I'm not sure if this is really the right forum for this, but as the propane is primarily to run my generator, I'll give it a shot.

I had a single 120g tank and I had the gas company add two more last fall. They seem to be ganged together properly, from what I can tell, and all 3 tanks are "on" but only one tank seems to have been going down. I just looked and the original tank (also first in line) is down to 30%, whereas the other two are 70% and 80%. Shouldn't they all draw down more or less evenly?

Thanks.

 

How is the system plumbed? Is there a regulator on each tank or one regulator for the whole bank?

 

The really important question is do all three levels go down ?
Are there regulators on all three tanks ?

Can you post a couple pictures of the connections at/between the tanks.
How-to-insert-pictures.

 

There is only one regulator. I'm only showing two of the 3 tanks in the photo, but #3 is just like #2's connection except it dead ends. The generator has not run long enough to empty tank #1 (original tank). I hadn't given it that much thought until recently, but looking at it now, it sure seems to me like they should equalize...(yes, the valves are all in the "on" position).

 

Interesting..... I'm looking at what looks like an awfully sloppy flare fitting on tank one where the regulator attaches to the tee on the valve. Usually the nut fits snug to the pipe.

I'm not the pro here but I would think that as long as the primary tank could deliver the needed pressure the other tanks would go down slower. The only true way to equalize the tanks is to have a liquid connection between all three on the bottom of the tanks.

 

Yeah, I wondered about the whole liquid/gas thing with the tanks.

 

The liquid propane vaporizes to supply you with propane gas. If the primary tank could vaporize fast enough.... the two additional tanks consumption would be lower. That would change quickly in the cold where the primary tank would not be able to keep up with the demand as well.

 

Those lines joining the tanks are way too small.
The tanks are going down at diferent levels because the restriction in the lines to the tanks won't allow them to vaporize at the same rate.
The supplier should be able to size that for you but for vapor 1/2" or 5/8" OD copper tube would be more appropriate.
How big is your generator?

One problem you will have is that it would be difficult to maintain fuller tanks in case of an extended outage.
I would suggest having it where you can operate with one or more tanks at a time and leaving one or more turned off to have a full one/set just in case.
If for instance it would operate on one tank leave the other two off and drain the first to near empty.

 

All the connections and piping were done by the gas company. We specifically added the extra tanks because we are in central New Hampshire on side of a mountain, and we operate a small commercial wood shop on the property. The generator is 15KW Kholer, one year old. And to someone's comment about cold, it gets pretty darn cold up here.

As for having tanks controlled individually, they do all have shut off valves on them, I wonder if I should turn #2 & #3 off until #1 get low?

 

You don't "have to" shut off some of the tanks. They will naturally work it out. If one tank empties first then gas will stop coming from the empty tank and the others will provide.

If you had portable tanks that you took to town for filling it would be a good idea to shut off the others and only use one at a time. That way you could run one empty and take it out of the system to be filled while the others supplied your generator. Since your tanks are permenant and can be topped off anytime it's not terribly important.

The tubing connecting your tanks is small but it's on the high pressure side so it doesn't have to as big as the low pressure side. I am more concerned about the vaporization rate. That is how fast the liquid propane can boil to create vapor. Double check my numbers but at full load your generator can consume up to 300'000 btu of gas per hour. That's a pretty thirsty rate.

First of all, your 360 gallons of tank size won't last long since completely full you only have about 288 gallons of liquid propane. After that you need to check the numbers to see how much liquid you have to keep in the tank to produce 300'000 btu of vapor.

Summer is no problem but things look worse when it gets cold. The amount of vapor a propane can produce depends on the amount of liquid in the tank and it's temperature. It doesn't help that you are using vertical tanks because the reduced surface area produces less vapor than a horizontal tank. So, come winter if your generator seems to be having trouble the first thing I would check is the fuel supply. Your propane tank setup may not be able to supply vapor fast enough to allow the generator to run at full load when the tanks are low.

So, on a really cold night even though the gauges on your tanks say you still have 20% left there might not be enough liquid to produce enough vapor to keep your generator running at full output. This would cause the generator to run lean and may sputter, run rough or shut down altogether. All you have to do though is reduce the load on the generator or get more fuel in your tanks.



If you look at the above chart you can see that a 120 gallon tank at -10f (the top chart) when 60% full can provide about 81'000 btu of vapor. All three of your tanks then are closeish to providing the 300'000 your generator could use at full load. But when you get down to only 20% left those three tanks can only provide about 150'000 btu of vapor which is only enough to allow your generator to run at half load.

 

Wow, that's some information! I guess the gas company is just like "sure, you can get more tanks"....And so, 3 seems better than one. Cold weather operation is really the crux of the biscuit here. I would have like to have put a big horizontal tank in when we moved in, but the house sits on ledge, so burying it is out, and the terrain is rough enough that if we put in an above ground tank far enough from the house for code, they probably couldn't reach it to fill! I've heard stories up here from old timers about how they used build wood fires around outdoor propane tanks in the winter to keep them vaporizing! Yikes!

So, what's my solution?

 

Add more tanks. Each tank will increase the standby capacity.

 

You could ask your propane supplier about them supplying a vaporizer.
Your set up uses propane that boils off a supply of liquid in the tank that is supplied to your propane equipment.
For a vaporizer system you would connect a liquid supply instead of vapor to the vaporizer which from heat generated by a small flame would flash the liquid propane into vapor form.
They are fairly common here on systems that can not install enough tanks to get capacity in sub-zero temperatures.