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Topic # 240615 17-Sep-2018 09:58
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Hi,

 

 

 

We are on tank supply, and a few yearas back we added a 10,000L tank to our system (already have a 20,000L tank).

 

They are connected together with a thick alkathene pipe via their outlets at the bottom of the tanks, and a t-junction goes from this pipe to the pump under the house to supply water for the house. Both tanks have a tap into this pipe, and on the pipe leading to the house off the T-junction, there is a non-return valve.

 

Generally, we draw water from the original tank, and leave the 10,000L tank turned off once full (in case we have a leak, or a tap is left on - it does not use all the water, and we use the water from the 20,000L tank as that is where the rain water goes (guttering only goes to the big tank).

 

Problem is, when we switch over to use the smaller tank, we get air bubbles in our water system. For the most part it works fine, but sometimes when turning on a tap it spits and bursts a bit till it evens out, but you can hear air in the system. It does not do this when drawing water form the main tank, only when the main tank tap is off, and the supplementary tank is on (if both on, it is OK, and the levels of both tanks comes down equally, as it should).

 

Plumbers, or other enlightened people - any idea how this air is getting into the system and how to stop it?

 

The bottoms of both tanks is about the same level, both tanks have access for air at the top.


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  Reply # 2091783 17-Sep-2018 10:07
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When you change over do you turn the 10k tank on before turning the 20k tank off or has the supply line emptied before the smaller tank is on?



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  Reply # 2091796 17-Sep-2018 10:48
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I turn the smaller one on before turning the bigger one off.




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  Reply # 2091802 17-Sep-2018 10:59
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Here is a dirty ole sketch of the tanks:

 

 

 


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  Reply # 2091804 17-Sep-2018 11:00
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What type of fittings have you used on your pump inlet pipework? As some types of fittings leak under negative pressures.

Could also be dissolved air coming out of solution again due to the negative pressure.

And if you only switch the tanks over when the first tank is completely empty, could be air in the branch pipe to that tank being drawn into the supply.







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  Reply # 2091823 17-Sep-2018 11:28
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The fittings at the pump end have not changed.

 

On the tank outlets it is those blue screw on fittings (for Alkathene). Both the same.

 

I do not wait until the other tank is empty, I switched it over just to use the water in the smaller tank (I don't like to let it sit for too long).

 

 

 

I am wondering if it might be the right angle bend (off the pipe between the two tanks, into the NR valve and up to the house) Wondering if it is cavitating there?

 

It is odd that the big tank doesn't do this at all, but the smaller tank does. Both have the same size outlets and taps, the only difference is the length between outlet tap and the pipe up to the house. Somehow when on the smaller tank, air is getting onto the hose between the house and the T-Junction.


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  Reply # 2092032 17-Sep-2018 16:02
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Could be pressure drop over the longer pipe causing something to leak that wouldn't otherwise leak. It might even be the shaft seals on the pump.

Unfortunately vacuum leaks are very difficult to trace.





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  Reply # 2092038 17-Sep-2018 16:08
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Not a plumber, but I'd think if you included a pipe vented to the atmosphere just after where the single pipe starts its run to the house, it would let the air escape. Make the pipe vertical and higher than the top of both tanks


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  Reply # 2092048 17-Sep-2018 16:37
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I had issues with my system (2x 12,000lt concrete tanks in the ground) setup similar to yours
Draw of one tank and other tank has the stormwater input from roof it either overflows or I pump it across to other tank
But the plumber made a booboo, he put on a flapper valve not a non-return valve on the line to house/pump
So at the pump end in house the level would drop in the inlet pipe if pump not used for a wee while
When pump turned on it would suck up air first, caused all sorts of issues with the controller
Once I changed to a proper non return valve all was good, not had an issue in 10 years since

 

Now for it to allow the level to drop it must be able to suck in air on the inlet side (where there is no pressure for it to leak water)
Check the lines are sealed, a suction line can let in air but water can't get out 
You may have an air leak from the smaller tank to the NR Valve


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  Reply # 2092150 17-Sep-2018 20:00
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Possibly you have lower head pressure from 2nd tank and pump is starving and cavitating, producing air. 




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  Reply # 2092325 18-Sep-2018 09:28
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JeremyNzl:

 

Possibly you have lower head pressure from 2nd tank and pump is starving and cavitating, producing air. 

 

 

This could be it, and I did wonder about that, but before we had the second tank, it never did it on the original tank, even when the water level got down to less than a metre in the bottom. The new tank can be full, and it still does it.

 

It isn't a major issue, water still gets through (although, for the first time the other day, the pump did error out and stop because I guess it thought there was no water), and I can equalise the tanks (transfer water from the full 10,000L to the nearly empty 20,000L) if I have to.

 

I think @Aredwood may be onto something with the shaft seals on the pump or pipe diameters. I don't think the pump is leaking (there is no water around the pump anywhere) and it is only about 3-4 years old. If I knew of a way of reducing the amount of water it is trying to suck up the hill (our water pressure is great in the house) I may be able to do some tweaking, but it is more of an annoyance than a critical water supply issue.

 

 


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  Reply # 2092342 18-Sep-2018 09:43
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What's the height difference between the tank low water level and the pump suction?  Is it a centrifugal pump?  Have you checked that there's no restriction between the small tank and the check valve - maybe the valve not opening as you think or an obstruction at the tank connection? Is the tank definitely venting?





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  Reply # 2092358 18-Sep-2018 10:06
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Height difference is negligible. The pipe between the two tanks' outlets runs pretty much level between them.

 

It may be possible that the tank is not venting properly. It is definitely venting, but maybe the airflow through the vent on the tank lid is not sufficient. I will double check that this weekend.


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  Reply # 2092394 18-Sep-2018 10:50
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Not a plumber but done plenty of work with pipes, pumps and valves in aquaculture facilities.

 

My first suspicion would be the NRV.  They can be problematic if they get any sort of growth/obstruction in them.

 

If you want to test the inadequate venting theory (also good a suggestion) I suggest you try doing a change over with the inspection lids on both tanks open temporarily.





Mike



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  Reply # 2092414 18-Sep-2018 11:14
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I was wondering about the NRV, but it behaves perfectly normally when the main tank is supplying the water instead of the supplementary. If it was the NRV, wouldn't it behave the same irrespective of which tank the water was coming from.

 

I am leaning more to the venting. Will remove the inspection cover completely on the weekend and switch it over to see what happens.


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  Reply # 2092433 18-Sep-2018 11:52
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trig42:

 

I was wondering about the NRV, but it behaves perfectly normally when the main tank is supplying the water instead of the supplementary. If it was the NRV, wouldn't it behave the same irrespective of which tank the water was coming from.

 

 

The NRV might be getting sticky and the pressures from the smaller tank may not be enough to keep it open all the time when the pump is "sucking"

 

 


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