Patent Publication Number: US-9841338-B2

Title: High integrity process fluid pressure probe

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a Divisional of and claims priority of U.S. patent application Ser. No. 13/930,583, filed Jun. 28, 2013, the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Industrial process control systems are used to monitor and control industrial processes used to produce or transfer fluids or the like. In such systems, it is typically important to measure “process variables” such as temperatures, pressures, flow rates, and others. Process control transmitters measure such process variables and transmit information related to the measured process variable back to a central location such as a central control room. 
     One type of process variable transmitter is a pressure transmitter which measures process fluid pressure and provides an output related to the measured pressure. This output may be a pressure, a flow rate, a level of a process fluid, or other process variable that can be derived from the measured pressure. The pressure transmitter is configured to transmit information related to the measured pressure back to the central control room. Transmission is typically provided over a two-wire process control loop, however, other communication techniques are sometimes used. 
     Generally, the pressure is coupled to the process variable transmitter through some type of process coupling. In many instances, a pressure sensor of the transmitter is fluidically coupled to the process fluid either through an isolation fluid or by direct contact with the process fluid. The pressure of the process fluid causes a physical deformation to the pressure sensor which generates an associated electrical change in the pressure sensor such as capacitance or resistance. 
     A pressure barrier is a mechanical structure that contains process fluid pressure. As such, pressure bathers are key requirements for process fluid pressure measurement system. In order to provide a safe and robust system, some manufacturers provide redundant pressure barriers. Thus, if a primary barrier fails, the process fluid is still contained by the secondary barrier. 
     One particularly challenging environment for pressure measurement is applications which have very high working pressure. One such application is the subsea environment. In such applications, the static pressure to which the process equipment is exposed can be quite high. Moreover, the process fluid can corrode many known metals. For example, some subsea applications are now being considered that require a 20,000 psi maximum working pressure (MWP). By requiring a 20,000 psi MWP, manufacturing approval standards typically require the pressure barriers of pressure sensors in such environments to withstand 2.5 times the maximum working pressure. Thus, a pressure barrier in such an application would need to be able to withstand 50,000 psi. The design criteria for pressure barriers are important in that they ensure the integrity of the process. Specifically, if the pressure barrier or barriers fail, it is possible for the process fluid to enter the environment. This is highly undesirable because the process fluid may be flammable or even explosive, or may generally cause environmental contamination. Thus, for subsea applications, it is desirable to provide two pressure bathers between the process fluid and the seawater, or the process fluid and the electronic compartment of the process fluid pressure transmitter. 
     SUMMARY 
     A process fluid pressure measurement probe includes a pressure sensor formed of a single crystal material and mounted to a first metallic process fluid barrier and disposed for direct contact with a process fluid. The pressure sensor has an electrical characteristic that varies with process fluid pressure. A feedthrough is formed of a single crystal material and has a plurality of conductors extending from a first end to a second end. The feedthrough is mounted to a second metallic process fluid barrier and is spaced from, but electrically coupled to, the pressure sensor. The pressure sensor and the feedthrough mounted such that the secondary metallic process fluid barrier is isolated from process fluid by the first metallic process fluid barrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic perspective view of a single-crystal pressure sensor with which embodiments of the present invention are particularly useful. 
         FIG. 2  is a diagrammatic view of a single-crystal pressure sensor being employed in an environment with a pair of pressure barriers. 
         FIG. 3  is a diagrammatic view of a high pressure, high integrity process fluid pressure probe in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagrammatic view of a single crystal feedthrough acting as a secondary pressure barrier in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagrammatic cross-sectional view of a high pressure, high integrity single-crystal process fluid pressure probe coupled to a pressure transmitter in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagrammatic exploded view of the high pressure, high integrity process fluid pressure fluid probe in accordance with an embodiment of the present invention. 
         FIG. 7  is a cross sectional diagrammatic view of a high pressure, high integrity process fluid pressure probe in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Pressure barriers can take various forms. For example, a process isolation diaphragm generally works well as a primary pressure barrier. Additionally, remote seal capillary systems can be an effective secondary barrier. Glass or ceramic headers allow effective electrical connections while also providing a useful pressure barrier. Finally, pressure sensors themselves can be designed to contain pressure and thus serve as a pressure barrier. 
     As set forth above, pressure barriers are extremely important in process fluid pressure measurement because they ensure the integrity of the process fluid. However, pressure barriers create a number of challenges. Such challenges include costs, complexity, size, reliability, and compliance. 
     In order to effectively address the subsea environment, a number of design criteria must be considered. Reliability, safety, size and cost are all important design considerations. 
     Reliability is very important because the design lifetime of the process fluid measurement system may be on the order to 30 years. Moreover, failed units often cannot be easily replaced or repaired. Further, providing units that can be replaced can drive the cost of such designs very high and the replacement process itself can cost over a million dollars. 
     Safety is important because it is critical that the pressure and the process fluid be contained. Subsea process fluid pressure measurement systems typically require two pressure barriers between the process fluid and the seawater. 
     Size is another important design consideration. Generally, smaller components and systems are favored because it is easier to maintain the pressure. Further, with smaller designs there is more room for other instruments and devices. Further still, given the use of relatively exotic materials in order to combat corrosion in the subsea environment, smaller designs help reduce costs. 
     Thus, embodiments of the present invention generally provide an extremely high integrity, high pressure transmitter that may be lower cost, safer, and more reliable than previous devices. Embodiments of the present invention generally utilize a small, single-crystal pressure sensor that is suitable for direct contact with the process fluid itself. Such pressure sensors are known. For example, pressure sensors constructed of sapphire have been employed by Emerson Process Management, of Chanhassen, Minn. These sensors can withstand high pressure and high temperatures. Moreover, the sapphire pressure sensors can be disposed for direct contact with the process fluid. Sapphire pressure sensors generally enable a unique architecture that can be integrated into the process vessel (such as a pipe or flow element). The advantage of this architecture is that the process pressure is better contained within the vessel. While embodiments of the present invention will generally be described with respect to a pressure sensor formed of a single-crystal material, embodiments of the present invention can be practiced with any pressure sensor mounted to a substrate as set forth below. 
       FIG. 1  is a diagrammatic perspective view of a pressure sensor formed of a single-crystal material with which embodiments of the present invention are particularly useful. Pressure sensor  10 , shown in  FIG. 1 , is known. For example, U.S. Pat. No. 6,520,020 discloses such a sensor. On the right side of  FIG. 1  is the process pressure, illustrated diagrammatically at reference numeral  12 . The process fluid pressure acts in the directions illustrated by arrows  14  to compress substrate  16 , which in one embodiment is formed of sapphire. This compression of sapphire substrate  16  causes a change in the distance between the layers  16 ,  17  of the sapphire substrate. Conductors  19 ,  21  are deposited on the inside surfaces of the pressure sensor such that deflection of sapphire substrates  16 ,  17  causes a change in capacitance between conductors  19 ,  21 . This change in capacitance is detected by suitable circuitry coupled to electrical terminations  18 . Process barrier  20  is shown in the middle of  FIG. 1 . This may be a pipe or tank wall but is typically a structure that may be welded into a process pipe or tank, or any other structure that contains process fluid  12 . Pressure sensor  10  passes through an aperture in process barrier  20  and is then brazed thereto as illustrated at reference numerals  22  and  24 . To the left of  FIG. 1  is nominally atmospheric pressure as indicated at reference numeral  25  where electrical terminations  18  are provided. Additionally, in some embodiments, pressure sensor  10  may include a temperature sensor, such as a resistance temperature detector, that provides an electrical indication, such as resistance, that varies with process fluid temperature. 
     In one commercially-available implementation of the pressure sensor  10 , sold under the trade designation Model 4600 available from Emerson Process Management, a process diaphragm is a primary pressure barrier that separates process fluid from an oil-filled container. The oil within the oil-filled container contacts sapphire substrates  16 ,  17 . In such case, the process diaphragm is the primary pressure barrier, and the brazed-feedthrough is the secondary pressure barrier. Both barriers can withstand extremely high pressures. Accordingly, it is believed that the sapphire-brazed barrier is an effective pressure barrier in part because it is proven, low cost, and small. However, in embodiments where an isolation or process diaphragm is not used or is simply too large or represents too much expense, allowing the process fluid pressure sensor  10  to directly contact the process media would cause the brazed feedthrough to become the primary pressure barrier. In high integrity process pressure measurement environments it is still necessary to have a secondary pressure barrier. 
       FIG. 2  illustrates a single-crystal sapphire sensor being employed in a pressure measurement environment with a pair of pressure barriers. As illustrated in  FIG. 2 , a sensing portion of the pressure sensor is substantially the same as that illustrated in  FIG. 1 . Moreover, the brazed junction through process fluid container wall  20  is also similar. However, a secondary wall  30  is provided through which the pressure sensor also passes. A brazed connection with this barrier is also provided. While this arrangement represents a high integrity double pressure barrier system, it is not without various drawbacks. The first drawback is that axial stresses caused by temperature changes between the brazed metal barriers and the single-crystal material may likely cause catastrophic failure. The second drawback is that the barriers themselves can fail to hold pressure with a common mode sapphire failure sensor such as a leak between the top and bottom halves of the sensor. 
       FIG. 3  is a diagrammatic view of a high pressure, high integrity process fluid pressure probe in accordance with an embodiment of the present invention. As shown in  FIG. 3 , the problems of the double pressure barrier embodiments set forth above with respect to  FIG. 2  are solved by imposing a gap  41  between the two pressure barriers. In this embodiment, process fluid  12  acts directly upon substrate  40  and the interior of process vessel  42 . Sensor  40  passes through an aperture process fluid vessel  42 , and is brazed thereto at connection  44 . A second process containment structure is illustrated diagrammatically at reference numeral  46  and a single-crystal material feedthrough  48  is provided that extends through an aperture in  46 . Feedthrough  48  is brazed to wall  46  in much the same manner that sensor  40  is brazed to the wall of process vessel  42 . Electrical interconnections  50  are provided between structure  48  and sensor  40 . In this manner, second pressure barrier  48  can be built on a simple, smaller structure with the appropriate number of connections. One suitable arrangement for structure  48  is illustrated with respect to  FIG. 4 . Feedthrough  50  includes a single-crystal substrate that passes through an aperture in secondary pressure barrier  46 . Substrate  50  includes a plurality of conductive pads  52  that are configured to be coupled, via welding or any other suitable manner, to conductors  50  (shown in  FIG. 3 ). Traces, or other suitable structures  51  on substrate  50  connect pads  52  to respective pads  54 , which are configured to be coupled to a plurality of conductors that are ultimately coupled to a process pressure transmitter (shown in  FIG. 5 ). Substrate  50  is sealed to barrier  46  at the aperture through barrier  46  by any suitable manner, such as brazing. 
     Referring back to  FIG. 3 , however, second structure  48 , unlike feedthrough  50 , can also include a sensor to detect a failure of the first barrier. Suitable sensors for structure  48  includes a pressure sensor or surface resistance sensor. Accordingly, if process fluid should pass through sensor  40  or breach the brazed junction  44 , the pressure between walls  42  and  46  would increase. The secondary sensor would accordingly respond to such pressure and/or the presence of process fluid. 
       FIG. 5  is a diagrammatic cross-sectional view of a high pressure, high integrity single-crystal pressure sensor probe in accordance with an embodiment of the present invention. Probe  100  is coupled to transmitter  90  and is mounted to and extends through process barrier  102 , which may be a pipe or tank wall. In the embodiment shown in  FIG. 5 , transmitter  90  is coupled to a single probe, however, transmitter  90  can be coupled to any suitable number of high pressure, high integrity probes in accordance with embodiments of the present invention. For example, using a pair of such probes allows transmitter  90  to provide an indication of differential pressure, or a redundant indication of absolute or gauge pressure. Using three such probes provides at least some redundancy as well as the ability to provide differential pressure. Transmitter  90  can be any suitable pressure transmitter, now known or later developed. Probe  100  is coupled to suitable electronics within transmitter  90 . The electronics are configured to measure the changing electrical characteristic of the pressure sensor of probe  100  to determine process fluid pressure. Moreover, the electronics preferably include controller electronics to transmit, or otherwise convey, digital information indicative of the pressure over a process communication loop, such as a Highway Addressable Remote Transducer (HART®) loop or a FOUNDATION™ Fieldbus segment. In some embodiments, transmitter  90  may be loop-powered and thus may be wholly powered through the same conductors over which it communicates. 
     Probe  100  includes an outer tube  104  coupled to a weld ring  106  at a proximal end and to process interface screen  108  at a distal end. Process interface screen  108  is disposed for direct contact with process fluid  110 , but protects single-crystal pressure sensor  112  from damage due to movement of particles and/or solids within the process fluid flow. An inner tube  114  is disposed within outer tube  104  and extends to secondary barrier  116 . Secondary barrier  116  is formed by welding a metallic disc  118  to end  120  of inner tube  114 . A single crystal interconnect  122 , preferably formed of sapphire, passes through disc  120  and is brazed thereto. Interconnect  122  provides an electrical connection between conductors  124  and conductors  126  while passing through a high pressure, high integrity pressure barrier  116 . Similarly, pressure sensor assembly  128  includes a disc  130  that is welded to a tubular member that itself is welded to disc  118 . Further, disc  130  includes an aperture through which pressure sensor  112  passes. Pressure sensor  112  is brazed within the aperture to create another pressure barrier. Additionally, as illustrated in  FIG. 5 , there is no rigid interconnect between single-crystal interconnect  122  and sensor  112 . 
       FIG. 6  is a diagrammatic exploded view of the high pressure, high integrity process fluid pressure probe in accordance with an embodiment of the present invention. The process of assembling the probe includes assembling sensor assembly  128 . Sensor assembly  128  is formed by from three distinct components. First, disc  130  is provided having an aperture therethrough. Next, pressure sensor  112  is passed through the aperture through disc  130 , and pressure sensor  112  is brazed to disc  130 . Next, sensor assembly tube  132  is welded to disc  130  at weld  129  to complete pressure sensor assembly  128 . Sensor assembly tube  132 , in one embodiment, has an outer diameter that is the same as that of inner tube  114 . Process interface screen  108  is welded to sensor assembly  128  as indicated at reference numeral  140  (shown in  FIG. 5 ). Next, secondary barrier  116  is formed by brazing single crystal interconnect  122  to metallic disc  120 . Sensor assembly  128  is then welded to barrier assembly  116  at weld  142  (shown in  FIG. 5 ). Next, barrier assembly  116  is welded, at reference numeral  144  (shown in  FIG. 5 ), to the end of inner tube  114 . Outer tube  104  is then attached to sensor assembly  128  via weld  146  (shown in  FIG. 5 ) at its distal end  131 . Next, the proximal end  133  of outer tube  104  is welded to weld ring  106  at weld  148  (shown in  FIG. 5 ). Weld ring  106  is also welded to inner tube  114  at weld. The outer diameter of pipe  104  is then welded to weld ring  106  as indicated at weld  150  (shown in  FIG. 5 ). Next the inner diameter of inner tube  114  is welded to weld ring  106  at weld  152  (shown in  FIG. 5 ). 
     As indicated in  FIG. 6 , inner tube  114  is not contacted by process fluid and thus can be made with any suitable standard material, such as 316 stainless steel. Outer tube  104  is process-wetted and is thus made of a more expensive, exotic material such as Inconel or Alloy C276. Alloy C276 is an example of a material suitable for corrosive fluids. Alloy C276 is available from Haynes International Inc. of Kokomo, Ind. under the trade designation Hastelloy C276. Alloy C276 has the following chemical composition (by percent weight): Molybdenum 15.0-17.0; Chromium 14.5-16.5; Iron 4.0-7.0; Tungsten 3.0-4.5; Cobalt 2.5 maximum; Manganese 1.0 maximum; Vanadium 0.35 maximum; Carbon 0.01 maximum; Phosphorus 0.04 maximum; Sulfur 0.03 maximum; Silicon 0.08 maximum and Balance Nickel. Alloy C276 provides excellent corrosion resistance to corrosive applications, and very high strength. The outer tube  104  can be made with a smaller diameter and thinner material because sensor  112  is small and because inner tube  114  assists in supporting the pressure load. Moreover, machining is less expensive because these parts can be turned. 
     Once assembly of probe  100  is completed, probe  100  may be installed in a pipe or other suitable conduit. In order to do so, weld ring  106  is welded to the process fluid conduit at weld  154  (shown in  FIG. 5 ). This results in a double pressure barrier, high integrity pressure probe for high pressure process measurement environments. The process interface is preferably a screen or similarly constructed assembly. 
     Embodiments of the present invention may include the utilization of an oil-filled system (such as that shown in  FIG. 7 ), or a system in which the process fluid contacts the pressure sensor directly (such as that shown in  FIG. 5 ). For an oil-filled system, the primary pressure barrier consists of the process diaphragm and welds  140 ,  146 , and  150 . The secondary pressure barrier consists of the sensor assembly braze and welds  142 ,  144 ,  148 ,  152 , and  154 . For an oil-less system, the sensor assembly braze joint becomes part of the primary pressure barrier and the single-crystal material barrier braze joint becomes part of the secondary pressure barrier. An important aspect of embodiments of the present invention is the utilization of the inner tube/outer tube combination. This enables significantly lower cost, small size, and redundant pressure barriers. These tubes are easily customized to length and enable installation into different sized vessels. Moreover, weld ring  106  enables the assembly to be directly welded into the vessel without the expense and space needed for a flange mounted unit. However, if the end user desires a flanged assembly, the weld ring can be replaced with a flange. 
       FIG. 7  is a cross sectional diagrammatic view of a high integrity, high pressure probe in accordance with an embodiment of the present invention. Probe  200  includes process interface screen  202  having a plurality of apertures  204  therethrough. Within region  206 , the process fluid contacts a foil isolator  208 . The process fluid bears against isolator diaphragm  208  and pressurizes fill fluid  210  in region  212 . Region  212  is fluidically coupled, via passageways  214 ,  216  to region  218  proximate single-crystal pressure sensor  220 . In this way, process fluid pressure acting on isolator diaphragm  208  generates a corresponding pressure on sensor  220 . 
     Isolator screen  202  is welded to isolator plug  222  at weld  223 . Isolator plug  222  is welded to both inner and outer conduits (such as tubes)  224 ,  226  at welds  225 ,  227 , respectively. Additionally, isolator plug  222  is also welded, at reference numeral  228 , to tapered pressure sensor module  230 . Tapered pressure sensor module  230  includes an aperture through which pressure sensor  220  is inserted. A disc to which pressure sensor  220  is brazed is then welded to tapered module  230  to create a sealed chamber within which pressure sensor  220  will sense the process fluid pressure. Electrical terminations on pressure sensor  220  can be made in any suitable manner, including utilization of a ceramic lead extender, or any other suitable electrical conductors. Each of inner conduit  224  and outer conduit  226  is also welded to flange  232  which can be attached, in any suitable manner, to the process. 
     In one embodiment, a suitable sensing structure, such as a strain gauge, indicated in phantom at reference numeral  240 , is coupled to the inside surface of inner conduit  224 . Thus, if weld  227  fails and process fluid enters the region between outer conduit  226  and inner conduit  224 , the pressure of the process fluid will strain inner conduit  224 . This strain will be detectable by strain gauge  240  and thus remedial action can be taken before the secondary pressure barrier fails as well. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.