Critical speed range

Dear experts,

We are having a turbine rotor which during High Speed balancing shows a critical speed range from 1500 RPM to 4000 RPM(actual critical speed 3200. I have read that critical band is +/-20%. But considering the above data it is about 50%. 

We do not have earlier records as the rotor is more than 20 years old and gone under several repairs.

I just want to know is it possible that during time course the band has increased. If yes then what can be the reasons

Secondly can multiple repairs be a reason for this increase in band? If yes how to avoid it.

Original Post

Casing vibration is measured in routine monitoring however during high speed balancing proximity probes were installed to measure shaft vibration.

During high speed balancing this range is observed. In routine monitoring transient data is not collected.

I would consider two possible issues:

1) Damping has increased which tends to broaden a resonance peak. If this were the case, then amplification factor is less and peak vibration amplitude would be lower.

2) More than one vibration mode is present within the speed range. This is more likely with casing measurements. If the speed runup is too fast, then vibration response can be smeared and not show 2 or more individual peaks. Typical coast-down vibration response can be different from run-up, because the speed change is less and possibly steam admission is not present. I am sure there are technical papers that can give a better description; or Mr. Foiles can step in on this one!

Walt

Not possible to have a critical speed for that length of speed. What is turnine actual operating speed? Are you sure the shaft mode does not change during this speed change? Are you sure this is the 1st critical or 2nd critical l? Does your phase change 90 degrees or more during this speed change? How many mils are you talking about during range or at peek? Many factors involved in evaluation and high speed balancing

 

Sorry for lack of data.. but this balancing activity was carried out at vendor works and no plots were available. If I manage to get any plots in next such opportunity i will definitely share them. Thanks to all for inputs. 

Some questions were asked in the thread:

Rated RPM-5200

Phase change 100º during critical

Max amp during critical-29 microns

Before critical amp-12 microns

At rated speed amp-10 microns

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