What are the causes of inter-stage surge in steam turbine air compressors?

There is a concept of inter-stage surge, which is related to temperature differences; however, there should be an upper limit for the temperature at the second stage. As long as this limit is not exceeded, surge generally does not occur. Moreover, the temperature differences between winter and summer are also different. If the temperature difference is large, and either the pressure at the inlet of the second stage increases (due to overheating or reduced efficiency of the second-stage impeller) or it decreases too much (due to reduced efficiency of the first-stage impeller or too low cooling temperature in the inter-stage cooler), surge may occur； Inter-stage surge is generally caused by blockages on the water side or air side of the inter-stage cooler, as well as reduced efficiency due to dirty impellers

When a compressor experiences surge, the rotor and stator components are subjected to alternating dynamic stresses. Imbalances in pressure between stages cause severe vibrations, which can damage seals and bearings; in extreme cases, it may lead to collisions between the rotor and stator components, leakage of the compressed gas, and even catastrophic accidents such as explosions. The fundamental cause of surge is low flow rate. There are many factors that lead to low flow rate during operation, which can be summarized as follows: (1) The pressure at the compressor outlet increases, and the system pressure exceeds this outlet pressure, causing the gas flow rate to drop to the surge flow rate. The high pressure in the system causes pressure buildup at the compressor outlet, allowing gas to flow back into the compressor and resulting in a low gas flow rate inside it. (2) The inlet flow is below the specified value, and the backflow control valve has failed. At a certain rotational speed and gas density, a specific pressure can be maintained. Low gas flow rates during startup or shutdown, or an excessively wide opening of the flare valve, are the most common causes of low inlet flow to the compressor. (3) Changes in gas density: at a certain rotational speed, the centrifugal force decreases, resulting in a drop in outlet pressure and flow rate; this is often mistaken for a pumping-out phenomenon. (4) Unstable operation of the distillation system causes oil to be present in the gas entering the compressor (for example, due to malfunctions in the liquid level or interface sensors), allowing liquid components to enter the compressor. (5) The steam pressure of the turbine is low or of poor quality (low temperature), resulting in the unit operating at full load while its speed decreases. (6) The speed control system fails, auxiliary systems malfunction, vacuum efficiency declines, and the unit is unable to perform at its rated capacity. To effectively prevent surge, it is necessary to control the vent valve so that its flow rate remains at a level not lower than that determined by the set pressure. Additionally, specific measures must be taken during operation: (1) When the source of compressed gas changes, the frequency should be adjusted appropriately and the counter-rotational momentum increased; the flare should not be used during startup or shutdown. When the gas flow rate at the compressor inlet is low, the counter-rotational momentum needs to be increased. During startup, gas should also be transferred from the stabilization system to the distillation system to supplement the flow rate and maintain it within the specified range. (2) Strengthen the regulation of system pressure to prevent overpressure. (3) Strengthen the control of the liquid level and interface level in the oil-gas separator of the distillation system, and enhance dehydration. (4) Strengthen the drainage at the inlet and outlet of the compressor; it is absolutely necessary to prevent oil from being carried in the gas. (5) Ensure that the steam pressure of the turbine remains stable and is not lower than the design value. (6) When the reaction pressure is high, the inlet flare valve can be opened; when the compressor outlet pressure is high, the outlet flare valve can be opened. However, care should be taken to ensure that the inlet and outlet flare valves are not opened simultaneously.

3# rwjlx Thank you, it’s extremely detailed. I’ve taken note of it!