DESIGN AND TESTING OF CRYOGENIC CONFORMABLE VALVE SEALS

Michael Wood

doi:10.7273/000007240

Cryogenic valve sealing technology is notoriously more challenging than what is used for traditional fluids due to the combination of small atoms, extreme temperatures, constant cycling, and foreign object debris. Industry standards currently require valve seals to be evaluated with a designed allowable leakage measured by bubbles over a specified number of cycles but do not account for leakage created by foreign object debris. This thesis guides the reader on the design requirements for developing cryogenic valve technologies. The research then utilizes this information to explore a nascent sealing paradigm that uses multiple conformable polymer layers to conform to a mating surface in cryogenic conditions. Layering these thin (0.0254 mm to 3.175 mm) discs in series with a spacer allows maximum bend around the seat while creating multiple sealing surfaces to influence resilience to foreign object debris. This paradigm was first applied to static seals as pressure relief valve seats. An experimental test stand was developed to measure and evaluate leakage when submerged in liquid nitrogen, with and without foreign object debris, and then compared with off-the-shelf re-closable pressure relief valves. This experimental study found conformable seals to reliably measure less leakage exposed to foreign object debris at 2.45 bar and 10.3 bar from 15% to 27% and 13% to 63%, respectively. Seven individual tests with off-the-shelf valves showed leakage increasing over 10 cycles, whereas only 2 conformable seal designs show this trend. This gives weight to conformable seals providing a repetitive and reliable seal at cryogenic temperatures with resilience to foreign object debris. Conformable seals are then applied to dynamic cryogenic valve packing to remove the additional weight of thermal standoffs and evaluate the relationship between disc thickness and sealing performance. The chapter begins by exploring how this paradigm was chosen, followed by the necessary modifications to the test stand to record measurements. Results indicate disc thicknesses between 0.254 mm and 0.794 mm are viable with conformable technologies. However, trends are insufficient to determine the optimal disc size. At ambient temperatures, it was observed that leakage tended to increase with decreasing disc thickness. At cryogenic temperatures, the opposite was observed. The minimum recorded leakage in LN2 was 471.8 sccs with the 0.254 mm discs with a static system, so the 50 parts per million volumetric standard is exceeded. The primary reason for these results is that increasing the disc thickness only increased the friction between the stem and seal, not conforming, leading to seal failure. These experimental results are the first iteration of these paradigms and should be considered within this context. Findings also suggest that, with recommended improvements, this technology has the potential to be a soft seal that is in direct contact with the cryogenic liquid and provides resilience to foreign object debris. However, the following limitations for the conformable static seals that restricted this research include manufacturing of the sealing materials, surface roughness of the seal surfaces, spring sizing, pressure relief valve set pressure, measuring instrument range and bias, statistical analysis of the results, test stand flow capacity, and the volume between measuring devices and outlet. Improvements for conformable dynamic seals include gland fill volume, surface roughness of seal surfaces, measuring devices that measure in parts per million by volume, and stepper motors strong enough to actuate the assembly at 77K. Despite these constraints, conformable seal technology outperformed the off-the-shelf valves under real-world conditions when tested with the same procedures.

DESIGN AND TESTING OF CRYOGENIC CONFORMABLE VALVE SEALS

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