Regarding compressor surging, I need an expert to explain it
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I came across a passage that read: \"When the compressor is in operation, if the resistance in the process system’s piping network increases, this raises the pressure at the compressor’s outlet while reducing the flow rate. As a result, the angle of attack of the air flow on the blades increases, leading to gas separation on the back side of the blades and the formation of separation zones. This causes a sudden drop in the pressure at the compressor’s outlet. Generally, this separation disappears as the pressure rises again, but this phenomenon repeats itself as pressure rises, resulting in surge.\" In severe cases, there can also be backflow of gas. Surge causes intense vibrations in the blades and a sharp rise in temperature inside the compressor casing; if this persists, it can damage the compressor. ”I don’t quite understand it; I always thought that rotational separation was caused by a decrease in inlet flow. Have you encountered any cases of surge caused by outlet resistance?
There is significant resistance at the outlet, resulting in high pressure; as pressure increases, the pressure ratio also rises, which in turn leads to a decrease in inlet flow and causes surge
Surge is an inherent characteristic of centrifugal compressors.
If there is resistance at the outlet, the pressure will definitely increase. Centrifuges have an outlet pressure setting; once that pressure is exceeded, the system will unload. If unloading does not occur, surging will take place, and in severe cases this can damage the impeller
Activate anti-surge function to prevent low pressure and low flow at the inlet, as well as control the rotational speed
When operating under normal conditions, centrifugal compressors generally do not experience surge. Surge can occur due to changes in operating conditions resulting from process changes or equipment failures (i.e., changes in flow rate). The fundamental reasons lie in two aspects: First, when designing a compressor, the geometric shape of the impeller should be determined based on the operating conditions, that is, the required flow rate. This ensures that, within a certain range of air intake volumes, the gas enters the impeller at the optimal angle, resulting in minimal energy loss and maximum velocity energy; this allows the impeller to convert as much energy as possible into the gas, thereby increasing its speed and pressure. It is only when the aerodynamic parameters of the gas and the geometric parameters of the impeller are in harmony that normal operating conditions are achieved. When the operating conditions deviate beyond the allowable range, the pneumatic parameters and geometric parameters become inconsistent, which in turn leads to disruptions in the way the impeller does work on the gas. This results in a lower efficiency of energy conversion, as well as severe rotational separation of the gas within the impeller, leading to sudden stall. The gas flow becomes very poor and irregular. At this point, although the impeller is rotating and doing work on the gas, it is unable to effectively increase the gas pressure, resulting in a decrease in the pressure at the compressor outlet. II. However, surge does not occur just because the pressure at the compressor outlet decreases. Another reason is that the compressor outlet is usually connected to a piping system; if the capacity of this piping system is large, it is not very sensitive to changes in pressure. In such cases, the pressure in the piping system may not decrease immediately, and instead it might even exceed the pressure at the compressor outlet. As a result, gas starts to flow back, until the pressure in the piping system drops below that at the compressor outlet, after which the compressor can supply air again. After some time, the pressure in the piping system will once again exceed that at the compressor outlet, and the same phenomenon repeats itself over and over. The system experiences periodic axial low-frequency, high-amplitude airflow oscillations, which give rise to surge. Summary: There are two aspects to the reasons. The internal reason is that under certain conditions, the gas flow inside the compressor experiences a severe sudden stall ； External factors: Related to the capacity and characteristic curve of the pipeline network system. Internal factors can only cause surge to occur when external conditions are met.
An increase in the resistance of the exhaust pipeline leads to a reduction in the amount of air intake, resulting in surge.
Open the anti-surge valve and adjust the pressure ratio.