Thread Content
The last edit to this post was made by LQ198619 on 2017-5-25 at 13:29. VI. Surge in Centrifugal Compressors: A centrifugal compressor generates a P-G curve at different rotational speeds; each of these curves has a maximum pressure value. The gas flow rate corresponding to this maximum value represents the surge point of the compressor. When connected into a curve, this forms the surge limit; if the flow rate falls within the shaded area on the left, surge will occur. Phenomenon: During the production process, the compressor sometimes experiences sudden intense vibrations; the flow rate and pressure of the gas medium also fluctuate significantly. There is a periodic, dull \"calling\" sound, as well as loud \"whooshing\" noises caused by the fluctuations in airflow within the piping system. The piping system undergoes periodic oscillations with large amplitudes and low frequencies, accompanied by periodic roaring sounds. The compressor vibrates intensely; the casing and bearings experience strong vibrations as well, accompanied by a loud, periodic airflow noise. This phenomenon is collectively referred to as compressor surge condition, and the compressor cannot operate for long periods under such conditions. Once surge conditions are reached, the operator should immediately take corrective actions by reducing the outlet pressure or increasing the inlet flow, in order to move the compressor’s operating point out of the surge zone and achieve stability. Consequences of surge: Due to the intense vibrations, the liquid lubrication conditions in the bearings are disrupted, the bearing shells can be damaged, and even the shaft may break. Frictional collisions between the rotor and stator occur, and the sealing elements suffer severe damage. Additionally, it has a severe impact on the process system. Reasons for surge: From an internal perspective, when the flow rate decreases to a certain level, severe rotational separation of the gas within the impeller’s flow channels is the underlying cause of surge. From an external appearance perspective: 1. The system pressure increases to the maximum pressure corresponding to a certain rotational speed. 2. The gas outlet pressure rises above the maximum pressure corresponding to a certain rotational speed, entering the surge zone. A. The compressor shuts down urgently, with no time for reflux or venting. Case where the anti-surge valve is not set to automatic mode. The gas exiting has no way to go, resulting in pressure buildup at the exit ； Either the check valve plate falls off or the valve cannot be opened, preventing the gas from escaping. B. If the system’s capacity is reduced suddenly and the check valve responds slowly or fails to close properly, then when the compressor slows down rapidly and the outlet pressure drops, the high-pressure gas from the downstream system will flow back, causing surging. 3. Surge caused by improper operation: Failure to put the anti-surge valve in automatic mode, incorrect setting of its parameters, or delayed activation can all lead to surge. For a two-cylinder compressor, when starting up, the pressure ratio of the low-pressure cylinder should be increased first, followed by that of the high-pressure cylinder. That is, during the pressure increase process, the anti-surge valve of the low-pressure cylinder should be closed first, then that of the high-pressure cylinder; otherwise, the low pressure at the outlet of the low-pressure cylinder combined with an excessively high pressure ratio in the high-pressure cylinder can cause surging in the high-pressure cylinder. Similarly, when parking, turn on the high-pressure anti-surge device first, and then the low-pressure anti-surge device. It is also dangerous to increase the speed and voltage too quickly, or to fail to reduce the voltage before reducing the speed. Therefore, when driving, increase the speed first and then the pressure; when stopping, reduce the pressure first and then the speed. Otherwise, surge is likely to occur under low speed and high pressure conditions. 4. Surge caused by damage to mechanical components: Aeration between sections or stages due to mechanical seals, flat disk seals, O-rings, etc., can lead to surge and also affect the air injection volume. B. A clogged filter screen, or a check valve plate that does not close properly or has fallen off, can all cause compressor surging. Prevention and elimination of surge 1. Increase gas flow rate. The most important method is to avoid operating the compressor in the surge zone at flow rates below the surge threshold; when surge occurs, it is necessary to immediately increase the flow rate by opening the outlet relief valve ； Reduce outlet pressure ； Open the anti-surge valve), then identify the cause and eliminate it. Principles for handling surge: increase flow at the inlet, maintain unobstructed flow at the outlet, reduce back pressure, and use recirculation or venting. If the system needs to maintain pressure, after opening the anti-surge valve to allow gas to flow back, increase the speed appropriately to raise the outlet pressure to its original level; before shutting down, the reflux or venting valve for anti-surge should be opened in advance. 2. Control the anti-surge margin based on the performance curve, and adjust the anti-surge valve appropriately. Before increasing the speed and voltage, the operating condition change points are determined based on the performance curve, and control is carried out in accordance with the anti-stall safety margin. The anti-surge safety margin refers to the ratio of the actual flow rate to the surge flow rate at a certain speed; that is, Anti-surge margin = Actual flow rate at that speed / Surge flow rate at the same speed. The control range is generally between 1.05 and 1.33. During startup and shutdown, as well as when increasing speed or pressure, it is necessary to follow the performance curve, adhering to the principle that \"pressure must be increased first when increasing speed, and pressure must be decreased first when reducing speed.\" Regulation principle 1: To prevent compressor surge, anti-surge valves are installed at the inlets of each stage of the compressor. In other words, a portion of the exhaust gas is cooled and then returned to the inlet, thereby compensating for any deficiency in the inlet gas flow under abnormal conditions and ensuring that the required inlet gas flow is maintained. During start-up and shutdown, operating the anti-surge valve manually facilitates frequent adjustments and rapid, substantial adjustments; once stability is achieved, automatic control can be engaged. 2. Increase the speed first when boosting pressure, otherwise surge is likely to occur. 3. There are various methods for adjusting the air volume of a compressor; during normal operation, using speed control is the most energy-efficient and quickest method. Under normal operating conditions, the compressor’s anti-surge valve should be set to automatic mode and kept fully closed. 4. Slowly open and close the anti-surge valve alternately. Do not operate the anti-surge valves too quickly or forcefully, in order to avoid increased shaft displacement, heightened vibration, and disruptions to the sealing system. If the unit is equipped with more than two anti-surge valves, they should be operated alternately so that the pressure changes in each cylinder are even; this is beneficial for the stress on each cylinder, anti-surge performance, and overall system coordination.