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High speed balance for steam turbine rotor of centrifugal compressure Login/Join
 
posted
Dear BalanceSpecialist,
Can you advise me to decide do properly balance for steam turbine rotor of centrifugal compressor?

This rotor has 1st critical speed = 6,400 RPM, 2nd critical speed =16,000 RPM, operation speed 13,246 RPM.

Last year, the rotor had high vibration after decreased RPM to 2000 RPM and rotor blade were rubbing and shaft bow.

Vendor recommends to repair and bend the shaft and check balance by low speed.

I think it should be balanced by high speed balancing for ensure not problem after 1st speed , what do you think?
 
Posts: 6 | Location: RAYONG | Registered: 18 September 2012Reply With QuoteReport This Post
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Yes, I would balance the rotor at low speed first then trim it at high speed. Low speed balancing can only get so close and keep the rotor speed away from that critical.
 
Posts: 5 | Location: Hamilton, ON, Canada | Registered: 03 November 2011Reply With QuoteReport This Post
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What do you mean by "bend shaft"? Is vendor recommending some form of straightening operation?
 
Posts: 2545 | Location: Exton PA | Registered: 22 February 2005Reply With QuoteReport This Post
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What is the rotor diameter?
 
Posts: 4285 | Location: Borneo | Registered: 13 February 2005Reply With QuoteReport This Post
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How much is the shaft bow?
 
Posts: 4285 | Location: Borneo | Registered: 13 February 2005Reply With QuoteReport This Post
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TING,

so, the rotor in operation now is the spare rotor. i can assume the 'damage' rotor is now on its way for refurbishment. rotor straightening is one of the repair scope. depending upon the degree of damage on the blade (i guess it is on the low pressure section blade rows), blade replacement may also be required.

now, if it is now at oem shop, i would expect the vendor recommends for hsb. even for general refurbishment scope (without this rotor straightening scope) they would normally include hsb in their proposal.

nway, imo, the history of 1x vibration performance and the impact of bent & straightening repair method would be the main factors for you to assess if hsb is required. if indeed the rotor has long history of 1X vibration issue, i would go for hsb. if no concern on the 1x history and oem can guarantee the magnitude of the bent had not resulted in rotor lateral change and rotor straightening procedure will not result in any change too, then low speed balancing may be sufficient. the last thing you do not want to be in is to find out your your speed governing range is in critical speed range.

again, if the rotor is at oem shop, i would expect they will include hsb in their scope due to simple reasons; they gain more profit and assurance (they normally provide guarantee period). with your problem description, if oem is recommending low speed balancing only, i have a thought that the rotor is at 3rd party service center.

at 13K speed, it could be in the range of 20-25MW hp (100 barg steam inlet) turbine. some of the turbine in this range do have field balancing provision (weight placement). even with this provision, i will never include this in the decision making. 'oh, we dont need hsb. we can always perform field balancing should problem arose during start-up'.

if the rotor is still in operation and vibration value is normal, it indicates the rotor had been straightened during subsequent (after the problem) slow roll or at its idle speeds. so, nothing to be done.
 
Posts: 488 | Location: Jubail, KSA | Registered: 09 August 2006Reply With QuoteReport This Post
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Smiler Thank you everybody of your answer,
Run out of shaft is 0.04 mm that is out of tolerance at 0.0058 mm.
I think it should be done hsb and Mr.Valve opinion.
 
Posts: 6 | Location: RAYONG | Registered: 18 September 2012Reply With QuoteReport This Post
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are you sure 0.0058mm is the run-out tolerance? it is very tight imo

maximum readout is one thing, how about the shape of the rotor based on the full (along the shaft) max runout/angle readings?
 
Posts: 488 | Location: Jubail, KSA | Registered: 09 August 2006Reply With QuoteReport This Post
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What standard was the steam turbine rotor manufactured based on?
 
Posts: 4285 | Location: Borneo | Registered: 13 February 2005Reply With QuoteReport This Post
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quote:
are you sure 0.0058mm is the run-out tolerance? it is very tight imo


Somewhat dependent on shaft size, 0.0058mm (.2 mil) seems reasonable to me. On shafts less than 250 mm (approximately 10 inches) we regularly made gear rotors at 1/2 that value (TIR).
 
Posts: 2545 | Location: Exton PA | Registered: 22 February 2005Reply With QuoteReport This Post
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this tolerance is MHI standard and the runout maximum 0.04 mm at 6 PM of middle of shaft dia 50 mm, length 1.35 m.
 
Posts: 6 | Location: RAYONG | Registered: 18 September 2012Reply With QuoteReport This Post
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John,

I always thought that the tolerance is always higher than the probe landing area TIR. This is considering the procedure required to attain such surface finish at probe landing area.

With this turbine speed of 13K, the probe landing area max run-out is 6.3 microns. To find out 5.8 microns as maximum rotor radial run out is just odd. I have never use dial indicator with 1 micron precision. I would expect it would be a challenge to differentiate between surface imperfection and an outcome from deflection.

0.02mm to 0.05mm is general mech runout tolerance that i used to hear/experienced. Even for MHI turbine.
 
Posts: 488 | Location: Jubail, KSA | Registered: 09 August 2006Reply With QuoteReport This Post
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TING,

for me, it is not only the maximum, it is the shape that matters most. if you have maximum run-out located at 6 o'clock along the middle span, yes you do have sign of bow. even if 0.05mm at scattered angles along the middle spam, bow is unlikely. with 0.2-0.7mm laby clearances, 0.0058mm run-out tolerance is just too tight imo.
 
Posts: 488 | Location: Jubail, KSA | Registered: 09 August 2006Reply With QuoteReport This Post
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Valve, just to clarify, surface runout and surface finish are two separate and distict parameters and your statement "...probe landing area max run-out is 6.3 microns" (0.25 mil) makes me think you may be confusing the two or not realize the difference. At my former gear employer (high speed gear OEM) we would regularly "super finish" probe target areas with a diamond burnishing tool in an effort to minimize electrical runout. The finishing method yielded a mirror like surface finish; you could actually see your reflection in a large pristine journal. That had nothing to do with runout which was commonly held to 0.0025mm (0.1 mil) on journals up to 250 to 300mm. Such values of runout were easily attained and are absolutely needed in high speed rotors. TING states the shaft is 50mm diameter - at that size 2.5 microns (0.1 mil) runout tolerance is easily attained and should be the "norm" for any shop. Indeed I would say if a shop can't maintain that level of runout I would likely look elsewhere.

You further state "0.02mm to 0.05mm is general mech runout tolerance that i used to hear/experienced. Even for MHI turbine". I sincerely doubt that is the case, especially as applied to a 50mm journal. For those reading this that are familiar with English units, these levels of 0.02 mm (0.79 mil) and 0.05 mm (1.97 mil) are actually far in excess of most acceptance criteria for running vibration! API for instance only allows 25.4 µm direct (1.0 mil PP) or overall vibration at 12000 RPM.

For more on surface finish I suggest you review http://www.engineershandbook.c...surfaceroughness.htm or Google for other sources.
 
Posts: 2545 | Location: Exton PA | Registered: 22 February 2005Reply With QuoteReport This Post
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John,

Am fully aware of the difference and that is the reason i explicitly bring up probe landing area TIR tolerance in this discussion. Please provide me the part of API where it provide us the surface runout tolerance accross a rotor. If none, I think you are wrong to use API p-p limit as surface run out 'reference' for a rotor. I have yet to obtain the general rule of thumb but i strongly believe that for the full radial surface runout tolerance of a rotor, it is the seal clearances that is referred as to set/calculate the tolerance.

Btw, I have seen many technical professionals have commented that diamond burnihsing is used to improve electrical run out. I am yet to able to digest it since what i understand, the objective of diamond burnishing is majorly on attaining the mirror like surface finish (MECHANICAL). As for electrical run out, degaussing or demagnetizing is the procedure involved. If this understanding is true on burnishing (mainly mechanical), so, does this mean that the radial surface for the surface runout measurement accross rotor shall be burnished or honned in order to attain 5.8 um tolerance???

I guess those that have strong experience in machienry maintenance and inspection should come forward and comment.

rgds
 
Posts: 488 | Location: Jubail, KSA | Registered: 09 August 2006Reply With QuoteReport This Post
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John,

I suggest you have a look on this article. Been trying to find extablished paper that suits this discussion. The turbolab paper is the closest i could get. The operating speed is 8676 rpm.

Excerpted from the article; "except for the instance when the runout at the centre of rotor is performed, 5 mils of runout were measured, which was acceptable for this rotor".

what do you think of this?

http://turbolab.tamu.edu/proc/...oproc/T34/t34-02.pdf
 
Posts: 488 | Location: Jubail, KSA | Registered: 09 August 2006Reply With QuoteReport This Post
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The article you site is talking about mid-span rotor bow not allowable tolerances at bearing journals and/or probe target areas. Having said that excessive rotor bow will also likely evidence itself at the journals as excessive journal runout.

The removal of mechanical runout is simply handled by proper tolerancing and the selection of the machine process, i. e., a lathe can't do as good a job as a journal grinder. As far as electrical runout degaussing is the first step but often doesn't get the rotor in spec.

In a proximity probe the magnitude of current supplied and the depth of the eddy current flow into the shaft are a function of the conductivity and permeability of the target material. Inclusions, cracks and carbon segrations cause discontinuities in the eddy current flow, which changes the eddy current response. Keep in mind that whenever there is a change in the electrical properties of the material there is likely a change in the mechanical properties as well. This is often seen as a change in hardness around the periphery of the journal. Inclusions below the surface of the steel may be too deep or localized to easily identify by mechanical means. A point to make here is the eddy probe "sees" below the surface, depending on the material about 300 to 380 µm (12 to 15 mils) in depth. When a shaft is diamond burnished the surface is cold worked and becomes significantly more uniform in surface hardness. This adds significant compressive stress to the outer skin of the shaft that can be seen by the eddy current magnetic field. BN has long stated "...is not fully understood why this reduces runout, but the fact that burnishing generally reduces runout implies that stress variations in the surface of the rotor have a significant impact on electrical runout." The diamond burnishing tool doesn't really remove material. Burnishing is a relatively inexpensive process as it can be done in a lathe. The tool is run into the surface and is loaded against the surface to about 60 kg (125#) by a compressed spring in the tool. It is rotated at low speed in the presence of significant lubricant and results in a mirror finish. Burnishing after grinding will reduce runout by about 60% to 80% but those numbers are highly dependent on the type of steel. High carbon steel (AISI 4140 or 4340) responds nicely while something like 9310 (very high in mangenese) has minimal response to the burnishing tool.

Go to http://www.ge-mcs.com/download...05/3q2005_runout.pdf for an Orbit article (Vol. 25, No. 3, 2005, pp. 5-17) titled "Understanding and Mitigating Shaft Runout” by N. Littrell. It will provide some good insight into mechanical and electrical runout and their reduction. I'd attach it here but it is likely too large.

This message has been edited. Last edited by: John from PA,
 
Posts: 2545 | Location: Exton PA | Registered: 22 February 2005Reply With QuoteReport This Post
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quote:
The article you site is talking about mid-span rotor bow not allowable tolerances at bearing journals and/or probe target areas.


Again, i want to stress that i am fully aware of probe landing area runout tolerance. The MAIN REASON i brought up probe landing area runout tolerance is just for compartive figure to the discussed subject here i.e the rotor radial surface runout tolerance. It is brought up since the tolerance the op has (5.8 um) is lower than its probe landing area tolerance of 6.3 um. Is this a valid tolerance? I am still with my initial comment i.e it is too tight. As clearly pointed in the turbolab article, 5 mils runout is acceptable for their rotor.

As said earlier, those with strong machinery maintenance should come forward and share their exp.

John, btw, thanks for the insight on how burnishing can improve electrical runout.
 
Posts: 488 | Location: Jubail, KSA | Registered: 09 August 2006Reply With QuoteReport This Post
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To minimize the runout error, API 541 and API 546 define limits for the maximum probe track runout. In simple terms, these runout limits are 25% of the unfiltered vibration limits, or 0.45 mils (thousandths of an inch) for most induction machines and 0.5 mils for most synchronous machines. These correspond to 11.4 and 12.7 μm respectively.

Taken from: http://www.ge-mcs.com/download...t_v27n207_runout.pdf
 
Posts: 4285 | Location: Borneo | Registered: 13 February 2005Reply With QuoteReport This Post
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If the area on which one is measuring is a good machined surface, true with the centerline, then 5 mils seems large to me. I thought the article talked about the runout being measured near the shaft end.

I know that John has seen much larger GT's (much much larger) whose measured runouts near the center would be tiny compared to this figure. Now, shaft sag on the larger rotor is much more than this, but you don't measure it as runout.

A temporary bow of 5 mils is very believable. Roll out the bow before measuring.

Low speed balance of a steam turbine with the criticals as described may be taking a chance.

Move the mass 5 mils (or 2.5 if mearing pp, tir) and use a good rotordynamics model to see what can happen. What is the damping at the first critical? Use a simple approximation of amplification factor times mass eccentricity for a first guess (The vibration would be measured pp.).


Regards,
Bill

 
Posts: 2951 | Location: Houston, TX USA | Registered: 23 February 2005Reply With QuoteReport This Post
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