09,22,2026 17 views
Rotating stall and surge are vibration faults of high-speed centrifugal compressors. This type of malfunction is caused by fluid flow separation, and the equipment itself generally does not have obvious structural defects, so there is no need to shut down for maintenance. By adjusting the flow rate, the vibration can be reduced to the allowable value.
When the rotation disengages and further develops into surge, it not only causes a decrease in unit efficiency, but also poses a serious threat to the machine. Surge can cause damage to internal seals, bearings, and even result in rotor bending and coupling damage in severe cases. Surge is one of the most dangerous operating conditions for fluid machinery such as centrifugal compressors, which poses a great threat to the machine. The role and benefits of status monitoring and fault diagnosis can be best demonstrated for faults that pose great harm but can be handled without the need for shutdown.
1、 The mechanism and characteristics of rotational stall 1 rotating stall The mechanism of rotational stall was first proposed by H.W. Emmons in 1995. The formation process of rotational stall is roughly as follows. The impeller structure and size of centrifugal compressors are designed according to the rated flow rate. When the compressor operates at normal flow rate, the direction in which gas enters the impellerB1 Installation angle with blade inletBSConsistently, the gas can smoothly enter the impeller, as shown in Figure 1 (a). At this point, the relative velocity of the airflow isOh1The radial flow velocity at the inlet isC1When the gas flow rate entering the impeller is less than the rated flow rate, the radial velocity of the gas entering the impeller decreases toC1The direction angle of the relative velocity of gas entering the impeller decreases accordinglyB1Therefore, it is installed at an angle with the blade inletBSInconsistent. At this point, the gas will impact the working surface (convex surface) of the blade, forming an airflow vortex near the concave surface of the blade. The vortex gradually increases, reducing the effective flow area of the channel. Due to manufacturing, installation, maintenance, or operating conditions, the distribution of airflow entering the compressor is not uniform in each channel, and the number of vortices in the airflow also varies. If there are many vortices in a certain channel (channel 2 in Figure 1 (b)), the amount of air passing through this channel will decrease, and the excess air will be diverted to adjacent channels (channels 1 and 3). When turning towards the front channel (channel 1), due to the incoming gas hitting the concave surface of the blade, a portion of the vortex on the original concave surface is washed away, and the airflow in this channel tends to be smooth. The airflow turning towards the rear channel (channel 3) is directed towards the convex surface of the blade, causing more vortices to be generated at the concave surface of the blade, blocking the effective flow area of the channel and forcing the airflow in the channel to turn towards the adjacent channel. In this rotational development, airflow blockage clusters composed of vortices (known as stall clusters or stall zones) will appear alternately in various flow channels along the opposite direction of impeller rotation. Because the stall zone propagates in the opposite direction at a speed lower than the rotational speed of the impeller, from a reference frame outside the impeller, the stall zone still rotates in the direction of the impeller's rotation, which is the mechanism of rotational stall. Although the actual airflow situation is quite complex, the rotating stall mechanism proposed by H.W. Emmons still provides a basis for future research work.
2 Rotating stall frequency The propagation speed or stall frequency in the rotating stall zone is a concern for many people, as it is of great significance in diagnosing whether the vibration of the compressor is caused by rotating stall. In addition to conducting extensive theoretical research, domestic and foreign research institutions have also conducted a large number of practical tests in the laboratory. B. F.J. Cossar et al. conducted extensive tests on axial compressors, and the results showed that the rotational stall zone first appeared at the tail of the blade and then moved forward, rotating approximately 20 ° relative to the leading edge of the blade.
In fact, the formation of the stall zone is a rather complex fluid dynamic process. The stall frequency is also related to whether there is distortion in the inlet airflow of the blades and the direction angle of the inlet airflowB1 Installation angle with blade inletBSThe difference in angle of attack between them is closely related to factors such as the number of stages of the compressor.B. F.J. Cossar used the method of installing low porosity metal wire mesh at the inlet of the compressor in the experiment, and measured the stall frequency to be 1/2 of the speed frequency, which is slightly different from the theoretically calculated stall frequency of 1/3 of the speed frequency.
N. A. Cumpsty's experimental model indicates that within the range of 1/5 to 1/2 of the rotational speed frequency, as the number of compressor stages increases, the propagation speed in the rotational stall zone gradually approaches 40% of the rotor speed.
According to Japanese vibration expert Hirobumi Shiraki, depending on the type of machine, the propagation speed in the rotating stall zone is 0.2 to 0.5 times the rotor speed. The compressor units of Italy's NUOVO PIGNOVE company are widely used in China's petrochemical industry. The company uses CO in large fertilizer urea plants manufactured according to their drawings2 The propagation speed of the rotating stall zone in the compressor can be calculated according to the following empirical formula:
In the equation,Q0pTo determine the actual flow rate during rotational stall,Q0 Design the operating flow rate for the compressor,uThe circumferential velocity of the rotor.
The propagation speed in the rotating stall zone calculated based on this formula is approximately 0.3-0.45 times the rotational speed of the rotor. In addition, the hub to hub ratio (i.e. the ratio of the inner diameter to the outer diameter of the impeller passage) has a significant impact on the propagation speed in the stall zone. A large wheel hub compared to an impeller will experience a stall in the entire radius direction, that is, the stall zone will occur throughout the entire range from the blade root to the blade top, which is called a full radius stall (Figure 2 (a)); The small wheel hub generally stalls only in the radial direction compared to the impeller, that is, the stall zone only occupies a part of the length of the flow channel (Figure 2 (b)). In terms of stall state, full radius stall is more severe than partial radius stall, that is, the fluid inside the blade will cause stronger pressure pulsation.
3 Vibration mechanism of rotational stall The pressure fluctuation generated by rotational stall inside the impeller is the excitation force that excites the rotor to undergo abnormal vibration. The magnitude of the excitation force is related to the molecular weight of the gas. If the molecular weight of the gas is large, the excitation force is also large, which has a significant impact on the operation of the machine.
From the perspective of the relative coordinate system fixed on the impeller, the rotational detachment of the mass occurs at an angular frequencyOhsPropagation between machine flow channels, the vibration frequency of the rotor excited by pressure fluctuations isOhsIts vibration frequency is lower than the angular frequency of the rotorOhFrom the perspective of the coordinate system outside the impeller, the detachment of the rotating mass is due to（Oh～Ohs）The frequency of rotation is the same as the direction of rotation of the rotor. Therefore, when fluid machinery experiences rotational stall, the abnormal vibration of the rotor is accompanied byOhsAnd（Oh～Ohs）Two harmonic characteristic frequencies.
When the unit experiences rotational stall, it may be due to a gas detachment mass on a certain stage impeller, or there may be several detachment masses on a certain stage impeller; Detachment from the group may occur on one stage of the impeller, or it may occur simultaneously on several stages of the impeller. Generally, when a machine experiences a rotational stall fault, two or more gases often detach from the cluster. In actual production, the angular frequency at which the machine experiences rotational stallOhsReferring to the above equation, it can be calculated as follows:
In the equation,OhFor the rotor angular frequency,NFor the number of gas detachment clusters,Q0pFor actual work flow,Q0 To design traffic.
The rotational stall fault of fluid machinery generally exists, but it may not necessarily excite the rotor to cause strong vibration of the unit. Only when the frequency of rotational stall is coupled with a certain natural frequency of the unit, can the machine resonate and produce dangerous vibration.
2、 The mechanism and fault characteristics of surge 1 surge Surge is one of the common faults in the operation of centrifugal and axial compressors, which is a further development of rotational stall.
As shown in Figure 3, the centrifugal compressor has the characteristic that for a certain speed, there is always a corresponding flow rate value, and the compressor efficiency reaches the point. When the traffic is greater or less than this value, efficiency will decrease. Usually, this flow rate is used as the design operating point.
The performance curve of the compressor is subjected to surge conditions on the left side（Qmin）Due to restrictions, the right side is experiencing blockage conditions（Qmax）The region between these two is called the stable operating condition region of the compressor. The size of the stable operating condition area is an important indicator for measuring the performance of the compressor.
When the compressor is running, if the flow rate continuously decreases due to external reasons, it will reachQmin When the value is reached, severe rotational detachment will occur in the compressor flow channel. If the air volume further decreases, the entire flow channel of the compressor impeller will be occupied by the vortex zone of the airflow, and the outlet pressure of the compressor will suddenly decrease. However, the pressure in the larger capacity pipeline system connected to the outlet of the compressor does not immediately decrease, and there may be a phenomenon of gas backflow from the pipeline to the compressor.When the pressure in the pipeline drops below the outlet discharge pressure of the compressor, the gas backflow will stop and the compressor will resume discharging into the pipeline. However, due to insufficient intake air, after the compressor returns to its original pressure in the outlet pipe network, a vortex zone will appear in the flow channel. This cycle repeats itself, causing periodic changes in the flow rate of the unit and pipelines, and significant fluctuations in the pressure at the inlet and outlet of the machine. Due to the backflow of gas at the inlet and outlet of the compressor, there will be huge periodic airflow roars and intense machine vibrations. These fluctuations can be clearly reflected in the pressure, flow rate, vibration signal display and other records on the instrument panel, and can also be immediately detected at the operation site.
The frequency and amplitude of machine vibration caused by surge are closely related to the size of the pipeline volume. The larger the pipeline volume, the lower the surge frequency and the larger the amplitude. The exhaust pipe network capacity of some machines is very large, and the surge frequency is even less than 1Hz at this time.
2 Fault characteristics of surge The main characteristics of compressor surge are as follows:
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When the compressor approaches or enters a surge condition, the cylinder body and bearings will experience strong vibrations, with amplitudes significantly increased compared to normal operation. The surge frequency is generally low, usually between 1-30Hz.
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When the compressor operates under stable conditions, there is little change in its outlet pressure and inlet flow rate, and the measured data fluctuates around the average value with a small amplitude. When approaching or entering the surge condition, there are significant changes in the outlet pressure and inlet flow rate, resulting in periodic and significant pulsations, and sometimes even gas backflow from the compressor inlet.
When the compressor operates stably, its noise is low and continuous. When approaching the surge condition, due to the periodic oscillation of the airflow generated by the entire system, the noise emitted by the airflow in the airflow pipeline also varies from high to low, resulting in periodic changes. When entering the surge condition, the noise increases dramatically, and even explosions may occur.
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