Abstract:
A method for detecting leaks in a fluid pipeline where the pressure supply pump remains ON during the test procedure and pressure on both sides of a closed shut-off valve in the pipeline is maintained equal. A novel flow meter is provided comprising a parallel combination of a solenoid value and a pressure differential switch connected across the shut-off valve.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to leak detectors and more particularly to a method and apparatus for detecting very small leaks, that is, the leak rates which are so low as to be undetectable by a conventional flow meter. 
     BACKGROUND OF THE INVENTION 
     Increasing environmental awareness and respect for valuable natural resources has created an abiding concern in the area of underground leak detection. This is especially true with regard to fuel tank storage and dispensing systems. The industry has been continuously in search of new technologies and enhancements to existing technologies for underground leak detection, particularly pipeline leak detection. 
     Conventional leak detection methods for pipelines involve pressurizing the line and then observing a pressure decay when there is a leak. This method is subject to false readings due to temperature changes of the pressurized liquid caused by heating or cooling from the environment. A small temperature change, specifically a reduction in temperature, can cause a large change in pressure of liquid, which is, for most purposes, incompressible. In pipelines, where a relatively few number of gallons of liquid may be involved, a very small amount of liquid volume, that is, flow, causes a large change in pressure. For example, a typical gasoline station may have 120 feet of pipe connected to one storage tank. There may be about 20 gallons of fuel in that length of pipe. A one-degree F reduction in liquid temperature at a typical temperature coefficient of expansion results in a detectable reduction in pressure. However, that one degree of temperature reduction results in a volume change of only 0.014 gallons of liquid. 
     Attempts to use currently available flow meters or flow sensors have been satisfactory for some purposes but they are insufficiently accurate to detect fluid flow at the minimum level required by the Environmental Protection Agency with respect to fuel storage and dispensing systems. This minimum level is currently 0.1 gallon per hour and any leak detection system for either fuel tanks or piping which does not achieve this minimum detection level will not be acceptable. 
     The apparent leak rate of 0.014 gallons per hour, if the one degree F reduction took place over the course of one hour, would be undetectable by a conventional flow meter. By way of further example, a leak rate of 0.1 gallon per hour amounts to only two drops per second. That is below the threshold leak rate detectable by currently available flow meters which themselves typically leak at more than two drops per second. 
     The description of some of the prior art pipeline leak detectors is set forth in a paper entitled &#34;Service Station Pipeline Leak Detectors,&#34; by N.D. Wolff, presented at the Underground Storage Tank Leak Prevention Seminars conducted by the California State Water Resources Control Board in July and Aug. 1988. This describes some of the pressure related leak detectors and some of the alternative leak detector devices. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, this invention overcomes the temperature-related deficiency of the pressure decay type devices by maintaining a constant supply pressure and measuring the flow caused by a leak in a pipeline, however low the leak rate. This invention provides significantly greater accuracy than has been previously available and can detect leak rates at any level, well below 0.1 gallons per hour. 
     The basic invention comprises a shut-off valve positioned in the main pipeline between the dispenser valve and the pump. A flow meter is connected to the main line in parallel with the shut-off valve. Testing is accomplished by energizing the pump to create fluid pressure in the pipeline with the dispenser valve closed. As soon as delivery pressure is achieved in the pipeline, the shut-off valve is closed and the flow meter indicates whether or not there is flow past the shut-off valve caused by liquid loss due to a pipeline leak. 
     In an alternative embodiment, a flow meter verification device is added which includes a valve having one end connected to the main line between the shut-off valve and the dispenser valve, and an orifice having a predetermined flow rate at the dispensing pressure connected to the other end of the verification valve. To determine whether or not the flow meter is operating properly, a leak is simulated by opening the verification valve and allowing flow through the orifice and back into the tank. If the flow meter indicates a leak, it will indicate that all aspects of the leak detector are operating properly. 
     Another alternative embodiment provides a new flow meter apparatus which is coupled in the same position across the shut-off valve and comprises a solenoid valve connected across the shut off valve. A pressure differential switch is connected in parallel with the solenoid valve and controls operation of the valve. The valve is normally closed and is opened under the influence of the pressure differential switch when there is a pressure differential across the shut-off valve. This last embodiment is particularly adept at measuring extremely small leaks. This flow meter has no flow until there is a slight pressure differential across the solenoid valve and, commensurately, across the shut-off valve. When the pressure switch detects a difference, the solenoid valve is opened to immediately equalize the pressure. By measuring the time the valve is open it is possible to calculate the flow through the solenoid valve and thereby calculate the leak rate in the pipeline. 
     Note that in all of these processes, the main pump remains ON, supplying pressure to the shut-off valve and to the main pipeline. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, advantages and features of the invention will be more clearly perceived from the following detailed description, when read in conjunction with the accompanying drawing, in which: 
     FIG. 1 depicts the basic pipeline leak detector apparatus of the invention; 
     FIG. 2 shows a portion of FIG. 1 with an alternative arrangement; 
     FIG. 3 is similar to FIG. 2 showing yet another alternative embodiment; and 
     FIG. 4 shows an automated embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the drawing, and more particularly to FIG. 1 thereof, there is shown tank 11 buried beneath ground grade level 13. The tank is filled to liquid level 12. Pump 14, which may be a submersible pump in tank 11, delivers liquid from the tank through lines 15 and 18 to main pipeline 16 in which is connected shut-off valve 17. At the end of pipeline 16 is dispenser valve 21 from which dispensing nozzle 22 and hose 23 are connected. There may be several such dispensing valves and nozzles. Also connected in pipeline 16 across valve 17 is flow sensor 24. 
     The process of determining pipeline leaks commences with closing dispenser valve 21 and energizing pump 14. In a very short time delivery pressure will have been achieved in pipeline 16, at which time shut-off valve 17 is closed. Pump 14 remains ON during the leak testing process. If flow meter 24 indicates no flow, it can be assumed that there is no product loss in pipeline 16 due to a leak. It may be necessary to wait several minutes in the event that there is cooling of the liquid in the pipeline so that temperature reductions do not contaminate the readings. Such cooling might occur due to a cold ground environment and a warmer product which could have been caused by pump heat and compression. This cooling would cause the liquid to compress slightly thereby reducing the pressure in pipeline 16. Because the flow sensor responds to changes in pressure on either side of valve 17, a reduction in pressure in main pipeline 16 will cause fluid from charged pump 14 to flow through the flow meter and thereby stabilize the pressure. The flow meter includes an indicator device such as a dial or a digital readout to show flow rate, typically in gallons per hour. 
     It is possible that the flow meter could be stuck and thereby not indicate when a flow takes place or should take place. In order to check or verify the performance of the flow meter, leak simulation apparatus as shown in FIG. 2 may be provided. Verification valve 25 is connected in pipeline 16, which is on the downstream side of shut-off valve 17. The opposite end of valve 25 is connected to line 26 which includes therein an orifice which has a predetermined flow rate at dispensing pressure. Line 26 leads back to tank 11. With dispensing valve 21 and shut-off valve 17 closed and the system in a quiescent condition, it may be desired, especially at the beginning or end of the day, to check the operability of the flow meter. Valve 25 is opened so that liquid flows through line 26 back to the storage tank. This causes a flow in flow meter 24, indicating a leak. It may be desired to cycle valve 25 open and closed so that it can be clearly determined that flow meter 24 shows a corresponding leak/no leak condition. 
     This cycling of valve 25 is employed to check for proper operation of the entire leak detecting system. If any of the valves or sensors are not operating properly, flow meter 24 will not switch between the ON and OFF positions as solenoid valve 25 is opened and closed. 
     Another embodiment, comprising an improved flow meter, is shown in FIG. 3. This embodiment comprises solenoid valve 31 connected in parallel with pressure differential switch 32, both of which are connected in parallel with shut-off valve 17 as before. Indicator 30 visually shows such parameters as the length of time valve 31 was open and the amount of fluid that passed therethrough. This structure car measure extremely small leak rates, effectively down to zero. As soon as there is a slight leak, a pressure differential exists between lines 16 and 18, effectively across the parallel combination of valve 31 and switch 32. Until that time there is no flow around valve 17. The pressure differential switch senses a small pressure drop, as low as two to five inches of water (0.072-0.181 psi), and opens valve 31. Since pump 14 continues to operate, this immediately equalizes the pressure and pressure switch 32 again closes valve 31. 
     The automated version of the invention is shown in FIG. 4. Control system 33 is electrically connected to flow meter 24 which may be any flow meter, including the embodiment of FIG. 3. It is also connected to pump 14 for control of and detecting duration time of operation thereof. By measuring the time the flow meter, which comprises solenoid valve 31 and pressure switch 32, remains open, it is possible to calculate the amount of flow through the flow meter. For example, if valve 31 remains ON for 10 seconds, it will represent 10 times as much flow as for a one second ON time. After two or three readings over a predetermined period of time, it is simple for control 33 to calculate the flow rate. If the flow rate exceeds a predetermined minimum level, indicator 34 will show a leak or a &#34;Fail&#34; indication. Otherwise a &#34;Pass&#34; or no leak indication will appear on indicator 34. Control system 33 includes appropriate means for establishing a leak rate above which the pipeline system fails, that is, a flow rate set point. It also measures flow time, and integrates a series of calculations over a predetermined period of time to determine total flow and flow rate through the flow sensor. 
     It can be seen that the principle of operation of the leak detector of this invention is to maintain pressure on main line 16 and measure the volume of fluid necessary over a measured period of time to maintain that pressure. This requires that the main fuel delivery pump remain on during the leak test procedure. This apparatus is so sensitive that it can detect leak rates at least as low as 0.05 gallon per hour, which is equivalent to one drop per second. 
     In view of the above description, it is likely that modifications and improvements will occur to those skilled in the art which are within the scope of the accompanying claims.