E-book material~Starting and stopping of gas-condensing turbines and compressors (VIII)

Thread Content

This post was last edited by LQ198619 on 2017-5-25 13:29. VII. Startup and Shutdown of Gas-Concentrating Turbines and Compressors – Preparation for Startup: First, the utility systems must be ready for operation: water, electricity, and gas (nitrogen, circulating water, drive steam, seal steam, process gas). Instrumentation, electrical systems, and inspections are all satisfactory (including proper functioning of alarms and interlocks, as well as successful purging of the process system). All water coolers are operating normally, the compressor outlet valve is closed, and the anti-surge valve is fully open. Introducing dry seal gas and backflow isolation gas (taking the ammonia compressor in an ammonia plant as an example): 1. The backflow isolation gas should be introduced at least 10 minutes before the oil system is put into operation. If the compressor is under maintenance or is started up in winter, the oil system must be operated in advance. The backflow isolation gas can be stopped only after at least 10 minutes have passed since the oil supply has been stopped, ensuring that there is no oil left in the oil return lines. 2. Before introducing the medium into the compressor unit, primary seal gas must be supplied first to prevent the medium inside the unit from contaminating the primary seal surface. At the same time, it is necessary to ensure that the pressure in the primary sealing chamber is at least 0.15 Mpa higher than the pressure in the flare line and balance pipes before the compression unit can be started. 3. Nitrogen is used as the sealing gas during operation; the pressure for the primary sealing is set at 3.0 Mpa, while the pressure for the secondary sealing and the rear isolation gas is 0.46 Mpa. After the compressor starts up normally (when the gas pressure at the outlet of the high-pressure cylinder exceeds the pressure in its equalizing line by 0.5 MPa), it is possible to switch to supplying process gas. The process gas consists of two streams: one from the third-stage outlet that has been cooled by a cooler, and the other that remains uncooled. These two streams are mixed together to reach a temperature of 100°C before being supplied to the high-pressure cylinder; after pressure reduction, it is then fed to the low-pressure cylinder. Oil system preparation: 1. Check the oil level in the oil tank; it should be above half full. 2. Turn on the heater to raise the oil temperature above 35–45°C, as it will drop during operation; once normal operation is achieved, the oil temperature should be kept within the 35–45°C range. 3. Open the inlet and outlet valves of the oil cooler and oil filter; exhaust air until oil is visible. 4. The inlet and outlet valves of the oil pump are open. 5. The high-level tank valve should be opened (both the upper valve and the oil return valve need to be open). 6. Start the oil pump: Start the main oil pump; the auxiliary oil pump must be set to automatic mode. Oil pressure regulation: Adjust the check valves before and after the control valves in the lubricating oil and control oil circuits, close the bypass valves, and operate the valves so as to achieve the desired parameters for automatic regulation. Confirm that the following items need to be checked: oil cooler, oil filter; oil presence in the exhaust lines to prevent interference with heat exchange. Check the oil level on the sight glass (verify whether the oil level on the sight glass of the oil pipeline is normal). Is the pressure difference of the oil filter below the alarm value? (It is advisable to clean and replace it during shutdown.) Remember that it is necessary to see oil overflowing in the high-level oil tank (with oil in the return pipe) before proceeding to the next step, in order to ensure safety in the event of a compressor failure. Starting up the condensation system: 1. Open the cooling water valve for the surface cooler, and also open the return water valve (to supply cooling water). 2. Open the make-up water valve of the surface cooler to achieve a liquid level of 50%. 3. Add water to the water seal of the atmospheric safety valve; ensure a continuous flow to prevent air leakage. 4. Open the inlet and outlet valves of the condensate pump; start the pump – one in operation and one as a backup. The auxiliary pump should operate in automatic mode (to maintain the liquid level, sometimes both pumps are used; if the level is too low, it’s not possible to maintain it, while if it’s too high, it affects the condensation of waste steam, leading to a decrease in vacuum). 5. Set the flow control valve and the level control valve to automatic mode. 6. Direct the soft water to Pumps A and B; after venting, start the main pump, and set the auxiliary pump to automatic mode. 7. Open the cooling water side vent of the extraction condenser, and close it after venting. IV. Steam pipeline warming and drain removal (taking the ammonia compressor in an ammonia plant as an example): Before warming the pipeline, ensure that the rotation of the compressor is normal. It involves allowing a small amount of steam to enter the pipeline, which is then discharged through venting, while also removing water vapor. 1. Why warm the pipes? Since the steam pipes and valves are cold while the vehicle is in operation, a small amount of steam is allowed to enter the pipes, so that the pipes can expand evenly as they heat up gradually; this prevents condensation from forming once the steam enters the pipes. 2. Disadvantages of inadequate pipe heating: Excessive condensate water is generated in the pipes; steam carrying water causes water hammer, which can damage the impellers of the turbine. The pipe warming time is insufficient, resulting in inadequate metal expansion, which makes the flange prone to leakage. 3. Pipe warming method: A. Starting from the pipeline from which steam is supplied, open all the drain valves along the line; also open the vent valve before the main gate valve. The pipe warming is considered successful when the steam temperature is at least 30–50 degrees higher than its saturation temperature. B. The pilot drains in front of the quick-shut valve are opened one after another; the turbine rotates properly. The bypass valve of the main gate valve is opened to warm up the pipeline ahead of the quick-shut valve, allowing the temperature of the pipeline to rise gradually – the warming process at low pressure takes about 30 minutes, with a temperature increase rate of 5 ℃/min and a pressure increase rate of no more than 0.1 Mpa/min. C. Once no more condensate is produced within the pipeline and the pressure difference between the main gate valve and the quick-shut valve is less than 1 bar, the main gate valve is gradually opened while the bypass valve is closed. At the same time, the temperature should reach 50℃ above its saturation temperature. During the warming process, if the quick-shut valve is not airtight, gas can easily enter the turbine, causing the rotor to bend due to heating. Therefore, shaft turning must be carried out during the pressure-raising and heating process of the pipes; in particular, it is necessary to perform shaft turning once the sealing steam is introduced. (When the barring gear is activated, the oil system must already be operating normally; it is strictly prohibited to rotate the rotor without any lubricating oil.) ) The driving time and vacuuming time must be short. Prerequisites for introducing shaft seal steam: The turning gear must already be in operation; heat the pipes for 5–10 minutes, pay attention to the drain water, then introduce the shaft seal steam and enable automatic control, adjusting the steam volume according to changes in vacuum. After the shaft seal is supplied with air, the turbine should be started as quickly as possible (within five minutes) to prevent a large temperature difference between the upper and lower cylinders at the shaft seal area. To set up the vacuum system, first confirm that air has been supplied to the shaft seal. 1. Ensure that the vacuum system is airtight (water can generally be used for testing); the condensate pump and liquid level are operating properly, meeting the conditions for starting up the system. 2. The manual valves before and after the condenser in the extractor (steam extraction condenser) steam trap are opened. 3. Start the start-up pump (also known as the initiation pump): open the steam valve first, then the exhaust valve. (Because air cannot be cooled down to water; it is considered an inert gas with respect to water vapor, so it is simply vented after being extracted.) When the vacuum level reaches around -0.04 MPa, the main exhaust pump (the primary and secondary exhaust pumps) can be started. First, open the steam valve of the secondary exhaust pump, then open the steam valve of the primary exhaust pump, and finally open the exhaust valve. 4. Once the vacuum level meets the requirements, the start-up pump can be removed; first close the vacuum pump valve and then the steam valve. Warm-up and overspeed tests: 1. Items to verify before starting up: 1) Oil temperature and pressure; 2) Main steam temperature and pressure – no water allowed; 3) Vacuum system (liquid level, vacuum degree); 4) Verify that the turbine governor is in its reset position, with no alarms and no alarm indicator lights on. 2. Stop the turbine shutdown device. 3. Slowly open the main steam valve; there may be a slight sticking effect, but if it is opened too quickly, the speed can rise to over a thousand revolutions per minute. Observe for any abnormal noises as soon as the rotor starts rotating; if no abnormal noises are heard, close the valve immediately (this can be done using a manual control valve). 4. Then proceed with warm-up by increasing the speed according to the specified speed curve. The warm-up times for cold start and hot start are different. Shut-down test: Reduce the oil pressure on-site (or press the emergency stop button) to see if an emergency shutdown can be achieved. Check whether the emergency shutdown device functions properly (this is done during the initial testing of the turbine or after maintenance). 5. The dwell time is checked in accordance with the requirements specified in the speed-up curve: 1) Main steam temperature and pressure; 2) Oil temperature and pressure; 3) Vacuum level and liquid level of the surface cooler; 4) Whether the rotor shaft displacement and vibration levels are within normal ranges; 5) Any abnormal noises; 6) Check for leaks in the oil pipelines and steam pipelines. If anything abnormal is detected, stop the operation immediately and address it. Speed-up: 1) Proceed according to the speed-up curve; 2) Check the items listed under “4” above before each increase in speed. 3) The critical rotation speed must not be maintained; otherwise, excessive shaft vibration will trigger an alarm, and in severe cases it may even cause the machine to shut down. 4) The main steam valve (operated on-site) – the transition from throttle valve control (operated via the dashboard) represents the switch from manual to automatic control. (Some manufacturers use fully automatic control, so this operation is not required.) ) What is the critical speed? 　　　The impeller mounted on the shaft, along with other components, together constitute the rotor of a centrifugal compressor. Although the rotor of a centrifugal compressor is carefully balanced, extremely slight eccentricities still exist inevitably. Furthermore, due to its own weight, the rotor always develops a certain degree of deflection between the bearings. Due to these two factors, it is impossible for the center of gravity of the rotor to coincide exactly with its axis of rotation; as a result, a periodically varying centrifugal force is generated during rotation, and the frequency of this force is undoubtedly consistent with the rotational speed of the rotor. When the frequency of variation of the periodically varying centrifugal force equals the natural frequency of the rotor, the compressor experiences severe vibration, known as \"resonance\". Therefore, the critical speed of the rotor can also be considered as the speed at which rotor resonance occurs during the operation of the compressor. Load reduction by stopping the machine: reduce the compressor’s capacity, lower its speed, and activate anti-surge measures to gradually bring the speed below 8,425 revolutions; use the manual valve to reduce the speed to 800 revolutions and then shut it off completely. When the vacuum is nearly 0, turn off the pump first and then the seal steam (otherwise cold air will enter the shaft end and cause damage to it). For shaft rotation after shutdown: carry it out as specified. Hot rotors are prone to deformation, so the turbine must be rotated continuously; after 2–3 hours, the condensation system is shut down. After the oil system is shut down, wait at least 10 minutes until no oil is present in the return pipe before shutting off the isolation gas.