Skimmer Suction Air Leak - Hypothetical?

[JamesW]

Got me to wondering if it's able to sustain the pull of about 40' through approximately 10 elbows.

On low, the flow won't be high enough to be a problem.

On high, it depends on the flow rate whether it gets cavitation, but I doubt that cavitation is the issue.

 

[mas985]

Looks like an air leak to me. You don't get air in the pump basket without an air leak somewhere. But a pressure test will not always show an air leak if it is the pump basket lid or other o-rings/seals. The reason being is that reciprocity does not apply to seals and air leaks so they may leak under suction but not under pressure.

Also, did someone mention CFD? I have been doing a little work in that area:

Pump Basket 90 Location - Google Drive

drive.google.com

This simulation shows the impact of having a 90 close to or far from the pump basket.

 

[JamesW]

This simulation shows the impact of having a 90 close to or far from the pump basket.

Can you explain the effect of the 90 at different velocities and different flow rates?

You are going to get a chaotic flow in the basket in any case.

The basket affects the flow characteristics.

In my opinion, the computation is going to be too complex to model effectively.

 

[JamesW]

I'm not sure how accurate the models actually are.

CFD is incredibly complex and most models oversimplify to the point of being mostly useless.

The water going to the impeller will be drawn from the space in front of the impeller in all directions.

The flow into the basket will go into the basket in all directions.

In any case, I am not aware of any case where a 90 immediately before the pump had any significant effect.

Maybe at high velocity, it can make some difference, but I doubt that it is going to cause cavitation or drop the NPSHa below the NPSHr at the impeller.

As long as the velocity is below 6 feet per second, you should not have a problem.

If you have a sweep 90, you can probably go up to 8 feet per second without a problem.

Over 8 feet per second might cause a problem.

 

[mas985]

Can you explain the effect of the 90 at different velocities and different flow rates?

Doesn't really change the animation, only U (Velocity) scale as long as flow rates are above the turbulence threshold (non-laminar).

You are going to get a chaotic flow in the basket in any case.

Yes, except for very low flow rates in laminar region.

The basket affects the flow characteristics.

Perhaps, but mostly near the outlet. A mesh could be added in that location.

In my opinion, the computation is going to be too complex to model effectively.

Depends on what you mean by effectively. If it is directionally correct, then it gives a good illustration as to what is going on in the pump basket. Simulations don't always need to give 6 9's in accuracy to be helpful.

The point of these simulations was to show that the velocity gradients are not all that different between the various locations of the 90. Adding the pump basket mesh is only going to add to the diffusion which will reduce the velocity gradients and lessen any differences.

 

[mas985]

I'm not sure how accurate the models actually are.

CFD is incredibly complex and most models oversimplify to the point of being mostly useless.

I disagree. Again results do not need to be 100% accurate to be helpful.

The water going to the impeller will be drawn from the space in front of the impeller in all directions.

The simulation captures that. However, the stills and movie do not fully show the flow from all of the directions but they are there. There is very little "still water".

The flow into the basket will go into the basket in all directions.

The simulation captures that as well.

In any case, I am not aware of any case where a 90 immediately before the pump had any significant effect.

Maybe at high velocity, it can make some difference, but I doubt that it is going to cause cavitation or drop the NPSHa below the NPSHr at the impeller.

As long as the velocity is below 6 feet per second, you should not have a problem.

If you have a sweep 90, you can probably go up to 8 feet per second without a problem.

Over 8 feet per second might cause a problem.

As long as pressure in the impeller is above the water vapor pressure, there should not be an issue under any velocity.

 

[JamesW]

As long as pressure in the impeller is above the water vapor pressure, there should not be an issue under any velocity.

You will get some dissolved gasses coming out before the pressure gets to the vapor pressure.

That will affect flow some, but I do not know how much.

It depends on the amount of dissolved gas, water temperature, turbulence, pressure etc.

 

[DavidLast]

Yes, it is low speed. For 10 years I had my Waterway Supreme. Always on low. It maintained a 98% full pot up until this season where it started to drop a little. I did some suction line inspection/repairs (pressure tested) as of late and it helped, but then I decided to install the new Power Flo Matrix I had laying around. New pump, valves, etc. I can post motor labels later, but the Matrix seems to be a slightly lower rated pump than the Waterway. Got me to wondering if it's able to sustain the pull of about 40' through approximately 10 elbows.

FYI - I took that 10 yr old Waterway last night and rigged it up next to the pool on the decking with a short in & out PVC line directly to/from the pool. Pump pot is 100% airtight and water full with not a spec of air in it. But to be fair, it's right next to the pool and no filter resistance.

Different pumps will act differently, even if the pump curves are identical.

My understanding is pool pumps are tested to a 2.4 m (8 feet) lift. This means they are placed in an elevated position (or equivalent), primed and tested. When I worked in this area a few years ago (probably 30) we used a mezzanine floor and a pond for testing.

However, as a rule of thumb, the vacuum gauge reading is added to the discharge gauge pressure to plot the pump performance. It may have changed, but this is how it was done at the place I worked.

I think it likely the new pump doesnt have as good an ability to create a vacuum as the previous pump, consequently the hair and lint is showing more air.

I'm experiencing some unusual air and in the past few months I have literally replaced just about everything (most of the line, pump, valves, etc) but the skimmer which I know has shifted (tilted) and has some stress cracks. My dye tests in static water we not conclusive. I've done some epoxy & silicone touch-up work inside the skimmer, but I am thinking I may have missed a spot. There wasn't anything that ever leaked water that I could tell. Nothing I could find when I had the skimmer exposed during my suction line repair in August, but I'm suspicious of that lower area in the skimmer. Since a pressure test only covers the line itself (skimmer hole to the pad), it got me to thinking about the lower half well area of the skimmer.

However, this post suggests the skimmer may have cracks etc in it from movement. It is possible under 'suction' the skimmer is opening up a little and sucking air. Never seen it, but could happen.

I think it would be worth doing an evaporation test in the skimmer.

Drop pool level just below skimmer weir.
Remove the basket.
Block the pipe(s) from the skimmer with an expanding test bung.
Fill the skimmer to a water level below the pool (dont want to measure entire pool loss). Mark it.
Float a plastic cup or similar two thirds full with water in the skimmer. Mark its water level.
Wait 24 hours.
Measure the difference in levels in the plastic cup and the skimmer. If they are the same then no leaks from skimmer itself.


It is important to use a plastic cup not polystyrene, it is important the cup floats in the skimmer it needs to be at same temperature as the water in the skimmer.

HTH

 

[JamesW]

The reason being is that reciprocity does not apply to seals and air leaks so they may leak under suction but not under pressure.

Get a vacuum pump and a vacuum gauge and do a whole system vacuum test.

 

[JamesW]

 

[mas985]

We already know there is an air leak due to the volume of air in the pump basket unless that is a weeks worth of outgassing. But given there is an air leak for both pumps, it is most likely in the plumbing somewhere.

@Texas Splash, if you run the pump on high until all the air is gone, and then switch to low speed, how long does it take to get the amount of air shown in the video?

 

[bdavis466]

Not sure this link will work. If not, I'll try another one when I get home.

New video by V F

photos.app.goo.gl

I've seen pumps do that with dirty filters or obstructed returns. It seems like the return restriction would help the pump fully prime but that isn't always the case. Sometimes you need the flow to clear the pump lid air.

Even if the skimmer wall is cracked, I don't see how that could draw air into the plumbing. There really isn't any vacuum once the pipe terminates in the skimmer, it's atmospheric pressure pushing the water into the pipe.

 

[Texas Splash]

how long does it take to get the amount of air shown in the video?

With this Power Flo Matrix, about an hour. The Waterway Supreme water level didn't drop this much and it too longer. About 2-3 hours. I left the waterway running all night and day today, albeit with no filter and at teh edge of the pool. But the pot remained airtight with not a lick of air in it. Tomorrow I'll try to post some more pump info, details about my plumbing, and perhaps try to do another pressure test before it gets dark. Last pressure test I did was in late Aug/early Sept and the suction line held at about 17 psi for an hour.

 

[Texas Splash]

I've seen pumps do that with dirty filters or obstructed returns. It seems like the return restriction would help the pump fully prime but that isn't always the case. Sometimes you need the flow to clear the pump lid air.

I agree. I have experienced that a bit before with my other pump, but my gut tells me this is something different.

Even if the skimmer wall is cracked, I don't see how that could draw air into the plumbing. There really isn't any vacuum once the pipe terminates in the skimmer, it's atmospheric pressure pushing the water into the pipe.

Appreciate that observation. I started looking outside the box for more answers as this situation got more involved.

 

[JoyfulNoise]

So I have a 1.5HP Pentair WhisperFlo single speed pump on my waterfall feature. It pulls water in from dual wall drains and then returns water to the waterfall pipe and a wall return pipe. The waterfall pipe is 2” as is the return line to the wall but then that transitions to a standard 1-1/2” threaded wall fitting. There’s a diverter valve in the return path that splits the water between wall return and waterfall. When that pump runs the wet end is always 100% full with water all the way up to the clear plastic lid. No bubbles, no air, nada. The pump never loses prime either, there is always water in it all the way up to the lid. There is no filter at all on this feature. Just a pump.

My main pool pump is a OG Pentair IntelliFlo (2013 model). It obviously has a more complicated plumbing setup - spa/pool on the suction side with split drain and addition suction line on the pool side. 2-1/2” plumbing. On the return path is all the other equipment - check valve, filter, heater, SWG. Then the split return path to the pool/spa with an additional wall return in the spa for overflow/makeup. When the main pump runs at high speeds (cleaning, startup priming, high speed for spa, etc) the strainer pot will be full of water with maybe the tiniest of bubble floating at the top of the clear lid. When the pump runs on very low speed (basic skimming function), the water in the pot will go down slightly over time and there will be maybe 2 cups worth of air up at the lid. The pump never loses prime when it shuts off, water always remains.

The wet ends of these two pumps are identical - the IntelliFlo is just the WhisperFlo wet end with a VS motor on it.

This has always been the condition of this system since day 1 of life. I can get rid of the air bubbles in the IF when it is on high speeds but they will always come back at low speed. Nothing I’ve done has ever made a difference. I don’t believe I have an air leak but I’ve never gone to the lengths that Pat has. It seems these VSPs just develop air in the lines. Perhaps it’s just something you should expect at low pump speeds.

 

[DavidLast]

When the pump runs on very low speed (basic skimming function), the water in the pot will go down slightly over time and there will be maybe 2 cups worth of air up at the lid. The pump never loses prime when it shuts off, water always remains.

I actually thought this is the case with OP, was just being a belts and braces. Seen some very odd things in 30+years.

 

[mas985]

There are only two ways that I know of where air can accumulate in a pump basket while the pump is running. A suction side air leak or through outgassing. The later takes much longer than the former but even if it takes a very long time for the air to accumulate, it could still be an air leak, just very slow. It is very difficult to know for sure which you may be dealing with. Given that for the OPs case, the air accumulates so quickly, the pumps are small so at low speed the pressure in the pump basket is not likely to be that much lower than atmospheric, I am more inclined to believe that it is an air leak as outgassing should be extremely slow.

 

[Texas Splash]

Thanks very much guys for the feedback. While I have conducted several tests on my suction side since August, I plan on doing more inspections and tests this weekend. I'll post back a summary of everything once completed with those results and any specific details related to the pumps, plumbing, etc, to give a full pictures of my scenario.

 

[wireform]

So I have a 1.5HP Pentair WhisperFlo single speed pump on my waterfall feature. It pulls water in from dual wall drains and then returns water to the waterfall pipe and a wall return pipe. The waterfall pipe is 2” as is the return line to the wall but then that transitions to a standard 1-1/2” threaded wall fitting. There’s a diverter valve in the return path that splits the water between wall return and waterfall. When that pump runs the wet end is always 100% full with water all the way up to the clear plastic lid. No bubbles, no air, nada. The pump never loses prime either, there is always water in it all the way up to the lid. There is no filter at all on this feature. Just a pump.

My main pool pump is a OG Pentair IntelliFlo (2013 model). It obviously has a more complicated plumbing setup - spa/pool on the suction side with split drain and addition suction line on the pool side. 2-1/2” plumbing. On the return path is all the other equipment - check valve, filter, heater, SWG. Then the split return path to the pool/spa with an additional wall return in the spa for overflow/makeup. When the main pump runs at high speeds (cleaning, startup priming, high speed for spa, etc) the strainer pot will be full of water with maybe the tiniest of bubble floating at the top of the clear lid. When the pump runs on very low speed (basic skimming function), the water in the pot will go down slightly over time and there will be maybe 2 cups worth of air up at the lid. The pump never loses prime when it shuts off, water always remains.

The wet ends of these two pumps are identical - the IntelliFlo is just the WhisperFlo wet end with a VS motor on it.

This has always been the condition of this system since day 1 of life. I can get rid of the air bubbles in the IF when it is on high speeds but they will always come back at low speed. Nothing I’ve done has ever made a difference. I don’t believe I have an air leak but I’ve never gone to the lengths that Pat has. It seems these VSPs just develop air in the lines. Perhaps it’s just something you should expect at low pump speeds.

@JoyfulNoise One last thing to try buy I'd pass on it in a heart beat is to swap the two pumps and see if the problem persists with the same pump at the new location or does the problem stay with the location where it initially began. I know it's confusing but can't writ it out any simpler.

 

[Jimrahbe]

This has always been the condition of this system since day 1 of life. I can get rid of the air bubbles in the IF when it is on high speeds but they will always come back at low speed. Nothing I’ve done has ever made a difference.

This is exactly how my old IntelliFlo works...

I believe that air is actually coming in around the lid O-Ring.. I even bought a new lid assembly, including the clear lid, the locking ring that holds the lid, and a new O-Ring. Used plenty of pool lube.. It made zero difference.

On both the old and new lids, if I closely watch the water in the basket, I can see very, very small bubbles randomly coming up from the basket area and collecting under the lid. I estimate that I see one bubble every minute or so. It takes a long time, but the bubble under the lid gets slightly bigger..

I "assume" that when the pump is running at a low RPM, the suction that pulls the lid down is not as great as it would be at faster RPMs, so this is allowing a small amount of air to get into the basket area.

I also notice that as my filter gets dirty, the bubble under the lid gets a little bigger. Not much, but enough to notice.

I consider this normal operation and just ramp my pump speed up once a day to keep the air under the lid in-check..

I guess I could use epoxy and glue the lid in place, to check my theory, but suspect that might be an issue the next time I tried to clean the pump basket.

Thanks,

Jim R.