Liquid ingress in Axial flow compressor
Liquid ingress in Axial flow compressor
(OP)
Hi all,
I am interested the reasoning behind liquid ingress being allowed in an axial compressor (on a gas turbine unit), but not in a centrifugal compressor. Obviously liquid carry over can be devastating in a reciprocating compressor, and I believe is quite damaging to a centrifugal compressor as well. I am just wanting to know the theory behind why liquid can enter an axial flow compressor without any negative effect(an example on my plant is compressor blade water washes on gas turbine units, or alternatively aircraft turbojet/turbofan engines which can ingest large amount liquid without damage). The principle of all 3 types of compressor are the same (gas is forced into a smaller space, decreasing its volume), so any liquids, being incompressible, should theoretically not be able to enter the compressor. What am I missing?
Is it just the high mass flow rate that makes liquid ingress negligible, or something else?
Thanks in advance for any responses.
I am interested the reasoning behind liquid ingress being allowed in an axial compressor (on a gas turbine unit), but not in a centrifugal compressor. Obviously liquid carry over can be devastating in a reciprocating compressor, and I believe is quite damaging to a centrifugal compressor as well. I am just wanting to know the theory behind why liquid can enter an axial flow compressor without any negative effect(an example on my plant is compressor blade water washes on gas turbine units, or alternatively aircraft turbojet/turbofan engines which can ingest large amount liquid without damage). The principle of all 3 types of compressor are the same (gas is forced into a smaller space, decreasing its volume), so any liquids, being incompressible, should theoretically not be able to enter the compressor. What am I missing?
Is it just the high mass flow rate that makes liquid ingress negligible, or something else?
Thanks in advance for any responses.





RE: Liquid ingress in Axial flow compressor
- In a centrifugal compressor the gas enters the eye of an impeller and is rapidly accelerated (at near constant pressure) into a volute. In the volute the high velocity is converted to increasing pressure by putting the gas into an increasing space.
- An axial compressor looks very much like a driven version of a reaction turbine. Within each set of wheels the gas velocity is increased by the driven wheel and then enters a divergent section that trades the increased velocity for increasing pressure in an increasing space
Centrifugal compressors have several very severe direction changes that gas can make without excessive force, liquid making the same turns has markedly more specific mass and since force is mass times acceleration, the forces increase dramatically at the turns and can break stuff.Axial compressors do not have the dramatic direction changes and can tolerate liquid ingestion without excessive local forces.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Liquid ingress in Axial flow compressor
Thanks again.
RE: Liquid ingress in Axial flow compressor
On centrifugal compressor, the liquid ingestion is tolerated however this may create erosion and/or corrosion. actually a certain fraction of liquid carry over through machine is normally specified by machine manufacturer as an acceptable limit. But centrifugal was not the question as I understand.
So for axial blades, the example that comes to my mind is condensing type steam turbines. I think that when the steam has some mist there is a risk of damaging the blades by erosion phenomenon. I think 80 percent humidity is a sort of acceptable limit I have in mind from some project, but I have to confirm it. Actually this sets the condensing pressure level. With air cooled condenser, the condensing pressure is a sizing criteria for the condenser, together with the humidity limit specified at the condensing stages (therefore mitigation of erosion problem) these things are intimately connected.
"If you want to acquire a knowledge or skill, read a book and practice the skill".
RE: Liquid ingress in Axial flow compressor
I've never looked for any manufacturers specified tolerances for liquid carry over..I've just always been told 'no liquid in the compressor', which I think is a good rule to live by in general. I'll check out the data sheets for some centrifugals and find out.
I don't know too much about steam turbine units, but the principle is the same so I get what you're saying, although that answer seems like it is more in relation to mist/saturation. How about liquid slugs or a constant liquid water stream through an axial compressor? How much is too much?
An example that comes to mind is Qantas flight 32 a few years ago...the aircraft engine wouldnt shut down (Rolls Royce trent 900 series, running full speed) and fire fighters at the airport introduced a high flow of water into the engine trying to cause flame out. How would internal components handle this kind of large liquid volume?
It's getting very hypothetical now, I know, but I am interested in whether water/liquid is an issue for compressors in regard to stator/rotor damage etc.
Thanks again.
RE: Liquid ingress in Axial flow compressor
I think you mean "steam quality of 80%" instead of "Relative Humidity 80%" since by definition in a saturated system the RH is always 100%.
The how much liquid is too much question is something that the manufacturer of a given machine would have to answer. I occasionally see centrifugal air compressors (most dynamic air compressors are axial), and each manufacturer has very detailed and specific limits on liquids ingress--and they are all different.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Liquid ingress in Axial flow compressor
RE: Liquid ingress in Axial flow compressor
RE: Liquid ingress in Axial flow compressor
Yes, correct, this is what I meant.
Thanks for correcting me.
"If you want to acquire a knowledge or skill, read a book and practice the skill".
RE: Liquid ingress in Axial flow compressor
RE: Liquid ingress in Axial flow compressor
I wonder whether the higher PR in centrifugal machines accounts for the lower tolerance of liquids? The relative velocity would need to be significantly higher to achieve the required PR in a single stage whereas the multi-stage axial machine accelerates then decelerates the working fluid in each stage.
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