HIGH AXIAL VIBRATION

Hello guys please help me to solve this critical problem.

please go through the attached file.

High vibration in Preheater Fan

 

Case: During our monthly scheduling we are continuously observing high axial vibration in preheater ID fan at DE end. Radial vibration is under 1mm/s or 1.2mm/s.

Description: 1-Fan R.P.M-900

          Motor K.w-1500kw

Vibration in motor:  NDE    H: 0.8MM/S    V:0.6 MM/S      A-0.4MM/S

          DE     H: 0.4 MM/S    V: 0.9 MM/S    A: 1.1MM/S  

 Vibration in Fan:          DE     H: 1.1 MM/S    V: 1.5 MM/S    A: 5.1MM/S  

          NDE    H: 0.9MM/S    V: 0.6 MM/S      A-1.8 MM/S

 

Cross Phase measurement:

  • Test for misalignment:

Radial phase across the coupling-erratic

Axial phase across the coupling: erratic

  • Test for bent shaft:

Radial phase across the fan bearing-9 degree

Axial phase across the fan bearing: 115 degree

  • Test for cocked bearing:

Cross phase at opposite side of DE bearing-180 degree

  • Test for unbalance:

Cross phase at fan DE bearing: 90 degree phase sift b/w the sensor at fan DE bearing from H TO V 

 

 

 

 

 

Attachments

Original Post

dear kishore 

its simple supported centrifugal fan.  at present my software is not working so i am not able to post the spectrum but by taking vibration with skf microlog we can able to see the spectrum in microlog that showing only 1x with an amplitude 4.2 mm/s. overall vibration is 5 mm/s

please confirm your phase analysis with a real world data using dial indicator

take radial and axial readings for alignment and check it to be within the tolerance 

check the runout at the bearing end to ,,usually runout for the id fans could reach 0.1 mm./sec 

if the fan was stopped before suddenly and the started after while it could make some axial vibration due to bending but with operation it will correct itself.

please assign some spectrums 

please confirm your phase analysis with a real world data using dial indicator

take radial and axial readings for alignment and check it to be within the tolerance 

check the runout at the bearing end to ,,usually runout for the id fans could reach 0.1 mm./sec 

if the fan was stopped before suddenly and the started after while it could make some axial vibration due to bending but with operation it will correct itself.

please assign some spectrums 

Ralph Stewart posted:

Vishal,

Is this an overhung fan with anti-friction bearing? Direct drive?

A sketch of the unit would help a lot especially if the measurement locations are also indicated.

Has the fan been in service for a long time or it was recently installed?

I'm not familiar with the process of this unit and its criticality. But from the overall values provided, the maximum of 5.1 mm/sec (0.21 in/sec) is not too bad for the machines I deal with is the reading is stable. BTW, is it 0-p or rms?

Regards- Ali M. Al-Shurafa

Shurafa posted:
Ralph Stewart posted:

Vishal,

Is this an overhung fan with anti-friction bearing? Direct drive?

A sketch of the unit would help a lot especially if the measurement locations are also indicated.

Has the fan been in service for a long time or it was recently installed?

I'm not familiar with the process of this unit and its criticality. But from the overall values provided, the maximum of 5.1 mm/sec (0.21 in/sec) is not too bad for the machines I deal with is the reading is stable. BTW, is it 0-p or rms?

Regards- Ali M. Al-Shuraf

this fan is in service from last 8 years.

Rotating Guy posted:

Vishal,

in relation to my above post, could you do phase reading for this attached picture from left and right side view. 

regards,

rg

U r talking about to do cross phase by putting one sensor top of the bearing housing and move other sensor on marked position 

Has the axial vibration always been high or is it increasing with time? My first check with increasing axial vibration on the DE of the motor would be to inspect the coupling for wear / dry and ensure the hub gap is sufficient. What type of coupling is it?

moily posted:

Has the axial vibration always been high or is it increasing with time? My first check with increasing axial vibration on the DE of the motor would be to inspect the coupling for wear / dry and ensure the hub gap is sufficient. What type of coupling is it?

Vibration is increasing and decreasing with time. But minimum vibration sometimes 2 mm/s and sometimes 7mm/s. We check already damper position.

Did you check how far is the fan critical speed from the major structural natural frequencies of DE bearing pedestal? If not, I suggest to perform the bump test on DE bearing pedestal when the fan is off and compare the major structural natural frequencies in axial direction with the fan critical speed. If they are not apart by more than 20 %, stiffening the fan DE pedestal in axial direction may reduce the axial vibration.

Vishal,

haven’t you check the silencer? With regard to your comment decreasing and increasing, is this happen during normal operation? I mean under steady state? Maybe the air flow is restricted.

regards,

vishal sharma posted:
moily posted:

Has the axial vibration always been high or is it increasing with time? My first check with increasing axial vibration on the DE of the motor would be to inspect the coupling for wear / dry and ensure the hub gap is sufficient. What type of coupling is it?

Vibration is increasing and decreasing with time. But minimum vibration sometimes 2 mm/s and sometimes 7mm/s. We check already damper position.

 

Its been a while since I posted anything on here but I have seen something like this before with several possible causes.

1)  If there has been an overhaul of the motor or there has been a bearing change recently and the 22238E is not set at motor magnetic centre that would cause an axial vibration with amplitude dependent on load (time erratic).  Check by switching off power to the motor and see if the vibration instantly drops.  This will only be true if the 22238E bearing is the primary thrust bearing on the system.

2)  The turbulence could easily be setting up a high axial vibration.  Check this by recording axial vibration on the 22238E while you change the damper setting

3)  You could have changed the natural frequency by adding all that steel to the fan frame.  Check by doing a bump test axially on the 22238E and on the fan casing while the machine is stopped.

4) Are the fan and the inlet/outlet passageways clean?  Fouled gas passages can cause excess turbulence in any direction. 

Copies of spectra would really be an advantage to the analysis.

Ron

RonFrend posted:

Its been a while since I posted anything on here but I have seen something like this before with several possible causes.

1)  If there has been an overhaul of the motor or there has been a bearing change recently and the 22238E is not set at motor magnetic centre that would cause an axial vibration with amplitude dependent on load (time erratic).  Check by switching off power to the motor and see if the vibration instantly drops.  This will only be true if the 22238E bearing is the primary thrust bearing on the system.

2)  The turbulence could easily be setting up a high axial vibration.  Check this by recording axial vibration on the 22238E while you change the damper setting

3)  You could have changed the natural frequency by adding all that steel to the fan frame.  Check by doing a bump test axially on the 22238E and on the fan casing while the machine is stopped.

4) Are the fan and the inlet/outlet passageways clean?  Fouled gas passages can cause excess turbulence in any direction. 

Copies of spectra would really be an advantage to the analysis.

The fan is 

the fan is running at constant speed 900 rpm. Now next month we are planning to take shut down for our cement plant. Then I will check the above mentioned point like natural frequency test and also check for turbulence flow

fan is running at constant speed 900 rpm?

Specifics matter when attempting to solve an issue like this.  Very likely it is not running at 900 RPM and this is the nominal speed.  Has to be some slip on what appears to be a induction motor.  Is the input freq 60hz?  If you are unable to download the spectrum data consider taking a picture of the screen of your SKF collector.  Information on the coupling type would be helpful.  Long term trend of the 1X vibration levels are also of interest.

vishal sharma posted:
Ralph Stewart posted:

what is thisWhat is all that paraphernalia around the fan "housing"?

It was welding to fan casing because sometimes due to air turbulence there is more vibration in fan casing so we add stiffness to fan casing.

since you had added stiffness plates to the casing to bring down casing vibration it will act like vibration dampener.but you didn't solve root cause for air turbulence.now the air turbulence increased  to an level where your stiffness plate no longer support the casing vibration.so remove those stiffness plates with original OEM casing first solve the air turbulence.check the discharge pipe flow restriction and damper valve restriction or temp to the impeller etc.,then check the axial vibration. 

vishal sharma posted:
Donnie MacInnis posted:

Sounds like a two plane balance may reduce the axial vibration.

But vibration is not constant increasing and decreasing

Quote: "But vibration is not constant increasing and decreasing"

Does the quote: "it is not constant increasing and decreasing" mean that "it does not have a relatively constant amplitude, Example: of 5 mils, but instead is constantly increasing and decreasing from, Example: 5 mils to 8 mils to 4 mils."

If it is constantly changing , how often does it change from increase to decrease and from the decrease to the increase?

Sorry for not quite understanding your quote. Your English is far better than my attempt to write and read your native language. No offense intended. Just trying to clear up my bogged down mind.

pal posted:

since you had added stiffness plates to the casing to bring down casing vibration it will act like vibration dampener.but you didn't solve root cause for air turbulence.now the air turbulence increased  to an level where your stiffness plate no longer support the casing vibration.so remove those stiffness plates with original OEM casing first solve the air turbulence.check the discharge pipe flow restriction and damper valve restriction or temp to the impeller etc.,then check the axial vibration. 

this is a nice reply, I was thinking this might be the erratic vibration.

looseness can give you 1x and erratic phase measurements, since you have an overhaul coming, check bearings and bearing caps clearence.

hey vishal,

If im not mistaken, bearing used in a Preheater ID fan is the self aligning roller bearings, I think we will go to the easiest way down to the hardest, 

 

1. if your company have plans for shutdown, kindly ask personnel to clean the blades of the fan after it cooled down. sometimes the abnormal vibration might due to uneven material coating of blades during operation especially at high temperature.

2. At cooled down, check clearance of bearing both DE and NDE of fan. Dont measure at high temp since metal is at expansion stage. if high clearances compare to allowable set clearance, replace both DE and NDE.

3. after that, decouple it and rotate the fan manually to check alignment or shaft bend, use those gauges its very useful.

4. If you did not find any abnormality, try to test run it at gradual increasing speed using the motor ( from low rpm - medium - high )

5. Sometimes due to high temperature and highly abrasive materials passing through the fan. it might worn out the blades. if you know how to conduct fan balancing, you may do so.

5. If the problem still exist, consult experts. your company is rich. use those money.

 

just trying to help.

 

Let's review your findings and the suggestions so far, and look at their implications.

0) The fan is a large between bearings fan, directly driven. The DE bearing should be the axially locating bearing. The DE bearing is pedestal mounted and the pedestal appears to be separate from the casing.

1) You have low radial vibration at all points on the motor and fan and low axial vibration at all but the fan DE bearing.

2) At the Fan DE bearing the 1x is dominant.

3) There is a 180 Deg phase difference axially across the pedestal left to right.

4) There is an erratic phase change axially between the motor DE and the fan DE bearings.

5) there is a 115 deg axial phase change between fan bearings

6) Axial vibration amplitudes at the fan DE bearing are not steady, but change appreciably from reading to reading.

7) A previous problem with high casing vibration led to the casing being stiffened.

Suggestions so far:

Unbalance - Unlikely to be the root-cause. Whilst the rotor of a fan of this type is axially offset from the centre of the shaft span, hence unbalance will generate some axial vibration, the radial vibration will normally be much higher. So, as the radial vibration is low, I would expect the unbalance forces to be modest at best. This would be the case even with significant couple unbalance. If the force generating the vibration is unbalance, this implies axial resonance of the DE bearing pedestal.

Flow Turbulence - again, why is the flow turbulence generating such high 1x axial vibration? Flow turbulence generally creates a broad-band non-synchronous excitation. For it to generate high axial 1x vibration at the pedestal, the pedestal would have to be axially resonant.

Motor running off the magnetic centre - If the motor has plain bearings, this is a possibility. In a motor with plain bearings, the DE bearing axial clearance is quite high and the fan DE bearing would be the locating bearing,  if the coupling is axially stiff, the force would be transmitted to the fan DE bearing. If, on the other hand, the motor has rolling element bearings, the force would be felt at the motor DE bearing.

Misalignment / Locked coupling - Unlikely - the symptoms don't match - the motor axial vibration is low and the phase change erratic. It is possible if the motor has plain bearings as axial shaft forces may not couple well to the motor bearing housing.

Axial resonance of the pedestal - as Ron has suggested. This is likely in my opinion - the erratic vibration trends and large difference between the Fan DE axial and other readings support this. Either do a bump test or look at the spectrum in a log-amplitude format and look for a raised noise floor in the vicinity of the 1x peak. A pedestal resonance may be caused by a crack, by insufficiently tightened pedestal holding down bolts or improperly flat mating faces on the pedestal. A mini-ODS on the pedestal and floor would help identify this. Note, As I believe the pedestal and casing should be separate you might want to check the two are not bridged in some way.

I would also look strongly at 1 further possibility:

A cocked bearing - The Fan DE bearing is a spherical roller bearing and so should overcome this, however I have personally seen very similar symptoms earlier in my career on a larger ID fan than this in Blast Furnace cast-house floor extraction service. On opening the bearing housing we found a taper-bore bearing had been installed on a parallel shaft without the taper collar!

Hope this helps

Dear all experts the above mentioned problem has been solved. Last month we had a big shut down in that shut down we clean the fan impeller, inlet and outlet damper was inspected.after that now our fan is having axial vibration under 2mm/s. But I am confused why not radial vibration increased if there was dirt deposit on fan impeller?? Because we study in books and also in practical we normally found if there is dirt deposit in any impeller our radial vibration increase not axial in case of simple supported fan.

I wish I had a dollar for every time a system didn't react like the text  books said it should.  This is the same reason it takes years to get good at this job.  One could pay someone to come in and do a modal analysis and possibly explain the axial component.   It is solved and most, if not all, companies will not spend the money.

Call it good and move on to the next issue, there is always one!

vishal sharma posted:

Dear all experts the above mentioned problem has been solved. Last month we had a big shut down in that shut down we clean the fan impeller, inlet and outlet damper was inspected.after that now our fan is having axial vibration under 2mm/s. But I am confused why not radial vibration increased if there was dirt deposit on fan impeller?? Because we study in books and also in practical we normally found if there is dirt deposit in any impeller our radial vibration increase not axial in case of simple supported fan.

Glad it's sorted. Did ya bump test the pedestal? The probable scenario is that you have an axial resonance at or close to 1x. The vibration may return as the impeller becomes fouled again.

vishal sharma posted:

Dear all experts the above mentioned problem has been solved. Last month we had a big shut down in that shut down we clean the fan impeller, inlet and outlet damper was inspected.after that now our fan is having axial vibration under 2mm/s. But I am confused why not radial vibration increased if there was dirt deposit on fan impeller?? Because we study in books and also in practical we normally found if there is dirt deposit in any impeller our radial vibration increase not axial in case of simple supported fan.

I'm not satisfied with the solution, anyhow it is a good case study tho.

regards,

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