Source: Inlink Smart Diagnostics (ID: gh_7bfa6a26e890)
Most of the vibrations of the turbine-generator set belong to the forced vibration, and its magnitude is proportional to the unbalance and inversely proportional to the stiffness of the bearing seat. It can be expressed in a simple relational formula
where A is the amplitude, P is the amount of unbalance, and k is the housing stiffness.
Poor expansion creates a gap between the housing and the platen, resulting in a reduction in support stiffness. From this point of view, it is the same principle as the general stiffness reduction. The difference is that the stiffness changes during the operation of the unit, which will lead to the instability of the vibration during operation.
Expansion problems often occur after a long shutdown of the unit. The prolonged shutdown makes the unit completely cool and fully contracted, and the sliding pin system is also prone to corrosion, so the resistance increases after start-up, resulting in poor expansion.
During the operation process after a long shutdown, if the vibration shows a gradual upward trend, the poor expansion should be investigated as a suspicious point. Further diagnosis should be combined with the following characteristics:
1. Amplitude and phase characteristics
Poor expansion leads to a decrease in the stiffness of the housing, which plays a role in amplifying the unbalanced vibration. The frequency of the vibration is still 50Hz.
In the process of poor expansion, it is mainly reflected in the change of amplitude, while the change in phase is relatively small. Because the expansion is not smooth, the stiffness of the bearing, and the phase is mainly determined by the angle of unbalance, and the stiffness has little effect on the phase.
2. Duration
The heat capacity of the cylinder is much larger than that of the rotor, and the change in temperature is much slower than that of the rotor. As a result, the vibration-rising process lasts for a long time when the expansion is not smooth, which can reach more than several hours.
3. Trend characteristics
There are two types of poor expansion of the unit: one is irreversible, that is, the expansion is always blocked during operation and cannot disappear by itself, such as the complete jamming of the sliding pin, the stress of the pipeline, and the thermal deformation of the cylinder block; The other is recoverable, that is, it can disappear by itself through a stage of operation, generally there is a temporary jam in the sliding pin, and it can gradually disappear after the action of expansion force and vibration during the operation.
1. The first type is poor swelling
As shown in Figure 1(a), the vibration tendency of the first type of poor expansion can be roughly divided into the following stages:
Initial phase, section of curve 1 in Figure 1(a). At this time, the vibration is generally stable, and the problem of poor expansion has not yet occurred.
Ascending phase, section 2 of the curve in Figure 1(a). This is the stage of the emergence and development of poor expansion, which is generally the initial stage of the unit's operation. At this stage, the temperature changes greatly and the expansion is large. Once the expansion is blocked, it will gradually develop, so that the stiffness of the bearing will become lower and lower, which is manifested as a continuous increase in vibration.
Stabilization phase, section 3 of the curve in Figure 1(a). At this time, the temperature and expansion of the unit are generally stable, but the existing expansion is not smooth, and the vibration is always maintained at a high level.
(a) Type I vibration tendencies with poor expansion; (b) Type II Expansion Vibration Tendency
Fig.1 Two types of bloat problems
2. The second type is poor swelling
As shown in Fig. 1(b), the vibration trend of the second type of poor expansion is roughly divided into five stages: the curve 1~3 part is similar to the stage corresponding to Fig. 1(a); 4 on the curve is the recovery stage, when the stuck parts are gradually dredged and the vibration is gradually reduced; 5 on the curve is the normal phase, when the vibration returns to the level before the change.
Fourth, the fluctuation of vibration
Figure 1 shows only the general trend of vibration. In fact, in most cases, the vibration changes in waves (wave up, wave down, even at a high level), the reason for which is that jamming and dredging, deformation and recovery often coexist. For example, in the early stage of operation, although in general it is a stage when the jamming is getting more and more serious, the stiffness is gradually reduced, and the vibration is increased, there is also local dredging to reduce the vibration. As a result, the vibration is generally rising, but fluctuating during periods.
In the presence of poor expansion, there is sometimes static friction, which can also cause fluctuations in vibration.
5. Reproducibility of vibration
The volume of the cylinder is large, and the temperature rises slowly when heated and falls slowly when cooled. Due to this characteristic, if the machine is started again after a short period of time after the shutdown, the vibration at the fixed speed will be greater than that at the last fixed speed because the temperature of the cylinder has not returned to the state when the last time the machine was turned on and the deformation of the cylinder has not been recovered. Table 1 shows the vibration of the low-pressure bearings at constant speed after the overhaul of the two units.
Table 1 Vibration of low-pressure bearings at constant speed after overhaul of two units
Sixth, the relationship between shaft vibration and seat vibration
When the deformation of the cylinder block causes the stiffness of the bearing seat to decrease, it is often manifested that the vibration of the bearing seat changes greatly, while the shaft vibration changes little, and even the shaft vibration is smaller than the vibration of the bearing seat.
Table 2 shows the vibration of bearings No. 5 and No. 6 of unit 9 of XX power plant. The unit is Dongqi D029 unit, No. 5 and No. 6 are low-pressure rotor bearings, and each bearing has a seat vibration measurement point and two shaft vibration measurement points. Due to the thermal deformation of the low-pressure cylinder, the vibration gradually increases during operation, but the change of shaft vibration is much lower than the change of housing vibration.
Table 2 Vibration of bearings 5 and 6 of unit 9 of XX power plant
7. The difference between thermal bending and dynamic and static friction of the rotor
The thermal deformation of the cylinder block leads to a decrease in the stiffness of the bearing seat and an increase in the imbalance caused by the thermal bending of the rotor, both of which will exhibit an increase in vibration with load. However, it can be distinguished from the following aspects:
- The change of the housing vibration of the former is more obvious, while the change of the shaft vibration of the latter is more obvious.
- The former has no effect on the shaking, while the latter causes a change in the rotor shaking.
- The heat capacity of the cylinder is much larger than that of the rotor, and the change in temperature is much slower than that of the rotor. Therefore, the vibration change process caused by poor expansion lasts for a long time, which can reach more than tens of hours. The vibration increase process caused by the thermal bending of the rotor lasts for a short period of time, and can be stabilized in about 1 hour.
- Vibrations caused by poor expansion are generally noticeably different after two start-ups, because the cylinder temperature is different on both occasions. Vibrations caused by thermal bending of the rotor do not, as the thermal bending resumes quickly after a shutdown.
- When dynamic and static friction occurs, the vibration is in a continuous and constantly changing state, while the expansion is not smooth, and there is a stage of relatively stable vibration.
- Other features: When the expansion of the high (medium) pressure cylinder is not smooth, the expansion indication can be reflected from the head, and the expansion value and vibration may jump. When the low pressure expansion is not smooth, it can be reflected by the abnormality of the expansion difference (large positive expansion difference). When expansion is not smooth, the contact surface between the housing and the platen can sometimes be measured for clearance.
Beijing Shenzhou Xiangyu Technology Co., Ltd