Bottom Connected Multiplexed Fuel Tanks

[EnergyRat]

Hi Guys:

I hope this is the correct forum for this question, here goes...

I have 'inherited' a design from a colleague for a fuel system
consisting of four 250 gallon fuel oil tanks feeding one 250 kVA emergency
diesel generator in a commercial office building.

I responded to a complaint that the machine prematurely ran out
of fuel, and discovered that the four tanks are piped in parallel, with
no bottom connected balance pipe. The top-connected oil pick-up pipes from all tanks are manifolded together into a common feed to the generator. The return from the fuel pump is arranged similarly. Unfortunately, the installer provided tanks with the incorrect number of connections and was forced to connect the fuel pump returns to the common vent pipe. I have reviewed all codes, and have found that other than the incorrect connection of the return to the vent pipe, there are no code violations.

Apparently, the generator runs out of fuel because the tanks all establish different fuel levels because the return does not refill all tanks evenly.


Personally, I see two options:

1) Add a day tank with fuel transfer pump from the larger combined tank. The day dank would have level switches to run the transfer pump, and the generator supply and return would be to and from the new day tank. I would also need to add an oil cooler to the day tank, secondary containment, and a oil detection sensor in the containment.

2) Scrap the tanks altogether, put in new ones with a bottom-connected balance pipe of appropriate size, and re-pipe the whole thing with the supply from the first tank, and the return to the last tank.

In your collective professional opinion, am I missing a simpler option? And could the existing tank arrangement (without bottom-connected communicating pipe) ever be made to work?

Best regards,

Daver
Ontario, Canada

 

[GregLocock]

I don't think you've found the root cause of the problem, that system should equalise, unless the pick-up -> pump pipes are marginal for size (my guess). Your solutions sound bizarrely complex, is there a reason why a balance pipe can't be used?

Is there a substantial height difference between the pump and the tanks?

Is access to the tanks difficult?





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[turbocohen]

The answer may be to use a Simplex digital level controller.
They make all sorts of tank and valve goods that defintely will solve your problem at the least cost.

Their website is

http://simplexdirect.com/FuelSupply/FSMain.html

and their systems are used by a lot of oem's.


Good Luck, Turbo

 

[MikeHalloran]

I assume you don't want to risk welding in bungs for the balance pipe, now that the tanks have had fuel in them.

On a boat, the tanks would never be cross- connected anyway. They'd be set up with manual switching valves and/or manifolds so that one tank could feed the day tank, two could be just sitting there settling out crud, and one could be connected to a circulating pump/ polishing filter/ water separator loop. They wouldn't even share a vent.

In a building, maybe it's less critical, but if you get one barrel of bad fuel, you've got no power supply.



Mike Halloran
Pembroke Pines, FL, USA

 

[EnergyRat]

Thanks everyone for your replies.

Greglocock: Why do you say that the tanks should equalize? They way they are piped, they do not communicate. I would think that for the tanks to equalize, the pipe friction from all pickups to the fuel pump would have to be identical. What are the chances of that (the pipe runs, as built, are not symetrical and perfectly identical)? There are no stop cocks or balancing valves (their can't be, as the head available from the engines fuel pump is limited). Same goes for the return piping from the pump back to the tanks.

 

[GregLocock]

Yes they do. They communicate where they are teed together ahead of the fuel pump. On re-reading your first post perhaps I am mistaken and you have more than one pump, in which case they won't equalise.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

I have stayed out of this to date because it all seems so simple that I can only think I am misunderstanding the question.

I have seen twin tanks connected by the draw of pipes and a "T" to the pump. These often draw more from one than the other, depending on pipe size and symmetry and consumption rate. The cure is to use larger pipes and/or a collector tank, from which the pump draws fuel. Basically a simple manifold and plenum. Effectivness and symetry of breathers can also have an influence.

The fuel return should go to manifold or collector.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[waross]

There may be an air leak in your piping. It sounds like it used a siphon principal to equalize levels in the tanks.
This system was used for fany years on over the highway diesel trucks. Yes I know some trucks have an equallizer line connecting the bottom of the saddle tanks but many trucks wre made without the bottom tie, and depended on the Siphon principle to maintain equal levels in the tanks.
I found this out years ago, hitching a very long ride with a brand new semi-trailer rig being delivered. This was a brand new rrig with the first fill of fuel in the tanks. Two saddle tanks, one on each side with a top connection line. about 400 miles into the trip, the engine quit. We coasted down and the driver got out and checked the tanks. One full, one emty. Shucks, they didn't fill the eqalizer line when they fuelled up. He disconnected one of the trailer brake lines and stuck it in the full tank. We stuffed some rags around it and I held it while he gave the trailer charging valve a shot for a 10 or 15 seconds.
That's all it takes said the driver, as long as the line is full and there's enough fuel in the tank to cover the bottom of the pickup. I rode with him for another 200 miles or so with no further fuel problems.
Four tanks is a challenge. Look for an air leak that is breaking your siphon. Also, after running out of fuel, the equallizer line probably needs to be refilled with fuel.
Remember, It worked before.
Any air leak in the piping system between the bottom of the fuel pickup tubes and the fuel pump will probably cause the siphon to fail.
If the set was run out of fuel on all tanks the manifold line would have to be refilled.