The 6th impeller of a twin stage wet gas compressor increases in size (width) than the previous

[Arshad Ahamed]

Greetings,

I came across a Wet Gad Compressor (2MCL-406 : Twin Stage, Horizontally Split, 40cm Impeller Dia. and 6 No. of Impellers). It is understood that the impeller size (width) would decrease as the compression progresses. The Compressor take suction from one end, has an intercooler, then the suction for the next stage is taken from the other side and discharged from the centre.

Of the First Stage, the impeller width decreases as required. In the Second Stage, the first two impellers decrease in size and the third has a width greater than the second impeller (of Second Stage).

Thanks and Regards,
Arshad Ahamed.

 

[rotw]

This GE Nuovo Pignone machine. This generally the case when there is injection in the 2n stage but the tag would be 3MCL in that case.
If this is really just a back to back and 2MCL tag is correct, it may be due to old design whereby the impellers were not "continunous" but discrete - so impellers are picked from a library with a pre-set design flow ;if situation is lucky enough the actual flow would match design flow; if not there is volume shift - so the width can be bigger (or smaller) to what is currently needed and you have to live with.
Also it may be the case that there is switch to different impeller family (1st and 2nd impellers stringent in terms of Mach number and then Mach number went back to "normal").
Another thing to consider irrespective of the fact wether continuous or discrete stage are employed (or even swith to new impeller family) is that designers are often required to widen the operating range; so typically what you would is look at which impeller is "surging first" and then try to fix that one because it is the locus for the overall operating envelop.
PS: I assumed that the width increased while diameter stayed constant for all impellers otherwise you need to be more precise.
Edit/ If by smaller size of 1st and 2nd impeller you mean smaller impeller tip diameter, keep in mind that there is also consideration of space so that you wont jump into the next frame size. If 3rd impeller has bigger diameter than 1st and 2nd, keep also in mind that the width would tend to be smaller not bigger.

 

[Arshad Ahamed]

Thank you rotw. You were right in the assumption - the impeller dia's are constant at 40 cm.

 

[awhicker84]

Maybe I'm not reading closely enough, but is there a intercooler between the 2nd impeller exit and the third impeller inlet? I couldn't figure that out from the post above.

Edit: I guess you're saying: Impellers 1 thru 3 -> Intercooler -> Impellers 4 thru 6 and Impeller 6 is larger discharge tip width than Impeller 5?

If so, I agree with ROTW. This may be either a manufacturing decision or may be dictated by performance range. Seems odd though.

 

[rotw]

By the way, wet gas compressors are machines sometimes difficult to select; the gas is often heavy with large swing of molecular weight at varying operating conditions. Most importantly it may be required on top of that interstage pressure has to match some imposed value (process or licensor requirement). Focus is more likely on operating range (flexibility / robustness to varying conditions) and control method(s) to accomodate all conditions in compressor performance envelop, making efficiency optimization secondary or a nice to have.

 

[Arshad Ahamed]

awhicker84, the configuration is exact to what you have guessed.

On talking with an Engineer from our company, his guess is that the peculiar configuration would be to dynamically balance the system (he wasn't sure because he wasn't familiar with the system).

 

[rotw]

Dynamic balancing of compressor involves improving mass distribution of rotor which is generally done by grinding / material removal and methods for that are quite standard (most likely shall be done to API617).

Impeller with bigger width will INCREASE the bearing span which in return decreases shaft stiffness and causes the machine to become LESS STABLE rotor-dynamically, considering everything else being equal.

IF this is really the case that for balancing reasons (or shaft mounting / other mechanical considerations??), a solution was adopted that impacts thermodynamic outcome (selecting larger width impeller just for that purpose interfers with performance) THEN first of all this would be something that needs to be done with prior consent of the User/Owner, also I suppose as a last resort method and - unless contractor has done poor job in specification phase, something that does not speak very well of the quality of the design.