Hi Guys,

 

Usually the turbo machinery rotors are hang vertically for long term storage.I know some theories such as L3/D4 ratio,API 686 also has one clause says bearing span/Diameter ratio but no mentioned about any values for reference for Horizontal Or vertical storage of turbo machinery rotors.

Do you guys know about any standard which suggests the criteria "which rotors can be stored Horizontally also and for which rotors Vertical storage is mandatory"?

 

Original Post

Going beyond the question of vertical or horizontal is the question of rebalancing the rotor that has been horizontally stored for an extended period of time.  In my experience many rotors are removed from a machine after some period of service, refurbished and then stored horizontally.  On needing the machine some time later, all too often the rotor is removed from storage, cleaned of any preservative used, and immediately placed on a balancing machine to check the balance.  Well the rotor was balanced when stored, so why not trust that previously completed balance effort?  During this balance verification, you are now in the hands of the people doing the rebalancing effort, and all too often if they are not aware the rotor has been stored horizontally they immediately apply balance corrections, which do nothing more than correct for a balance state that includes bow in the rotor due to storage.  After installation of the stored and recently balanced rotor the machine is started up, often exhibiting good levels of vibration initially.  In short order the vibration starts to increase and may reach undesirable levels.   That is often caused by the bow "relaxing" itself.

 

Good balance shops will chart runouts on the rotor for the purpose of establishing the presence of a bow and if found roll the rotor at slow speed in a balance machine to work out any bow that has been found.  Any balance shop of decent quality, having been told the rotor was stored horizontally and may have a bow, should check and take appropriate steps to minimize the effects of residual bow on the rotor prior to balancing.

 

It has never made much sense to me to perform the verification balance of a rotor that has been stored when it had been refurbished and balanced prior to its storage.  Do whatever steps are needed to detect and roll out any residual bow that has resulted from storage and "yes" perhaps in some machines skip the balance verification phase.  Perhaps, if being used in a variable speed machine, simply trust the balance done a few years back after rotor refurbishment and simply install the rotor, then roll it out at slow speed in-situ.

 

There is an old but relatively good article at http://turbolab.tamu.edu/proc/...roc/T14/T1435-45.pdf that you may find of interest.


If you store horizontally, use care to protect proximity probe target areas and do not let these areas become load bearing areas while in storage.  Use a preservative on the target areas and pay attention to the areas where the rotor is physically supported.  I know of many shops that store rotors by allowing them to rest on lead sheet.  Problem methods of support direct contact with wooden v-blocks, which contain moisture and thus induce the possibility of localized corrosion (rusting).  I have also observed the use of teflon sheeting for support; this should in general be avoided in proximity to bearing journals.  Teflon should not contact bearing journals.  Depending on the length of time of contact lubrication issues can be an issue.    

Originally Posted by vibramac:

John educate me - why not use Teflon on bearing journals?

There is a term called wettability that is defined as "an affinity for lubricants so that they adhere and spread to form a protective film over the bearing surface."  Contact with teflon, especially long term, can upset the wettablility in the localized areas of contact and later affect the running characteristics, with respect to lubrication, of the bearing.
 
You can Google wettability journal bearings and come up with many resources, many of which are quite complex.  For just a good all round understanding of journal bearings I would suggest

http://edge.rit.edu/edge/P1445...Journal_Bearings.pdf.

 

Scroll to the last page (Fluid film bearings do's and don'ts), specifically item 12 where it is stated "Do not roll shafts in Teflon strips or “V” blocks due to micro embedment. This could result in the shaft being unable to properly "wet" and causes failure.

 

Related to wettability, many years ago when I was involved with the manufacture of high speed precision gearing we often used diamond burnishing as a means to minimize/treat electrical runout in proximity probe target areas.  These areas, being just outside the defined bearing journal were often "treated" with the diamond burnishing tool which gave a mirror-like finish to the bearing journal.  We immediately noticed significantly higher than predicted bearing temperatures.  We had inadvertently destroyed the wettability of the journal surface.  Recall that a journal bearing acts as a pump and in a sense "pulls" the lubricant around the shaft in the clearance volume.  By providing a very smooth finish, we had eliminate the ability of the bearing to properly pull the lubricant around its periphery.  

  
 
Originally Posted by candy:

So JohnPA, are you saying it is not quite necessary or mandatory to store spare rotors in vertical position and in inert gas/nitrogen metal compartment?  

Not at all!  I am solely questioning the practice of removing a rotor from a machine, then having it totally refurbished including balancing (often by the OEM I might add), then after storing it for some extended period, upon removal the first thing done is a balance check.  If the rotor was stored horizontally, it is almost certain to be out of balance due to residual bow and unless the balance shop is aware of this the corrections applied will be compensating for the residual bow induced by storage.  Then, once installed and the machine is started, the residual bow relaxes often resulting in a rotor that is out of balance.

 

If the rotor prior to storage is refurbished to a near new condition, including a shop balance, why not trust that balancing years later when you remove that rotor from storage?  If you chose not to trust the balance work done when the rotor was refurbished, then at least advise the balance shop doing the "check" balance that the rotor was stored horizontally and they need to take appropriate measures to eliminate residual bow.

 

All this is more practical on variable speed machines where in operation you have the ability to slow roll the newly installed rotor at some slow RPM and work out any residual bow present.

 

I saw this on many occasions at refineries in Philadelphia.  Refurbished rotors would be installed after having a check balance accomplished and on startup high vibration would be present.  If the check balance records were available on some occasions it was possible to plot the balancing corrections and vibration vectors and make a correlation.

 

I guess this is one instance where at least in some cases, I'm saying trust, but don't verify.

So JohnPA, you are saying it is necessary to do the balance check for spare rotors stored horizontally only? How about if stored vertically? 

 

I agree that the balance check is a good practice to comply with the ABC rule of thump for rotating equipment.

Originally Posted by candy:

So JohnPA, you are saying it is necessary to do the balance check for spare rotors stored horizontally only? How about if stored vertically? 

 

I agree that the balance check is a good practice to comply with the ABC rule of thump for rotating equipment.

I thought I was very clear that in many instances I am saying that a balance check is not necessary!  Since the my reasons for making that statement seem difficult to understand, let me outline this in a scenario; please follow the timeline.

 

1.  I have a steam turbine driven compressor and I swapped out the rotor in 2010.

 

2.  The rotor that was removed was totally refurbished by the OEM or other reputable shop.  Part of the refurbishment included re-balancing the rotor to the original specification.

 

3.  After the total refurbishment, I store the rotor horizontally.

 

4.  Fast forward to today and an outage.  I want to install that rotor that has been stored for five years.

 

Is there some compelling reason to not trust the balance effort that was done when the rotor was refurbished and stored in 2010?  There might be good reason to do another balancing; one reason might be a coupling that has to be replaced.  But ask yourself, if it was balanced in 2010, do we need to do it again, and do it a high potential of residual bow in the rotor?  If so, then make the shop doing the balance fully aware that the rotor was stored horizontally for five years and some residual bow may be present.  A good balance shop at that point should know what to do.  

 

As for vertical rotors, if the rotor went through a prior refurbishment that included a balance by a reputable shop, then why re-balance?

 

The key in either of these scenarios is the rotor was balanced as part of the refurbishment process prior to storage.  If this was done, is another balance necessary?  What does it accomplish?

 

Also, as I said earlier, this is most applicable to variable speed machines where the opportunity exists to slow roll the newly installed rotor as part of the startup process.  Proximity probes can indicate the residual bow (at slow roll) and one can actually observe the bow "relaxing" itself.  Obviously you would not necessarily do this on some machine driven by an induction motor that reaches full speed in 10 seconds.

Originally Posted by candy:

About the slow roll, what would be the rpm as an example? How long to slow roll to allow the "relaxing" to take place? 

I can't answers those questions as the answers are dependent on the machine and its operational limits.  I have seen many steam turbine compressors rolled around 200 to 500 RPM, often for an hour to just warm things up and that will be sufficient.  If bow exists you can also watch the proximity probes and watch the bow relax; some larger machines have an eccentricity monitor.  When you think the bow has relaxed ramp the machine up carefully, paying particular attention to amplitudes through any critical speeds.  If residual bow is still present, the rotor will be out of balance to some degree, and you don't want excessive amplitudes to damage seals, bearings, etc.  You may be able to reliably get through the critical but sometimes you can't.  In those instances back the speed down and consider rolling for another hour.

 

Use common sense, it never hurts to go slow on a startup.  Mistakes often happen when you are in a hurry.   

Originally Posted by John from PA:
Originally Posted by candy:

About the slow roll, what would be the rpm as an example? How long to slow roll to allow the "relaxing" to take place? 

I can't answers those questions as the answers are dependent on the machine and its operational limits.  I have seen many steam turbine compressors rolled around 200 to 500 RPM, often for an hour to just warm things up and that will be sufficient.  If bow exists you can also watch the proximity probes and watch the bow relax; some larger machines have an eccentricity monitor.  When you think the bow has relaxed ramp the machine up carefully, paying particular attention to amplitudes through any critical speeds.  If residual bow is still present, the rotor will be out of balance to some degree, and you don't want excessive amplitudes to damage seals, bearings, etc.  You may be able to reliably get through the critical but sometimes you can't.  In those instances back the speed down and consider rolling for another hour.

 

Use common sense, it never hurts to go slow on a startup.  Mistakes often happen when you are in a hurry.   

John,For rotors stored horizontally in wooden box, Do periodic shaft rotation is required?If required then what would be the frequency for rotation?

I am not aware of anyone doing rotation of stored rotors on a regular basis that are stored in a wooden box.  That is not to say it isn't done at some facilities.  But we are talking "turbomachinery" rotors in this thread which is separate and distinct from rotors supported on rolling element bearings, often in a stored spare machine.  For information on that I suggest you search false brinelling or go to https://www.maintenance.org/top...0#399590942963856010.

 

OK, away from the case of rolling element bearings and more toward rotors that in operation will be using fluid film bearings.  I would first of all suggest you go back to the OEM for storage recommendations.  Many will advise you of detailed procedures to follow based on anticipated length of storage.  Some are very specific about preservatives for instance.  Some form of preservative may be needed when storing rotors in a wooden box.  Depending on the wood, there may be a considerable amount of moisture present that can cause corrosion.  I have also seen wooden boxes that have v-block support areas, and the v-block is lined with rubber.  All well and good except the rubber is often nailed into place, and as luck would have it the nails fall on proximity probe target areas creating another issue.   

 

 

 

   

Originally Posted by John from PA:

I am not aware of anyone doing rotation of stored rotors on a regular basis that are stored in a wooden box.  That is not to say it isn't done at some facilities.  But we are talking "turbomachinery" rotors in this thread which is separate and distinct from rotors supported on rolling element bearings, often in a stored spare machine.  For information on that I suggest you search false brinelling or go to https://www.maintenance.org/top...0#399590942963856010.

 

OK, away from the case of rolling element bearings and more toward rotors that in operation will be using fluid film bearings.  I would first of all suggest you go back to the OEM for storage recommendations.  Many will advise you of detailed procedures to follow based on anticipated length of storage.  Some are very specific about preservatives for instance.  Some form of preservative may be needed when storing rotors in a wooden box.  Depending on the wood, there may be a considerable amount of moisture present that can cause corrosion.  I have also seen wooden boxes that have v-block support areas, and the v-block is lined with rubber.  All well and good except the rubber is often nailed into place, and as luck would have it the nails fall on proximity probe target areas creating another issue.   

 

 

 

   

I have gone thru the OEM recommendations but it says nothing about the rotors stored in wooden box,but As per best practices do we need to periodically rotate/change position the rotor to avoid sagging?

I am asking about the horizontally  stored turbomachinery rotors(supported by wooden crates) in wooden box

I have gone thru the OEM recommendations but it says nothing about the rotors stored in wooden box,but As per best practices do we need to periodically rotate/change position the rotor to avoid sagging?

I am asking about the horizontally  stored turbomachinery rotors(supported by wooden crates) in wooden box

I know absolutely nothing about your plant and/or your machinery and have further stated "I am not aware of anyone doing rotation of stored rotors on a regular basis that are stored in a wooden box."

 

Sometimes you have to develop your own best practice.

What JohnPA said is that you don't have to rotate turbomachinery rotors which are stored horizontally because any sagging can be "relaxed" by slow rolling during the initial start up process. Is my understanding about his statement is right?

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