1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-17a71 Center Point of Contact: MSFCa71 Submitted by: Wil HarkinsSubject: Leak Testing of Liquid Hydrogen and Liquid Oxygen Propellant Systems Practice: Leak testing is a nondestructive test method that provides the capabil
2、ity to detect and measure the amount of liquid or gas escaping from a sealed pressure system and to locate the individual leaks for possible repair.Programs that Certify Usage: This practice has been used on Saturn I, IB II, IV, IVB, V, and Space Shuttle External Tank (ET).Center to Contact for Info
3、rmation: MSFCImplementation Method: This Lesson Learned is based on Reliability Practice number PT-TE-1424 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Leak testing of a Liquid Hydrogen (LH2) and a Liquid Oxygen (LO2) propellant system prior
4、to flight insures that the flight leakage rate does not exceed allowable leakage established for flight. Leak testing also insures the quality and reliability of a Space Shuttle Element and reduces the probability of system failure. Leak checks also prove that seals and sealing surfaces at joints ar
5、e defect free and seals are seated correctly.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Implementation Method:A leak is the result of cracks, holes, porosity, permeability, surface aspirates or scratches in a pressurized container or joint that
6、allows the escape of liquid or gas. Leak test methods and procedures are designed for detecting, locating leaks, and ensuring the quality, serviceability and safety of components, structures and the entire propellant system.Leak testing can be a time consuming and expensive task but should be implem
7、ented when necessary to protect product integrity or personnel safety. One of the most important steps in leak test development is to establish the specification for acceptable and unacceptable leak rates. Choosing the most effective and reliable leak testing method is also important.Leak testing me
8、thods and sensors include the following:1. Acoustical - escaping gas that is audible or can be detected using acoustical instrumentation.2. Odor - from tracer gas.3. Leak test solution - bubbles form when applied over escaping gas.4. Pressure decay - pressure loss over time.5. Pressurized liquid - n
9、ormally water, leakage can be seen.6. Trace gas - color/radiation/odor.7. Gas chromatography.8. Halogen leak detection - requires use of alcohol torch (blue flame) with a sniffer tube when small amounts of fluorine, chlorine bromine, etc. enter the tube then the blue flame turns to a different color
10、.9. Helium leak detectors - detects leakage of small amounts of helium.10. Flow meter - detects the volume of gas escaping over a specified time.11. Bubble gauge - monitors inter seal leakage between primary/secondary when primary pressurized.12. Light refraction.13. Mass spectrometer.14. Hydrogen s
11、ensors.15. Oxygen sensors.16. Electronic detectors.17. Liquid sensitive electrical aluminum tape.This reliability Practice covers MSFCs experience for ensuring that the External Tank, as an element of the Space Shuttle Transportation System, has developed an acceptable leak detection system and leak
12、age verification program for the cryogenic propulsion oxidizer and fuel systems, Liquid Oxygen (LO2) and the Liquid Hydrogen (LH2) tanks.EXTERNAL TANKProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Extra effort is devoted to the procurement of defect
13、-free raw material. Aluminum plate has been ultrasonic tested at the mill. The skin panels are placed into a weld fixture and welded using the variable polarity plasma arc (VPPA) or TIG welding method. After welding, the welds are x-rayed and penetrant inspected (defects are repaired as required). W
14、hen the LO2tank has been assembled, the welds are wrapped with an aluminum tape which has an adhesive that is soluble in water. The LO2tank is then hydrostatically tested. If a leak is encountered during hydrostatic testing, the water will dissolve the adhesive and provide electrical continuity thro
15、ugh the aluminum foil tape. A leak could then be isolated and repaired. After the LH2tank has been assembled, it is pneumatically proof tested with gaseous nitrogen (GN2). Leakage is checked with bubble solution at a reduced pressure after proof test. After the LO2tank/intertank assembly is joined t
16、o the LH2tank, the propulsion delivery systems are installed on the tanks. The propulsion delivery systems consist of the LO2and LH2feed lines, recirculation line and pressurization lines. The propulsion delivery system lines are joined with bolted flanges containing Naflex or Raco/Creavey seals to
17、control leakage.The Raco/CreaveyTMseal consists of a primary and secondary seal set. The primary seal is a Racoseal that is a pressure activated seal consisting of a Teflon jacket over an Inconel 718 spring. The secondary seal is a CreaveyTMseal, which is a tubular Teflon cover with a stainless stee
18、l spring core that forms an “O“ ring type seal. For Raco/CreaveyTMseal arrangement, see Figure 1. This two-seal arrangement provides for means for leak checking the primary seal or the total leak rate of the primary and secondary seals. For the Naflex seal arrangement, see Figure 2. The bolted flang
19、es for both types of seals have a leak test port that has been proved to be blockage free. The allowable leakage rate for the Raco/CreaveyTMseals is 0.183 SCIM of helium at 50 5.0 PSIG and ambient temperature. The propulsion line mechanical joints are leak checked in final assembly by pressurizing t
20、he leak check port to either 6 or 50 PSIG through a flow meter. The LH2& LO2tank and propulsion lines are pressurized to 6 PSIG with GN2and the leakage rate of b nuts and K seals are checked using the leak test solution bubble test (no bubbles in three minutes).Provided by IHSNot for ResaleNo reprod
21、uction or networking permitted without license from IHS-,-,-refer to D descriptionD Figure 1. Raco/CreaveyTMSeal Configuration (not to Scale) refer to D descriptionD Figure 2. Naflex Pressure Assisted Seal (Not to Scale)The LO2and LH2tanks remain pressurized to 6 PSIG during storage and could stay p
22、ressurized for several years. The tank pressure is monitored and if it decays too rapidly action will be taken to find the leak. During the entire ET program, no repairs have been required for flight hardware.Technical Rationale:Provided by IHSNot for ResaleNo reproduction or networking permitted wi
23、thout license from IHS-,-,-The rigorous leak check and nondestructive testing on the External Tank program have contributed to more than fifty successful flights of the Space Shuttle. Leak testing methods performed on the ET including bubble solution, water soluble adhesive on aluminum tape, flow me
24、ter testing and pressure decay testing. These techniques have proven to be effective in maintaining 100 percent mission success.References:1. MMC-ET-SE 25-0: “System Definition Handbook, Space Shuttle External,“ Volume I & II, National Aeronautics and Space Administration, Martin Marietta, August 19
25、87.2. Bray, Don E. and Don McBride: “Nondestructive Testing Techniques,“ John Wiley & Sons, Inc., 1992.3. Metals Handbook, Volume 17: “Nondestructive Inspection and Quality Control,“ ASM International, Metals Park, OH, 1989.4. “Introduction to Helium Mass Spectrometer Leak detection,“ Varian Associa
26、tes, Inc., Palo Alto, CA 94303, 1980.5. McMaster, Robert C.: “Nondestructive Testing Handbook, Leak Testing,“ Volume 1, American Society for Nondestructive Testing, Inc., 1985.6. Hammock, W. R. Jr., P.E. Cota, Jr., B.J. Rosenbaum, and M. J. Barrett: “Investigative Techniques Used to Locate the Liqui
27、d Hydrogen Leakage on the Space Shuttle Main Propulsion System,“ NASA, Johnson Space Center, AIAA SAE/ASME 27th Joint Propulsion Conference, Sacramento, CA, June 24-26, 1991.7. Szemenyei, B., R. Delcher, M. Randall, E. Schmidlin, and S. Barkhoudarian: “Ground-Based and In-flight Leak Detection for R
28、ocket Engines,“ X91-10319 Rockwell International/Rocketdyne Division, Canoga Park, CA.8. Barile, Ronald G.: “Hazardous Gas Leak Analysis in the Space Shuttle,“ N92-19308, 1991 NASA/ASEE Summer Faculty Fellowship Program, Kennedy Space Center, University of Central Florida, August 22, 1991.9. Barile,
29、 Ronald and William Helms: H2 Leak Detection Technical Interchange Meeting at KSC, Proceedings, April 29, 1992.10. S-69-117: “Leakage Testing Handbook,“ JPL/GE NTIS N-69-38843, July 1969.11. Reliability Preferred Practice PD-ED-1208, Static Cryogenic Seals for Launch Vehicles.12. Reliability Preferr
30、ed Practice PD-ED-1205, Weld Practices for 2219 Aluminum and Inconel 718.Impact of Non-Practice: Nonpractice could result in an unacceptable leakages of the cryogenic fuels from the LO2and LH2tanks and the propulsion system lines. Excessive leakage could ignite in the presence of an ignition source
31、and could result in catastrophic loss of vehicle and crew. Excessive leakage could cause launch delays and result in excessive program cost.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-17a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
