Dominant Peak at 6 times RPM

[mtigos]

I have a problem with a dominant velocity peak at 6 times the rpm. What symptom does this indicate. This peak is on the NDE of a 400 kW Booster fan. The fan uses journal bearings. The DE of the fan shows 2 times rpm vibration but on the motor DE there is no indication of 2 times which would point to misalignment.
I need help with the 6 times component. And also whether taking measurements across the coupling should both have 2 times component to point to misalignment. Thanks.

 

[GregLocock]

How many blades are there on the fan? How many duct supports? we need a photo of the installation

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mtigos]

The Fan has 9 blades. There are six duct supports on the discharge side and four duct supports on the Inlet side.
On the discharge side however, there is an extra small mechanical (screw-like) support rod, type that looks like adjusting rods for accelerator linkages on old heavy trucks. No one has ever touched it though, even during shut-downs. I have attached a photo, dont know if it will be any help. Please let me know about the picture. I can always get another one.

Thanks GregLocock.

 

[JJPellin]

Some non-lubricated couplings such as Thomas Series 71 often have 6 bolts through each disk pack. Is the vibration at exactly 6 times run speed, synchronous with a consistent phase angle? If the fan is running pinched back at low flow, it could be surging which could excite a resonance in the rotor, duct work, structure, pedestals, etc. Some other part of the fan system could be resonating at about 6 times run speed. The vibration could be transferring from other equipment.

Johnny Pellin

 

[mtigos]

The coupling is non-lubricated TurboFlex HBS-540 with 8 bolts. Yes the vibration is at 298.9 Hz, close to 6 times the rpm. I will have to take newer readings with phase angle.
I dont understand when you say "fan running pinched back at low flow"?

 

[JJPellin]

Most of our booster fans are on heater service for induced draft or forced draft. In these fans, it is common to control the flow through the fan with inlet louvers. The louvers act like a control valve to reduce flow in order to achieve target values for fire box pressure, stack temperature, etc. I don't know your process. But, if there are louvers on the inlet or outlet of the fan or a stack damper or other device that restricts the flow, it could be possible to restrict the flow so much that the fan goes into a surge or stall. This condition is characterized by high vibration at unusual frequency. The fan I am working on right now surges at 13 Hz with a running speed of 20 Hz. When it goes into surge, the entire structure, duct work, foundation, piping and conduit all shake at about 13 Hz. When they open the louvers and increase the flow, this 13 Hz vibration goes away.

Johnny Pellin

 

[GregLocock]

In my opinion you don't have a sixth order problem, you have a first or second order problem with harmonics.

Why do you think the sixth harmonic (300 hz) is dominant?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mtigos]

Greg, If you look at the NDE axial and Horizontal, the is a peak at or about 300 Hz. The DE horizontal does show a 2nd order with harmonics. What I dont understand is why the 6th harmonic is significantly big on the NDE axial & Horiz. Also what is indicated by the 2nd order with its harmonics on the DE? Thanks.

 

[GregLocock]

where is the axial measurement?

On the Horizontal measurement you have a big spike at 50 Hz bigger again at 100 hz, and then a descending series of harmonics. No way would I describe 6th order as dominant.

The sixth order is just part of the harmonic series which I suspect is based on 1st order , 50 Hz.

Have you balanced the fan recently?


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mtigos]

yes I have balanced it on the 16 Nov, on the DE. The vibrations on the Bentley was reading 4 m/s2 rms. They are now reading 10 m/s2 rms. I had brought it down from 35m/s2 rms.

 

[mtigos]

Brought it down from 35m/s2 to 4m/s2. The vibes picked to +-13m/s2 three days ago. There was a leak (flue gases) blowing onto the DE bearing housing, but that was rectified (carbon seal replaced) and the vibes have since remained at +-10m/s2.

 

[MikeyP]

You need to answer Greg's question.

Where is the evidence that this is a 6th order vibration problem? Your graph in your first post does not show "domninant" 6th order vibration.

M

--
Dr Michael F Platten

 

[TPL]

Quote:
Brought it down from 35m/s2 to 4m/s2.
Unquote

If correct, your vibration units are in terms of acceleration, so you've balanced it from nothing to less than nothing.

The data you show doesn't indicate a problem: an overall amplitude of <6mm/sec pk is OK. Take any machine vibration spectrum and, look hard enough you will find things that cannot and do not need to be explained - they just are!!

Can you explain your concerns a little better?

 

[mayt4u]

Bad bearing on nde.

could be worse!! Could be me.