Case analysis of compressor seizure

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Case: The LG-6.2/8G twin-screw compressor head used by a food processing company of a certain brand from Zhejiang seized up, preventing the equipment from operating properly. It was determined that the screw in the head had stuck, and upon inspection it was found that the main cause of the jamming was a crack in the internal bearing support frame. 　　The most serious consequence of a compressor failure is the sudden \"stall\" of the main unit. Once such a situation occurs, if the electrical protection system does not respond in time or fails to provide protection, it can cause severe damage to the main motor and the electrical system. Regarding the handling of a ‘stuck’ main unit, on one hand, the repair costs for a major overhaul will be much higher than those for a regular overhaul; on the other hand, since the components of the main unit are damaged, its overall performance after repair will also be lower compared to that of a main unit that has undergone a regular overhaul. If the damage to the host is so severe that it is no longer worth repairing or cannot be repaired at all, the only option is to discard it and replace it with a new one, resulting in even greater losses. The cost of a newly purchased host is usually about one-third of the cost of buying an entire compression unit, which is far higher than the cost of overhauling a host under normal circumstances. Under normal conditions, the overall technical performance of a host that has been overhauled is very similar to that of a new host! 　　We should understand that even the highest-quality compressor units can, over time or due to improper use, have a certain chance of experiencing jamming in their compressors. While the jamming of the compressor head is indeed related to the quality of the compressor itself, it is not the only factor; the influence of other objective factors cannot be ignored either. Therefore, in order to reduce costs for businesses, it is necessary to analyze and address other causes that lead to the jamming or sticking of the compressor head beyond the compressor itself. Identifying the cause and then taking appropriate action is necessary to better protect the compressor. 　　The screw compressor main unit is the most important component of the system, and it mainly consists of the casing assembly, a pair of intermeshing male and female rotors, bearings, shaft seals, etc. Since the compressor head is the most important component of the unit, and all the components inside it are highly precise moving parts, it is not allowed for any solid debris to enter the unit during operation, in order to avoid malfunctions that could affect its performance. The engine is the core of the unit; to maintain its functionality, in addition to ensuring the necessary clearance for movement, it is also necessary to have a suitable working environment, a proper oil supply and lubrication system, as well as an appropriate amount of fuel injection. 　　When a screw compressor is operating normally, there is no contact between the screws themselves, nor between the screws and the compressor housing, or between the screws and the high/low pressure ends. This is ensured by three main conditions: 1. The machining of the screws, the compressor housing, and the high/low pressure ends is carried out with high precision. This precision ensures that, after the host is assembled, there are appropriate gaps between the screws as well as between the screws and the host housing. The size of these gaps takes into account the fact that the screws and the housing may deform when the host operates at high temperatures ； 　　2. The gap between the screw and the high/low pressure end faces is ensured during screw assembly in accordance with technical requirements. During operation of the host, the screw does not experience any axial movement, while the allowable radial displacement of the screw is ensured by high-precision bearings ； 　　3. When the machine is operating normally, the oil film formed by the lubricating oil prevents direct contact between the screws. 　　In summary, we can see that the internal and external reasons that may cause the compressor screw compressor unit to get stuck could include: 1. Poor clearance at the exhaust end of the compressor head. During one or two heavy workdays when the user encounters freezing issues, the fault can be easily identified by measuring the clearance values during disassembly and inspection. 　　2. Poor clearance at the air intake end of the machine head, causing jamming. Since the clearance at the intake end of the nose cone is determined by the manufacturer of the nose cone, and this intake clearance is much larger than the exhaust clearance, the likelihood of jamming at the intake end is relatively low. For most models, it is possible to visually check at the air inlet whether there is any sintering at the intake side. The cause of this fault is mostly excessive wear of the exhaust-side positioning bearing due to oil issues, which leads to axial displacement of the rotor toward the intake side and results in an excessively small clearance at the intake side. It is also possible that the loosening of the positioning nut at the exhaust end causes the rotor to shift axially toward the intake end, resulting in an excessively small clearance at the intake end or even direct metal-to-metal contact (this is commonly seen in engines that have been repaired manually, due to an inappropriate locking torque for the positioning nut or an incorrect type of thread sealant). 　　3. Reverse jam. Since the vast majority of screw compressors are unidirectionally positioned, during normal operation, the conical thrust bearing bears the force exerted by the compressed air on the rotor in order to push it toward the intake side, thereby maintaining the exhaust clearance. When reversed, the thrust generated by compressed air pushes the rotor toward the exhaust side, causing metal contact and sintering, which results in jamming. Although most heads are equipped with disc springs or similar elements that provide reverse floating positioning to counteract this thrust, they only allow short periods of no-load reversal to determine the correct direction of rotation. All complete machine manufacturers incorporate appropriate protection against reverse rotation, so this generally only occurs when the protection fails, or when the motor wiring is changed or the switch is replaced. 　　4. Lubricant-related issues: a. Prolonged idling (including during transportation) can result in insufficient oil inside the compressor’s components, leading to a sudden lack of oil and subsequent jamming. Therefore, for equipment that has been idle for a long time, it is necessary to fill the components with enough oil before starting the equipment again ； 　　b. Insufficient oil supply to the operating machine, including delayed replacement or addition of lubricant, or blockages and leaks in the oil circuits, resulting in the machine head running out of oil and getting stuck ； 　　c. An excessively low oil level causes the bearings at both ends of the screw rotor to lose lubrication during operation, leading to severe bearing wear and resulting in the male and female screws rubbing together and sticking together. 　　d. There are too many impurities in the lubricating oil. It may be due to reasons such as the user failing to change the oil in a timely manner, poor quality of the oil, mixing different types of oil, or overly dirty oil, which results in poor cooling of the engine head. Oil carbon deposits cause the rotor to fuse together with the casing; this usually occurs at the exhaust end face of the rotor and the casing, or at the outer surface of the rotor shaft and the inner hole of the casing. Therefore, choosing the right lubricant and replacing it regularly is crucial; using inferior lubricants or using them beyond their expiration date increases the likelihood of such accidents. 　　5. Severe aging and wear of the compressor bearings. The degree of bearing wear can be determined by attempting to move the needle rollers or balls on the lower side of the bearing without removing it; if they can be moved easily, then the bearing needs to be replaced. As the compressor operates for an extended period of time, the bearings in the compressor head undergo wear, which in turn leads to an increase in the axial and radial movement of the screws. This change results in alterations in the gaps between the screws, as well as between the screws and the main housing and its front and rear end surfaces. 　　Such variations in the gap are normal and acceptable throughout the bearing’s service life, and the resulting decrease in the compressor’s gas production volume and increase in the load on the drive motor are also normal and acceptable. However, when the bearings of the screw compressor accessories wear out and the clearance between the main components exceeds the allowable limits, serious consequences can occur. 　　6. Foreign object has entered and caused jamming. It often occurs when care is not taken during maintenance, with foreign objects being drawn in through the air intake, resulting in jamming. In some cases, the oil filter is damaged as well; larger impurity particles can enter through the fuel injection ports and cause jamming. In the case of larger units with gears, the gears may be damaged, and the resulting debris can be drawn into the suction chamber via the oil return port, leading to rotor jamming. When repairing such heads, a copper mesh is generally installed at the oil return port to prevent the failure from worsening in case the gears are damaged. 　　For example, in June 2015, a compressor inspector in the power plant of a large enterprise discovered that a 90kW screw compressor had stopped running while performing cleaning tasks; upon attempting to start it, the compressor would not start. The on-duty electrician was called to inspect the electrical system. The inspection revealed that the motors and electrical circuits were in good condition, but the machine still wouldn’t start; it was initially suspected that there was a fault with the PLC. Upon inspection, the electrical system of the compressor and the PLC were found to be normal. When attempting to start it, the current reached 800–900 A, which indicated a fault in the compressor head. After separating the motor from the compressor head, it is not possible to manually rotate the screw compressor rotor. It was initially assumed that the disassembly and jamming of the head bearing was preventing the rotor from rotating, but upon opening the head bearing cover, it was found that the bearing was intact and not disassembled; the reason why the head could not rotate was actually the jamming of the male and female rotors inside the housing. 　　The compressor head was sent to the compressor manufacturer, where it was disassembled for inspection. It was found that the reason why the compressor head could not be removed was that an M12 bolt had fallen in and gotten stuck there. According to the maintenance technicians from the compressor manufacturer, the reason for the bolts falling inside was bolts that were left behind in the compressor during maintenance. Under the effect of the compressor’s vibration and the suction force of the air at the intake valve, these bolts were drawn into the interior of the compressor head, eventually causing the head to get stuck. 　　7. Gears are stuck. Since the gears in the geared head are lubricated and cooled by oil sprayed from dedicated oil injection holes, poor quality oil can easily cause carbon buildup in these injection holes, leading to their blockage and consequently the jamming of the gears. 　　When a screw compressor head gets stuck, the first step is to disconnect the motor from the compressor head, and then check whether the head can rotate on its own to determine if it is indeed stuck. This is a serious fault; it is recommended that users do not attempt to disassemble it themselves, but rather contact the dealer or manufacturer promptly to determine the appropriate repair approach. Carrying out maintenance on screw compressors helps to extend their service life. The maintenance of air compressors is also an important expense; therefore, it should be carried out by professional compressor operators. 　　The main solutions for a stuck compressor head include: (1) disassembly and inspection after cleaning with chemical agents ； (2) Manual disassembly and inspection ； (3) Hydraulic disassembly and inspection ； (4) Disassembly by electrolysis. 　　Proper compressor maintenance is not only a fundamental requirement for the proper operation of the equipment, but it is also essential for companies to control maintenance costs, avoid unnecessary financial losses, and ensure the smooth continuation of production. Therefore, it is not only necessary but also essential to carry out regular and standard maintenance on the compressor, as well as to monitor its operating condition