Abstract:
A process transmitter includes a hydraulic system with a fill fluid for transmitting pressure from a process fluid to a pressure sensor. The fill fluid contains hydraulic fluid and a self-activating stop-leak composition for preventing loss of fill fluid caused by leaks in the hydraulic system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to self-sealing hydraulic systems for use in process transmitters.  
         [0002]     Process transmitters are used to monitor process variables, such as pressure, temperature, flow and level of process fluids used in a variety of industrial processes. For example, process transmitters are widely used in the chemical manufacturing and oil refining industries to measure variables of process fluids situated throughout production facilities. Process transmitters are typically employed throughout manufacturing or processing facilities at multiple locations to monitor a variety of process variables.  
         [0003]     Process transmitters include sensors that produce an electrical output in response to physical changes in the process variable. For example, capacitive pressure transducers or piezoresistive pressure transducers produce an electrical output as a function of the pressure of a process fluid. Typically, the pressure of the process fluid is transmitted to the sensor through a closed hydraulic system that is in contact with the process fluid at one end and the sensor at another end. As the pressure is sensed, the electrical output of the sensor is processed by the transmitter&#39;s circuitry so it can be monitored as an indication of the process variable magnitude.  
         [0004]     The accuracy of the process transmitter depends on the ability of the closed hydraulic system to convey the magnitude of the process fluid pressure to the sensor. Typically, the hydraulic system is comprised of a hydraulic passageway that is filled with a precise level of fill fluid. At a first end of the hydraulic passageway is an isolation diaphragm that separates the fill fluid from the process fluid. At a second end of the hydraulic passageway is a sensor diaphragm that links the sensor with the fill fluid. The fill fluid typically comprises a hydraulic fluid that conveys the process fluid pressure from the isolation diaphragm to the sensor diaphragm. In other embodiments, remote seals serve as extensions of the passageway and comprise a capillary tube having additional fill fluid. As the process fluid pressure fluctuates, the process fluid exerts a corresponding force on the isolation diaphragm at the first end of the hydraulic system. The fill fluid present in the hydraulic system transmits the force to the first end of the hydraulic passageway to the isolation diaphragm. At a second end of the hydraulic passageway is a sensor diaphragm that links the sensor with the fill fluid. The force deflects the sensor diaphragm, thereby causing the sensor to alter its electrical output. Thus, the electrical output is directly related to the hydraulic nature of the fill fluid. The accuracy of the process transmitter output is related to the quantity and quality of fill fluid in the passageway and remote seal of the hydraulic system. Small leaks can reduce the quantity of fill fluid, which can introduce several inaccuracies such as diaphragms bottoming out. Leaks also allow air to enter the system which can introduce inaccuracies because gas is compressible.  
         [0005]     Due to harsh usage, corrosion, material variations, manufacturing variations, etc., the hydraulic systems of process transmitters can develop microscopic leaks. Over time these microscopic leaks can lead to reduced accuracy performance. Since the leaks are insubstantial, there is little physical evidence of the leaking fill fluid. There is also little initial evidence in the decline in performance of the process transmitter from such small leaks. Until a substantial amount of fill fluid has leaked from the hydraulic system such that the deficiency in the process transmitter&#39;s performance is readily apparent, inaccuracies in performance can go undetected. If not discovered, the microscopic leaks will ultimately lead to a complete inability of the hydraulic system to transmit the magnitude of the process variable to the sensor.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention is directed toward a process transmitter for measuring a process variable of a process fluid. The process transmitter includes a sensor, a hydraulic system and a fill fluid. The sensor measures the magnitude of a process fluid variable. The hydraulic system provides a communication channel between the process fluid and the sensor. The fill fluid comprises a hydraulic fluid and a stop-leak composition. The fill fluid transmits a change in the process variable of the process fluid to the sensor. The stop-leak composition is self-sealing and seals leaks in the hydraulic system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  shows a block diagram of a process control system in which the process transmitter of the present invention is used.  
         [0008]      FIG. 2  shows a process transmitter having a remote seal in which the present invention is used. 
     
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  shows process control system  10 , which includes control room  12 , process transmitter  14  and process fluid  16 . Process transmitter  14  is comprised of transmitter circuitry  18 , sensor circuitry  20 , sensor  22  and hydraulic system  24 . Process transmitter  14  is used to monitor the level of a process variable, such as pressure, in process fluid  16 .  
         [0010]     Sensor  22  of process transmitter  14  is hydraulically connected with process fluid  16  through hydraulic system  24 . Hydraulic system  24  includes a passageway that is coupled with process fluid  16  at a first end and sensor  22  at a second end. Hydraulic system  24  includes a first fill fluid that is comprised of a first hydraulic fluid and a first stop-leak composition. The first hydraulic fluid transmits the pressure of process fluid  16  to sensor  22 . The first stop-leak composition seals leaks that develop in the passageway of hydraulic system  24 .  
         [0011]     In another embodiment, hydraulic system  24  includes a remote seal that allows the passageway of hydraulic system  24  to be extended beyond the confines of the housing of process transmitter  14 . The remote seal includes a capillary charged with a second fill fluid, comprised of a second hydraulic fluid and a second stop-leak composition, that is isolated from the first fill fluid of the passageway.  
         [0012]     The first hydraulic fluid and the second hydraulic fluid can be any suitable hydraulic fluid that is known in the art. In various embodiments, the first and second hydraulic fluids are comprised of DC 200, DC 704 ® or Syltherm XLT ® silicone oil as is commercially available from Dow Corning Corporation, Midland, Mich., USA. In other embodiments, similar fluids can be used such as Halocarbon ® from Halocarbon Products Corporation, River Edge, N.J., USA; Neobee ® M-200 from Stepan Company, Northfield, Ill., USA; glycerin and water or propylene glycol. The first hydraulic stop-leak composition and second stop-leak composition are described in greater detail with respect to  FIG. 2 . Both the first stop-leak composition and the second stop-leak composition are self-sealing. Upon the formation of a microscopic leak in either the passageway or remote seal, the self-sealing stop-leak compositions work to quickly seal the leak without any external activation. In one embodiment, the first and second fill fluids are comprised of a mix of hydraulic fluid to stop-leak composition in a ratio of about 100:1.  
         [0013]     Sensor  22  senses a physical change in a process variable of process fluid  16  through hydraulic system  24 . Typically, sensor  22  is a transducer that produces an electrical signal in response to a change in the pressure of process fluid  16  as presented through the first fill fluid. Sensor  22  is in electronic communication with sensor circuitry  20 . Sensor circuitry  20  conditions the output of sensor  22  into a usable format whereby it is displayed for local monitoring and sent to transmitter circuitry  18 . Transmitter circuitry  18  of process transmitter  14  is used to convey the output of sensor circuitry  20  to an outlying location for remote monitoring of the process variable at control room  12 . In one embodiment, transmitter circuitry  14  is in communication with control room  12  through control loop  26 . In other embodiments, transmitter circuitry  18  communicates over a wireless network. In still other embodiments, process transmitter  14  does not include transmitter circuitry  18  and is not connected with control room  12 . In yet another embodiment, the conditioned output of sensor  22  is readable by a handheld device linked by wires or wirelessly with process transmitter  14 .  
         [0014]      FIG. 2  shows an embodiment of process control system  10  in which process transmitter  14  of the present invention is used. Process control system  10  includes control room  12 , process transmitter  14  and process fluid  16 . Process transmitter  14  is comprised of sensor  22 , hydraulic system  24  and housing  25  and is connected to control room  12  by control loop  26 . Hydraulic system  24  is comprised of passageway  28  and remote seal  30 .  
         [0015]     Passageway  28  extends between sensor diaphragm  32  and first isolation diaphragm  34  and is filled with first fill fluid  36 . Remote seal  30  comprises capillary  38 , isolator assembly  40 , connector  42  and second isolation diaphragm  44  and is filled with second fill fluid  46 . First fill fluid  36  comprises a first hydraulic fluid and a first stop-leak composition. Second fill fluid  46  comprises a second hydraulic fluid and a second stop-leak composition.  
         [0016]     Process transmitter  14  is used to detect the pressure of process fluid  16 . Process transmitter  14  is shown as an absolute pressure gauge having only one hydraulic system  24 . In other embodiments, process transmitter  14  is fitted with a second hydraulic system so that differential pressure can be sensed. Process transmitter  14  generates an electrical signal based on the response of sensor  22  to the pressure applied by first fill fluid  36  to sensor diaphragm  32 . First fill fluid  36  is influenced by the pressure of second fill fluid  46 , applied to first isolation diaphragm  34 . The pressure of second fill fluid  46  is responsive to the pressure applied process fluid  16  to second isolation diaphragm  44 . Process transmitter  14  relays the electrical signal to control room  12  utilizing control loop  26 . The magnitude of the electrical signal is based on the pressure of process fluid  16  detected by sensor  20 , as presented through fill fluids  36  and  46  of hydraulic system  24 . The magnitude of the electrical signal is monitored at control room  12  as an indication of the magnitude of the process pressure of process fluid  16 . Control room  12  also supplies power to process transmitter  14  and communicates with process transmitter  14  over control loop  26 .  
         [0017]     Isolation diaphragm  44  segregates process fluid  16  from second fill fluid  46  used in remote seal  30 . Process fluid  16  has an associated pressure P 1  that exerts a force on isolation diaphragm  44 . The force is transmitted from isolation diaphragm  44  by second fill fluid  46  of capillary  38  to first isolation diaphragm  34  of passageway  28 , such that the pressure in capillary  38  equals pressure P 1 . The force associated with P 1  is transmitted from first isolation diaphragm  34  to sensor diaphragm  32  and sensor  22  by first fill fluid  36 , such that the pressure in passageway  28  equals pressure P 1 .  
         [0018]     The amount of force that is transmitted to sensor  22  depends on the quality and quantity of first hydraulic fluid and second hydraulic fluid present in hydraulic system  24 . Process transmitter  14  is calibrated having a fixed amount of first and second hydraulic fluids present in hydraulic system  24 . In the event any hydraulic fluid leaks out of hydraulic system  24 , the accuracy of process transmitter  14  is reduced, and an inaccurate output is produced by sensor  22 .  
         [0019]     To minimize the affect of microscopic leaks, first fill fluid  36  and second fill fluid  46  contain first and second self-activating stop-leak compositions, respectively. Passageway  28  is filled with first fill fluid  36  comprising the first hydraulic fluid and the first stop-leak composition. Capillary  38  is filled with second fill fluid  46  comprising the second hydraulic fluid and the second stop-leak composition. The two fill fluids are separated by first isolation diaphragm  42 . The stop-leak compositions seal microscopic leaks that form in hydraulic system  24 , and in turn maintain a higher accuracy in process transmitter  14 .  
         [0020]     Since passageway  28  comes into contact with sensor diaphragm  32  it is not feasible to use metallic based stop-leak compositions as the first stop-leak composition. Metallic based stop-leak compositions could interfere with the electrical output generated by the capacitive or piezoresistive transducer of sensor  22 . For example, metallic based stop-leak compositions could affect the capacitance between capacitor plates of capacitive transducers.  
         [0021]     In one embodiment, the first stop-leak composition is a non-metallic based stop-leak composition. Particularly, in one embodiment, the first stop-leak composition is polymer based. Preferably, the first stop-leak composition is a long-chain polymer based composition such as a polytetrafluoroethylene (e.g. Teflon ®) based composition.  
         [0022]     The second stop-leak composition seals microscopic leaks that form in capillary  38 . The second stop-leak composition prevents microscopic leaks from slowly bleeding capillary  38  of second fill fluid  46 . Second fill fluid  46  is isolated from process fluid  16 , sensor  22  and the circuitry of process transmitter  14 . Therefore, second fill fluid  46  can include a metallic based stop-leak composition as is readily known in the art. In one embodiment, second stop-leak composition comprises a metallic flake based stop-leak composition. Particularly, in one embodiment, second stop-leak composition is an aluminum flake based stop-leak composition. In one embodiment, the second stop-leak composition is RSL/2 Radiator Stop Leak ® commercially available from Justice Brothers, Inc., Duarte, Calif., USA. In one embodiment, the second-stop leak composition is Wynn&#39;s radiator sealant, commercially available from Wynn Oil Company, Azusa, Calif., USA. Aluminum based stop-leak compositions work well for very small, pinhole type leaks. Particularly, aluminum based stop-leak compositions having aluminum particles ranging in size from micro-inch sized particles to approximately 0.033 inches.  
         [0023]     For either type of stop-leak composition, the size of the polymer or metallic particles should be extremely small so they remain in solution and/or suspension. Special formulations in the sub-microinch scale would be preferable.  
         [0024]     In other embodiments of the present invention, first and second stop-leak compositions are supplemented with a marker that is noticeable on the exterior of process transmitter  14  after a leak has been sealed. For example, a color additive or a scent additive can be used. In other embodiments, the stop-leak is supplemented with a chemical additive that is emitted into the air after a leak has been sealed. The chemical additive can then be detected using a chemical detector such as a mass spectrometer. Additionally, in other embodiments, the stop leak composition is supplemented with an ultraviolet light sensitive material, whereby after a leak has been sealed the ultraviolet sensitive material could be detected with ultraviolet light.  
         [0025]     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.