## 5 Conclusions  
With the aim of real-time monitoring and early detection to prevent catastrophic consequences of piston rod fracture, this work proposed an AE-based approach for crack propagation monitoring and the early warning of reciprocating compressor piston rod fractures, with the hope of contributing to ensuring the operational safety of core process gas equipment in UGS. Several conclusions can be drawn. 1. Accelerated life experiments have effectively reproduced the occurrence of piston rod fracture accidents and their subsequent effects. In one particular scenario, the fracture of the piston rod resulted in secondary effects, such as the impact, deformation, and further fractures at the fracture site. In addition, the inertia of the piston could cause it to collide with the cylinder wall, resulting in cylinder destruction and a significant leakage of compressed gas. The severity of these failures highlights the factors that contribute to the potential explosion in a natural gas compressor resulting from a fractured piston rod, and emphasizes the importance of real-time crack detection and monitoring. 2. The AE signal in the 70-150 kHz range is sensitive to the appearance and intensive development of crack propagation within an operating cycle. In the cyclic operation of a compressor, the appearance of transient impact-type AE crack signals is correlated, in the angular domain, with the moments of switching between piston rod load stretching and compression, and with the moments before the piston rod is subjected to the peak of the tensile load. 3. The AE count, energy, peak-to-peak, and impulse factor show a sensitive response to cracking when the crack enters a rapid propagation phase. Qualitatively, these parameters can be used as effective indicators for early crack identification and the warning of piston rod fracture failures during compressor operation. The crack propagation warning can be provided approximately 10,000 cycles before fracture, providing sufficient operating time to shut down the compressor for maintenance. In addition, it is expected that further research will enable the application of this methodology to other fracture-prone compressor components, such as valve plates, crosshead sliders, connecting rods, bolts, crankshafts, etc. 4. The linear relationship established between the rate of change in the AE energy and the crack growth rate was demonstrated, and a predictive model for the crack growth rate was established. This model proves beneficial for the early-warning and real-time assessment of the severity of piston rod cracking failures. Quantitatively, the calculation of the crack propagation length at specific time intervals allows comparison withthe critical fracture size of the material, thus allowing the assessment of the urgency of immediate shutdown, serving as an effective tool for diagnosing compressor failures and supporting predictive maintenance in underground gas storage (UGS) facilities. Conceptualisation: X.L., X.P. and Y.G; methodology: X.L. and Z.C. ; software: X.L. ; validation: [PERSON] and S.W. ; writing--original draft preparation, X.L. ; writing--review and editing, [PERSON] and X.P. ; visualisation: [PERSON] and Z.C. ; supervision: [PERSON]; project administration: [PERSON] and X.J. All authors have read and agreed to the published version of the manuscript. This work was supported by the China Petrochemical Corporation Project \"Research on the core technology of the first and second compressors of a high-pressure polyethene plant\" (nos. 3000000-22-ZC0607-0605 and 13300000-22-ZC0607-0030). No relevant Institutional Review Board Statement and approval number. This study did not involve human research. This work was supported by the China Petrochemical Corporation Project \"Research on the core technology of the first and second compressors of a high-pressure polyethene plant\" (nos. 3000000-22-ZC0607-0605 and 13300000-22-ZC0607-0030). No relevant Institutional Review Board Statement and approval number. This study did not involve human research.