1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-06a71 Center Point of Contact: KSCa71 Submitted by: Wil HarkinsSubject: Oil-Free Vacuum Pumps in the LOX/LH2 Transfer System Practice: Oil-free pumping is the latest state of art in vacuum pump technology. At Kennedy Space
2、 Center, this technology is used to maintain approximate 650 sections of the LOX/LH2 vacuum jacketed transfer lines, at a pressure between 0-1000 Torr. This insures the capability to deliver cryogenic propellants on time and at a pre-defined temperature and pressure to the Shuttle vehicle.The use of
3、 oil-free pumps eliminates the risk of contaminating the LOX/LH2 transfer system (vacuum jacketed lines). It enhances the reliability of the vacuum pumping system.Programs that Certify Usage: This practice has been used on the Space Shuttle Program, Shuttle Main Propulsion System (LOX/LH2),Ground Su
4、pport Equipment.Center to Contact for Information: KSCImplementation Method: This Lesson Learned is based on Reliability Practice number GSE-3010 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:The major benefits for the use of oil-free vacuum p
5、umps in the LOX/LH2 transfer system is that Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-these pumps eliminate the danger of oil or oil molecule contamination and can be safely operated unmanned. This results in both cost saving and a reliability
6、improvement in the pumping process.Implementation Method:The implementation of oil-free vacuum pumps has resulted in eliminating the danger of oil contaminants, increased reliability and has resulted in a reduced support costs.Technical Rationale:Oil-free pumps offer many advantages over the oil sea
7、led pumps. The oil-free pumps are light-weight, truly portable, highly versatile, and the absence of fluids means the pump can be operated in any orientation - sideways or upside down.There are a number of considerations that affect the decision of whether to use an oil-free or an oil-sealed pump. F
8、irst, and most important, is the determination of whether the application is sensitive to oil contamination. If it is determined that the application is oil sensitive, the next consideration is whether or not a foreline trap is sufficient to prevent the backstreaming oil from entering the process ch
9、amber in quantities that will cause contamination problems, if not then consideration should be given to use of oil-free pump.Oil-Sealed Type Pumps:The foreline traps (oil-sealed mechanical pump) is one type of pumping system that the LOX/LH2 system engineers considered for use in the LOX/LH2 transf
10、er system vacuum jacketed lines. The following paragraphs discuss some of the characteristics of the foreline trap pump and figures 1, 2, and 3 show the creeping surface that can cause oil contamination.Foreline or roughing traps have been used for a number of years in an attempt to stop or reduce t
11、he oil backstreaming from oil-sealed mechanical pumps. Although they have had great commercial success in terms of number of units used, they have a number of problems that need be weighed carefully if they are to be considered as a method of stopping the pump oil from entering the chamber.Three typ
12、es of foreline traps are commercially available:1. Condensation traps, Cryogenic2. Absorption traps, absorbent trapping medium3. Absorption traps, active surface trapping mediumProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-All have a potential prob
13、lem that needs constant consideration. They have room temperature walls which will allow liquid oil to coat the inner surfaces and once the oil has passed through into the system side of the trap, it becomes a vapor source that can no longer be prevented from entering the system. The amount of time
14、required for the surface oil to work its way through the trap will vary with the application. The best way of determining when oil is passing through is to occasionally remove the trap from the pumping line and placing a drop of water inside the tubulation on the system side to see whether it beads
15、or spreads. Once oil is detected on the system side, its time to solvent clean the trap or to bake out the entire trap, walls and all.The time to fully penetrate through the trap will be extended if the mechanical pump is shut off and the trap (fore or roughing line) is air released when the pump is
16、 not being used.refer to D descriptionD The LOX/LH2 System Engineers selected the oil-free type pump shown in figure 4, because this pump incorporates new vibration isolation techniques, new 3-stage pumping technology, simplified construction for easy maintenance and increased reliability. This type
17、 pump will probably have higher initial costs than an oil-sealed mechanical pump. Figure 4, shows a picture and Block Diagram of an oil-free pump.Oil-Free PumpsThe following paragraphs briefly discuss some of the characteristics and advantages of an oil-free pump.Unique design: The key to this type
18、pump performance is a unique three-stage design, which incorporates a molecular drag module backed by two customized diaphragm modules to provide a constant, dynamic flow of gas. Each module functions selectively in series, automatically carrying the pumping burden within its own pressure/flow envel
19、ope. As each successive stage acquires the Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-primary pumping burden, the previous stage continues pumping in a support capacity.Entirely oil-free: Since no module uses pump fluids, roughing or backing wit
20、h this type pump introduces no oil to backstream into a system. They require no traps, oil filtration accessories, or special oils as all mechanical pumps do. Since traps are unnecessary, oil-free pumps without a trap can deliver their full pumping speed. Mechanical pumps with traps inevitably exper
21、ience speed losses.Clean, efficient backing: These type pumps provide highly efficient backing for turbo or diffusion pumps, improving their performance by providing a lower foreline pressure. In most cases, an oil-free pump will help achieve vacuum up to an order of magnitude higher than mechanical
22、 type pump.Versatile: Oil-free pumps will pump any gas, including hydrogen and helium, and any condensables, e.g., solvents or water vapor. They never load up with “memory“ gases.Easy to use and repair: The oil-free pump requires no cryogenic or bakeout procedures. Because the molecular drag and dia
23、phragm modules are separate, each can be easily repaired in the field or, if necessary, completely replaced.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD References:1. KSC Problem Report Number S70-0817-00-001-0553, dated 5/
24、9/89Impact of Non-Practice: All oil-sealed mechanical pumps backstream oil to some extent. As the oil becomes hot from the operation of the pump and the pressure falls into the molecular flow regime hydrocarbon vapors will Provided by IHSNot for ResaleNo reproduction or networking permitted without
25、license from IHS-,-,-rise from the pump.Surface creep will also occur right out of the pump itself with the oil reservoir within the pump acting as the source. In many cases, this constant surface flow of oil from the mechanical pumps are the greatest single source of hydrocarbons that can enter the
26、 vacuum chamber.Oil-sealed pumps can go into “hydraulic lockup“ if a power outage would occur and the pumps have to be constantly manned.The use of an oil operated type vacuum pump has disadvantages from both a cost and reliability stand point. A single vacuum jacketed section recently contaminated
27、cost $4,031 when returned to the vendor for oil removal. This was the vendors charge only, and did not include shipping, removal and replace or testing cost.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-03-06a71 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-,-,-
