Failure analysis and optimization of high-pressure pipe in carbon dioxide heat pump air conditioning system

Abstract

Carbon dioxide heat pump air conditioning system is a promising technique due to its ODP of zero and GWP of one. However, high operating pressure induces extra challenges on refrigerant leak-proofness, limiting its practical applications. This study applied high-pressure metal pipes to enhance the leak-proofness of the CO2 heat pump system. The endurance of the metal pipes is increased by optimizing the thermal treatment process which can reduce the residual stress and optimize the grain size of the alloy. Based on a laboratory pipe leakage test, the failure mechanism was analyzed and clarified by macroscopic and microscopic inspection on the tested pipe. The results indicate that the cause of the leakage is the cracking of the metal corrugated pipe, which occurred at nearly half of the pipeline length. The failure mode of the corrugated pipe cracking is mainly bending fatigue damage caused by local stress concentration. At the same time, the unevenly distributed grain structure of the metal material is also one of the causes. By optimizing the thermal treatment process, the reliability of high-pressure metal pipes was improved. The gas tightness verification test results show that the average leakage rate can reach 0.00315 g/a. The temperature pulse cycle durability test can be conducted 250,000 times without leakage, which is five times better than before. The maximum tensile strength can reach 6.585 kN, and the bursting pressure can reach 85.79 MPa. This study demonstrates that the high-pressure metal pipes with optimized thermal treatment can overcome the leak-proofness challenge caused by the much higher pressure in carbon dioxide heat pump air conditioning system.