Abstract:
The present invention is directed to a system for determining a fluid level in a vessel. The system comprises a first differential pressure transducer, a second differential pressure transducer, a pressure delivery system, and a fluid passage. The first differential pressure transducer includes a first side and a second side. The first side is selectively in fluid communication with a portion of the vessel above the fluid level. The second differential pressure transducer includes a third side and a fourth side. The third side is selectively in fluid communication with a portion of the vessel below the fluid level. The fluid passage is in fluid communication with the second side of the first transducer, the fourth side of the second transducer, and the pressure delivery mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This non-provisional patent application claims the benefit of U.S. Provisional Patent Application No. 60/792,694, entitled “FIBER OPTIC DIFFERENTIAL PRESSURE LEVEL MEASUREMENT APPARATUS WITH ACTIVE REMOTE SEAL,” filed Apr. 18, 2006, which is hereby incorporated in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates in general to differential pressure measurement apparatus and, more particularly, to differential pressure measurement apparatus for measuring the level of fluid in a vessel.  
       BACKGROUND  
       [0003]     It is known in the prior art to use differential pressure measurements to determine the level of fluid in tanks and vessels. A conventional practice for using differential pressure measurements is to attach a differential pressure transducer near the bottom of a vessel and to expose one side of the differential pressure transducer to the fluid in the vessel. To account for the static vapor pressure in the vessel, a remote seal is positioned near the top of the tank. The remote seal directly exposes the other side of the differential pressure transducer to the static vapor pressure in the vessel. This arrangement is illustrated in the prior art system shown in  FIG. 1 .  
         [0004]     Such prior art arrangements have encountered a number of problems that prevent the measurement of liquid levels in vessels or result in highly inaccurate measurements. A first problem is that such arrangements are sensitive to temperature changes. A second problem is that a remote seal may be susceptible to plugging or clogging. A third problem is hydrogen migration or permeation when such arrangements are used with vessels containing hydrogen. A fourth problem is condensation within the measurement apparatus. A fifth problem is leakage of fill-fluid used in measurement apparatus.  
         [0005]      FIG. 1  illustrates a system  10  with a differential pressure transducer  12  located near the bottom of a vessel  14  and a remote seal  16  located near the top of the vessel  14 . The system  10  includes an external fill-fluid line  18  extending from the differential transducer  12  to the remote seal  16 . The vessel  14  is designed to store fluid and normally includes vapor or other such gases occupying the space  20  above the fluid level  22 . In the arrangement illustrated in  FIG. 1 , the pressure produced by such vapor, i.e., the static vapor pressure of the vessel  14 , is directly accessed by the remote seal  16  coupled to the vessel  14  near the top of the vessel  14 . The fill-fluid line  18  provides a fluid path between the remote seal  16  and the differential pressure transducer  12 .  
         [0006]     The static vapor pressure is transferred to the differential pressure transducer  12  by applying a force to fill-fluid in the fill-fluid line  18  through the remote seal  16 . The pressure produces a force on the fill-fluid, which in turn produces a force on the differential pressure transducer  12 .  
         [0007]     The system  10  remains sensitive to temperature changes because the remote seal is a closed hydraulic system. If the system  10  experiences a drop in temperature, the fill-fluid contracts and when the temperature increases, the fill fluid expands. Unless the diaphragm at the remote seal is very compliant, significant error is introduced to the measurements. Under such conditions, the pressure sensed by the differential pressure transducer  12  due to the static vapor pressure of the vessel  14  will not be reflective of the actual static vapor pressure in the vessel  14  nor the actual liquid level.  
         [0008]     When the system  10  of  FIG. 1  is exposed to a cold environment, the viscosity of the fill-fluid may cause a failure of the system  10 . The viscosity of the fill-fluid may become high enough to clog the fill-fluid line  18 . In such a state, the fill-fluid will not transmit forces due to the static vapor pressure of the vessel  14  to the differential pressure transducer  12 .  
         [0009]     If the vessel includes a hydrogen-rich fluid, hydrogen will dissolve into the fill-fluid. The addition of hydrogen to the fill-fluid may change the density of the fill-fluid, thus decreasing the accuracy of any measurements. In addition, as the static vapor pressure in the vessel  14  is lessened, the hydrogen in the fill-fluid will expand and bubble out of the fill-fluid, which also decreases accuracy of measurements and may cause the diaphragm to burst.  
         [0010]     The accuracy of the system  10  may also be compromised due to very small leaks of fill fluid from the remote seal  16  or fill fluid line  18 . Any of these occurrences will jeopardize the accuracy of any measurements produced by the system  10 .  
         [0011]     As the prior art includes a number of drawbacks, there is a need for systems and methods for accurately determining the level of fluid in a vessel.  
       SUMMARY OF INVENTION  
       [0012]     The present invention is directed to a system for determining a fluid level in a vessel. The system comprises a first differential pressure transducer, a second differential pressure transducer, a pressure delivery system, and a fluid passage. The first differential pressure transducer includes a first side and a second side. The first side is selectively in fluid communication with a portion of the vessel above the fluid level. The second differential pressure transducer includes a third side and a fourth side. The third side is selectively in fluid communication with a portion of the vessel below the fluid level. The fluid passage is in fluid communication with the second side of the first transducer, the fourth side of the second transducer, and the pressure delivery mechanism. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]     Operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:  
         [0014]      FIG. 1  is a schematic of a prior art system for measuring a level of fluid in a vessel;  
         [0015]      FIG. 2  is a schematic of an embodiment of a system for measuring the level of fluid in a vessel in accordance with the present invention;  
         [0016]      FIG. 3  is a schematic of another embodiment of a system for measuring the level of fluid in a vessel in accordance with the present invention;  
         [0017]      FIG. 4  is a schematic of another embodiment of a system for measuring the level of fluid in a vessel in accordance with the present invention; and  
         [0018]      FIG. 5  is a schematic of another embodiment of a system for measuring the level of fluid in a vessel in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     While the present invention is described with reference to the embodiments described herein, it should be clear that the present invention should not be limited to such embodiments. Therefore, the description of the embodiments herein is illustrative of the present invention and should not limit the scope of the invention as claimed.  
         [0020]      FIG. 2  illustrates a system  50  with a differential pressure transducer  52  located near the bottom of a vessel  54  and a remote seal  56  located near the top of the vessel  54 . The system  50  includes an external fill-fluid line  58  and an external fill-fluid reservoir  60 . The vessel  54  is designed to store fluid and normally includes vapor or other such gases occupying the space  62  above the fluid level  64 . In the arrangement illustrated in  FIG. 2 , the pressure produced by such vapor, i.e., the static vapor pressure of the vessel  54 , is directly accessed by a tube  66  coupled to the vessel  54  near the top of the vessel  54 . The tube  66  is also coupled to the fill-fluid reservoir  60  and provides a fluid path between the vessel  54  and the reservoir  60 .  
         [0021]     The static vapor pressure is transferred to the differential pressure transducer  52  by applying a force to fill-fluid in the fill-fluid line  58  and reservoir  60 . The pressure produces a force on the fill-fluid, which in turn produces a force on the transducer  52 . The tube  66  is arranged at an angle such that any condensation forming from the vapor drips back into the vessel  54 . Fins  68  may be positioned around the tube  66  to assist in the condensation process.  
         [0022]     Although the fill-fluid reservoir  60  is designed to accommodate expansion and contraction of fill-fluid as temperatures increase and decrease, the system  50  remains sensitive to temperature changes. If the system  50  experiences a drop in temperature, the fill-fluid contracts to the extent that the reservoir may be emptied. Under such conditions, the pressure sensed by the differential pressure transducer  52  due to the static vapor pressure of the vessel  54  will not be reflective of the actual static vapor pressure in the vessel  54  nor the actual liquid level.  
         [0023]     When the system  50  of  FIG. 2  is exposed to a cold environment, the viscosity of the fill-fluid may cause a failure of the system  50 . The viscosity of the fill-fluid may become high enough to clog the fill-fluid line  58  or reservoir  60 . In such a state, the fill-fluid will not transmit forces due to the static vapor pressure of the vessel  54  to the differential pressure transducer  52 .  
         [0024]     The accuracy of the system  50  may also be compromised due to condensation or leakage of fill-fluid. Although the system  50  does limit condensation, some moisture may still enter the reservoir  60  and add to the fluid level, thereby changing the fluid head pressure of the fill-fluid. In addition, any leakage of fill-fluid or spillage of fill-fluid from the open reservoir  60  will change the level of fill-fluid in the system  50 . Any of these occurrences will jeopardize the accuracy of any measurements produced by the system  50 .  
         [0025]      FIG. 3  schematically illustrates an embodiment of the present invention.  FIG. 3  shows a system  100  that includes positioning a differential pressure transducer  102 , a fill-fluid line  104 , and a fill-fluid reservoir  106  inside a fluid filled vessel  108 . The reservoir  106  is positioned above the fluid level  110  and is exposed to the static vapor pressure of the vessel  108 . The differential pressure transducer  102  is positioned below the fluid level  110  and near the bottom  112  of the vessel  108 . The differential pressure transducer  102  is arranged such that a first side or process side  114  of the transducer  102  is exposed to the fluid in the vessel  108  and a second side or reference side  116  of the transducer  102  is exposed to the fill-fluid and ultimately to the vapor pressure in the head of the vessel  108 .  
         [0026]     The reservoir  106  is exposed to the static vapor pressure of the vessel  108 . As such, the static vapor pressure is transferred to the differential pressure transducer  102  though the fill-fluid. The process side  114  of the transducer  102  is exposed to the pressure of the fluid in the vessel  108  and the static vapor pressure of the vessel  108 . The reference side  116  of the transducer  102  is exposed to the pressure of the fill-fluid and the static vapor pressure of the vessel  108 . In such an arrangement, the static vapor pressure is exposed to both the process side  114  and reference side  116  of the transducer  102  and will cancel each other out. The system  100  may be arranged such that the fill-fluid is a constant and known quantity; therefore, the only substantial unknown in the system  100  is the pressure exposed to the process side  114  of the transducer  102  due to the fluid in the vessel  108 . Thus, the transducer  102  may directly determine the pressure due to the fluid in the vessel  108 , and the level of fluid corresponding with that pressure may be calculated. As will be readily appreciated by those skilled in the art, such a calculation may be made by knowing the density of the fluid in the vessel  108 , the dimensions of the vessel  108 , and the placement of the transducer  102  with respect to the bottom  112  of the vessel  108 .  
         [0027]     As will be readily appreciated by those skilled in the art, the system  100  as illustrated and described overcomes some of the problems with prior art systems. As the differential pressure transducer  102 , fill-fluid line  104 , and fill-fluid reservoir  106  are positioned inside the vessel  108 , errors due to temperature changes and clogging due to extreme cold temperatures are reduced or eliminated.  
         [0028]      FIG. 4  schematically illustrates another embodiment of the present invention.  FIG. 4  shows a system  200  that includes a force transfer rod  202  for transferring static vapor pressure from a vessel  204  containing a fluid to a differential pressure transducer  206 . As shown, the system includes a tube  208  coupled on one end to the vessel  204  and coupled on the other end to a sleeve  210  housing the transfer rod  202 . The tube  208  exposes a first end  212  of the rod  202  to the static vapor pressure of the vessel  204 . The tube  208  may optionally include fins  214  to enhance the condensation properties of the tube  208 . The tube  208  is angled downward and generally arranged such that any condensation formed from vapor entering the tube  208  will drip back into vessel  204 .  
         [0029]     The transfer rod  202  includes an upper bellows  216  and a lower bellows  218  that isolate the rod  202  in the sleeve  210 , while allowing the rod  202  to move relative to the sleeve  210 . The rod  202  may be constructed or fabricated from steel or other such rigid material. Such construction provides a rod  202  that transfers forces though the rod  202  without substantial deformation and that contracts and expands negligibly over most temperature ranges.  
         [0030]     The sleeve  210  further includes a chamber  220  for holding fill-fluid. The chamber  220  is located below the lower bellows  218  and supports a lower end  222  of the rod  202 . The differential pressure transducer  206  is positioned near the bottom  224  of the vessel  204 . A first side or process side  226  of the transducer  204  is exposed to the pressure of the fluid filling the vessel  204 . A reference side or second side  228  is exposed to the fluid filled change chamber  220 .  
         [0031]     In the system  200  described, the tube  208  is exposed to the static vapor pressure of the vessel  204 , and the static vapor pressure is applied to the first end  212  of the rod  208 . The pressure acts as a force on the transfer rod  202 , which transfers the force plus its own weight to the fill-fluid in the chamber  220 . The rod  202  and bellows  216  and  218  may be arranged such that the transfer of force along the rod  202  is accomplished without significant displacement of either of the bellows  216  and  218 .  
         [0032]     In such an arrangement, the process side  226  of the transducer  206  is exposed to both the static vapor pressure and the pressure of the fluid in the vessel  204 . The reference side  228  of the transducer  206  is exposed to the static vapor pressure of the vessel  204 , the weight of the rod  202 , and the weight of the fill-fluid in the chamber  220 . As the static vapor pressure is applied to both sides  226  and  228  of the transducer  206 , the static vapor pressure is cancelled out of the system. The weights of the rod  202  and the fill-fluid are known quantities; therefore, the only remaining unknown is the pressure of the fluid in the vessel  204 . Thus, the transducer  206  may directly determine the pressure due to the fluid in the vessel  204 , and the level of fluid corresponding with that pressure may be calculated. Such a calculation may be made by knowing the density of the fluid in the vessel  204 , the dimensions of the vessel  204 , and the placement of the transducer  206  with respect to the bottom  224  of the vessel  204 .  
         [0033]     Optionally, a second force transfer rod  230 , a second upper bellows  232  and lower bellows  234 , a second sleeve  236  to house the rod  230 , and a second fill-fluid chamber  238  may be positioned between the process side  226  of the transducer  206  and the vessel  208 . In such an arrangement, the spring constant of the two lower bellows  218  and  234  are substantially the same, and the volume and temperature of the two fill-fluid chambers  220  and  238  are substantially the same; therefore, thermal effects on the fluid volume density or the elasticity of the bellows are negligible.  
         [0034]     Optionally, the rods  202  and  230  and sleeves  210  and  236  are manufactured or fabricated from the same material, such as high-strength steel, and are protected by insulating material wrapped around the sleeves  210  and  236 . Thus, when the sleeves  210  and  236  and rods  202  and  230  are exposed to the same temperature, they expand and contract together without causing any significant displacement or increase in load on the fill-fluid. To further increase the equilibrium between the process side  226  and reference side  228  of the transducer  206 , a fluid passage  240  may be formed between the chambers  220  and  238  to place the chambers  220  and  238  in fluid communication. A valve  242  may be included in the passage  240  to selectively open the passage  240  between the chambers  220  and  238 .  
         [0035]     In the arrangement illustrated in  FIG. 4  and described herein, measurement errors due to thermal expansion are minimized or eliminated by the use of force transfer rods to transfer the static vapor pressure from the vessel to the transducer. In addition, such an arrangement minimizes the volume of fill-fluid needed. When the bellows are manufactured or fabricated from high strength materials, the vapor is sealed away from the fill-fluid, and hydrogen migration or permeation problems are eliminated. In addition, the valve  242  and the fluid passage  240  allow for the establishment of a zero position, enabling precision field offset calibration.  
         [0036]     When the force transfer mechanism of this embodiment is combined with a temperature-tolerant fiber optic-based differential pressure transducer, the advantages are multiplied, as the system can operate at temperatures exceeding 500° F. with inherent safety even in explosion hazardous areas.  
         [0037]      FIG. 5  illustrates another embodiment of the present invention. In this embodiment, a system  300  utilizes two differential pressure transducers  302  and  304 . A first or measurement differential pressure transducer  302  is similar to the transducers described above. The measurement transducer  302  has a first or process side  306  exposed to a pressure caused by fluid in a vessel  308  and a second or reference side  310  exposed to a pressure based on the static vapor pressure in the vessel  308 . The measurement transducer  302  is positioned near the bottom  312  of the vessel  308  and generally below the fluid level  314  of the fluid in the vessel  308 . A second or feedback transducer  304  is positioned near the top  318  of the vessel  308 , with a first side or process side  316  exposed to the static vapor pressure of the vessel  308 .  
         [0038]     The system  300  further includes a pressure delivery system  320 , a signal processor  322 , and a fluid passage  324 . The fluid passage  324  is arranged to maintain a pressure and places a second or reference side  326  of the feedback transducer  304 , the reference side  310  of the measurement transducer  302 , and the pressure delivery mechanism  320  into fluid communication with each other. This is to say that any pressure in the fluid passage  324  will be applied to the reference side  326  of the feedback transducer  304 , the reference side  310  of the measurement transducer  302 , and the pressure delivery mechanism  320 . As will be readily appreciated by those skilled in the art, the fluid passage  324  may be any arrangement of pressure lines, piping, tubing, or the like capable of maintaining a pressure.  
         [0039]     The pressure delivery system  320  is arranged to selectively pressurize the fluid passage  324 . As shown in  FIG. 5  and described above, when the pressure delivery system  320  pressurizes the fluid passage  324 , the pressure in the fluid passage  324  is applied to the reference side  326  of the feedback transducer  304  and the reference side  310  of the measurement transducer  302 . In the arrangement illustrated, the pressure delivery system  320  includes a piston  328  located within a cylinder  330 . The piston  328  may be actuated by an actuator  332  to move in a first direction within the cylinder  330  so as to increase pressure in the fluid passage  324  and may be actuated in the opposite direction so as to decrease pressure in the fluid passage  324 . The pressure delivery system  320  as described herein is a piston and cylinder arrangement; however, it will be readily appreciated by those skilled in the art that the illustration and description in no way limit the structure of the pressure delivery system  320 . The pressure delivery system  320  may be any system or mechanism capable of pressurizing a fluid passage.  
         [0040]     The signal processor  322  is arranged to accept signals and deliver signals within the system  300 . In addition, the signal processor  322  may be arranged to calculate values or other parameters, or generally execute logic based on input signals and stored information. The signal processor  322  may include a microprocessor, digital storage, or other such equipment to receive signals, deliver signals, calculate values, and the like.  
         [0041]     The signal processor  322  is in communication with the feedback transducer  304  and the pressure delivery mechanism  320 . The processor  322  is arranged to receive a signal from the feedback transducer  304 , analyze the signal, and deliver another signal to the pressure delivery system  320 . In one embodiment, the signal processor  322  directs the pressure delivery mechanism  318  to pressurize the fluid passage  324  to a pressure that is equivalent to the static vapor pressure of the vessel  308 . Similar to the descriptions above, pressurizing the fluid passage  324  to the equivalent of the static vapor pressure of the vessel  308  will cancel the effect of the static vapor pressure on the process side  306  and reference side  310  of the measurement transducer  302 . Since the process side  308  of the transducer  306  is exposed to the combination of the static vapor pressure and pressure resulting from the fluid in the vessel  308 , applying a pressure equivalent to the static vapor pressure to the reference side  310  of the transducer  306  cancels the vapor pressure from the measurement, thus leaving the pressure due to the fluid in the vessel  308  as the only unknown quantity. Such an arrangement allows the level of the fluid to be calculated from the pressure measurement of the measurement transducer  302 . The feedback transducer  304  may monitor the pressure in the fluid passage  324  and the static vapor pressure of the vessel  308 . As the two pressures become unbalanced, the feedback transducer  304  may provide feedback to the signal processor  322  to balance the pressures.  
         [0042]     Optionally, the fluid passage  324  may be divided into two legs or portions. For example and with reference to  FIG. 5 , the passage  324  may be divided into a first leg  334  extending from the reference side  324  of the feedback transducer  304  to the pressure delivery mechanism  320  and a second leg  336  extending from the first leg  334  to the reference side  310  of the measurement transducer  302 . In one embodiment, the second leg  336  is at least partially filled with hydraulic fluid, and the first leg  334  is filled with air or other similar gas. The second leg  336  optionally may include a valve  338  to insure hydraulic fluid does not leak into the first leg  334 . In such an arrangement, the signal processor  322  may take the volume of hydraulic fluid, and thus its pressure effect on the measurement transducer  302 , into account when directing the pressure delivery mechanism  320  to pressurize the fluid passage  324 . A processor  322  may perform a calculation to insure that the sum of the pressure in the fluid passage  324  and the pressure due to the weight of the hydraulic fluid equals the static vapor pressure of the vessel  308 . The feedback transducer  304  again may monitor the pressure in the fluid passage  324  relative to the static vapor pressure and provide feedback to the signal processor  322  to continuously adjust the system.  
         [0043]     Optionally, the measurement transducer  302  may be placed in communication with the signal processor  322 . The processor  322  may evaluate a signal from the measurement transducer  302  to determine the fluid level in the vessel  308 . The density and amount of hydraulic fluid in the fluid passage  324  are known quantities, as are the placement of the measurement transducer  302  with respect to the bottom  312  of the vessel  308  and the density of the fluid in the vessel  308 ; therefore, the only remaining unknown is the pressure caused by the fluid in the vessel  308 . Thus, the transducer  302  may directly determine the pressure due to the fluid in the vessel  308 . Once a signal is relayed to the signal processor  322  by the measurement transducer  302 , the level of fluid corresponding with that pressure may be calculated by the processor  322 .  
         [0044]     As illustrated in  FIG. 5 , the measurement transducer  302  may be placed in communication with the signal processor  322  by a connector  340 . The connector  340  may be, for example, a wire arranged to carry an electrical signal based on movements within the transducer  302 . In another example, the connector  340  may be an optical fiber arranged to carry an optical signal based on movement within the transducer  302 . Similarly, the feedback transducer  304  may be placed into communication with the signal processor  322  by another connector  342  that is capable of carrying electrical signals, optical signals, mechanical signals, and the like.  
         [0045]     Optionally, a pair of valves  344  and  346  may be arranged between the vessel  408  and the transducers  302  and  304 . The valves  344  and  346  may be closes to isolate the transducers  302  and  304  from the vessel  408 . The closing of the values  344  and  346  may facilitate the temporary shutting down of the apparatus or repair and maintenance of the apparatus.  
         [0046]     The arrangement as illustrated in  FIG. 5  and described herein eliminates or minimizes problems associated with hydrogen migration and permeation, condensation, plugging or clogging, thermal effects, and leakage of fill-fluids and results in improved measurement of fluid levels within vessels.  
         [0047]     A differential pressure transducer, as used herein, may be any type of transducer. The transducer may relay pressure differentials in a variety of ways. For example, a transducer may produce an electrical signal that varies based on the stretching of a diaphragm of the transducer. In another example, a transducer may vary an optical signal based on the movement of a diaphragm or other element in the transducer and relay the signal along an optical fiber. In yet another example, a mechanical signal may be produced based on movements of a transducer. The examples of transducers illustrated and described herein will not limit the scope of a transducer as practiced with this invention.  
         [0048]     When any of the embodiments described herein is combined with a temperature-tolerant fiber optic differential pressure transducer, the advantages to industry are multiplied, as the differential pressure transducers can operate at temperatures exceeding 500° F., can withstand differential pressures of 3000 psi without rupture or the need for recalibration, and can operate with inherent safety even in explosion hazardous areas.  
         [0049]     The invention has been described above and, obviously, modifications and alternations will occur to others upon the reading and understanding of this specification. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.