RCM-2009 Reliability Centered Maintenance Managers' Forum - March 23-26 - Daytona Hilton Ocean Walk Village - Daytona Beach Florida Join or Manage Your Profile Posting Boards Machinery Condition Monitoring and Predictive Maintenance Posts About vibration/alignment/balance Thermal Growth
Join or Manage Your Profile Posting Boards Machinery Condition Monitoring and Predictive Maintenance Posts About vibration/alignment/balance Thermal Growth|
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Does anybody know wher I can find an article on thermal growth and alignment? I'm trying to get the higher ups to allow us to compensate for thermal growth on a replacement fan we are going to install. We had a bad experience on a alignment when we allowed for growth. On startup the outboard brg shifted on shaft. After investigating we found the shaft was undersized. It had beeen machined for babbit bearing way back when. Somebody decided to change to Cooper split bearings. Youi need a lot tighter spec's for cooper split than babbit. We are now replacing shaft and fan and I'm getting a lot of resistance against allowing for growth. We have the tools to measure actual growth, but management is afraid that another bearing will shift and are CYA. I thought if I can supply data on the advantages I might be able to convince them.
Thanks for the help. |
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i've worked on thousands of fans and have not found it necessary to hot align one. In fact I can't recall many alignment issues. Only gross misalignments. I don't know how many times i have had to align fans and even sheaves because someone saw 2x on it. In almost all cases 2x was not the result of misalignment.
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Here's what comes to mind:
Align the machine cold to your normal acceptance standards and document the "as-left" alignment. Use the tools that you already have to measure alignment changes. Measure vibration on startup. Monitor bearing temperatures on startup. If vibration and / or bearing temps indicate that the fan is operating unsatisfactorily use the data to determine route cause and appropriate correction (which may be hot alignment or cold offset alignment or something else entirely). If the machine is performing well then management was right! |
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We have documented that growth is a problem with this fan. This is a 8ft diameter bt 12 ft long center hung fan on a 20 ft shaft. It is a rrecirculating fan on a dryer that runs 700 degrees. We use a Vibralign lasr tool with the bracktes to measure offset from cold to hot. We have 0.090 growth, we also have very high axial data. We know it is a problem.
I'm looking for info to educate management on the benefits of running aligned hot rather than only being aligned when cold and not running. |
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It's the differential growth that matters. This is what needs to be documented.
How would I show that this needs hot alignment, assuming it does? Calculate shaft, coupling stresses (loads), and bearing loads. These numbers are cold hard facts, usually a presuasive argument. Also, one does not always align perfectly when hot, because you have to get there from cold. The above numbers will lead to acceptable alignments. Let the facts make the decision,not emotions. Regards, Bill Bill.Foiles@bp.com |
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 Hi The best way to do this type of critical equipment alignmemt with thermal growth , before you should the equipment always take the reading, there is facality in Laser alignment (OLR2)alignment can be done in hot and cold alignment . so that you can now extact reading of thermal growth , this is the easy method. |
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Once you know the thermal growth of the equipment, you still have to determine what an acceptable alignment is with tolerances. One does not automatically correct for the full thermal differential growth.
You must have acceptable forces and stresses at both cold and hot conditions and everything in between. Regards, Bill Bill.Foiles@bp.com |
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Bill makes extremely valid points! Type of coupling also has a play. You can come up with realistic tolerances via graph paper and plot your coupling dimensions perfect, then using realistic numbers +/- 3 mils or 5 depending on speed and 1 minute angle (span of defectable area of coupling X tan of 1 min). This should give you good tolerance numbers. Align to estimated and set cold and document. Next, allow to run until heat soak and take measurements. Take measurements at all bearings is perferred and can be done via Acculign. Or, if you have smaller rigid machines and want to use laser as an across-the-coupling method, you may want to shoot both PW brgs and plot all measurements. Once you have acceptle numbers with good vibrations, plot and put in maintenance folder by all means. You may also note time of year if this is outside and in the sun or not - take a note, write it down and store it as it may be valuable later on.
Cordially, Sam Pickens pdmsampickens@gmail.com |
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Read it (are 15 slides):
Thermal Growth |
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I think you will find this only works on small rigid machines. It is comparable to the old Dynalign system by Ray Dodd.
On large machines of large dimensions and mass and especially pedestal bearings you must monitor each bearing and it is a good idea to monitor axial direction as well. Hot alignment checks are generally best made with optics or acculign sytems as a rule. But the across-the-coupling method will work on rigid machines but always use caution. There are a lot of 'Hot Alignment Check' methods; what Ludeca referred to as "the so-called" method is just one method. Hot alignment checks are or may be calculated, checked of course via vibration analyses, and a not-perferred method of shut-down, lock-out and measure via alignment brackets (laser or dial), Dynalign method (another across-the-coupling method), monitoring with lasers across-the-coupling, water stands, laser prisiams to monitor every bearing, optics and acculign monitoring machine's bearings, piping, foundations and axial movements. So you see there are a lot of options available - some complex, not-so-ease and some expensive. Cost justifibility is always a consideration. Cordially, Sam Pickens pdmsampickens@gmail.com |
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