Surge is the vibration of turbine compressor (also called blade compressor) under abnormal working conditions when the flow is reduced to a certain extent.Centrifugal compressorAs a form of turbine compressor, surge is very harmful to centrifugal compressor.
Chinese name
surge
Foreign name
surge
Definition
Vibration of compressor under abnormal working conditions
Fluid machineryThe periodic oscillation of the medium in the pipeline is caused by the excitation of periodic suction and discharge of the mediummechanical vibration。
Compressor surgeIt refers to the low frequency (usually only a few hertz or ten hertz) and high amplitude (strong pressure and flow fluctuation) airflow oscillation phenomenon along the compressor axis.This low frequency and high amplitude air flow oscillation is a great source of exciting force, which will lead to strong mechanical vibration of compressor components and hot end overheating.It will also cause serious damage to components in a short time, so the compressor is not allowed to work in the surge area under any conditions.[1]
Surge process
For example,pumporcompressorThe surge process that may occur during operation is:
When the flow rate decreases to the minimum value, the outlet pressure will drop suddenly, and the pressure in the downstream pipeline will be higher than the outlet pressure, so the conveyed medium will flow back into the machine until the outlet pressure rises to transport the medium to the pipeline again;When the pressure in the pipeline recovers to the original pressure, the flow decreases again, and the medium in the pipeline flows back again and again.[1]
Cause
The generation of surge is related to the characteristics of fluid machinery and pipelines. The larger the capacity of the pipeline system, the stronger the surge and the lower the frequency.Generally, the products are attached with pressure flow characteristic curve, according to which the surge point, surge boundary line or surge area can be determined.The surge of fluid machinery will destroy the flow regularity of the medium inside the machine, resulting inMechanical noise, causing strong vibration of working parts, accelerating bearing andseal upIs damaged.Once surge causes pipeline, machine and its foundationresonanceIt will also cause serious consequences.In order to prevent surge, the fluid machinery must be operated outside the surge zone.In compressors, the minimumFlow type, flow speed control or flow pressure difference control anti surgeRegulating system。When multiple machines work in series or parallel, they should have their own anti surge regulating devices.
Surge, as the name implies, is like human asthma, and the fan appearsPeriodicityThe air outlet and reverse flow ofAxial flow fanSurge is more likely to occur, and severe surge will lead to fatigue damage of fan blades.[1]
Form of expression
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The general phenomenon of surge fan is as follows:
1 The current decreases and swings frequently, and the outlet wind pressure drops and swings.
2. The abnormal noise and vibration of the fan, the temperature rise of the casing, and the surge vibration of the induced draft fan make the furnace negative pressure fluctuate and the combustion unstable.[2]
Common causes
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The system resistance is too large due to ash accumulation and blockage in the flue duct or insufficient opening of the flue duct baffle.;When two fans are running in parallel, the deviation of guide vane opening is too large to make the fan with small opening fall into the surge zone for operation (we often encounter the situation that the connecting rod of the fan guide vane actuator is disengaged when lifting and lowering the load, which causes large deviation due to the unsynchronized adjustment of the guide vanes of the two fans);The fan operates under low output for a long time.[2]
Handling principles
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The general treatment principle is to adjust the load, turn down the guide vane opening of the high output fan to make the fan output similar, reduce the change rate of the load, strengthen the wind pressure detection and information feedback control of the inlet and outlet sections, and then find out the possible causes according to the above and make corresponding treatment.[2]
performance curve
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The so-called surge refers to the phenomenon that when the fan with a "hump" shaped Q-H performance curve works to the left of the critical point of the curve, that is, when it works in an unstable area, the flow and energy head of the fan will change periodically and unstably in an instant.The maximum energy head generated by the fan will be less than the resistance loss in the pipeline, and the fluid will start to flow back in the opposite direction from the pipeline into the fan (negative flow occurs). As the fan continues to operate, when the pressure in the pipeline decreases, the fan will start to output flow again. As long as the flow required by the outside is kept less than the critical point flow, the above process will repeat, which is surge.
There is a saddle shaped area in the left half of the performance curve of the axial flow fan. When operating in this area, the flow, pressure head and power of the fan will fluctuate significantly. The fan and pipes will produce strong vibration, significantly increased noise and other abnormal conditions, which are generally called "surge". This unstable working condition area is called surge area.In fact, surge is only a phenomenon that may be encountered in the area of unstable working conditions, and the inevitable phenomenon in this area is rotating flow shedding or rotating stall.These two working conditions are different, but they have some relationship.For Q-H performance curve of axial flow fan, if throttling adjustment method is used to reduce the flow of fan, if the working point of fan is on the right side of point K, the fan works stably.When the flow rate of the fan Q<QK, the maximum pressure head generated by the fan will then drop 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, so the pressure in the air duct is greater than the pressure head generated by the fan, making the air flow start to reverse in the opposite direction. The air duct flows into the fan, and the working point moves quickly from point K to point C.However, the airflow backflow reduces the air volume in the air duct system, so the pressure in the air duct drops rapidly, and the working point drops rapidly along the CD line to the D point when the flow Q=0, at which time the air volume supplied by the fan is zero.As the fan continues to operate, when the pressure in the air duct decreases to the corresponding point D, 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 flow required by the outside world remains less than QK, the above process repeats.If the working state of the fan is carried out in a cycle of F-K-C-D-F, if the frequency of this cycle matches the oscillation frequency of the fan system, resonance will be caused and the fan will surge.[1]
Conditions
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When the fan is working in the surge area, the flow fluctuates sharply, resulting in the impact of air flow, which causes strong vibration of the fan, increases the noise, and the wind pressure keeps shaking. The larger the capacity and pressure head of the fan, the greater the hazard of surge.Therefore, the following conditions shall be met for surge of fan:
a) The working point of the fan falls in the unstable region with hump shaped Q-H performance curve;
b) The air duct system has enough volume, and it forms an elastic aerodynamic system with the fan;
c) Resonance occurs when the frequency of the whole cycle is in harmony with the air flow oscillation frequency of the system.
The occurrence of rotating flow separation and surge is in the unstable region on the left side of the Q-H performance curve, so they are closely related. However, there are essential differences between rotating flow separation and surge.The rotating flow separation occurs in the whole unstable region to the left of the peak value of fan Q-H performance curve;However, surge only occurs in the upper right part of Q-H performance curve.The occurrence of rotational flow separation only determines the structural performance of the impeller blade, air flow and other factors, and has nothing to do with the capacity and shape of the air duct system.The impact of rotation on the normal operation of the fan is not as serious as surge.
When the fan surges during operation, the situation is different.During surge, the flow, total pressure and power of the fan produce pulsation or large amplitude pulsation, accompanied by obvious noise, sometimes even high decibel noise.The vibration during surge is sometimes very violent, which damages the fan and pipeline system.Therefore, when surge occurs, the fan cannot operate.[3]
Alarm device
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The axial flow fan is equipped with a surge alarm device at the impeller inlet, which is arranged in front of the impeller by a pitot tube, with the opening of the pitot tube facing the rotation direction of the impeller.Pitot tube is to bend the end of a straight tube to 90 ° (with the opening of the pitot tube facing the airflow direction), and connect a U-shaped tube to the pitot tube. The reading of the U-shaped tube (pressure gauge) should be the sum of the kinetic energy (dynamic pressure) and static pressure (full pressure) of the airflow.Under normal conditions, the air flow pressure measured by the pitot tube is negative, because it is the pressure in front of the impeller.However, when the fan enters the surge area for operation, the pressure measured by the pitot tube is also a fluctuating value due to the large fluctuation of air flow pressure.In order to enable the impulse 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 impeller measured with a U-shaped tube plus 2000Pa pressure is used as the alarm setting value of the surge alarm device.When the operating condition exceeds the surge limit, an audible and visual alarm signal is sent to the console through the pitot tube and the differential pressure switch, requiring the operator to deal with it in time to return the fan to normal operating condition.
In order to prevent the operating point of the axial flow fan from falling in the rotating flow separation and surge area during operation, when selecting the axial flow fan, carefully verify whether the regular operating point of the fan falls in the stable area. At the same time, when selecting the adjustment method, pay attention to the change of the operating point. The movable blade adjustable axial flow fan is adjusted by changing the installation angle of the movable blade, so when the fan reduces the flow,The change of the angle of attack of the air flow caused by the reduction of the axial speed due to small air flow is just compensated by the change of the installation angle of the moving blade, so that the angle of attack of the air flow will not increase, so that the fan will not produce rotational flow loss, nor surge.When the installation angle of the moving blade decreases, the unstable area of the fan becomes smaller and smaller, which is very beneficial to the stable operation of the fan.[3]
Preventive measures
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1) Make the flow of pump or fan always greater than QK.If the flow required in the system is less than QK, a recirculation pipe or an automatic discharge valve can be installed to ensure that the discharge flow of the fan is always greater than QK;
2) If the pipeline performance curve does not pass through the coordinate origin, changing the fan speed may also result in 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 side is the stable working area, and the left side is the unstable working area. When the pipeline performance curve passes the coordinate origin, changing the speed has no effect, so the working points at various speeds are similar working points.
3) The axial flow fan adopts adjustable blade adjustment.When the flow required by the system decreases, its installation angle decreases, the performance curve moves down, the critical point moves to the left and down, and the output flow decreases accordingly.
4) The most fundamental measure is to avoid the use of fans with hump shaped performance curves as far as possible, and to use fans with straight and downward sloping performance curves.[4]
Stall distinction
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Stall and surge are two different concepts. Stall is a hydrodynamic phenomenon caused by the structural characteristics of blades. Some of its basic characteristics, such as the rotating speed of the stall area, the starting point and the vanishing point of the flow separation, have their own laws and are not affected by the volume and shape of the fan system.
Surge is a form of oscillation characteristics after the coupling of fan performance and pipeline device. Its basic characteristics such as amplitude and frequency are affected by the fan pipelineSystem volumeThe fluctuation of flow rate and pressure power is caused by unstable operating conditions. However, experimental research shows that the occurrence of surge is always closely related to the flow separation in the blade passage, and the increase of incidence angle is also related to the decrease of flow rate.Therefore, in the unstable working condition area where surge occurs, there must be rotational flow separation.[4]
