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Dear xperts,

Please clarify that how to read unfiltered orbit plot. though its contains lots of frequencies. sometimes, by default the unfiltered orbit looks fuzzy like i attached here. in that case how can we judge the machine behaviour?

please help. i heard, distortion cause rub, but here orbit report says, compressor looks fine, i find lots of distortion. am i right? please need your advice!

thanks

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  • qwerty

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First of all I've attached the data in a PDF that shows it much enlarged so people can properly review and comment.

Looking at the data let me first state that I doubt there is anything seriously wrong with this machine. The 667Y2/667Y1 probe pair are showing approximately 11 Β΅m over all amplitude. That is well within most acceptability criteria for a machine with a speed of 5071 RPM. I do note however that this is direct data, and at the levels you are seeing shaft glitch may have a significant effect on the data. Correcting for the glitch (look up compensation in your ADRE or Sys1) may make the level lower, or sometimes even make it higher.

The second probe pair, 668Y2/668Y1 probe pair presents even lower amplitude, approximately 5 Β΅m so is of less concern.

In both of these cases you may be seeing the shaft finish in the probe target area. Only compensation will show with any certainty if that is the case.

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RM
quote:
Only compensation will show with any certainty if that is the case.

Thanks john,
sounds clear to me. but tell me how we do compensate?

so we may get different orbit plot type once we did compensating; right?

with this direct orbits, how we pinpoint the faults if the vibration amplitude is high.

i want to know the how we identify the faults from direct orbit?

thnaks again
RM
quote:
Originally posted by John from PA:
Adre Sxp or System 1?

In addition, you need data at "slow roll", in your case something around 200 to 500 RPM and a coastdown preferably. Do you have such data?


sorry john, for late reply. unfortunately we dont have such system. we just taken a reading from rack by using CSi2130. these report which i attached here provided by third party.
RM
quote:
Originally posted by William_C._Foiles:
Just think; can the rotor relative to the casing (or casing relative to the rotor) move as fast as some of these deviations show. NO!


ok william, i got you. when we use unfiltered orbit? sorry, im newbie to this orbit. whenever i look direct orbit, it makes me crazy.. i really have no idea how to get there. share your valuable tips. please

thanks again.
RM
quote:
Originally posted by spherical:
quote:
Originally posted by Abu ehan:
[QUOTE]Originally posted by John from PA:
Adre Sxp or System 1?

QUOTE]
sorry john, for late reply. unfortunately we dont have such system. we just taken a reading from rack by using CSi2130. these report which i attached here provided by third party.


But the plot you sent was coming from a System 1 not from CSI?


yes you right, but we called BN people here to support. Smiler
RM
There are some unstated assumptions that go into the loop rules.

1. if the loops are internal this is a forward precession
2. If the Loops are External Then This is a Backward precession

Most of these rules (including other rules) assume that 1X is present, likely dominant, and forward.

Internal loops say that the two frequencies have different orientations of forward and backward. What if the forward orbit were backward or mostly backward?

Shaft trigger dots help with orbit analysis. However, suppose one has a dominant harmonic or sub-harmonic and a smaller 1X (significantly smaller but healthy size 1x). Each outer or shaft revolution causes the shaft trigger mark (maybe not on the smaller loop itself but the bigger loop). The 1X could be the sub-harmonic for the larger harmonic, or the harmonic for the larger sub-harmonic.

There can be much variety. Don't over simplify.
RM
quote:
For shaft vibration precession, I rely solely with the filtered orbit trigger dot. Is their any chance that the direct orbit is different?

The precession is well defined for any single frequency (different frequencies may have opposite progression).

I don't think the precession of a direct / unfiltered orbit with multiple frequencies is well defined although you might look at it and have an intuitive feel for what you think it is. If the other frequencies have magnitudes much less than 1x then I think your intuition would be to describe the overall orbit precession the same as the 1x filtered precession. If on the other hand you have something like a figure 8 then I don't think there is any well defined precession for the overall orbit by math or by intuition.
RM
quote:
Originally posted by electricpete:
quote:
For shaft vibration precession, I rely solely with the filtered orbit trigger dot. Is their any chance that the direct orbit is different?

The precession is well defined for any single frequency (different frequencies may have opposite progression).

I don't think the precession of a direct / unfiltered orbit with multiple frequencies is well defined although you might look at it and have an intuitive feel for what you think it is. If the other frequencies have magnitudes much less than 1x then I think your intuition would be to describe the overall orbit precession the same as the 1x filtered precession. If on the other hand you have something like a figure 8 then I don't think there is any well defined precession for the overall orbit by math or by intuition.


In a GE/BN convention the Keyphasor dot is preceeded by a blank area and thus the visual interval seen as blank-bright defines precession. Admittedly this can be difficult to see, especially in an orbit generated by Excel. A GE/BN training tool is the Full spectrum simulator and I've attached a case where the 1X forward and 2X reverse are about equal in amplitude and the 2X forward and 1X reverse are non-existent ("zero").

quote:
If on the other hand you have something like a figure 8 then I don't think there is any well defined precession for the overall orbit by math or by intuition.


In response to what Pete has stated about a figure-8 orbit, I'll add such an orbit in a separate post. Note the relative amplitudes of both the forward and reverse components of the 1X and 2X components to generate this orbit. Precession of a direct orbit can be visualized, at least by the blank-bright convention.

Visualizing all these things is highly dependent on just how "busy" a real orbit is. In ADRE or System 1, depending on plot configuration 8 revolutions are shown. The option does exist to show 1, 2, 3 or n revolutions in which case the apparent precession can often be more readily seen.

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  • 1X_2X
RM
quote:
Originally posted by John from PA:
In response to what Pete has stated about a figure-8 orbit, I'll add such an orbit in a separate post. Note the relative amplitudes of both the forward and reverse components of the 1X and 2X components to generate this orbit. Precession of a direct orbit can be visualized, at least by the blank-bright convention.

The blank-bright convention tells you which direction the instantaneous position travels along the curve over time. While I might be misunderstanding your point or arguing semantics, I'd say that in the case of figure 8 knowing this information (direction that position travels along the curve) is not enough to define any precession (forward or backward) of the direct orbit. After all, traversing half of the figure 8 we're going CW and the other half we're going CCW. In other words forward precession in half of the orbit and reverse in half of the orbit. So which half of the orbit are you going to pick to characterize the precession of the entire direct orbit? It seems arbitrary/undefined to me. (on the other hand characterizing precession of individual frequencies is very well defined).
RM
Last edited by Registered Member
quote:
Originally posted by electricpete:
The blank-bright convention tells you which direction the instantaneous position travels along the curve over time. While I might be misunderstanding your point or arguing semantics, I'd say that in the case of figure 8 knowing this information (direction that position travels along the curve) is not enough to define any precession (forward or backward) of the direct orbit. After all, traversing half of the figure 8 we're going CW and the other half we're going CCW. In other words forward precession in half of the orbit and reverse in half of the orbit. So which half of the orbit are you going to pick to characterize the precession of the entire direct orbit? It seems arbitrary/undefined to me. (on the other hand characterizing precession of individual frequencies is very well defined).


For the figure-8, Pete you are correct that half of the direct orbit will be going CW and half CCW. Hence the reason I stated "Precession of a direct orbit can be visualized, at least by the blank-bright convention." But knowing that half of the direct orbit will be going CW and half CCW is in itself meaningful information because, at least in the presence of a single Keyphasor dot, it tells me there is relatively high 2X.
RM
quote:
Originally posted by spherical:
quote:
Originally posted by John from PA:
Figure 8 orbit


John, I still see forward precession in the orbit but Full FFT showing reverse components are somewhat level for 1X but higher for 2X. Should the orbit show the same? Also what those smaller 4 orbit box, looks individual (filtered)orbit components for me?


Based on what you are asking, I think you are making reference to the figure 8 shaped orbit but I'm not sure of what you are asking when you state "Should the orbit show the same". Can you clarify?

The four smaller box are predefined orbit shapes. I've attached a PDF that shows each case and the relative amplitudes used in the generation of the orbit.

Keep in mind a few basic rules of full spectrums.

1. At any given frequency the sum of the forward and reverse components is the major axis of the filtered orbit.

2. At any given frequency the difference of the forward and reverse component is the minor axis of the filtered orbit.

3. At any given frequency the larger of the forward or reverse component defines precession for that frequency.

Note that now we know the ellipticity and precession of the filtered orbit. We also know this in the absence of a Keyphasor.

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RM
John,

Thanks for the attachment & rules. Actually i have it in my notes from Bently Book which i had read before but i dont have it now.
Let me also also add the axis are measured perpendicualr to the probes

(3. At any given frequency the larger of the forward or reverse component defines precession for that frequency.) This was what im asking. In the figure 8, the Full FFT showing somewhat equal 1X & clearly higher 2X but the trigger dot clearly looks forward. I was expecting it to be reverse.
RM

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