Is vibration analysis over rated ? 1

[rQuestionEngineering]

New turbine tripped on high vibrations during startup.

Phd vibrations analysis came back with 40 pages of studies about undamped critical speeds, damped , Bode , centerline , waterfalls. But unable to tell what was wrong apart of a list of possibilities.

Tradesman turbine technician was also clueless about that , but just did trim balance on the coupling end. Vibration was reduced to below trip limits and unit went for endurance run.

Its such an irony the skill gap and pay gap in this sense, a low rank skilled worker outperforms s specialist.

Are Phd and Vibration Analysis over rated in turbomachinery field ?

 

[3DDave]

Who decided to hire the PhD for a balance problem? PhDs aren't technicians. Besides, if there was a design problem the technician would likely have failed - but the lack of a design problem suggests that the PhD who designed it did a good job and it was the technicians who installed it who failed.

 

[JohnRBaker]

Sounds like the technician simply made an adjustment which brought the system into compliance with the design specifications and the proscribed manufacturing/installation tolerances.

John R. Baker, P.E. (ret)
Irvine, CA
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[SWComposites]

Many PhD’s have never touched real hardware. Sigh.

 

[CWB1]

Most of the (many) Phd's I've worked with were dedicated researchers, not regular design engineers. Asking them to troubleshoot an issue in the plant or design a new system would be rather problematic bc its not something they have a lot of experience in.

 

[Scuka]

Its such an irony the skill gap and pay gap in this sense, a low rank skilled worker outperforms s specialist.

It's not a question of who outperformed who. It's not a contest.

The PhD guy designed the turbine.
The technician installed and adjusted the turbine.

They each did their jobs. Everything is working as intended here, as far as I can see.
Different people in different positions have different kinds of education and experience.

Would the technician be able to solve the problem by "just trimming balance on the coupling end" if the PhD skipped on his 40 pages of vibration analysis?

 

[TugboatEng]

Was the trim balance on the coupling end in the list of possibilities produced by the engineer? If so, the engineer did their job.

 

[IRstuff]

Its such an irony the skill gap and pay gap in this sense, a low rank skilled worker outperforms s specialist.

The converse question is whether the skilled worker can design and analyze a system? The answer is almost always no; hence the skill/pay gap is valid. There are few true generalists that can span analysis, design, fabrication, and test. That's the nature of our evolved world, starting with the first specializations of hunter/gatherer societies. Rather than have everyone make their own knives and arrow points and having inconsistent quality, the best person for that job makes all of edges, and the others remunerate them for their services and goods.

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[geesaman.d]

Ha, I experienced the exact same thing once - some consultant spent days banging with hammers and wires strung all over the machine. Cost us $50k. A balancing expert came out and decided to set the high speed coupling keyways in opposite directions. The machine was in spec just like that. (Given the rotating mass and RPM, I was amazed it had such a drastic effect, but now we know and it's a simple thing to do.) Granted, the balancing expert was prone to look for balancing issues, but it does always circle back to fundamentals.

I've also read vibration reports provided by my customers, and it's clear that their "reliability consultant" is just someone using a canned vibration software puking pages of data. Not a clue anywhere in those reports, just lots of alarms based on inappropriate limits.

Vibration analysis solves certain problems very neatly, others not at all. In my experience the vibration signatures are often not as clean as the book examples. A good vibration analyst is quite valuable.

 

[GregLocock]

OP, can you poste the cambpells diagram or waterfall of the original problem?


Cheers

Greg Locock


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[spciesla]

There is not enough information in your post to make an assessment of the situation.

"New turbine tripped on high vibrations during startup."

Did it trip when attempting to transition through a critical speed, at full speed no load, after being synchronized to the grid and at X% of base load? Starting a cold turbine is not trivial, especially with a new machine. Vibration is a symptom, not a root cause of an underlying issue or malfunction.

My employer has 150+ vibration analysts with focus on the turbomachinery field. This is our core work. There is a mix of engineering education background, but most have a BS in mechanical engineering (some in electrical), many have MS or Ph.D. degrees. Determining if unbalance is the problem and then field balancing a turbine rotor or train is also not trivial. The vibration data needs to be analyzed and assessed to determine if it exhibits unbalance characteristics. Then an appropriate balancing strategy needs to be implemented (static/couple at both ends of the rotor, single end shot(s), coupling shot(s)). This is usually performed under a significant time pressure from the customer to complete the balance in one run (or fewer, haha). Vibration analysis on turbomachinery is a rewarding, difficult, and humbling career. The running joke at work is that the machines don't "read the book".

A forty page report (including plots) regarding the vibration behavior in various machine conditions (slow roll, transient, and steady-state) isn't unusual. Now typically, that report wouldn't be written until the unit was successfully started and operating.

If a customer has a critical piece of turbo machinery that can't be started/operated due to vibration, then how can vibration analysis be over rated?

Here are a couple of reference books that you can review and assess if vibration analysis on turbo-machinery is over-rated.

https://www.amazon.com/Machinery-Ma...&sprefix=machinery+malfunction,aps,529&sr=8-1

https://www.amazon.com/Fundamentals...amentals+of+rotating+machinery,aps,150&sr=8-1

Steve