Research on structural resonance of large axial fan in power plant

According to the structural characteristics of the axial fan, the causes of equipment resonance are analyzed. Four reference methods are proposed, including strengthen the structural stiffness, change the exciting frequency, weaken the exciting force, and strengthen the damping. The characteristics of the response treatment measures are discussed. Take the two-stage axial-induced draft fan of a 600 MW unit as an example, the axial resonance problem has been dealt with well by improving the support stiffness and reducing the excitation force. The research results and treatment can provide an experience reference for the analysis and treatment of the resonance problem of large axial fans.


Introduction
In recent years, China has carried out large-scale energy saving and emission reduction transformation [1] of the boiler system, added a variety of dust removal, desulfurization, and denitration systems, and carried out integration transformation, so the fan power of the boiler smoke exhaust system continues to increase [2].With the large flow, small starting torque, strong adaptability to changes in the air duct system, and other advantages, axial fans gradually become very important [3][4].However, the large capacity of axial fans' weight and size increase more, and the supporting material is often weak, especially with the large capacity fan frequency conversion transformation, which expands the working speed range, so the frequency range of exciting force increases.The resonance problems of its rotor, blade, foundation, and support system are very common [5], and the common analysis and management of the resonance problems of this kind of fan are of very important significance.Generally speaking, the fan resonance phenomenon generally needs to focus on the frequency analysis [6] of two aspects, one is the natural frequency of the rotor or structure, and the other is the vibration frequency inside the equipment.When the two frequencies are too close or have a multiple relationship, the fan equipment may have a resonance phenomenon [7].By analyzing the mechanism of the resonance problem, this paper puts forward the measures to deal with resonance fault from four aspects and discusses its advantages and disadvantages.Finally, it is applied through the field case.

Working principle and resonance mechanism
The main components of an axial fan include an intake box, inlet and outlet flexible connection, main casing, impeller and supporting main bearing group, diffuser, hydraulic regulation system, etc. [8].When the fan is in operation, the air flowing into the fan from the system pipeline changes direction through the intake box and flows to the impeller to achieve pressurization after converging and accelerating through the collector.The air flowing through the impeller is spiral movement, which is converted into axial flow into the diffuser through the rear guide vane.Part of the kinetic energy of the gas in the diffuser is converted into static pressure energy and then flows to the system to meet the operation requirements.Thus, completing the working process [9] of the fan output, the axial fan has two forms adjustable rotor blade angle and adjustable stator blade angle [10].The impeller is usually a single-stage impeller, and the high-power fan also uses a two-stage impeller.Resonance is a component or more components of the natural frequency and excitation frequency close to caused [11], each structure in the three-plane direction of the resonant frequency (horizontal, vertical, and axial) [12], no matter the vibration force in which resonance frequency, could make the vibration enhanced.Triggering resonance requires two conditions at the same time: x The exciting frequency is the same or close to the natural frequency of the system or components.
x The exciting force energy is large enough to overcome the system damping.Resonance does not cause vibration but enhances vibration, and the mechanical gain obtained by the structure through the system is called the amplification factor (Q).
f is the frequency of exciting force, Hz; fn is the natural frequency, Hz; ߞ is the damping coefficient, which is the ratio of actual damping to critical damping.
It can be seen from Equation (1) that under the condition of constant damping of the system, the closer the frequency of the exciting force is to the natural frequency of the structure, the larger the amplification factor is, and the stronger the benefit of increasing the vibration is.
In the resonance state f=fn, then According to Equation (2), when the excitation frequency is the same as the natural frequency of the system or component, the inertial force and the elastic force cancel out, and the system response is only the damping force.
The exciting frequency of axial fans mainly comes from the fault characteristic frequency, such as unbalance, misalignment, blade passing frequency, bearing fault characteristic frequency, etc.The main vibration source components of a typical two-stage movable vane adjustable axial flow-induced draft fan are shown in Figure 1, and the fault frequency range of each vibration source is shown in Table 1.When the natural frequency of the fan components and the inlet and outlet pipes is the same or similar to the fault characteristic frequency of the fan during operation, resonance may occur.With the change in speed, the exciting frequency of the fan also changes.If the frequency of the exciting force under certain or several speeds is the same or similar to the natural frequency of the fan component or the inlet and outlet pipes, it may also cause resonance.For example, the fan's upper and lower shells, support plates, blades, etc.All have one to several natural frequencies, which are usually determined by the percussion test.When the natural frequency is close to the common fault frequency of the fan, it is easy to cause local resonance.

Characteristics of structural resonance
The resonance of the axial fan is usually determined according to the amplitude and phase change characteristics of vibration, and its main characteristics are as follows: x In the direction in which the resonance frequency may be excited, the amplitude of the resonance frequency increases disproportionately, and the vibration value in a certain direction is much larger than the vibration value in the other two directions orthogonal to it and it is unstable.x There is an elevated background noise band near the peak of the spectrum.
x After changing the speed and crossing the resonance region, the peak amplitude is significantly reduced, but the frequency position is unchanged.x There is a phase difference of 180° from below the resonance frequency to above the resonance frequency, and there is a phase difference of 90° at the resonance frequency.

Treatment measures
The management of an axial fan resonance problem is usually considered from four aspects.The first is to change the natural frequency of the rotor or structure, so there is no resonance point in the whole working speed range.The second is to reduce the exciting force.When the external excitation is very small, under the action of system damping, even if the exciting frequency and the natural frequency of the system or components are in the resonance region, the vibration value can be controlled in the normal range.The third is to change the frequency of the exciting force, so the frequency of the exciting force is far away from the natural frequency of the rotor or structure.The fourth is to apply enough damping to the system to reduce the amplification factor.

Changing the natural frequency of the structure
The natural frequency of a structure is its inherent property and is related to the mass and dynamic stiffness (including the structural stiffness of the components themselves and the stiffness of the connections between the components), which is calculated as follows.
k is the stiffness of the structure, N/m; m is the mass of the structure, kg.
From Equation (3), it can be seen that changing the structural stiffness and mass of the component can change its natural frequency.
Changing the quality of the component requires a new design, which generally needs to be demonstrated by the manufacturer and requires a long construction period.It is relatively easy to change the rigidity of the structure.The higher the rigidity is, the higher the natural frequency is.If the rigidity is high enough, the natural frequency can rise to outside the working speed range.The site usually adopts a way to increase the reinforcement or increase the support to change the natural frequency of the parts.When the foundation bolt is not firmly connected or the support rust crack leads to insufficient stiffness, the connection stiffness can be improved by tightening the screws and leveling the contact surface.

Reducing the exciting force
For a vibrating body, in the absence of external force excitation, even if there is an obvious structural resonance, there would be no vibration in theory, because the implementation of this method is relatively simple, often used in solving the actual structural resonance problem on site.From the effect point of view, although the structural resonance still exists, after taking measures to reduce the rotor excitation force, the role of system damping can significantly reduce the vibration amplitude of the resonance region to effectively solve the vibration problem, such as improving the dynamic balance accuracy to reduce the power frequency excitation force, checking the consistency of blade and the uneven wear of blade, reduce the force of blade through the frequency, replace the faulty bearing to reduce the amplitude of the bearing failure frequency, improve the coupling alignment accuracy, and reduce the one or two times frequency excitation force caused by the misalignment.

Change the frequency of the exciting force
For axial fans, change the frequency of exciting force is usually not easy, there are usually two schemes: one is to avoid work in the speed range of the resonance zone, which is feasible for frequency conversion equipment.But if the speed range of the resonance is just in the normal load zone, this scheme is difficult to implement.The second is to change the frequency of the excitation force by transform the structure of the components.For example, by change the number of blades to change the blade through the frequency and replace the bearing model to change the bearing fault characteristic frequency, this program is effective for the resonance caused by the specific fault frequency.

Increase the system damping
As can be seen from Equation (2), in the resonance state, increasing damping can effectively reduce the amplitude amplification factor and thus reduce the vibration, but increasing system damping often reduces the fan efficiency.The field usually adopts the method of adding a damping shock absorber to avoid direct contact between the excitation source and the structure to eliminate the resonance.For axial flow fans, the determination of the resonance components, the placement position of the shock absorber, and the frequency setting need to be carried out by a specialized company.

A fault diagnosis case of an induced draft fan
The original single-stage booster fan and induced draft fan were combined into a two-stage adjustable axial-flow induced draft fan for a 600 MW unit in a thermal power plant.After putting it into operation, the vibration of each part is relatively stable under a high load.When the unit frequently deepens the peak load to 400 MW or below, the axial vibration fluctuates greatly, from 4.5 mm/s at the lowest to 29 mm/s at the highest, which is more than 3 times the horizontal and vertical directions.After the load is increased, the vibration gradually recovers, but there is a long-term slow growth trend.Spectrum analysis shows that its frequency component is mainly doubled frequency, as shown in Figure 2.For this reason, combined with the diagnostic opinions of relevant units, the power plant carried out some maintenance, include optimize the outlet flue of the induced draft fan to reduce the axial flue gas turbulence, washing the heat storage element of the air preheater to reduce the problem of large differential pressure of the air preheater caused by clogging, reinforcing the ribs of the bearing chamber to improve the support stiffness, and replace the rotor, etc.During the maintenance, cracks were found in the support rib of the bearing room many times, which were reinforced during the maintenance.The center review was carried out after each shaft re-installation, and a high-speed dynamic balance test was done to improve the accuracy of the neutralization dynamic balance.
After many maintenance methods, the radial vibration of the induced draft fan has been significantly reduced, but the axial vibration problem has not been completely solved, and then the radial vibration has gradually increased, leads to frequent failure repair of induced draft fan.Which seriously affects the safe and stable operation of the unit.
According to the fan system structure, operate parameters, and maintenance records, after in-depth discussion, the main reasons lead to axial vibration are finally determined as follows.The rotor itself is too heavy, the rotor thermal deflection changes or the rotor offsets the working center point under different working conditions caused by excessive exciting force.The bearing room support strength is insufficient, not enough to limit the rotor and bearing room vibration within the allowed range.Therefore, excessive exciting force and insufficient stiffness cause axial resonance, and the axial damping of the fan itself is small, which further increases the axial vibration.

Treatment measures
The weight reduction of the fan rotor removes the balance hammers on the internal blade adjustment handles of the first and second hubs, about 18 kg each, a total of 22 balance hammers of about 396 kg, and a total of about 782 kg of two-stage hubs are removed to reduce the excitation force of the rotor during rotation.Fan shell reinforcement removes the bearing room support rib plate of the upper and lower shell of the fan, keeps the front guide blade of the two-stage blade of the shell unchanged, and increases the bearing room support plate of the upper and lower shell by two, that is, changing the upper shell from three to five, changing the lower shell from nine to eleven, and replacing the original 30 mm steel plate with a steel plate of 36 mm thickness, as shown in Figures 3 and 4   In the reloading process, we break the foundation about 400 mm, adjust the shell levelness and shaft coaxiality, grout the lower shell twice after meeting the technical requirements, adjust the cylinder servo valve center after complete solidification, reinstall the fan blade, install the fan cover to measure the clearance of the bearing room and the tightness of the flange surface, adjust the center of the motor side coupling, etc.After the cold start of the fan, the spectrum characteristics at both ends are horizontal frequency, the amplitude is about 4.0 mm/s, and the phase is stable.The rotor of the fan is balanced, and the processing process is shown in Table 2.

Treatment effect
After dynamic balancing treatment, the horizontal frequency doubling is reduced from 4 mm/s to 0.15 mm/s, the axial frequency doubling is synchronously reduced from 4 mm/s to 1.20 mm/s, and the vibration value is at an excellent level.Under different loads, the axial and radial vibration is stable, and no abnormal changes occur.

Conclusions
The resonance of a large axial fan is a type of fault that is difficult to deal with.Improper handling measures often have a poor effect.Before determining the treatment plan, the cause of resonance should be determined by various test means.When selecting the treatment plan, it should be based on the actual situation of the equipment from easy to difficult, to avoid a wide range of equipment transformation.This paper analyzes the cause of resonance and the correspond treatment measures and combines with the field case comprehensive use of reduce the excitation force and improve the support stiffness measures to make a two-stage axial fan axial resonance that has been completely cured, to provide a reference for the future similar vibration problems analysis and treatment.

Figure 1 . 5 Figure 2 .
Figure 1.Schematic diagram of common axial flow fan vibration source components.

6 Figure 3 .
Figure 3. Support structure diagram of the original case of the induced draft fan.

Figure 4 .
Figure 4. Schematic diagram of the support structure of induced draft fan reinforced casing.

Table 1 .
Main characteristic frequency of each component failure.

Table 2 .
Data record of the dynamic balancing process of the induced draft fan.