We have a turbine driven blower with quite abnormal vibration pattern. The common operating speed of machine is 2600-3100 rpm. There are a lot of fractional harmonics in the 2400-2900/min interval, while the 0,5×f0 subharmonic travels trhrough a 20-24 Hz resonance-band. Above 2900 rpm all the fractional harmonics disappear, integer harmonics remain.

Seems to be a bearing looseness or clearance/oil instability broblem, doesn't it?

turbo4.doc (64 Kb, 41 downloads) turbine spectra

Libra,

Not sure if points 1 & 2 are on Blower or Turbine. Either way, it surely is looseness and I doubt it is bearing looseness/clearance. The amplitude is at an acceptable level but, the signs warrants attention.
I'd look for loose anchor bolts that need to be torqued.

Regards...Rajan

Best bet: Rub facilitated by looseness...

RajSha!

These spectra are from turbine side. 1 V, 1 H mean NDE vertical and horizontal, 2 V, 2 H mean DE vertical and horizontal.

What type of turbine, steam? Sub-synch occurs during starts, but not shutdowns?

Have you looked to see if this is related to steam emission – causing an instability?

The presence of the integral harmonics are puzzling, but this could relate to a rub. The seismic levels are relatively low. Instabilities often lead to rubs. Fluid film bearings? If so try shaft relative measurements.

Yes, it is a fifty years old steam turbine with fluid film bearings. We could measure during coast down 3100-500, and spin up 500-3100 rpm. Fractional harmonics are present only between 2400-2900 rpm. It is seems to be related to a structural resonance 20-24 Hz. Waterfall plot shows this very clear. After the subharmonic peak leaves the mentioned band, there're no fractional harmonics anymore. Several harmonics are present always. Vertical readings are often higher than horizontal.