Abstract:
Accordingly, a tank, preferably on a wheeled vehicle such as a railcar or trailer, is provided with a protective liner. Two electrodes are preferably permanently mounted within the tank and respectively connected to a monitor. Another two ground connections are made from the monitor to the tank. At least one of the two probes in the tank are utilized with the monitor to measure the voltage between the tank and the selected probe. When the tank is filled with an ionic solution, a breach of integrity of the liner results in a voltage corresponding to the difference in potentials of the metals forming the probe and tank is read by the monitor. This triggers an alarm. Furthermore, the monitor provides a voltage or current from probe to probe, probe to shell, and shell to shell on a periodic basis to measure and then record the performance of the liner. The equivalent resistance of the liner will be recorded in a memory of the monitor which may be downloaded to a computer for monitoring the performance of the liner. Any breaches of the liner whether obtained from the galvanic cell measurement or resistance measurement are provided to an alarm.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a lined storage tank equipped with a leak detection and monitoring system, and more particularly a leak detection and monitoring system for use with ionic solutions stored in mobile storage tanks. 
     2. Prior Art 
     Over the road tanks, such as tanks carried by tractor trailers and rail cars, are often utilized to carry ionic solutions from one place to another. Some of the solutions are hazardous, and could dissolve through the steel or aluminum tanks if exposed to them. Accordingly, a liner is utilized to line the inside of the tanks to prevent exposure of the transported solution with the metal making up the tank. 
     Over time, and especially if solutions are carried which are incompatible with the lining in the tank, the lining may wear out. When the solution contacts the metal tank, it may start a chemical reaction to rapidly corrode a hole through the tank which could result in a spill of hazardous material. Avoiding this problem is a concern for over-the-road transportation companies. 
     Presently, there are two primary ways which are utilized to test over-the-road tanks for leaks when full of liquid. First, a conductivity test may be conducted. A Milliamp (mA) meter is connected to a battery (a direct current source), such as a five volt battery. The meter may be a Simpson analog meter, or any other suitable device. The meter is put into test mode and adjusted until it reads 3 mA. The meter is then connected to a probe which is placed in a liquid and the other connection is grounded to the rail car, or shell of a tank. The test button is then pushed and if the reading is higher than 3 mA, a leak is present, but if it is lower than 3 mA, no leak is reported. 
     The conductivity test is not a particularly precise test and it provides little advance warning before having a relatively large problem. Additionally, the test is performed by taking an access cover off the top of the tank, and dropping an electrode into the transported solution. Many items inadvertently end up in a tank including watches, wrenches, bolts, etc. . . . over time when the tanks have an exposed opening. Furthermore, since the system operates on direct current, the possibility of polarization exists, as the resistant increases, the current decreases. Polarization of the probe occurs through use which would provide indications that the liner is good, while actually defective. If the operator were aware of the polarization, the leads could be reversed, but a need exists for a monitoring system which does not necessarily rely on the skill or experience of the employee to operate properly. A large spill could subject the transportation company to large liabilities. 
     The second way commonly utilized to test tank liners is to perform a megohm (megger) check where a large voltage is applied across a first electrode placed in the solution in the tank, and another electrode is placed in contact with the tank (opposite the liner) from the first electrode. The current passing through the electrodes is measured and a resistance value is provided for the “circuit”, i.e., through the liner. If the resistance drops below a certain value, such as 10,000 ohms, then a leak is present. If above, the cutoff, then no leak is present. 
     The problem with the megger test is that the resistance measured is not direct resistance but an equivalent resistance through all parallel circuits. Accordingly, if a pin hole leak were present offering a minimal amount of resistance, such as 0.1 ohms and the remainder of the liner provided excellent resistance such as 10,000 ohms, then in a large tank, the equivalent resistance may be on the order of 9,000 ohms, which would not be reported as a leak. 
     The most accurate way presently utilized to check for leaks in an empty tank is to perform a spark test. A probe having 15,000 volts is passed across the liner. If a leak is found, even a pin hole leak, a visible spark travels from the probe through the hole to the tank. The problem with this method is that most liners are applied in sheets like wallpaper and overlap adjacent sheets. The spark will not travel very far between two sheets, while a leak may travel a few feet through the adjacent liner layers to the tank wall. Government Regulation No. 4 M 183 requires a certified inspector to perform this check and DOT requirements require the test to be performed yearly on tractor trailer tanks. There are no known regulations addressing rail car tests. 
     Accordingly, a need exists for a leak detection system for use with tanks, especially mobile tanks. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a lined tank equipped with a monitor able to detect very small leaks in a liner. 
     It is another object of the present invention to provide a method and apparatus for monitoring and recording data relating to the performance of a tank liner. 
     Another object of the present invention is to provide an alarm to an operator of a vehicle in the event of a breach of a liner in a mobile tank. 
     Another object of the present invention is to provide a regular monitoring of a lined tank to check for a breach in the integrity of a liner. 
     Another object of the present invention is to utilize the natural potential difference between dissimilar metals to provide a voltage in the event of a breach of a liner. 
     Another object of the present invention is to provide a method and apparatus for monitoring a mobile lined tank for breaches in the liner when the tank is filled with an electrically conductive liquid. 
     Accordingly, a tank, preferably on a wheeled vehicle such as a railcar or trailer, is provided with a protective liner. Two electrodes are preferably permanently mounted within the tank and respectively connected to a monitor. Another two ground connections are made from the monitor to the tank. At least one of the two probes in the tank are utilized with the monitor to measure the voltage between the tank and the selected probe. When the tank is filled with an ionic solution, a breach of integrity of the liner results in a voltage corresponding to the difference in potentials of the metals forming the probe and tank is read by the monitor. This triggers an alarm. Furthermore, the monitor provides a voltage from probe to probe, probe to shell, and shell to shell on a periodic basis to measure and then record the performance of the liner. The effective or equivalent resistance of the liner will be recorded in a memory of the monitor which may be downloaded to a computer for monitoring the performance of the liner. Any breaches of the liner whether obtained from the galvanic cell measurement or equivalent resistance measurement are provided to an alarm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a perspective view of a tank supported by a trailer and driven by a truck with the tank equipped with a monitoring system according to the preferred embodiment of the present invention; 
     FIG. 2 is a circuit schematic of a presently preferred monitoring system; and 
     FIG. 3 is an indicator used in the presently preferred monitoring system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While various substances may be transported in liquid form, the particular solutions of concern with the system described herein are ionic in nature. While almost everyone is familiar with the ability of solid conductors to carry electricity, some solutions also conduct electricity. These electrically conductive liquids utilize the principle of ion conductance and include such solutions as water (except not ultrapure water), milk, most acids, bases and salts, including, but not limited to, hydrochloric acid, sulfuric acid, hydrobromic acid, sodium hydroxide, sodium chloride, etc. . . . When each of these solutions are diluted with water, they can conduct electric current through ion conductance. Gasoline, alcohols and most hydrocarbons are not electrolytic solutions. 
     Another electrical principle which is utilized by the present invention is the electrical potential which exists between dissimilar metals. A voltage difference exists between metals of two different chemical compositions. Two metals may be selected so that the potential between them is significant enough to be measured. When they are in electrical contact with one another, such as through an electrically conductive liquid, a galvanic cell is created and current flows from the higher to the lower potential. This current and voltage can be measured. 
     A lined tank containing an electrically conductive ionic solution, with an intact liner and with no electrodes present, is not an electrochemical cell. This is the usual non-test condition of a loaded lined tank such as a trailer, lined car, or stationary lined tank. It would be very difficult to detect electrical characteristics with no electrode located within the tank. 
     When electrodes are submerged in the fluid in the tank, either temporarily or permanently, as long as the fluid is an electrically conductive ionic solution, and then connected to an external power source whose electromotive force is stronger than the electromotive force exerted by the cell, the lined tank can then be considered to be an electrolytic electrochemical cell. 
     When a breach occurs in the lining of the tank, a portion of the electrode is exposed to a portion of the steel shell, but no outside electromotive force is present or the outside electromotive force is less than that exerted by the cell, the tank can then be described as a galvanic or voltaic electrochemical cell. 
     Referring to FIG. 1, a tractor trailer truck  10  is illustrated with a trailer  12  in tow. The trailer  12  has a tank supported thereon. Tanks  12  are often utilized to carry various liquids ranging from gasoline to milk to acids, etc. The truck and trailer  10 , 12  are preferably utilized together to house portions of a leak monitor system according to the preferred embodiment. 
     The trailer  12  carries a tank  14  thereon. The tank  14  has an airtight chamber  16  therein. Since the majority of liquid chemicals utilized with the teachings of this disclosure are somewhat toxic, especially in quantity, a sealed tank  14  is believed to be a feature of the preferred embodiment. Inside chamber  16  is stored a quantity of electrically conductive liquid  18 . 
     First and second probes or electrodes  20 , 22  extend from an access hole  24  into the cavity  16  of the tank  14 . Preferably, the electrodes extend toward the bottom of the tank so that a significant quantity of liquid  18  need to be contained for the electrodes  20 , 22  to be submerged. The tank  14  has a liner  26  which surrounds the cavity  16 . It is preferably for the liner  26 , or non-conductive material to also extend up into the access holes  24  as well. Otherwise when liquid  18  sloshes as is likely to occur during transport, if conductive portions of tank are exposed to the liquid  18 , alarms can be triggered as explained in detail below since the electrodes  20 , 22  would be in contact with the conductive tank portions. 
     The electrodes  20 , 22  are connected to a monitor, shown in FIG. 2, which is preferably housed on the trailer  12 . With the monitor  28  housed in a weather proof and tamper proof housing  28 , a history of the contents of the trailer may be monitored. The monitor  28  is also connected to grounds  30 , 32  on the tank  14 . It is important that the grounds  30 , 32  be in electrical contact with the metal of the tank and not hindered by paint or other coating. In the preferred embodiment, the tank was sanded with a grinder and the grounds  30 , 32  were welded to the exposed tank portions. A lead from each of the grounds  30 , 32  was then connected to the monitor  34 . 
     The first and second electrodes are preferably selected so that they can withstand the harsh environment they will be subjected to in the liquid  18  in the tank  14 . A carbon probe for electrodes  20 , 22  has a sufficient potential difference from steel (iron) in the tank  14 . For most electrolytes (electrically conductive solutions) used as liquid  18 , about 1 volt of potential would be formed in the event of a liner breach so as to place the electrode  20  or  22  in electrical contact through liquid  18  with tank  14 , and thus ground  30 , 32 . 
     FIG. 2 shows a schematic of the monitor  34 . The monitor may have an internal power source  36 , such as in the form of batteries or otherwise. The preferred embodiment also has terminals  38 , 40  which connect with the power source of the vehicle  10 . While this would be 12 volts DC in a tractor trailer rig or rail car, it could be 120 V AC or other source. A transformer may also be included to step voltage from the vehicle power source to the internal power source voltage. Recharging of the internal power source  36  is believed to be advantageous so that the monitor  34  can operate as long as possible when the trailer  12  is disconnected from a vehicle  10 . Of course the terminals  38 , 40  may also have the capability of connecting to a stationary source, such as an extension cord connected to an outlet. 
     The power source  36  drives a processor  42  which receives leads  44 , 46 , 48 , 50  which are respectively connected to the first and second electrodes  20 , 22  and the grounds  30 , 32 . The processor  42  may have a meter  52 , no meter, or otherwise be equipped to measure voltage and/or resistance current, a logic portion  54 , and a memory  56  connected thereto. The monitor  34  also has communication ports  58 , 60  which provide a way to access the data retained in memory  56 , instruct the processor  42  to perform specific commands, and to receive alarm signals sent to the remote indicator  62  shown in FIG.  3 . Communication ports  58 , 60  may communicate with devices such as infrared devices, radio transmitter/receiver signals, cell phone technology such as General Packet Radio System (GPRS), or other protocol. Of course, the monitor  32  could be located in the vehicle  10  rather than on the trailer  12  in some embodiments, and the remote indicator  62  could be integral, attached or otherwise proximate to the monitor  32 . 
     The processor  42  is preferably configured to be able to perform at least two subroutines in with the logic portion  54 . It is preferred that the processor  42  be a custom designed microprocessor based device, but other components may be utilized in other embodiments. The first subroutine is the reading of voltage between the first electrode  20  which provides an input at lead  44  and a ground,  30  or  32 , which provides an input at one of leads  48 , 50 . The meter  52  reads the voltage between the leads  44 , and  48  or  50 . When the liner  24  is not breached, there is no voltage difference expected. However, if the liner  24  is breached a voltage will be recorded by the processor  42 . If the processor  42  reads greater than a predetermined setting, such as about 0.6 volts DC, then an alarm condition is satisfied and a signal is sent from the processor, out of the communication port  58  to the remote indicator  62  to alert an operator of the vehicle  10 . The process may then be repeated for leads  46 , and the other of  48 , 50 . 
     Since the tank  14  is an airtight enclosure about the cavity  16 , even humid or moist air filled with electrolye solution has been found to be a satisfactory conductor to indicate a liner  26  breach. The voltage reading may be stored in memory  56  for later use and/or retrieval from the communication port  58 , however in the preferred embodiment, this subroutine is utilized to drive a red alarm signal  64 , in the event of a liner  26  breach, on the remote indicator  62 . 
     While some monitor embodiments may be limited to taking voltage readings across some or all of leads  44 , 46 , 48 , 50 , other monitor embodiments may also provide a known voltage across some of the leads  44 , 46 , 48 , 50  as will be explained in detail below. Since voltage equals resistance multiplied by current, the application of a known current while measuring the voltage allows the processor  42  to calculate a resistance value for the effective resistance through the liner  24 . 
     Various electrolytic solutions will have different conductivity readings depending on the amount of ions dissolved in solution. For instance a solution of sodium hydrochloric acid might have a conductivity reading of 0.85 while a solution of sodium hypochlorite might have a conductivity reading of 1.42 and a solution of sodium hypochlorite might have a conductivity reading of 1.5. In general the less conductive a solution is, the more resistive that solution will be to conducting current. Accordingly, in the event of a liner  24  breach, a less conductive fluid  18  will report a higher effective resistance value than a reading taken when the tank  14  contains a fluid  18  having a higher conductivity. 
     The second subroutine of the processor  42  is the calculation of the resistance. The processor  42  preferably records the time and date. An internal clock (not shown) may be utilized. Next a reading is taken probe-to-probe. A known current is applied across leads  44 , 46 , the voltage is measured by the processor  42  and the resistance value is calculated and recorded. When there is no liquid  18  in the tank  14 , this resistance value will be higher than when liquid connects the two electrodes  20 , 22  together in the cavity  16 . 
     Next a probe-to-tank reading is taken across leads  44  or  46  and  48  or  50 . Once again the resistance is calculated and recorded. Finally, a tank-to-tank reading is taken across leads  48 , 50 . It is expected that the tank to tank reading will be near zero, and if a higher resistance is obtained, an open circuit exists along one of the leads  48 , 50  extending to the grounds  30 , 32  on the tank  14 . 
     It is preferable to use alternating current as opposed to direct current signals due to the preferable feature of somewhat continuously monitoring cell resistance. Measurements of ionic conduction are normally made with AC techniques to avoid complications due to the Faradaic processes taking place at the electrodes. If a direct current is imposed upon a chemical cell, chemical reactions will occur at the electrodes in accordance with Faraday&#39;s laws. If an alternating current rather than a direct current is used, that Faradaic reaction which takes place on one half-cycle is reversed on the following half cycle. If, in addition, no product can escape from the inter phase regions, no net Faradaic current can flow. There are still flows of current, however, and such currents, which do not produce chemical changes in materials, are called non-Faradaic current. 
     The probe-to-probe measurement involves electronic conductance from the monitor  34  to the electrode surface  20 , 22 . This is in series with the many parallel circuits ionic conductance through the electrolye solution  18  to the surface of the other probe. This is also in series with the electronic conductance from the probe surface back to the monitor  34 . With the next signal, the probe-to-probe circuit is reversed. Since the surface areas exposed to the electrolyte are constant, the equivalent resistance of circuits does not vary appreciably. 
     The probe-to-tank circuit involves electronic conductance from the monitor  34  to the surface of the electrode  20 , 22 . This is in series with the many parallel circuits of ionic conductance through the electrolyte to the inside of the liner. This is then in series with the electronic conductance through the liner to the inside of the tank. Then this is in series with the electronic conductance through the tank and back to the monitor  34 . With the next signal, the probe-to-tank is reversed. A decrease in resistance is observed upon filling and an increase in resistance is observed upon pumping out the fluid  18 . 
     The tank-to-tank circuit involves electronic conductance form the monitor  34  to the tank  14 . This is in series with the electronic conductance through the tank and then in series with the electronic conductance back to the monitor  34 . With the next signal, it is preferable to reverse the polarity of the signal sent. 
     The impedence of both the probe-to-probe and probe-to-tank circuits is thought to involve the following circuit constants, inductive reactance, equivalent resistance of a parallel circuits and capacitive reactance. They are in series. The measuring of the equivalent resistance is the intended measurement to be made, but it is difficult to separate the three. 
     In the preferred embodiment the signal strength output of the monitor is only a few milliamps, the frequency is less than about 60 Hz and the maximum regulated voltage is less than six volts. The coefficient of self inductance may be known in some embodiments, but has not been calculated in test devices. Furthermore, the instantaneous counter emf due to inductance has not been calculated. 
     In any event when reading the full scale, the maximum output voltage of five volts is not reached when the maximum circuit impedance is reached, thus indicating that the two circuit constants inductive reactance and capacitive reactance are preferably maintained small or otherwise accounted for. 
     An advantage of using processor  42  is to have the ability to switch probe-to-tank readings between electrodes  20 , 22  in the tank  14  as well as direct which terminal, positive or negative is supplied to each of the leads  44 , 46 , 48 , 50  so that polarization of any of the electrodes  20 , 22  or grounds  30 , 32  does not occur. 
     The processor  42  preferably repeats the resistance check on a first periodic basis and records values for retrieval on a second periodic basis in the memory  56 . Of course, the first and second periodic bases could be the same interval, but they need not in all embodiments. 
     The preferred remote indicator  62  is shown above the driver&#39;s seat in FIG.  1  and in detail in FIG.  3 . Of course, the indicator  62  may be a portion of the monitor  34  as well. The red light  64  flashes or stays steady on an alarm condition, such as if the conductivity reading from a probe-to-tank reading drops below a limit. The limit may or may not be adjustable depending upon the liquid  18  in the tank. The probe-to-probe reading may be utilized to calculate the limit as it will correspond to the conductivity (as measured by resistance) of the fluid  18 . When the limit is dropped below, the alarm signal is triggered and the light  64  or other indicator alerts the operator or other person. The yellow light  66  is utilized to indicate a fault in the system, such as if ground-to-ground readings are not near zero. A problem exists with the system. Finally, the green light  68  may be utilized to indicate normal operation of the system as well as the liner apparently operating as designed. A push to test button  70  may be utilized to insert a short across the probe-to-tank test so that the red light  64  will indicate, but it is preferred that no record be made of this event. 
     Different readings may be obtained from the monitor  34  and printed out. Furthermore, a graph may be made of data. An RS  232  download may be performed on the memory portion of the monitor  34 . E-Prom, programmable memory or other memory storage may be utilized. If the remote indicator  62  is part of the monitor  34 , input and output  72 , 74  may correspond to the communication input and output ports  58 , 60  on the monitor  34  and connector  76  may be used for a power supply inlet and/or a communication terminal for use with a computer. 
     The tanks are equipped with liners  24  which may be rubber, whether natural or synthetic, frp (fiberglass reinforced plastic), pvc (polyvinyl chloride), coatings of  10  mil or greater such as may be sprayed or rolled, high baked phenolics, vinyl esters, epoxy, fluorinated hydrocarbon resins or other non, or low conductive liner  24 . 
     The preferred monitor  34  is capable of operating in various modes, either automatically or on command. It performs a galvanic voltage check as described above. It also is utilized to calculate the resistance across the liner  24  as described above. Finally, if voltage is induced, it may be reversible to operate with a system switching from an electrolytic to a galvanic cell. The voltage or current may be supplied to read a null and whenever it switched to voltaic conditions, a reading would be recorded and whenever it switched to generating conditions a voltage could be recorded. 
     The monitor  32  also conditions and controls the frequency and level of various electrical output and input signals through leads  44 , 46 , 48 , 50 , scans the system circuits for electrical continuity and faults, processes and retains pertinent data in memory for alarm events and data acquisition, and initiates alarms in the event of a continuity or a leak failure is detected. Tests may be performed every few seconds or at other desired frequencies. The monitor  34  may require a plurality, such as three, occurrences in a row of exceeding a predetermined limit to report a fault or alarm condition. 
     Numerous alternations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present disclosure relates to the preferred embodiment of the invention which is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.