Patent Description:
Industrial processes are used in the manufacturing and transport of many types of materials, as set forth above. In such systems, it is often useful to measure process fluid pressure using a pressure sensor that is typically contained in, or coupled to, a process fluid pressure sensor which is electrically coupled to or part of a process fluid pressure transmitter that transmits pressure-related information to one or more remote device, such as a control room. The transmission is frequently over a process control loop.

In the measurement of process fluid pressure, it is relatively common, to operatively couple a pressure sensor to the process fluid using an aperture or intrusion in a process fluid conduit. In many instances, an isolation diaphragm is placed in direct contact with the process fluid and flexes in response to process fluid pressure. An opposite side of the isolation diaphragm is in contact with a fill fluid in a fill fluid conduit that conveys the isolation diaphragm movement to a sensing diaphragm of the pressure sensor. An electrical structure on the pressure sensor (such as a resistive, capacitive, or piezoelectric element) is responsive to movement of the sensing diaphragm of the pressure sensor and provides a signal that is measurable using measurement electronics of the process fluid pressure transmitter. 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, process intrusions to couple a pressure sensor to the process fluid 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, while such techniques are useful for providing accurate process fluid pressure indications, they have some limitations. A pressure sensor for detecting a fluid pressure inside a pipe is known from <CIT>.

A system for non-intrusively measuring process fluid pressure within a process fluid conduit is provided as defined in claim <NUM> and a calibration method is provided according to claim <NUM>.

Embodiments described herein generally leverage the ability to reliably and accurately measure the actual deformation of the process fluid conduit itself and characterize such deformation in such a way that an accurate process fluid pressure can be estimated and provided.

<FIG> is a diagrammatic view showing an exaggerated process fluid conduit deformation. As shown on the left most portion of <FIG>, process fluid conduit <NUM> (shown in cross section) has an internal pressure P of <NUM>. A mirrored or opposing structure <NUM> is attached to an external surface <NUM> of process fluid conduit <NUM>. In one example, each half is a mirrored-Z structure that is welded at welds <NUM> to external diameter <NUM> of process fluid conduit <NUM>. As shown, at internal pressure P = <NUM>, the mirrored-Z structure <NUM> has a relatively small gap <NUM>. In the right portion of <FIG>, an internal pressure P (where P > <NUM>) is provided that is exerted equally upon the inner diameter <NUM> of conduit <NUM> thereby increasing the outer diameter <NUM> of conduit <NUM>. When this occurs, each half of mirrored Z-structure <NUM> moves apart slightly and gap <NUM> (also labeled as G<NUM>) increases. Accordingly, it can be seen that process fluid pressure can be inferred or estimated using a specialized assembly comprised of a pair of brackets that can be tack welded, or otherwise attached, to an exterior of the pipe with a relatively small gap separating the brackets. The gap will separate as the internal pressure of process fluid conduit <NUM> increases. An important aspect of this operational technique is the design of the brackets or other suitable structure in such a way as to maximize pressure sensitivity while reducing temperature-induced effects.

<FIG> is a diagrammatic view of the structure illustrated with respect to <FIG>, showing various quantities in order to further illustrate a theory of operation. In <FIG>, process fluid conduit <NUM> has an internal radius a and an external radius b. Additionally, the mirrored-Z structure has a height defined in terms of the distance of gap <NUM> from center <NUM> of process fluid conduit <NUM>. In the illustrated example, this height is defined as Rs. Further, the angle of separation between the radially extending walls of mirrored-Z structure <NUM> is defined as sweep angle Θ. Embodiments described herein generally quantify the change in the outer circumference of a thick-walled conduit, defined as ΔS, when a pressure P is introduced inside the conduit. The assumption is that the outside pressure is zero. However, the expressions described below can be easily adapted to accommodate situations where the external pressure, such as ambient atmospheric pressure, is different. Under the no-exterior pressure assumption, the change in circumference (ΔS) is determined from the following equation:
<MAT>.

Where E is the Young's Modulus of the conduit and a and b are the inner and outer pipe radii, respectively.

The quantity in curly brackets is the change in hoop strain on the outer surface of the process fluid conduit as pressure is applied. It can be seen that the gap spacing <NUM> between the brackets goes from G<NUM> to a new value G(P) when a pressure P is applied according to:
<MAT>.

Equation <NUM> above can be rewritten as:
<MAT>.

Where K is an amplification factor defined by:
<MAT>.

One can see in Equation <NUM> that K will increase whenever θ or Rs increases, or when Go decreases. <FIG> is a diagrammatic cross-sectional view illustrating a capacitance-based gap change detector in accordance with an embodiment of the present invention. As can be seen, electrodes <NUM> and <NUM> are attached to each respective portion of the mirrored-Z gap measurement structure via insulators <NUM> and <NUM> respectively. The gap surfaces are the plates of a parallel plate capacitor, which separate as pressure increases. The formula for how the capacitance varies with gap spacing is provided below.

In the equation above, A is the area of the end of the bar, while G(P) is the gap at pressure P, and ε<NUM> is the permittivity of free space and has the value of <NUM>×<NUM>-<NUM> F/m (<NUM> pf/in).

In order to understand how temperature changes can affect the type of sensor described above, it is important to consider the geometry in order to minimize thermal effects. <FIG> is a diagrammatic view of the trapezoidal structure shown in <FIG> and shows what happens as the temperature increases. One important consideration in the minimization of thermal effects, is to match the sensor brackets to the pipe material. Accordingly, if the process fluid conduit is formed of, for example, <NUM> stainless steel, then the sensor brackets should also be made from <NUM> stainless steel. Similarly, if the process fluid conduit is formed of carbon steel, then the sensor brackets should also be made of carbon steel, and so forth. When this is the case, the gap will only expand by an amount equal to material of the same width.

The temperature-induced change in the gap (ΔG) can be considered to be G·αmetal·ΔT. With pressure, the sensor brackets begin to separate determined by the change in arc length swept by the angle Θ. This arc length change is translated into a gap change via equation <NUM>, which is measurable, as shown above. With a temperature change, the arc length also expands, however, the sensor bracket pieces that project back towards the gap are expanding inwardly. The net effect is that the gap only appreciably changes by an amount proportional to the gap spacing (i.e., G) and not proportional to the arc length. Accordingly, the design amplifies pressure effects (via arc length changes) without a commensurate increase in temperature sensitivity. Additionally, the use of thermal shields, insulation, and copper thermal traces would help reduce the impact of temperature transients on the sensor. Furthermore, an additional bracket disposed adjacent to the measurement bracket, but positioned to not be responsive to pressure, would help eliminate temperature transient effects by way of common-mode rejection or simple ratioing techniques.

<FIG> illustrates model results for a particular embodiment described herein. The theoretical (model) gap change at an Rs value of approximately <NUM> (<NUM> in). away from the center with applied pressure on a <NUM> (<NUM> in). outside diameter <NUM> stainless steel schedule <NUM> pipe is shown. In the illustrated example, a <NUM> kPa (<NUM>,<NUM> psi) pressure change produces a gap change of <NUM> (<NUM> mils (<NUM> inches)), which is large enough to be easily detected by a variety of means well understood by those skilled in the art, of which capacitance displacement is one of them.

While embodiments described above have generally provided a mirrored or opposing structure that is attached directly (i.e., welded or otherwise adhered) to the outside surface or diameter of a process fluid conduit, it is expressly contemplated that embodiments described herein can be practiced with a structure that clamps directly to a process fluid conduit.

<FIG> illustrate conduit deformation that leads to a further embodiment of a clamp-on sensor shown in <FIG>. In <FIG>, there is a thick band <NUM> encircling a pipe <NUM>, which can be considered to be a rigid clamp. The clamp is designed to be robust such that it will not deflect appreciably when the pipe is pressurized. <FIG> shows a finite element simulation of a <NUM> stainless steel pipe (schedule <NUM>) pressurized to <NUM> kPa (<NUM>,<NUM> psi) having a rigid clamp <NUM> in the middle. The shape is exaggerated to see the shape better. What is interesting is that the pipe diameter resumes its full deflection a relatively short distance away from clamp <NUM>, only <NUM> inches (in the longitudinal direction) in this example. This property can be used to construct a useful pressure sensor such as the one illustrated in <FIG>.

<FIG> are simplified perspective and side views, respectively, showing the location of sensors <NUM>, <NUM> that detect the gap change relative between arms <NUM>, which are fixed to the relatively immobile bracket, and the pipe's outer surface <NUM>, which deflects according to <FIG> with pressure.

For this design, the gap being detected changes according to:
<MAT>
where the variables, a, b, E, P are defined the same as before.

Band <NUM> is formed as a hinged clamp or a pair of clamp halves that can be bolted or otherwise secured about pipe <NUM>. Arm(s) <NUM> are affixed to band <NUM> and extend transversely therefrom. This transverse extension is preferably at least three inches beyond the edge of band <NUM> such that deformation of pipe <NUM>, in response to internal pressure, is fully developed, as shown in <FIG>. The band <NUM> and arm(s) <NUM> are preferably constructed from the same material as the pipe in order to minimize thermal expansion effects as already explained following the arguments cited regarding <FIG>. Accordingly, with changes to temperature, clamp <NUM> and arms <NUM> will change in size in much the same way as pipe <NUM>. In the illustrated example, arms <NUM> extend both upstream and downstream from band <NUM>, with gap measurement system or detectors <NUM>, <NUM> disposed proximate an end of each arm <NUM>. In one example, each detector <NUM>, <NUM> is a capacitive sensor that has a capacitance with pipe <NUM> that varies with the gap between the detector and pipe <NUM>.

By way of example, <FIG> tabulates the expected gap changes per <NUM> kPa (ksi (<NUM>,<NUM> psi)) for a <NUM> (<NUM> in). OD <NUM> stainless steel pipe having various thickness wall schedules.

<FIG> is a diagrammatic cross-sectional view of another embodiment of a clamp-on sensor <NUM> disposed about pipe <NUM> in accordance with the present invention. As shown, clamp-on sensor <NUM> is configured to physically attach to the external diameter of pipe <NUM> at attachment points <NUM>, <NUM>. Clamp <NUM> is preferably constructed from the same material as pipe <NUM> in order to compensate for thermal expansion/hysteresis effects. Accordingly, with changes to temperature, clamp <NUM> will change in size in much the same way as pipe <NUM>. However, clamp <NUM> includes a pair of proximity sensors <NUM>, <NUM> disposed at approximately <NUM>° from attachment points <NUM>, <NUM>. In this way, as the pressure within pipe <NUM> increases and pipe <NUM> deforms outwardly, the proximity sensors <NUM>, <NUM> will measure a reduced distance to the external surface of pipe <NUM>. Proximity sensors <NUM>, <NUM> are coupled to suitable measurement circuitry, such as measurement circuitry <NUM> (shown in <FIG>) and provide an indication of proximity to the pipe and thus an indication of process fluid pressure within the pipe.

<FIG> is a diagrammatic perspective view illustrating clamp-on sensor <NUM> with proximity sensors <NUM> and <NUM> disposed diametrically opposite one another.

From the description set forth above, it is apparent that the change in the circumference of the outside diameter of the process fluid conduit varies based, not only on the applied pressure, but also on the material of the process fluid conduit, and the thickness of the pipe wall. Accordingly, embodiments described herein, generally include the calibration of a non-intrusive process fluid pressure measurement system once it is mounted to a particular process fluid conduit.

<FIG> are simplified diagrammatic finite element analysis models illustrating the behavior of the clamped assembly as pressure is applied internally to a pipe on which the sensor is clamped. In the illustrated example, the deformation scale is exaggerated by a factor of <NUM>. The circled areas highlight how the gap spacing decreases as pressure is applied. Note, that two effects are present. One effect is that the pipe is expanding outwardly, and secondly, the sides of the brackets due to a mechanical effect, are moving inwardly. This amplifies the gap change beyond simple changes in the conduit diameter. Moreover, the output of the device can be configured to be the sum of the two sensors, which doubles the signal as well as minimizing side-to-side disturbances, since the sum of the two gaps will remain constant under side-to-side movement. Hence, the net gap only changes when pressure is applied inside the conduit. The result is a robust measurement that has a signal output almost four times larger than the single bracket approach described above with respect to <FIG>. Lastly, the thermal effects are still low provided the clamp material is made from the same material as the pipe.

<FIG> illustrates an example where a clamp-on sensor <NUM> is clamped to process fluid conduit <NUM> and then electrically coupled to process fluid pressure transmitter <NUM> which is somewhat spaced from clamp-on sensor <NUM> along conduit <NUM>. Clamp-on sensor <NUM> is configured to provide an indication relative to one or more gaps that change as process fluid conduit <NUM> reacts to changes in process fluid pressure. The electrical signals from the one or more gap sensors within clamp-on sensor <NUM> are provided to electronics within transmitter housing <NUM> to be processed to provide a process fluid pressure output that is then communicated to remote electronics, such as a control room.

The embodiment shown in <FIG> and <FIG> are believed to be particularly advantageous in that they do not require any particular permanent mounting (i.e., welding) to the process fluid conduit. Thus, the embodiments shown in <FIG> and <FIG> can be attached to the process fluid conduit anywhere that it is desired to measure process fluid pressure. The spacing of transmitter <NUM> from clamp-on sensor <NUM> is beneficial in order to minimize disturbances that might arise from the transmitter mounting. The separate mounting also affords the possibility of having additional process measurements, such as temperature and/or corrosion and pipe wall thickness measurements.

<FIG> is a diagrammatic view of electronics within housing <NUM>. As shown, a power module <NUM> is configured to provide power to the various components of the transmitter as indicated by arrow <NUM> labeled "to all. " In embodiments where the transmitter is a wireless transmitter, power module may include a battery, rechargeable or non-rechargeable, and suitable power conditioning components to provide the appropriate voltage and current levels to the various components within the transmitter. In embodiments where transmitter <NUM> is configured to couple to a wired process communication loop or segment, power module <NUM> may be adapted to derive all power required to operate transmitter <NUM> from electrical energy provided over the process communication loop or segment.

Communications module <NUM> is coupled to controller <NUM> and provides controller <NUM> with the ability to communicate in accordance with a process communication standard protocol. Examples of wired process communication standard protocols include the Highway Addressable Remote Transducer (HART®) protocol and the FOUNDATION™ Fieldbus protocols. An example of a wireless process communication protocol for which communication module <NUM> may be adapted is the known WirelessHART protocol (IEC62591).

Controller <NUM> may be any suitable electrical device or arrangement of logic that is able to execute instructions or programmatic steps to determine a process fluid pressure estimation output based on a gap measurement. In one example, controller <NUM> is a microprocessor having associated timing and memory circuitry disposed therein. Controller <NUM> is coupled to measurement module <NUM>, which may include one or more analog-to-digital converters that allow controller <NUM> to obtain information indicative of electrical signals provided the gap sensors. One example of a suitable gap sensor is the capacitive plate arrangement described above. However, it is expressly contemplated, that any suitable technology that is able to accurately and reliably obtain information indicative of the gap can be used. Examples of such technology include optical techniques (interferometry, attenuation, et cetera); eddy current proximity detection; acoustic echo location, strain gauge technology; and magnetic technology (variable reluctance, inductance, hall sensors, et cetera).

Measurement module <NUM> is coupled to one or more gap sensors <NUM> that provide an electrical indication indicative of the varying gap. In the embodiment described above, the gap sensor may be a single parallel-plate capacitor. However, other forms of gap measurement, as described above, can be used.

<FIG> is a flow diagram of a method of calibrating such a system in accordance with an embodiment of the present invention. Method <NUM> begins at block <NUM> where the non-intrusive process fluid pressure measurement system is attached to a particular process fluid conduit. The attachment may be in the form of welding a mirrored-Z structure to such a conduit or clamping a clamp-on structure to the process fluid conduit. Next, at block <NUM>, a first known pressure (P<NUM>) is generated within the process fluid conduit. While the internal pressure P<NUM> is present within the process fluid conduit, a gap measurement is obtained using one or more gap sensors, as indicated at block <NUM>. Next, at block <NUM>, a second known pressure (P<NUM>) is generated within the process fluid conduit, and, at block <NUM>, the gap is measured again. With the two known pressures and two measured gaps, the system is solved for the constants (related to pipe wall thickness and Young's Modulus). Note, in some embodiments, these quantities may be entered directly into the system via a user interface or via process communication, or they may be selected by a user when the system is ordered or otherwise manufactured. At block <NUM>, the known quantities for pipe wall thickness and Young's Modulus <NUM> are stored <NUM> in memory, such as memory of controller <NUM> for use during operation.

<FIG> is a flow diagram of a method of non-intrusively measuring process fluid pressure. Method <NUM> begins at block <NUM> where a gap of a structure that is attached to an outside surface of the process fluid conduit is measured. Next, at block <NUM>, information related to the Young's Modulus and conduit wall thickness are obtained. At block <NUM>, the measured gap, and obtained Young's Modulus and pipe wall thickness are used to compute a process fluid pressure within the process fluid conduit. Finally, at block <NUM>, the computed process fluid pressure is provided as an output. This output may be in the form of a local indication, such as via a user interface, or communicated to a remote electronic device, such as over a process communication loop, either wired or wirelessly.

Embodiments described herein generally lend themselves to enhanced multi-variable measurements as well. The clamp-on sensor could integrate a process thermal measurement via either a temperature sensor located within the clamp-on pressure sensor assembly, or into the transmitter housing attachment <NUM> using non-intrusive process fluid temperature estimation technology. Additionally, information could be obtained from a sensor integrated into housing attachment <NUM> which would provide property pipe characterization (i.e., wall thickness, material type), as well as corrosion information.

<FIG> illustrates a pair of examples with multiple non-intrusive process fluid pressure sensors disposed on opposite sides of a flow restriction. The use of a second clamp-on sensor could also be configured to measure the pressure drop across the restriction in the pipe. Based on knowledge of this restriction, the two pressure measurements could provide an indication of process fluid flow.

While embodiments have generally been described with respect to a mirrored assembly that is welded or otherwise permanently affixed to an outside surface of a conduit, or a clamp-on assembly, it is expressly contemplated that a spool assembly could be provided where a section of pipe could have a pair of mounting flanges, and a non-intrusive sensor pre-mounted to the short spool section of the pipe. Then, installation of the non-intrusive process fluid pressure measurement system to the process would be as simple as mounting the spool using the pair of mounting pipe flanges.

Claim 1:
A system for non-intrusively measuring process fluid pressure within a process fluid conduit, the system comprising:
a measurement bracket configured to couple to an external surface of the process fluid conduit, the measurement bracket generating a variable gap (<NUM>) based on deformation of the process fluid conduit in response to process fluid pressure therein;
a gap measurement system (<NUM>) coupled to the measurement bracket and configured to provide an electrical signal based on a measurement of the variable gap (<NUM>); and
a controller (<NUM>) coupled to the gap measurement system (<NUM>) and configured to calculate and provide a process fluid pressure based on the electrical signal and information relative to the process fluid conduit
characterised in that the information relative to the process fluid conduit includes a wall thickness of the process fluid conduit.