How Pool Pump Cavitation Sucks!

I’m afraid there’s not going to be anything pretty about this one. At least not with the diagrams. I want to talk about Why Pool Pump Cavitation Sucks and the pictures I want don’t exist. So, I apologize in advance. Hopefully, my drawings are not too horrible.

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Why Pool Pump Cavitation Sucks!

Why Pool Pump Cavitation Sucks!

For the most part, it’s easy enough to identify; the system starts to sound like there is gravel running through it, the flow rate fluctuates, and you may also have a steady stream of tiny bubbles coming back into the pool through the return jets. That’s not the mystery. It’s the reason a pump cavitates where the internet spews bull squat.

The impeller is the part of the pump that ‘throws’ the water. This is also the part of the system that takes the brunt of the damage when cavitation occurs.

The influent (inflowing) water is drawn into the wet end by the spinning impeller’s suction. After the water passes through the pump pot where the strainer basket is kept, it must enter the pump volute before it reaches the impeller. Or through a diffuser, if a diffuser pump is directed to the impeller’s eye.

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Brass is 67% Copper and 33% Zinc

Why Pool Pump Cavitation Sucks!

Bernoulli’s equation.

As the impeller spins (counterclockwise), centrifugal force exerts on the water. The rotation increases both its pressure and velocity, driving the liquid through the discharge and into the filter. This force creates low pressure at the eye, water from the pump pot rushes to fill, giving us the suction that draws the water from the pool.

Why Pool Pump Cavitation Sucks!

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If something were to restrict the flow of water to the impeller, the pressure at the eye would drop further. This could be due to a full pump basket, incorrect valve position on the suction side, clogged suction pipe, stuffed skimmer basket, blocked main drain, or a vacuum filter (if present) in need of cleaning.

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A low-pressure boil.

If the pressure drop were significant enough, the water would flash to steam. These steam bubbles are whisked along the veins of the impeller. Some get caught in the effluent (discharge) stream of water and may make it back to the pool. This explains the floor returns bubbles when a vacuum DE filter is long overdue for a cleaning.

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Collapsing of air bubbles.

Most of the bubbles won’t make it past the veins of the impeller. Once they reach the water with higher pressure, the bubbles begin to implode. The steam bubbles’ violent collapse sends shock waves through the water that can damage the pump’s internal components. This will cause pitting, and the impeller typically takes the bulk of the damage.

Why Pool Pump Cavitation Sucks!

It is important to note that discharge cavitation also exists. When something, such as a closed valve, restricts the flow to the point it cannot easily leave the pump. Or if the flow is restricted downstream in the system (see video below).

When water trapped in the pump circulates within the wet end at a high velocity, it is forced through the narrow clearance between the impeller and the seal plate and the impeller and the volute, or diffuser if a diffuser pump. This will cause damage to the impeller, pump shaft, and other internal components when the steam bubbles collapse.

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Can Cavitation Kill Algae & Bacteria?

This question comes up a lot. Can cavitation kill algae? The answer is, in many cases, yes. But this only applies to the organisms that come in contact with the shock waves from the bubble implosion. Then, consider the cost due to equipment damage and leaks caused by a continuous vibration through plumbing. That said, there is a lot of scientific literature available on cavitation for algae and bacteria control.

In everything I have read to date, the results have been pretty much in line with one another. A 90% eradication rate exists for most algae and bacteria, though kill time varies with species. Eradication rate is poor for some species, others see no effect, and at least one algae species growth rate increased. Not all studies look at both algae and bacteria – I attached the pdf for one that does below.

Mojca Zupanc, Žiga Pandur, Tadej Stepišnik Perdih, David Stopar, Martin Petkovšek, Matevž Dular,
Effects of cavitation on different microorganisms: The current understanding of the mechanisms taking place behind the phenomenon. A review and proposals for further research,
Ultrasonics Sonochemistry,
Volume 57,
2019,
Pages 147-165,
ISSN 1350-4177,

Abstract: A sudden decrease in pressure triggers the formation of vapour and gas bubbles inside a liquid medium (also called cavitation). This leads to many (key) engineering problems: material loss, noise, and vibration of hydraulic machinery. On the other hand, cavitation is a potentially useful phenomenon: the extreme conditions are increasingly used for a wide variety of applications such as surface cleaning, enhanced chemistry, and wastewater treatment (bacteria eradication and virus inactivation). Despite this significant progress, a large gap persists between the understanding of the mechanisms that contribute to the effects of cavitation and its application. Although engineers are already commercializing devices that employ cavitation, we are still not able to answer the fundamental question: What precisely are the mechanisms how bubbles can clean, disinfect, kill bacteria and enhance chemical activity? The present paper is a thorough review of the recent (from 2005 onward) work done in the fields of cavitation-assisted microorganism’s destruction and aims to serve as a foundation to build on in the next years.

How to Get Rid of Black Algae

A Few Things That Work

We have found Black Algae in some pretty interesting places. Here, Ashley takes us through several different methods that work on eliminating it.

2018 was a pretty exciting year for us as we dove deeper into swimming pool black “algae”. Not only did we confirm that black algae was actually cyanobacteria, we were able to break it down even further identifying the constituents (yes, there was more than one) of the colonies by genus.

the Black Algae myth ?

In doing this, we had found that we were the first known folks to research swimming pool black algae in over thirty-five years. The last known research document Laboratory Comparison of the Effectiveness of Several Algicides on Isolated Swimming Pool Algae written by R. P. ADAMSON AND M. R. SOMMERFELD of the Department of Botany and Microbiology, Arizona State University, Tempe, Arizona 85281 appeared in the APPLIED AND ENVIRONMENTAL MICROBIOLOGY, in February of 1980. I hadn’t even made it into High School yet when this publication came out ? and I’m old!!! Interesting to see ? that no one else has really even looked at this stuff since I was thirteen.

So, next we decided to see if we could investigate some methods in How to Rid a Swimming Pool of black “algae”. We were actually able to locate a 230,000 gallon test pool with over fourteen years of black algae growth for this experiment.

Black Algae Copper Experiment ?

By now, I was getting a flood of phone calls from my industry peers asking if, in my research, I had found black algae to be harmful. The truth is at this point I hadn’t. Honestly, my research had not taken me this far yet. I felt that I had opened the door to this and that I had an obligation to take my investigation to the next level.

For this phase, we located a public swimming pool with black algae colonization in Jacksonville (Florida), one hundred miles from where we had taken our initial samples.

All “Black Algae” Are NOT Created Equal ⚖

Unfortunately, we did not get the results we were looking for and would be unable to test to see if the black algae (cyanobacteria) in this swimming pool was toxin producing. However, we did find that the cyanobacteria in the samples we had taken from this pool were an altogether different genus from the three we had identified in the first. The black alga in this pool was also identified as cyanobacteria, withal a new genus not present in the first and at only one hundred miles apart. We were able to determine that geographic location would play a factor in the constituents of black algae.

We could not let things go there. Determined to find an answer, we located a third swimming pool with black algae growth. This one was within a mile of the pool where we had taken our initial samples.

☣ Does “Black Algae” Produce Toxins?

This time we would get the results we were looking for! One of the genus discovered in this third pool was known to be toxin producing, so we would be able to roll forward with our tests for the presence of cyanotoxins. The big question is, in the pool soup of chemicals we use to balance water, would we even be able to detect toxins if they were there? By neutralizing the chlorine in the sample and lysing the photosynthetic prokaryotes, we were able to detect a trace amount of the toxin Microcystin; though in an amount below the acceptable maximum set in the EPA Drinking Water Health Advisory.

 

Still, this was a relatively large chlorinated swimming pool that literally had two to three quarter sized dots of black algae. This begs the question of what would be found in a poorly maintained swimming pool with massive colonization… We will keep you posted as our research continues into 2019…

 


☣ Does “Black Algae” Produce Toxins?

A friend once told me that if they were ever to witness my getting into a public pool to collect…

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Black Algae Toxicity ⚗️

MICROCYSTIN RESULTS Due to the level of chlorine in our sample, testing the pool water for microcystin would be futile.…

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All “Black Algae” Are NOT Created Equal ⚖

And The Biofilm Thickens If you have been following along you already know that we have been doing some things with…

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Black Algae Copper Experiment

a less than lethal dose at an extended contact time⏱ To determine whether a less than lethal dose of copper…

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