Patent Application
20110284742
1. Field of the Invention
This invention relates to the detection of degradation or failure of pressurized-bottles and in particular to the positive verification of mass inside pressurized bottles.
2. Description of the Related Art
Pressurized bottles are used to store coolant as well as actuator gases for long periods of time. Typically, the pressurized bottles are impossible to check for fill between the time they are installed and when called upon to function. If any latent fault or damage has occurred in the intervening years, the bottle can leak, causing the catastrophic failure of the machine depending on it. The bottles are typically single-shot devices and are consumed if opened, and thus cannot be sampled and refilled. Pressurized bottles include Joule-Thompson Cryo coolers for IR focal plane arrays (FPAs), pneumatic actuators for fins and nozzles and gasses for fire suppression systems.
Previous devices have attempted to measure pressure effects on the bottle to determine if it is loaded with high pressure gas. The most common version is to attach a Bourdon tube type pressure gage directly to the bottle. The device can read out the pressure in the bottle directly, though the automated version uses an electrical switch to denote if a bottle has dropped below a reference pressure. The Bourdon gage itself is the source of several potential leak paths in the Bourdon tube as well as the joints needed to attach it. A diaphragm type pressure gage can be installed, which uses a strain gage on the back side of a thin metal diaphragm. This technology is subject to long term creep effects which cause the reading to drift, and like the Bourdon tube, introduces additional potential leak paths. Applying strain gages directly to the bottle wall can directly measure the strain from being loaded to infer pressure. This technique has been attempted and found to produce false leak detections due to reading drift with time. “Ping” testing uses a mechanical impact to ring the bottle. A fast Fourier Transform of the resulting ringing frequencies detected by an accelerometer is used to infer pressure. This technique has been found to be greatly complicated by useful bottle geometries, and highly susceptible to shifts caused by installation constraint. All of these methods must be compensated for the bottle temperature to be able to determine if the actual proper mass of material is in the bottle.
The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.
The present invention provides a non-invasive technique for the positive verification of mass in a pressurized bottle over the lifetime of the bottle.
This is accomplished by mixing a Kr-85 tracer gas with the carrier gas in a pressurized bottle. External detection of the gamma rays that penetrate through the walls of the bottle provides positive verification of mass inside the bottle. In addition, external detection of beta rays from Kr-tracer gas outside the bottles provides positive verification of the occurrence of a gas leak from the bottle.
In an embodiment, a pressurized bottle comprises a mixture of a carrier gas and a Kr-85 tracer gas. The mixture is initially pressurized to at least 3,500 PSI. The Kr-85 tracer gas emits gamma rays that penetrate through the bottle.
In another embodiment, a pressurized bottle comprises a mixture of a carrier gas and a Kr-85 tracer gas. A tag provides a calibration date, a calibrated mass and a calibrated Kr-85 gamma count. The tag may, for example, comprise a bar code, an RF tag or an electronic file associated with a bottle identification number.
In another embodiment, a pressurized bottle comprises a mixture of a carrier gas and a Kr-85 tracer gas. A tag provides a calibration date, a calibrated mass and a calibrated Kr-85 gamma count. A gamma detector external to the bottle counts gamma rays emitted by the Kr-85 tracer gas inside the bottle through the bottle. A processor calculates from the gamma count and the half-life properties of Kr-85 a test mass. The processor compares the test mass to the calibrated mass to provide positive verification of mass in the pressurized bottle.
In another embodiment, a method of positive verification of presence of mass in a pressurized bottle comprises providing of a mixture of a carrier gas and a Kr-85 tracer gas in a high-pressure bottle. The bottle is tagged with a calibration date, a calibrated mass and a calibrated Kr-85 gamma count. The bottle is emplaced in-situ in a system to provide, for example, cooling, and actuation or fire suppression of a sub-system. Gamma rays are detected external to the bottle to measure a test gamma count of gamma rays emitted by the Kr-tracer gas inside the bottle through the walls of the bottle. Based on the test gamma count and the half-life properties of Kr-85 a test mass is calculated. The test mass is compared to the calibrated mass to provide positive verification of mass in the pressurized bottle. The test process may be repeated periodically or based on the occurrence of certain events.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
a and 3b are diagrams of an embodiment of a gamma detection system for a bottle that serves to cool a focal plane array detector for a kill-vehicle;
The present invention provides a non-invasive technique for the positive verification of mass in a pressurized bottle over the lifetime of the bottle. This is accomplished by mixing a Kr-85 tracer gas with the carrier gas in a pressurized bottle. External detection of the gamma rays that penetrate through the walls of the bottle provides positive verification of mass inside the bottle. In additional, external detection of beta rays provides positive verification of a gas leak from the bottle.
Known techniques such as the Bourdon pressure gage, strain gages or the “ping” test do not provide positive verification of mass in the pressurized lifetime. They rely on evidence on which to draw negative inferences of presence of mass. The reliability and accuracy of such negative inferences is suspect, particularly in very high-pressure bottles (e.g.>3,500 PSI) over long life times, e.g. several years to decades.
A non-invasive technique for the positive verification of mass is useful for all types of pressurized bottles and environments. However, such a technique may find particular import in demanding environments. These environments may require very high pressures, in excess of 3,500 PSI. They may require this pressure level to be maintained for 10 or more years prior to use of the pressurized gas. Proper operation of the bottle may be critical to successful execution of the mission. The bottle may be located “in-situ” where access to the bottle by a technician is very difficult. The environment may demand an accurate measurement over the lifetime. If the technique cannot provide this accuracy, the bottle may have to be over designed, expending valuable resources on volume, weight and cost for a given system. The environment may demand or at least prefer that the technique is non-invasive as invasiveness may compromise both the test results and the integrity of the bottle.
Referring now to
The processor compares the test mass to the calibrated mass to provide positive verification of mass in the pressurized bottle. The positive verification of mass may then be provided in a report 26 via, for example, a display 28.
Krypton is a colorless, odorless, tasteless gas about three times heavier than air. As a noble gas, krypton is generally inert and forms very view chemical compounds. It occurs in nature as six stable isotopes of which Krypton-84 is the most prevalent. Eleven major radioactive isotopes of krypton exist of which only two—Kr-81 and Kr-85 have appreciable half-lifes. Kr-81 has a half-life of about 210,000 years and Kr-85 has a half-life of 10.76 years. Kr-85 is produced by the fissioning of uranium and plutonium and is present in spent nuclear fuel. Kr-85 is also present in the atmosphere due to neutron capture reactions from cosmic ray neutrons interaction with stable krypton isotopes.
Mixture 10 may have an initial calibrated pressure of a several hundred to a few thousand PSI or greater than 3,500 PSI depending upon the intended application. Typical carrier gases 12 include Nitrogen (N), Argon (Ar), Krypton-84 (Kr-84 that is not radioactive) and Helium (He). The tracer gas 14 is the radioactive isotope Kr-85, which has a half-life of approximately 10.76 years. Other radioactive isotopes do exist but they are not well suited for providing positive verification of mass over the expected life times of pressurized bottles. The half-life of some isotopes is simply too short to provide monitoring over typical periods. The half-life of other isotopes is simply too long to provide a gamma count in a reasonable period of time with an acceptable signal-to-noise ratio. The amount of Kr-85 tracer gas (specified in mole percent) in the mixture will depend on several factors including anticipated background radiation levels, period to measure the gamma rays, safety issues and volume of the bottle In particular, if the percentage of tracer gas is too high the mixture may freeze up when the gas is expelled from the bottle during its intended use. For example, in an embodiment of pressurized bottle configured for cooling a FPA the concentration of Kr-85 tracer gas in a Nitrogen carrier gas is less than 1 mole percent. This threshold may vary with the type of carrier gas and the configuration of the pressurized bottle to perform its intended function e.g. coolant, actuation or fire suppression.
The mixture inside the bottle is governed by the gas law pV=nRT where p is pressure, V is volume, n is the amount of substance in moles, R is the gas constant (8.314471J/K*mo) and T is the temperature in degrees Kelvin. The mass is equal to the number of moles (n) times the molar mass M. In the mixture, the number of moles (n) is apportioned between the number of moles of the carrier gas nc and the number of moles of the tracer gas nKr-85, which have different molar mass.
Pressurized bottle 16 is typically made of steel with walls between approximately 1/10″ and ¾″ depending on the volume of the bottle and the pressure of the gas mixture. Other materials may be used to form the bottle including any metal or composite structures such as carbon with sufficient strength to contain the high pressure. The gamma rays emitted by the Kr-85 isotope from the tracer gas inside the bottle will penetrate through the walls to a distance that can be detected by detector 20. If the volume/pressure dictate walls that are too thick to allow penetration, a “window” may be formed in the bottle and aligned to detector 20. The beta rays emitted by the Kr-85 isotope from the tracer gas inside the bottle will not penetrate through a metal wall of any appreciable thickness. Bottle 16 comprises a valve 30 to both fill the bottle with pressurized gas and to expel the pressurized gas to cool, actuate or suppress fires. The bottle is typically configured to release the pressurized gas in “1-shot”. Alternately, the bottle could be configured to release gas in multiple shots, which would require recalibration after each shot. This valve, and other penetrations of the bottle such as tubes, windows etc. can have exhibit defects that create failure points where the pressurized gas may leak and escape to the external environment or may rupture and cause catastrophic failure. Beta rays emitted by the Kr-85 isotope that has leaked outside the bottle are detectable. The pressurized bottle may be put in the field and emplaced “in-situ” in a system such as a missile, a kill vehicle, etc to provide coolant for a sub-system such as a FPA, actuation of a sub-system such as a wing or fin or fire suppressant.
Tag 18 may take any one of several different forms to provide the calibration data for the mixture in a particular bottle. For example, tag 18 could be a bar code placed on the bottle, an RF (radio frequency) tag, written documentation or a computer file stored elsewhere and associated with an identification number on the bottle. Typically, the tag will uniquely identify the date, mass and gamma count for that particular bottle. However, if a batch of bottles is filled on the same date with the same mass and amount of Kr-85 the tag could provide calibration data for the entire batch. The calibration data may be specified when the bottle is filled initially or perhaps if and when the bottle is recalibrated. For example, if a bottle was placed in storage it may be recalibrated before being incorporated into a system.
The calibrated mass may be provided in one or more ways including measuring the empty and filled bottle, measuring the pressure and temperature and calculating the mass or directly monitoring the mass that is placed into the bottle. The calibrated gamma count is suitably provided by measuring the actual gamma count outside the bottle for the tracer gas inside the filled bottle. Alternately, the calibrated gamma count may be calculated based on a measurement of the amount of Kr-85 tracer gas placed in the bottle.
Gamma detector 20 detects and counts gamma rays 22 that are emitted from the Kr-85 tracer gas inside the bottle and penetrate through the bottle to the detector. The gamma detector is preferably non-invasive with respect to the bottle. Any type of invasiveness can effect the positive verification of mass and may affect the integrity of the bottle. The gamma may be a form of a Geiger counter, also referred to as a Geiger-Muller counter, in which an inert gas-filled tube briefly conducts electricity when a gamma ray makes the gas conductive. The tube amplifies this conduction and outputs a current pulse. Another device for detecting gamma rays is a scintillation counter. Scintillation detectors use crystals that emit light when gamma rays interact with the atoms in the crystals. The intensity of the light produced is proportional to the energy deposited in the crystal by the gamma ray. The detectors are joined to photomultipliers that convert the light into electrons and then amplify the electrical signal provided by those electrons. Common scintillators include thallium-doped sodium iodide (NaI(Tl))—often simplified to sodium iodide (NaI) detectors—and bismuth germanate (BGO). See for example, Kwang Hyun Kim et al. “Signal and noise performance of large-area PIN photodiodes and charge-sensitive preamplifiers for gamma radiography” Nuclear Instruments and Methods in Physics Research A 591 (2008) 63-66.
Gamma detector 20 is positioned near the bottle to detect and count the gamma rays emitted through the bottle. The detector may be fixed in-situ with the monitored subsystem and/or provided as a man-portable unit. The gamma detector counts the gamma rays over a period of time long enough to provide an acceptable SNR. The raw count is suitably calibrated to compensate for any background gamma radiation due to other sources and the efficiency of the detector. Not all gamma rays emitted by the source and pass through the detector will produce a count in the system. The probability that an emitted gamma ray will interact with the detector and produce a count is the efficiency of the detector. The gamma detector may be configured to perform the measurement every N units of time where the unit could be a day, a month or a year for example or may be configured to perform the measurement upon the occurrence (or planned occurrence) of a certain event such as the use of the bottle for its intended purpose.
Processor 24 may include one or more computer processors and any processor memory required to store and process the calibration and measured data to provide positive verification of mass. The calibration data (date, mass, gamma count) is provided to the processor. For example, upon emplacement of the bottle into a system a bar code may be read and the data stored in the processor or an electronic file corresponding to the bottle ID may be downloaded to the processor. Alternately, an RF tag may broadcast the data to the processor. Given the calibration date of the bottle and the half-life properties of Kr-85, the processor can normalize the measured test gamma count to the calibrated date or vice-versa. Knowing the calibrated gamma count and calibrated mass, the processor can compute the test mass currently inside the bottle. The processor compares the test mass to the calibrated mass to provide positive verification of mass inside the bottle.
The processor may then report out the positive verification of mass. The processor may be configured to report out after every test or only if the mass inside the bottle has changed by a threshold amount. The processor may report out a simple status such as “Passed” or “Failed” or a more complete report 26 as shown in
a and 3b illustrate a pressurized bottle 80 including a mixture of Nitrogen gas and Kr-85 tracer gas emplaced in situ with a gamma detector 82 to cool a focal plane array (FPA) on a kill-vehicle 84. To achieve the SNRs needed for terminal guidance of the kill-vehicle to a target, the FPA must be cooled. At the appropriate time, the valve on the bottle is opened and the high-pressure gas expands through a nozzle becoming cold as it is sprayed onto the FPA. The pressurized bottle is a critical failure point. One or more kill-vehicles are carried as, for example, the third stage of a ballistic missile to launch them into space to intercept enemy missiles. The ballistic missile may be stored underground or in a submarine. Access to the kill-vehicle and pressurized bottle is quite limited. The Kr-85 tracer gas provides a non-invasive capability to provide positive verification of mass in the bottle while in-situ.
In an embodiment, gamma detector 84 may comprise a scintillating optical fiber 86 wrapped around pressurized bottle 80 and a photo detector 88 that is optically coupled to the end of the fiber. When a gamma ray 90 interacts with a properly doped fiber, a light pulse is generated and transported down the fiber to the photo detector. The photo detector converts the optical pulse into an electrical pulse that is registered by a counter 92. The processor may be located in-situ or remotely. The detector is coupled to a communication link of the kill-vehicle/ballistic missile to report out either the raw count if the processor is remote or the processed results of the positive verification.
In the high-pressure accumulator 122 and low-pressure reservoir 124, the cross-hatching represents a mixture of Nitrogen gas and Kr-85 tracer gas 126, and the dots represents hydraulic fluid 128. Usually a bladder 130 of some sort is used to separate the hydraulic fluid from the gas. The bladder contains the mixture so that it contracts and expands as hydraulic fluid enters and exits the accumulator, respectively.
Hydraulic fluid is much easier to pump than a gas would be, but it cannot be compressed. However, a gas can be compressed and is much better at storing mechanical energy than a fluid. Therefore, a gas-fluid combination is ideal. The Nitrogen/Kr-85 gas mixture acts as a gas “spring” which stores and releases energy as the hydraulic fluid shuttles back and forth, in and out of the high-pressure accumulator. Nitrogen gas is used because it is inert and non-explosive at high pressures.
To deliver the necessary power, the pressure inside the high-pressure accumulator must be very high, as much as 5000-7000 psi. The pressure inside the low-pressure reservoir is much lower, 100-200 psi, and serves to provide the necessary pressure differential as the hydraulic fluid is pumped into and out of the high-pressure accumulator. The accumulator and reservoir are typically constructed out of carbon fiber material which is high-strength and much lighter than steel.
Gamma detectors 132 and 134 may be positioned to detect and count gamma rays emitted by the mixture inside the low-pressure reservoir and high-pressure accumulator, respectively. In this system, the gas should be conserved, therefore any loss of mass is indicative of a leak.
For pressurized bottles that are made out of metal, the beta particles that are emitted by the Kr-85 tracer gas do not penetrate through and out of the bottle. As such, detection of beta particles as positive proof of mass inside the bottle is not possible. However, the presence of beta particles is positive proof that the gas mixture is leaking out of the bottle. Because the gas tends to rapidly disperse once outside the bottle the count of beta particles is not generally accurate enough to make a negative inference regarding how much mass is left inside the bottle. But the presence of any beta particles is proof of a leak. The combination of gamma detection of gamma rays emanating from Kr-85 tracer gas inside the bottle as positive verification of gas inside the bottle and beta detection of beta particles emanating from Kr-85 tracer gas leaking outside the bottle as positive verification of a leak provides a more robust system for monitoring the pressurized bottles.
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While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
This invention was made with United States Government support under Contract Number HQ0147-09-D-0001 with the Department of Defense. The United States Government has certain rights in this invention.
