Diagnosis and treatment of vibration increase of t

2022-10-04
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Diagnosis and treatment of vibration increase of induced draft fan

one 300 MW unit boiler is equipped with two an25eb axial-flow induced draft fans with adjustable stationary blades. Since the fan was put into operation, the panel assembly industry has faced considerable market space problems due to excessive vibration, and some measures have been taken. However, the vibration characteristics of the fan are still small in no-load or low load operation, increased in high load and full load, and forced to reduce the load or stop the fan for treatment for many times. The vibration threatens the safe and economic operation of the unit

1 vibration diagnosis

1.1 cause analysis

(1) generally speaking, the vibration source of induced draft fan should come from the fan itself, such as the non-uniformity of rotating parts; Rotor mass imbalance caused by manufacturing error; Poor installation and maintenance quality; The operation adjustment of induced draft fan is poor when the boiler load changes; The rotor is worn or damaged, and the front and rear guide vanes are worn and deformed; The opening of inlet and outlet baffles is not adjusted in place; Bearing and bearing seat failure can make the induced draft fan vibrate under the action of small interference force

however, due to the adoption of a series of corresponding treatment measures, such as the anti-wear treatment of the fan impeller and the rear guide vane, the use of imported high-quality products for bearings, the anti loosening measures for the high-strength bolts connecting the bearing housing and the end plate of the core barrel, the support and fixation of the core barrel have been improved, and lacing tendons have been added; Strict maintenance of process quality and the addition of vibration monitoring devices for induced draft fan operation have solved some practical problems. The fan operates well at low load, but the phenomenon of increased vibration at high load has not been solved

(2) when the fan is started in cold state, the vibration is small when it rises to working speed and low load, indicating that the vibration caused by rotor mass imbalance has little effect with the change of speed; From the frequency spectrum analysis of fan vibration, it can be seen that the fan vibration is mainly power frequency vibration, which can eliminate the effects of rotating stall, surge and so on

(3) measure the natural vibration frequency of the fan blade by hammering. The working frequency of the fan (after blade wear prevention) is 16.5 Hz, and the first-order frequency of the blade has been greater than k=7, so it is safe for the first type of exciting force; There are 24 guide vanes at the inlet of the fan, and the frequency of the second type of exciting force is 1. The first end of the device is a solid rod 6.5 connecting one pedicle screw × 24=396 Hz, but no frequency of about 400 Hz was found in the spectrum analysis, so it can be considered that the second kind of excitation force has little effect on blade vibration and fan vibration

(4) fan vibration mainly refers to the increase of high load or full load vibration, and the vibration is unstable, with fluctuations or periodic vibration

① unstable vibration may be related to boiler combustion adjustment, flue gas flow rate, flow distribution of two parallel running fans, etc., and also reflects the problems of poor fan support stiffness and possible local looseness. The vibration of the fan increases when it enters high load. If it continues to operate for a long time under this condition, the main bearing may be damaged, and its foundation, bedplate, impeller and main shaft connecting parts may be loosened by vibration, which further worsens the vibration, and finally leads to the disassembly and maintenance of the stopped fan

② the most common items tested by the universal experimental machine for fan operation are tensile strength and tensile modulus, flexural strength and modulus line bearing capacity. When the load is high, the fan output increases. According to the principle of force and reaction, the result is that the force supporting the rotor increases and the load of the fan supporting foundation increases. If the stiffness of the fan supporting foundation or related connection stiffness is insufficient, its bearing and anti-interference performance is poor. Although the vibration source of the fan comes from the fan itself, due to the structural characteristics of the fan, there is vibration under no-load or low load, but it does not exceed the standard; When the fan support stiffness is insufficient and operates at high load, the existing but not exceeding standard vibration of the fan will provide conditions to amplify the vibration, and the above characteristics of high load vibration increase appear. Therefore, the analysis shows that the increase of high load vibration of the fan is caused by insufficient support stiffness

1.2 on site detection and diagnosis

1.2.1 on site detection

concrete foundation support is under the fan steel support, and the relevant structure is shown in Figure 1. In the figure, there are three groups of supports, one on the left and one on the right of each group

in order to further judge the relationship between vibration and insufficient support stiffness of the fan, in November 2000, when the fan was operating at a generating load of 240 MW in 2006, a vibration meter was used to measure the vibration of the fan along the circumference of the casing and the fan support foundation at the rear guide vane at its outlet. The results are shown in Figure 2 and figure 1 respectively (Note: the length unit in the figure is mm, and the vibration measurement unit is µ)

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