Key operating points for centrifugal compressors

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Key points for operating centrifugal compressors: Proper operation of the compressor is crucial for preventing failures, ensuring good performance, and maintaining the economic efficiency of the factory. For a compressor to operate properly, in addition to the good performance of the unit itself (including auxiliary equipment and its systems), the proper coordination of the process piping network, and high-quality installation, careful operation is also necessary. Various monitoring and inspections must be carried out thoroughly during operation, and the data obtained must be analyzed and processed carefully. Depending on the type of compressor, there are also many types of drives, such as electric motors, steam turbines, and gas turbines; as a result, the methods of operation vary. In particular, the operation of chemical compressors is closely related to the processes involved in chemical manufacturing. Here, only some common issues in the operation of turbine-driven centrifugal compressors are discussed; this is not a manual for operating specific compressors. For instructions on operating particular compressors, please refer to their respective operation manuals. Before starting up, the compressor unit must undergo a series of inspections or tests to ensure that it is in a condition suitable for operation. These inspections or tests mainly include the following. ①Check to confirm that the main and auxiliary machines are properly installed, and that the gas, steam, water, and oil circuits have been installed and cleaned in accordance with requirements ； When driving the machine for the first time, it is necessary to ensure that the pipelines have been purged and are free of foreign substances such as welding slag, waste yarn, sand, and gravel. A temporary conical filter should be installed on the inlet pipe of the compressor cylinder, and it should be removed after the machine has been operating for some time ； Check the quality of pipeline installation; it is not allowed for the expansion and contraction of pipelines, vibrations, and weight to affect the cylinder body ； Check to confirm that the compressor and turbine rotors can rotate smoothly, that any liquid accumulated in the pipes and cylinders has been drained, and that the cooling water in the intercooler is flowing properly. ②Check to confirm that the oil circuit system has been properly cleaned, that the fuel tank contains an appropriate amount of oil, that the oil samples meet the required standards after testing, that the cooling water in the oil cooler flows smoothly, that the accumulator has been filled with nitrogen at the specified pressure, and that the oil pump can deliver oil properly, providing lubricating oil, seal oil, and oil for the turbine control system at the required pressure, temperature, and flow rates. Before the unit was put into operation, the oil system was tested, with all parameters adjusted to meet the specified requirements. All interlock instruments and valves were also tested to ensure they functioned properly and sensitively. ③Check to confirm that the electrical wiring and instrument air system are in good condition, that the instrument valves operate smoothly and accurately, that the automatic control and safety systems have been tested and function properly, and that all measuring and display instruments are properly adjusted. The main operational steps for a turbine-driven compressor unit are: preparation before startup ； Start ； Operate under load ； Shut down. 1. Preparations before startup: Before starting the compressor, the following preparations should be made. (1) Preparation of the oil system. After connecting various external power sources (such as electricity, instrument air, cooling water, steam, etc.), put the oil system into operation. First, check the oil temperature; if it is low, the steam coil at the bottom of the tank should be used for heating. Generally, it is required that the auxiliary oil pump be started for oil circulation when the temperature is above 15°C; the main shaft pump is started once the temperature reaches above 24°C. The auxiliary oil pump should then be stopped and placed in a standby position, while the oil pressure in all sections is adjusted to the specified levels. (2) Preparation of the compressor’s gas system. When the process gas cannot be mixed with air, after the oil system is operating properly, air is displaced by nitrogen; the oxygen content in the gas within the compressor system must be below a specified value, such as 0.5%, after which the nitrogen is replaced by the process gas to meet the requirements, and the process gas is pressurized to the specified inlet pressure. The pressurization should be done slowly, so that the seal oil pressure matches the gas pressure. Adjust the gas pipeline valves to the specified positions according to the specific requirements of the unit. For turbine-driven centrifugal compressors, the inlet valve is generally fully open, the backflow valve or vent valve used for anti-surge is fully open, and the outlet valve to the process system is closed. (3) All instruments and interlocks shall be put into use before startup, and the intermediate cooler of the compressor shall be supplied with cooling water. (4) Prepare the turbine for startup in accordance with the turbine operation procedures, and activate the barring gear, warm up the pipes, and warm up the turbine. 2. Start-up: Spin up the rotor in accordance with the turbine operation procedures to start the unit, operate it at the specified speed for a certain period of time, and conduct a comprehensive inspection of the unit. The contents include: the oil temperature and oil pressure of the lubricating oil system ； Seal oil system, control oil system, vacuum system, steam seal system of the turbine, thermal expansion of the turbine, etc., as well as whether the gas temperatures and pressures at the inlet and outlet of various sections of the machine are abnormal ； Is there any unusual noise from the machine? If everything is normal, acceleration can begin. The speed increase should be carried out in accordance with the turbine’s speed-up curve. While increasing the speed, a comprehensive inspection of the unit’s operation is carried out; only if everything is normal can the speed be increased further, until reaching the lowest speed at which the governor takes effect (around 85% of the design speed). The critical speed should be reached promptly, with close attention paid to the vibration of the unit. 3. Loaded operation: Once the compressor starts operating under load, it gradually increases its pressure. With the inlet valve of the compressor fully open, the outlet pressure is increased step by step by gradually closing the anti-surge backflow valve or the vent valve. For compressors equipped with a simple anti-surge circuit, the sequence for closing the anti-surge valves is from low to high; that is, the exhaust valve at the outlet of the low-pressure cylinder is closed gradually first, followed by the anti-surge valve at the outlet of the high-pressure cylinder. When increasing the pressure, the speed must also be increased simultaneously, and close attention should be paid to the changes in the inlet and outlet pressures at each stage, ensuring that the pressure ratio for each stage does not exceed the surge pressure ratio corresponding to the operating speed, thereby preventing compressor surge due to too rapid pressure increase. The sequence in which the anti-surge valves of compressors equipped with complex anti-surge circuits are closed shall be carried out in accordance with the unit operation regulations. For crews driving the machine for the first time, after 8 hours of operation under design conditions, it is generally necessary to shut down the machine to check the alignment, and at the same time remove the bearings for inspection. Air is supplied to the process system in accordance with the process requirements; to prevent large fluctuations in outlet pressure when the compressor outlet valve is opened, the anti-surge valve can be closed slightly at the same time. Under normal gas supply conditions to the process system, and within the safe operating range to prevent surge, all surge return valves or vent valves should be closed in order to reduce energy consumption. Once normal production is underway, all manual operations should be switched to automatic control. At the same time, it is necessary to regularly check the operation of various components of the machine in order to have a full understanding of its condition. This is not only crucial for ensuring the safe operation of the machine but also essential for developing an effective maintenance plan for it. When the compressor is in operation, the following routine checks are generally carried out: (1) Turbine inlet pressure and temperature ； (2) Pumping flow rate, temperature, and pressure ； (3) Condenser vacuum level ； (4) Oil pressure drop of the lubricating oil filter and pressure drop of the seal oil filter ； (5) Bearing oil pressure, oil temperature, bearing bush temperature ； (6) Sealing oil and reference pressure difference ； (7) Oil discharge temperature at the outer seal of the compressor ； (8) Compressor shaft displacement, axial thrust value (some compressors directly measure the axial thrust) ； (9) Unit vibration value ； (10) Gas pressure, temperature, and flow rates at the inlet and outlet of each stage of the compressor. There are also regular tests to analyze the quality of the oil, etc. The items listed above are the main inspection items; other inspection items can be found in the specific specifications for each compressor. 4. Normal shutdown: The sequence for normally shutting down the compressor is the reverse of the startup sequence. (1) Upon receiving the shutdown command, close the air supply valve to disconnect the compressor from the process system; at the same time, open the outlet anti-surge return valve or vent valve to allow complete self-circulation. (2) Reduce the speed of the unit through the governor speed control mechanism, down to the lowest operating speed at which the governor functions. Open all anti-surge valves or vent valves; the sequence for opening them is the reverse of the sequence for closing them, starting with high pressure and then low pressure. The valve should be opened slowly to prevent the pressure at the preceding inlet from rising too rapidly, which could result in excessive reverse axial thrust. (3) Use the main steam valve to reduce the speed manually, and operate at around 500 r/min for half an hour; again, care must be taken to pass through the critical speed quickly. Finally, stop the machine using the manual shutdown switch or manual brake. After shutdown, the barring device should generally be activated to turn the shaft for a period of time. (4) After shutdown, the oil system should continue to operate for a while until the temperatures of all components have dropped; generally, it is required that the temperature of the return oil drop to around 40°C before shutting down the oil system. Turn off the cooling water for the compressor intercooler. If the process gas is flammable, explosive, or toxic, it is necessary to continue supplying oil to the sealing system even after the unit is shut down, to ensure that the gas does not leak outside. If the unit is to be shut down for an extended period, after closing both the inlet and outlet valves, the gas inside the unit must be relieved to atmospheric pressure; it should then be replaced with nitrogen, and after that with air, before the oil supply system can be shut off. Various faults may occur during the operation of a compressor; it is necessary to be adept at analysis and judgment, identify the causes, adopt appropriate corrective measures, and promptly resolve the faults.