Basic knowledge of compressor surge

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Compressor surging: One particular phenomenon in the operation of compressors is surging. Preventing surge is an extremely important issue in compressor operation. Numerous facts have shown that many compressor accidents are related to surging. Surge can cause severe damage because during surge, the airflow generates strong reciprocating pulses that repeatedly impact the compressor rotor and other components ； The intense and irregular oscillations of the airflow cause severe vibrations in the unit, leading to various serious consequences. Surge can cause the rotor main shaft to bend ； The seal is damaged, resulting in severe air and oil leaks ； Surge increases the axial thrust, causing the thrust bearing pads to burn out ； It undermines centering and installation quality, thereby increasing vibrations ； Severe vibrations can cause instruments to malfunction ； Severe and prolonged surging can cause the rotor to collide with stationary components, resulting in the fracture of the shaft and diaphragms, or even rendering the entire compressor unusable; this has happened both domestically and abroad. Surge is a problem that must be constantly guarded against during operation. During operation, signs of surging generally include a significant drop in flow rate, a marked decrease in compressor displacement, fluctuations in outlet pressure, the pressure gauge needle swinging back and forth, intense vibration of the unit, and intermittent low-frequency roaring noises. In addition to relying on human perception, detecting surge can also be done by using instruments and operating parameters in conjunction with performance curves. 1. Conditions for surge occurrence: According to the principles of surge, it occurs under the following conditions: 1.1 It happens when the flow rate is low, dropping to the surge flow rate at that speed; this is determined by the compressor’s characteristics. At a constant speed, a certain flow rate corresponds to a specific outlet pressure or pressure increase ratio, and there exists a limit flow rate – the surge flow rate – at that speed. When the flow rate is below this surge flow rate, the compressor cannot operate stably and surging occurs. The combined relationship between the aforementioned flow rate, outlet pressure, speed, and surge flow rate constitutes the characteristic line of the compressor, also known as the performance curve. At a certain rotational speed, if the flow rate is greater than the surge flow rate, surge will not occur. 1.2 Surge occurs when the pressure of the gas within the pipeline network exceeds the maximum pressure corresponding to a certain rotational speed. When a compressor operates in conjunction with a pipeline network, if the system pressure **exceeds the maximum allowable pressure for the compressor at that rotational speed**, the high-pressure gas within the system creates a constantly high “back pressure” at the compressor’s outlet. This blocks the outlet, reduces the flow rate, and may even cause the gas in the pipeline network to flow backward, resulting in compressor surge. 2. Causes of surge during operation The various reasons that can lead to surge during operation include: 2.1 Excessively high system pressure. This situation can be caused by an emergency shutdown of the compressor, resulting in the gas being vented or returned ； The check valve on the outlet pipeline does not operate smoothly and fails to close tightly ； Or, the check valve is located too far from the compressor outlet; the volume of gas before the valve is quite large. When there is a sudden drop in system flow, the compressor fails to adjust in time, and the anti-surge system does not operate automatically, etc. 2.2 Insufficient suction flow: Due to external factors, the suction volume decreases below the surge flow rate; this, along with the rotational speed, causes the compressor to enter the surge region, resulting in surging. As shown in Figure 1 below. The causes of this situation are: blockage of the compressor inlet filter, excessive resistance, and the inability to adjust the compressor speed, resulting in surging ； This can happen if the filter element is too dirty, or if ice forms in winter ； The incoming air supply is reduced or interrupted, such as insufficient air supply from the compressor, or the compressor having no backup air source, etc. All these situations need to be identified and adjusted in a timely manner. Surge can occur in compressors. 2.3 Damage or detachment of mechanical components: Incomplete installation of components such as dry gas seals, balance disk seals, and O-rings, as well as incorrect installation positions or detachment of these components, can lead to air leakage between different stages or sections, which may cause surging ； Excessive filter resistance, as well as failure or damage to the check valve, can also cause surging. 2.4 During operation, the speed and voltage should not be increased too rapidly; before reducing the speed, the voltage must first be reduced. Speed and voltage increases should be done slowly and evenly, and pressure relief measures such as venting or backflow should be taken before reducing the speed ； To prevent backflow of air flow when the speed decreases. 2.5 Operating conditions change, causing the operating point to fall into the surge region. When operating conditions change—such as a change in speed, flow rate, or pressure—the characteristic curve is not checked in advance, resulting in the compressor’s operating point ending up in the surge region. 2.6 During normal operation, the anti-surge system is not in automatic mode; when external conditions change, such as a drop in steam pressure or fluctuations in gas flow rate ； The turbine speed drops while the anti-surge system does not have time to make manual adjustments ； Or interruption due to anger, etc ； Failure to use the automatic anti-surge device may cause surging. 2.7 Surge caused by changes in the medium condition. The occurrence of surge is closely related to the condition of the gas medium. Since the state of the gas affects the flow rate, it also affects the surge flow rate; naturally, it affects surging as well. Factors such as intake temperature, intake pressure, and gas composition, i.e., molecular weight, all have an effect on surging. When the rotational speed and outlet pressure remain constant, an increase in gas inlet stability can easily lead to surge ； When the rotational speed is constant, the higher the intake pressure, the greater the surge flow rate ； When the inlet pressure is constant, the rotational speed remains unchanged, and the molecular weight of the gas decreases significantly, surging is likely to occur. 3. Methods for preventing and eliminating surging
3.1 The fundamental measure for preventing and eliminating surging is to try to increase the inlet gas flow rate to the compressor. For generally non-toxic and non-hazardous gases such as air and CO2, venting can be employed ； For gases such as syngas, natural gas, and ammonia, a reflux cycle can be employed. After applying the aforementioned method, the gas flow rate through the compressor can be increased, thereby eliminating surging ； However, the pressure subsequently decreases, wasting power and reducing economic efficiency. If the system needs to maintain constant pressure, the rotational speed should be increased after venting or recirculation, so that the discharge pressure returns to its original level. Before boosting pressure and before slowing down or shutting down, the vent valve or return valve should be opened in advance to reduce backpressure, increase flow rate, and prevent surging. 3.2 Based on the compressor performance curve, the anti-surge margin anti-surge system should be set to automatic mode during normal operation. Before increasing the speed or pressure, it is essential to first examine the performance curve and select the appropriate operating point for the next stage. The increase in pressure and speed should be controlled based on the anti-stall safety margin. The anti-surge safety margin is the ratio of the normal operating flow rate to the surge flow rate at a given operating speed. Generally, the normal operating flow rate should be 1.05 to 1.3 times greater than the surge flow rate. If this margin is too large, although surge is less likely to occur, there will be a significant drop in pressure, resulting in considerable waste and reduced economic efficiency. In actual operation, it is best to set the setting value of the anti-surge valve based on the anti-surge margin. If it’s too large, it’s not very economical; if it’s too small, it’s unsafe. After the anti-surge system has been tuned based on the safety margin, the anti-surge valves should remain closed during normal operation and be placed in automatic mode; this ensures both safety and cost-effectiveness. In some units, the anti-surge device is not set to automatic mode but is operated manually; as a result, surge may occur, and operators are reluctant to close the anti-surge valves fully. During normal operation, a large amount of gas flows back or is vented, which is neither economical nor safe ； Since manual operation is too late when surging occurs, it is impossible to prevent surging as a result. 3.3 During pressure increase and speed change, it is important to adhere to the principle that \"pressure must be increased first by increasing speed, and speed must be reduced first by lowering pressure.\" The compressor’s pressure increase should take place after the turbine governor starts operating ； Before boosting, check the performance curve to determine the desired rotational speed; after reaching that speed, increase the pressure ； The compressor speed reduction should be initiated only after the anti-surge valves are properly arranged ； Do not increase the speed or voltage too rapidly or excessively ； The reduction in speed and pressure should also be gradual and even. 3.4 The anti-surge valves must be opened and closed slowly and alternately. Do not operate the anti-surge valves too abruptly, to prevent excessive shaft displacement, increased axial thrust and vibration, as well as malfunction of the oil sealing system. If a compressor unit has more than two anti-surge valves, they should be opened and closed alternately in order to ensure a uniform change in pressure across the various cylinders. This is beneficial for the stress on each cylinder, as well as for the coordination of the anti-surge and sealing systems. 3.5 Use of the “constant pressure ratio” pressurization method and the “safe pressure ratio” pressurization method. For safety reasons, the “constant pressure ratio” pressurization method can be employed during pressurization; this method was introduced earlier and helps prevent surging. “The \"safety pressure ratio\" boosting method is effective in preventing surge during boosting. Its basic principle is that, based on the performance curves of each cylinder in the compressor, there is a surge flow rate value at a certain rotational speed; the point where this value intersects the speed curve corresponds to a “surge pressure ratio” (or discharge pressure). At this speed, surge occurs when the boost ratio (or discharge pressure) reaches this value. Therefore, controlling the pressure ratio is equivalent to controlling the flow rate at a certain speed. If, based on the surge margin, the normal flow rate—that is, the safe flow rate—at various rotational speeds is calculated, and the corresponding pressure ratio (or discharge pressure) is determined, then during pressure increase, by ensuring that the compressor’s discharge pressure does not exceed the calculated discharge pressure derived from the safe pressure ratio according to the rotational speed, surge can be prevented. The normal flow rate and discharge pressure at different rotational speeds can be plotted as graphs and curves. During ramp-up of speed and pressure, the safe outlet pressure can be determined based on the rotational speed; as long as the pressure rise does not exceed this value, surging will not occur.