Abstract:
A float level sensing system comprises a float and an elongated probe for sensing position of the float. The float is mounted proximate the probe so that the float floats atop the process material. The float drops outside a sensing range of the probe responsive to a failure of the float. A sensing circuit is operatively associated with the probe for measuring a characteristic of the probe representing position of the float and is operative to indicate a fault condition if the float is outside of the sensing range of the probe.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    There are no related applications. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to level sensing instruments and, more particularly, to float diagnostics. 
       BACKGROUND OF THE INVENTION 
       [0003]    Sensing instruments are used for sensing various different process variables, such as level of a process fluid or material in a process vessel. Many such instruments are of the intrusive type in which a sensing apparatus is exposed to the process material for sensing level. A common technique for measuring level uses a float. A float is an independent device designed to always stay on the surface of the material being measured. The level measurement is made by determining the location of the float. Floats also can be designed to settle at the interface between two materials where the top material is not gaseous. For instance, a float can be designed to stay at the interface between oil and water. 
         [0004]    A float can be constructed with an internal magnet. The location of the float is determined by sensing the magnetic field from the float. One technology used to sense the magnetic field of a float is magnetostriction. The term magnetostriction refers to the tendency of some materials to change physically in the presence of a magnetic field. Magnetostrictive devices consist of a wire of magnetostrictive material. The wire is contained in a tube, or waveguide. The float can either surround the waveguide or be located in the vicinity of the waveguide. An electrical pulse is applied to wire. When the pulse reaches the magnetic field of the float the wire twists generating a strain pulse that travels back up the wire at the speed of sound. A pickup sensor at the end of the wire senses the return signal. The time between the generation of the electrical pulse and the return of the strain pulse is a measure of the distance to the float. This time measurement is typically done by a combination of analog and digital electronics attached to the wire. These electronics may include a microprocessor that makes the time measurement, converts it into a distance, and finally into a level. The electronics can use two wires, four wires or digital communication. 
         [0005]    Many applications for level measurement exists in the process industry. Some of these applications have safety requirements defined by industry standards such as IEC 61508 and IEC 61511. These standards describe methods to measure the appropriateness of devices, such as level transmitters, for these applications. One such method uses the calculation of the Safety Integrity Level (SIL). The higher the SIL value, the lower the likelihood a dangerous undetected fault will occur. An important aspect of determining the appropriateness is the ability of the device to determine and indicate the difference between an actual measurement and a false measurement. In the case of magnetostrictive level devices the float is measured at the end of the waveguide when the level is at or below the end of the waveguide. A float will also stop at this point if it collapses or ruptures and fills with the material that is being measured. The level of the material can now rise above this point but the float will not rise to the surface. The result is an incorrect indication of the level which could result in material overflowing the vessel. The possibility of such a condition lowers the achievable SIL and such an event could have significant safety implications. 
         [0006]    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 
       [0007]    In accordance with the invention there is described a float level sensing system for measuring level of a process material and including float diagnostics. 
         [0008]    In accordance with one aspect of the invention a float level sensing system comprises a float and an elongated probe for sensing position of the float. Means are provided for mounting the float proximate the probe so that the float floats atop the process material. The mounting means enables the float to drop outside a sensing range of the probe responsive to a failure of the float. A sensing circuit is operatively associated with the probe for measuring a characteristic of the probe representing position of the float and is operative to indicate a fault condition if the float is outside of the sensing range of the probe. 
         [0009]    In accordance with one aspect of the invention the float comprises a magnetic float and the probe senses a magnetic field of the float. The probe may comprise a magnetostrictive wire and a tube having a near end and a distal end supporting the wire. The mounting means may comprise the float being carried on the tube and a distal end of the wire is spaced from the distal end of the tube so that if the float is at the distal end of the tube it is out of the range of the wire. 
         [0010]    In accordance with another aspect of the invention the mounting means comprises a chamber receiving the float and supporting the probe and wherein the chamber extends below the probe so that if the float is at the lower end of the chamber the float is out of range of the probe. 
         [0011]    There is disclosed in accordance with another aspect of the invention a float level sensing system for measuring level of a process material and including float diagnostics, comprising an elongated probe for sensing a magnetic field. The probe has a near end and distal end and defines a select sensing range ending at an intermediate position between the near end and distal end. A magnetic float is carried on the probe between the near end and the distal end so that the float floats atop the process material. The float drops outside the select sensing range of the probe responsive to a failure of the float. A sensing circuit is operatively associated with the probe for measuring a characteristic of the probe representing position of the float and is operative to indicate a fault condition responsive to the float being outside the select sensing range of the probe. 
         [0012]    There is disclosed in accordance with a further aspect of the invention a float level sensing system for measuring level of a material in a process vessel and including float diagnostics comprising a chamber for mounting to the process vessel and having an elongated interior space receiving the material. A magnetic float in the chamber floats atop the material. The float drops to a bottom portion of the chamber responsive to a failure of the float. An elongated probe is mounted to the chamber for sensing a magnetic field. The probe has a near end and a distal end. The distal end is above the bottom portion of the chamber so that the bottom portion is outside a select sensing range of the probe. A sensing circuit is operatively associated with the probe for measuring a characteristic of the probe representing position of the float and is operative to indicate a fault condition responsive to the float being outside the select sensing range of the probe. 
         [0013]    Further features and advantages of the invention will be readily apparent from the specification and the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view of a level measuring system in accordance with one embodiment of the invention; 
           [0015]      FIG. 2  is an elevation view of the float level system of  FIG. 1 ; 
           [0016]      FIG. 3  is a block diagram for a sensing circuit of the float level system; 
           [0017]      FIG. 4  is a flow diagram illustrating a program implemented in the microprocessor of  FIG. 3 ; 
           [0018]      FIG. 5  is a perspective view of a float level sensing system according to another embodiment of the invention; 
           [0019]      FIG. 6  is an elevation view the level sensing system of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Referring to  FIG. 1 , a float level sensing system  10  for measuring level of a material M in a process vessel V includes float diagnostics in accordance with the invention. 
         [0021]    The float level sensing system  10  comprises a chamber or cage  12  for fluidic coupling to the vessel V via a first horizontal pipe coupling  14  near the top of the vessel V and a second horizontal pipe coupling  16  near the bottom of the vessel V. Although not shown, the vessel V can be isolated from the chamber  12  using valves or the like. 
         [0022]    Referring also to  FIG. 2 , the chamber  12  comprises an elongated pipe  18  closed at a top end by a cap  20  and having a bottom flange  22  coupled to a bottom cover  24  to define an interior space  26 . The described arrangement allows the material level in the vessel V to equalize with the level in the chamber  12 , as illustrated in  FIG. 1 , while largely isolating the chamber  26  from agitation, mixing or other activities in the vessel V. 
         [0023]    The float level sensing system  10  comprises a float  28  in the chamber space  26 . The float  28  rides up and down in the chamber  12  at the surface of the material M. The float  28  is typically hollow so that it rides freely on the surface of the material M. The float  28  may be made of stainless steel or the like and houses a magnet  30  adapted to be positioned at the surface of the material M. As such, the float  28  is also referred to herein as a magnetic float. The float  28  is sized and weighted for the specific gravity and pressure of the application. 
         [0024]    The float level sensing system  10  includes a level transmitter  32  for measuring position of the float  28  representing level of the material M in the vessel V. The transmitter  32  comprises a measurement instrument including a probe  34  connected to a housing  36  containing a sensing circuit  38 , see  FIG. 3 . Straps  40 , or the like, mount the transmitter  32  to the chamber  12 . 
         [0025]    In the illustrated embodiment to the invention, the transmitter  32  comprises a magnetostrictive level transmitter. The housing  36  comprises a dual compartment instrument housing as described in Mulrooney et al. U.S. Pat. No. 6,062,095. The probe  34  comprises an elongated stainless steel tube  40  having a near end  42  and a distal end  44 . The distal end  44  is closed by an end cap  46 . A coupling  47  mounts the housing  36  to the probe  34  at the near end  42 . Referring also to  FIG. 3 , a magnetostrictive wire  48  has a first end  50  and a second end  52 . The wire second end  52  is secured by a fixture or the like (not shown) in a conventional manner proximate the end cap  46 . The wire first end  50  is electrically connected to the sensing circuit  38 . A return wire (not shown) may be connected to the wire second end  52  and the measuring circuit  38 . Alternatively, the tube  40  may be used as a return, as is known. 
         [0026]    A pickup sensor  54  is positioned proximate the tube near end  42  or in the housing  36  and is connected to a return pulse sensing circuit  56 . The magnetostrictive wire  48  is connected to a pulse launching circuit  58 . The circuits  56  and  58  are connected to a logic and timing circuit  60  which is in turn connected to a microprocessor  62 . The microprocessor  62  is also connected to a memory  64 , a display/push button interface  66  and an I/O circuit  68  which drives a two wire 4-20 mA interface circuit  70 . The interface circuit  70  is conventional and not described herein. As is known, power to the transmitter  32  is received on the two wire connection to the interface circuit  70 . 
         [0027]    The basic operation of the transmitter  32  is as follows. The microprocessor  62  periodically commands the logic and timing circuit  60  to drive the pulse launching circuit  58  to generate a pulse applied to the wire  48 . When the pulse reaches the magnetic field of the float  28  the wire twists, as is known, generating a strain pulse that travels back up the wire at the speed of sound. The pickup sensor  54  senses the return signal, as determined by the return pulse sensing circuit  56 . The time between the generation of the electrical pulse and the return of the strain pulse is measured by the logic and timing circuitry  60  and the microprocessor  62 . The microprocessor makes the time measurement, converts it into a distance and finally into a level which can be displayed and/or transmitted to external devices via the interface circuit  70 . 
         [0028]    In accordance with the invention, the float level sensing system  10  includes float diagnostics. Particularly, in accordance with the first embodiment to the invention, the probe  34  is mounted to the chamber  12 , as shown in  FIG. 2 , with the probe distal end  44  being spaced above the chamber bottom flange  22 . Particularly, the spacing is sufficient so that the magnetostrictive wire is above the float  28  when the float is at its lowest position, as generally illustrated in  FIG. 2 . More particularly, the probe distal end  44  would be at a position representing the lowest level to be used in the vessel V. The chamber  12  extends below this point. Thus, under normal conditions, the float  28  will never drop below the probe distal end  44  as the level in the chamber  12  should not drop below such a level. However, if the float  28  collapses or ruptures and fills with the material M it will drop to the bottom of the chamber  12  so that it will no longer be sensed. The transmitter  32  is adapted to sense such a condition and indicate a fault. 
         [0029]    Referring to  FIG. 4 , a flow diagram illustrates operation of a program implemented by the microprocessor  62  for level measurement and float diagnostics. The routine begins at a block  72  which initiates an electrical pulse down the wire  48 , as described. A block  74  starts a timer. A decision block  76  determines if a return pulse has been received. If so, then the timer is stopped at a block  78 . The elapsed time is converted into distance to float at a block  80 . The distance is converted to level at a block  82 . The level is indicated on the current loop, the local display and any digital communications at a block  84 . 
         [0030]    Returning to the decision block  76 , a block  86  determines if the timer has timed out. If not, then control returns to a block  76  to continue waiting for a return pulse. If the timer does time out, indicating that the distance would be greater than the sensing range of the probe, a block  88  indicates a no float failure. This happens if the float  28  is out of the range of the probe  34 , as discussed above. A block  90  then sets the loop current to the fault state, indicates no float on the local display and sends a “no float” message through digital communications. Control then returns to block  72  for the next measuring cycle. 
         [0031]    Thus, rather than simply indicating that the tank level is at the lowest level, the float diagnostics provide an indication that the float is no longer being sensed and the level measurement is not reliable. 
         [0032]    Referring to  FIG. 5 and 6 , a float level sensing system  100  in accordance with a second embodiment of the invention for measuring level of the process material M in the vessel V is illustrated. A transmitter  102  includes a control housing  104  and a probe  106 . A float  108  comprises a magnetic float captured on the probe  106 . The float  108  rides up and down the probe  106 , as is known, with the material surface. A coupling  110  connects the probe  106  to the housing  104 . The coupling  110  is threaded into a flange  112  of the vessel V. 
         [0033]    The probe  106  comprises a tube  114  having a near end  116  connected to the coupler  110  and a distal end  118  closed by an end cap  120 . The end cap  120  is enlarged to prevent the float  108  from falling off the probe  106 . The tube  114  receives a magnetostrictive wire  122 , as above. The magnetostrictive wire  122  is connected to a measuring circuit in the control housing  104 . The measuring circuit will be identical to the measuring circuit  38 , discussed above. 
         [0034]    In accordance with the invention, the tube  114  includes an annular ridge  124  which indicates location of a conventional fixture at a lower end for the magnetostrictive wire  122 . The ridge  124  is located at an intermediate position between the near end  116  and the distal end  118 . Thus, the probe defines an active span  126  between the near end  116  and the ridge  124  representing a range where level is being measured; a dead band  128  just below the ridge  124  where the magnetic field of the float will be sufficient to be measured by the wire  122  but not indicate any change in level, and an inactive zone  130  wherein the magnetic field of the float  108  is out of range and will not be sensed by the magnetostrictive wire  122 . The level sensing system  100  is designed so that the float  108  would only enter the inactive zone  130  upon failure of the float. 
         [0035]    The operation of the level sensing system  100  is similar to that discussed above relative to  FIGS. 3 and 4 . Particularly, the transmitter  102  indicates a no float fault if the float  108  is positioned in the inactive zone, out of sensing range of the wire  122 . 
         [0036]    The transmitters  32  and  102  described above comprise magnetostrictive transmitters. As will be apparent, other types of transmitters could also be used for sensing the magnetic field of the magnetic float. 
         [0037]    The present invention has been described with respect to flowcharts and block diagrams. It will be understood that each block of the flowchart can be implemented by computer program instructions. These program instructions may be provided to a processor to produce a machine, such that the instructions which execute on the processor create means for implementing the functions specified in the blocks. The computer program instructions may be executed by a processor to cause a series of operational steps to be performed by the processor to produce a computer implemented process such that the instructions which execute on the processor provide steps for implementing the functions specified in the blocks. Accordingly, the illustrations support combinations of means for performing a specified function and combinations of steps for performing the specified functions. It will also be understood that each block and combination of blocks can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions. 
         [0038]    Thus, in accordance with the invention, the float level sensing system for measuring level of a process material includes float diagnostics which indicate a fault condition responsive to a float being outside a sensing region of a probe.