Why is Rotor Dynamics so Important?

Welcome back for the 3rd installment of our introduction to rotor dynamics! If this is your first time having a look at this series, hello! Feel free to have a look at the previous installments if you want to play catch-up or get a refresher.

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Otherwise, let’s get into a question I’m sure a few of you have been asking. Why is rotor dynamics analysis so important?

Steam Turbine View and Train Lateral Model
Steam Turbine View and Train Lateral Model in AxSTREAM RotorDynamics

Let’s start with a basic premise. As we’ve previously established, rotor dynamics is the behavior of rotating equipment and the analyses of said behavior. Rotating equipment tends to be very expensive to design, develop, and manufacture, so from a financial standpoint, it is prudent to ensure that the behavior of the equipment as it operates does not jeopardize itself or any other. A machine like an aero engine cost hundreds of thousands or even millions of dollars for a team to design, analyze and refine the flowpath, therefore, an analysis which costs a fraction of that money and also ensures the rotor-train is properly supported is a prudent use of time and engineering resources.

From a preventative maintenance standpoint, proper rotor dynamics analysis can ensure that excessive maintenance and repair costs are avoided. Rotor dynamics analyses will help to ensure that engineers aren’t constantly trying to diagnose unprompted equipment failures, which in and of itself can be extremely costly.

From a dependency standpoint, rotor dynamics is critical in preventing excessive downtime and unexpected breakdowns due to vibrational issues. Turbomachinery and rotating equipment are used in everything from power generation to aircraft propulsion, and even in cardiac pumps. These machines can’t just have unscheduled shutdowns when lives depend on them. A multimillion-dollar turbine can’t suffer a breakdown at 34,000 feet in the air while going 500+ miles per hour because no time was taken to ensure that the engine wasn’t operating at the design’s critical speeds where vibrations would be too high.

Now let’s look at it from the worst case scenario; what happens when rotor dynamics analyses aren’t performed during the design phase? The best thing that can happen, is the machine wears excessively in a short period of time, and during an inspection, the turbomachine is forced out of service due to premature excess wear. In the absolute worst case, lives are lost, as we have seen in the past, either directly from the failure of the machine, or because the machine was being depended on to operate without failure.

A Coupling Failure in an Induced Draft Fan System
A Coupling Failure in an Induced Draft Fan System
Source: Wang Q., Feese T.D., Pettinato B.C. Torsional natural frequencies: measurement vs. prediction. Proceedings of the 41st Turbomachinery Symposium Sept. 24-27, 2012, Houston, Texas

As my colleague Dr. Pastrikakis described in his blog a while back, people can die because rotor dynamics analyses were not performed. For those who aren’t able to access the article, there have been cases where a turbofan engine’s blades were ejected from the engine, and the ensuing shrapnel killed two passengers aboard the aircraft. In other cases outside the aviation world, turbine generator shafts have suffered catastrophic failures because the machine was run at its critical speed. Had people been nearby at the time of the failure, they could very easily have lost their lives.

In other cases, a high-speed rotating machine like a hard drive can fail because it was not properly balanced and supported, and important data can be lost as a result. A pump in a person’s heart can suddenly fail, sending them to the hospital, or potentially their grave. A medical gas compressor in a hospital can unexpectedly shut down, and if there are no backups in place, it could bring a halt to important surgeries and other operations going on.

By now it should be clear that rotor dynamics is not a trivial engineering discipline to be taken lightly. Although there may not be too many engineers in the world who specialize in these analyses, those who do are well aware that the consequences of incomplete or improper lateral and torsional analysis, can have disastrous or even fatal results.

A Destroyed Multistage Compressor Rotor

So, how can these terrible consequences be avoided? Easy (on paper, not so much in practice, unless of course you’re an expert!) properly perform lateral, torsional, and axial vibration analyses on your turbomachinery during the design phase. In comparison to other stages of the design process, this one is not as labor and time intensive, but it is critical. Model the entire rotor train, and the proper bearings, supports, seals, etc. On that note, ensure that you are accurately modeling and calculating the support bearings’ stiffnesses for your rotor train. Accurate models mean accurate results. Accurate results will help you to determine if your machine’s design will meet the proper API (and other governing bodies) standards. Next time, we’ll discuss this more in depth.

If you want to learn more about the importance of rotor dynamics, or about the tools our engineers and thousands of others around the world rely on for their turbomachinery designs, reach out to us at info@softinway.com

Interested in using AxSTREAM or our AxSTREAM RotorDynamics package for your project? Shoot us a message at sales@softinway.com