Regarding the critical speed of steam turbines
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The turbine is a multi-stage condensing turbine of the NH25/04 model. Its rated speed is 12,300 r/min, while its critical speed is 5,085 r/min. During testing, it reached 4,200 r/min for the first time; immediately thereafter, vibration increased sharply, leading to an emergency shutdown. On the second attempt, when it reached 3700 r/min, the vibration suddenly increased and the machine stopped. The manufacturer says it’s a drift in the critical speed.
The critical speed is a relatively fixed range – why does it keep changing?
It might be abnormal if this happens. If it is the critical speed, it is possible to simply operate beyond this speed. It’s impossible to have two critical speeds! Unless it has been overhauled, but since the poster is testing a new unit, this situation is impossible. I don’t know how it was resolved later on.
I’ve never heard of that. Isn’t the critical speed always fixed? How could it change?
A centrifuge is a high-speed rotating device, and its rotating components cannot be perfectly balanced due to factors such as manufacturing defects, assembly issues, and uneven material properties. Therefore, when the frequency of the vibrations caused by these forces acting on the rotor coincides with or is close to the rotor’s natural frequency, the system experiences severe vibrations. This phenomenon is known as resonance, and the rotational speed at which resonance occurs is called the critical speed, which is equal to the rotor’s natural frequency. If there is another rotor on the same shaft, then there is one critical speed; if there are two rotors, then there are two critical speeds. Under normal circumstances, it is required that the machine’s speed be kept away from its critical speed
If the turbine experiences high vibration, it may be due to insufficient warming up. When starting the machine, ensure adequate warming up before increasing the speed. For a single shaft, the critical speed remains constant; by passing through that range quickly when increasing the speed, the vibration will drop to normal levels.
It’s not normal; it only vibrates strongly at critical moments!
In general, the critical speed is theoretically related only to the rotor, and the calculation formula reflects this as well. I have never encountered any drift in critical speed; at least not in compressors that have been overhauled numerous times and have been in operation for many years. Based on experience, I don’t agree with what the manufacturer says.
It is possible that rotor bending occurred during startup, resulting in an increased imbalance, which led to severe vibration at lower speeds. Or, during startup, the structure of the turbine is affected, its stiffness decreases, and its critical speed drops. The specific issues need to be analyzed carefully.
This post was last edited by Longkongzhi Eagle on 2010-5-31 at 15:11. The first occurrence might be due to insufficient warming up, while the second occurrence could be caused by bending of the rotor. If possible, run it at 2000 revolutions per minute for 3 to 4 hours, and then see what happens after reaching the critical point.