This invention is a method and design for the conveyance of instrumentation lead wires from one pressure boundary to another pressure boundary in cryogenic process systems. Such a device or article is commonly referred to as a feedthrough. The novelty of the present invention is the extreme low-temperature conditions commensurate with extreme leak-tightness requirements that are managed by a relatively simple and economical approach. The design is directly applicable to any process system or instrumentation device operating below approximately 300°F. The novel feedthrough design is very cost-effective and easy to produce, yet provides solutions to sealing problems under severe conditions or for extremely demanding requirements.
The feedthrough design includes a set of process fluid pressure fittings modified to accommodate the lead wires sealing package. For one design, the fittings include a back-to-back union fitting with a tube extension, a tee or other male fitting, and a plug type fitting. In this case, VCR fittings with copper (or silver-plated nickel) gaskets were used. The feedthrough package consists of, from bottom to top, a reaction pin with potting cavity, reaction bearing, lower load pin, packing stack, upper load pin, and plug with potting cavity. The packing stack consists of packing buttons made from expanded PTFE (Teflon) foam sheet material. The dimensions are proportional to the diameter of the flow area of the tube and fittings and the compressed thickness of the stack. The buttons are punched out of the sheet material and stacked in a preferential (common) direction with the same orientation. A jig tool is used to further punch the hole(s) in each button for the wire/feed. Each button is punched individually, or systematically staggered during assembly, to later effect the proper sealing function.
The feedthrough is then built into the lower fitting, from bottom to top. Optionally, potting in the lower section (reaction pin) and/or the upper section (plug) can be done using a suitable two-part epoxy. The lead wires are sealed by the packing stack by its compression between the two load pins. The number of buttons and lengths of load pins are tailored to the size and type of fittings that are used. Complete hermetic sealing is provided by the packing stack; any potting is entirely optional for providing additional leak barriers. Without the potting, the feedthrough can be easily rebuilt for a different configuration (new packing buttons are required).
The fittings are torqued to normal values for the completion of the assembly, which is then installed in its intended location. In the present example, four assemblies were installed into a heat exchanger system within a 33,000-gallon vacuum-jacketed liquid hydrogen tank. These assemblies house silicon diode sensors that measure in-line flow temperatures of the cold helium refrigerant. The heat exchanger system is fed by a large cryogenic refrigeration system (850 W at 20 K) located adjacent to the tank. The refrigeration system is used to thermally condition the liquid hydrogen, allowing it to be effectively managed for future cryofuel applications.
Control and monitoring of the heat exchanger requires temperature sensors at strategic locations along the process flow lines mounted within the large storage tank. This provision requires a penetration between the cold helium and liquid hydrogen pressure boundary (a 1⁄4" stainless steel tube) so that the sensor wires can be successfully routed through a port on the tank and connected to the computer data system. This novel feedthrough design provides an efficient, cost-effective solution for extreme conditions and demanding requirements, and has been fully demonstrated in field applications down to temperatures of 20 K and vacuum pressures below 1 psia.
This work was done by James Fesmire and Adam Swanger of Kennedy Space Center. For more information, contact the Kennedy Space Center Technology Transfer Office at 321-867-7171. Refer to KSC-13956.