System for monitoring storage tanks

• US 5,132,923 A
• Filed: 02/23/1990
• Issued: 07/21/1992
• Est. Priority Date: 02/23/1990
• Status: Expired due to Fees

• Current Assignee: Beda C. Calhoun, Susan D. Calhoun
• Original Assignee: J.A. King & Company, Inc., Major Oil Company, Inc.
• Inventors: Randolph, John H., Crawford, Charles D., King, James A. Jr.
• Primary Examiner(s): Dixon, Joseph L.
• Application Number:
  US07/484,209
• Time in Patent Office: 879 Days
• Field of Search: 364/509, 364/558, 364/571.02, 364/751.03, 364/571.07, 73/309, 73/322.9, 73/40.7, 73/49.2 T
• US Class Current: 702/51
• CPC Class Codes: G01F 23/0038 using buoyant probes with floats G01F23/30 - G01F23/76
• CPC Class Codes: G01M 3/3245 using a level monitoring device G01M3/3272 takes precedence

Abstract

A system for monitoring a storage tank containing a fluid. The system measures the buoyancy force acting on a displacer suspended in a storage tank by means of a load cell. The resulting force acting on the displacer is proportional to the mass of fluid in the tank. A look-up table is first developed by making periodic entries of the resulting load cell output during the initial filling of the tank. This data is then stored in a computer memory. By utilizing this data and interpolating between entries, a very accurate measurement of the fluid mass in the tank is calculated. In addition, by making periodic measurements over time, small leaks can be detected. In the preferred embodiment, the monitor includes an A/D converter located adjacent to the sensor assembly, thereby substantially eliminating measurement errors due to thermoelectric effects. The sensor assembly includes a generally cylindrical displacer extending from near the tank bottom or at least below the lowest level reached by the fluid in the tank to the maximum level reached by the fluid. In one preferred embodiment, the displacer has a tapered cross-section on at least one of its ends to substantially eliminate measurement errors which occur due to changes in fluid temperature.

Claims

1. A sensor assembly for a monitoring system for a storage tank containing a fluid, said sensor assembly comprising:

(a) a body positioned within said tank and extending into the fluid contained by said tank;

(b) sensor means for sensing the force exerted by said fluid upon said body and providing a first signal representative of said force; and

(c) means mounted within said tank adjacent to said sensor means for receiving said first signal and providing a second signal, said second signal being digitally encoded, whereby said means is maintained at substantially the same temperature as said sensor means and the fluid in said tank, thereby eliminating measurement errors, for example the Seebeck Effect, which occur due to differences in temperature between said sensor means, the fluid in said tank, and said means mounted within said tank.

2. The apparatus according to claim 1, wherein said body adapted to be positioned within said tank and extending into the fluid contained by said tank is attached to said sensor means at one end and the other end is freely suspended in said fluid.

3. The apparatus according to claim 1, wherein said sensor means for sensing the force exerted by said fluid upon said body and providing a first signal representative of said force is a load cell.

4. The apparatus according to claim 1, wherein said means mounted within said tank adjacent to said sensor means for receiving said first signal and providing a second signal, said second signal being digitally encoded, is an A/D converter.

5. A monitoring system for a storage tank containing a fluid, said system comprising:

(a) a body positioned within said tank and extending into the fluid contained by said tank;

(b) sensor means for sensing the force exerted by said fluid upon said body and providing a first signal representative of said force;

(c) means mounted within said tank adjacent to said sensor means for receiving said first signal and providing a second signal, said second signal being digitally encoded, whereby said means is maintained at substantially the same temperature as said sensor means and the fluid in said tank, thereby eliminating measurement errors, for example the Seebeck Effect, which occur due to differences in temperature between said sensor means, the fluid in said tank, and said means mounted within said tank; and

(d) first storage means for storing a plurality of first values in a look-up table, each of said first values corresponding to a periodic entry of the force exerted by said fluid upon said body during initial filling of said tank, said first values defining a relationship between the force exerted upon said body at a given fluid depth in said tank and the corresponding fluid mass within said tank, said first values being based on the unique characteristics of said tank, said characteristics including tank shape, volume, orientation, location of said sensor means, and the shape of said body.

6. The apparatus according to claim 5, further including means for computing a second value representative of the fluid mass within said tank by comparing a subsequent signal representative of the force on said body with said plurality of first values in said look-up table and providing its corresponding fluid mass.

7. The apparatus according to claim 6, further including second means for storage of said second values representative of the fluid mass within said tank over a predetermined time interval.

8. The apparatus according to claim 7, further including means for detecting a change in fluid mass in said tank by comparing said second values over said predetermined time interval.

9. The monitoring system according to claim 7, further including third storage means for storing a plurality of third values, each of said third values corresponding to a periodic entry of the force exerted by said fluid upon said body at a predetermined time interval.

10. The apparatus according to claim 9, wherein said third storage means is connected between said sensor means and said means for computing said second value representative of the fluid mass within said tank, said third storage means supplying said third values to said means for computing in response to a control signal from said means for computing.

11. The apparatus according to claim 10, wherein said third storage means is located at said tank.

12. The apparatus according to claim 5, wherein said first storage means is remotely located from said tank.

13. The apparatus according to claim 5, wherein said body is attached to said sensor means at one end and the other end is freely suspended in said fluid.

14. The apparatus according to claim 13, wherein said body includes a generally elongated, cylindrically-shaped portion and at least one end portion shaped along its length proportionally to the horizontal cross-sectional area of said tank.

15. The apparatus according to claim 14, wherein said at least one end portion is the upper end of said body.

16. The apparatus according to claim 15, wherein said at least one end portion further includes the lower end of said body.

17. The apparatus according to claim 5, wherein said body extending into the fluid contained by said tank extends into the fluid at least to the lowest fluid level in said tank.

18. The apparatus according to claim 5, wherein said body extending into the fluid contained by said tank extends substantially to the bottom of said tank.

19. The apparatus according to claim 5, wherein said sensor means for sensing the force exerted by said fluid upon said body and providing a first signal representative of said force is a load cell.

20. The apparatus according to claim 5, wherein said means mounted within said tank adjacent to said sensor means for receiving said first signal and providing a second signal, said second signal being digitally encoded, is an A/D converter.

21. A method for sensing the fluid mass in a storage tank containing a fluid, said method including the steps of:

(a) extending a body into the fluid contained by said tank;

(b) sensing the force exerted by said fluid upon said body and providing a first signal representative of said force; and

(c) receiving said first signal and providing a second signal, said second signal being digitally encoded, thereby eliminating measurement errors, for example the Seebeck Effect, which occur due to differences in temperature between the sensor, the fluid in the tank, and the processing circuitry mounted within the tank.

22. A method for monitoring a storage tank containing a fluid, said method including the steps of:

(a) extending a body into the fluid contained by said tank;

(b) sensing the force exerted by said fluid upon said body and providing a signal representative of said force;

(c) providing a second signal, said second signal being digitally encoded, thereby eliminating measurement errors, for example the Seebeck Effect, which occur due to differences in temperature between the sensor, the fluid in the tank, and the processing circuitry mounted within the tank;

(d) storing a plurality of first values in a lock-up table, each of said first values corresponding to a periodic entry of the force exerted by said fluid upon said body during initial filling of said tank, said first values defining a relationship between the force exerted upon said body at a given fluid depth in said tank and the corresponding fluid mass within said tank, said first values being based on the unique characteristics of said tank, said characteristics including tank shape, volume, orientation, location of said sensor means, and the shape of said body; and

(e) computing a second value representative of the fluid mass within said tank by comparing a subsequent signal representative of the force on said body with said plurality of first values in said look-up table and providing its corresponding fluid mass.