What Is a Compressor Surge?

Surge is a kind of vibration under abnormal working conditions that occurs when a turbine compressor (also called a vane compressor) reduces its flow rate to a certain extent. Centrifugal compressors are a form of turbo compressors. Surge can cause serious damage to centrifugal compressors.

The general phenomenon of a fan with surge is as follows:

1 The current decreases and swings frequently, and the outlet wind pressure drops and swings.

2 The sound of the fan is abnormal. The noise is large, the vibration is large, the temperature of the casing is increased, and the suction fan vibrations make the furnace negative pressure fluctuate. ^[2]

The dust accumulation in the flue duct is blocked or the opening of the flue baffle is insufficient to cause the system resistance to be too large. ; When the two fans run side by side, the deviation of the opening of the guide vanes is too small. The fan with a small opening falls into the surge zone. Large deviation caused by unsynchronized adjustment); the fan runs under low output for a long time. ^[2]

The general processing principle is to adjust the load and close the opening of the guide vane of the small and high output fan to make the fan output close, reduce the change rate of the load, strengthen the wind pressure detection and information feedback control of the inlet and outlet sections, and then according to the above Find the possible causes and then deal with them accordingly. ^[2]

The so-called surge is when the fan with a "hump" QH performance curve is operated to the left of the critical point of the curve, that is, when the fan is working in an unstable region, the fan's flow rate and energy head are unstable and repeatedly change in an instant. The phenomenon. The maximum energy head produced by the fan will be less than the resistance in the pipeline. The fluid will start to flow backward in the opposite direction and flow back into the fan from the pipeline (a negative flow occurs). Because the fan continues to run, when the pressure in the pipeline decreases, the fan The output flow is restarted. As long as the externally required flow remains less than the critical point flow, the above process repeats and surge occurs.

The left half of the performance curve of the axial flow fan has a saddle-shaped area. In this section, large fluctuations in the flow, head and power of the fan sometimes occur. The fan and pipeline will generate strong vibrations, and the noise will increase significantly. The abnormal working condition is generally called "surge", and this unstable working area is called the surge area. In fact, surge is only a phenomenon that may be encountered in unstable operating conditions, and what is bound to occur in this area is the phenomenon of spin-off flow or spin stall. These two conditions are different, but they have a certain relationship. Figure 17 is shown in the figure below: Q-H performance curve of axial flow fan. If the flow rate of the fan is reduced by using the throttling adjustment method, if the fan's operating point is to the right of point K, the fan operation is stable. When the flow rate of the fan Q <QK, the maximum pressure head generated by the fan will decrease and be less than the pressure in the pipeline, because the capacity of the air duct system is large, the pressure in the air duct is still HK at this moment. Therefore, the pressure in the air duct is greater than the pressure head generated by the fan, so that the air flow starts to flow backward, and the air duct is poured into the fan. The working point is quickly moved from point K to point C. However, the backflow of the airflow reduces the air volume in the air duct system, so the pressure in the air duct drops rapidly, and the operating point rapidly drops along the CD line to point D at the flow rate Q = 0. At this time, the air volume supplied by the fan is zero. Because the fan continues to run, when the pressure in the air duct decreases to the corresponding D point, the fan starts to output flow again.

In order to balance with the pressure in the air duct, the operating point jumps from D to the corresponding operating point F. As long as the required external flow remains less than QK, the above process repeats itself. If the working state of the fan is repeatedly performed according to F-K-C-D-F, if the frequency of this cycle matches the oscillation frequency of the fan system, it will cause resonance and the fan will surge. ^[1]

When the fan is working in the surge area, the flow fluctuates sharply, and the impact of the air flow causes the fan to vibrate strongly, the noise increases, and the wind pressure continues to sway. The larger the capacity and head of the fan, the more harmful the surge is. Big. Therefore, the following conditions must be met for the fan to generate surge:

a) The operating point of the fan falls in an unstable region with a hump-shaped Q-H performance curve;

b) The air duct system has a sufficiently large volume, and it forms a flexible aerodynamic system with the fan;

c) When the frequency of the entire cycle coincides with the airflow oscillation frequency of the system, resonance occurs.

The occurrence of rotational deflow and surge are both unstable areas on the left side of the Q-H performance curve, so they are closely related, but there is an essential difference between rotational deflow and surge. Rotational deflow occurs in the entire unstable area to the left of the peak value of the Q-H performance curve of the fan shown in Figure 5-18; while surge only occurs in the upper-right slope of the Q-H performance curve. The occurrence of rotary deflow only determines the structural performance and air flow of the blade of the impeller itself, and has nothing to do with the capacity and shape of the air duct system. Rotation does not affect the normal operation of the fan as much as surge.

The situation is different when the fan surges during operation. During surge, the flow, full pressure and power of the fan produce pulsations or large-scale pulsations, accompanied by obvious noise, sometimes even high decibel noise. Vibration during surge is sometimes very severe, damaging fans and piping systems. When the surge occurs, the fan cannot run. ^[3]

The axial flow fan is equipped with a surge alarm device at the inlet of the impeller. The device is arranged by a pitot tube in front of the impeller, and the opening of the pitot tube faces the direction of rotation of the impeller. Pitot tube is to bend the end of a straight tube to 90 ° (the opening of the pitot tube is facing the airflow direction), and a U-shaped tube is connected to the pitot tube, then the reading of the U-shaped tube (pressure gauge) should be airflow Sum of kinetic energy (dynamic pressure) and static pressure (full pressure). Under normal circumstances, the air pressure measured by the pitot tube is negative because it measures the pressure in front of the impeller. However, when the fan enters the surge area to work, the pressure measured by the pitot tube is also a fluctuating value because the air pressure fluctuates greatly. In order for the pulse pressure sent by the pitot tube to send an alarm signal through the pressure switch, the alarm value of the pitot tube is specified as follows: when the moving blade is at the minimum angle position (-30 °), the pressure in front of the fan wheel is measured with a U-shaped tube In addition, 2000 Pa pressure is used as the alarm setting value of the surge alarm device. When the operating conditions exceed the surge limit, the pitot tube and differential pressure switch are used to send an alarm signal to the console using sound and light, and the operator is required to handle them in time to return the fan to normal operating conditions.

To prevent the working point of the axial flow fan from falling in the rotating deflow and surge area during operation, carefully check whether the frequent working point of the fan falls in the stable area when selecting the axial flow fan. At the same time, when selecting the adjustment method, it is necessary to Pay attention to the change of the working point. The adjustable-blade axial-flow fan is adjusted by changing the installation angle of the moving blade. Therefore, when the fan reduces the flow rate, the change of the airflow impact angle caused by the small air volume reducing the axial speed is caused by the moving The change of the blade installation angle can be compensated, so that the angle of attack of the air flow will not increase, so the fan will not produce rotating outflow, and will not produce surge. When the blade installation angle decreases, the unstable area of the fan becomes smaller and smaller, which is very beneficial to the stable operation of the fan. ^[3]

1) Make the flow of the pump or fan constant greater than QK. If the required flow rate in the system is less than QK, a recirculation pipe or an automatic discharge valve can be installed to make the discharge flow rate of the fan constant greater than QK .;

2) If the performance curve of the pipeline does not pass through the origin of the coordinates, the speed of the fan may be changed to obtain stable operating conditions. The performance curve of the fan is divided into two parts by the parabola of the highest pressure point in the performance curve of the fan at various speeds. The right is the stable working area and the left is the unstable working area. When the pipeline performance curve passes the coordinate origin, the speed is changed It has no effect, so the operating points at each speed are similar operating conditions.

3) Axial flow fans are adjusted with adjustable blades. When the required flow of the system is reduced, the installation angle is reduced, the performance curve is moved downward, the critical point is moved to the lower left, and the output flow is reduced accordingly.

4) The most fundamental measure is to avoid using fans with a hump-shaped performance curve, and use fans with a straight downward slope of the performance curve. ^[4]

Stall and surge are two different concepts. Stall is a hydrodynamic phenomenon caused by the structural characteristics of the blade. Some of its basic characteristics, such as: the rotational speed in the stall zone, the starting point of the outflow, and the vanishing point, etc. Its own law is not affected by the volume and shape of the fan system.

Surge is a manifestation of the oscillation characteristics after the performance of the fan is coupled with the piping device. Its basic characteristics such as amplitude and frequency are dominated by the volume of the piping system of the fan. The fluctuations in flow, pressure and power are caused by the unstable working area However, experimental studies have shown that the occurrence of surge is always closely related to the outflow of airflow in the blade channel, and the increase in the angle of attack is also related to the decrease in flow. Therefore, in the unstable working area where surge occurs, rotational deflow must occur. ^[4]