WO1992005408A1

WO1992005408A1 - Apparatus for measuring water bottom level and leakage of a tank

Info

Publication number: WO1992005408A1
Application number: PCT/US1991/006868
Authority: WO
Inventors: Randy J. Longsdorf; Robert C. Hedtke; Richard L. Nelson
Original assignee: Rosemount Inc
Current assignee: Rosemount Inc
Priority date: 1990-09-26
Filing date: 1991-09-20
Publication date: 1992-04-02
Anticipated expiration: 1993-03-26
Also published as: CN1061660A

Abstract

A differential pressure transmitter (42) is used to measure the level of settled water (14) residing below a process fluid (12) and at the bottom of a tank (10). The pressure transmitter (42) measures a pressure difference between a pressure tap (48) of the tank located below an upper level of the higher specific gravity settled water and a second pressure tap (24) of a reference column (22, 86, 88) containing only lower specific gravity process fluid and opening to the tank (10) at a level (26) above the settled water. The differential pressure is a function of the specific gravity or density of the water (14) and a pressure head of the waterc (14). Utilizing the differential pressure and specific gravity data the height of the water (14) in the tank (10) can be determined. The reference column (22, 86, 88) is capable of being purged to remove any water or other impurities. A portion of the reference column (22, 86, 88) extends at least as high vertically as the lighter fluid (12) in the tank (10). The reference column can be a separate column (86, 88) that can be closed off from the tank (10) to permit the differential transmitter (42) to sense differential pressure changes due to level changes of fluid in the tank (10).

Description

APPARATUS FOR MEASURING WATER BOTTOM LEVEL AND LEAKAGE OF A TANK BACKGROUND OF THE INVENTION The present invention relates to a highly sensitive tank monitoring system capable of measuring an upper level of an impurity, such as water, settled at a bottom of a storage tank. The invention is also capable of tank leak detection.

In large storage tanks containing process liquids, for example petroleum products, undesirable impurities such as water having higher specific gravities than the process liquid settle out and collect at the tank bottom, commonly known as a water bottom.

The water bottom is an accumulation of denser impurities, such as rain water that falls on a floating roof and leaks through a seal into the tank, injection water used to aid pumping of products from the ground to the tank, and sea water used for ballast by ocean going tankers transporting products. It is extremely important to know an upper water level for if water level reaches a tank outlet, which is typically only one to three feet above the tank bottom, any outflow of the process liquid would contain a high percentage of water.

Further, the water and other undesired dense liquids or solids can be corrosive to the tank bottom which shortens tank life and must be removed from time to time.

One method of measuring water level in a tank involves using two air bells, one disposed inside the other, each having an opening at different levels and communicating with a manometer, such as taught by U.S. Patent 4,084,435. The drawback of this method is that the system must be inserted in the tank, and the bells need to be accurately positioned near the tank bottom, which may not always be a uniform surface.

Another method of measuring level is taught in U.S. Patent 1,622,794 which also implements a manometer. The drawback to this technique is that the tank must always be full of liquid and the reference pipe is filled with a fixed amount of impurity(water) thus created a fixed reference head.

Many tanks have a hydrostatic tank gauging (HTG) system to measure the amount of process liquid in . the tank by mass, such as the Series 7500 manufactured by Rosemount Inc. of Eden Prairie, Minnesota. Present HTG systems typically utilize two separate pressure transmitters at different elevations below a hydrostatic head created by the process liquid and separated a predetermined distance apart. A first pressure transmitter measures hydrostatic pressure near the tank bottom, and a second pressure transmitter measures hydrostatic pressure at a known distance above the first pressure transmitter. A mass of the process liquid is proportional to pressure measured by the lower transmitter multiplied by a horizontal tank area. Density is proportional to a difference between the two measured pressures divided by the height or distance between pressure taps for the two transmitters. Level and volume can be calculated from mass and density calculations.

However, when water at the tank bottom having a higher specific gravity than the process liquid reaches the first(lower) transmitter, errors are introduced in the mass and density calculations since the HTG assumes the contents above the first pressure transmitter comprises only process liquid having a uniform density. Hence, while HTG is accurate for inventory control of a uniform density process liquid and capable of total automation, current HTG systems lack the capability to compensate for and accurately measure water levels at the bottom of the tank. A further problem with liquid storage tanks is leakage. When large tanks are involved, for example in the diameter range of 20 to 50 feet, a substantial amount of process liquid can be lost into the ground with very little change in process liquid level in the tank. One method of leak detection utilizes a visual sight glass, however, accuracy is poor to detect small level changes and is temperature dependent. Another technique uses a pressure transducer measuring pressure changes at a tank bottom, such as taught by the cross- referenced application. While volume of liquids can change with temperature, mass does not change.

An instrument capable of accurately measuring water level at a tank bottom that can be easily converted to a highly accurate tank leak detector is desirable. Further, an instrument that is compatible with current HTG systems is desirable.

SUMMARY OF THE INVENTION The present invention provides a simplified and highly accurate apparatus for determining an upper level of a denser impurity, such as water, settled at a bottom of a storage tank below a less dense process liquid. The apparatus can automatically maintain an impurity upper level below a product outlet to avoid discharging t ^"■ -^~t impurity with the process liquid. The apparatus can easily be converted to a leak detection system capable of automated control and is compatible with HTG systems.

In a first preferred embodiment, a single low range differential pressure transmitter of conventional design has a high pressure port at a first level receiving a first hydrostatic head pressure created by both process liquid and denser impurity residing at and above the first level. A low pressure port of the transmitter at substantially the first level receives a second hydrostatic head pressure from a purged column containing only process liquid. The purged column opens into the tank at a level above an upper level of the impurity. The differential transmitter senses differential pressure across the high and low pressure ports created by the denser impurity residing at and above the first level and provides a transmitter output representative of the impurity upper level with respect to the first level. Means for calculating receives the transmitter output and connects to a device capable of receiving and storing specific gravity data of the process liquid and the impurity. The means for calculating calculates the upper impurity level from specific gravity data and the transmitter output in a known manner. Common head effects from the process liquid across the transmitter cancel allowing a low range differential pressure transmitter having a higher accuracy to be used.

In the second preferred embodiment, a valve is interposed between the purged column and the tank for selectively isolating pressure in the purged column from the tank, permitting the apparatus to accurately detect tank leaks. A portion of the purged column extends upward opening to the tank at a point above liquid in the tank. Upon closing the valve, a fixed reference level is established in the purged column equal in height to total liquid in the tank. Any subsequent changes in the height of the tank liquid generates a proportional change in differential pressure across the differential pressure transmitter. The transmitter output is not an absolute measurement of volume of leakage, but is proportional to level changes. After a predetermined time period since closing the valve, a start reading of the transmitter is established. Upon opening the valve a second subsequent reading is established, where the pressure difference across the differential transmitter between the start reading and the second subsequent reading is proportional to level changes. Errors due to temperature differences during the predetermined time period as well as errors created by transmitters that drift over time are reduced thereby providing improved leak detection accuracy. Keeping the transmitter and the purged column at a constant temperature aids in stability of readings. Leak measurements are extremely accurate using a differential transmitter since common head effects cancel and a low range highly accurate transmitter can be used.

The invention is compatible with conventional hydrostatic systems since the transmitter can be connected to previously installed tank flanges used in conventional hydrostatic tank gauging systems. The invention is non-intrusive, meaning the entire apparatus can reside exterior to the tank, but, if desired, the apparatus can be placed interior to the tank.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a water bottom measurement system of the present invention illustrating the principles of determining water level in a tank.

Figure 2 is a schematic representation of a second form of the invention which includes an ability to convert the water bottom measurement system to a leak detection system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a liquid storage tank indicated generally at 10 contains liquids of two different specific gravities or densities, including a lighter or less dense than water process liquid 12, and a denser impurity or water 14 residing below process liquid 12, which can include sediment. In this embodiment, tank 10 has a bottom section 16 for collecting water impurities, commonly known as a water bottom. Water bottom measurement system 20 connects to tank 10 and can reside inside tank 10, or outside tank 10 as shown in FIG. 1.

Purged column 22 taps into tank 10 at port 24 residing above water level 26, which is an interface between process liquid 12 and water 14, and ideally, below outlet port 28 at level 30 and therefore always below upper tank level 32 in tank 10. Purged column 22 extends downward to purge valve 34 which can be automatically controlled by controller 36 via control line 38 to purge column liquid 40 of any contaminants such as water 14 prior to any measurements.

Differential pressure transmitter 42, such as Model 3051C manufactured by Rosemount Inc. of Eden Prairie, Minnesota has a high pressure port 44 connecting via passageway 46 to port 48 of tank 10 at a point very near tank bottom 16 sensing a pressure head P₁ created by both higher specific gravity water 14 and lower specific gravity process liquid 12. Low pressure port 50 opens via passageway 52 to purged column 22 at substantially the same elevation as high pressure port 44 and senses pressure head P₂, serving as a reference pressure, created by only lower specific gravity process liquid 12 in purged column 22 and tank 10 since purged column 22 bypasses water level 26. Differential pressure sensor 54 in transmitter 42 senses differential pressure Pι-P₂ between high and low pressure ports 44 and 50, respectively, generated by a presence of higher specific gravity water 14 above port 48. Transmitter 42 generates a transmitter output signal on line 56 proportional to differential pressure, for example, a DC current in a range of 4-20 A or a digital output, which routes to receiver 58 in electrical circuit 60 which can comprise logic circuits or a microprocessor based system. Since high pressure and low pressure ports 44 and 50, respectively, are at substantially the same level, common head effects due to process liquid 12 cancel, hence, a low-range differential transmitter having a high sensitivity can be used to measure small pressure differences due to denser water 14.

Specific gravity data 62, comprised of stort Λ suitable look-up tables, measured specific gravity, entered specific gravity, or the like, provides density or specific gravity data of water 14, as well as of process liquid 12, to calculator 64 in electrical circuit 60. A linear relationship exists between differential pressure P_}-^ and height of water level 26 making calculations simple. Calculator 64 calculates water level 26 from specific gravity data 62 and the transmitter output signal provided on line 56 in a known manner. Computation storage 66 stores calculations from calculator 64 and provides output 68, where output 68 is representative of water level 26. If no water 14 resides above port 48, then both pressures ₂ and P₂ are created by only process liquid 12 and are equal, therefore, differential pressure P_i- ₂ is zero and computation storage 66 generates output 68 correspondingly. If, however, water 14 rises above port 48, pressure P-_j^ generated by higher specific gravity water 14 and process liquid 12 is greater than pressure P₂ generated by process liquid 12 only and a differential pressure is generated. Output 68 can be used, for example, to drive a recorder, or, to sound an alarm when water level 26 exceeds a predetermined level. Calculator 64 can initiate controller 36 to open drain valve 70 automatically via control line 72, where valve 70 connects to drain port 74 opening to water 14 in tank 10, to reduce water level 26 by draining water 14 and any other sludge residing at bottom section 16, ensuring water level 26 remains below outlet port 28. Controller 36 also controls valve 76 via control line 78, where valve 76 connects to tank outlet port 28 to selectively dispense process liquid 12. Mass calculations of water 14 and process liquid 12 can also be calculated by calculator 64 by multiplying known horizontal area of water 14 and process liquid 12 as a function of vertical elevation in tank 10.

To minimize measurement errors due to temperature, which causes liquids to expand, pressure transmitter 42 should be connected to tank 10 via passageway 46 as close to tank bottom section 16 as possible, and valves 34 and 70 should reside at substantially the same level. Passageways 46 and 52, and purged column 22 should comprise the same material as tank 10 and be insulated, and should be attached to tank 10 opposite the sun, which is the north side in the northern hemisphere.

In FIG. 2, a second preferred embodiment of the invention is shown capable of measuring both height of water level 26 and tank leakage. In this embodiment, measurement system 80, similar to measurement system 20 described in the first preferred embodiment, connects to tank 10.

High pressure port 44 of transmitter 42 opens to port 48 of tank 10 via passageway 46 and two-way valve 82. Low pressure port 50 of differential pressure transmitter 42 opens to purged column 86 via passageway 52 and two-way valve 84. Purged column 86 extends downwardly to purge valve 34, which can be automatically controlled by controller 36 via control line 38 as previously explained, and upwardly to passageway 88, which opens to tank 10 at port 89 above liquid level 32 thus exposing column liquid 40 to same vapor pressure as process liquid 12. Purged column 86 also opens to passageway 90 which connects to passageway 92 via valve 94. Passageway 92 opens to port 24 of tank 10 at a point above water level 26 and, ideally, below output port 28 whereby only lower specific gravity process liquid 12 is drawn into purged column 86 when purging purged column 86 prior to any measurements. When valve 94 is open, measurement system 80 measures water level 26 as described in the previously described first preferred embodiment. Upon initiating a tank leakage measurement, valve 94 is selectively closed, for example by controller 36 via control line 96, communication of pressure between purged column 86 and tank 10 via passageways 90 and 92 is discor inued, and measurement system 80 serves as a tank leak detector. Reference liquid 40 comprising process liquid 12 remains at a constant level 98 corresponding to level 100 while any chang . in liquid level 32 due co, for example tank leakage, evaporation, condensation, valve leakage, etc., causes a change in pressure P-^ and, hence, a change in differential pressure across high pressure port 44 and low pressure port 50. Differential sensor 54 senses changes to differential pressure across high and low pressure ports 44 and 50, respectively, which changes are proportional to changes in liquid level 32, and transmitter 42 provides a proportional change of transmitter output signal to line 56. To calculate tank leakage after a predetermined time period since closing valve 94, a start reading is established by transmitter 42 corresponding to liquid level 32. When valve 94 is selectively opened, for example by controller 36 via control line 96, communication of pressure between purged column 86 and tank 10 is resumed and shortly thereafter a second reading is established by transmitter 42 corresponding to liquid level 32. Calculator 64 calculates an absolute change in liquid level 32 during the predetermined time period utilizing the change in transmitter output signal on line 56 between the start reading and the second reading and specific gravity data 62, and computation storage 66 provides output 68 representative of tank leakage. Since the time period between the start reading and the second reading is substantially short, errors due temperature errors and drifting of transmitter 42 are reduced substantially. If tank 10 has a horizontal surface area uniform in the vertical direction, such as a cylinder positioned on end, volume of leakage can be calculated by multiplying level change of liquid level 32 by surface area.

Differential pressure transmitter 42 can be a very sensitive low-range transmitter since common head effects cancel and only small changes in differential pressure are encountered when measuring ref rence level 98 and tank leakage, thereby providing highly accurate leak measurements. Two-way valve 82 interposes between high pressure port 44 and port 48, connecting to passageway 102 which opens to low pressure port 50. Two-way valve 84 interposes between low pressure port 50 and reference column 86, connecting to passageway 104 which opens to high pressure port 44. Upon selectively changing valves 82 and 84 to alternate positions, such as by controller 36 via control lines 106 and 108 respectively, valve 82 now directs heavier specific gravity water 14 at port 48 via passageway 102 to low pressure port 50 and discontinues pressure communication to high pressure port 44. Similarly, valve 84 now directs lower specific gravity reference column liquid 40 via passageway 104 to high pressure port 44 of differenticl pressure transmitter 42 and discontinues pressure communication to low pressure port 50. By switching valves 82 and 84 to their alternate positions, communication of pressure heads P^_j^ and P₂ to high p^assure port 44 and low pressure port 50, respectively, are interchanged. This provides calculator 64 an ability to average differential pressure measurements of sensor 54, as represented by transmitter signal on line 56, eliminating any errors due to zero shifts in differential pressure sensor 54. Plumbing internal to differential transmitter 42, such as a valved manifold, can also permit pressures P_A and P₂ to route to opposite pressure ports in a similar manner.

The invention disclosed is compatible with existing HTG systems, including those that have been previously installed.

Although the present invention has been described with reference to preferred embodiments. workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for coupling between a first pressure tap connected to a tank through a first port at a first level and a second pressure tap connected to a purged column at substantially the first level, the tank holding a first fluid of a first specific gravity and a second fluid having a higher specific gravity than the first fluid such that the first fluid settles substantially above the second fluid with an interface layer formed therebetween and above the first level, and the column holding the first fluid in fluid communication with the tank through a second port of the tank, the second port opening to the tank below an upper level of the first fluid to provide a substantially common first fluid pressure head to the first and second pressure taps; the apparatus providing a product output indicative of the height of the interface layer, the apparatus comprising: means coupled between the first and second pressure taps for sensing pressure difference between the first and second taps and providing a pressure output indicating the pressure difference; means for receiving and storing data indicative of relative specific gravities of the first and second fluids; and means coupled to the pressure output f-r computing the height of the interf a layer as a function of the pressure output and the data, and for providing the product output indicative of the height of the interface layer.

2. The apparatus as specified in Claim 1 further comprising outlet valve means connected to an outlet port opening to the tank for selectively discharging the first fluid from the tank.

3. The apparatus as specified in Claim 2 wherein the means for computing the level of second fluid further comprises means for providing a first signal to the outlet valve means for selectively controlling the outlet valve means.

4. The apparatus as specified in Claim 1 further comprising tank drain valve means opening to the second fluid for selectively draining the second fluid from the tank.

5. The apparatus as specified in Claim 4 wherein the means for computing the level of second fluid further comprises means for providing a second signal to the tank drain valve means for selectively controlling the tank drain valve means.

6. The apparatus as specified in Claim 1 further comprising purge valve means connected to the purged column for selectively purging the purged column.

7. The apparatus as specified in Claim 6 wherein the means for computing the level of second fluid further comprises means for providing a third signal to the purge valve means for selectively controlling the purge valve means.

8. The apparatus as specified in claim l further comprising an isolating valve means disposed between the second port and the purged column for selectively preventing the first fluid in the purged column from communicating with the first fluid in the tank, whereby changes in the pressure output are indicative of the level changes of first fluid in the tank.

9. The apparatus as specified in Claim 1 wherein the purged column resides external to the tank.

10. The apparatus as specified in Claim 1 further comprising zero effect means connected to the means for sensing pressure difference for interchanging the first and second pressure taps to the apparatus, and for providing an averaged pressure output which cancels zero shift errors of the means for sensing pressure difference.

11. Apparatus for coupling between a first pressure tap at a first level of a tank holding first and second liquids, and a second pressure tap of a purged column holding the first liquid, the second liquid having a greater specific gravity and settling below the first liquid with an upper level above the first level, and where the second tap is at substantially the same level as the first pressure tap, the apparatus comprising: conduit means coupled between the purged column and a tank opening to the first liquid in the tank at a level above an interface formed between the first and second liquids; means coupled between the first and second pressure taps for sensing pressure difference between the first and second pressure taps and providing a pressure output indicative of the pressure difference between the first and second pressure tap; and means coupled to the conduit means for selectively closing off the conduit means to isolate the first liquid in the purged column from the first liquid in the tank, whereby changes in pressure outputs are indicative of changes in level of the first liquid in the tank.

12. The apparatus as specified in Claim 11 further comprising: means for receiving and storing data indicative of relative specific gravities of the first and second fluids; and means coupled to the pressure output for computing the level of second fluid as a function of the pressure output and the data, wherein when the means for selectively closing off the conduit means is opened the means for computing provides a product output indicative of the level of second fluid.