adjusting water flow from tanks

[sunnybob]

heres a puzzle for you lot.
First, the background;
2 water storage tanks, side by side.
Both have ball valves on the inlets.
1 tank is taller than the other.
Both outlets are joined into one pipe to the pump for pressurising the system.
The lower tank has a check valve fitted to stop the taller tank over flowing the lower tank.

Now the puzzle;
When water is drawn off, only the taller tank flows. The lower tank does not flow any water at all.

What needs to done / fitted, to make both tanks flow equally?

 

[CHJ]

If you want both ball valves to charge the tanks then raise the lower tank up until its upper surface level matches the taller tank and remove the check valve.

Or do away with the lower tank ball valve and let the taller tank recharge it.

How are you judging that the lower tank is not contributing/supplying, just because its ball valve does not operate or that its level never goes down?
Unless the lower tank outlet is drawing water quicker than the taller tank check valve link is supplying then surely you will see little movement in the lower tank.

The system as you describe seems to me to be just providing a larger quantity of reserve not an increased flow potentetial.

 

[Sideways]

Water pressure is related to the depth of water (head).
Assuming both outlets are at vaguely the same height, the pressure at the outlet of the taller tank is greater than at the outlet of the smaller tank because of the extra head of water above the smaller tank.
At the point where the outlets are joined, the water can't flow out of the smaller tank into a flow driven by the higher pressure from the larger tank.
As the water is drawn out of the taller tank and it's level falls towards that of the small tank, then the pressures will get closer and water will draw from both. At some point, the level of the two tanks equalise and their levels will drop together until you run them dry.
I don't see a problem in this. It means that the water equivalent to the excess head in the large tank is always used first, but if you want to cycle the water in the smaller tank from time to time, you just need to fit a valve on the outlet of the big tank so that you can isolate it and force the water to draw from the small tank whenever you choose.

The other aspect is that when water starts to flow, there's a drop in pressure due to drag in the pipes etc. So installing wide bore pipes and outlet from the small tank and narrower ones in the large tank would offset some of the pressure difference due to different heads but this would depend on how fast you were trying to draw off water relative to the pipe sizes. No idea whether the effect would be large or small in your particular installation. You could always put a valve on the outlet of each tank and alter them to balance the water flow at whatever your normal demand is.

I assume in all of this that you have enough mains water pressure to fill either tank from it's ball valve but probably not enough flow to meet demad when you are filling a bath / running a power shower / using a large hosepipe.

 

[sunnybob]

I have a situation, bought about by all you tourists :shock: :roll:
We had a drought last winter, the reservoirs are almost empty, and rain isnt even due till october at the earliest.
The government keep all the tourist towns supplied, so the inland villages and towns are now on a water cut of 1 day a week. I expect that to get worse before it gets better.
I had a 1000 litre tank already (the lower one) but that only has a day or two supply at best, so I bought another tank. Unfortunately I couldnt get the same shape again so I bought a 750 litre (the taller one). These are theoretical totals, I actually have about 1400 litres stored now. The two combined will keep us going for 3 or 4 days easily even with guests.
But I dont really want one tank only in use. Ambient temp is 38 plus and i dont want any bacteria problems due to standing water. I want both tanks to be constantly moving water through the system.

I have fitted valves to both, and its not too big a deal to remember to switch them over every couple days, but I'm lazy and would prefer an automatic system provided it wasnt stupid expensive.

Even when we have water, the pressure is rarely above 2 bar, so the entire house uses a pressurised system to give me 3.5 bar.

 

[Lons]

The other 2 guys have explained it Bob and you clearly understand what you have and what you need to do so stop being lazy. :lol:

You can get electrically operated solenoid valves, even a central heating zone valve will work but you would have to weatherproof it if outside and route electrics to it. Only need to fit one to the larger tank to shut it off and be careful what you buy as some are closed if no power and open when power applied.
You would also need either switches to operate manually ( too lazy to do that? :wink: ) or a control timer for automatic operation. I don't think it would be silly money.

Your economy would be in deep doodoo without the tourists btw.

Bob

 

[sunnybob]

nah, according to brussels we're awash with russian mafia money =D>
the hotel chains are foriegn, all offering all inclusive so the local bars are dying out, and all the workers are east european who send money home then claim dole during the winter. (hammer) (hammer)
i had given some thought to a venturi system, but i'm too lazy to do that also :?

Guess I need to do some more thinking in the pool 8) 8) 8) soon be tiffin time.

 

[Just4Fun]

The lower tank has a check valve fitted to stop the taller tank over flowing the lower tank.

Some (all?) check valves work by have a movable flap that, when water attempts to flow backwards, is pushed against a restriction to make a seal. For the water to flow in the desired direction there must be enough pressure to push that flap "forwards" out of the way. If you have little pressure and/or a sticky flap there may not be enough pressure to ensure a flow.

In your situation when both tanks have water in them the pressure from the bigger tank with more head will tend to force water back up the pipe to the small tank, closing the check valve and preventing water flowing out of the smaller tank. If this is the source of the problem water will flow first out of the larger tank and then out of the smaller tank.

 

[sunnybob]

I was expecting the taller tank to take precedence at first, but then I expected the lesser pressure of the water flowing away down the pipe to allow the lower tank to let water through the valve.
I was not expecting the flow from the the tall tank to back up and keep the valve closed even when the other end of the pipe is open into the pump.
Oh well, no big deal #-o #-o

 

[jimmy_s]

With the 2 tanks piped in parallel its not uncommon for one to provide more flow than the other as the flowrates can vary depending on the header arrangement etc. As you are not needing a main/ standby tank arrangement you would be better piping the tanks in series as you get less bother with stagnation. Float valve in the first one with draw off on the bottom to the 2nd tank or if sitting at a higher level it can serve a float valve in the 2nd tank. Draw off to the house at the other end. Thats guidance from the British Standards to prevent stagnation. Most of the time we dont do that here because we have 2 in parallel to allow cleaning/ chlorination of 1 tank which can be taken off line leaving the other to serve the building.

 

[sunnybob]

Hmmm. good idea but wont work in my situation.
The tall tank is a good 2 ft taller than the low one. If I ran mains to the top tank, then from the bottom of that tank to the top inlet of the lower tank, and from the bottom outlet of the lower tank to the pump I would never get the lower half of water out of the tall tank.

 

[finneyb]

Reduce the ball valve height in the bigger tank so that it is at the same level smaller tank.

Brian

 

[sunnybob]

That would reduce the tall tanks capacity to half. defeats the object of the storage tank

 

[Sideways]

Hmmm. good idea but wont work in my situation.
The tall tank is a good 2 ft taller than the low one. If I ran mains to the top tank, then from the bottom of that tank to the top inlet of the lower tank, and from the bottom outlet of the lower tank to the pump I would never get the lower half of water out of the tall tank.

No Bob, it would. The water would follow a path like going around the trap under a sink. It would go in at the top, gradually move down, out of bottom of tall tank into top of small, work its way down and out the bottom to the pump. Nothing would stagnate. Relies on the weight of water in tall tank to push the water from its bottom up to the top of the smaller. Just make the pipes wide enough that flow isn't restricted.

 

[sunnybob]

IN a normal situation, I would agree completely, but these tanks are my emergency supply for when the water mains are cut off.
With no further water entering the top of the tank, once the level in the tall tank equaled the level in the lower tank, the water would back syphon when the pump went off, allowing air into the ball valve on the lower tank and isolating the 300 litres still in the tall tank.
I need both tanks to empty without any replenishment.

 

[Sideways]

Ah ! I see where you're coming from Bob. Completely agree.

 

[Fitzroy]

Without a common level in the two tanks it’ll never* work. Time to raise that short tank up on a platform!

I’d also:
- Install a cross over at a level c. 10cm below the normal level and only have a fresh water feed to one tank.
- Install trim valves on the outlet of each tank and conduct some experiments to tune the valve positions to give similar flow from each.

F.

* it would actually work as configured if the flow rate resulted in a frictional pressure drop between the two tank outlets of greater than the head difference between the two vessels. At .5m delta head in a 25mm inlet pipe with a 1m between the two outlets with a this would need a flow of c. 10m3/hr. Which having run the numbers is not as great as my gut told me it would be!

 

[sunnybob]

Solved it.
Sometimes brains are not required :shock: :shock: (hammer) (hammer)

Turn the bottom valve of the tall tank to half closed, and both tanks are now syphoning.