Bubbles out of a Negative Edge Return

[Homebuilder]

First post here and appreciate any advice. A quick backgound. I am a 20+ year, large custom home builder, most homes over 5mill. Seem to be at a stale mate with my pool builder on an issue and seeking some feedback.

The Pool:
8' deep foot ball shaped
65 ln ft of neg edge
Approx. 70 feet from pool to equip.
Neg edge trough is approx. 11' lower than pump equipment
Jandy pumps/variable speed

The issue:

There are bubbles comig out of the wall mounted return in the pool. Pool contractor attempted fixes, all pipes were under pressure prior to equipment install. Pool has bee up and going about 2 months.

The bubbles were reduced about 40% by his fixes, but still unacceptable as it is very noticable when pool water is flat (no wind). After thier last fix, the tech tells me that the drop in elevation and size of the pump needed is actually pulling air out of the water it is so strong. I am not quite buying into that theory.

Any suggestions on what we may check?

 

[linen]

Welcome to tfp, Homebuilder

Can you see bubbles forming in the pump basket?

If yes, then it is likely you have a suction side leak that could be exasperated by the vacuum required to raise the water up. I would first check/relube the cover and check reseal the pump basket drain plug. These two places are the most common culprits.

Does the pool have a Saltwater Chlorine generator (swg)?

Swg's can also create small bubbles that can show up at the return.

 

[Homebuilder]

Thanks for the welcome and advice.

I believe he changed the seals during his repairs.

I will check the basket for bubbles. It should be the pump as I had the lines under pressure for 6 months prior and no leaks.

It does have a Salt Water system, but dont know the brand or method. I think a cell.

Is It plausable that the pump is so strong it could pull air out of the water and bubble?

Thanks again for the advice

 

[linen]

Is It plausable that the pump is so strong it could pull air out of the water and bubble?

I doubt it.

The 11 feet of water column is not causing this alone. 11 feet of water is ~ 5 psig which would be ~10 psia. Vapor pressure of water at 20C is 0.33 psia.

What type of pump is on the system?

It does have a Salt Water system, but dont know the brand or method. I think a cell.

If you do not have air in the pump basket, then I think this is your culprit. Sometimes speeding up the flow rate through the swg can reduce the bubble size (caused by the swg) returning to the pool making them less noticeable.

 

[duraleigh]

Is It plausable that the pump is so strong it could pull air out of the water and bubble?

doubtful.

Tell us more about that negative edge.....does all the pool water spillover and then return to the equipment from the negative edge? Got a pic?

 

[RobbieH]

Installing an SWG in my system creates bubbles in my returns. It's part of it, at least in my experience.

 

[mas985]

Is It plausable that the pump is so strong it could pull air out of the water and bubble?

I am going to disagree with the others on this one. It is possible to pull dissolved air out of water and is typically called out gassing or deaeration. It doesn't require much in the way of a vacuum to do so, unlike cavitation which requires pressures lower than the vapor pressure of water. Just put a glass of pool water out in the sun and and you can see the same effect. Bubbles will start forming in the glass with just a slight increase in temperature. Since temperature and pressure are related, the same thing can occur with a slight drop in water pressure.

Reference: http://www.engineeringtoolbox.com/air-s ... d_639.html

However, it doesn't mean this is what is going on in you case. My guess is that it is probably the SWG but how big are the bubbles? Champagne size or are they bigger? If you shut off the SWG, do the bubbles stop?

 

[Homebuilder]

Thanks for all the replies.

I am meeting with the pool contractor, pump rep and plumber tomorrow to get a meeting of the minds.

The pump is a Jandy variable speed, 2hp.

The pool water spills over a 60lf long negative edge down 9 feet to a 4 foot wide by 60 foot long trough about 4 feet deep and returns to the equipment 60 feet away and up hill about 11 feet.

The salt system I will check into tomorrow and see if we can disconnect it and run the pool. if we can dissconnect/reroute it, how long of running the pool should it take to get the air out? and see results if this is it.

The pump rep mentioned trying a 1 hp unit and see if that works. I feel we need the volume the 2hp can push to keep the negative edge flowing. The pool has a 60lf neg. edge and the wall is about 9 feet down to the trough so I want it to be fully wet at all times on the backside to stop unsightly calcium build up and void the pepple sheen warranty.


Thansk again for the great responses, much appreciated.

 

[mas985]

After stopping the SWG, the bubbles should last no more than a minute or two.

Also, the 1 HP should work fine for a neg edge. The only difference will be not as much water going over the edge so the dynamics will be a little different. As long as the edge is level, the entire edge should remain wet.

 

[linen]

I am going to disagree with the others on this one. It is possible to pull dissolved air out of water and is typically called out gassing or deaeration.

I agree that components of air dissolved in water can come out of solution when a vacuum is applied, but does this happen instantly?
And...If it were to happen quickly/instantly, then does this air coming out of solution then coalesce quickly into visible bubbles?
How quickly will a significant volume happen?...quick enough to show up before reaching the impeller? (I suppose the longer the pipe run for a given head it is possible that more air comes out of solution and the longer it has to coalesce.)
In addition, if a significant volume of air were to come out of solution before reaching the impeller, wouldn't some (or maybe most) of the air go back into solution on the other side of the impeller due to the pressure increase?

Theorizing here, it seems like any air that comes out of solution at -5 psig would go at least partially back into solution at the typical pressures seen on the pressure side of the impeller, and any air that doesn't would likely be at a very small bubble size.

Sorry, this part of the discussion probably belongs in the deep end

 

[mas985]

My only point was that it is possible for a pump to pull air out of solution but I also said that it was unlikely to be the cause of the air bubbles in this case. However, I do believe this may be the cause of the accumulating bubble on low speed of a multi-speed pump.

 

[JamesW]

Is there a separate pump just for the vanishing edge?

What size pipe is the suction pipe to the vanishing edge pump?

How many main drains in the trough, and what size are the main drains?

Do the bubbles happen at low speed?

Does the pump hold prime when off?