Cryogenic Valve Seal

 
Current fluid control units operating in European and worldwide launcher propulsion systems are mainly hydraulically or pneumatically activated. Recent studies on system level performed in the US by NASA and companies involved in space propulsion activities show substantial advantages of electro-mechanically actuated valves. The study results are valid for a wide range of pressure levels - from several up to 500 bar - and temperature levels including cryogenic operation. The evaluation criteria applied: Performance, Mass, Cost (development, production, operational).

The primary problem in the design of a valve for fluid control and shut-off application is to achieve the specified leakage requirement under all operational conditions. The leakage is a direct function of the seat loading stress, the surface finish - assuming a continuous contact between the poppet and seat over the sealing surface - and the fluid itself.

Objective(s)
The objective of this study is to investigate the potential of new materials (metals or others) and sealing designs combining the compatibility with a cryogenic environment. This includes a moderate sealing force and a good oxygen compatibility at high pressure. The proposed study defined possible candidates including material pairings and design concepts. Respective tests were performed in a relevant environment to gain experimental data in order to judge on further application options.

Achievements and status
Characterizing materials able to improve cryogenic high-pressure valve sealing by using a reduced actuator force, which is the key factor to switch for electromechanical actuated valve. Only the Nitinol and the Scalmalloy achieved a good tightness with an actuator force reduction for each pressure and temperature cases. By choosing an appropriate design and using a Nitinol seal, it would be possible to reduce the closing force up to 50 % and to reach very good performance even at extreme conditions: cryogenic temperature, pressure up to 500 bar and large diameter valve. We investigated the potential of new materials to reduce the sealing force, which is the main driver to switch for electrically actuated valve. The project proves that designs using metal seals are comparable or even better than soft design. In view of the NGL design, which means to have high-pressure operation at the valve inlet, soft material does not fit requirement in view of mechanical properties therefore, metallic seat seal material is needed to comply with the requirements. For this project, only one material option was used in order to check the basics as concept proof. Three types of alloy (Nickel-Titane alloy, Aluminium alloy with scandium and Bulk metallic glass) appear to be of interest to find an optimized solution for future applications.

Benefits
The activities aim for preparing the next European launcher - Ariane 6 - in a competitive world-wide environment. Recent studies and investigations especially in the US resulted in a substantial advantage of motor driven valves compared to pneumatic and hydraulically driven valves. The investigation has shown that especially operational cost at the pad are substantially lower in case of using electrically actuated valves. A further advantage is the fact that to get rid of supply fluids like helium and nitrogen on the one hand will reduce system complexity because of the possibility to cancel components like e.g. latching valves. In addition, the non-availability of the fluids itself including necessary tanks will contribute to reduce the launcher mass.

Next steps
Astrium and ET propose to continue the activities and further evaluate the performance of additional sealing and seat materials. The materials proposed are part of the list provided to ESA\ESTEC in the frame of part one of the running program. The goal of the investigation is to optimize design solutions for future motor driven valves operating at high pressure under a cryogenic environment in order to lower further life cycle cost and mass.