Abstract:
A redundant level measuring system comprises a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. A float including a magnet in the chamber rises and falls with material level in the chamber. A magnet actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A measurement instrument includes a probe and a measurement circuit. The instrument is mounted atop the chamber with the probe extending downwardly into the chamber. The measurement circuit measures a characteristic of the probe representing level of the material in the chamber. A shield in the chamber isolate the float from the probe.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Numerous technologies exist for measuring level of liquids or solids in an industrial process environment. Among these are transmitters which measure the level and transmit a signal representing actual level. The techniques for measuring level include guided wave radar, magnetostrictive, capacitance and the like.  
           [0002]    A magnetic level indicator is another type of commonly used level sensing device. A magnetic level indicator, also known as a flipper gauge, is constructed of a chamber, a float and a visual indicator. The chamber, also known as a cage, is essentially a pipe or similar device external to a process tank or vessel which is usually mounted horizontally and which is usually connected to the tank through two or more horizontal pipes. One of the horizontal pipes is near the bottom of the chamber and the other is near the top of the cage. This arrangement allows the material level in the chamber to equalize with the material level in the tank, largely isolating the cage from agitation, mixing or other activities in the tank. The chamber, which is usually a pressure vessel, can be isolated from the tank using valves. The float is sized and weighted for the specific gravity and pressure of the application and contain magnets which actuate a visual indicator on the outside of the chamber to indicate level.  
           [0003]    In certain applications it is desirable to transmit a level signal to a remote device in addition to the local visual indication of a magnetic level indicator. Currently, magnetic level indicators are used with magnetostrictive transmitters or with a series of reed switches, either of which provides an indication of continuous level which is redundant to the primary visual indication provided by the magnetic level indicator. Both the magnetostrictive and reed switch sensors are located on and external to the chamber and are actuated by the magnet placed inside the float in the chamber. A significant drawback to these redundant systems is that the float may fail, in which case both the primary visual and secondary transmitter signals are lost.  
           [0004]    The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the invention, a redundant level measuring system includes a probe-type measurement instrument with the probe mounted in the chamber.  
           [0006]    Broadly, there is disclosed herein a redundant level measuring system comprising a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. A float including a magnet in the chamber rises and falls with material level in the chamber. A magnet actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A measurement instrument includes a probe and a measurement circuit. The instrument is mounted atop the chamber with the probe extending downwardly into the chamber. The measurement circuit measures a characteristic of the probe representing level of the material in the chamber. Shield means in the chamber isolate the float from the probe.  
           [0007]    It is a feature of the invention that the probe comprises a coaxial probe having a signal rod contained in an outer tube and wherein the outer tube defines the shield means.  
           [0008]    It is another feature of the invention that the shield means physically isolates the probe from the float.  
           [0009]    Still another feature of the invention is that the shield means comprises an elongate screen extending longitudinally in the chamber providing an electromagnetic shield and the screen is disposed between the probe and the float. The screen comprises a ferrous metal screen. In one aspect of the invention the probe comprises a twin rod probe. In accordance with another aspect of the invention the probe comprises a single rod transmission line. In accordance with still a further aspect of the invention the probe comprises a transmission line and the screen comprises a return for the transmission line.  
           [0010]    It is a further feature of the invention that the shield means comprises an electromagnetic shield cage housing the float in the chamber.  
           [0011]    It is still a further feature of the invention wherein the probe comprises a capacitance probe.  
           [0012]    There is disclosed in accordance with another aspect of the invention a redundant level measuring system comprising a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. A float including a magnet in the chamber rises and falls with material level in the chamber. A magnet-actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A guided wave radar measurement instrument includes a probe defining a transmission line. The instrument is mounted atop the chamber with the probe extending downwardly into the chamber. A measurement circuit is connected to the probe for generating pulses on the transmission line and receiving reflected pulses returned on the transmission line, the reflective pulses representing the level of the material in the chamber. Shield means in the chamber electromagnetically isolate the float from the transmission line.  
           [0013]    Further features and advantages of the invention will be readily apparent from the specification and from the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an elevation view of a redundant level measuring system in accordance with the invention mounted to a process vessel;  
         [0015]    [0015]FIG. 2 is a cutaway view of a magnetic liquid level indicator of the redundant level measuring system of FIG. 1;  
         [0016]    [0016]FIG. 3 is an elevation view similar to FIG. 1, with a portion of a chamber cut away to more particularly illustrate the redundant level measuring system according to the invention;  
         [0017]    [0017]FIG. 4 is a detailed, cutaway partial sectional view illustrating the elements of the redundant level measuring system of claim  1  located within the chamber;  
         [0018]    [0018]FIG. 5 is a sectional view taken along the line  5 - 5  of FIG. 4;  
         [0019]    [0019]FIG. 6 is a view similar to that of FIG. 4 for a redundant level measuring system according to a second embodiment of the invention;  
         [0020]    [0020]FIG. 7 is a sectional view taken along the line  7 - 7  of FIG. 6;  
         [0021]    [0021]FIG. 8 is a view similar to that of FIG. 4 for a redundant level measuring system according to a third embodiment of the invention;  
         [0022]    [0022]FIG. 9 is a sectional view taken along the line  9 - 9  of FIG. 8;  
         [0023]    [0023]FIG. 10 is a view similar to that of FIG. 4 for a redundant level measuring system according to a fourth embodiment of the invention;  
         [0024]    [0024]FIG. 11 is a sectional view taken along the line  11 - 11  of FIG. 10;  
         [0025]    [0025]FIG. 12 is a view similar to that of FIG. 4 for a redundant level measuring system according to a fifth embodiment of the invention; and  
         [0026]    [0026]FIG. 13 is a sectional view taken along the line  13 - 13  of FIG. 10. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    In accordance with the invention, a redundant level measuring system  20 , see FIG. 1, is provided. The redundant level measuring system  20  is used for providing redundant level measurement of a tank or vessel  22  having a material  24 , the level of which is to be sensed. The level measuring system includes a chamber  26  for fluidic coupling to the vessel  22  via a first horizontal pipe  28  near the top of the vessel  22  and a second horizontal pipe  30  near the bottom of the vessel  22 . The vessel  22  can be isolated from the chamber  26  using valves  32  in each of the top pipe  28  and the bottom pipe  30 .  
         [0028]    Referring also to FIG. 3, the chamber  26  comprises an elongate pipe  34  closed at a bottom  36  and having a top flange  38  to define an interior space  40 . The described arrangement allows the material level in the vessel  22  to equalize with level in the chamber  26 , as illustrated, while largely isolating the chamber  26  from agitation, mixing or other activities in the vessel  22 .  
         [0029]    In accordance with the invention, the redundant level measuring system  20  comprises a magnetic level indicator  42  and a level transmitter  43 .  
         [0030]    The magnetic level indicator  42 , see FIGS.  1 - 3 , includes a float  44  in the chamber interior space  40  and an external visual indicator  46 . The float  44  rides up and down in the chamber  26  at the surface of the material  24 . The float  44  is typically hollow so that it rides freely on the surface of the material  24 . The float  44  may be made of stainless steel or the like and houses a magnet  48  adapted to be positioned at the surface of the material  24 . As such, the float  44  is also referred to herein as a magnetic float. The float  44  is sized and weighted for the specific gravity and pressure of the application. The visual indicator  46  is strapped to the chamber  26  and is totally isolated from the process material  24 . The visual indicator  46  includes rotating flags  50 . Each flag  50  contains an alignment magnet which reacts to the float magnet  48  and protects against false actuation. With rising level, the flags  50  rotate, changing color. The floats are positioned alongside graduated markings  52  on the level indicator  46  to indicate level of the material  24 .  
         [0031]    The transmitter  43  comprises a measurement instrument including a probe  52  connected to a housing  54  containing a measurement circuit  56 . A coupling  58  mounts the probe  52  to a flange  60  mounted atop the chamber flange  38 . As such, the probe  52  extends downwardly into the interior space  40  of the chamber  26  to a level at or below level of the bottom pipe  30 . In accordance with the invention, the transmitter  43  comprises a guided wave radar transmitter that uses time domain reflectometry. Such a transmitter may be as generally described in Carsella et al., Ser. No. 09/336,194 filed Jun. 18, 1999, owned by the assignee of the present application, the specification of which is hereby incorporated by reference herein. As described therein, the probe  52  defines a transmission line. The measurement circuit  56  is electrically connected to the probe  52  for generating pulses on the transmission line and receiving reflected pulses returned on the transmission line. The reflected pulses represent level of the material in the chamber  26 . Particularly, assuming the chamber  26  is empty, then the geometry of the probe  52  and chamber  26  are selected to provide a select impedance which may be on the order of  50  ohms in air. With the material  24  located on the probe, the dielectric changes along the transmission line based on the dielectric constant of the material  24 . This change of impedance causes the reflected pulses which are returned on the transmission line.  
         [0032]    As described, the signal on the probe  52  is only dependent on the level of the material  24  being sensed. In a case of a failure of the float  52 , and resulting loss of visual indication, the transmitter  43  continues to sense material level on the probe  52  and the level signal from the transmitter  43  continues to operate. A guided wave radar transmitter is not float-actuated and is therefore unaffected by changes in product density. Also, a guided wave radar transmitter does not have to be calibrated in each application by raising and lowering the product level.  
         [0033]    The electromagnetic field produced by the guided wave radar probe  52  can interact with the magnetic float  44 . In accordance with the invention, a shield is provided in the chamber  26  for isolating the magnetic float  44  from the probe  52 .  
         [0034]    Referring to FIGS. 4 and 5, the probe  52  comprises a coaxial probe including a signal rod  62  completely contained in an outer tube  64 . The outer tube  64 , which is the signal return, is sometimes referred to as a stillwell. This is similar in construction to a common coaxial cable, except the dielectric in the case of a level measurement instrument is air in the space  66  between the rod  62  and outer tube  64 . Spacing between the rod  62  and outer tube  64  is maintained by suitable spacers  68 , as illustrated in FIG. 4. In this embodiment, the outer tube  64  provides an electromagnetic shield. The probe field is completely contained and the pulse traveling down the probe  52  is not reflected off of the float  44 .  
         [0035]    Referring to FIGS. 6 and 7, a probe  152  according to a second embodiment of the invention is illustrated. The probe  152  comprises a twin rod probe including a first rod  154  and a second rod  156  separated by spacers  158 . The rod  154  acts as the signal rod while the rod  156  acts as the return. The twin rod probe  152  can be constructed from almost any geometry which provides signal and return rods side by side in space. In this embodiment, a shield in the form of an elongate screen  160  extends longitudinally in the chamber  26 . The screen  160  is disposed between the probe  152  and the float  44 . In this embodiment of the invention, the screen  160  comprises a ferrous metal screen.  
         [0036]    Alternatively, the geometry of the system could be designed to obtain the desired impedance such that the shield  160  becomes the return.  
         [0037]    Referring to FIGS. 8 and 9, a probe  252  according to a third embodiment of the invention is illustrated. The probe  252  comprises a single rod probe. A single rod probe can be used when the geometry is such that proper impedance is obtained by using the chamber  26  as the return. A shield in the form of the screen  160  is provided between the float  44  and the probe  252 .  
         [0038]    As is apparent, a variety of probe configurations could be used, with proper shielding, to protect the probe from the float. Still a further approach, illustrated in FIGS. 10 and 11, is to shield the float  44 . In the illustrated embodiment of the invention, the float  44  is contained in a screen or cage  254  which provides electromagnetic shielding.  
         [0039]    Referring to FIGS. 12 and 13, still another embodiment of the invention is illustrated. In this embodiment, the single rod probe  252  extends downwardly near the center of the chamber  26 . A float  260  has a through opening  262  for receiving the rod  252 . Thus, the float  260  is generally donut shaped. In this embodiment, the float  260  comes in contact with the signal wire defined by the single rod probe  252 . The single rod probe  252  returns a signal representing level of the float  260 , rather than directly the level of the liquid. Particularly, the return signal provided by the float  260  is substantially stronger than a return signal provided by level of the material  24 . Signal gain utilized in the guided wave radar transmitter  43  can be adjusted to effectively ignore return signal produced by the material and thus respond only to the substantially stronger return signal produced by the float  260 . Moreover, the transmitter  43  can be configured to transmit an error signal if the float  260  fails. With failure, the float  260  will sink to the bottom of the chamber  26  and fall of off the single rod probe  252 . This would result in no return signal being received by the transmitter  43 . The transmitter thus senses the absence of a return signal and indicates a failure condition.  
         [0040]    In the above described embodiments, the measurement transmitter  43  comprises a guided wave radar instrument. Alternatively, the measurement transmitter  43  could use other techniques such as, for example, capacitance. In use as a capacitance level measurement instrument, the measurement circuit  56  measures capacitance between the probe and the vessel or shield, or the like. As is known, the measured capacitance represents level. In such applications, the shield provides a physical shield from the float, rather than an electromagnetic shield as with a guided wave radar instrument.  
         [0041]    As is apparent, the shape of the chamber  26  may be different from that shown. Likewise, the chamber  26  may be connected to the vessel  24  by only one pipe. The vessel  22  may be pressurized or nonpressurized. The present invention is not directed to any particular tank or vessel configuration or chamber configuration.  
         [0042]    Thus, in accordance with the invention, there is provided a redundant level measuring system comprising a probe-type measuring transmitter and a magnetic level indicator.