1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-15a71 Center Point of Contact: MSFCa71 Submitted by: Wil HarkinsSubject: Static Cryogenic Seals for Launch Vehicle Applications Practice: Practice: Deflection actuated, pressure assisted coated metal seals, or spring energ
2、ized Teflonseals, along with prudent flange joint designs, should be used for high pressure static cryogenic sealing applications in launch vehicle engines and related propulsion system components.Programs that Certify Usage: This practice has been used on Saturn I, Saturn V, Space Shuttle External
3、Tank (ET), and Space Shuttle Main Engine (SSME).Center to Contact for Information: MSFCImplementation Method: This Lessons Learned is based on Reliability Practice No. PD-ED-1208; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Leak-free joints
4、can be achieved in cryogenic lines, joints, valves, and pumps for launch vehicles through the use of proven, state-of-the-art static cryogenic seals. These seals adapt to wide ranges of temperature and continue to seal when subjected to high pressures, in-flight static stresses, and in-flight dynami
5、c loads.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Implementation Method:1. Introduction:Low or zero fluid or gas leakage in flight and ground-based cryogenic systems can be achieved through meticulous joint design and testing, selection of the
6、proper seal configuration and materials, thorough cleaning and inspection of seal and flange surfaces, carefully controlled installation, and carefully controlled fastener tightening procedures. The most widely used and successful cryogenic seal for NASA space flight applications has been the deflec
7、tion actuated, pressure assisted coated metal seal. High nickel content steel alloys coated with a thin layer of Teflonor plated with gold, silver, indium, palladium, lead, copper, nickel, or aluminum have provided good sealing properties at elevated as well as cryogenic temperatures. This practice
8、covers experience with pressure assisted and spring energized cryogenic seals in the SSME and ET. Experience was derived from earlier programs (Saturn I and Saturn V) to develop these effective seals. Although the subject of this practice is cryogenic seals, the pressure assisted and spring energize
9、d seals described are also effective over the broad temperature ranges from liquid hydrogen (-423 deg.F) to hot gas (1000/1200 deg.F).2. Nonspacer Type, Deflection Activated, Pressure Assisted Sealsrefer to D descriptionD Provided by IHSNot for ResaleNo reproduction or networking permitted without l
10、icense from IHS-,-,-The nonspacer type seal shown in Figure 1, fits into a groove in the flange. It can be used with a separate spacer to eliminate the need for a seal groove, but a retaining groove is preferred.As shown in Figure 1, these seals have two sealing surfaces that mate with adjoining fla
11、nges. Diameters range from 0.55“ to 16.75“ as used in the SSME. Cross sections of the seal ring vary from 0.200“ x 0.164“ to 0.150“ x 0.120“ in radial width and installed length, respectively, and the seals can be made in other diameters and other cross-sectional configurations. They are found throu
12、ghout the SSME in both cryogenic and hot gas applications. The seals are machined from high nickel alloy steel and coated with either silver or silver with rhodium overcoat. The silver coated seals have a temperature range of -423 deg.F to +1000 deg.F, while the silver with rhodium can be used over
13、a -423 deg.F to +1200 deg.F range. The seals are used in both fuel and oxidizer systems.In installing both the nonspacer type and the spacer type seals, the seals are compressed during joint assembly, which provides a load at the sealing circumference to effect sealing at low pressures. As the press
14、ure increases, it acts on the internal surfaces of the seal, increasing the force on the seal tips to augment sealing capability as pressure increases. The seal coating presses into the flange surfaces, filling microscopic asperities and irregularities in the flange sealing surfaces. The combination
15、 of the installation deflection and the pressure on the internal surfaces permits the sealing faces to compensate for joint separation under system pressure and for shrinkage during exposure to cryogenic temperatures.3. Spacer Type, Deflection Activated, Pressure Assisted SealsProvided by IHSNot for
16、 ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD This type of seal was originally used on the Saturn program and was later adapted for use on the Space Shuttle. The seal incorporates a flange, drilled to match the mating parts, which provides a posi
17、tive stop to control seal compression and secondary pressure barriers on each side of the seal to facilitate leak checking. While some seals were originally silver plated, present use is confined to Tefloncoated high nickel alloy steel seals. The seals are used on the ET and on the piping connecting
18、 the Tank to the Orbiter. Most have rated temperatures of -423 deg.F to +350 deg.F except for one which has a -423 deg.F to +800 deg.F rating. A typical seal installation as it is used on the ET is shown on Figure 2. Notice that the seal has both a dual-sided primary seal located at the interior per
19、iphery of the seal and a dual-sided secondary pressure barrier just inside the bolt circle. A Tefloncoated seal is used in the LH2and GH2systems while a silver plated seal is used in the LO2and GO2systems.4. The Raco/CreaveyTMSeal ConfigurationProvided by IHSNot for ResaleNo reproduction or networki
20、ng permitted without license from IHS-,-,-refer to D descriptionD Figure 3 shows the combination Raco/CreaveyTMseal as used for 17-inch diameter feed lines on the ET. The primary Racoseal consists of a metal hoop-spring inside an energized Teflonjacket. The secondary CreaveyTMseal is a metallic coil
21、 spring housed within an energized tubular Tefloncasing.5. Recommended Practicesa. Design PracticesIn general design practice, the development of a good leak-free joint design requires an integral look at the design of all the parts: seal(s), flanges, and fasteners. It also requires some foreknowled
22、ge of the degree of access required for leak checking, inspection, and potential disassembly and reassembly during downstream operations and particularly on the launch pad. Leak-free joint design is based on the seal maintaining contact between a surface on one flange and the mating surface on the o
23、ther flange under all operating conditions. The fasteners take the dynamic loads and are installed in a preloaded condition to maintain seal contact with the flange surfaces. The seal in the joint must prevent leakage in excess of the allowable limit. The advantages of deflection actuated, pressure
24、assisted seals are that they maintain a nearly constant fastener loading under pressurized and nonpressurized conditions and that they result in minimum flange deflection at the sealing surface. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Sealing
25、 surfaces on flanges can be recessed to protect them from damage, and seal grooves can be configured for easy seal installation, centering of the seal, and for error-free assembly. The seal and joint can be designed with detents to prevent misaligned or reverse installation.The design of joint assem
26、bly and seal installation tooling, equipment, fixtures, and procedures should proceed concurrent with joint design. The designer must remember that a separable joint is used to permit later disassembly, inspection, and reinsertion of seals or refurbishment/replacement of systems or components either
27、 in the manufacturing shop, on the test stand, or on the launch pad. The joint design and the assembly tooling or fixtures should provide: (1) protection to the seal and mating surface; (2) concentric and accurate seal positioning; and (3) even pressures around the periphery of the seal and joint du
28、ring the fastener tightening process. Circumferential indexing that will ensure relocation of impressed seal surface deformities over corresponding flange deformities is desirable if the seal is to be reused. A good design practice is not to depend entirely on flange bolt locations to position seal
29、components radially. A notch or groove should be provided to retain the seal. Gaskets, seals, parts, and subassemblies should be designed to preclude improper alignment or rotation. Using seals very close to the same size in the same area should be avoided. The design should be adaptable to using th
30、e same size seal (or a very different size) in all locations which are in close proximity. This practice will reduce the potential of installing the incorrect size seal.The following design suggestions pertain to seals for cryogenic and gaseous hydrogen and oxygen:a71 Where feasible, secondary seals
31、 with a vent for direct measurement of leakage should be provided.a71 a71 The materials in cryogenic/gaseous hydrogen seals should be resistant to hydrogen embrittlement.a71 a71 Walls should be provided in the seal groove to carry the seal hoop load when pressurized.a71 a71 Designs of liquid oxygen
32、seals should include LO2compatible materials.a71 a71 Designs of liquid hydrogen seals should include LH2compatible materials.a71 a71 Potential flange ovality resulting from flange stresses or temperature cycling must be taken into account in establishing the width of flange sealing surfaces.a71 a71
33、The seal material(s) must be compatible with any anticipated purge or cleaning material that may contact the seal during its intended use. Purge or cleaning material restrictions should be Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-noted on engi
34、neering drawings and in procedural documentation.a71 a71 A seal alignment provision should be incorporated in the design process.Design optimization in metal seals for cryogenics can be accomplished with currently available general purpose finite element analysis programs such as ANSYS (produced by
35、Swanson Analysis Systems, Inc., see reference #13) or by specially programmed finite element models (reference #14). Modeling of seals can take into account surface texture, gas transmission flow methods, seal load distribution, material properties, and dynamic environmental conditions (temperature,
36、 pressure, vibration, shock, etc.). New seal designs should be evaluated using these analysis and modeling techniques and qualified before use. (see below).b. Qualification PracticesPrior to incorporation into the production design, the entire joint system, which includes the flanges, seal(s), and f
37、asteners, should be qualified for use in the specific environments expected to be encountered in operations. As part of the qualification procedures for SSME seals, seals of 0.8“, 1.1“, and 3.8“ diameter were chilled to -250 deg.F and pressure cycled from ambient pressure to 8,970 psig for 240 cycle
38、s while demonstrating their ability to continue to meet leakage requirements. Seals were also subjected to structural verification at pressures up to twice operating pressures after completion of 240 pressure cycles, while still meeting leakage requirements. If different temperatures, pressures, or
39、gases are used for qualification and/or leak checking, leak and nonleak conditions must be carefully calibrated and correlated with leak and nonleak conditions in the actual environments expected. These calibrations must be meticulously adhered to in interpreting leak test results.c. Manufacturing P
40、racticesCleanliness and inspection at intermediate manufacturing steps are extremely important in the manufacture of deflection actuated pressure assisted seals as well as for other types of seals used in liquid oxygen and liquid hydrogen environments. Nonspacer type seals are usually silver plated
41、with an initial gold undercoat. The gold undercoat prevents oxidation of the substrate at temperatures above 600 deg.F, when used in hot gas environments, and this prevents blistering of the silver plating. Silver is used for its low compressive yield strength and ductility required for effecting a
42、seal, and for its corrosion resistance. Rhodium overplate is used to prevent bonding of the silver plate to the mating flange surfaces at high temperatures. A chromate coating is used to prevent discoloration of the seal or flange due to tarnishing of the seals silver plating. Care must be taken to
43、thoroughly clean and inspect each seal between the plating and coating operations. Adherence of the Teflonprimer coat and subsequent final coat to spacer type seals also requires stringent cleaning and inspection between each operation to prevent inclusions, voids, contamination, and surface Provide
44、d by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-defects.d. Inspection PracticesSeal and flange mating surfaces should be visually inspected after manufacture and immediately before installation with a 10X magnification device. Very tiny scratches across the
45、 face of the seals coated or plated sealing surface can cause leaks. In one instance, a scratch .001“ wide and .0005“ deep extending across the radial length of the sealing surface was of sufficient size to cause a Class I leak. The inspector should look carefully not only for nicks and scratches in
46、 seals, but also for metal or foreign particles on the seal or on the mating flange surfaces. NASA problem reports have indicated that, in at least one incidence, metal flange faces were allowed to contact each other and to rub together prior to seal installation, causing fine metal particles to be
47、created which interfered with the seals ability to seat properly against the flange sealing surface. Optical microscopy up to a power of 50X has been used to detect very small flaws and irregularities in flange and seal surfaces when leakage tests failed. Seal flatness or waviness can be confirmed o
48、r detected by the glass test in which the seal is placed against a plate glass sheet and observed from the underside. This method can be used to detect potential nonparallelism or small deviations in seal contact or coating thickness. Precision flat bars can be used with a light source to verify tha
49、t flange faces are flat.e. Protection PracticesMany of the leaks detected from joints of cryogenic and gaseous lines were possibly caused by scratching or nicking of the flange sealing surfaces or the seal, after initial manufacturing and inspection, but before assembly. Post manufacturing, in-transit, and preassembly protection of b
