[Objective] This study aims to solve the problems of easy failure of back pressure valve seals and poor stability of wide pressure difference pressure regulation in supercritical carbon dioxide closed cycle systems. 

[Methods] In this paper, sealing failure and pressure regulation characteristics tests were carried out to compare the service performance of two types of rubber seals, quantify the valve pressure regulation law and dynamic response, and provide experimental basis for sealing material selection and stable operation of high-pressure S-CO2 back pressure valve. Based on the self-built closed S-CO2 cycle experimental platform, the step-by-step pressure rise, pressure stabilization and pressure relief cycle tests were completed in the range of 5~30 MPa. The macroscopic damage morphology of nitrile rubber (NBR) and hydrogenated nitrile rubber (HNBR) O-ring after service was compared. The pressure before and after the valve, the temperature of the outer wall and the number of handwheels were collected synchronously. The linear relationship of pressure regulation, dynamic pressure fluctuation, pressure rise and fall hysteresis and throttling thermal effect were analyzed. The repeatability of pressure regulation was tested four times under the condition of 30 MPa. 

[Results] The NBR sealing ring produces bulges, cracks and holes under the action of high pressure CO2 infiltration, swelling and rapid decompression, and the failure mechanism is explosive decompression failure. The HNBR only swells slightly, the sealing surface is intact, and the ability to resist decompression failure is stronger. The pressure in front of the valve is approximately linear with the number of handwheel rings, and the adjustment slope is 3.56 MPa/ring. The pressure behind the valve fluctuates significantly under the high pressure difference of 30 MPa, and the maximum temperature difference of the outer wall of the valve body is 15.5 °C. When the pressure is increased, the pressure behind the valve goes down for a short time, and rises instantaneously when the pressure is relieved. There is obvious pressure regulation hysteresis in the process of pressure increase and decrease. HNBR is more suitable for high-pressure S-CO2 sealing conditions, and NBR cannot withstand 30 MPa long-term cycles. The back pressure valve can achieve basic pressure stabilization in the range of 5~30 MPa, and the linear adjustment law can be used to predict the opening of the project. The unsteady flow of the fluid under high pressure difference, the friction of the valve core and the nonlinear thermal properties of S-CO2 together aggravate the fluctuation and hysteresis. It is necessary to optimize the throttling structure, sealing pair and thermal management scheme to improve the long-term operation reliability of the valve. 

[Conclusion] The research results can provide experimental basis for the selection of sealing materials, structural optimization, and operational stability evaluation of high-pressure S-CO2 back pressure valves.