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The relationship between steam turbine shaft vibration and shoe vibration 2020-04-06


The shaft vibration is the radial vibration of the rotating shaft. At present, the shaft vibration of the steam turbine unit is generally measured by an eddy current probe. When a high-frequency current passes through the coil in the probe, a high-frequency electromagnetic field is generated and an induced current is generated on the surface of the measured rotor journal, which is converted into a voltage and expressed. And this voltage changes with the distance between the shaft surface and the sensor, so that the vibration of the rotating shaft is measured. Shaft vibration is generally expressed in terms of displacement, in microns. If the eddy current sensor is fixed on the bearing shell, the relative vibration between the rotating shaft and the bearing is measured; if the sensor is fixed on the basis, the measured vibration is approximately regarded as the absolute vibration of the rotating shaft.

 

Tile vibration is the bearing seat vibration, also known as bearing vibration. It is generally obtained by a contact-type speed or acceleration sensor, which is usually directly fixed on the bearing cover or adsorbed on it through the magnetic seat, so it is sometimes called shell vibration and cover vibration. The measurement of tile vibration is mainly in the vertical direction, followed by the horizontal direction, and the axial vibration is used as a reference.

 

The turbine or generator rotor is supported by the bearing, and the vibration of the rotating shaft will inevitably be transmitted to the bearing, so there is a certain relationship between the two, including amplitude, phase, frequency, etc.

 

1. The relationship between the amplitude of the two. The proportional relationship between the amplitude of the shaft vibration and the tile vibration has a great relationship with the stiffness of the bearing seat. Generally, if the bearing seat is rigidly supported (such as a general floor bearing), the amplitude of the shaft vibration is considered to be about 3 of the tile vibration ~ 6 times. If the support stiffness is weak, the ratio will be reduced accordingly, and even the case where the shoe vibration is greater than the shaft vibration (such as the low-pressure rotor seat cylinder bearing seat of Dongqi 60 watt unit);

 

2. Phase relationship. The tile vibration is generally a velocity value, and its phase is ahead of the displacement value of the shaft vibration by 90 °. When the velocity value is converted to a displacement value, its phase angle needs to be increased by 90 °. If the vibration is caused by unbalance, the phase of the unbalanced mass has a fixed relationship with the phase of the shaft vibration, and it also has a similar relationship with the phase of the tile vibration. It is precisely because of this relationship that the dynamic balance through the tile vibration on the spot becomes may.

 

3. Frequency relationship. The two have almost the same frequency components. The difference is that when the speed value is integrated into the displacement value, part of the high-frequency component will be lost. Therefore, when measuring the mechanical vibration of complex structures such as rolling bearings, fan hubs and pump impellers, it is customary to use The vibration speed value is used for fault analysis because it can provide more abundant spectrum information.

 

Domestic turbo-generators mostly use relative shaft vibration, which reflects the vibration of the rotor relative to the support or the cylinder. The rotor's shaft vibration is large, indicating that the rotor has a large displacement from the balance distance during one revolution. The tile vibration is an absolute vibration, which reflects the vibration of the bearing housing etc. relative to the foundation. In modern steam turbine vibration detection, both shaft vibration and shoe vibration are necessary. If the relative shaft vibration of a rotor is small and the tile vibration is large, it means that standing on the motion coordinate fixed to the cylinder, the displacement of the rotating shaft relative to the support system changes little, and the relative contact between the two can be avoided; The dynamic stress of the rotor itself and the dynamic stress received by the bearing support also depend on the vibration value of the cylinder itself; the vibration of the cylinder and the support is large, and the dynamic stress sustained by the rotor and components must be high. Is unacceptable.


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