Pump Cavitation When on Main Drains

[CivengPE]

Last weekend we got into our new pool and while swimming I noticed that I could feel no flow into the main trains when I dove down to them and covered them with my hands. Since our PB installed 4 main drains I didn't think too much about this, but decided to perform some more tests.

First, I took a small piece of grass and placed it next to and on top of all 4 different drains. The grass floated away from the drains in each instance. I then decided to turn off my skimmers and only have the main drains supply the pump. This is what happens when I do that:

[youtube:1apxg838]O_8OPBBLPl4[/youtube:1apxg838]

Each skimmer is connected with its own 2" line to the pump pad and the 4 main drains all share one 2" line back to the pump pad.

The PB has come out and says everything is working normally, but I am not convinced.

Any thoughts?

 

[Prof100]

My new pool does the same thing! I called the pool builder and he(and the folks here) think the pipe is restricted. he is going to come out when the water is warmer and take the covers off and force water thru in reverse.

 

[CivengPE]

I also took a video of the pressure drop when I switch to the main drains:

[youtube:1n7hoayj]JIA16PCXP-8[/youtube:1n7hoayj]

 

[Melt In The Sun]

You are correct, it is not working. Something is blocking the suction from the drains. Possibly stupid question: there isn't another valve, is there?

 

[CivengPE]

You are correct, it is not working. Something is blocking the suction from the drains. Possibly stupid question: there isn't another valve, is there?

Not that I am aware of.

 

[Melt In The Sun]

He's right that you shouldn't be able to feel suction with your hands, but you should be able to watch a piece of grass get sucked down. Can you post a picture of your equipment, where the lines come from the ground to the pump?

 

[CivengPE]

 

[Melt In The Sun]

Yep, looks to me like you should be able to turn that one valve and pull just from the drain. Sounds like your PB has a problem to fix.

What kind of pipe is that? We don't typically see black hard pipe used for pools. The type of plastic should be printed on the fittings...i can see the stamp just to the left of the top jandy valve in your first picture.

 

[CivengPE]

Yep, looks to me like you should be able to turn that one valve and pull just from the drain. Sounds like your PB has a problem to fix.

What kind of pipe is that? We don't typically see black hard pipe used for pools. The type of plastic should be printed on the fittings...i can see the stamp just to the left of the top jandy valve in your first picture.

It's just sch80 PVC painted black by the PB to protect from UV and look cool.

 

[Plankton]

I have the same problem, although I don't have a specific valve to switch from the skimmer to the main drains, when I close off the skimmer diverter valve and have the pump pull from the main drains the exact same thing happens. I inherited the pool with the house and I though it was because of the skimmer restricting the flow with the diverter valve closed.

 

[mas985]

Running a 2 HP pump on just the main drain line alone, especially a small pipe, can be problematic on it's own. I am not saying there isn't a blockage but just that the head loss alone could cause issues with the pump. This can be compounded by a low head loss return side.

CivengPE, is the line 2" all the way to the main drains and how long is the line?

It appears from the picture there may be multiple return paths, is this correct and if so how many?

One thing to try is running just on one of the two skimmers and see if you have a similar issue.

 

[CivengPE]

Running a 2 HP pump on just the main drain line alone, especially a small pipe, can be problematic on it's own. I am not saying there isn't a blockage but just that the head loss alone could cause issues with the pump. This can be compounded by a low head loss return side.

CivengPE, is the line 2" all the way to the main drains and how long is the line?

It appears from the picture there may be multiple return paths, is this correct and if so how many?

One thing to try is running just on one of the two skimmers and see if you have a similar issue.

I believe the line is 2" all the way to the main drain area, it then splits to pick up the 4 individual drains. I will pace it off when I get home, but I imagine that the total length is probably 80' or so.

There are 4 possible return paths. 6 return eyeballs and one for the ozonator which are always on, then one each for the weeping wall and bubbler in the tanning ledge.

I did try the one skimmer experiment last night. It ran perfectly...

The PB is in the pool right now supposedly flushing the main drain line to check for an obstruction.

Shannon

 

[mas985]

Based on the description of your plumbing, it sounds like a setup that would be conducive to high suction head loss with low return head loss. That unbalance can cause problems with pumps, especially large pumps and I would expect the problems you are experiencing. The solution of course is not to run with just the main drain. Or if you do, close all but one of the return paths. That should help balance out the head loss.

 

[CivengPE]

Based on the description of your plumbing, it sounds like a setup that would be conducive to high suction head loss with low return head loss. That unbalance can cause problems with pumps, especially large pumps and I would expect the problems you are experiencing. The solution of course is not to run with just the main drain. Or if you do, close all but one of the return paths. That should help balance out the head loss.

When I ran the test last nigh, the only return paths were the 6 eyeball jets and the ozonator. I guess my next step is to close off the ozonator path and see what happens.

 

[mas985]

The main drain path of 80' plus, I would assume, many fittings has quite of bit of head loss so even a single return path may not be enough to balance the head. If the single path of 6 eyeballs don't work, then try the ozone path alone. Does that have a single eyeball return or some other type of return in the pool?

 

[CivengPE]

The main drain path of 80' plus, I would assume, many fittings has quite of bit of head loss so even a single return path may not be enough to balance the head. If the single path of 6 eyeballs don't work, then try the ozone path alone. Does that have a single eyeball return or some other type of return in the pool?

It is plumbed to a single eyeball return. I will give that a try.

Thanks for all of the advice on this.

 

[CivengPE]

Just to close the loop on this, my PB came out last week and blew compressed air through the main drain lines. Lo and behold, there was debris in the lines.

I am now able to confirm that flow is in fact going through the main drains. I still have a cavitation issue when I switch to main drains only, but now believe that that is indicative of the 2" line coming from the mains.

Thank you all for your advice!