Centrifugal Compressor - IGV closure percentage vs vibration 1

[awhicker84]

Ladies and gents,

I'm trying to find some correlation (if any) between the amount the IGV is closed versus vibration of a centrifugal compressor rotor. So far, I'm coming up blank. I have a feeling that as long as 1) the passing frequency is safe and 2) the compressor is not in surge that rotor vibration caused by 60% closed (for example) IGV's will be minimal. However, I'm not finding any studies to back this up.

Thanks for any help. Cheers,

Andy

 

[georgeverghese]

You've got to tell us more before you get some useful responses. A process control sketch will help, as will a startup sequence. Keeeping away from surge requires monitoring flow / dp across the compressor and speed, so it is not as simple as only keeping the IGV at a min permissible setting.

 

[rotw]

One correlation I would assume is that by varying the IGV angle, necessarily the process conditions of the compressor are changing. The gas density for instance would vary. When the density changes the cross coupling stiffness effect (destabilizing effect) through the machine (say this is a between bearing design rotor) will also be impacted. In a level I stability analysis, the Wachel formula account for the cross coupling effect which in practice has the consequence to reduce the logarithmic decrement. In a level II analysis the contribution of the seals (balance piston labyrinth for example) normally contribute to additional destabilizing forces, on the other hand it adds some damping (certain technologies take advantage of this effect).
In short, the higher the density, the higher the cross coupling effect and the lower the log dec. In practice the point of operation can also be plotted on a Fulton diagram which would show flexibility ratio (rigid bearing first critical speed) versus the average gas density density and the cut off for various chartered areas (safe, severe, worst).
I would suppose that the lower the stability, the lower the damping capability of the rotor system which ultimately would translate in increase vibrations. By the way, except degradation of performance, wear and tear, offset / off design conditions etc - a machine "built to standards" should normally be - by design - verified and qualified for operation across its full control range / operating envelop while satisfying passing critera for rotor-dynamics.

 

[awhicker84]

Thanks! I'm looking up the rotordynamic study now to confirm the rotor is stable. I'm sure it is, but it gives me a way to quantitatively show the customer.

Thanks again.

 

[awhicker84]

What's interesting is that the API 684 section on Stability never mentions inlet guide vanes as a possible cross coupling force. I can only assume that IGV's have a very minimal direct effect on the magnitude of the cross coupling force.

Your statement on log dec vs density is correct. It's also true the required HP decreases with increased closure of IGV's. HP is in the denominator of the cross coupling force equation. So there will be some additional cross coupling. However, this single stage overhung unit is extremely stable. The anticipated cross coupling is about 15 times lower than the force required to cause a log dec = 0.

I'd like to say that this machine is fine.

(Honestly, the customer is pestering us on a non-issue and I need to give a solid 'IGV's aren't strongly correlated to vibration' statement. I think I now have enough fire power to get this done)

Thanks again for your insight.

edit: added HP note.

edit 2: clarified HP.

 

[awhicker84]

I would like to state my results in case this subject gets searched in the future.

The anticipated cross coupling actually went down to about half of the original guarantee conditions. The cross coupling equation includes two main drivers that changed from new machine to now: Power required and density ratio. The density ratio decreases as the IGV's close (less head delivery) and the power required also decreases (again, head goes down).

In other words, assuming that the IGV's have a very weak correlation to cross coupling (which appears to be the case considering how hard I looked for studies and how much I sifted through API) then we can also assume that the worst case cross coupling scenario for a constant speed IGV machine is with the IGV's 100% open (or greater, if the OEM allows).

*NOTE* this is only one data point that suggested that density ratio decrease was more important than the horsepower loss. This unit also operates with ~30 MW, which is where the cross coupling equation is most accurate.

Please feel free to correct me if I'm wrong.

Cheers,

Edit: added CYA clause.

 

[rotw]

Machines equipped with IGV's are fixed speed - this is most common. If am correct, they are handled rotor-dynamically as such.
API 617 - suggested here as reference only, states for the cross-coupling effect per level I screening - Excerpt>
....For this analysis, the machine inlet and discharge conditions shall be at either the rated condition or another operating point unless the vendor and purchaser agree upon another operating point.

A sound lateral analysis report should screen the min/max lube oil temperature, min/max support stiffness combined and the maximum density case (see API quote above for which point to consider), and if not by standard - at least as a good practice.

Back to your discussion with your client, your finding can help you build a story and kill a possible non-subject.

Weather there could be other ways for IGV's to impact rotor-dynamic behavior / vibrations is something maybe to investigate. This goes far beyond my knowledge.