Bubbles in Strainer Basket - Cavitation?

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syntax53

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Apr 12, 2016
95
Philadelphia, PA
Pool Size
28000
Surface
Plaster
Chlorine
Liquid Chlorine
I spent at least an hour reviewing threads on this forum and reading everything from "cavitation cannot be seen in the strainer" to "cavitation can occur with too much velocity / turning just before the inlet" and also describing cavitation as "champagne bubbles" which I believe is what I see. I believe it may be that. I also here strange "cracking noises" or more accurately, "knocking noises" but they are few and far between and haven't been able to locate where it is coming from. I tried the "run the hose" method over the valves, joints, and pump basket to no avail. But I'm not ruling out a suction side air leak...

I just finished re-doing my entire filter / pump setup and all of the plumbing associated with it. I even tore apart and re-did a portion of it to get as much straight pipe into the inlet of the pump as I could as I previously had a 90 right into it. No water leaks! :)

I am not getting any air out of my return jets. I also don't appear to be getting any build up at the top of my filter. I had this same problem with my last setup when I had everything full-open, valve wise. I found that if I closed the flow a bit coming in from the main drain that it would get rid of the bubbles in the strainer. This is the reason I am thinking it's not an air leak. Unless of course the leak is so small that it just requires a certain amount of pressure to start penetrating. With the new setup if I try closing the flow from either the skimmers or the drain, the problem gets WAY worse. Like, insanely worse. Leading me to believe maybe the pump is too strong. Yet, the flow into the skimmers is good and the GPM I am seeing is what the filter wants as well.

All of of me new equipment is in my signature. My old equipment was also a 1-1/2 HP pump (century ST1152), but with almost the same service factor HP (2.20 vs 2.25 with the new one). I wanted to up the HP a tad as I had always thought my flow rate was a little low. Took a long time to prime, skimmers seem to have sub-par suction, etc. I actually would reduce the flow from the main drain in order to increase it in the skimmers to make them more effective. However, it was/is obvious that a larger pump was overkill.

I also upped the plumbing between the original intake lines and original return lines (1.5") to 2" piping. So I have two 1.5" pipes feeding into a jandy valve exiting into a 2" pipe. Visa versa for the return lines. I'm seeing 70-90 GPM flow rate. It started out pushing 90, but then I reduced flow to one of my return lines which brought it down to about 85. Unfortunately I have one 1.5" return line that feeds two return jets on one side of the pool and then a single 1.5" line feeding another return on the other side of the pool. This was originally used as a vacuum port for a Polaris cleaning robot. I have a jandy valve there so I can even out the flow between the 3 lines. However, this does mean that my discharge is slightly smaller than my intake (two full-open 1.5" lines vs one full open and one half open 1.5" on the discharge). Could that be the problem? If so, I have no solution other than to reduce the intake which makes the problem worse.

Anyways, the GPM is now down to 70-75 after running the pump for 12 hours and getting dirty (going from 18 to now 23 psi). The pump has worked awesome. Took a very cloud and dirty pool to crystal clear water in 12 hours. Really, other than the bubbles in my pump, everything is working great.

So I think I gave enough back story and detail. Here is a video I took of what's going on in the pump. I took it in slow motion mode on the iPhone (at night last night) so the beginning and end of the video show real-time speed: File_000.mov - Google Drive

Here is a picture of the new setup:
IMG_0514.JPG - Google Drive

And here is a picture of the old setup:
IMG_0515.JPG - Google Drive

Long story short -- should I be concerned about this? Are these bubbles enough to try and go crazy finding their source? Leave well enough alone?
 
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You cannot see cavitation in the pump basket because it occurs in the impeller vanes. Once the cavitation bubbles form, they move along with the water flow towards the outer part of the impeller where pressure rises and the vapor bubbles implode. So you will never see cavitation in a pump's basket. Also, if cavitation does occur, you will hear it because it sounds like the pump is pumping gravel. But yours does not sound like that.

If there is air in the pump basket, either there is an air leak or your pump is just not fully priming. If there is no air in the filter, then it is most likely the pump is not fully priming (i.e. pushing out the remaining air to the returns).

But why did you get such a large pump? Do you have a spa?
 
mas985 said:
But why did you get such a large pump? Do you have a spa?
Click to expand...
No, no spa. I was afraid to get something smaller than what I already had because it felt underpowered before (what I mentioned with the poor skimmer performance, etc). My system has a ton of feet to head. It's a very far distance to the filter pad, and it's like 3-4 feet above the water line.
 
I have two skimmers and use a 1/2 HP pump on low speed most of the time. Takes longer to clear the pool on low speed but that's ok since the energy use is 1/4th as much.
 
mas985 said:
A small bubble like that is not an issue unless it continues to grow. Does it?
Click to expand...

No, it's not getting bigger. But what I can't tell is whether or not bubbles just steadily keep coming and going, keeping the size the same. I have been consistently checking the air release on the filter and there hasn't been any. Or at least not more than a fraction of a second worth.
 
Do you still think my pump is oversized, given the GPM I'm seeing? According to the "head in ft / gpm" chart in the manual I'm looking at 65-75 feet of head at 80 GPM. Though, they don't talk about pipe size in that chart so I'm not sure how that factors in.
 
Any pump larger than you need is over sized so yes, it is over sized but only from an energy use perspective. The filter should be fine. You will just be paying more than you need to for energy.
 

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What do you think it means when I get ridiculous amounts of air and/or sloshing in the basket (haven't left it in that position long enough to tell what's going on because it's so loud and scary, haha) when I cut off the path from either the skimmers or main drain? I have been seriously considering buying the 1-hp version of my pump (Pentair 011513). Going by the manual's flow rate chart, it would reduce my GPM from ~80 to ~60 which would still turn over my pool in 8 hours. Figuring I would keep the seal plate / basket housing part in tact, swap the motor/impeller parts, and then just try to resell it as "like new - used 3 days".
 
syntax53 said:
What do you think it means when I get ridiculous amounts of air and/or sloshing in the basket (haven't left it in that position long enough to tell what's going on because it's so loud and scary, haha) when I cut off the path from either the skimmers or main drain?
Click to expand...
Here are two more videos. This one is with the main drain off and only water coming from the skimmer lines (single 1.5" pipe which gets tee'd off somewhere to two 1.5" lines): IMG_6765.MOV - Google Drive ...a lot more air in the basket there, but still nothing crazy, and still almost seeming like it's just whatever is already in there floating around in the grids of the lid or just not fully priming. I checked the skimmers while the main drain was off and they weren't sucking air at all.

And here is one with the skimmers turned off and only the main drain (single 1.5" line): IMG_6768.MOV - Google Drive ...now this is a horse of a different color. Crazy amounts of air and whatnot going on in the basket.

Is all this being caused by restriction or is it definitely an air leak somewhere under my deck / in the ground?

syntax53 said:
I have been seriously considering buying the 1-hp version of my pump (Pentair 011513). Going by the manual's flow rate chart, it would reduce my GPM from ~80 to ~60 which would still turn over my pool in 8 hours. Figuring I would keep the seal plate / basket housing part in tact, swap the motor/impeller parts, and then just try to resell it as "like new - used 3 days".
Click to expand...
Still debating this as well.
 
So I just got done replacing my 1.5 HP single speed pump (2.2 total HP) with a 1 HP (1.65 total) dual speed pump. Oddly enough, the GPM is about the same and on the high speed, I'm seeing the same bubble scenario as before. I cleaned an re-lubed the basket o ring as well to rule that out.

However, after running it on low speed all night I had significantly more bubbles in the basket this morning. Leads me to believe there is obviously a suction side leak now, but one that is so small that on high speed the bubbles just get sucked right through without being noticeable anywhere but the basket. And now on low speed there isn't enough force to pull them through.

Im still screwed though because I believe the leak is in an inaccessible place, either under my deck or in the ground. And therefor I assume not worth fixing at this point in time. However, it also means I will not be able to properly close the pool as I assume this leak is at least in the main drain line, if not both. That's going to be my next step-- determine which lines.
 
Have you disassembled the valve that is right before the pump? It has o-rings that can be replaced.

And going down only 1 size in a pump does not change flow rate much but it does make a big difference in energy use.
 
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