Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M3/00—Investigating fluid-tightness of structures
  • G01M3/02—Investigating fluid-tightness of structures by using fluid or vacuum
  • G01M3/26—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors
  • G01M3/32—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors for containers, e.g. radiators
  • G01M3/3236—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors for containers, e.g. radiators by monitoring the interior space of the containers
  • G01M3/3245—Investigating fluid-tightness of structures by using fluid or vacuum by measuring rate of loss or gain of fluid, e.g. by pressure-responsive devices, by flow detectors for containers, e.g. radiators by monitoring the interior space of the containers using a level monitoring device

Definitions

• the present invention is directed towards a method and apparatus for providing a safe, precise, and cost-effective storage tank leak detection system and more particularly, to a method and apparatus wherein the containment integrity of a storage tank is determined by mass measurements of the stored product.
• Background Information Storage tanks play a vital role in today's economy. The economy, on a global scale, depends on the proper function of these tanks as they are prevalent in several industries and virtually every geographical region in the world. In light of the vital role these storage tanks play, the integrity of the tanks is placed at a premium. That is, storage tank owners are willing to invest huge sums of money in both the maintenance and inspection of such tanks.
• Storage tank leak detection systems are known in the art; however, these products are fraught with problems.
• the present systems are imprecise, or provide erroneous data for any or all of reasons including: the consistency of the soil acting as the tank's foundation, the temperature stratification of the in-tank product, extraneous noise sources, thermal expansion of the tank's contents, water table level, previous soil contamination, and/or tank shell dynamics.
• some detection devices can only be used when the storage tank is empty, and no known system or method ensures a comprehensive inspection of the tank.
• the most common form of such a system is "vacuum box testing"; however, this system is intended only for weld joints and is not usually applied to the entire tank bottom.
• Magnetic flux floor scanning is also used, but is not effective at examining the area of the floor surface close to the surface walls or where there are physical obstructions.
• Ultrasonic detection is used, but this is only effective for small areas of the surface.
• Gas detection is also used, but the types of materials stored in the tank can obstruct this method.
• mass measurement detection systems are known in the art.
• the presently available systems and associated methods are not capable of the precision, which is indicated above as crucial at the present time (and which, as described below, is afforded by the systems and methods of the present invention).
• Present mass measurement leak detection systems in the art are limited by tank shell variations resulting from temperature effects on tank shell plating.
• known mass measurement detection systems are only sensitive enough to be used in smaller tanks, typically underground storage tanks.
• the present invention overcomes tank shell variations and other shortcomings of presently known technology in this field through data collection and data correction apparatus, techniques and interpretation.
• the system represents an attractive alternative to hand strapping and other volume based (through flow meter) product custody transfer measurement methods and apparatus.
• accuracy of custody transfer measurements is of significance due to the quantities, commercial terms and value of the product, this system can be directly employed to accurately measure the transfer amounts.
• Embodiments and practice of the present invention provides a safe, extremely precise, and cost-effective solution to the problems mentioned above.
• Test results produced through practice of the present invention provide a determination of containment integrity, transfer quantities, and in the event of leakage, a precise volumetric leak rate, all at accuracy levels unprecedented in the art.
• Use of the present invention is not restricted by fluid type, fluid temperature, fluid level, or tank size.
• the present invention obviates the need for physical inspection of storage tanks. As such, there is no need to drain, clean, or enter a to be inspected/monitored tank, therefore, neither inspectors nor the environment are exposed to the contents of the tank as a result of the evaluation. In addition to eliminating hazardous byproducts associated with the draining/cleaning process, also avoided is the danger of transporting and storing the drained product.
• practice of the present invention eliminates manual inspection of a tank, it is non-intrusive. Therefore, operation of the tank is not hindered, and production downtime is altogether avoided. Costs related to the handling, transport, disposal, or storage of removed hazardous material (required by, among other methods, any method requiring internal, physical inspection of a tank) are likewise avoided.
• the unprecedented utility afforded by the present invention arising from the unprecedented accuracy of the system of the present invention, is the product of substantially improved sensitivity (compared to prior technologies) with which the present system is able to measure mass variations.
• the present invention by employing a combination of techniques and components not known in the art, provides a leak detection threshold that is much lower than any known device. This is important because, the lower the leak detection threshold level of such a system, the more effective it will be at detecting leaks.
• One feature of the present system involves its maintaining the mass measurement component's temperature at a substantially constant level during an entire measurement process. Further, certain features of the system correct errors in the collected data which are attributable to storage tank shell dynamics, as well as to the inherent imprecision in the mass measurement devices. This data correction process will be discussed in detail in the specification to follow.
• the systems and methods of the present invention overcome tank shell variations (a major "weak link" in mass measurement-based systems) through data collection and data correction techniques.
• data is collected through use of a quartz crystal type pressure transducer (the specifications and use of this transducer will be explained in more detail in the Detailed Description of the Preferred Embodiment).
• a PLC connected to the pressure transducer records pressure data over a period of time (preferably one to five nights).
• the atmospheric temperature and barometric pressure are recorded and precisely analyzed to calculate any changes in the mass of the fluid within the tank.
• the atmospheric temperature is recorded and precisely analyzed to correct for indicated changes in the observed mass which arise only from tank shell temperature variations.
• the corrected data is regressed to give a line slope that is converted to a leak rate of volume over time (the system usually being configured for reporting in gallons per hour).
• Data generated by the transducer is, according to the preferred mode of the present invention, collected on a 24-hour basis. Only data containing a sufficiently low amount of extraneous noise is analyzed. Such data is, accordingly, usually obtained at nighttime and during fair weather conditions. Such selective data gathering eliminates, among other adverse factors, the effect of the sun's radiant energy on the surface area of the tank, which may adversely affect the mass measurement of the stored product. Also, data correction software accounts for the coefficient of thermal expansion for any given storage tank construction material. The nighttime data is corrected for atmospheric conditions and variations in the tank shell. These measurements and corrections allow the system to repeatedly achieve the stated accuracy in real world conditions on a routine basis.
• the leak detection system of the present invention provides for an independent barometric measuring means to constantly record the barometric pressure during the data collection process.
• This independent barometric pressure measuring means used in combination with data correction software, corrects any zero drift associated with the individual pressure transducer. That is, this system corrects for the inherent error present in any transducer when that transducer deviates from its initial reference pressure.
• the tank bottom pressure is determined through use of a "bubbler" which is ideally positioned at or near the bottom of a tank and to which is provided a regulated inert gas.
• tank bottom pressure The pressure, measured at the tank floor ("tank bottom pressure") and atmospheric and vapor pressure measured just above the liquid surface, is recorded by a highly accurate differential pressure transducer on a real time basis and post processed using a data analysis routine to accurately calculate any changes in the mass of fluid contained within the tank to determine if there is a loss.
• the present system using the specified transducer, and when used in the manner and with the data interpretation described herein, is capable of detecting above ground storage tank leaks at a threshold of less than 0.9 gallons per hour with a probability of detection of 95% in a 120 foot diameter tank-far more accurate than is possible with any presently available quantitative leak detection system.
• This quantitatively, amounts to detecting pressure differentials equivalent to less than 1/10,000th inch of water column pressure, a tolerance level necessary to achieve such detection thresholds.
• the method and apparatus of the present invention provides a safe and effective way to detect very small leaks in very large tanks.
• the present invention provides a tremendous improvement in accuracy and leak detection threshold, allowing its operators to achieve greater results than presently thought possible.
• Figure 1 is a block diagram depicting the general layout of the present leak detection system.
• Figure 2 is an elevational, sagital cross sectional view of the bubbler of the leak detection system.
• Figures 1 and 2 generally depict a preferred configuration and constituency of a storage tank leak detection system according to the present invention, which system is generally designated by the reference number 10.
• the preferred embodiment of the present invention includes an inert gas pressure reduction 52 and flow rate regulator 54 which provide a clean and steady supply of an inert gas, such as nitrogen, from a compressed cylinder 18 to an in-tank bubbler 12 via bubbler tube 15.
• the in-tank bubbler 12 which is placed substantially at the bottom of a to-be- evaluated storage tank, releases inert gas bubbles in a consistent manner with minimal pressure variation.
• the minimum inert gas pressure required to consistently form and release bubbles at the bottom of the tank's contents serves as a proxy for the hydrostatic pressure at the bottom of the tank.
• inert gas bubbles in a consistent manner with minimal pressure variation is accomplished by way of a flow rate regulator 54 and, in the preferred embodiment of a system according to the present invention, a specially shaped orifice at the terminus of metal tubing 14, which is used to convey the inert gas within the in-tank bubbler 12.
• This orifice is in the form of a notch cut into the side of the metal tubing at an approximately 30 degree angle to the tubing's vertical orientation.
• In-tank bubbler 12 is made of a substantially non-corrosive metal (stainless steel, for example), however, any material that is corrosion resistant and of sufficient density is adequate for use with the present invention.
• In-tank bubbler 12 is directly immersed in storage tank 60 and rests on storage tank bottom surface 62 in the preferred mode of practice of the present invention.
• bubbler tube 15 forms a gas tight seal with in-tank bubbler 12. Extending from in-tank bubbler 12, bubbler tube 15 passes through storage tank top surface access 65 to an area outside of the class 1 region of storage tank 60 (class 1 region refers to the National Electric Code designated hazardous areas). Bubbler tube 15 serves as a conduit for pressurized inert gas flowing to the in- tank bubbler 12.
• MCDC 100 Placed outside of the aforementioned class 1 region, but no more distant from tank 60 than is necessary to so reside, is a measurement, control and data collection unit 100 (MCDC for short).
• MCDC 100 includes a differential pressure transmitter 22 which, in the preferred embodiment, is a highly precise quartz crystal pressure transducer 24.
• MCDC 100 also includes a highly precise quartz crystal temperature transducer 25.
• Transducer 24 contains an oscillating quartz crystal and has a pressure resolution of 1x10 8 of full scale. The ultimate resolution achievable with a transducer is limited by its stability and repeatability. System 10 greatly improves upon the stability and repeatability of the transducer thereby increasing the true resolution of transducer 24. In this regard, in system 10, transmitter 22 is to be insulated and, as will be further described in this section, transducer 24 is to be held at a constant temperature. This, in turn, achieves the desired high stability and repeatability.
• Quartz crystal type pressure transducer 24 includes a transducer low side 26, through which is determined a first differential reference-the atmospheric pressure value at the liquid surface (atmospheric pressure and vapor pressure directly above the liquid surface).
• Transducer low side tube 28 forms a gas tight seal at its proximate end with transducer low side 26 and extends through the tank access 65 to a location just above the tank 60 liquid contents.
• Transducer low side tube 28 allows transducer low side 26 to receive the atmospheric pressure from the reference point at the liquid surface.
• Quartz crystal type pressure transducer 24 also includes a transducer high side 30 which receives input reflecting the sum of the atmospheric (barometric) and hydrostatic pressure near the tank bottom surface 62.
• Transducer high side tube 32 forms a gas tight seal at its proximate end with transducer high side 30 and extends to a gas tight "tee" connection with bubbler hose 15 near tank access 65.
• Transducer 24 measures the pressure differential between the transducer low side 26 and transducer high side 30 to arrive at the pressure exerted by the mass of the tank contents while eliminating the pressure variations due to change in atmospheric pressure by way of their inclusion in both the high side and low side pressure measurements.
• Transmitter 22, communicating digitally, then sends this processed information to PLC 34. This data is transmitted along data transfer means 23.
• data transfer means 23 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.
• Quartz crystal temperature transducer 25 serves as a part of a temperature regulation scheme used to keep the pressure transducer 24 at a constant temperature during the data gathering process. Quartz crystal temperature transducer 25 communicates digitally via transmitter 22 with the PLC 34. This data is transmitted along data transfer means 23.
• data transfer means 23 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.
• the PLC through a control loop, generates an output which activates resistive heater 36 which in combination with heat sink 38 regulates the temperature of pressure transducer 24.
• resistive heater 36 which in combination with heat sink 38 regulates the temperature of pressure transducer 24.
• MCDC 100 further includes barometric pressure measuring means 40.
• Barometric measuring means 40 serves as an independent reference for true atmospheric pressure.
• barometric pressure measuring means 40 may be any standard barometer that sends signals to be processed by PLC 34 (each respectively being configured for communication with the other).
• Barometric measuring means 40 is very useful for increasing the precision of system 10.
• the present invention employs barometric measuring means 40 to serve as an independent measure of true atmospheric pressure above the tank's liquid contents, thereby allowing for data correction over any extended period of time. As will be discussed in this section, data correction using values taken from barometric pressure measuring means 40 greatly increases the precision of the current invention.
• Barometric measuring means tube 42 forms a gas tight seal at its proximate end with barometric measuring means 40 and forms a gas tight seal at its distal end where it "tees" into transducer low side tube 28.
• Barometric measuring means tube 42 forms a gas tight seal at its proximate end with barometric measuring means 40 and forms a gas tight seal at its distal end where it "tees" into transducer low side tube 28.
• System 10 in its preferred embodiment, also includes an ambient temperature measurement and reporting means 50, or "ambient temperature transmitter 50." Ambient temperature transmitter 50 is advised to be mounted outside of the class I region of storage tank 60, so as to provide an accurate measure of the ambient temperature of the air surrounding tank 60. This, in turn, facilitates data correction, over any extended period of time, for such tank shell expansion and contraction as attends ambient temperature variations.
• Temperature data is transferred along data transfer means 53 to the PLC 34.
• Ambient temperature transmitter 50 is, as employed in the manner described herein, another very useful element for increasing the precision of system 10.
• data transfer means 53 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.
• man-machine interface computer 70 Also contained within MCDC 100 is the man-machine interface computer 70.
• the PLC 34 and the computer 70 are typically housed in a common enclosure, such as field unit 100.
• the PLC processes data received from transmitter 22, atmospheric pressure measuring means 40 and ambient temperature measurement means 50.
• the PLC also controls the temperature of pressure transducer 24 by means of resistive heater 36 and heat sink 38.
• the PLC 34 communicates with man-machine interface computer 70 by data transfer means 72.
• data transfer means 72 is a standard bus communications cable. However, one could easily envision a data transfer means such as wireless communication that would work equally as well.
• Man-machine interface computer 70 provides the means whereby the operator interacts with the system to: log data, monitor system operation, enter temperature set point, record job specific physical site and client data, collect and process custody transfer data, trouble shoot detected errors and enter password keys to authorize data collection.
• the software of the present system commences operation with the initialization of data collection at the tank bottom, along with the atmospheric and environmental conditions. Data is automatically collected via industrial computer controlled programming over some length of time, preferably 36 to 60 hours. The length of the test is dependent on tank size and site weather conditions.
• remote computer 80 contains software that performs linear regressions of data downloaded from the man-machine interface computer 70. This regression detects minuscule changes in the mass of the stored product, thereby indicating the presence of the smallest of leaks. As the compilation of data grows, the more precise the regression becomes.
• the post processing module and software of remote computer 80 is independent of the PLC 34 and the man-machine interface computer 70. There are three software programs or modules involved with the storage tank leak detection system of the present invention: the PLC program, the man-machine interface computer program and the post processing program operated on remote computer 80.
• the PLC program is performed by the PLC 34 and is responsible for obtaining (subroutine Measure) data from transmitter 22, controlling the temperature of transmitter 22 (subroutine Temp Ctrl), obtaining transmitter 22 differential pressure and temperature (subroutine Measure), and backup data storage.
• the data acquired by the PLC program is stored within the PLC 34 in non-volatile memory.
• the PLC program interrogates the differential pressure transmitter (transmitter 22) via a serial connection.
• the pressure read from differential pressure transmitter 22 is the difference in pressure read from transducer low side 26 and transducer high side 30. That pressure value is modified by two additional variables in order to improve the accuracy of the reading.
• the post processing program performs measured head corrections for (a) tank shell temperature changes based on measurements of ambient temperature and (b) atmospheric pressure changes which otherwise would skew the data interpretation. This post processing is intended solely to detect variations of contents of storage tank 60 due to leakage and eliminate variations due to environmental changes. Any change in tank diameter is accommodated in the calculations thus properly attributing substantially all variations in differential pressure to variations in the content of storage tank 60, such as through leakage.
• the PLC subroutines Measure and Tx Cmplt lnt obtain pressure readings and transducer temperature readings from transmitter 22. This may be performed every one minute.
• Subroutine Temp Ctrl controls pressure transducer 24 temperature, which is performed as follows: the operator entered temperature set point is compared to the digitally communicated transducer temperature, the difference or error is then used to establish the resistive heater output.
• the remaining recorded variables are also obtained on a one minute time frame. This is accomplished in the Measure subroutine.
• the PLC program is responsible for data storage. This is accomplished in subroutine Record.
• One record per minute is stored. The organization of the data is by date and time. The record for every minute will include: (1) the differential pressure representing the hydrostatic pressure produced by the fluid mass (as a floating-point number, IEEE 32 bit format), (2) the barometric pressure (as x 1000- 16 bit integer), (3) the ambient temperature (as x 100- 16 bit integer and (4) the transducer temperature (as x 100- 16 bit integer).
• the system features a man-machine interface computer 70 to allow direct operator interaction with the unit, to accept the operator's transducer temperature setpoint, to allow the operator to graphically visualize the differential pressure data collection via a time based chart, to allow the operator to determine the state of the machine, to allow the operator to trouble-shoot any system recognized errors, to provide a second (primary) means for logging the collected data, to allow the operator to authorize data collection by way of entry of a predetermined "run key,” to allow the operator to record client, location, tank and fluid physical characteristics and to allow the operator to initiate and conclude mass based fluid custody transfer measurements.
• a man-machine interface computer 70 to allow direct operator interaction with the unit, to accept the operator's transducer temperature setpoint, to allow the operator to graphically visualize the differential pressure data collection via a time based chart, to allow the operator to determine the state of the machine, to allow the operator to trouble-shoot any system recognized errors, to provide a second (primary) means for logging the collected data, to allow the operator
• the third software program of the storage tank leak detection system of the claimed invention is the post processing program.
• Remote computer 80 performs this program. Linear regression of logged data is performed as follows.
• the data files created by the PLC program are read in, these include time stamped values for measured hydrostatic pressure created by the fluid mass, ambient temperature, fluid temperature, transducer temperature, and atmospheric pressure, along with the tank diameter and the specific gravity of the tank contents.
• Tank shell temperature corrections are made on the basis of the coefficient of thermal expansion of the construction material of the tank and the calculated tank shell temperature.
• the calculated tank shell temperature is derived from a combination of the fluid temperature and the ambient temperature at a ratio entered by the software operator, typically in the range of 0.2 to 1.0.
• Atmospheric pressure corrections are made based on the application of a barometric correction coefficient applied to the hydrostatic pressure data (this barometric correction coefficient is determined empirically through the observation of head variations as a function of barometric pressure changes in an otherwise leak free and well insulated test tank during initial system commissioning).
• the operator selects data periods, typically consistent nighttime periods where the data is relatively free from the significant effects of solar radiant energy, rain and other significant weather events and the software performs linear regressions of the three hydrostatic pressure data series over these periods.
• the software performs a validation of the data by calculating the theoretical tank shell temperature on the basis of the measured hydrostatic pressure change throughout the duration of the test.
• This theoretical tank shell temperature is presented graphically superimposed upon the measured ambient temperature data and manually scaled to align the first nights theoretical tank shell temperature to the region bounded by the measured ambient temperature and the manually measured fluid temperature for the same time period.
• the operator can identify whether or not the theoretical tank shell temperature is consistent with the measured ambient and fluid temperature for all data throughout the duration of the test and similarly verify the appropriateness of the operator selected fluid and ambient temperature ratio. More specifically if the theoretical tank shell temperature calculated on the basis of the measured head change does not fall within the region banded by the ambient and fluid temperature on subsequent nights the excursion is of interest and observed for linearity.
• the operator can apply a simulated leak rate to verify the linearity of the excursion and to confirm the leak rate measured through linear regression of the hydrostatic pressure data.
• This method allows the operator to correlate changes in measured head with changes in ambient and fluid temperature and if the correlation is strong, validate the measured head data. Where the correlation is not strong and the excursion from night to night is not linear the change in head can be attributed to other outside influences and thereby the use of irrelevant data avoided (these could include effects such as rain, the bubbler settling in tank sediment, tank pumping operations, etc).
• the present system and method of its use will obviate significant inconvenience and provide substantial utility to those who wish to detect leaks in storage tanks.
• the present device will allow very small leaks to be detected in very large storage tanks in a consistent and cost- effective manner.

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• Examining Or Testing Airtightness (AREA)

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• 2006
  • 2006-12-01 EP EP06851195A patent/EP2089703A1/de not_active Withdrawn
  • 2006-12-01 CA CA002671247A patent/CA2671247A1/en not_active Abandoned
  • 2006-12-01 WO PCT/US2006/061526 patent/WO2008066554A1/en active Application Filing

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