###### Abstract  
Ensuring the reliability of process gas compressors is critical for underground gas storage, as piston rod fractures can lead to serious accidents, such as natural gas leaks or explosions. On-time monitoring and early detection play a vital role in preventing catastrophic consequences, minimising costs, and reducing production losses due to unplanned downtime. This study presents a novel accelerated life-testing method designed to replicate the fracture events of reciprocating compressor piston rods. By accelerating the induced crack initiation and propagation to the final fracture, comprehensive analyses of the fracture results are performed to reveal the piston rod fracture mechanism and the resulting secondary damage to the unit. The research further presents an innovative approach for identifying piston rod crack propagation by means of acoustic emission. Through kinetic analysis and time-frequency domain analysis, the study elucidates two mechanisms responsible for triggering crack signals during the compressor operation: the contact impact between the crosshead pin and the bearing due to the piston rod load reversal, and crack propagation occurring before the maximum tensile load is reached. In addition, the study identifies the piston rod crack expansion signal frequency band and achieves a high-sensitivity identification of crack dynamic growth by extracting signal sub-band features associated with crack propagation. Then, a prediction model of the fatigue crack growth rate was established based on the AE energy release rate, which provides a quantitative assessment of dynamic crack propagation during compression. This method aims to provide a maintenance strategy for piston rod fractures, thereby increasing the operational safety of critical dynamic equipment in underground gas storage. process gas compressor; piston rod fracture; underground gas storage; acoustic emission; crack propagation +
Footnote †: journal: Applied Sciences
The process gas compressor system designed for UGS has distinctive characteristics, characterised by a wide range of gas injection volumes, significant variations in the gas injection pressure, and frequent transitions between the gas injection and withdrawal. In this context, reciprocating compressors emerge as the preferred pressurisation equipment for natural gas storage systems due to their ability to achieve high-pressure ratios, optimum efficiency, increased flow rates, and adaptability to a wide range of operating conditions [6; 7]. However, the use of reciprocating compressors presents challenges associated with unplanned downtime and significant maintenance costs. These challenges stem primarily from the frequent and abrupt nature of core component failures, including failures such as valve leakage and fractures [8; 9], bearing bush wear issues [10; 11], and piston rod fractures [12]. In particular, piston rod fractures account for a significant proportion of mechanical failures in reciprocating compressors, estimated at around a quarter of reported incidents [13]. The consequences of fractured piston rods can be severe, ranging from catastrophic chain reactions to extreme outcomes, such as explosions and fires, highlighting the need for the rapid and accurate detection of such failures [14; 15; 16]. There is a pressing need for effective solutions that facilitate the real-time monitoring and early warning of faults and failures during the operation management of machine systems [17]. While considerable attention has been paid to compressor fault diagnosis [18; 19; 20] to distinguish abnormal from normal conditions [21], the intricate character of reciprocating compressors presents significant challenges. These challenges arise from the complex structural and motion patterns inherent in such mechanical devices, resulting in signals with multiple sources of influence [22; 23]. The existing literature on this topic is limited, with few studies dealing with the diagnosis of piston rod fractures during operational phases. [PERSON] et al.