Abstract:
A process fluid pressure sensing system includes a process fluid pressure transmitter and a process manifold. The process fluid pressure transmitter has first and second pressure inlets and is configured to obtain a measurement relative to pressures applied at the first and second pressure inlets and provide a process variable output based on the measurement. The process manifold is operably coupled to a process fluid and has first and second pressure outlets. A first high-pressure coupling joins the first pressure outlet of the process manifold to the first pressure inlet of the process fluid pressure transmitter. A second high-pressure coupling joins the second pressure outlet of the process manifold to the second pressure inlet of the process fluid pressure transmitter. The first and second high-pressure fluid couplings are configured to accommodate misalignment between the respective pressure outlets and inlets.

Description:
BACKGROUND 
     Process pressure transmitters are used to monitor pressure of process fluids used in industrial processes. Process pressure transmitters include a pressure sensor that typically provides an electrical output in response to a change in process fluid pressure. Each process pressure transmitter includes transmitter electronics for receiving and processing the electrical output of the pressure sensor. The transmitter electronics are also typically configured to transmit a signal, digital, analog, or a combination thereof, over a control loop or network to a central monitoring location such as a control room. 
     Pressure sensors used in pressure transmitters generally include a flexible sensor element, such as an electrode plate or a piezo-resistor that deflects in response to a pressure change. The sensor element is fluidically coupled to the process fluid typically through an isolation system. The isolation system includes a metal diaphragm that is configured to contact the process fluid. The isolation system also includes a sealed passageway that extends from the isolator diaphragm to the pressure sensor. The sealed passageway is typically filled with a substantially incompressible fill fluid such as silicone oil. As the pressure of the process fluid changes, the position of the isolator diaphragm changes thereby transferring a pressure change through the isolation fluid to the pressure sensor element. When the pressure sensor element moves in response to the pressure change, a corresponding change in an electrical characteristic of the pressure sensor, such as capacitance or resistance, changes as well. The electrical characteristic of the pressure sensor element is measured by the pressure transmitter electronics and is used to compute the pressure of the process fluid. 
     Differential pressure transmitters are used in a variety of applications where a difference between two pressures must be measured. Examples of such applications include level measurement in a container and flow measurement across a differential pressure producer such as an orifice plate or venturi. Differential pressure sensors typically require two isolation systems to convey separate process pressures to opposite sides of a single differential pressure sensor element. Typically, a differential pressure transmitter is installed with an integral manifold/valve body that enables both zero calibration of the transmitter and removal/replacement of the transmitter without having to shut off pressure to the transmitter/manifold assembly. The interface between the transmitter and the manifold is defined by International Standard IEC 61518, entitled “Mating dimensions between differential pressure (type) measuring instruments and flanged-on shut-off devices up to 413 BAR (41.3 MPa).” 
       FIG. 1  is a diagrammatic view of a process fluid differential pressure transmitter coupled to a manifold assembly in accordance with the International Standard set forth above. Transmitter  10  is coupled to manifold  12  by four bolts (not shown) that extend from surface  16  of manifold  12  into transmitter  10 . By using the four bolts, no fittings or additional hardware are used or required to hold the assembly together. This arrangement provides for simple assembly by the end user. 
     It is sometimes desirable to connect differential pressure transmitters to processes having extremely high static pressures. For example, deeply penetrating oil wells require large line pressures to transport the oil to surface levels. In applications above 413 bar, the manifolds tend to be spaced from the differential pressure transmitter with impulse piping or lines coupling the manifold to the differential pressure transmitter. This is due, at least on part, to the stresses that would be placed on the four clamping bolts if the differential pressure transmitter were bolted directly to the manifold. Given that known isolator diaphragms can exceed 0.8 inches in diameter and that two such isolators are required for differential pressure measurement, a static pressure of 10,000 psi can generate a pressure on the bolts in excess of 5,000 pounds. 
       FIG. 2  is a diagrammatic view of a differential pressure transmitter coupled to a manifold that is suitable for applications that exceed line pressures of 413 bar. Differential pressure transmitter  20  is coupled to manifold  22  via a pair of impulse lines  24 ,  26 . The fluidic couplings between manifold  22  and impulse lines  24 ,  26  and between the impulse lines  24 ,  26  and differential pressure transmitter  20  are generally configured to support high line pressure. For example, such couplings sometimes use tapered fittings of the type disclosed in U.S. Pat. No. 3,362,731. However, current high pressure coupling systems require the user to route the impulse piping between the manifold and the differential pressure transmitter and to employ fittings on each end of each impulse line. Providing a high-pressure fluidic coupling system that could better accommodate coupling short runs of impulse piping would facilitate installation of differential pressure transmitters in high line pressure applications. 
     SUMMARY 
     A process fluid pressure sensing system includes a process fluid pressure transmitter and a process manifold. The process fluid pressure transmitter has first and second pressure inlets and is configured to obtain a measurement relative to pressures applied at the first and second pressure inlets and provide a process variable output based on the measurement. The process manifold is operably coupled to a process fluid and has first and second pressure outlets. A first high-pressure coupling joins the first pressure outlet of the process manifold to the first pressure inlet of the process fluid pressure transmitter. A second high-pressure coupling joins the second pressure outlet of the process manifold to the second pressure inlet of the process fluid pressure transmitter. The first and second high-pressure fluid couplings are configured to accommodate misalignment between the respective pressure outlets and inlets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of a differential pressure transmitter coupled to a manifold assembly. 
         FIG. 2  is a diagrammatic view of a differential pressure transmitter coupled to a manifold that is suitable for applications that exceed line pressures of 413 bar. 
         FIG. 3  is a diagrammatic view of a known high pressure fluidic coupling that uses coned and threaded fittings. 
         FIG. 4  is a diagrammatic view of portions of two devices being coupled together with known cone and threaded connections. 
         FIG. 5  is a diagrammatic view of a high pressure fluidic coupling in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagrammatic view of portions of two devices being coupled together in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagrammatic cross-sectional view of a differential pressure transmitter coupled to a process manifold in accordance with an embodiment of the present invention. 
         FIG. 8  is an enlarged view of a high-pressure fluidic coupling in accordance with an embodiment of the present invention. 
         FIG. 9  is a diagrammatic view of a high pressure fluidic coupling in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     While embodiments of the present invention will generally be described with respect to a differential pressure transmitter, those skilled in the art will appreciate that embodiments of the present invention are practicable with any high-pressure fluid coupling application where precise axial alignment of the fluid couplings is impossible, difficult, or simply time-consuming. 
       FIG. 3  is a diagrammatic view of a high pressure fluidic coupling that uses coned and threaded fittings such as are known and commercially available from such manufacturers as Parker Autoclave Engineers of Erie, Pa., and BuTech, of Burbank Calif. Typically, the high-pressure fluidic coupling consists of three components: a tube  32  with a thrust collar  34  and a gland  36  to hold tube  32  into female fitting  38 . The sealing mechanism is a metal-to-metal seal  39  between cone  40  machined on the end of tube  32  and a cone  42  machined into female fitting  38 . This metal-to-metal seal  39  is essentially a line contact formed by the intersection of cone  40  on the end of tube  32  and cone  42  in the female fitting  38 . Collar  34  and gland  36  not only hold tube  32  into female fitting  38 , but also ensure that tube  32  enters female fitting  38  at the correct angle. Precise axial alignment is required between male and female couplings in order to ensure a leak-free connection. If the collar and gland did not maintain the proper angle of the tube in the female fitting, the line contact would become two points and a gap could be caused that would allow leaks. For example, if a conical end tube is inclined 5 degrees to the axis of the coned portion of the female fitting, the seal area will consist of two points and two gaps of approximately 0.002 inches wide (for a 0.250 inch OD tube) will result. Thus, if there is any misalignment between devices that are to be coupled with coned and threaded couplings, a bend in the tubing must be provided to compensate for such misalignment. In instances where the two devices are closely-spaced, it can be difficult or even impossible to create the appropriate bends. 
       FIG. 4  is a diagrammatic view of portions of two devices being coupled together with known cone and threaded connections. Device  44  is separated from device  46  by approximately 1.74 inches. The two devices  44 ,  46  are to be coupled with a known cone and threaded coupling for a 0.250″ outside diameter tube. As shown in  FIG. 4 , even a relatively small misalignment of 0.090″ over a 1.74″ span requires that a bend  48  be introduced into tube  32  in order to ensure that tubing  48  approaches each device  44 ,  46  at precisely 90 degrees. The bending step itself can be time-consuming. Moreover, since the bend results is a permanent deformation of the tubing, it is possible that improper bending operations could damage the tubing. For reference, the coupling shown at reference numeral  50  is that described above with respect to  FIG. 3 . 
     In accordance with embodiments of the present invention, a high-pressure fluidic coupling is provided that can accommodate some misalignment without requiring tubing to be bent. Embodiments of the present invention still provide the robust seal of metal-to-metal contact, but can allow the tubing to enter fitting even when not completely axially aligned with the fitting. 
       FIG. 5  is a diagrammatic view of a high pressure fluidic coupling in accordance with an embodiment of the present invention. Coupling  100  includes tube  102  having spherical end  104  that is received by conical recess  142  of female fitting  138 . While conical recess  142  is illustrated in  FIG. 5 , other geometries such as a straight bore, ellipse, or sphere can be used for the internal recess structure of female fitting  138  as long as a suitable line contact can be formed with spherical end  104 . In the embodiment shown, tube  102  has an externally threaded portion  160  that receives internally threaded thrust collar  162 . However, in order to accommodate slight axial misalignment, some inside dimensions of gland nut  136  have been enlarged. Specifically, internal surface  164  has a diameter that sized to provide a gap  166  between surface  164  and outside diameter  168  of tube  102 . Thus, tube  102  is allowed to move within gap  166 . Similarly internally threaded thrust collar  162  has an outer surface  167  with a diameter that provides a gap  170  between surface  167  and internal surface  172  of gland nut  136 . Another adaptation of the gland nut/thrust collar interaction is a curved surface  174  that defines the gland nut/thrust collar interface. As, different axial misalignments are accommodated, thrust collar  162  may be slid off-center. The gaps illustrated in  FIG. 5  are exaggerated for the purposes of illustration and clarity. Those skilled in the art will recognize that any suitable dimensions for the outer diameter of the thrust collar; inner diameters of the gland nut; and radius of curvature for interface  174  can be changed and adjusted as long as a robust interface  174  can be maintained to reliably cause spherical end  104  to sealingly bear against conical recess  142  at all possible misalignments. 
       FIG. 6  is a diagrammatic view of portions of two devices being coupled together in accordance with an embodiment of the present invention. Device  200  is coupled to device  202  through a high-pressure fluidic coupling. For contrast, the dimensions of separation and axial misalignment for  FIG. 6  are identical to those of  FIG. 4 .  FIG. 6  includes a tube  204  having spherical ends  206 ,  208 , which are received in respective conical recesses  210 ,  212 . Thus, each of the two fluidic connections illustrated in  FIG. 6  can be in accordance with that shown with respect to  FIG. 5 . However, the two degree misalignment can be accommodated by the fluidic coupling without requiring a bend to be introduced into tube  204 . Thus, the overall fluidic connection may require less time and effort to complete than that shown in  FIG. 4 . 
     As set forth above, embodiments of the present invention can be advantageously used to provide high-pressure fluidic couplings in a variety of applications where strict axial alignment is difficult or impossible. However, embodiments of the present invention are particularly applicable to coupling differential pressure transmitters to process fluid manifolds. While embodiments of the present invention can be used for impulse line connections, embodiments of the present invention can also enable direct coupling of a differential pressure transmitter to a process manifold for pressures exceeding 413 bar. 
       FIG. 7  is a diagrammatic cross-sectional view of a process fluid pressure transmitter coupled to a process manifold in accordance with an embodiment of the present invention. Differential pressure transmitter  250  has a plurality of high-pressure fluidic couplings  252 ,  254  with process manifold  256 . For clarity, the internal details of transmitter  250  and manifold  256  are not shown. Each coupling can include a female interface that can be in accordance with known high-pressure couplings, such as the known Autoclave FC-250 interface that provides an internal cone recess. Typically, such internal cone recess would couple to a corresponding male cone surface. However, the male portion of the coupling includes a spherical surface (shown in  FIG. 8 ) that seals against internal cone recess of the female fitting. The arrangement still provides a robust metal-to-metal seal, but can accommodate less stringent manufacturing tolerances and surface finishes. Advantageously, the entire assembly can still be bolted together with four bolts (similar to the IEC 61518 interface). However, the bolted assembly may be rated to pressures that significantly exceed 413 bar. 
       FIG. 8  is an enlarged depiction of rectangle  258  shown in  FIG. 7 . Each of female connector portions  260  can be in accordance with known designs, such as those having an internal cone recess  262 . Each portion  260  may also include a threaded portion  264  of female fitting  260  that is intended to mate with the male threads on a gland used in typical coned-and-threaded fittings. Alignment feature  267  uses threads  264  to center tube  266  in fitting  260  as it is brought into contact with a respective internal cone recess  262 . As can be appreciated, when two or three such high-pressure fluidic couplings are required, ensuring the precise alignment of each fitting becomes very difficult. However, using seal tubes having spherical ends  268  enables seal tube  266  to enter the female fitting at a slight angle, and thus allows for manufacturing with relaxed tolerances for individual fittings on both the differential pressure transmitter and the process manifold. Another advantage provided by embodiments of the present invention is due, at least in part, to the small surface area on the ends of the tubes and the rigid nature of the tubes. Specifically, similar bolts and torques used for flanged IEC 61518 connections can be used to affix the differential pressure transmitter to the process manifold at much higher pressures. Moreover, the entire transmitter/manifold assembly can still be bolted together with as little as four bolts. This provides simple assembly for end users, but still allows the completed assembly to perform at higher pressures. 
       FIG. 9  is a diagrammatic view of a high pressure fluidic coupling in accordance with an embodiment of the present invention.  FIG. 9  bears some similarities to  FIG. 5 , and like components are numbered similarly. High pressure coupling  300  includes a tube  302  having a conically-recessed end  304  that contacts spherical surface  306  of female fitting  308  in device  310 . Coupling  300  includes thrust collar  162  and gland nut  136 , much like the coupling illustrated with respect to  FIG. 5 . So arranged, tube  302  is allowed to be slightly axially misaligned with female fitting  308 , but can still maintain a high-pressure seal. Spherical surface  306  can be manufactured or otherwise provided in any suitable manner. For example, surface  306  can be cast as part of device  310  or surface  306  can be provided by a ball having a passageway therethrough that is welded to device  310 . Alternately, spherical surface  306  can be part of a metal-injection molded (MIM) insert that is welded within a bore in device  310 . 
     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.