Pool pump cavitation under load

Nashan1313 · Postby **Nashan1313** » Thu 16 Oct, 2014 02:56

Hi guys,

I have just bought a house with a concrete in ground pool. It is a large pool at roughly 13x8 meters. The pump has been running with no problems since we moved in 2 weeks ago.

A couple of days ago I got given a new creepy crawly. It's a zodiac t3 and hardly been used. When I plug it in the pump starts to moan and the filter with the clear lid attached to the pump starts to fill with air and cavitates and eventually the pump free spins. I make sure there is no air in the cc hose when it's put into the pool. I have tried the neighbours cc too and same story.

I thought maybe the pump was drawing too much so I drilled some holes in the skimmer fitting but as the air slowed, the cc stopped all together. With the flow monitor only reading 1.5.

Today I tried the cc attached with the filter set to rinse and this seemed to work fine.

I have come to a brick wall so some help would be much appreciated.

Cheers

Nathan

Postby **mas985** » Fri 17 Oct, 2014 09:23

It sounds like the cleaner line or head may be blocked which is causing air to leak into the pump basket.

What is the filter pressure with and without the cleaner?

Nashan1313 · Postby **Nashan1313** » Fri 17 Oct, 2014 22:22

Thanks for the reply. I have looked at both of them and can't see any blockages I have also tried another hose and still no joy.

With the creepy crawly disconnected I get max suction (ie 8+) on the suction gauge.

My next step will be to remove the pump and inspect the impeller. Any other ideas would be great. Surely someone has had a similar problem.

Cheers

Postby **mas985** » Sat 18 Oct, 2014 14:05

mas985 wrote:What is the filter pressure with and without the cleaner?

Nashan1313 · Postby **Nashan1313** » Wed 22 Oct, 2014 03:24

Hi, sorry i misunderstood the question.

With the pump off its at 100 kpa

Pump on and nothing connected 165kpa

Pump and CC connected 145 kpa

Pump on, Hose connected but no CC head 155 kpa

Any help? And sorry for the kpa, old school sand filter :lol:

Cheers

Denniswiseman · Postby **Denniswiseman** » Wed 22 Oct, 2014 04:48

Try silicon grease or vaseline on the pump basket lid "O" ring
It works OK when just filtering but the extra resistance throught the CC and hose can cause air to be sucked in
I have seen this numerous times

Failng that try and find where the air is being sucked in from (it will be on the suction side of the pump)

Nashan1313 · Postby **Nashan1313** » Wed 22 Oct, 2014 16:03

Thanks Denis

I took a took at the oring a few days ago, it looked ok but I siliconed it anyway. Also the inlet oring. Neither looked damaged or flat. Even my pool guy is stumped with this one!

Denniswiseman · Postby **Denniswiseman** » Wed 22 Oct, 2014 16:51

It will definately be sucking in on the suction side of the pump when under load
Try soapy bubbly mixture all around the basket, lid, body and fittings to see if it gets sucked in

symmetrical · Postby **symmetrical** » Fri 11 May, 2018 03:09

I have a similar problem with cavitation occurring in the pool pump's basket due to the pool cleaner and its flexible pool hose. Flexible, ribbed pool hoses cause a high friction to water flow, it's 2.5 x smooth pipe friction. My pool has 11m flexible pool hose + pool cleaner. When reduced to 2m flexible pool hose + pool cleaner OR no hose/ cleaner (as experiments), cavitation stops - this test proves there's no air leaks in suction. This correlates to your pool cleaner/ no pool cleaner scenarios.

Other causes: clogged skimmer filter or pump filter or sand/ cartridge filter; suction pipe is too small or too long, pump too powerful; pump basket lid is loose or O-ring defective; any pipe unions are loose or O-ring defective, check flexible pool hoses are not leaking (lift hose out of water and hear suction).

Solutions: check above causes, reduce no. flexible pool hoses (do a test), use variable speed pump, adjust pool cleaner's suction bypass to maximum (in skimmer).

Cavitation is caused by pump trying to suck lots of water from pool, but pipe's total friction (called equivalent head) is too high thus restricting water flow. This causes a low pressure in basket and steam bubbles form because water boils at ambient temp. and it sounds like gravel rattling. There many educational articles on internet.

Denniswiseman · Postby **Denniswiseman** » Fri 11 May, 2018 04:43

[quote="symmetrical"When reduced to 2m flexible pool hose + pool cleaner OR no hose/ cleaner (as experiments), cavitation stops - this test proves there's no air leaks in suction. This correlates to your pool cleaner/ no pool cleaner scenarios. [/quote]
That's not quiet correct. Shortening the hose will reduce the resistance in the system and not suck in air
The full length of hose creates a lot more resistance allowing air to be sucked in through a weak spot in the suction side