[Haichuan Illustrated Chemical Equipment] Series Posts – Surge Phenomenon in Centrifugal Compressors

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Get to know the equipment, understand it, and make good use of it. [Haichuan’s Illustrated Guide to Chemical Equipment] series posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719196. Everyone is welcome to participate in the discussions. ----------------------------------------------------------- The centrifugal compressor is the “power heart” of the entire system, and surge can be considered the most dangerous “arrhythmia” for this heart. As senior chemical engineers, one of our primary tasks when designing and operating compressors is to prevent such incidents from occurring. 1. What is surge? In simple terms, surge is a severe aerodynamic instability that occurs in centrifugal compressors when the flow rate is too low. During normal operation, the compressor impeller rotates at high speed, transferring kinetic energy to the gas, which is then continuously discharged as it overcomes the high pressure (back pressure) in the downstream piping system. However, when the required flow rate of the system decreases (for example, when the downstream valve is closed), and the flow rate of gas passing through the compressor drops to a critical level, the exhaust pressure generated by the compressor will no longer be able to overcome the backpressure downstream. At this point, the airflow reverses direction: high-pressure gas rushes back into the compressor from the outlet piping system. Backflow causes the pressure in the downstream piping network to drop instantly; when the pressure falls below the exhaust pressure that the compressor can provide, the compressor discharges gas again. Then the flow rate decreases again, the back pressure rises once more, and backflow occurs again... This low-frequency, large-amplitude periodic oscillation of gas involving \"exhaustion – backflow – further exhaustion – further backflow\" is known as surge. 2. Typical symptoms of surge: Once surge occurs, the noises at the scene are extremely frightening; you certainly won’t ignore them: a deafening noise – a periodic, deep roaring sound similar to an ox’s bellow, or a loud hissing sound. Severe mechanical vibration: The entire compressor unit and the connected pipes experience intense shaking. Severe parameter fluctuations: On the screens in the control room, the pointers (or values) for outlet pressure, flow rate, and motor current exhibit large-scale periodic swings. Abnormally high temperature: Due to the repeated compression and friction of gas inside the casing, the temperature inside the compressor rises sharply. 3. The destructive form of surge is known as the \"number one killer\" of compressors; if not addressed promptly, it can cause catastrophic damage to the equipment within seconds to minutes: damage to the rotor and bearings – the alternating aerodynamic forces can lead to the burning out of bearings and the bending of the rotor. Seal failure: Intense vibration can damage precise dry gas seals or labyrinth seals, resulting in fluid leakage (and the consequences can be catastrophic if it is a flammable or explosive gas). Impeller fracture: Extreme alternating stresses can cause the blades to fatigue and break, completely destroying the interior of the casing. 4. How to prevent it: Anti-surge control. In chemical processing plants, we typically use anti-surge control systems (ASCC) and anti-surge valves (ASV) to protect compressors. The principle is simple: since surge is caused by \"too low a flow rate through the compressor,\" when the system detects that the flow rate is approaching the critical level (the surge line), it automatically opens a bypass circuit (a surge valve) to send the high-pressure gas exiting back to the inlet (usually after being cooled), thereby increasing the flow rate of gas passing through the compressor and preventing it from entering a surge condition. It’s like creating an internal roundabout during traffic congestion to prevent the flow of vehicles from coming to a standstill.