Centrifugal compressor Compression Ratio

[AndreChE]

Dear all

Our Raw Gas Compressor, in an ethylene plant, has been experiencing lower compression ratios in two stages when compared to previous operation, before plant shutdown.
Rotors and impellers are switched every 5 years. The removed rotors are sent to Dresser Rand for recovery and previous rotors (most recently recovered) are installed.

We don't see vibrations nether axial displacements.
I notice lower polytropic efficiencies and lower Rc are an evidence.

What can be origin of this phenomena? Because of this we are limited in plant load. The turbine is at its maximum since discharge of the last stage has to be kept at 35barg. Other stages are compensating these lower Rc.

The compressor has three rotors, each stage has 3 impellers (6 impellers for the 1st stage only).

ROTOR 1 - 1st stage
ROTOR 2 - 2nd and 3rd stages
ROTOR 3 - 4th and 5th stages

We have this problem in stages III and IV, different rotors!!??!!

I cannot understand this. If the phenomena was only in one rotor but on two...

Does anyone have experienced something simular?
Labyrinths between impellers?

AndreChE

 

[dcasto]

Switched impellars between 3 and 4?

 

[AndreChE]

All impellers were switched.

We have two pairs of everything.

 

[dcasto]

I mean the actual impellars switched, where they istalled a 3rd stage on 4th stage

 

[ash9144]

Switched impellers is good possibility.
Also the clearances on those two stages may have been improperly set.