Case analysis of jamming in York propane industrial screw compressors

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1 Introduction: The propane compressor is a screw compressor imported from York Company in the United States. This unit is designed and manufactured in accordance with the standards of the American Refrigeration Association, equipped with a highly intelligent microprocessor, and features intermediate gas injection. This compressor was put into operation in October 2001, and its main function is to provide cooling for natural gas and ethylene. 2 Main features and technical parameters of the unit: This compression unit is part of the propane system, which includes a propane storage tank, propane compression units, an economizer, a propane cooler, a propane evaporator, etc. Propane compression units have the following features: (1) They adopt a skid-mounted design, with the compressor, lubricating oil system, and control system all located on the same skid, resulting in low floor space requirements. (2) By using an intermediate air injection compressor, it is possible to achieve the functions of the two compressors originally designed, while also saving on investment costs. (3) It features an advanced microprocessor control system that regulates the compressor inlet pressure via a slide valve to meet the requirements of the process; the microprocessor can be connected to a DCS. (4) This compressor uses specialized oil designed for refrigeration compressors, allowing it to operate at low temperatures. Pre-lubrication of the compressor is not required when starting up, and the lubrication system is controlled by pressure difference. It features an efficient oil separator system; after separation by this oil separator, the oil content in propane is ≤ 5×10^-6, which helps reduce oil consumption and prevent oil from entering the propane system. (5) It has high operating requirements; before the system is put into use, the entire system must be vacuum-dehydrated to prevent water from entering the system and affecting its cooling performance. The process operation parameters of the unit are shown in Table 1. 3 Compressor failure symptoms and handling 3 1 Failure symptoms In December 2001, the propane compressor was ready to be restarted after being shut down for 2 days. About 3 seconds after pressing the start button, the compressor begins to vibrate violently and then stops. Due to the self-protection mechanism of the electrical control cabinet, the microprocessor of the compressor reports a low-current shutdown (power-off protection). At the same time, the compressor unit exhibits the following symptoms: (1) there is no oil level visible in the sight glass of the oil separator; (2) the microprocessor indicates that the temperature of the lubricating oil is dropping at a rate of 1 degree per minute, and frost forms rapidly at the inlet of the compressor; (3) when attempting to manually rotate the compressor shaft, the compressor does not respond. 3 2　Accident handling: Since there was no lubricating oil in the compressor unit’s separator, the manual rotation of the unit did not work; therefore, it was necessary to address the compressor’s fault before it could be restarted. (1) No lubricant treatment in the separator: The loss of lubricant is mainly due to the lubricant entering the inlet pipeline after the compressor stops, as a result of poor sealing of the compressor’s inlet check valve. Drainage was carried out at the lowest point of the system compression inlet pipeline, and no lubricating oil was found. The auxiliary oil pump was started; lubricating oil appeared in the lower view mirror of the separator, but no oil level was visible in the upper view mirror. The inlet and outlet valves of the compressor were closed to cut off the gas supply, thereby reducing the pressure in the compressor unit, and the lubricating oil returned to the compressor separator. (2) No movement when manually turning the compressor: Remove the coupling of the compressor and manually turn the motor and the compressor; it rotates smoothly when powered electrically, but the compressor still does not move, which indicates that the compressor may be stuck, and it is necessary to disassemble the compressor. (2) The compressor was disassembled under the on-site guidance of the technical professionals from YORK Company. After disassembly, no metal impurities were found inside the compressor. The journal of the compressor’s rotor was in severe contact with the sealing surface of the housing; there were slight scratches on the rotor and the housing, and metal buildup occurred in two areas within the housing due to contact with the rotor. The journal of the compressor’s male rotor, the sealing surfaces of the casing, as well as the metal buildup areas on the casing were repaired. All of the compressor’s seals and bearings were replaced, and the compressor was reassembled. 4 Compressor accident analysis: No hard metal objects entered the compressor’s interior, yet the compressor seized up. What caused this? Our technical staff conducted a detailed analysis and determined that it was due to liquid slugging affecting the compressor rotor. (1) Based on the conditions of the lubricating oil after a failure occurred in the compressor, it can be seen that the lubricating oil cannot return quickly to the compressor separator. This is because, after the inlet valve of the compressor was opened, a large amount of propane liquid entered the compressor unit. Once the compressor started operating, the lubricating oil entered the compressor, and part of the propane liquid vaporized to form gas, thereby blocking the lubricating oil channels. The temperature of the lubricating oil in the separator dropped, and frosting appeared on the inside of the separator within 6 hours; this indicates that a large amount of liquid entered the compressor separator, causing the temperature of the lubricating oil to drop from 25°C to -3°C. (2) As can be seen from the propane system flow diagram, the propane system is a cyclic system with a relatively complex process. After the propane compressor stops operating, there is a large amount of propane gas in the system. At the time the compressor stops, the ambient temperature is 15°C; when it restarts, the ambient temperature is -3°C. The ambient temperature dropped by 18°C over 2 days, causing a large amount of propane gas to turn into liquid and remain in the system pipelines. (3) Since the adverse effects of liquid on screw compressors were not adequately taken into account during the design and construction of this system, multiple U-tubes are present at the compressor inlet. When liquid accumulates in these U-tubes, the pressure there decreases, causing more gas to flow toward that area; as a result, a large amount of liquid accumulates in the U-tube sections during the shutdown process of the compressor. Although the system was depressurized while driving, theoretically the pipeline should contain a saturated propane gas-liquid mixture; however, thanks to the excellent insulation of the low-temperature pipeline, the propane liquid cannot vaporize rapidly. After removing the insulation from the U-shaped pipe section and heating the pipeline with hot water, the pressure in the system rose rapidly, indicating that a large amount of liquid propane was still present in the pipeline. Frost formation at the inlet after the compressor stopped operating confirms that a large quantity of liquid entered the compressor. 5 Improvement measures: Since the liquid is an incompressible fluid, all users should pay sufficient attention to the possibility of large amounts of liquid entering the screw compressor, which could lead to serious accidents. When a continuous \"squealing\" sound is heard at the compressor inlet and the compressor current increases, it indicates that a small amount of liquid has entered the compressor. In terms of operation, strict adherence to the operating procedures is required for conducting pre-startup inspections of the system and all components of the compressor. Heat the lubricating oil to 50°C and maintain it at that temperature for at least 24 hours to reduce the amount of propane gas dissolved in the lubricating oil. To prevent liquid from entering the compressor, the following measures can be taken: (1) Before starting up, remove part of the insulation around the U-tube and heat that section of the tube to ensure that no liquid enters the compressor; or after shutting down the compressor, isolate the system and release the pressure in the pipes to atmospheric level. (2) Modify the process pipeline by removing the U-tube, so that the liquid can flow back to the propane storage tank. (3) Installing a gas-liquid separator at the compressor inlet and the intermediate air injection port can completely solve the problem of liquid entering the compressor.