Patent Description:
A process fluid temperature transmitter provides an output related to a process fluid temperature. The temperature transmitter output can be communicated over a process control loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled.

Traditionally, process fluid temperature transmitters were coupled to or employed thermowells which thermally couple a temperature sensor to a process fluid, but otherwise protect and isolate the temperature sensor from direct contact with the process fluid. The thermowell is positioned within the process fluid in order to ensure substantial thermal contact between the process fluid and the temperature sensor disposed therein.

Thermowells are typically designed using relatively robust metal structures such that the thermowell can withstand a number of challenges provided by the process fluid. Such challenges can include physical challenges, such as process fluid flowing past the thermowell at a relatively high rate; thermal challenges, such as extremely high temperatures; pressure challenges, such as process fluid being conveyed or stored at a high pressure; and chemical challenges, such as those provided by a caustic process fluid.

There are a number of factors that increase the difficulty of working with thermowells in process fluid measurement systems. One issue is that thermowells are intrusive and invasive to the process fluid system. This is because the thermowell must extend through a wall of a process fluid conduit, such as a pipe, in order to couple a temperature sensor to the process fluid. Additionally, thermowells typically require regular evaluation for many related factors. Further, thermowells generally increase the response time for temperature measurement.

Thermowells are used in a number of environments for a variety of reasons. Thermowells provide a robust and effective conduit to allow a temperature sensor to be placed in thermal communication with a process fluid. Additionally, thermowells provide protection for the sensor capsule from materials flowing in the process fluid conduit that could easily bend, break, or erode the temperature sensor capsule. Moreover, thermowells allow the temperature capsule to be easily removed for calibration and replacement without having to shut down the process.

<CIT> discloses a system for correcting an installation error when using a thermowell for measuring the temperature of a process fluid in a conduit by using measurements of a plurality of temperature sensors spaced apart from each other. <CIT> discloses a system for a non-invasive determination of the temperature of a fluid inside a vessel. The system comprises two temperature sensors which are spaced apart from each other by a thin flexible sheet with known thermal conductivity, and the temperature is determined from the measurement of the two temperatures and the known thermal conductivities of the vessel wall and the spacer, respectively. <CIT> discloses a process fluid temperature measurement system comprising a thermowell which is coupled to a process fluid conduit wall via a thermally insulative collar. <CIT> discloses a system for measuring an ambient temperature close to a heat source. Two temperature sensors are mounted at a distance from each other on a carrier circuit board. The ambient temperature is determined based on the difference between the temperatures measured by the two sensors, and the ratio of the thermal resistance for the heat transfer from the surroundings to first temperature sensor and the thermal resistance of the carrier circuit board.

The invention relates to a process fluid temperature measurement system according to claim <NUM>, and a method for detecting process fluid temperature within a conduit according to claim <NUM>. Embodiments of the invention are defined by the dependent claims.

One design consideration for using thermowells in process fluid measurement systems is that the thermowells should generally be inserted into the process fluid with a length that is approximately one third of the pipe diameter in order to achieve the highest accuracy. The main reason for this design consideration is to reduce or minimize the influence on the temperature sensor element from the process fluid conduit temperature. Additionally, to achieve the desired insertion depth for large pipe diameters, longer thermal wells are needed. However, factors such as vortex shedding, material impacts, and water hammering become a larger concern in the strength of the thermowell. With this and other design considerations in mind, users typically require thermowell lengths specified with millimeter resolution thereby requiring manufactures of thermowells to maintain significant inventory in the various possible lengths of thermowells. Thermowells are typically made robust using a significant amount of material. While such robust designs improve thermowell longevity, they can slow the thermowell's responsiveness to a process temperature change. In some applications, a fast-changing temperature relates to a fault in the process such as a reaction runaway. In this case, it is very important to understand as soon as is feasible that this is occurring so that materials can be added to the process to slow the reaction. Accordingly, at least some embodiments described herein may reduce response time.

<FIG> is a diagrammatic view of a process fluid temperature measurement system coupled to a thermowell in accordance with the prior art. As illustrated in <FIG>, process fluid <NUM> flows through process fluid conduit <NUM>. A thermowell <NUM> is mounted within an aperture <NUM> or process intrusion through the wall of conduit <NUM>. Typically, thermowell <NUM> will have an externally threaded portion that engages internal threads of aperture <NUM> to form a robust coupling. However, in some implementations that the thermowell may be welded to the pipe wall, or coupled thereto using other arrangements, such as a pair of mating flanges and a gasket. Regardless, thermowell <NUM> is generally formed of a robust material such as stainless steel, and has an internal chamber <NUM> that is sized to allow temperature sensor assembly <NUM> to extend to therein. Temperature sensor assembly <NUM> includes a temperature sensor that is positioned within thermowell <NUM> in order to measure the temperature of process fluid <NUM>.

<FIG> is a diagrammatic view of a process fluid temperature measurement system in accordance with another embodiment of the present invention. Process fluid temperature measurement system <NUM> includes a thermowell <NUM> coupled to conduit wall <NUM>. Thermowell <NUM> is relatively short (in comparison to thermowell <NUM> shown in <FIG>) and is mechanically coupled directly to pipe wall <NUM>. As shown in <FIG>, system <NUM> includes temperature sensor assembly <NUM> that includes a plurality of temperature sensitive elements <NUM>, <NUM>. Temperature sensitive elements <NUM>, <NUM> can be formed of any suitable temperature sensing devices. The temperature sensitive elements can be any suitable device or apparatus that has an electrical characteristic that varies with temperature. Suitable examples include resistance temperature devices (RTDs), thermistors, thermocouples, or other suitable devices. The temperature sensitive elements <NUM>, <NUM> of sensor assembly <NUM> are generally coupled to transmitter circuitry <NUM> within transmitter housing <NUM>. The transmitter circuitry (described below) is generally configured to measure or otherwise detect the electrical property of the temperature sensitive elements <NUM>, <NUM> and generate a process fluid temperature output related to the measured temperatures.

The output from process fluid temperature measurement system can be provided over a process communication loop, such as a <NUM>-<NUM> milliamp loop, or provided digitally, such as in accordance with the Highway Addressable Report Transducer (HART®). Other examples of process communication protocols include the Profibus-PA Communication Protocol and the FOUNDATION™ Fieldbus Protocol. Further still, suitable wireless technologies can be used in addition to or in place of a wired process communication protocol. One example of a suitable wireless process communication protocol is that in accordance with the WirelessHART standard (IEC <NUM>).

As shown in <FIG>, temperature sensitive element <NUM> is disposed adjacent a distal end <NUM> of thermowell <NUM> and temperature sensitive element <NUM> is spaced apart from temperature sensitive element <NUM> along sensor assembly <NUM> by element <NUM> having a known or relatively constant thermal conductivity. By placing more than one temperature sensitive element in sensor assembly <NUM> at different locations spaced by spacer <NUM>, electronics <NUM> within transmitter housing <NUM> can use a heat flux measurement to infer the temperature of process fluid <NUM>. As shown in <FIG>, temperature sensitive elements <NUM>, and <NUM> are separated in temperature capsule <NUM> by spacer <NUM> and placed at locations that are significantly impacted by both pipe wall temperature and process fluid temperature. As used herein, a "spacer" is any physical structure or arrangement that controls or sets a distance and thermal conductivity between two elements. Accordingly, a spacer may be formed of a solid, a powder, such as Magnesium Oxide powder, or even an air gap. Any change on temperature sensor element <NUM> will also affect the temperature at temperature sensitive element <NUM> and vice versa. Given this correlation, the heat flux calculation can be simplified to what is shown in <FIG>. While <FIG> shows thermowell <NUM> extending into process fluid conduit <NUM>, it is noted that heat flux measurement can be performed effectively for embodiments where the thermowell does not protrude into the process vessel or conduit whatsoever.

<FIG> is a diagrammatic view of heat conduction modeled using resistive components. Specifically, the heat of process fluid <NUM>, shown at node <NUM>, flows to node S2 at temperature sensitive element <NUM> through the thermal impedance of the thermowell as indicated diagrammatically at Rtwell <NUM>. Then, the heat flows along spacer element <NUM> to node S1 where the temperature is sensed by temperature sensitive element <NUM>. The thermal impedance through spacer <NUM> is modeled diagrammatically as Rsnsr <NUM>. Finally, heat at node S1 may flow to or from pipe wall <NUM> indicated at node <NUM>. The thermal impedance from node S1 to pipe wall node <NUM> is illustrated diagrammatically as Radapter <NUM>. Given these quantities, the temperature of process fluid is equal to the temperature measured at temperature sensitive element <NUM> plus the difference between the temperature measured at element <NUM> and <NUM> multiplied by Rtwell / Rsnsr.

<FIG> is a diagrammatic view of the process fluid temperature measurement system shown in <FIG> and <FIG>. Specifically, sensor assembly <NUM> includes temperature sensitive elements <NUM>, <NUM> that are separated by spacer <NUM>. Each of temperature sensitive elements <NUM>, <NUM> is operably coupled to measurement circuitry <NUM> within transmitter housing <NUM>. Measurement circuitry <NUM> generally includes any suitable arrangement of electrical circuits that are able to engage each of temperature sensitive elements <NUM> and <NUM> to measure the temperature-sensitive electrical property thereof. Measurement circuitry <NUM> can include one or more analog-to-digital converters as well as suitable switching circuitry, such as a multiplexer. Additionally, measurement circuitry <NUM> can also include any suitable linearization and/or amplification circuitry. Measurement circuitry <NUM> generally provides a digital indication of the electrical properties of temperature sensitive elements <NUM>, <NUM> to controller <NUM>. In one embodiment, controller <NUM> may be a microprocessor or microcontroller, or any other suitable circuitry that is able to receive the digital indications from measurement circuity <NUM> and execute the heat flux calculation described with respect to <FIG>. Additionally, as shown in <FIG>, controller <NUM> is coupled to communication module <NUM>.

Communication module <NUM> allows the temperature measurement system to communicate the process fluid temperature output over a process communication loop. As set forth above, suitable examples of process communication loop protocols include the <NUM>-<NUM> milliamp protocol, HART®, FOUNDATION™ Fieldbus Protocol, and WirelessHART (IEC <NUM>). Process fluid temperature measurement system <NUM> also includes power supply module <NUM> that provides power to all components of the system as indicated at arrow <NUM>. In embodiments where the process fluid temperature measurement system is coupled to a wired process communication loop, such as a HART® loop, or a FOUNDATION™ Fieldbus process communication segment, power module <NUM> may include suitable circuitry to condition power received from the loop to operate the various components of system <NUM>. Accordingly, in such wired process communication loop embodiments, power supply module <NUM> may provide suitable power conditioning to allow the entire device 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.

<FIG> is a diagrammatic view of a process fluid temperature measurement system <NUM> in accordance with an embodiment of the present invention. As shown in <FIG>, a thermowell <NUM> is mounted to and extends through pipe wall <NUM> into process fluid <NUM>. In comparison to the arrangement shown in <FIG>, thermowell <NUM> does not extend into the process fluid conduit nearly as far as thermowell <NUM>. Part of this difference is enabled by the thermal insulation that is provided by collar <NUM>. Collar <NUM> is configured to engage an internal surface of aperture <NUM>. For example, collar <NUM> may include external threads that engage internal threads of aperture <NUM>. Collar <NUM> then includes suitable internal structure to engage thermowell <NUM> to reliably and sealingly mount thermowell <NUM> to pipe wall <NUM>. However, collar <NUM> is constructed from a material that reduces thermal conduction, in comparison to the material from which thermowell <NUM> is constructed. Using collar <NUM> around thermowell <NUM> allows the temperature from the process to propagate to the temperature sensor with reduced influence from conduit <NUM>. By using a thermally resistive material between the stem of thermowell <NUM> and process conduit <NUM>, the thermowell length can be reduced while still maintaining requisite measurement accuracy. The thermally-resistive material can be any suitable organic or inorganic material that is able to withstand the process fluid pressures involved, as well as provide suitable chemical resistance to the process fluid. Examples of suitable materials include ceramics as well as organic materials, such as polytetrafluoroethylene (PTFE). By providing collar <NUM> around thermowell <NUM>, thermowell <NUM> will respond faster with a reduced mass, which will also significantly reduce the need for wake frequency calculations. Further still, it is believed that fewer such "short" thermowell sizes will be needed, in comparison to current commercially-available sizes, which are generally specified down to the millimeter. Additionally, a thermally-insulative material, such as a gasket, may also be disposed between a mounting flange of the thermowell and a pipe standoff.

<FIG> is a diagrammatic view of a process fluid temperature measurement system in accordance with an embodiment of the present invention. Process fluid temperature measurement system <NUM> bears many similarities to system <NUM> (described with respect to <FIG>), and like components are numbered similarly. The main difference between system <NUM> and system <NUM> is that system <NUM> has a sensor assembly that has a single temperature sensitive element <NUM>. Thus, it is expressly contemplated that by using a thermally-insulative collar, such as collar <NUM>, enough heat flow to/from the process fluid conduit may be prevented that a short (with respect to thermowell <NUM>) thermowell may be used.

Claim 1:
A process fluid temperature measurement system (<NUM>, <NUM>) comprising:
a thermally-insulative collar (<NUM>) configured to couple to a process fluid conduit wall (<NUM>);
a thermowell (<NUM>, <NUM>, <NUM>) configured to couple to a process fluid conduit via the thermally-insulative collar and extend through the process fluid conduit wall (<NUM>);
a temperature sensor (<NUM>) assembly disposed within the thermowell (<NUM>, <NUM>, <NUM>), the temperature sensitive assembly including:
a first temperature sensitive element (<NUM>) disposed within the thermowell (<NUM>, <NUM>, <NUM>) adjacent a distal end of the thermowell (<NUM>, <NUM>, <NUM>);
a spacer (<NUM>) having a known thermal conductivity, the spacer (<NUM>) being formed of a powder;
a second temperature sensitive element (<NUM>) spaced apart from the first temperature sensitive element (<NUM>) along the spacer (<NUM>); and
transmitter circuitry (<NUM>) coupled to the first and second temperature sensitive elements (<NUM>, <NUM>) and configured to perform a heat flux calculation to provide a process fluid temperature output,
wherein the second temperature sensitive element (<NUM>) is disposed within the thermowell (<NUM>, <NUM>, <NUM>),
wherein the heat flux calculation is based on the thermal conductivity of the distal end of the thermowell (<NUM>, <NUM>, <NUM>) and the thermal conductivity of the spacer (<NUM>) between the first and second temperature sensitive elements (<NUM>, <NUM>),
wherein the heat flux calculation is based on a ratio of the thermal conductivity of the distal end of the thermowell (<NUM>, <NUM>, <NUM>) to the thermal conductivity of the spacer (<NUM>),
wherein the process fluid temperature output is equal to the temperature measured by the first temperature sensitive element (<NUM>) added to a quantity that is a difference in measurements between the first and second temperature sensitive elements (<NUM>, <NUM>) multiplied by the ratio,
wherein the transmitter circuitry (<NUM>, <NUM>, <NUM>) includes a microprocessor (<NUM>) configured to perform the heat flux measurement, and
wherein the transmitter circuitry (<NUM>, <NUM>, <NUM>) includes communication circuitry configured to communicate the process fluid temperature output to a remote device.