liquid carry over to centrifugal compressors

[upm]

Gents,
i know the risk of carrying over liquid to comressors and what damage does it cause (and even explosion). from technical point, the compresor will act as pump and try to cpmpress liquid, but any body can explain how does this take place? is it due to heavier liquid weight thatn gas where the load on the bearing increases excessivliy?any body aware of technical paper on that matter?
pls. explain
upm

 

[pmover]

please note responses in heat transfer and thermodynamics forum.
-pmover

 

[scalleke]

I am not aware of a technical paper on the matter but I can confirm that you are right in your main idea.

Bearing load is the most important factor. Immagine the difference in specific weight between a gas and a liquid. Most compressors are designed to take bearing loads that correspond to an operating range close to BEP.that means that shaft support surfaces are designed to closely match the expected load. Not too small a support area as this would cause the oil film to be broken, not too wide as this would make the shaft behave unstable on the oil film due to too light a load.

Increased load due to increased specific weight will cause the shaft and bearings to run into each other. it will cause the impeller to run out of position and possibly touch. But most likely what will happen is that the impeller to shaft connection is not strong enough and will snap.

Note that my experience is with high speed centrifugal compressors. I once had the question how much liquid can the compressor take? If liquid comes in as a mist or droplets the compressor will see it as an increase in specific weight and load will increase accordingly. the unit can accept this within the limits of the design. However a glut of liquid entering an impeller running at high speed will have the impeller almost stop at entry anything goes at that point... bent and snapped shafts ... ripped off blades melted bearings...etc etc...

I like to think in analogies: Immagine waving a paddle through the air, no problem Immagine waving a paddle through the air while the fire department is exercising and spraying a heavy water mist, This would be more difficult. immagine slapping a paddle as hard as you can on a plain water surface, if you have enough power the paddle will snap.

Best Regards
Scalleke

 

[upm]

Scalleke,
Thanks for the info. i like th example you had and it was very clear to what impact does the liquid have on a compressor. i am trying to reflect this on technically speacking and convert it into numbers i.e. the predicted internal pressure when liquid is carried over? this will give an idea of what we are talking about.
if you came a cross any, pls. let me know.
upm

 

[scalleke]

Calcs will be difficult in this caser as all the dat that you have will be guesswork.

You do not know how much liquid will come in and then tghere is the issue of the liquid characteristics. Once the liquid enters the compressor temperature will be hotter and what kind of a phase will the matter be in. will it stay liquid? will it become gas? How much will come in? will it mix with gas condensate? Too many variables, too few equations.

Best regards.

Scalleke

 

[Bensley]

Has anyone had any experiences with outer cylinder head bolts breaking or shearing on a gas recip compressor and the many possible reasons?

Like liquid slugs, loose bolts, cylinder head stud bolt not being installed properly, (bottomed out and torqued) metal fatigue, embrittlement etc?

The gas compressor in question is Dresser Rands latest Valve in Piston (VIP) these run at around 1000~1500 rpm.