Surge analysis and prevention examples for centrifugal compressors

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When a centrifugal compressor experiences surge, the rotor and stator components are subjected to alternating stresses. The severe vibrations caused by pressure imbalances between stages can damage the 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. Therefore, centrifugal compressors must not operate in the surge region. I. Mechanism of surge: The occurrence of surge is influenced by two factors: the internal factor is the \"rotational separation\" of the airflow within the centrifugal compressor under certain conditions” ； External conditions are the characteristics of the compressor pipeline system. Surge occurs when external conditions are favorable for internal factors. 1. Internal factors of surge: When operating at the design condition point M, Q = Qset (as shown in Figures 1 and 2). The inlet angle of the airflow is essentially equal to the installation angle of the impeller’s inlet, allowing the airflow to enter the flow channel smoothly; there is little to no separation of the airflow boundary layer, resulting in minimal losses. After Q P2, the gas in the pipeline system flows back toward the compressor. This process repeats continuously: the compressor sometimes outputs air flow, and at other times gas flows back into the machine through the pipelines, resulting in periodic fluctuations in air flow and surging. The frequency and magnitude of parameter changes during surge are highly dependent on the capacity of the piping network. The capacity of the pipeline network is equivalent to the fundamental resonator of the entire system. The larger the capacity of the pipeline network, the lower the frequency of surge and the greater the amplitude ； The smaller the capacity of the piping network, the higher the frequency of surge and the smaller the amplitude. It can be seen that the fundamental cause of surge is low flow rate. There are many factors that lead to low flow rates during operation, which can be summarized as follows: (1) The pressure at the compressor outlet increases, and the system pressure exceeds this outlet pressure, resulting in the gas flow rate dropping to the surge flow rate. The high pressure in the stable 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 vent 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, leading to a drop in outlet pressure and discharge volume; this is often mistakenly considered to be a pumping action. (4) Unstable operation of the distillation system causes oil to be present in the gas entering the compressor, with liquid components entering the machine. (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 cannot perform at its rated capacity. II. Typical surge incidents: Example: The MB-CH type 7-stage in-series horizontal centrifugal gas compressors in a catalytic unit at Qiange Oil Refinery. 1. Surge caused by changes in rotational speed: Under normal conditions, changes in the compressor’s rotational speed are controlled by the pressure signals generated by the system. However, when a fault occurs in the machine, these pressure signals cannot allow the turbine’s rotational speed to be adjusted freely. One winter, due to insufficient steam supply and low pressure in the steam pipeline network, the steam pressure supplied to the turbine often ranged from 0.7 to 0.8 MPa. The unit frequently operated at full load; its speed could not increase, sometimes reaching only 80% to 90% of the set value, and surge phenomena occurred regularly. 2. Reduced molecular weight of gases causes surging. Testings of the catalytic unit were conducted using residue oil; after 20 days, the high content of heavy metals in the residue oil led to catalyst poisoning, which altered the composition of the cracking gases. The H2 content in the rich gas reached 40% (by volume), and the molecular weight of this rich gas dropped to 35 (the originally designed molecular weight was 50). After the molecular weight decreases, the compressor experiences surge. 3. Surge caused by throttling in the compressor outlet pipeline. Washing water is introduced into the compressor outlet pipeline before it reaches the container; the inner diameter of this pipeline is 150 mm, but it reduces to 30 mm due to scaling, resulting in blockage of the outlet pipeline. This causes the pipeline’s performance curve to shift upward, and the operating point enters the surge region, leading to surge. 4. Inlet throttling (low inlet pressure) causes compressor surge. Once, the defoaming screen in the oil-gas separation tank in front of the compressor fell off and was drawn into the compressor inlet pipe, creating a throttling effect; as a result, the inlet pressure dropped, leading to surge. III. Measures to prevent surge The basic principle for preventing surge is to keep the flow rate and pressure away from the surge point, that is, to ensure that the flow rate remains within the stable operating range at Qmin