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Discussion Starter #1
I have 2 separate 1200 gal poly round cistern tanks that I would like to connect together with a 3 or 4 inch pvc pipe.

There are bulkhead fittings at the bottom of the tanks that water goes out of, and there is a manway at the top of each tank that water goes into, but they fill at different rates and I would like to equate the levels with the aforementioned 3 or 4 inch pipe say 3/4 of the way up each tank tying the 2 tanks together.

Since the tanks are round I don't think a bulkhead fitting would be good to use. I searched and heard pretty good things about "Uniseal's" that are supposed to work well on curved surfaces.

Has anyone ever used a Uniseal type fitting and did you like it?

In addition to tying the 2 tanks together I would possibly like to install a riser in the same pipe and make an overflow prevention pipe that goes up and then turns 90 degrees and goes outside.

These cisterns are for drinking water and are located in a room slightly below grade.

Any help or advice is appreciated!
 

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Discussion Starter #2
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How big is the pipe that fills each tank? I would think that the pipe that equalizes them would not have to be any bigger than the biggest fill pipe. It would be nice if you could do something easy with flexible tubing. Maybe even a piece of high quality garden hose. Or a washing machine hookup hose. Those are tough.
 

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The curve of the tank would imply to me that welding it is easiest method.i have not witnessed 3-4 inch being done,but I saw 2 inch get done on mine.they had bunches of sizes,12 being the biggest I saw.what the mfg did was put my two inch in a 10 amp router ,spun to 20k,pushed against the side of my tank and 3 seconds later it was attached.i asked if that is how the bigger stuff gets done,he said yes.predrill a slight smaller hole and he had a custom adapter for the dia to fit in a 1/2 collet of the ordinary wood router. standing close,to watch, wearing sandals,shorts and no shirt for some reason didn't seem like that bad of a idea,it was
 

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Discussion Starter #5
They are 3 inch supply lines. The tanks do equate over time as they are "connected" at the bottom. Its just as they are being rapidly filled they don't equate and 1 always fills faster than the other (and it varies as to which one fills first, its very weird).

I have some space in each corner of the room, maybe I can put an overflow line from near the top of each tank and feed another smaller tank (1 for each current cistern) into those tanks instead of just dumping the water outside. The amount they don't equate is maybe 150 to 200 gals at most, so if I had 2 overflow tanks of 200 gals each that would probably work. By then the tanker truck will probably be empty (which is what I want, take all the water from the truck and have the truck go empty before the tanks overflow. The reason for that is because the tanker truck will be about 150 to 200 ft away and I just want the driver to fill and forget about it and not walk into the cistern room and then have to run back to the truck to stop the flow from the truck before they overflow).

So between the 2 current tanks, the 2 I add, the amount of water in say 200 ft of 3 inch line, that may drain the truck before any overflow occurs?
 

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OK, my 2-cents for what it's worth.

Is the run to each tank being fed by the tee's branch level, and the same length? It looks level in the photo; but is it really? Are your tank vents sized equally and unblocked?

Instead of a level supply pipe I'd use 45-degree fittings to create a sloped supply to each tank. If 22.5-degree fittings are available, consider those too.

I'd avoid cutting into those expensive tanks at all costs, especially since there aren't flat faces for bulkhead fittings where you want to put them. I've never heard of Uniseal tank fittings; but that doesn't mean they are good or bad in my book.
 

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Rather than adding a new hole to your tank, why not use the existing drain pipe?

Remove the existing supply piping back to where it's reduced. Add a T where it comes out of the tank, add a section of pipe with a valve (if you need to isolate one tank from the other) to a T over to the other tank. Continue from the T with a reduced line to your existing supply. The tanks will reach their own level in pretty short order.
 

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I guess I am confused!! It looks like the two tanks are connected at the bottom by a connection pipe tee'd to each tank. If both tank valves are open at the bottom, the head pressure in each tank should force equalization in each tank if each tank is vented, which i do not see. Is each tank vented with a separate vent?
 

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3 inch spin weld fittings. We use a router
 

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Discussion Starter #10
OK, my 2-cents for what it's worth.

Is the run to each tank being fed by the tee's branch level, and the same length? It looks level in the photo; but is it really? Are your tank vents sized equally and unblocked?

Instead of a level supply pipe I'd use 45-degree fittings to create a sloped supply to each tank. If 22.5-degree fittings are available, consider those too.

I'd avoid cutting into those expensive tanks at all costs, especially since there aren't flat faces for bulkhead fittings where you want to put them. I've never heard of Uniseal tank fittings; but that doesn't mean they are good or bad in my book.
The supply pipe coming in from the wall is sloped then the T and branch lines are perfectly level. There is no vent per se, but the manway has a large hole in it that the supply pipe ends just above, then the water "freeflows" into the hole in the manway. Maybe I will completely remove the manway cover all together (thus making a 16 inch diameter opening) from each cistern to allow air to escape easier.

I agree, I absolutely would prefer to not have to cut into the tanks at all.
 

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Discussion Starter #11
Rather than adding a new hole to your tank, why not use the existing drain pipe?

Remove the existing supply piping back to where it's reduced. Add a T where it comes out of the tank, add a section of pipe with a valve (if you need to isolate one tank from the other) to a T over to the other tank. Continue from the T with a reduced line to your existing supply. The tanks will reach their own level in pretty short order.
I'm not 100% sure I follow what you are saying exactly, but working on the downstream side of the cisterns makes a lot of sense. I could replumb the downstream piping and incorporate a riser that would fill another smaller tank I suppose. Also, if I made the downstream connections a 2 inch line (I believe the bulkhead fitting is 2 inch) it may help with the equating of the tanks (since right now the downstream pipe is 1 inch and although the 2 cisterns are plumbed together I don't think contributes much to the equating initially (but it does after like half an hour).

I just reread your post. Do you basically mean to fill the tanks from the bottom up?
 

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Discussion Starter #12
I guess I am confused!! It looks like the two tanks are connected at the bottom by a connection pipe tee'd to each tank. If both tank valves are open at the bottom, the head pressure in each tank should force equalization in each tank if each tank is vented, which i do not see. Is each tank vented with a separate vent?
You are correct, the 2 tanks are connected together and they do equate over time (like about 30 mins). The venting is basically a hole cut into the manway cover (the hole is where the water goes through to fill the tank. The hole is like 6 inches, piping is 3 inches.) I will try to remove the entire manway cover to allow for a 16 inch opening to improve venting.
 

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Discussion Starter #13
Thanks to everyone for their input so far!

The spin on welding methods a few of you mentioned sure seems very cool but I definitely don't have the guts to do it myself, lol!
 

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I would change the fittings at the bottom from 1 to 2 inch with tees as you suggested. The difference in flow rate for 2 inch pipe is about 3 1/2 times that of 1 inch. So equalization should take more like 8 minutes instead of 30 if the holes in the tops of the tanks are equal to or larger than the cross sectional area of a 2 inch pipe.
Ron
 

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I think, you are trying to equalize the FILLING of two tanks,,
and you are concerned one tank will fill faster than the other when the water comes into the tank.

So far, the discussion has been how to equalize the tanks with a large pipe connecting the tanks.

'how about,,, if there was a way to equalize the filling? :dunno:
The size pipe you are using is used for septic drainage fields.
the flow there must be equalized, so that one run does not "get stuck" taking all the flow.

A simple plug, with an off center hole, is installed. The plug is rotated, until there is equal flow.



If the plugs get you 90% of the way to equal filling,, the small interconnect pipe will handle the remaining inequality.

You can increase the size of the hole if needed to handle greater flow.

In a septic field, the rotating valve system can control as many pipes as needed to get somewhat equal in-flow.
 

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Discussion Starter #16
Sorry if I'm not explaining it quite properly.

My cisterns hold say 2500 gallons. A full tanker truck load will deliver 3000 gallons.

My first problem is to install some type of overflow prevention to capture the excess 500 gallons (either plumb it to the outdoors, or better yet install an additional tank(s) to catch the excess).

The second problem is that since the tanks don't fill at the same rate I need to ensure that the overflow can be handed by the overflow tank(s).

The tanks will equate over time (say 30 minutes), but I would like them equate nearly instantly as they are filling up (which I can maybe improve by opening the manway to allow for better air escape and also by increasing the outlet pipe connecting the 2 tanks from 1 inch to 2 inch (or maybe even 3 inch or whatever)

The water delivery company can't/won't supply me with just 2500 gallons as that would solve everything.

I could just let the current tanks overflow and spill onto the gravel base, but the cistern room is attached to the side of the house and is insulated and not heated and I'm not sure if its cool to have 500 gallons of water rapidly dump onto the ground right beside my house/foundation, especially in winter with freezing temperatures.
 

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The supply pipe coming in from the wall is sloped then the T and branch lines are perfectly level. There is no vent per se, but the manway has a large hole in it that the supply pipe ends just above, then the water "freeflows" into the hole in the manway. Maybe I will completely remove the manway cover all together (thus making a 16 inch diameter opening) from each cistern to allow air to escape easier.

I agree, I absolutely would prefer to not have to cut into the tanks at all.
Slope these 'branch lines' to each tank. In the piping world, the direction without any change of flow direction in a tee is called the 'run' because it's a straight shot through the fitting. You have the tee's branch 'bull heading' into the run, which is best to avoid if possible because it causes turbulence; but it does happen and I've done it when I had no other design options open to me.

As another poster has suggested, increase your tank discharge interconnect line to the maximum size your bulkhead fittings will allow. You can choke down the size to a normal residential pipe size at the tee's branch.

Absolutely put in an overflow collection tank of say 750 gallons to give you some cushion for a potential 500-gallon overflow that may be more than 500-gallons. You can connect the overflow pipes to near the top of your main tanks above the max fill mark. Use a pipe the same size as your current inflow line, or at most, one pipe size less; though the cost won't be a huge savings by going one size lower.

Since the overflow pipes will normally be in air, the chances of them leaking due to an overflow condition are small should an overflow occur as there will be minimal water weight against the fittings connecting the main tanks to the overflow.
 

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Sorry if I'm not explaining it quite properly.

My cisterns hold say 2500 gallons. A full tanker truck load will deliver 3000 gallons.

My first problem is to install some type of overflow prevention to capture the excess 500 gallons (either plumb it to the outdoors, or better yet install an additional tank(s) to catch the excess).

The second problem is that since the tanks don't fill at the same rate I need to ensure that the overflow can be handed by the overflow tank(s).

The tanks will equate over time (say 30 minutes), but I would like them equate nearly instantly as they are filling up (which I can maybe improve by opening the manway to allow for better air escape and also by increasing the outlet pipe connecting the 2 tanks from 1 inch to 2 inch (or maybe even 3 inch or whatever)

The water delivery company can't/won't supply me with just 2500 gallons as that would solve everything.

I could just let the current tanks overflow and spill onto the gravel base, but the cistern room is attached to the side of the house and is insulated and not heated and I'm not sure if its cool to have 500 gallons of water rapidly dump onto the ground right beside my house/foundation, especially in winter with freezing temperatures.
I think you are on the right track if you make the outlet piping larger and vent each tank sufficiently. The water level in each tank will find its own level if the water can free flow between the tanks without restriction. The key is to have a large enough pipe connecting the outlets together and have sufficient venting in each tank.

It will be hard to create equalization of flow into the tank since the water will follow the path of least resistance, which ever direction that is.
 

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You are correct, the 2 tanks are connected together and they do equate over time (like about 30 mins). The venting is basically a hole cut into the manway cover (the hole is where the water goes through to fill the tank. The hole is like 6 inches, piping is 3 inches.) I will try to remove the entire manway cover to allow for a 16 inch opening to improve venting.
I would think a 6" hole with a 3" pipe would provide plenty of venting. I think the issue is, the outlet pipe is allowing the tank levels to equalize at a rate based on the size of the existing outlet pipe. If this pipe were large, the tanks will equalize quicker. :good2:
 

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Discussion Starter #20
Thanks everyone, there is some great advice here!

I think I will slope the inlet piping to reduce the turbulance. Also I will increase the interconnected discharge piping. Hopefully this will result in a more even filling/equating of the tanks.

Also an overflow tank will be installed. I'm waiting to hear back on what the absolute maximum amount of water is that could potentially be delivered, then ensure the overflow tank can handle it.

I'm a couple of months away from doing the work since I obviously need the tanks to be empty when I do the work, but I'll be sure to update everyone with the results.

Thanks again, and if anyone has any more advice in the meantime I would be happy to hear it!
 
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