Why isn’t the shaft vibration high?
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The steam source driving the turbine in our facility is SS (500°C, 10.5 Mpa). A few days ago, there was a fluctuation in the SS temperature, which dropped as low as 450°C. At that time, the temperature of the turbine thrust bearing rose by 10°C, and the shaft displacement increased accordingly, but there were no changes in shaft vibration or the temperature of the radial bearings. Could it be that large temperature fluctuations only affect the axial balance of the rotor, without affecting its radial balance? I’m confused and would like to ask everyone for advice. Thank you.
If the strain is caused solely by temperature, it should occur in one direction primarily. In the case described by the original poster, it is mainly due to axial unevenness; as for the radial direction, the temperature of the bearing shells rises overall, and the temperature difference is not significant. Even if there is slight deformation, it is absorbed as axial strain, so the radial effect is not very noticeable.
The pressure difference between the inlet steam and the exhaust gas generates a portion of the axial force. Therefore, the main cause of the change in axial displacement is likely the variation in thrust bearing load resulting from pressure fluctuations in the steam pipeline network. To determine if this is the reason, you can check whether the direction of the change in pressure difference matches the direction of the change in displacement. If a stricter standard is required, the theoretical displacement change can be calculated based on (the projected area of the SS section of the turbine rotor * the change in pressure difference) and the stiffness of the thrust bearings. If the radial vibration increases, it is either due to uneven thermal expansion; in most cases it should be only temporary.
Thank you all for your replies. A sharp drop in the SS temperature over a short period of time, in my opinion, should also affect the radial thermal expansion of the rotor, leading to increased vibration and higher temperatures in the radial bearings.
Although radial expansion/contraction inevitably occurs, such deformation is not necessarily asymmetric, and therefore it does not necessarily lead to an increase in the rotor’s imbalance. As for what was said on the second floor, it seems a bit inappropriate – uniform material undergoing uniform thermal expansion should be essentially isotropic, with only the axial dimensions changing. Typically, this thermal contraction affects the bearing load in a dynamic manner; here, dimensions are not the main factor, but rather the bearing load is.
Changes in the steam source temperature cause changes in pressure difference and shaft displacement. It may cause uneven radial expansion, leading to changes in radial vibration; this is related to the structure and mass of the rotor, but it is not inevitable.
Yes, it’s the axial displacement, and the thrust balance disk factor also needs to be taken into account.