Abstract:
A monitor for remote reading of liquid volumes in pressurized tanks utilizes a hollow buoyant displacement probe extending downwardly from a load cell carried adjacent a top port in the tank. The buoyant probe is optionally formed in detachable segments so that it can be assembled on site at the time of installation. The load cell is suspended from a flange cover acting as a closure for the tank port so that the load cell is freely pivotable on perpendicular horizontal axes, insuring that it is always oriented horizontally. A temperature-reading string is positioned in the open hollow center of the probe and has three spaced temperature sensing units along its length. Force data from the load cell, giving the apparent weight of the probe immersed in the liquid tank contents, and temperature data, from which volumetric data may be calculated, are fed to an external battery-operated microprocessor which periodically radios its data to a central computer which may be polled by phone lines from a remote monitoring station.

Description:
IDENTIFICATION OF RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 10/159,136, filed May 30, 2002. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    This invention relates to apparatus mountable to pressurized liquid hydrocarbon storage tanks for remote monitoring of the volume of contents therein, and more particularly to improvements in monitors which function by suspending a buoyant displacement probe in liquid tank contents from a load cell to measure the apparent weight of the probe.  
         BACKGROUND OF THE INVENTION  
         [0003]    Storage tanks for liquefied hydrocarbon products such as butane and propane present special problems for the safe and ready monitoring of tank volume levels. This is particularly so where the tanks are located in remote or relatively inaccessible locations, making tank inspections inefficient and inconvenient.  
           [0004]    LP gas storage tanks are classified as explosion hazards by the National Fire Protection Association, requiring special care in the design and installation of any ancillary equipment. The LP Gas Code(NFPA 58) defines the area within 5 feet of any tank, fill opening or point where LP gas is dispensed, loaded, vented or the like as a Class I, Division 1, Group D hazard.  
           [0005]    Despite the daunting nature of the problems involved in safely installing a volume monitor for such a tank, it would be desirable to provide for the remote reading of volume levels so that fuel supplies can be maintained in adequate amounts without the necessity of site trips to ascertain the amount of fuel on hand.  
           [0006]    It is known in the measuring art to determine the weight of liquid contents in a tank by suspending a buoyant probe in the liquid from a load cell to measure the apparent weight of the probe. By well-known calculational formulas, such measurement yields the weight of tank contents. Examples of such systems in the prior patent art may be found in U.S. Pat. Nos. 5,614,672, 5,157,968, 5,132,923 and 4,244,218.  
           [0007]    U.S. Pat. No. 5,157,968 discloses a buoyant displacement probe mounted through a top tank port via a load cell for the determination of the liquid tank content weight. It also provides a second buoyant probe mounted via a load cell through a second tank port, so that the specific gravity of the liquid may be calculated from the second load cell reading and the content weight thus be converted to a volume. U.S. Pat. No. 5,614,672 likewise determines content weight by a load cell-mounted buoyant displacement probe. It, however, relies for volume determination on an assumption that the specific gravity of the liquid in the tank is a constant, known value.  
           [0008]    Despite these efforts to provide tank monitors employing buoyant displacement probes for monitoring tank contents, there remains a lack of suitable devices employing this principle for safe and ready installation on remote hazardous storage tanks. This invention is directed to apparatus which will satisfy this need by providing an accurate and reliable monitoring system which can be safely installed for monitoring of the liquid contents of hazardous storage tanks.  
         SUMMARY OF THE INVENTION  
         [0009]    The object of this invention is to provide a volume monitoring apparatus which may be readily and safely installed on both new and existing tanks used to store liquid propane, liquid butane or like hazardous liquids. An important consideration in this invention is to maintain a high degree of accuracy in the volume determination.  
           [0010]    In accordance with the invention, there is provided a device for measuring the liquid volume in a tank, including a load cell positioned adjacent a port in the top wall of the tank. A downwardly extending displacement probe is supported by the load cell, and has sufficient length so that its lower end is adjacent to, but not touching, the tank bottom. A suspension mechanism for the load cell includes a universal joint means allowing the load cell to be oriented horizontally without regard to the orientation of the tank and port. The load cell monitors the apparent weight of the probe, and thus the weight of the liquid contents in the tank. An elongate temperature probe is mounted in the same port and has at least one temperature sensor for measuring the temperature of the liquid in the tank. This temperature reading may then be used with the known specific gravity-temperature curve of the stored liquid to utilize the specific gravity of the stored liquid at the measured temperature to calculate the volume of the liquid content.  
           [0011]    In a specific embodiment of the invention, the displacement probe is an elongate hollow member. The temperature probe is a string extending through the open center of the displacement probe, the string carrying a plurality of spaced thermometric units for measuring the temperature of the tank liquid at spaced levels of the tank. The preferred probe of the invention is in modular forn, comprising plural discrete segments which may be assembled on site during installation of the monitor by the use of joining collars, one positioned between each adjacent segment.  
           [0012]    The preferred form of the invention includes a riser pipe secured in the tank port, a mounting flange threaded on the riser pipe and a flange cap secured to the flange to close the port, the flange cap supporting a hanger bracket from which the load cell and displacement probe are suspended by way of a universal joint assembly which insures that the load cell, in the form of a planar beam sensor is maintained in true horizontal orientation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following Description of the Preferred Embodiments taken in conjunction with the accompanying Drawings in which:  
         [0014]    [0014]FIG. 1 is a plan view of a monitor constructed in accordance with this invention mounted on a pressurized tank;  
         [0015]    [0015]FIG. 2 is a perspective view of the hanger bracket, universal joint assembly and load cell of the device of FIG. 1;  
         [0016]    [0016]FIG. 3 is a plan view of the portion of the apparatus shown in FIG. 2;  
         [0017]    [0017]FIG. 4 is a plan view of the displacement probe and temperature probe of the device of FIG. 1, taken at right angles to the depiction of the probe in FIG. 1;  
         [0018]    [0018]FIG. 5 is a horizontal cross-section taken along line  5 - 5  IN FIG. 4;  
         [0019]    [0019]FIG. 6 is a schematic illustration of a monitoring system utilizing the invention;  
         [0020]    [0020]FIG. 7 is an exploded perspective view of a preferred modular form of buoyant displacement probe for use in the system depicted in FIGS.  1 - 6 ; and  
         [0021]    [0021]FIG. 8 is a plan view of the assembled probe of FIG. 7. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    As illustrated in the drawings, a storage tank  10  for liquefied propane, butane or similar hazardous liquid is provided with a monitoring apparatus constructed in accordance with the invention by installation through a single top port  12 . Existing tanks are conventionally provided with such an inspection port having a two inch diameter, and the preferred form of apparatus of this invention may be readily and safely installed in such a pre-existing port.  
         [0023]    A threaded riser pipe  14  is secured and sealed in port  12 , and extends above the port a few inches. Typically, pipe  14  may be about six inches in length. A heavy-duty flange  16  is threaded and sealed to riser  14 . Flange may be provided with a circular array of eight bolt holes. A flange gasket and flange cover  18  having conventional pressure-proof electrical cable pass-through is secured to flange  16  by conventional means such as bolts  20  to close the port  12  in sealed, pressure-proof fashion.  
         [0024]    A hanger bracket  30  is provided for suspending the in-tank elements of the apparatus. Hanger bracket  30  has an upper flange  32 , a lower flange  34 , and a vertically extending web  36  connecting the flanges  32  and  34 . A circuit board  38  is mounted on web  36 . The details of construction of board  38  are conventional. Board  38  is provided with a plug  37  for connecting to the cabling of the pass-through flange cover  20  to communicate the data received by board  38 . Web  36  also carries a temperature sensor  41  for measuring the air temperature in the upper portion of the tank, connected to circuit board  38 . This permits temperature compensation of data from the load cell described below. An aperture  40  is provided in upper flange  32  for alignment with the pressure measuring port of the flange cover  18  for measuring tank pressures. Upper flange  32  also has a pair of mounting holes  42  for bolting bracket  30  securely to flange cover  18 .  
         [0025]    A universal joint assembly  43  is suspended below hanger bracket  30 . The assembly  43  may be any suitable commercially available universal joint assembly, such as Part Number 64565K1 from McMaster-Carr Supply Company, depicted here. The upper body  44  of assembly  43  is secured to the lower face of flange  34  by bolt  46 . A pair of spaced legs  48  extend downwardly from upper body  44 , and carry an upper horizontal pivot pin  50 . The lower body  52  of assembly  43  has upwardly extending spaced legs  54  which carry a lower horizontal pivot pin  56 . Pins  50  and  56  are oriented so that the vertical planes through their axes are mutually perpendicular. Each of the pins  50  and  56  extend through a pivoting central body  58  of the universal joint assembly  43 , positioned between legs  48  and  54 . This arrangement permits lower body  52  to hang vertically plumb from pin  56 , even if the lower flange  34  of hanger bracket  30  is not oriented horizontally because of a tilt in the tank  10 , the tank port  12 , or for any other reason.  
         [0026]    An upper load cell-mounting u-bracket  60  is secured to the universal joint assembly  43  at its lower body  52  by means of a clevis pin  62  secured by a cotter pin  64 . U-bracket  60  has a horizontal leg  66  extending therefrom. One end of a load cell  70  in the form of a planar beam sensor is secured to leg  66  by means of a first compression clamp  72 . The opposite end of load cell  70  is secured to leg  76  of lower u-bracket  78  by second compression clamp  79 . Thus, downward force on lower u-bracket  78  will produce an electrical signal from planar beam sensor  70  which measures the magnitude of the force. The wiring harness  73  of planar beam sensor  70  is connected to circuit board  38 .  
         [0027]    The effect of universal joint assembly  43  is to insure that planar beam sensor  70  is oriented horizontally. This eliminates the need for measurement and correction for any variation of the load cell  70  from the horizontal. Were the cell  70  permitted to be oriented out of horizontal, its measurements of force would be reduced by the sine of the angle of deviation. Universal joint assembly  43  eliminates this source of error, and the necessity of compensation.  
         [0028]    Lower u-bracket  78  is provided with a clevis pin  82  secured by cotter pin  84  for mounting a buoyant displacement probe  90 . Probe  90  may be a hollow tubular aluminum extrusion, and includes a vertically extending central passage  92 , as well as vertically extending side chambers  94  provided to lighten probe  90  and increase its buoyancy. Annular covers  96  are secured to each end of probe  90  to close chambers  94  while leaving central passage  92  open to the liquid contents of tank  10 . A mounting neck  100  extends from the upper end of probe  90 , and is provided with a through-hole  102  for receiving the clevis pin  82  to suspend probe  90  from lower u-bracket  78 . A second through-hole  104  is provided in neck  100 , so that a screwdriver or the like may be placed therethrough to support probe  90  on the riser pipe  14  during installation, while the installer makes the wiring connections to circuit board  38 .  
         [0029]    Probe  90  houses a flexible temperature probe string  110  which passes downwardly through open central passage  92 . A plurality of temperature sensors  112  are spaced along temperature probe  110  for measuring the temperature of the liquid contents at spaced levels. In the preferred embodiment, the sensors  112  are spaced so that they are suspended at the 5%, 35% and 65% of tank height levels within the tank. Each sensor  112  communicates separately with a signal connector  114  located at the upper end of probe  110 . Connector  114  plugs into circuit board  38  at temperature plug-in  39 . This plug-in connection is adequate to support the temperature probe string  100 , because of its light weight.  
         [0030]    The data conveyed from the load cell  70  to circuit board  38  through wiring harness  73 , and the temperature data coming to the board through connector  114 , is communicated externally of the tank to a microprocessor housed in control box  120  secured atop flange. The microprocessor calculates the volume of contents in the tank from: (1) the apparent probe weight data from load cell  70 , compensated for air temperature surrounding load cell  70  as measured by temperature sensor  41 ; (2) the liquid temperature data from probe  110 ; and (3) the specific gravity curve for the stored liquid. Control box  120  also houses a radio frequency transmitter/receiver which can transmit the data to a master computer. This eliminates the need for a power hook-up within the hazardous area of the tank, as the microprocessor and radio may be conveniently operated on safe battery power.  
         [0031]    A suitable arrangement of the monitoring station is depicted in FIG. 5. A plurality of tanks with monitors installed as described above communicate by radio to a master computer housed under roof at a nearby location outside the hazardous zone. The master computer can be polled periodically by telephone from a remote monitoring station located many miles away. Of course, when remote downloading is not required, as where an on-site manned facility exists, the data can be accessed directly at the master computer. Indeed, the master computer may be a pc used for office or other functions.  
         [0032]    [0032]FIGS. 7 and 8 depict a preferred form of displacement probe for use in the system described above. This probe, which is in a modular form of discrete plural segments which may be assembled during installation, is especially versatile. In the event that a tank to be monitored is indoors, for example, a probe in this form may be installed even though the vertical clearance above the tank would be insufficient to permit a one-piece probe to be put into place. Also, even in the case of outdoor tanks, the height of the tank may be so great as to make it difficult to handle a one-piece probe of sufficient length for the job. The probe of FIGS. 7 and 8 may be transported in pieces to the job site and assembled during installation of the system.  
         [0033]    The modular displacement probe of FIGS. 7 and 8 is comprised of discrete segments  130 , which may be substantially identical. The segments are chosen in number and length so that they may easily be handled and connected on site to form the completed probe of FIG. 8 having a length to extend to near the bottom of the tank to be monitored. The illustrated displacement probe is shown with three such segments  130   a,    130   b  and  130   c,  each of which has an upper end  132  and a lower end  134 . Each segment  130  includes a central tube  136  and an outer cylinder  138 . The annular void space between each tube  136  and its corresponding outer cylinder  138  is sealed by end caps  140  which join tube  136  and cylinder  138  adjacent each end of segment  130 . Tube  132  extends beyond each end cap  140  to provide a location for joining adjacent segments  130  together during installation of the monitor. The portion of each tube  136  at the upper end  132  is provided with a diametrical upper through-hole  142  adjacent its end. A second diametrical through-hole  144  of larger size is formed through tube  132 , at an axial location between through-hole  142  and the adjacent end cap  140 .  
         [0034]    Adjacent the lower end  134  of each segment  130 , tube  136  is provided with a diametrical lower through-hole  146 , which is oriented so that it is parallel to the upper through-hole  142 . Adjacent segments  130  are secured together during installation by a joining collar  150 . Collar  150  has a central bore corresponding to the outer diameter of tube  136 , so that it slides snugly around tube  136 . It is provided with a pair of axially spaced threaded diametrical through-holes  152 , which are a match for holes  142  and  146  on each tube  136 . Set screws  153  are provided for extending into both ends of the collar holes  152  into the tube  136  at its through-holes  142  and  146 . A deep diametrical slot  156  is formed in the lower face of collar  150 .  
         [0035]    It will be seen that the displacement probe of FIGS. 7 and 8 may be formed in the appropriate number and length of segments  130  and transported to the installation site unassembled. Assembly of the probe and installation proceeds with the lowermost segment  130   a  first. This segment  130   a  is extended into the tank through the tank port  12 , and through-hole  144  is used to receive the blade of a screwdriver so that the screwdriver may rest on the top of the tank riser  14  and support segment  130   a  while a joining collar  150  and second segment  130   b  are secured thereto. This proceeds by sliding collar  150  on the upper end of tube  136 , with the slot  156  sliding over the screwdriver blade. Set screws  153  are secured in each end of the lower hole  152  of collar  150 , extendimg into upper through-hole  142  of tube  136  so that the collar is firmly secured to the lowest segment  130   a.    
         [0036]    With the collar  150  secured in place on the lowest segment  130   s,  another segment  130   b  is inserted into the collar so that its tube  136  at its lower end  134  abuts the tube  136  of the lowest segment  130   a  at the upper end  134  thereof. Set screws  153  are then secured through each end of the higher of the collar threaded holes  152  and into through-hole  146  of segment  130   b.  With this accomplished, the two segments  130   a  and  130   b  are securely joined and form a continuous hollow bore therethrough on the inside of their respective registered tubes  136 . The joined assembly can then be lowered further into the tank by removing the screwdriver blade.  
         [0037]    The procedure can be repeated identically for each segment  130  to be added to the displacement probe. After all segments have been joined, and the complete probe lowered into the tank, the hole  142  through the tube  136  at the upper end  132  of the uppermost segment ( 130   c  in the example illustrated) may be used to suspend the displacement probe from the monitor&#39;s load cell. Also, the temperature sensor array may be fed down through the completed probe through its central bore.  
         [0038]    The collars  150 , in addition to enabling easy assembly of the modular probe on site, contribute to the versatility of the probe design by permitting the user to readily vary the probe weight. The collars  150  may be fabricated to increase the mass of the modular probe for applications in which the tank liquid has a relatively high specific gravity requiring a heavier probe. Where this is the case, the mass of the probe may be readily increased by using a collar material of higher density and/or by using collars of larger outside diameter. This flexibility enables the basic modular probe design to serve for monitoring fluids with a wide range of specific gravities, with simple adjustment to the collar fabrication.  
         [0039]    Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art and it is intended to encompass such changes and modifications as fall within the scope of the appended claims.