Bubble formation pressure of water

[jwray]

I am running several pumps in parallel that are pumping water. A major concern for our application is that bubbles will form because the bubbles will cause our accuracy to be destroyed. I have found a little information on the bubble point of liquids but I am not familiar with the topic of bubble point. I was wondering if anyone could let me know the pressure at which bubbles form in water at a temperature slightly higher than room temperature. Any help will be greatly appreciated.

 

[phex]

are you talking about cavitation? if yes, the pressure to be looking at would be vapor pressure at your temp. going below this pressure will allow the vaporization, which causes bubble formation.

other way of forming bubbles i can think of would be gas leaving solution. for this to happen, temperature had to be higher before your pumps (which is not really the most probable thing to happen...).

give a little more info in line with my comments above, then i'd be able to help further.

hth,
chris

 

[BobPE]

phes is right we need a bit more information....try flows and pressures, npsha and npshr, piping configuration and diameters...things like that...it may not seem relavent, but we usually find a way to figure things out by looking at information like that....
BobPE

 

[jwray]

the flow rate currently is 2 ml/sec and an average pipe size is around 2 mm. There are many different sized pipes thoughout the manifold that connects these pumps together. I have already done analysis of the fluid system and arrived at a pressure at the pump. I also have performed experiments on lab bench set up with these pumps. The experiment was done by varying flow rates until bubble formation occured. I want to know what the pressure should be when these bubbles are forming so that I can corrolate my analysis and observations. My analysis is obviously not very accurate due to the fact that there are many minor losses due to bends that I had to account for with approximations.
Thanks guys

 

[phex]

as i mentioned before, the pressure you are looking for is the vapor pressure of the liquid in question at the temperature in question.

in other words, the pressure at suction side of your pump is low enough for the liquid to boil at the working temperature. this means, your npsh required is higher than your npsh available.

as your process liquid is water, you can get the neccessary information from steam tables. a freeware calculator is available here:

http://www.atkinsopht.com/eng/asmetabl.htm

hth,
chris

 

[BobPE]

I would think your concern is going to be more with phex's second concern of gasses leaving the liquid. This will occur before the onset of cavitation which should not be a dramatic problem in this case.

You will have to know the disolved gas makeup of the stream and detrmine the partial pressures of these gasses. You will discover the limiting gas pressure from your analysis and this will become your minimum pressure on the suction side. The next thing to cause bubbles will be the liquid constituants of the stream...but this wont be a concern since you would have already generated bubbles from off gassing....

BobPE

 

[jwray]

Thanks guys you have been quite helpful. You raised a good point that I had forgot to mention earlier as well. That the water supply being used is a 'millipore' water supply, which is de-gassed in order to help prevent bubbles from forming.

 

[phex]

then it's clear... you ARE in fact dealing with cavitation.

may i do some dancing for guessing right?

 

[25362]

CMIIW, vapour-filled bubbles are generally those that cavitate. Pirouetting is a good exercise. <J> :]

 

[MortenA]

The vapour pressure of water depends on temperature. Most data books have list of the vapour pressure at various temperatures. At around 15 deg C i belive its about 0.03 bar (a) and at a 100 deg C im sure its 1.01325 bar (a)

Best Regards

Morten

 

[wotuwish]

I am currently designing a pumping station for effluent at 90 deg C. The vapour pressure at this temp is a problem, that is how I stumbled on this site, looking for the vapour pressure of water at 90 deg.
What is required is to look at the NPSH, or the required net pressure at impeller to avoid cavitation. this pressure is pump and system specific. NPSH increases with flow and pump impeller speed. Also some manufacturers allow some cavitation.
Your pump manufacturer should be able to tell you what the NPSH is for your particular conditions.
Also thanks guys (all of you!) Thanks to your postings i found a site distributing software for calculating vapour pressures. I have joined and will be participating in forums!
Wotuwish

 

[BobPE]

wotuwish:

I would say one thing, pump manufacturers do not allow cavitation, engineers allow cavitation...If the manufacturer tells you that, then their pump is junk and you should find another....They go throug great pains to hide and alter their curves so that we engineers have no clue what they are selling us, but we can figure out what they are doing....and when you get to the bottom of their curves, they tell a far different story sometimes....not that said, not all pump manufacturers are like this...it takes years of experience to figure out who sells the junk....

BobPE

 

[wotuwish]

BobPe,
Thanks. However I have seen some literature (it could be from a manufacturer!) that new coatings have been invented that can resist the pressure of the bursting gas bubbles! You are right however, it is better to ensure that the available NPSH exceeds required NPSH by a good 0.5 m at least