in the lookup table or compensation curve in order to accurately estimate process fluid temperature. Each material, accordingly, would have different thermal properties and would affect the process fluid temperature estimation, and such variations can be accommodated in the Rother parameter of the calculation.

However, in one example, a single block of homogenous material is used to mount elements <NUM>, and <NUM> and thus corrections for heat flow between the sensing elements <NUM> and <NUM>, are easily made. For example, sensing element <NUM> may be considered a skin temperature sensor and sensing element <NUM> may be considered a reference temperature sensor. The difference in the measured temperatures will be related to the magnitude of the heat flow through the block of homogenous material and its thermal conductivity. Increasing the precision of placement of elements <NUM> and <NUM> allows the spacing between elements <NUM> and <NUM> to be reduced thus reducing the overall size of sensor capsule <NUM>. It is believed that this will improve the linearity of the heat gradient across elements <NUM> and <NUM> and make the response less affected by external influences. It is preferred, in one example, that temperature sensitive elements <NUM> and <NUM> be RTDs, since such devices are generally believed to have higher accuracy and repeatability than other types of temperature sensitive elements, such as thermocouples.

<FIG> shows a portion of sensor capsule <NUM> where epoxy or some other suitable encapsulation <NUM> has been applied. Epoxy <NUM> ensures that temperature sensitive elements <NUM> and <NUM> remain secured within their respective bores and also helps to provide strain relief where the lead wires of each individual temperature sensitive element attach to the temperature sensitive element. Accordingly, the machined tip and sensing elements could be prebuilt into a metal capsule with wires. This could allow late customization during manufacture to build a sensor to a final length while maintaining consistency of the measurement.

<FIG> is a diagrammatic view of a portion of a sensor capsule providing a staggered bore for a plurality of temperatures sensitive elements, as well as a curved end to match the curvature of a selected process fluid conduit. In this sense, the embodiment illustrated in <FIG> can be thought of as addressing both sources of error. By providing a curvature to end <NUM> that is matched to the curvature of the process fluid conduit, an air gap between end <NUM> and the process fluid conduit is minimized. Further, providing a staggering between bores <NUM> and <NUM> using machining techniques provides a highly reliable location technique for temperature sensors, such as elements <NUM> and <NUM>, to be mounted therein. Since the curvature of surface <NUM> must be selected during the ordering process, the embodiment shown in <FIG> is not particularly amenable to an end user making changes to process fluid conduit diameters without requiring an entirely new sensor capsule <NUM>.

<FIG> is a diagrammatic view of a portion of a sensor capsule in accordance with another embodiment of the present invention. Sensor capsule <NUM> includes bores <NUM> and <NUM> terminating further from flat surface <NUM> (i.e. the hot end) than the embodiments shown in <FIG>, <FIG>. Terminating bores <NUM> and <NUM> further from surface <NUM> may reduce high temperature exposure to the sensitive elements, which may provide additional advantages for higher temperature applications.

<FIG> illustrates another portion of a sensor capsule in accordance with another embodiment of the present invention. Specifically, sensor capsule <NUM> includes machined tip <NUM> that is larger or equal diameter to tube <NUM> to provide more space for sensing elements within bores <NUM>, <NUM>. Tip <NUM> is attached to tube <NUM> in accordance with any suitable techniques, such as welding.

<FIG> is a diagrammatic view of a portion of a sensor capsule <NUM> in accordance with another embodiment of the present invention. Sensor capsule <NUM> includes an insert <NUM> that that is formed to include bores <NUM>, <NUM>. Bores <NUM>, <NUM>, are sized to receive temperature sensitive elements, such as elements <NUM>, <NUM> (shown in <FIG>). Additionally, the walls of insert <NUM> may be tapered away from side wall <NUM> of a tube in order to provide better insulation from ambient conditions.

<FIG> is a flow diagram of a method of estimating process fluid temperature non-invasively in accordance with an embodiment of the present invention. Method <NUM> begins at block <NUM> where an end user provides an indication of conduit curvature. Such indication can be provided in the form of communication (e.g. process communication and/or interaction with a user interface) to a process fluid temperature estimation system, as indicated at block <NUM>. Alternatively, the indication can be provided to a manufacturer during acquisition of the process fluid temperature system, such that the curvature of the conduit is already entered into the system when it is shipped to the end user, as indicated at block <NUM>. The curvature can also be set by the end user ordering a system with a sensor capsule having a curvature that matches the curvature of the conduit, as indicated at block <NUM>.

At block <NUM>, the process fluid estimation system is installed on the process fluid conduit. Next, at block <NUM>, the Rother term is obtained or calculated. Note, for embodiments that do not have a sensor capsule with a conduit-matched curved end, the Rother parameter will have a value that is influenced by Rair based on the diameter of the process fluid conduit, as indicated at block <NUM>. As set forth above, the Rother parameter may be obtained from a lookup table based on the selected conduit curve. Further, other heat flow variables, such as end cap thickness and/or material can be obtained from a lookup table. Further still, thermal grease heat flow information can also be obtained. These other factors that affect Rother are indicated diagrammatically at reference numeral <NUM>.

Next, at block <NUM>, the system obtains a skin temperature of the process fluid conduit. At block <NUM>, a reference temperature is measured. The reference temperature may be obtained from a temperature sensor coupled to a terminal located within an electronics housing of the system or it may be obtained from an additional temperature sensitive element located within the sensor capsule but positioned in such a way that a known thermal impedance exists between the skin temperature sensor and the reference temperature sensor, such as shown in <FIG>.

At block <NUM>, controller <NUM> (shown in <FIG>) performs a heat flow calculation using the measured skin temperature, reference temperature, and Rother. In embodiments where the sensor capsule has a curved end that matches the curvature of the process fluid conduit, the Rother parameter may not include a value for Rair. However, in such cases, Rother may still model other heat flow characteristics, such as the heat flow through the curved cap as well as through the particular thermal grease employed. At block <NUM>, the estimation of process fluid temperature, based on the heat flow calculation performed at block <NUM>, is provided as an output. Then, method <NUM> repeats by returning to block <NUM> to obtain another skin temperature.

Claim 1:
A process fluid temperature estimation system (<NUM>) comprising:
a mounting assembly (<NUM>) configured to mount the process fluid temperature estimation system (<NUM>) to an external surface (<NUM>) of a curved process fluid conduit (<NUM>);
a sensor capsule (<NUM>) having an end configured to contact the external surface (<NUM>) of the process fluid conduit (<NUM>), said sensor capsule (<NUM>) having a relatively flat lower surface (<NUM>), to form an interface having a contact region and an air gap (<NUM>) between the relatively flat lower surface (<NUM>) and the curved process fluid conduit (<NUM>), the sensor capsule (<NUM>) having at least one temperature sensitive element (<NUM>, <NUM>, <NUM>) disposed therein;
measurement circuitry (<NUM>) coupled to the sensor capsule (<NUM>) and configured to detect an electrical characteristic of the at least one temperature sensitive element (<NUM>, <NUM>, <NUM>) that varies with temperature and provide at least process fluid conduit skin temperature information (Tskin);
a controller (<NUM>) coupled to the measurement circuitry (<NUM>), the controller (<NUM>) being configured to:
obtain the process fluid conduit skin temperature information (Tskin) from the measurement circuitry (<NUM>);
obtain reference temperature information (Treference),
obtain a heat flow parameter related to the air gap (<NUM>) of the interface, and
employ a heat transfer calculation with the process fluid conduit skin temperature information (Tskin), reference temperature information (Treference), and heat flow parameter to generate an estimated process fluid temperature output.