Vibration diagnosis and working principle of screw compressors

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I. Working principle of screw compressors: The male and female rotors of the compressor are arranged horizontally and parallel within the cylinder. Thrust bearings are installed on the outside of the exhaust ends of these rotors to withstand the axial force generated by the pressure difference between suction and discharge. Shaft seal devices are provided between the bearings on the suction and discharge sides and the screw rotors, in order to prevent the lubricating oil in the bearings from leaking into the cylinder and to stop gas from leaking out of the cylinder. Synchronous gears are provided on the outside of the suction ends of both the male and female screw rotors; their speed ratio is equal to that of the screw rotors. Thanks to the support provided by bearings and the adjustment of the thickness of the synchronous gear plates, an extremely small gap is maintained between the male and female rotors, as well as between the outer surfaces of the rotors and the cylinder body, and between the end faces of the rotors and the cylinder end faces. To reduce gas leakage between the two rotors as well as between the rotors and the casing, the vibration analyzer injects a certain amount of lubricating oil into the working chamber while the compressor is running. This improves airtightness, lubricates the tooth surfaces, and reduces exhaust temperature and noise. A screw compressor is a positive-displacement compressor, and its working principle is essentially the same as that of a reciprocating compressor. In actual operation, a pair of male and female rotors inside the screw compressor mesh with each other and rotate at a certain speed ratio; this rotation causes periodic changes in the volume of the device, thereby enabling the intake, compression, and discharge of the medium. The direction of gas flow inside a screw compressor as measured by a vibration analyzer is shown in Figure 1; the intake and exhaust ports are almost diagonal to each other, but in reality, gas enters from above and exits from above as well. The pressure ratio of a screw compressor depends on the length and shape of the screws, as well as the shape of the exhaust port. II. Vibration Analysis of Screw Compressors: At a manufacturing facility, there are 3 screw compressors designed to be used in parallel for the recovery of flare gas. Initially, it was planned that all 3 compressors would operate simultaneously to capture the gas from the low-pressure pipelines of 2 gas tanks on site. However, under current operating conditions, running 2 compressors to handle one gas tank results in relatively stable gas flow rates. When all 3 compressors are turned on at once, limitations in the downstream processing capacity cause pressure alarms in the exhaust pipeline network; as a result, it is not possible to keep all 3 compressors running for extended periods. This leads to frequent start-up and shutdown cycles of one of the compressors. When all 3 compressors operate together, the vibration levels of the units exceed acceptable limits, with the maximum vibration speed reaching 11.8 mm/s, and this vibration occurs in the horizontal direction at the bearings at the compressor inlet. Since the motion of the rotor inside a screw compressor and the meshing process are essentially similar to those in a gear drive, the diagnostic methods used to identify the causes of vibration in gear drives can be applied to diagnose the causes of vibration in screw compressors as well. During operation of a screw compressor, the meshing frequency between the male and female rotors is approximately 198 Hz. The greater the frequency components of the motion of these rotors, the more likely it is that poor gear meshing between them will occur, leading to vibration. The main reason for the excessively large meshing frequency component in vibration analyzers is related to aspects such as product design, manufacturing, and assembly. Given that the vibration level of the screw compressors remains within acceptable limits when two units are operating simultaneously at the production site, and more noticeable vibration occurs only when three units are running at the same time, it is inferred that the cause of the increased vibration may be a mismatch between the actual operating conditions under which three units operate and those specified in the design. This mismatch leads to fluctuations in the compressor inlet pressure, thereby changing the volume of air entering the compressor, and ultimately results in pulsations in the airflow at the compressor inlet, causing vibration in the compressor units.