MartinLe
Civil/Environmental
- Oct 12, 2012
- 394
I'm not at all sure that I really understand the hows and whys of cavitation in gas carrying media, here's some thoughts, if I'm way off please point out.
In water, the mecahnism as far as I understand is that the pressure in the pump is locally lower than the vapor pressure, small vapor bubbles form, these collapse, this can damage the pump and the whole process looses kinetic energy.
I'm interested in biogas slurries, the relevant gases are CH4 and CO2.
CH4 is practically insoluble, so a pressure change would mean that the tiny bubbles entrained will grow and shrink, correct?
CO2 will be mostly dissolved, lower pressure will mean that small bubbles form. Bubble formation will be limited in rate because the CO2 is dilute. With a pressure rise I'd expect the CO2 to dissolve again.
Either case, while bubble forming from the entrained/dissolved gasses will in all likelyhood occur at lower temp/high pressure than cavitation in pure water. In my mental image of gas bubbles collapsing there's somehow no shockwaves (like in vapor bubbles): CH4 bubbles don't vanish, they just shrink, CO2 dissolvesbut the kinetics is different and I somehow would expect these bubbles to collapse slower. I'd expect no damage to the impeller purely from this. But this is very much intuition and could be very wrong.
The observations I hear from our field guys is that when a pump cavitates, you get noise and performance losses, no one mentioned damage to the impeller (But it's entirely possible ). So I don't think my mental model is totally wrong. Or is it?
In water, the mecahnism as far as I understand is that the pressure in the pump is locally lower than the vapor pressure, small vapor bubbles form, these collapse, this can damage the pump and the whole process looses kinetic energy.
I'm interested in biogas slurries, the relevant gases are CH4 and CO2.
CH4 is practically insoluble, so a pressure change would mean that the tiny bubbles entrained will grow and shrink, correct?
CO2 will be mostly dissolved, lower pressure will mean that small bubbles form. Bubble formation will be limited in rate because the CO2 is dilute. With a pressure rise I'd expect the CO2 to dissolve again.
Either case, while bubble forming from the entrained/dissolved gasses will in all likelyhood occur at lower temp/high pressure than cavitation in pure water. In my mental image of gas bubbles collapsing there's somehow no shockwaves (like in vapor bubbles): CH4 bubbles don't vanish, they just shrink, CO2 dissolvesbut the kinetics is different and I somehow would expect these bubbles to collapse slower. I'd expect no damage to the impeller purely from this. But this is very much intuition and could be very wrong.
The observations I hear from our field guys is that when a pump cavitates, you get noise and performance losses, no one mentioned damage to the impeller (But it's entirely possible ). So I don't think my mental model is totally wrong. Or is it?
