Bubble test on pump impellers 1

[valleviento]

I would like to specify a bubble test requirement to check performance on a critical pump first impeller.

I would like to know if there is any laboratory or univ doing this kind of test.

 

[Artisi]

Ok, I'll be the first one to ask - what's a bubble test?

Naresuan University
Phitsanulok
Thailand

 

[valleviento]

On a centrifugal pump, there is a risk of cavitation phenomena taking place at the first impeller. Due to the geometry of the impeller and the liquid being pumped, the pressure could go below the vapour pressure and some bubbles may appear at certain critical areas.

Ok, on a bubble test we try to have visual access to this phenomena (usually with a transparent volute and with high frequency strobe light) and determine the areas, size and amount of bubbles that appear.

Sometimes a soft coating is applied to the impeller so that when the bubble implodes peel off this coating, being able to check the cavitation phenomena pattern

 

[Scipio]

What's wrong with a conventional 3% drop in differential to test for NPSHR? Unless this is a lab pump intended for experiment, I have a feeling your bubble test will wind up being cost prohibitive.

 

[sprintcar]

"Two things that are not the same are different"
- your clear volute and standard one will not be identical and could result in invalid test results when applied to the real world parts.

Scipio is right - the standard test should be accurate for the majority of applications.

Check the Hydraulic Institute recommendations for more info.

Racing and bullfighting are the only real sports...everything else is just a game.
Bob

 

[checman]

valleviento, I have had the opportunity to play with an acrylic pump with acrylic piping. It was difficult to see what was happening at the impeller because of the white water (for lack of a better term) in the casing. You could restrict the suction on the pump and degas the liquid in the suction pipe. If you further restricted the flow you would see precipit clouds of cavitation in the suction pipe. And further restriction you would have full blown cavitation. The problem with trying to use this for verification of pump impeller performance was that so many things influenced the performance of the pump and it would probably not duplicate the conditions that your pump will operate in. The acrylic casing would not preform the same as a cast iron one. The test systems operating conditions changed as we used it. The water in the loop became warmer and more entrained with air every pass through the pump.

So in short your money would be probably be better spent on improving your NPSHA. Stay away from low NPSHR impellers unless it is absolutely necessary. They generally have a more narrow window of smooth operation.

I have no experience with the coated impeller.

Regards checman

A side note: One of the most interesting things that we found with the test stand was that if you were able to create a vortex in the suction pipe you had to drastically change the operating condition to collapse it.

 

[PUMPDESIGNER]

The 3% drop HI method is a work around really. The 3% drop in Head at test conditions is most accurate (and therefore most useful) for low Ns and low Nss pumps, say Ns=450-2000, and Nss=2000 - 6000.

As Ns and Nss increase 3% value becomes less useful until you reach the High Energy Suction area where 3% is useless because the impellers pass fluid with such high efficiency that they can be completely enveloped in super cavitation bubbles and still move a ton of water with reduced head still below the 3% value.

Incipient cavitation is present in most pumps in most installations, and visual detection of incipient cavitation does reveal how much incipient is occurring.

Good luck eliminating incipient cavitation, takes tremendous amounts of pressure to totally supress all cavitation, not possible in most situations, but it is possible.

Concerned about a problem, use an inducer, increase suction head, locate pump differently, etc.

I will check back, but am very busy to write more at the moment.

PUMPDESIGNER

 

[vanstoja]

For detection of incipient cavitation, acoustic methods are an alternative to the usually costly process of determining cavitation inception visually using a tranparent, plexiglas pump casing. For a description of incipient cavitation acoustic testing see vanstoja's 2/25/05 reply in Thread254-116566.
Any major hydraulics testing laboratory can probably do visual cavitation inception testing. In the USA the Penn State University Garfield lab, St. Anthony Falls lab and Alden Hydraulic Laboratory are probable candidates. In Britain and Germany, some possibilities are the National Engineering Lab and the Pfleiderer Institute, respectively.
A recent visual cavitation test at Pleiderer Institute is reported in Friedrichs,J. & Kosyna,G.,2002, "Rotating Cavitation in a Centrifugal Pump Impeller of Low Specific Speed", ASME J.Fluids Engineering, Vol.124,June, pp.356-362.

 

[PUMPDESIGNER]

Hello vanstoja, been busy and missing you guys.
St. Anthony Falls is good, nice guy runs it, enjoys his work.


PUMPDESIGNER