Method for Detecting Leaks in an Above Ground Petroleum Storage Tank

The present invention relates to a method utilizing an electrical or thermal conductivity gauge to test for leaks in above ground storage tanks containing a liquid having a lower specific gravity and lower electrical or thermal conductivity than water in the tank.

Leaks in large above ground petroleum product storage tanks containing liquids such as regular gasoline, light fuel oil, diesel fuel and regular gasoline are a serious problem to the petrochemical industry from the standpoint of both inventory losses and environmental degradation.

The large size of such tanks makes leak detection difficult. In this respect, a leak rate of one barrel per hour in a 100 foot diameter tank will produce a level change rate of approximately 0.01 inches per hour and a direct measurement of the level change at the surface of the liquid in the tank is difficult due to the resolution required and to thermal expansion of the liquid, the tank and the detecting apparatus.

Furthermore, acoustic emission methods of leak detection previously utilized provide results which are clouded by the weakness of the leak as an acoustic source and by signal attenuation in the liquid or in the tank shell. Furthermore, background noise from wind and thermal expansion also appear to limit the reliability of leak detection determinations utilizing acoustic emissions.

Experience has shown that all significant leaks in above ground tanks occur in the area of the tank bottom. Also, experience has shown that thermal conditions near the bottom of the tank are stable. Furthermore, surface waves and convection currents in the liquid are effectively dampened by the time they reach the bottom of the tank.

With these observations in mind, the method of the present invention utilizes the environment adjacent the bottom of the tank for detecting leaks. As will be described in greater detail hereinafter, the leak detector utilized in practicing the method is adapted to be positioned in the stable zone adjacent the bottom of the tank.

The method of the present invention utilizes sensors which sense electrical or thermal conductivity in the stored liquid and in water introduced into the tank adjacent the bottom of the tank. The sensors and associated circuitry are not affected by outside temperature, pressure, etc. Furthermore, empirical tests indicate that the plane or interface between the petrochemical liquid and an induced water layer at the bottom of the tank is stable, except for a drop in the level of the water layer due to a leak. On rare occasions, the level may increase due to small and accountable amounts of water coming from condensation and rainfall.

According to the teachings of the present invention, there is provided a method for determining the presence of a leak in an above ground tank of liquid hydrocarbons having a lower specific gravity and a lower electrical or thermal conductivity than water, said method comprising the steps of: determining the level of water in said tank; sensing changes in the level of water in the tank over time by sensing changes in conductivity at points along a line extending across an interface between the liquid hydrocarbon and the water in the tank; and, determining the amount and rate of leakage relative to the capacity of the tank and the change in the level of the water in the tank over a selected time period.

In practicing the method, prior to making a test for leaks, water is introduced into the tank to form a water layer on the bottom of the tank. Typically, this is accomplished the night before testing to allow equilibrium in the water layer and water-liquid interface to take place.

The leak detector includes a sensor assembly adjustably mounted in a casing. The casing typically has a vertical layer of water sensing material thereon and is lowered into the tank and allowed to settle on the bottom of the tank. The portion of the layer of sensing material disposed in the water will change color so as to establish a water level indicating line.

The leak detector is lifted out of the tank and the position of the water level line is noted. Then the sensor assembly comprising an array of bare uninsulated conductor ends or thermal sensors which are arranged in a line, is positioned in the area of the water level with the line of the array extending at an angle to the horizontal and across or intersecting the plane containing the water level line.

The leak detector is then lowered back into the tank and placed on the bottom of the tank.

The wire conductors are insulated except for the end portions thereof so that electrical conduction between the conductor end portions can only take place through the product liquid or through the water, or through a combination of both. The electrical resistance or thermal resistance of the product liquid is several orders of magnitude greater than that of the water layer which contains electrolytes. With respect to electrical resistance, typically the electrical resistance of the product approaches infinite ohms and typically the electrical resistance of the water is 8 to 20 megohms. Typically, the uninsulated wire end portions forming electrical conductivity sensors are spaced approximately 0.05 inches from each other. To obtain higher sensitivity or higher resolution, one can tilt the line of the array of sensors or have a fixed line of the array of sensors located at an angle of inclination to the horizontal or interface. Resolution is typically defined as L tan .theta. inches where .theta. is the angle of the inclination of the sensor array line and L is the center-to-center spacing between sensors.

The electrical conductivity can be measured by a standard ohm meter, a conductivity meter or a hand held digital volt-ohm meter. An a.c. conductivity meter is preferred to avoid polarization at the electrode sensors. In one preferred embodiment of a leak detector system utilizing the leak detector of the present invention, a panel containing an array of lamps realized by light emitting diodes (LEDs) was utilized for indicating which sensors were in the product liquid and which were in the water layer.

By correlating changes in the level of the water layer, as determined by changes in the conductivity through the sensor profile with respect to the size of the tank and the time period during which the change in the level occurred, such as, by means of a simple calculation or optionally with a microprocessor, one can determine the leak rate.

A typical testing period will be from a half day to two days.

Old tanks which have been in use for some period of time may already contain a bottom layer of water. A matted layer of debris can be formed at the interface containing emulsified material probably produced by biological action. To ensure that this layer of stringy, matted emulsified material does not clog up the sensors and adversely interfere with a measurement, the array of sensors of the sensor assembly of the detector are preferably mounted within the casing and a structure is provided for minimizing, if not altogether preventing, the interface layer of stringy matted emulsified material from gaining access to the interior of the casing.

Additionally, in one preferred embodiment, a flushing system is incorporated into the detector for flushing or washing the sensors.

Typically, the above ground petrochemical storage tanks are of the type which have a bottom drainoff faucet coupled through a valve to a pipe extending from the bottom area of the tank. As a result, once a leak test is completed the water introduced into the tank can be drained off. A known rate of drain-off can simulate a leak and verify the working of the leak detector and the results obtained can be used for calibrating the leak detector.

FIG. 1 is a fragmentary vertical sectional view, with large portions broken away, of an above ground petroleum storage, tank showing the leak detector of the present invention resting on the bottom of the tank.

FIG. 2 is an exploded perspective view of a cylindrical casing for the leak detector showing a sensor assembly separated from the cylindrical casing for housing the sensor assembly.

FIG. 3 is a vertical sectional view through the leak detector shown in FIG. 1.

FIG. 4 is a top plan view of the leak detector shown in FIG. 3 and is taken along line 4--4 of FIG. 3.

FIG. 5 is a horizontal sectional view of the leak detector shown in FIG. 3 and is taken along line 5--5 of FIG. 3.

FIG. 6 is a fragmentary vertical view of the leak detector shown in FIG. 3 and is taken along line 6--6 of FIG. 3.

FIG. 7 is an enlarged vertical plan view with portions broken away of a sensor array of the sensor assembly shown in FIGS. 2 and 3.

FIG. 8 is a vertical plan view of another embodiment of the sensor array shown in FIG. 7.

FIG. 9 is a block, partially mechanical partially schematic drawing, not to scale, of one embodiment of the overall detecting system including the leak detector, a control panel with lamps and conductivity meter and an optional microprocessor.

FIG. 10 is a schematic circuit diagram of an electrical circuit that is mounted in the control panel shown in FIG. 9 and is connected to one sensor conductor in the sensor assembly shown in FIGS. 7 or 8.

FIG. 11 is a fragmentary vertical sectional view of an above ground storage tank, with major portions broken away, and shows a modified embodiment of the leak detector of the present invention resting on the bottom of the tank.

In FIG. 1 there is shown a fragmentary upper wall portion or top 8 of a petroleum storage tank 10 with an access hole 12 therein which is adapted to be sealed by a cap, and through which the leak detector 14 of the present invention can be inserted for being lowered to the bottom or bottom wall 16 of the tank 10 and then utilized for checking for leaks and subsequently withdrawn from the top 8 of the tank 10.

The leak detector 14 is shown resting on the bottom wall 16 of the tank 10.

Typically, in a petroleum storage tank 10 a small amount of moisture or water will accumulate on the bottom 16 of the tank 10, such as from condensation. However, in using the leak detector 14 a small amount of water is inserted into a tank 10 being tested for leaks sufficient to cover the bottom 16 of the tank 10. Also, experience has shown that if there is a leak in the tank 10, it usually is in the bottom wall 16 or in a lower end portion of the side wall (not shown) of the tank adjacent the bottom wall 16. Accordingly, when there is a leak, level 18 of a water layer 20 will tend to fall.

The hydrocarbon, petroleum or other petrochemical liquid 22 stored in the tank 10 typically has a low electrical conductivity (a resistivity of effectively infinite ohms - cm), while the water 20 at the bottom 16 of the tank 10 will typically have a much higher conductivity (a resistance of 8-20 M .OMEGA.- cm) as a result of the electrolytes therein. Also, at an interface 24 between the hydrocarbon liquid 22 which is situated above the water layer 20, there is a transition layer 24 which has a varying conductivity and which under certain conditions may be made up of a thin mat or layer 24 of debris that have somehow found their way into the tank 10 or have been formed therein.

As will be described in greater detail hereinafter, the leak detector 14 of the present invention enables one to accurately determine the presence of a leak by determining changes in the water level 18, i.e., changes in the height of the interface 24 above the bottom 16 of the tank 10. This is determined by detecting changes in thermal or electrical conductivity or current flow in sensor circuits 30 (FIG. 10) for example coupled to a sensor assembly 32 (FIG. 2) located at the interface 24 between the upper liquid hydrocarbon 22 and the lower water layer 20.

The leak detector 14 will now be described below with reference to a sensor assembly 32 for sensing electrical conductivity after which a thermal sensor assembly 322 for sensing thermal conductivity will be described in connection with the description of FIG. 8.

As shown in FIG. 2, the leak detector 14 of the present invention comprises the sensor assembly 32 which is received through an upper open end 36 of a cylindrical casing 38 having mounted at the lower end 40 thereof a base assembly 42. The base assembly 42 typically includes an annular ring member 44 having affixed therein a generally circular flat bottom plate or wall 46 (FIG. 3) which closes off the lower end 40 of the cylindrical casing 38. Mounted to the bottom wall 46 are three feet 51, 52, 53 which extend downwardly and which include threadably adjustably extendable, pointed prongs 61, 62 and 63 adapted to extend through any sludge at the bottom of the tank and rest on the bottom wall 16 of the tank 10.

The base assembly 42 also includes a skirt or flange 64, generally frustoconical in shape, which flares upwardly and radially outwardly from the annular ring member 44 and which extends in a vertical direction above four port openings 71-74 (FIGS. 1 and 3) in the lower end 40 of the cylindrical casing 38.

As best shown in FIGS. 2 and 3, the sensor assembly 32 extends downwardly through the open upper end 36 of the cylindrical casing 38 and is fixed at its upper end to a cap member 76 which includes a circular flat top portion 78 having at least one vent hole 79 (FIG. 3) and an annular portion 80 slightly larger in diameter than the diameter of the cylindrical casing 38. The cap member 76 is fixed to the cylindrical casing 38 by wing nuts 81-83 (FIG. 4).

Mounted to the top portion 78 of the cap member 76 is a generally inverted U-shaped bracket member 86 having two upstanding leg portions 87 and 88 and an upper bight portion 89 to which is affixed an eyelet 90 which in turn has a cable 92 (FIG. 1) connected thereto for raising and lowering the leak detector 14 out of and into the tank 10 of petroleum product (or liquid HC).

The top portion 78 also has an opening therethrough for receiving a tubular connector 94 which receives therethrough a multiconductor cable 96 which, at its lower end 98 has a plurality 100 of insulated wire conductors (typically 25) extending therefrom which extend to and are connected into the sensor assembly 32. The tubular connector 94 is of the type which will permit movement of the cable upwardly or downwardly into the cylindrical casing 38.

The sensor assembly 32 is mounted on a threaded rod 104 having a free lower end 106 and an upper end 108 which is journaled in the top portion 78 of the cap member 76 and which has a knob 110 at the upper end thereof for rotating the rod 104.

Also fixed to and extending downwardly from the top portion 78 are spaced apart guide rods 111 and 112 which extend on either side of the threaded rod 104 such that the two guide rods 111 and 112 (and the threaded rod 104 therebetween) lie generally in the same plane. The guide rods 111 and 112 are each fixed at their upper end to the top portion 78 of the cap member 76 and are each fixed at their lower ends to a circular plate 113 located adjacent a circular plate 114 of the base assembly 42.

A hollow space 115 is defined between the plates 46 and 114 for receiving a ballast material 116 such as lead shot or ball bearings.

Spaced slightly above the lower circular plate 113 is a guide block 118. The lower end of the threaded rod 104 is journaled in the guide block 118 which has two spaced apart openings therein for enabling the block 118 to be slidably received on the guide rods 111 and 112.

The sensor assembly 32 includes an upper block 120 which has two openings for slidably receiving the guide rods 111 and 112. It will be understood from FIGS. 2, 5 and 6 that the block 120 also has a central threaded opening through which the threaded rod 104 extends whereby rotation of the knob 110 to rotate the threaded rod 104 will cause raising or lowering of the block 120 on the guide rods 111 and 112.

The sensor assembly 32 further includes a cable connector block assembly 122 which is mounted on top of the upper block 120. Also mounted to the upper block 120 and extending downwardly therefrom is a sensor plate assembly 124 into which the multiple insulated conductors 100 extend and in which they are clamped in an equally spaced generally vertical parallel manner as shown in FIGS. 2, 3, 7 and 8. The lower edge of the plate assembly 124 has a bottom edge 126 which is inclined to the horizontal at a predetermined angle .theta. as shown in FIG. 7. The insulated wire conductors 100 are insulated from metallic parts of the plate assembly 124 which can be made of metallic conducting and/or nonmetallic nonconducting plate portions so long as the insulated wire conductors 100 are insulated from one another and from an outer metal plate 128 of the plate assembly 124 which serves as a ground (current return) element.

However, the lower ends of the wire conductors 100 such as wire conductor ends 201, 202, 203, 204 . . . 219, 220, 221, 222, 223, and 224 are bare, uninsulated, exposed end portions 201-224 spaced vertically above each other a predetermined distance H and horizontally a predetermined distance L from each other on the inclined bottom edge 126 of the plate assembly 124 as shown in FIG. 7 for providing an array of sensors (conductor end portions) 201-224 having a predetermined sensitivity as will be described in greater detail hereinafter.

The circular plates 46 and 114 are held to the circular plate 113 by a bolt 230 which extends through the center of the assembly 42.

Also mounted to the top portion 78 of the cap member 76 is a fitting 232 which receives a tubing 233 that extends through the cap member 76 to a coupling 234 (FIG. 6) mounted to the block 120 and connected to a tubing member 235 having a curved end portion 236 that is positioned so that an open end 238 thereof is aimed at and in line with the area adjacent to and parallel to the lower edge 126 of the sensor plate assembly 124. The tubing 233 is flexible and extends through the fitting 232 to the coupling 234 connected to the tubular member 235 to enable an operator to wash, scrub or flush off the array of sensors (bare conductor end portions) 201-224 periodically such as with the petroleum liquid in the tank 10.

In the use of the leak detector 14 (after a quantity of water is introduced into the tank 10 to cover the bottom 16), an operator will first lower the leak detector 14 through the hole 12 as shown in FIG. 1. The operator then will let the detector 14 settle through any sludge at the bottom 16 of the tank 10 until it is safe to assume that the detector 14 is resting on the bottom 16 of the tank 10. The detector 14 is allowed to rest in this position as shown in FIG. 1 for a short period of time after which it is raised out of the tank 10. A generally vertical layer of water sensing material 248 is provided on the exterior surface of the casing 38 as shown in FIG. 1. The portion 249 of the layer 248 situated in the water will change color to establish a water level line 250. Alternatively, a float type or optical sensor can be used to initially determine the water level line 250.

After the detector 14 is raised out of the tank 10, the operator will turn the knob 110 to adjust the position of the sensor assembly 32 so that the inclined lower edge 126 of the plate assembly 124 having the sensors (conductor end portions) 201-224 thereon will extend through the horizontal plane containing the water level line 250 and interface 24 between the water layer 20 and the liquid hydrocarbon 22 in the tank 10 when the detector 14 is reinserted into the tank 10.

Then the operator again lowers the leak detector 14 back into the tank 10 to the bottom 16 thereof so that the leak detector 14 is resting firmly on the bottom 16 of the tank 10. Then conductivity can be detected using a conventional conductivity meter, ohm meter or volt meter.

In one embodiment shown in FIG. 9, a display panel 254 can be utilized which will indicate the position of the interface relative to the inclined lower edge 126 of the plate assembly 124. This is accomplished by the electrical circuits 30 shown in FIG. 10. Each circuit 30 includes one of the conductors 100 and the sensor end portion thereof, such as the conductor end portion 201. Each circuit 30 also includes the liquid between the sensor (e. g., sensor 201) and the grounded plate 128, a common conductor 256 connected to the plate 128, a lamp circuit 258 including a lamp (LED) 260, a resistor 262 and a capacitor 264, and a signal processing circuit 266 including four resistors 271-274, a NAND gate 276 and an inverter 278.

In addition or as an alternative, the sensors or conductor ends 201-224 can be coupled to input ports of a microprocessor 280 (FIG. 9) which will note the conductivity or voltage levels on each of the conductors connected to each of the sensors (conductor ends) 201-224 and note the voltage levels at the respective input ports and what changes take place over a given period of time.

Another conductivity measuring alternative is a multi-position switch with an AC conductivity meter.

In this way, one can determine over a period of time whether there has been an increase in moisture or water in the tank 10 or whether there has been a leak by reason of a decrease of water in the tank 10.

Each of the circuits 30 provides a simple indication of the status of the water-petroleum liquid interface 24 and can be used in addition to or in place of a conductivity meter 282 on the control panel 254 shown in FIG. 9. The position of the interface layer 24 is indicated by a row or column 286 of twenty-four (24) light emitting diodes lamps 260, each one corresponding to and coupled to one of the sensor forming conductor end portions 201-224.

The LEDs 260 corresponding to the sensors immersed in the low conductivity hydrocarbon liquid 22 will flash on and off approximately twice per second. Those LEDs 260 associated with the water-contacting sensors 201-224 remain continuously energized. In this way, if the level of the interface 24 changes due to a leak, those ones of the LEDs 260 that were previously energized begin to flash on and off as the water level 18 in the tank 10 decreases.

Each of the circuits 30 can be energized by batteries so that the leak detector has a self-contained power supply.

As noted above, FIG. 9 illustrates a leak detector system 300 including the leak detector 14, the control panel 254 and the optional microprocessor 280. The control panel is shown with two rows of sixteen (16) lamps, although as noted above preferably one row or column of twenty-four (24) lamps are preferred. Also, the control panel 254 is shown with the conductivity meter 282 mounted thereon together with a calibration control knob 302. These items can be eliminated, if desired. Also an ON and OFF switch 304 can be provided for energizing the control panel 254.

Further, it will be understood that all monitoring can be effected solely with the microprocessor which can be programmed to poll the inputs thereof connected to the sensors 201-224 periodically, such as once every hour. Then, of course, any changes in conductivity over any number of the sensors or bare end portions 201-224 can be correlated with the distance H, the distance L, the angle .theta. and the time elapsed to determine the rate of leakage.

It should be noted that the sensitivity of the array of sensors can be altered by changing the angle .theta., the angle of inclination of the lower edge 126 of the plate assembly 124 to the horizontal.

From empirical tests, it appears that a preferred angle for the angle .theta. is 20.degree..

In FIG. 8 there is illustrated a modified embodiment of a sensor assembly 322 including a plate assembly 324 wherein the plate assembly 324 is adapted to be rotated about an axis perpendicular to the plate assembly 324 so as to alter the angle .beta.. In this assembly, a lower edge 326 of the plate assembly 324 is perpendicular to the side edges of the plate assembly 324. In this embodiment, the sensors 330 are thermal sensors each of which extends an equal distance from the edge 326. The sensitivity of this array of sensors is adjusted by rotating the plate assembly 324 so as to change the angle of the lower edge 326 relative to the horizontal. This will result in a change in the distance H measured vertically between the exposed ends of adjacent sensors of the plurality 330 of sensors.

In a thermal system, the same voltage is applied to each sensor 330 and the liquid medium will determine how much heat is dissipated by each sensor and the rate of heat dissipation. This will determine a given steady state current. Then when the liquid medium is changed, such as from petroleum product to water, the heat dissipation will change causing a change in the steady state current. This change in current is measured to determine a change in the water level.

Preferably, the exposed end portions 201-224 of the sensing conductors 100 are "tinned", i.e., coated with an anti-corrosive metal, such as silver, gold or tin.

In FIG. 11 there is illustrated an alternative embodiment of a leak detector assembly 414 of the present invention where, instead of having a rotatable plate assembly 324 as shown in FIG. 8 which lies in a generally vertical plane and is rotated in that plane, the modified leak detector 414 shown in FIG. 11 includes a plate assembly 426 which also lies in a vertical plane but which is rotatable about an axis lying in or parallel to the plane of the plate assembly 424. Here a plurality 430 of insulated wire conductors are mounted on one side of the plate assembly 424 and when the plate assembly 424 is rotated, the relative vertical distance between adjacent bare end portions of the insulated wire conductors 430 is changed in a manner similar to rotating the plate assembly 324 in FIG. 8 as described above.

It will be appreciated that in the embodiment shown in FIG. 7, the sensitivity is fixed whereas in the embodiment shown in FIGS. 8 and 11, the sensitivity of the array of sensors can be altered.

Further it will be appreciated that the frustoconical skirt or flange 64 provides a deflecting baffle for deflecting any debris at the interface 24 between the liquid 22 and the water 20 away from the ports 71-74 when the leak detector 14 is lowered into the tank 10. Additionally, in case any of this debris should gain access to the interior of the cylindrical casing 38, the tubing member 235 for washing or flushing the inclined lower edge 126 of the plate assembly 124 enables the sensors 201-224 to be washed or cleaned.

Extensive testing of the leak detector was conducted at an Amoco facility in Casper, Wyo. and produced surprisingly good results.

For example, leak tests for a tank with gasoline (tank No. 325) and a tank for light fuel oil (tank No. 322) maximum leak rates of 15 gal./hr and 10 gal./hr were detected using a leak detector constructed according to the above teachings. Accuracy of the detector is verified by determining the leak rate while drawing water from the tank using the detector and comparing that determined rate with the actual measured rate of water drained off from the tank.

From the foregoing description, it will be apparent that the method of the present invention using the leak detector 14, 414, has a number of advantages, some of which have been described above and others of which are inherent in the invention.

Also it will be apparent from the foregoing description that various modifications can be made to the method of the present invention without departing from the teachings of the invention. Accordingly, the scope of the invention is only to be limited as necessitated by the accompanying claims.