The 6 major component structures of reciprocating compressors, 5 major monitoring methods, and 10 major troubleshooting approaches!

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When a reciprocating compressor is in operation, the crankshaft drives the connecting rod, which in turn drives the piston, causing the piston to move up and down. The movement of the piston causes changes in the volume inside the cylinder. When the piston moves downward, the volume of the cylinder increases; the intake valve opens while the exhaust valve closes, allowing air to be drawn in, thus completing the intake process ； As the piston moves upward, the cylinder volume decreases, the exhaust valve opens, and the intake valve closes, completing the compression process. Typically, a piston ring is installed on the piston to seal the gap between the cylinder and the piston, with lubricating oil inside the cylinder serving to lubricate the piston ring. Due to the complexity of the reciprocating compressor structure, there are many components that can fail, and the causes of these failures vary. The characteristic parameter signals of reciprocating compressors mainly include thermodynamic signals, vibration signals, and noise signals, among which the thermodynamic signals encompass the temperature of various components, exhaust volume, exhaust pressure, and pressure inside the cylinders. The core idea of fault diagnosis technology is to identify and determine the type of compressor failure by monitoring and analyzing characteristic signals. The six main components of a reciprocating compressor. A reciprocating compressor is a type of positive-displacement compressor; its key components include the cylinder, crankshaft and connecting rod mechanism, piston assembly, packing (which serves as the seal for the compressor), gas valves, the compressor housing and foundation, as well as pipelines and auxiliary equipment. 1. Cylinder: The cylinder is one of the main components of a compressor. It needs to have a good surface quality to facilitate lubrication and wear resistance, as well as good thermal conductivity, so that the heat generated by friction can be dissipated as quickly as possible ； There must also be a sufficiently large area for the air flow channel and for the installation of the air valves, so that the volume of the valve chamber is sufficient to reduce the amplitude of pressure fluctuations in the air flow, thereby ensuring the proper operation of the air valves and reducing power consumption. The clearance volume should be smaller to improve the efficiency of the compressor. 2. Crank-connecting rod mechanism: This mechanism includes a crosshead, connecting rods, crankshaft, and guides—it is the main component responsible for motion and transmission, converting the rotational motion of the motor into the reciprocating motion of the piston; it is also the primary component that bears forces. (We recommend that you follow the \"Mechanical Engineer\" official account to get valuable knowledge and industry updates as soon as they become available.) 3. Piston assembly: It mainly consists of the piston head, piston rings, thrust plate, and piston rod. The shape and size of the piston are closely related to the cylinder, and it is divided into double-acting and single-acting pistons. Piston rings are used to seal the high-pressure gas inside the cylinder, preventing it from leaking through the gap between the piston and the cylinder. As the name implies, the role of the towar is to support the piston; therefore, it is also a wear-prone component. The quality of the material used for the towar directly affects the service life of the compressor. 4. Packing: The piston rod packing is primarily used to seal the gap between the cylinder bore and the piston rod, preventing gas from leaking radially along the piston rod. The manufacturing and installation of the packing ring involve “three clearances”. They are respectively the axial clearance (to ensure that the packing ring can float freely within the ring groove), the radial clearance (to prevent the packing ring from being compressed and deformed or damaged due to the sinking of the piston rod), and the tangential clearance (used to compensate for the wear of the packing ring). Currently, planar packing is mostly of the “three-six petal type” and “tangentially notched three-petal type”. 5. The air valve is the most important component of a compressor, as well as the part that is most prone to damage. The quality of its design directly affects the compressor’s displacement, power consumption, and operational reliability. A good air valve should possess the following characteristics: high efficiency and energy savings (accounting for 3%–7% of the shaft power), a perfect combination of airtightness and timely operation, a long service life (typically 8000 hours in practice), a small volume of clearance space, low noise levels, minimal temperature rise, and the ability to be refurbished for reuse. Currently, the materials used for air valves are divided into metallic and non-metallic types. As things stand now, valve discs made of non-metallic materials are being used more and more frequently. 6. Piping and auxiliary equipment: The rationality of the design of the compressor’s piping and the inlet and outlet buffers has a direct impact on the vibration levels of the unit. Five major methods for fault diagnosis and monitoring of reciprocating compressors. Currently, the main methods for fault diagnosis and monitoring of reciprocating compressors include the following: 1. Thermal performance monitoring method. Temperature is a characteristic parameter that is quite sensitive in reciprocating compressors; by monitoring temperature changes, it is possible to determine the operating condition of the internal components of the compressor. For example, if there is a leak in the exhaust valve, backflow will occur during the intake process, resulting in an increase in the temperature of the valve ； If the piston rod is damaged, the temperature of the stuffing box will also rise, among other things. When using temperature monitoring methods, the sensor can be placed on the outside of the machine body, without the need to modify the housing structure, making it easy to operate. One operating cycle of a reciprocating compressor includes four processes: suction, compression, discharge, and expansion. The pressure undergoes periodic changes during these four processes, and the pressure variation curve inside the cylinder can directly indicate whether the compressor is operating properly. If the intake valve leaks, the pressure during the intake process increases, the exhaust process shortens, and the expansion process curve also shifts downward. Since the pressure measurement point is located inside the cylinder, mounting holes must be provided at the cylinder head or other locations on the housing; this is something that requires special attention for pressure monitoring purposes. 2. Vibration monitoring method: Vibration signals are also sensitive characteristic parameters for diagnosing faults in reciprocating compressors; most faults, such as damaged air valves, sinking piston rods, loose crosshead bolts, and worn connecting rods, are accompanied by abnormal vibration signals. Thanks to the increasingly sophisticated signal analysis techniques, there is more and more research on the unsteady vibration signals of reciprocating compressors. For example, using acceleration sensors to measure vibration signals at locations such as the crosshead slide box, cylinder sidewalls, cylinder head, and bearings is an effective method for diagnosing performance-related faults. 3. Displacement monitoring method: The fracture of the piston rod in a reciprocating compressor usually leads to damage to other components, and in severe cases, it can even cause the entire unit to explode. The breakage of the piston rod occurs suddenly; it is very difficult to detect cracks before the breakage occurs, and analysis can only be carried out on the broken area after the fact. At present, there are no reliable and effective diagnostic warning methods available. By installing displacement sensors to monitor the settlement of the piston rod, it is possible to indirectly assess the wear condition of piston rings, crossheads, etc., which can serve as an auxiliary method. 4. Oil monitoring method: Oil monitoring involves analyzing the lubricating oil in compressors to determine the size, shape, and composition of wear particles present in the sample; it is a relatively effective supplementary technique. Methods such as ferrography, spectroscopic analysis, and particle counting can be used to monitor the wear of the moving parts in air compressors. Some scholars, by testing the copper content in the oil, discovered that the breakage of the large-head tile had successfully prevented an accident from occurring. 5. Noise monitoring method: Noise signals include those generated by the operation of mechanical equipment, as well as those from the surrounding environment and other noise sources; therefore, noise monitoring can also serve as a reliable auxiliary tool in the fault diagnosis of reciprocating compressors. Combining advanced noise sensors to separate and extract typical fault noise signals is a key focus and challenge in future research on the field of reciprocating compressor fault diagnosis. There are a wide variety of faults in reciprocating compressors, and one fault can cause changes in multiple characteristic parameters. Therefore, during fault diagnosis, it is necessary to take into account the relationships between these various parameters in order to more accurately identify the type of fault. In addition, artificial intelligence systems and neural network technologies are also being increasingly used in reciprocating compressor fault diagnosis systems, raising fault diagnosis technology to an intelligent level. Classified by the different components of reciprocating compressors that cause accidents, the proportions of various types of failures are shown in the figure below. Among them, the failure probability of intake and exhaust valves is the highest, at 36% ； Secondly, accidents caused by stuffing boxes, connecting rods, piston rods, etc. also account for a significant proportion. Table 1 shows the proportion of various faults in reciprocating compressors, as well as the monitoring and diagnostic methods used, providing a reference for the fault diagnosis of reciprocating compressors. 10 Common Faults of Reciprocating Compressors 10 and Their Solutions 1. Leakage in the suction valve or damaged gasket. Leakage in the suction valve or damaged gasket is characterized by: (1) increased temperature and heating of the valve cover ； (2) The temperature of the corresponding exhaust valve increases ； (3) Increase in pressure between the stage where the valve is located and the previous stage ； (4) Decrease in compressor discharge volume ； (5) Increase in intake air temperature. After compression, the temperature of the gas rises. If the intake valve leaks or the gasket is damaged, the hot gas returns to the intake chamber, causing the valve temperature to increase as well as the intake temperature; this in turn leads to an increase in the exhaust temperature after further compression. Furthermore, the backflow of compressed gas causes pressure to rise ahead, and the higher the pressure, the more the exhaust volume decreases. 2. Exhaust valve leakage or damaged gasket The problems associated with an exhaust valve leakage or a damaged gasket are mainly manifested as: (1) An increase in the temperature of the exhaust valve, with the valve disc becoming hot ； (2) Drop in exhaust pressure ； (3) The compressor’s discharge volume decreases. Due to a leak in the exhaust valve or damaged gasket. During the intake process of the cylinder, some of the compressed, high-temperature, high-pressure gas flows back into the cylinder, raising the temperature of the mixture. Upon being compressed again, the temperature rises further. This backflow also results in a decrease in flow rate and a drop in exhaust pressure. 3. Jamming of the load adjustment mechanism: The jamming of the load adjustment mechanism is mainly manifested as: (1) The load adjustment indicator does not function ； (2) The temperature of the corresponding intake valve rises, and the valve cover heats up ； (3) Corresponding increase in exhaust valve temperature ； (4) Increase in pressure between the stage where the valve is located and the previous stage ； (5) Decrease in compressor discharge volume ； (6) Increase in intake air temperature. If the load adjustment mechanism gets stuck in the unloaded position, it can cause leakage at the intake valve ； If it gets stuck in the loading position, it will cause the compressor to start under load, affecting the service life of the transmission components. 4. Common faults of piston rings include: (1) Piston ring breakage ； (2) The piston ring has become stiff, lost its elasticity, and is unable to expand ； (3) Excessive wear of the piston rings, resulting in an increased clearance. The main manifestations of piston rings failing to provide a seal are: (1) an increase in the exhaust temperature at that stage ； (2) The exhaust pressure at this stage decreases ； (3) The compressor’s discharge volume decreases. In double-acting reciprocating compressors, where one side of the cylinder is compressing while the other side is drawing in air, if the piston rings are damaged or seized, they can no longer provide a seal. As a result, the high-pressure, high-temperature gas compressed on the cover side or shaft side leaks through the piston rings into the low-pressure, low-temperature gas on the shaft side (or cover side). It mixes with the low-pressure, low-temperature gas drawn in; after mixing, the temperature of the gas rises. Furthermore, as the compressed gas leaks past the piston rings, the exhaust pressure for that stage decreases, and consequently the compressor’s exhaust pressure also drops. 5. Particulate matter entrained in the process medium: On-site inspections sometimes reveal large amounts of deposits in the compressor cylinders and packing seal chambers. These deposits consist of fine solid dusts or coked carbon particles carried in by the process medium, and they often have a high hardness. Its deposition at the sealing chamber will inevitably cause severe wear of the sealing packing, thereby **reducing the service life of the packing seal ring and the piston rod. By adjusting the process to bring the compressor parameters up to the design requirements, and by using a gas-solid separator to remove such particulate contaminants when necessary, particle-induced wear between the cylinder and piston rings, as well as between the piston rod and packing, can be avoided. 6. Excessive clamping force on the combined sealing ring of the piston rod or loss of elasticity in the spring: In a reciprocating compressor, the piston rod and the packing seal are in relative motion; the packing ring seals the medium by gripping the piston rod, and the gripping force of this ring is provided by the spring as well as the radial pressure difference. Obviously, the greater the tightening force of the spring, the stronger the grip of the packing on the piston rod, and the more severe the relative friction between the piston rod and the packing ring. This results in more heat being generated due to friction, which causes a very high temperature rise at the beginning of use of the packing ring, as well as significant wear. Since packing rings are typically made of polytetrafluoroethylene, which has a high coefficient of thermal expansion, if the frictional heat generated in the initial stage is not removed in time, the packing ring undergoes significant thermal expansion and deformation. Coupled with the strong compressive force exerted by the ring springs, this leads to increased frictional wear, creating a vicious cycle. After just a few days of intense wear, as the gripping force of the packing on the piston rod decreases, that is, as the friction force diminishes, the gap between the packing ring and the piston rod increases, leading to an increase in fluid leakage and ultimately to failure of the seal. The solution is to, without changing the overall structure, replace the piston seal rings to adjust the gap between them and the cylinder block; or use materials with better self-lubricating properties and wear resistance for manufacturing the piston rings and packing rings. Additionally, the tension of the springs can be reduced appropriately, and a clearance seal can be incorporated into the design. Most spring failures are caused by spring fatigue; quality issues with the springs account for only a small proportion of cases, and in such situations it is necessary to replace them with springs of better quality. 7. The cooling water flow rate to the packing seal box is too low. The temperature rise in the area of the packing seal box is mainly caused by intense friction between the packing rings and the piston rod, and this frictional heat needs to be removed promptly. In fact, since both the water used for the packing seal box and that used for the cylinder liner are supplied in parallel, a high pressure drop occurs at the packing seal, which results in insufficient flow of cooling water to the packing box. As a consequence, the frictional heat cannot be removed in time, affecting the normal service life of the packing. Therefore, the pressure and flow rate of the circulating water should be increased appropriately to allow it to carry away in a timely manner the heat generated by the friction between the piston rod and the packing ring. Keep the temperature at the packing seal box at no more than 60°C. 8. The amount of oil applied at the packing seal is either too small or too large. An excessive amount of oil can lead to excessive emulsification, resulting in the formation of deposits ； If it is too small, the lubrication effect of the packing ring is poor, the wear rate increases, and this affects its service life. In addition to injecting the oil according to the manufacturer’s standards, an appropriate amount should also be determined at the beginning of testing by checking the operation at the seal ring. After the test run, check whether there is any carbon-like substance on the piston rod at the packing to determine the amount of oil injected. 9. Failures caused by pipeline vibration. There are mainly two types of reasons that cause vibration in reciprocating compressors and their pipelines: one is due to an improperly designed foundation that results in uneven vibration of the unit. During the assembly process, compressors can suffer from significant assembly errors due to technical or quality issues, which leads to a deterioration in the unit’s balance and results in vibration. An excessively small mass of the compressor foundation can also cause vibration in the compressor itself. Another type is caused by the pulsations of the gas flow within the pipeline, which lead to changes in the suction and discharge volumes of the piston compressor; this results in pulsations in the gas flow – gas columns are formed when the compressor pipeline is filled with gas. This gas column is an elastic vibration system with continuous mass, and it begins to vibrate under certain operating conditions – at the elbows in the unit’s piping system, the direction of gas flow changes, which in turn subjects the pipes to gas-induced forces. If there are too many bends in the system pipes, the impact force on those pipes will be very high. If there is a lack of a fixing support at the elbow, severe vibration will occur. When the fluid flows steadily, the pipeline does not vibrate ； However, when the direction of fluid flow changes at the abrupt change in the pipeline cross-section, the fluid velocity changes, which in turn alters the forces acting on the pipeline – causing local pressure variations within the pipeline and resulting in certain pulsations that induce vibration. If pulsations are present in the pipe, the pressure in different sections of the pipeline will vary, which can also create sources of vibration. Due to the large number of elbows in the piping system, the fluid constantly changes direction as it flows through the pipes, exerting stress on them ； Moreover, the state of the fluid itself also changes—vibrations induced by these changes occur resonance when their frequency coincides with the natural frequency of the piping system. Measures taken: The most basic way to eliminate resonance is to reduce the pulsating pressure of the airflow and keep it within the allowable minimum value, so that the excitation frequency does not equal the natural frequency of the pipeline. The specific methods include: (1) Installing a buffer tank at each inlet and outlet of each stage of the compressor, which changes the natural frequency of the gas column in the piping system, prevents the vibration frequency of the source from coinciding with that of the piping system, and can reduce the amplitude of gas flow fluctuations. However, an improperly designed volume for the buffer tank can also cause vibration; experience shows that it should be 10 times larger than the cylinder’s stroke volume, and as close to the cylinder as possible ； (2) Add fixed supports at appropriate locations along the piping system, especially at the bends, and insert rigid rubber sheets between the pipes and the supports in order to alter the elasticity of the supports and change the vibration frequency of the piping system ； (3) Orifice plates are installed at appropriate positions along the pipeline to change the vibration frequency of the system. Using orifice plates for vibration reduction results in significant pressure losses; therefore, they are only used in cases where resonance has occurred and it is not possible to alter the pipe diameter, and their effectiveness is far inferior to that of surge tanks. 10. Cylinder collision: Cylinder collision is a serious and severe accident in reciprocating engines. It is characterized by loud knocking sounds inside the cylinder, and in severe cases it can cause extensive damage to the engine, such as the cylinder head being ejected, the bearing shells breaking, or even an explosion occurring. Cylinder collision occurs in two forms: water hammer and metal impact. The sound produced by water hammer is somewhat muffled compared to that of metal impact, but the consequences are equally severe. Preventive measures mainly involve standardizing daily operations to avoid large amounts of liquid present ； Strengthen inspections and promptly investigate any abnormal noises.