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I need to make a reserve of water for a fire truck to come and suck from. I have 3 tanks on my site that I plan to pipe together, to make one location to suck from all the tanks. Problem I have is that the tanks are at different elevations, so I will empty the highest tank and then the tank will draw air, and not draw from the lower tanks. Or possible that the upper tank will end up filling and flooding the lower tank.

60 cubic meter upper tank, 72 cubic middle tank that is at the same elevation as the draw point, and 36 cubic lower tank.

Note that I can not use electrical items in the system, as it is a emergency supply water and I need to pretend there is no power.

The photo shows the connection point, which is a existing concrete tank underground where tank #1 is already connected. Tank #1 is the easy tank as it is uphill to the connection point.

Tank #2 to the left is level with the connection point, and tank #3 is down hill from the connection point.

enter image description here

It comes down to, how do I take water out of all the tanks without the fire department sucking air, or running one tank empty?

My connection is both filling and unloading the tanks with water. I expect if I fill tank 1 uphill I can always fill the other tanks from that tank.

One other thing, I agree with all you have said, is there any way to eliminate the valve on each tank, by maybe putting some kind of a gravity or float ball to stop the flow when the water gets to low.

In my cad picture you can see I put a back flow valve to keep water from backing up into the lower tanks, but then when you want to fill all the tanks, can just open the valve.

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    I believe you're wanting to make your own "water tower" since you're in the country (no city water supply), but not have an actual, standing tower. Based on your text description, I'm having a hard time visualizing what you're describing, though. Could you possibly supply a drawing (just a readable sketch, nothing super fancy) to show the layout of tanks, elevation, desired inputs and expected outputs? – FreeMan Nov 23 '20 at 12:27
  • My connection to the system is at elevation 0.0. My upper tank is 16 ft higher in elevation with about 600 ft of 4 " pipe. My other tanks are at elevation 0 and minus 10 ft. I need to connect all the tanks together so that the fire department has 1 location to take water from, that has 170 cubic meters of water. No pumps, as the fire truck will connect and suck on the system. But it is difficult that I would like to do all three tanks in a connection so there are not 3 shut off valves. – toby Nov 23 '20 at 12:48
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    While Ecnerwal's answer is excellent and raises valid concerns that I didn't think about, I wouldn't be so quick to give it a check mark. Many people won't even look at a question if it has an accepted answer, so you may be shutting off other potential answers/solutions. Give it 48 hours or so, then give him the check mark if nothing better has cropped up. (Again, he does have very good points that I hadn't considered - I'm not criticizing his answer.) – FreeMan Nov 23 '20 at 14:30
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    I’m voting to close this question because it's not about home improvement – Ack Nov 23 '20 at 19:31
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    If this is for fire service, have you spoken with your local FD, and more importantly, your insurer about this whole idea? – ThreePhaseEel Nov 24 '20 at 0:12
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Filling the tanks should be easy. When the fire department shows up with a tanker, they should have a pump that will provide more than enough pressure to push the water uphill. Remember that it can take 2 or 3 men to hold a hose against the pressure coming out of it! If you hire a pool filling company, they may count on simple gravity flow to get the water out of their truck and into a pool, but there should be enough elevation to take care of your situation. If you want to go with this route, you may consider an extra fill-only point at the highest tank in case you run into trouble.1 Put a supply/fill point at the highest tank and start pumping in. Open all the valves to let the water flow into the other tanks. The lowest tank will fill first. Depending on how well sealed it is, either let the water backup into the higher tanks, or shut off the valve to the lower tank once it's full.

The emptying is a bit more complex, but should be doable:

  • The highest tank will simply gravity feed, no problem.
  • The "ground level" tank should gravity feed as the exit point should be at the bottom of the tank, and as the FD is sucking water out of it, it should suck through the lines and up whatever amount of plumbing is above the exit point. There may come a point when there's not enough water pressure to get the last few gallons out of the tank.
  • The "below grade" tank will be the difficult one.
    • I'd plumb the exit point on the bottom of the lowest tank.
    • The pressure of the water sitting above this point should push the water down the pipe and up hill, even if the connection point is above the highest point of the tank.
    • Once the water is flowing, siphon action should keep it flowing even without a pump pulling it. With the FD's pumps pulling the water, I wouldn't think you'd have any issue with the water flow.

When it comes to emptying the tanks, if you open all the valves at once, the tanks will (after a few moments of reasoning, but with no scientific method applied) empty roughly in the highest to lowest order, simply based on gravity. However, once a tank is empty, the FD will certainly be sucking air from that tank until they start to draw enough of a vaccume that either they're sucking air through leaks or there's enough low pressure that it's easier to pull water from the other sources. However, that could draw air into their pumps/hoses causing a pause in the water supply at a least, and possibly cavitation in the pump and loss of suction on the siphon from the lower tank (essentially cutting off supply, at least temporarily), at worst. I would think that you'd want (them) to manually coordinate the valve operation to ensure these situations don't happen.

Most importantly, I'd discuss it with the FD while they are there under calm circumstances filling your tanks up, to ensure that they're familiar with the set up and have them recommend a procedure. Put the procedure in writing, maybe ask them to print it on FD letterhead (get a fire marshal/chief or other "official" signature on it), and put a laminated copy in an easily accessible and visible position at the connection point so that if you're not there to direct operations, the firefighters on scene will know that they're looking at a department approved method of operation. Remember, staffing may change, you may be out of town, or even worse, may be incapacitated by an accident or fire itself. You don't want them to rely on memory of a one-off procedure (since your setup will be different than your neighbors) when they're under stress trying to save your property.

1My initial understanding was that the fire department would show up with a tanker to fill this system. It was later clarified by the OP that he is responsible for filling it himself. Since the OP may not have sufficient pumping equipment to push enough water uphill, I changed that part of the answer.

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  • Brings up another question, what is enough pumping power to push water into the upper tank? Saying that the upper tank is about 16 ft above the draw point, and the line to the tank is all of 500 ft. – toby Nov 23 '20 at 15:13
  • Well, you have 16 feet of static head (roughly 6 PSI, but most pump stuff is done in terms of feet of head) and then 600 feet of 4 inch pipe which will have a dynamic head (friction from moving water though the pipe) depending on how fast you are pumping. Practically speaking, if your pump manages more than 16 feet of head the system will settle into a rate where the pump head and dynamic head + static head balance. – Ecnerwal Nov 23 '20 at 15:24
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    You are correct, @toby, that's another question. Down here in the comments is a bad place to ask it. Actually, there are already questions here about pumping water. Search for "pumping water" and/or "head pressure" - you'll likely find the answer already exists (i.e. the formulas to calculate it for your situation). If not, ask a whole new question, referencing some existing questions, and detail why they don't fully answer your sitation. – FreeMan Nov 23 '20 at 15:24
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You appear to be setting this up to fail. Don't do that.

If you have three tanks at different elevations, your fire truck fill point is the bottom of the lowest one.

Your system fill point and vent is the top of the top one.

If the two lower tanks won't hold pressure (typical concrete tank that does not really have a tight seal at the top) you'll need float valves on the pipes that fill them from the higher tank so the water does not just leak out the top.

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  • "fire truck fill point is the bottom of the lowest one" - I presume you mean "fire truck draw point..." – FreeMan Nov 23 '20 at 14:26
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    The point from which you fill the fire truck. I do not assume that the fire truck ever fills the tanks - that may happen in some areas, but I'm used to the division of labor where the owner of the tanks gets to make sure they have water in them when the fire truck shows up. Though around here it's more likely to be a pond... – Ecnerwal Nov 23 '20 at 14:32
  • Yes the tank filling is done by my equipment, the fire trucks just draw from the pipe, as if they are sucking from a pond. – toby Nov 23 '20 at 14:45
  • I am American, but working in Romania, where things are a little old school. – toby Nov 23 '20 at 14:46
  • OK, filling the truck from the tank vs the truck drawing drawing from the tank. Same result, different perspective. – FreeMan Nov 23 '20 at 14:57
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This is a false economy

Obviously what you are trying to do here is preserve the sunk cost of the tanks that are too low to do the job. You are in pursuit of a fallacy. The simple fact is, 2 of these tanks are in the wrong place!

So if the Fire Department is requiring a certain amount of water onsite that comes out of one standpipe without operating valves, then you need to construct additional tanks so the one place has that much storage. The other place can also get a standpipe and be used for auxiliary/backup water, irrigation, potable water or what have you.

The only reliable way to save these tanks is to pressurize the lower ones. Most tanks aren't made for that.

What I would do

Honestly if it was my house, I'd want the firefighting tanks nearby to avoid the flow resistance of 600 feet of pipe. If you could extend the middle tank with 100 cubic metres more, that'd be great.

I would use the top tank for

  • lightly pressurized water (15 PSI/50 KPA) for irrigation and general use (e.g. my "potable water" tank), because flow rates will be low enough for 600' of resistance in a 4" line to be irrelevant. And,
  • "make-up" water for the firefighting-only tank. When it drops below the top, the float drops, opening the refill valve, and the top tank gravity-refills the middle tank. This will stretch the life of the middle tank for "awhile" but it may not be certifiably enough to meet Fire Dept. requirements. So I'd consider it "bonus water".

A 100-cubic-metre tank seems to be in the US$2000 neighborhood in the U.S. A 170-cubic-metre tank is $3500-ish.

Now, the problem with the above setup is, that if the firefighting tank system and standpipes develop a water leak, they will drain the top tank first. So we set it up so that's just about impossible: By having all pipes enter the TOP of the firefighting tank(s). The inlet tanks have a vacuum break (either a vacuum breaker or simply an air gap). The outlet draw, which will be a syphon, has a "tee" at the top of the syphon going to either a manual valve of a designed-in pinhole. That lets air into the syphon, so water won't naturally draw over it anymore. The big fire pump will apply active suction which will pull water through it. In fact, you could grade the standpipe downhill to a drain point, and simply have a dry standpipe. That takes freeze issues off the table.

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