Mitigation of Rupturing LO2 Vent line
- Friday, 01 April 2011
Stennis Space Center (SSC) has performed extensive large-scale (200 klbf to 7 Mlbf thrust) rocket engine propulsion testing beginning with the Apollo program, extending through the Shuttle program, and presently supporting ongoing NASA and commercial future launch systems propulsion technology development. Part of the test facility infrastructure to support this testing are LO2 (liquid oxygen or LOX) propellant vent systems that must be able to accommodate a wide range of flow demands to support a broad range of engine testing programs.
The LO2 vent line at the SSC B Stand is made of aluminum pipe that runs horizontally for several hundred feet. The pipe was installed by welding a series of pipe sections. The yield strength is significantly reduced, by almost half, to 18-22 ksi (kilo psi), in the heat-affected zone after welding. Recently, an unforeseen problem occurred as a result of low cycle fatigue from repeated low levels of cryogenic flow through this LO2 vent system. The problem occurred when the vent duct ruptured along a joint weld approximately midway on the vent system.
The low cycle fatigue is the result of regular, thermally induced, structural bowing that occurs when the line is partially filled with LO2 during chill down operations prior to engine testing. The piping system bottom begins contracting as the LO2 starts to flow through the vent line. However, the top of the piping system does not contract at the same rate as the bottom since the top does not immediately experience the cryogenic temperature. The resulting difference in the pipe’s natural movement causes structural bowing and generates significant internal stress. This recurring low-cycle transient stress is exacerbated during the summer months due to the larger temperature gradients experienced in the piping system. (Incidentally, all three vent duct ruptures experienced in the past 10 years have occurred during summer months.)
Several mitigation approaches have been considered to date, such as replacing the aluminum pipe with stainless steel and/or insulating the duct, friction-stir-welding the joints, and extensive thermal conditioning of the vent duct before introducing cryogenic liquid flow. Low-cost, innovative solutions are sought for options to minimize or reliably accommodate the vent pipe stress resulting from the thermal differential cycling in the pipes and the elimination of the line ruptures.
The technologies or solutions developed must comply with all safety and quality standards in use at SSC and be cost effective for implementation.