Hi all;

We have a vibration problem with an exhaust fan running at 2470 RPM, motor at 1490 RPM.
CAPACITY: - 22000 M3/HR. STATIC PRESSURE: - 5Omm
POWER: - 38.6 kW. TEMPERATURE: - 350°C
DENSITY: - 0.5425 kg/M3

History;
Fan requires maintenance as the oil seal in the bearing housing fails and is replaced with a new bearing housing, shaft and impeller on the 4th Sept-07.
On start up vibration levels are about the same level of the previous fan.

All vibration levels are at 1xRPM of the Fan speed. Vibration levels went through the roof about 2 weeks ago on all points with levels on the Fan NDE reaching 44mm/s in the horizontal plane. But the lowest vibration was at the Fan NDE, and Motor DE axial at 4mm/s.

Initial thoughts were that we had a damaged fan pulley, which was causing the high vibration.
But during a maintenance shut down, both pulleys and belts were replaced. All holding down bolts checked, fan impeller was inspected for damage via a small inspection cover, there was no evidence of a balance weight being thrown. Shaft runout on both sides of the pulley was 0.001”.
On start up after maintenance, the vibration levels are back to original levels, but soon start increasing as seen in the Rad-Tube Trend.pdf. There is no evidence of bearing defects in the data, and again it is all at 1xRPM of the Fan Speed (See Rad-Tube Spectrum.pdf). Phase data was taken and can be seen in the attached Rad-Tube Phase.pdf.


Question;
What would be causing the increase in vibration over time?

Rad-Tube_Trend.pdf (9 Kb, 46 downloads)

Here is the Spectrum information

Rad-Tube_Spectrum.pdf (374 Kb, 39 downloads)

And here is the Phase information, keeping in mind that both Accelerometers were moved in pairs. for the results

Rad-Tube_Phase.pdf (119 Kb, 35 downloads)

quote:

POWER: - 38.6 kW. TEMPERATURE: - 350°C

Fan requires maintenance as the oil seal in the bearing housing fails and is replaced with a new bearing housing, shaft and impeller on the 4th Sept-07.

Question 1: Is this 350 degree tempurature the air the fan is moving?

Question 2: Is the shaft/fan made from the correct metal to withstand such a high temp?

Question 3: Was the other fan and shaft running ok before the replacement of the fan and shaft?

This sounds like a tempurture problem causing a change in the shape of the fan and/or shaft.

OMOAICBTW

Were you able to see the impeller well enough to be sure that is not cracking.

Update to the Static Pressure.. it was meant to read 450mm

Thanks Ralph and Vibeguy for your questions, it has made us look further into operating details.
Some more details that will try to answer your questions.

Ralph;
Question 1: Is this 350 degree tempurature the air the fan is moving?
The 350-degree temp was directly taken off the drawing, after speaking with operations, the operating temp is between 510 and 700 degree’s depending on the product they run. (think its time to update the drawings)

Question 2: Is the shaft/fan made from the correct metal to withstand such a high temp?
Shaft is made of K1045 steel, there is no details of what the impeller is made of.

Question 3: Was the other fan and shaft running ok before the replacement of the fan and shaft?
Previous fan and shaft had no issues and run steady for a number of years. Both shafts’ was made as a pair when changing from DE & NDE bearing (grease lubricated) blocks to the current tunnel (oil Lubricated) bearing system now used.

Vibeguy:
Were you able to see the impeller well enough to be sure that is not cracking?
Visually and to the touch there was no evidence of damage or cracking to the outer diameters of the impeller, could not reach in or see the internal diameters.

Now this brings more questions. Would size for size fit between the impeller and shaft cause high vibration in the horizontal plane but not in the vertical plane when the operating temeperature increases?

Possibly fan wheel-hub gets loose on shaft at high temperature; check fit of impeller hub on shaft.

Possibly new wheel is changing shape due to high temperature; trim balance and run again.

Walt

David, does this fan have frequent start/stops?
I had an ID fan that was well balanced and aligned in a similar High Temp application. On initial startup, after trim balance, the fan would run fine. This fan would run continuous for 4-5 months. After restart, fan would increase 5 fold, or more.
Bottom line, the hub was NOT an interference fit, on initial start up, cold everything was smooth. When the unit was shut down, the hub had expanded, shifted on the shaft, and changed location. Fan manufacture had specified the incorrect style of material and mounting method of the fan hub. Changed the hub to a "Pinch Boss" style hub, with (4) clamp bolts and problem was solved.

Fan shaft thermal bow. How are the shaft bearings arranged? Is the floating bearing "floating"?

David,

Each of the fan bearings' phase data have about 180 deg difference between H and V. This tells me that there is a variable force significantly larger then the unbalance one and acting along a straight line. This might be a result of runout of the fan pulley itself. Runout also could result from a bowed shaft occuring over time. If there was a possibility to measure runout ( using a prox ) it would've helped.
Adding axial phase measurement will also help to identify bowed shaft.

I suggest taking phase readings ( only on the fan) and shaft runout over time and trend them.

As has been mentioned above bearing arrangement is critically important and runout could be derived from a "two fixed" bearings situation. Has bearing temperature been also trended?

David

This message has been edited. Last edited by: David_G,

Considering the fan operating temperatures are higher than the drawing, this sounds like a shaft that has been overtemped and now has lost it's temper.

In the original post you stated "350°C"

That corresponds to 662°F

Is it possible the band stated by the operators (between 510 and 700 degree’s depending on the product they run) is in degrees F?

Wondering if the phase between the bearings and all the other phase data you have taken, returns to the or close to the same thing after each shutdown and startup after it gets to the max temp?

I agree with the possibility of the shaft bowing over time due to heat. Would it be possible to do a cold and then hot run out of the shaft?

Thanks all for your comments and idea's.
Currently not at work so will only answer the questions that i can answer correctly from home, which wont be many.
The temperatures are all in celcius.
The equipment runs 24/7 until there is a failure in the system or there needs to be a potgear change. We are a metal coating line supplying zincalume and galvanised products.
The temperature chenge happens on the run as the product changes, from hrd iron, to soft iron and then to specific products.
We do take temperatures via a citect system but have not been trending these, but will be looking into that side of it on Monday.

Sorry for the confusion of the Temperatures. Was looking at the wrong figures when quoting 510 to 700 degree Celcius. When checking the trends on the Correct equipment, the temperature varies between 260 and 340 degree Celcius.
This fan has only been shut down once to complete the maintenace tasks stated earlier.
The next shutdown will be later Nov-07 if the fan holds together.