## 1 Introduction  
Large-scale reciprocating compressors are widely used in fields such as oil refining, the chemical industry, and natural gas transportation. During operation, the flow of a compressor must be adjusted according to changes in the process requirements [1]. Stepless flow control systems, based on the reflux of the suction valve, are widely used because of their wide adjustment range and small pressure fluctuation at the back end, which saves a significant amount of energy for the compressor [2]. However, because the system changes the dynamic behavior and gas flow characteristics of the valve, the compressor deviates from the normal design conditions, resulting in an increase in the valve failure rate [3]. As an important part of the compressor, the valve's motion characteristics directly affect the operating efficiency and reliability of the compressor [4]. [PERSON] [5] observed that the movement characteristics of a valve can be controlled by changing the key parameters of the unloader. Therefore, optimizing the parameters affecting the unloader and the system process parameters is vital to extending the service life of the valve and reducing the operating cost of the compressor. The stepless flow control system of a reciprocating compressor, based on the operating principle of the top-opening suction valve, is primarily composed of a mechanical system, an electronic control system, a hydraulic system, and the gas system of the compressor. Coupling and parameter interactions exist between the subsystems. The suction valve is the coupling point between the mechanical and compressor gas systems. The control system changes the movement characteristics of the suction valve through the mechanical system, which causes changes in the thermodynamic parameters and flow characteristics of the compressor gas system. Many research results based on the thermodynamic parameters and flow characteristics of compressors under the stepless flow control of the top-opening suction valve have been published. [PERSON] [6] established a theoretical model of a reciprocating compressor under stepless flow control conditions, obtaining the thermodynamic changes of the gas in the cylinder under different load conditions, and performed experimental verification. [PERSON] [7] established a reciprocating compressor valve dynamics model, based on the Lee-Kesler real gas state equation, and analyzed the valve action characteristics under variable flow conditions. [PERSON] [8] combined the suction valve of a reciprocating compressor, a hydraulic plunger cylinder, and an unloader to establish a comprehensive mathematical model under different load conditions; they analyzed the change in thermodynamic parameters in the cylinder and energy loss of the valve under different loads. [PERSON] [9] used computational fluid dynamics technology to establish a fluid domain model of a reciprocating compressor under variable load conditions; they analyzed the pressure change of the gas system under different loads and the influence of the valve return clearance on the thermodynamic parameters of the gas system. With respect to the influence of system control parameters on the operation state of the valve, [PERSON] [10] analyzed the influence of the inlet pressure of the hydraulic system on the action of a suction valve to optimize system adjustment parameters, such as the hydraulic pressure output of the hydraulic system. [PERSON] [11] obtained relatively reasonable values by quantitatively analyzing the hydraulic pressure of a hydraulic system, the influence of the displacement of a valve, and the speed of a compressor on the performance of the compressor. [PERSON] [12] conducted a finite element analysis of the suction valve of a reciprocating compressor under different loads. The decrease in the load caused an increase in the valve retreat speed, and the stress concentration was significant.