Abstract:
A process control instrument for coupling to a process is attachable to a metal flange. The flange has a first passageway adapted to be filled with process fluid. The process control instrument includes a body having an opening adjacent to the first passageway for receiving process fluid from the first passageway when the process control instrument is attached to the flange. A diaphragm is disposed across the opening for fluid communication with the process fluid. A seal is positionable against the flange to prevent process fluid from leaking past the flange and diaphragm. The seal includes a ring positioned in the opening and coupled to the body, the ring being substantially in contact with the diaphragm along an inner annular shoulder when the body is not attached to the flange (unloaded).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a process control transmitter. In particular, it relates to a process seal for a process control transmitter.  
         [0002]     Transmitters which sense pressure typically have a pressure sensor coupled to at least one isolation diaphragm. The isolation diaphragm isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm to the sensor having a sensing diaphragm through a substantially incompressible isolation fluid carried in a passageway. U.S. Pat. No. 4,833,922 entitled MODULAR PRESSURE TRANSMITTER and U.S. Pat. No. 5,094,109 entitled PRESSURE TRANSMITTER WITH STRESS ISOLATION DEPRESSION show pressure transmitters of this type.  
         [0003]     The process fluid sealing mechanism for a transmitter should be operable in a wide range of chemical environments, temperature ranges and stress conditions and work well over a broad range of pressures. Teflon® and other fluorocarbons are among the preferred sealing compounds. Hastelloy®,  316  stainless steel and other corrosion resistant materials are preferred as construction materials for wetted surfaces. While these materials have very good corrosion resistance properties, their mechanical properties, such as yield strength of the corrosion resistant alloys and the resistance to extrusion of the sealing materials, are marginal at best. Sealing material tends to extrude when subjected to high pressures and temperatures. For this reason, the sealing material must be treated as a gasket. To form effective seals with gaskets, it is usually necessary to have a sealing material with a large surface area under significant compression. The stress from compression is mechanically coupled to the isolation diaphragm and ultimately to the sensing diaphragm of the pressure transmitter. The amount of stress can vary over time as mounting bolts loosen or are re-torqued, and as the gasket sealing material extrudes. These changes result in instabilities in the pressure sensor output.  
         [0004]     To minimize the stress coupled to the process isolation diaphragm, it is preferred to separate the diaphragm from the sealing mechanism to provide stress isolation. However, practical considerations make stress isolation of the diaphragm difficult. Industry standards and the requirement of backward compatibility with existing products dictate the size, location and pattern of the bolts and pressure ports of the assembly. The overall geometry of the transmitter limits the space that must be shared by the process sealing gaskets and the isolating diaphragms. The process isolation diaphragms must fit within the boundaries defined by the bolt pattern. Space within the bolt boundary used for sealing is generally unavailable for isolation diaphragms. It is frequently undesirable to reduce the size of the isolation diaphragms because smaller isolation diaphragms are more sensitive to stress coupling and therefore instabilities result.  
         [0005]     Tradeoffs must typically be made among the several competing needs of the pressure transmitter design: 1) the need for large compliant diaphragms; 2) the need for diaphragms that are well isolated from the stresses of the sealing mechanism; 3) the need for a sealing mechanism that has sufficient surface area; 4) the need for a sealing mechanism held together with sufficient force to be reliable; and 5) the constraint that all structures fit within the boundary defined by the bolt pattern.  
         [0006]     One technique which addresses some of these concerns is shown and described in U.S. Pat. No. 5,955,675, which issued Sep. 21, 1999 to Peterson entitled SELF ENERGIZING PROCESS SEAL FOR PROCESS CONTROL TRANSMITTER which is commonly assigned with the present application. This reference describes a technique in which process pressure is used to assist in sealing a process seal to a flange. The process seal has a ring shape and sealing material is coupled to the ring along its inner diameter. The ring is adapted to force the sealing material into contact with the flange to prevent process fluid from leaking past the seal. U.S. Pat. Nos. 5,922,965 and 6,055,863, entitled PRESSURE SENSOR AND TRANSMITTER HAVING A WELD RING WITH AROLLING HINGE POINT, and PRESSURE SENSOR AND TRANSMITTER HAVING A WELD RING WITH A ROLLING HINGE POINT, issued Jul. 13, 1999 and May 2, 2000, respectively, also describe process seals.  
       SUMMARY OF THE INVENTION  
       [0007]     A process control instrument for coupling to a process is attachable to a metal flange. The flange has a first passageway adapted to be filled with process fluid. The process control instrument includes a body having an opening adjacent to the first passageway for receiving process fluid from the first passageway when the process control instrument is attached to the flange. A diaphragm is disposed across the opening for fluid communication with the process fluid. A seal is positionable against the flange to prevent process fluid from leaking past the flange and diaphragm. The seal includes a ring positioned in the opening and coupled to the body, the ring being substantially in contact with the diaphragm along an inner annular shoulder when the body is not attached to the flange (unloaded).  
         [0008]     A method of attaching a seal to a process transmitter is also provided which includes preloading the seal to urge an inner annular portion of the seal against an isolation diaphragm of the transmitter. The method further includes attaching the seal to the transmitter while applying the preloading and removing the preloading following the attaching whereby an annular shoulder of the seal remains in contact with the diaphragm. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a cross-sectional fragmentary view of a pressure transmitter having a process seal in accordance with the present invention.  
         [0010]      FIG. 2  is a cross-sectional view of a prior art seal.  
         [0011]      FIGS. 3A and 3B  are more detailed cross-sectional views of the prior art seal shown in  FIG. 2 .  
         [0012]      FIG. 4  is a cross-sectional view of a portion of the transmitter in flange in Figure which illustrates a seal.  
         [0013]      FIGS. 5A and 5B  are cross-sectional views of one embodiment of a seal according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]      FIG. 1  shows an exemplary pressure transmitter  10  having transmitter body  12 , coupling flange or manifold  13  and sensor body  14  in accordance with the present invention. Although the present invention is shown with a Coplanar™ flange, the invention may be used with any type of flange, manifold, or other coupling adapted to receive process fluid. Sensor body  14  includes pressure sensor  16 , and transmitter body  12  includes transmitter circuitry  20 . Sensor circuitry  18  is coupled to transmitter circuitry  20  through communication bus  22 . Transmitter circuitry  20  sends information related to pressure of the process fluid over a communication link such as a two wire process control loop  23  (or circuit). The transmitter  10  may be wholly powered over the control loop  23  by a controller  25 .  
         [0015]     In one embodiment of a transmitter, pressure sensor  16  measures a difference in pressure between pressure P 1  in passageway  24  and pressure P 2  in passageway  26  of flange  13 . Pressure P 1  is coupled to sensor  16  through passageway  32 . Pressure P 2  is coupled to sensor  16  through passageway  34 . Passageway  32  extends through coupling  36  and tube  40 . Passageway  34  extends through coupling  38  and tube  42 . Passageways  32  and  34  are filled with a relatively incompressible fluid such as oil. Couplings  36  and  38  are threaded into sensor body  14  and provide a long flame-quenching path between the interior of the sensor body carrying sensor circuitry  18  and process fluid contained in passageways  24  and  26 .  
         [0016]     Passageway  24  is positioned adjacent to opening  28  in sensor body  14 . Passageway  26  is positioned adjacent to opening  30  in sensor body  14 . Diaphragm  46  is positioned in opening  28  and is coupled to sensor body  14  adjacent to passageway  24 . Passageway  32  extends through coupling  36  and sensor body  14  to diaphragm  46 . Diaphragm  50  is coupled to sensor body  14  adjacent to passageway  26 . Passageway  34  extends through coupling  38  and sensor body  14  to diaphragm  50 .  
         [0017]     In operation, flange  13  presses against seals  48  and  52  when transmitter  10  is bolted to flange  13 . Seal  48  is seated on sensor body  14  adjacent to opening  24  and diaphragm  46 , and prevents process fluid leakage from passageway  24  and opening  28  past flange  13  to the outside environment. Similarly, seal  52  is coupled to sensor body  14  adjacent to opening  26  and diaphragm  50 , and prevents process fluid leakage from passageway  26  and opening  30  past flange  13  to the outside environment. Seals  48  and  52  are configured in accordance with the present invention. The configuration of seals  48  and  52  is discussed in greater detail below with reference to  FIGS. 4-5B .  
         [0018]      FIG. 2  is a cross sectional view of a portion of transmitter  14  and flange  13  showing a prior art seal  100 .  FIGS. 3A and 3B  are more detailed cross sectional views of prior art seal  100 . The seal  100  is adapted to be positionable against the surface of flange  13  for preventing process fluid from leaking past the flange. As illustrated in inset A of  FIG. 2 , which is a top plan view of seal  100 , the seal  100  comprises a metal ring having an outer diameter  103  and an inner diameter  106 .  
         [0019]     In the cross-sectional view of  FIG. 2  shown in  FIG. 3A , the seal  100  is shown to include an outer circumference  122  and an inner circumference  124  which forms a cavity therebetween filled by sealing material  120 . The seal  100  is coupled to transmitter  14  through, for example, a weld  128 .  
         [0020]     One aspect of the present invention includes the recognition that a gap  126  of a prior art seal can be formed during the manufacturing process between an interior diameter shoulder  130  of seal  100  and an outer circumference of diaphragm  46 . This introduces a variability in the force applied to the diaphragm  46 . This gap  126  is formed by weld distortions arising from weld  128 . For example, if the diaphragm  46  and seal  100  are held in position by a clamp which applies a load during a welding process, the contraction of the weld  128  (for example during cooling) causes the metal ring of seal  100  to pull inward, thereby lifting the metal ring along the shoulder  130  away from the diaphragm  46 . This changes the contact area and causes the contact area to become uneven.  
         [0021]      FIG. 3B  shows the same cross-sectional view as illustrated in  FIG. 3A  with the addition of process flange forced against seal  100 . This can be through clamping, bolting or other techniques. As illustrated in  FIG. 3B , the load from the process flange  13  causes the shoulder  130  to be pushed against diaphragm  46  thereby eliminating, or at least altering, the dimensions of gap  126 . Thus, the gap  126  causes the diaphragm and seal assembly to be highly susceptible to bending or other deformation in response to the application of differing bolt and flange loads. This bending or movement is transferred to the diaphragm  46  and ultimately introduces errors into pressure measurements.  
         [0022]     The present invention provides a technique to reduce errors introduced due to the gap  126  caused by weld contraction, or which may arise due to other sources. The weld contraction can be, for example, due to cooling of the weld or surrounding material. The present invention provides a configuration to ensure that the contact along the inner diameter shoulder of the diaphragm is substantially consistent and stable, regardless of the loading force applied to the seal. This is achieved by reducing, or substantially eliminating, the gap  126  shown in  FIG. 3A  when the seal is in an unloaded condition by insuring contact at an inner annular area. In some configurations, the present invention can reduce errors due to the mounting force from a flange by 50% to 75% when compared to the prior art configurations shown in  FIGS. 2, 3A  and  3 B.  
         [0023]      FIG. 4  is a side cross-sectional view and includes inset A of a seal  200  in accordance with one example embodiment of the present invention. Similarly,  FIGS. 5A and 5B  are side cross-sectional views of seal  200 . Seal  200  includes a metal ring  202  having an interior diameter or circumference  206  and an exterior diameter circumference  203 . The metal ring is preferably formed of a spring material having suitable corrosion resistance to allow exposure to the process fluid. For example, cold worked stainless steel or metal sold under the tradename Inconel, a high strength non-magnetic steel may be used.  
         [0024]      FIGS. 5A and 5B  more clearly show an interior or inner circumference  224  and an exterior or outer circumference  222 .  FIG. 5A  also illustrates an annular shoulder region  230 .  FIG. 5A  illustrates seal  100  prior to attaching the seal  100  to the transmitter  14 . An annular contact area  228  is formed along shoulder  230  by applying a loading force during the attachment process. A back taper  236  is provided which has height  226  which is greater than the weld distortion which arises due to the welding process. While the seal  200  is held in this position, the weld ( 232  in  FIG. 5B ) is applied. The weld  232  can be, for example, formed using a laser weld to thereby attach the seal  200  to the transmitter  14 .  
         [0025]      FIG. 5B  is a cross-sectional view showing the configuration of seal  200  following the welding process. As illustrated in  FIG. 5B , any welded distortion due to the shrinkage of weld  232  is counteracted by the preload such that shoulder  230  remains in a consistent and stable contact along its circumference with the surface of transmitter  14  and diaphragm  46 .  
         [0026]     The bevel or back taper  236  is preferably configured and of sufficient depth so that when the pre-loading force is applied during the welding process, some portion of the gap  226  provided by taper  236  remains. The weld  232  then preserves the contact at shoulder  230  and provides some residual load along the interior annulus formed by the shoulder  230 . This residual load eliminates, or substantially reduces, the gap  126  of prior art configurations such as that shown in  FIG. 3A . Thus, as the gap has been reduced, when different flanges and/or loads or conditions are applied to the seal  200 , the contact area along shoulder  230  does not change appreciably. By reducing any change or variation in the contact area, due to any extraneous force applied to the diaphragm  46 , errors in the measured pressure are reduced.  
         [0027]     In an alternative embodiment as shown in  FIG. 6 , the inner annular shoulder portion  330  is formed as an enlarged area such that it extends beyond the exterior annular area  336  of the ring. In this way, as the ring is preloaded, contact is assured at the inner shoulder  330  and the contact remains after the preload is removed.  
         [0028]     The present invention also includes a method of attaching a seal to a transmitter which reduces variations in the force applied to the diaphragm when the seal is placed under load. In accordance with the method, a seal  200  is preloaded during the welding or other attachment process such as shown in  FIG. 5A . Following the attachment, the preload is removed and the annular shoulder  230  remains substantially in contact with the diaphragm  46  despite any contraction in the weld  232 .  
         [0029]     Although a laser weld has been specifically described herein, the present invention is applicable to any attachment technique which would cause distortions in the seal  200  following the attachment process. Typically, the seal  200  comprises a metal, however other can be used as desired. A sealing or fill material  120  can be used with seal  200 . Any appropriate material can be used including, for example, glass filled Teflon®, graphite filled Teflon®, Viton®, or other materials known in the art for producing O-rings or the like.  
         [0030]     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. The present invention can be applied and used with other seal configurations and is not limited to the particular configurations shown herein. Other types of lips, hinged or coated flanges can be used or materials or fillings. The back taper along the outer diameter of the flange can be achieved using any technique including machining. In some embodiments, the taper is achieved due to the application of the preload force or other technique to achieve the desired profile.