Optimization and System Integration of EHD Seal for Nuclear Supercritical Power Cycles

Supercritical CO2 (sCO2) power cycles are superior to traditional water based, air-breathing, direct-fired, open Brayton cycles or indirect-fired, closed Rankine cycles in terms of energy efficiency and equipment size. To unlock the great potential of sCO2 power cycles in nuclear power production, technology readiness must be demonstrated at the scale of 10 ľ 600 MWe and sCO2 temperatures and pressures of 350 ľ 700 ºC and 20 ľ 35 MPa. Amongst many challenges at the component level, the lack of suitable shaft seals for sCO2 operating conditions needs to be addressed for DOEĺs next generation nuclear turbine and compressor development. Contact type sealing technologies, such as brush seals and compliant foil seals operate on metal-to-metal contact mechanisms, which are prone to wear and cannot withstand the delta pressures in sCO2 power cycles. ôLift offö mechanical face seals run in the hydrodynamic lubrication region when the shaft rotates at high speed. However, even for well-balanced ôlift offö seal, the hydrodynamic ôlift offö effect decreases with the increase of sCO2 pressures and can barely sustain continuous sCO2 film under those extreme sCO2 pressures and temperatures. Recently, Ultool, LLC, partnering with Sandia National Lab (SNL) developed an elasto-hydrodynamic (EHD) shaft seal for sCO2 cycles, which forms tighter sealing under higher pressure. A 2ö prototype EHD seal was successfully demonstrated at SNLĺs 2ö sCO2 seal test rig. While the seal clearly demonstrated the self-tightening EHD sealing mechanism, lessons were learned that high Youngĺs Modulus of the prototype material (Inconel 718) requires thinner seal sleeve wall (<2mm) for EHD deformation. This poses a challenge in manufacturing and vibration control. In addition, Inconel 718 also has limited wear-resistance. To that end, we propose a totally new EHD seal prototype to address the issues. The proposed solution firstly replaces Inconel 718 with carbon graphite, whose Youngĺs Modulus is approximately 1/10 of that of Inconel 718. This can increase the EHD seal wall thickness from 2mm to 20 mm, a huge improvement in manufacturability and anti-vibration capability. Carbon graphite is also an excellent self-lubrication material that will reduce the wear of the shaft. To further enhance the EHD seal, the Team will leverage Ultoolĺs patented multi-sleeve EHD seal and start seal for system integration. The benefits are: 1. Carbon graphite EHD seal provides a magnitude thicker seal sleeve (~20mm wall) that enables EHD seal for nuclear power cycles. 2. Thick-walled carbon graphite EHD seal can be easily integrated into sCO2 turbomachinery and provide stable operation under vibration conditions. 3. Carbon graphite material will reduce shaft wear. 4. Multi-sleeve EHD seal and start seal further optimize the EHD seal performance for nuclear turbomachinery. Phase I will begin with mapping the sCO2 nuclear power cycle specifications against the optimized carbon graphite EHD seal metrics, followed by extensive numerical nonlinear CFD analysis and tribological material study. A 2ö proof-of-concept prototype seal will be developed and tested. Phase II carbon graphite EHD seal prototype will be designed to fit into SNL 2ö sCO2 seal test rig.