Fluid vaporising, but not cavitating... 1

I think i once saw a study where N2 was added to the fuid at the suction side of a pump pumping a condensate. The theory being that the pump would be OK with the presence of bubble, it was the cavitation (implosion) of these bubble that actually cause the wear on the impellers. But i cant remember if the conclusion was clear or if this technique is used in this type of service.

You may be interested to consult (see Fig.3):

For the more detailed technical literature, I found this resource:

So with reference to operation beyond incipient cavitation level (NPSHi), this quote from the article may be of relevance to your question:

It is clear from the above analysis that all of the nuclei whose size, R, is greater than some critical
value will become unstable, grow explosively, and cavitate, whereas those nuclei smaller than that critical size will react passively and will therefore not become visible to the eye.

So my understading is that NPSHi is defined by criteria relative to bubble critical size. In other words, beyond NPSHi, cavitation inception does not occur, still you may have nuclei smaller than that critical size which will react passively. This is what I infer personally, keeping in mind I just have very generalist knowledge of pumps.

Are discussing water as your pumped fluid and how do you know it's vaporising?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

The fluid doesn't cavitate, the pump does.

If you mean it's two phase flow when it enters the pumps then generally it's not good news for the impellor, the bearings or the seals as a whole load of shock loads go through the pump on a regular basis. The head of the pump will also vary quite a lot...

Downstream the pump the vapour might then be absorbed or dissolved in the liquid

Can you add a bit more detail please.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Yes, the result is vapor locking the pump and moving no fluid.
This is common with dissolved gasses or solvents.

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P.E. Metallurgy, Plymouth Tube

I will give you my story. It was brought up a number of years ago in this forum and a resident pump expert had a lively discussion with me as to what I was saying. I was saying no cavitation was occuring...He said there was cavitation occuring…LOL…:

[ol ]
[li]An end of life brand name fan cooled cooling tower was replaced with a brand name fan cooled cooling tower in a water plant administration building[/li]
[li]The cooling tower was located indoors on the second floor. The recirculation pumps were located in the basement level some 25 ft below the cooling tower. Therefore the pumps had a flooded suction and YOU CANNOT BE HAVING CAVITATION[/li]
[li]I had flow meters on the recirculation pump discharge piping. The flow meter readings were bouncing all over the place[/li]
[li]I put my ear against the pump suction line and it sounded like it was not running full even though the pump was flooded. You could hear a hollow dripping sound with the water sloshing around![/li]
[li]The cooling tower rep set up the working water levels in the cooling tower to Setting B mentioned below. Cooling towers such as this are supplied all over Toronto. They said that is the way they have always been setting the working water levels[/li]
[li]The cooling tower fan is entraining air into the water to cool it down. Could the air be making its way into the suction of the pump? I decided to phone the US manufacturer of the cooling tower to find out what the settings of the working water levels in the cooling tower are supposed to be.[/li]
[li]A lady engineer from the cooling tower manufacturer was assigned to answer my question. She emailed me drawings on how to set it. I had to do some simple arithmetic with the numbers shown on the drawings. One of the drawings, we will call it Setting A, had the top water level 1.5" higher than what was shown on another drawing which we will call Setting B. Yes….only 1.5” lower! I asked her to check with the old timers in her company which setting it should be at. She advised I was the first person to ever notice that. She confirmed it should be at Setting A.[/li]
[li]With Setting B we were basically getting entrained air into the pump. This is not cavitation. It can, but not always, cause damage to the impellors and to the UNTRAINED eye it may look like or sound like cavitation. But it is not cavitation. To prove what I am saying, I throttled the pump discharge. There was no change in gravelly sound. If it was cavitation, the gravelly sound will disappear. The vibration from air being entrained into the pump was also shortening the life of the coupling mating the pump shaft and motor shaft.[/li]
[li]There was a lot of subsequent back and forth between the US manufacturer, their Toronto rep and myself. The Toronto rep had trouble believing the water levels should be at Setting A because THEY felt the cooled water sump would overflow. THEY KEPT RESISTING AND RESISTING RESETTING THE WORKING WATER LEVELS TO SETTING A EVEN THOUGH THEIR MANUFACTURER TOLD THEM TO DO SO! At the end of the day, the water level setting in the cooling tower was finally reset to Setting A. This miniscule 1.5" increase in working water level allowed the entrained air to float to the water surface IN THE COOLING TOWER and not be drawn into the pump suction 25' below. All of the reasons the Toronto rep had for using Setting B and resisting Setting A were proved to be totally unfounded. The cooling tower worked fine, the pump suction piping sounded full, the flow meter readings became stable, the pump and motor shaft couplings are fine and there is no more "cavitation". This incident happened about 6 years ago. I think there are a lot of screwed up cooling towers in Toronto...Good for after warranty service calls LOL...[/li]
[/ol]

When you are pumping gas and liquid remember that one is compressible and the other is non compressible. Because of that you will get pump vibration, wearing out of the seals, impellor damage (similiar to but not cavitation impellor damage), erratic flow readings, pump shaft/motor shaft coupling damage etc

Look at ways of preventing the fluid from vaporising before the fluid enters the pump. It may be as simple as having a flooded pump suction high enough to overcome the vapor pressure at the centreline of the pump. Remember that when you are in suction lift condition your liquid is under negative pressure and it is now more conducive to offgassing

"Therefore the pumps had a flooded suction and YOU CANNOT BE HAVING CAVITATION"

Why do people put those two things together as if the one results from the other?

"Therefore the pumps had a flooded suction and YOU CANNOT BE HAVING CAVITATION"

Why do people put those two things together as if the one results from the other?


Not sure what you mean? In this case I was advising there was no cavitation while the resident expert said it was cavitating. It sounded like cavitation but it was NOT cavitation. Using engineering logic it COULD NOT be cavitation so I went one step further and the problem was solved. Cases like this are NOW well written about on the internet and almost all involve cooling tower applications. There has been a number of past threads on eng-tips describing such problems on their projects. Except for me there never seemed to be a successful conclusion in those threads. The following explains what is happening (as does my previous writeup):



There was not very much written about the subject on the internet some 7 years ago on this subject. Now there is more information out there

My point was that many people believe that if you have a 'flooded suction', you cannot have cavitation. That is not true.

TenPenny:

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)