1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-18a71 Center Point of Contact: JSCa71 Submitted by: Wil HarkinsSubject: Design of an Improved Gas Transfer Valve for Leak Tight Testing Practice: A needle-point penetration gas transfer valve has been developed at JSC that
2、 is leak tight and gives very reliable results in transferring low or high pressure gases from sample containers to laboratory measuring devices such as chromatographs.Programs that Certify Usage: This practice has been used on the Space Shuttle ProgramCenter to Contact for Information: JSCImplement
3、ation Method: This Lesson Learned is based on Reliability Practice number PT-TE-1441 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:The reliability benefits of this new valve are that it can be used to transfer gas without risking the loss of t
4、he sample or the samples purity. This improved reliability is in comparision to normal refrigeration gas transfer valves which are not leak tight and are not suitable for gas transfer where limited and unique gas samples are available and absolute gas measurements are required.Implementation Method:
5、Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The purpose of this gas transfer valve was to provide a leak tight method of delivering gases from an Orbiter Aft Fuselage Gas Sampling System (OAFGSS) bottle assembly to a laboratory chromatograph. A c
6、ommercial gas transfer system was modified to accomplish this task. A tube piercing valve was redesigned to replace the dynamic shaft seal with a compressible hermetically-sealed metal bellows. The redesigned valve and the component parts are illustrated in Figure 1.refer to D descriptionProvided by
7、 IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-D Figure 1. Valve Assembly and Gas Transfer Set-up The redesigned valve incorporates existing technologies into a compact and most importantly leak-tight device. The metal bellows assembly was obtained from an off
8、-the-shelf valve which was readily available. The body of the valve had to be fabricated; however, the basic idea was obtained from commercial valves. A high vacuum fitting was used as the outlet fitting of the valve due to its excellent ability to provide a leak-tight connection.The redesigned valv
9、e also has an adjustable stem travel which allows the stem tip penetration to be set to the desired level without excessive deformation of the tube which could lead to valve leakage.Helium leak checks were made on the valve after its development to determine leak rates and are shown in Table 1. The
10、leak detector was configured to detect helium leaks as low as 1x10-8standard cubic centimeters per second (sccs) during valve functioning. The values shown in Table 1 indicate that the valve is leak tight.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,
11、-,-refer to D descriptionD Table 1. Leak Tests on the Gas Sampling Valve Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Technical Rationale:It was desired by the Space Shuttle Program to determine if and at what levels explosive exhaust gasses were
12、being collected in the Orbiter Aft Fuselage during ascent of the Orbiter. For this purpose, several gas transfer assemblies were developed and flown on the Orbiter. Each of the assemblies were programmed to sample exhaust gasses at several locations and altitudes along the Orbiter ascent trajectory.
13、 After a mission, these assemblies were downloaded and taken to a laboratory for gas analysis.Commercial tube piercing gas transfer systems were first used to penetrate the assemblies access tube for chromatography testing. This technique was not suitable in some cases because of valve leakage and n
14、ot reliable in others because of sample contamination, i.e., smaller samples were sometimes completely lost while other samples became contaminated with atmospheric constituents that rendered the test results meaningless. For this reason, a new gas transfer valve was developed for use that was leak
15、tight and preserved the sample and its integrity during testing.Impact of Non-Practice: The use of the redesigned tube-piercing valve in place of the commercial valve provided one with no leakage and more robust and reliable operational characteristics. Re-use of the commercial valve would give less
16、 than satisfactory results in OAFGSS testing.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-18a71 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-,-,-
