Two methods for preventing surge in compressors

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Surge is an inherent characteristic of centrifugal compressors. The cause of surge must first be sought in the characteristics of the system in question. As can be seen from Figure 1, both the curve of the compression ratio P2/P1 of the compressor and the curve of the flow rate Q have a maximum value for P2/P1. Working on the curve to the right of this point keeps the compressor stable. In the low-flow region on the left side of the curve, an unstable state arises due to the compressibility of the gas. As the flow rate gradually decreases to the surge line, once the compression ratio drops and causes the flow rate to decrease further, the gas under pressure in the output pipeline is forced to flow back into the compressor due to its higher pressure than that at the compressor outlet. Once the pressure in the pipeline drops, the direction of gas flow reverses, and this cycle repeats itself, resulting in surge. During surge, the compressor casing vibrates and this vibration affects the adjacent piping systems; in severe cases of surge, it can cause serious damage to the compressor. II. Feasibility analysis of the anti-surge automatic control system: To ensure the safe, efficient, and economical operation of the compressor, when it is operating at low loads, its gas flow rate must always remain on the right side of the surge zone, with a certain safety margin; generally, the control line is set at 5%–10% above the surge limit flow rate. As long as the suction flow rate of the compressor is greater than the critical suction volume Qp, the system will operate in the stable region without experiencing surge. That is, when the production load is reduced, some of the exhaust gas must be returned to the inlet via the outlet bypass valve, or part of the exhaust gas must be vented, in order to keep the system operating within a stable range. III. Several Implementation Methods for Anti-surge Automatic Control Systems Currently, two types of anti-surge methods are commonly used: the fixed limit flow rate (or minimum flow rate) method and the variable limit flow rate method. 1. Fixed Limit Flow Rate Method The anti-surge control system with a fixed limit flow rate ensures that the compressor’s flow rate remains above a certain specific value, as indicated by Qp in Figure 1, thereby preventing operation in the surge zone. The advantage of this method is a simple control system with fewer instruments required. The drawback is that when the compressor speed decreases and it is operating at low load, the anti-surge control system activates too early, resulting in a large amount of backflow and higher energy consumption. 2. Variable limit flow method: In situations where the compressor load can be adjusted by changing its speed, the limit surge flow varies depending on the operating speed – it decreases as the speed drops. Therefore, the most appropriate method for surge prevention is to provide an appropriate safety margin, allowing the surge control device to operate along a safe control line on the right side of the surge limit flow curve. This is what is known as the variable limit flow method. There are two common control schemes: one involves measuring the compressor speed and using a function generator to provide the set value as a flow regulator (Figure 2). Secondly, based on the mathematical expression of the anti-surge control line, conventional instruments are used to simulate expression (1), and the control process is shown in Figure 3. In recent years, with the development of digital instruments and microprocessors, such control systems have become easy to implement. Here, a and b are determined by the compressor manufacturer, and C is a constant. In the formula, M represents the molecular weight, z represents the compression coefficient, R represents the gas constant, and k represents the overall flow coefficient. IV. Methods for implementing an anti-surge control system: The water and gas plant is equipped with Ingersoll Rand air compressors of model C90M × 3, featuring three stages of compression. The flow rate is 11,942 m3/h, the inlet pressure (absolute) is 0.09 MPa, the outlet pressure (absolute) is 0.9 MPa, and the power output is 1,305 kW. The anti-surge control system is shown in Figure 4. This anti-surge system works by opening the bypass valve to release some air when the pressure at the outlet of the unit approaches the surge point (the point at which the bypass valve opens). It is important to set the opening point of the bypass valve. When set too high, the compressor consumes more energy at low loads. When set too low, the compressor will be allowed to cross the surge line and experience surge. The CMC of the compressor can automatically adjust the opening point of the bypass valve to a value higher than that of the surge line in order to make corrections. This compressor has also experienced surge. Based on an analysis of the automatic control system, there are several situations that can cause surge in the compressor: 1. A fault in the detection of the outlet pressure results in the CMC receiving false signals, which prevents the bypass valve from opening fully. 2. The bypass valve is faulty and cannot be opened. 3. The ramp time (the time it takes for the bypass valve to go from closed to open) is set too long, resulting in excessive delay in the opening of the bypass valve. 4. The inlet filter is dirty, resulting in high filter resistance and a reduced inlet flow rate. 5. CMC failure causes the bypass valve to lose control. V. Conclusion The anti-surge control system of centrifugal compressors plays an extremely important role in ensuring the stable operation of large-scale units. The implementation methods for the anti-surge control system include the fixed limit flow rate method and the variable limit flow rate method. Depending on the specific circumstances, some other security measures can also be added.