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Compressor critical speed

Compressor critical speed

(OP)
Hi All:

I have a compressor supplier proposing 1500 mm shaft, 125 mm D, tilting pad compressor/ bearing system, runs at 9000 RPM, is it usual and acceptable to have 2 critical speeds below the operating speed and the 3rd above within undamped analysis, and when he run the damped analysis, only one critical speed is below the operating speed, which eliminate the situation of the undamped?

What about bearing loading?


And what about loading on or between pads?

RE: Compressor critical speed

-You rely on the damped analysis as this one gives you the realistic behavior. An undamped analysis is just an overview which gives rough idea where the CS are located with respect to the operating range and respective to bearing clearances.

-So now the damped analysis : If you have a pick in the operating speed range and the amplification factor at the pick is below 2.5 then it is okay per API ;

- Bearing load : you don't have to care about that, it is up to the manufacturer to choose the most suitable configuration and yes it impacts the CS. Bearing with load on pad might be questionable when it comes to heavy shafts ; during start up the shaft needs to be lifted up upon rotation so that the hydrodynamic lubrication regime takes fully place.

- For your particular case you have a shaft that has a stiffness which is questionable so you need to pay attention.
It does not mean the machine is not okay but there are of course techniques you need to apply then in order to look more closely...

"If you want to acquire a knowledge or skill, read a book and practice the skill".

RE: Compressor critical speed

(OP)
Thank you so much, can you clarify more two points, why you are saying the I have a shaft with a question stiffness? Are we saying that my supplier picked a very high speed and compress the gas in two impellers only that result in super performance but very short shaft? Do I need to look at this? I'm really worrying about loosing my damping by time and these criticals ll be a problem.

My 2nd point, please add more from your knowledge on the differences between load on and between pads.

LAST Question, have you ever seen before a compressor runs through two criticals until it reach 100% fixed- MCS- Rated speed?

RE: Compressor critical speed

could you send the following :

- Type of casing (drive through or end drive), type of driver.
- Number of impellers and their diameters
- Average diameter of the shaft and bearing span
- Operating speed
- Journal bearing diameter and type of lubrication
- operating pressures and temperature, MW of the gas and actual flow (m3/h) at inlet

"If you want to acquire a knowledge or skill, read a book and practice the skill".

RE: Compressor critical speed

(OP)
Feed gas compressor, Methane, driven by Synch Motor, Barrel machine compressor.
The two impellers are 530 mm

The operating speed is 9000 rpm, motor is 1800.

ISO 46

600- 900 delta P

Over a million of SCFM

130 mm bearing span

RE: Compressor critical speed

(OP)
I'm sorry, pressure is in pound, gas power is ~ 15 MW

RE: Compressor critical speed

Your data are not consistent.

Supplier might have pushed a bit on the speed to optimize the performance but if their lateral analysis report is ok and if he could back up his claim with references then no need to go deeper and deeper.

If you really want to go further despite what I wrote above, then you should hire a qualified consultant and have him review this.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

RE: Compressor critical speed

(OP)
Thank you So much again, you are correct.They pushed the speed between 2nd and 3rd CS to eliminate an impeller. Having said that, in general, what is your experience with load on/between pad?

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