Fuel tank problem

[mvujica]

Our stand-by generator fuel tanks are four 2000 l plastic tanks in arrangement shown in attachment. Gen-set full load fuel consumption is around 230 l/h but average consumption is around 25-30 l/h so 200 l/h gets returned into tanks. After long power outage of 24h we noticed that tank levels are considerably different and if we didn't refill first tank(first to pump) it would run out fuel and consequently stopped the engine. What could be the problem with this configuration of tanks. Do we have to connect separate tanks by extra piping to avoid problems?

 

[BigInch]

Well.. that's an incredibly bad configuration to draw fuel equally from all tanks, if that is the objective.

Labeling each tank A to D from left to right,
and assuming you start with equal pressure in each tank,
you will have a large pressure drop in the pipe from tank D into the header, so most flow will come from tank D.

If there were equal flow Q, from each tank, you would have 1Q in the header between A and B, 2Q in the header from B to C, 3Q in the header between C and D and 4Q going to the generator. The pressure drop in AB = H, the pressure drop in BC, with twice the velocity, would have to be close to H^2. It won't try to make H^2 when it can simply pull all it wants from tank D.

I think you need to valve each tank and control the flow from each tank. You could possibly do better by connecting the generator take off to the center of the header, but then you would probably draw most all flow from tanks B&C. You might be able to balance flow from each tank, if you ran an individual pipe from each tank directly to the generator, but I don't think that would be an ideal solution.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

Why not connect all the tanks together with bottom piping and draw fuel from a sump on that piping?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[hydtools]

Along the lines of what BigInch has said, connect all tanks with bottom piping and draw from one tank. The bottom piping will keep all tanks at the same decreasing level. Effectively creating one big tank.

Ted

 

[pmover]

There may likely be a purpose of having 4 separate tanks (i.e. fuel contamination). Then again, the OP leads us to believe that the 4 tanks serve as 1 tank.

Having the pipe connected to bottom of each tank will work, and it is best to have valves installed so as to isolate the fuel supply from each tank (if desire to isolate the tanks).

Secondly, the fuel supply connection at each tank bottom should be inserted into tank (i.e. 2-4 inches) to avoid any heavy sediment, debris, or water into fuel system piping.

BigInch - please clarify your posting. I believe you meant to state "D to A from left to right" as the leftmost tank has greatest resistance or longest pipe path (assuming constant pipe diameter).

good luck!
-pmover

 

[BigInch]

No. D.
Assume all tanks have atmospheric pressure (say 15 psi) on equal fluid levels. There is some pressure, say 8 psi (near tank D) in the beginning of the pipe run to the generator.

Then Tank D has the largest pressure drop (pressure drop for me has psi/ft units), since ft is relatively very very short, (15-8)/(basically just the length from bottom of tank).

The largest pressure drop/unit_length will accelerate more fluid from tank D into the header.

The other tanks have the same net pressure drop from fluid level to the 8 psi point near D, but over a much longer length, hence acceleration of fluid out of those tanks is slower, if there is any at all.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[pmover]

BigInch . . . ok, my misunderstanding . . . now that i "carefully" read your initial response. yes the rightmost tank (tank D as described) will likely have greatest fuel withdrawal.

-pmover

 

[itsmoked]

Seems a great opportunity for redundancy is being wasted here. What if there's an earthquake and one tank is cracked or a fitting fails. The whole load is going to hit the ground.

What if you're running in emergency mode and get a new load of fuel and it's contaminated? Now ALL the fuel is contaminated.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mvujica]

I see your point BigInch, but after a while level in tank D will be lower than of A-B-C and the pump will draw more fuel from B and so on until there is again equality? Bottom piping is not an option since these are plastic tanks (originally used for heating oil, tanks and fuel supply manufactured by Shütz)and it is hard to make good joints. I am attaching Shütz manual for connecting the tanks- it is in German but you will see the picture. Only difference is that there is no return line in manufacturer's configurations. Maybe there is the problem?
My solution is to connect them from above using plastic pipes (without drilling the tanks) and purge air using some other pump eventually creating one large tank.

 

[BigInch]

There will be some draw from other tanks, but not significant until the pressure difference of the level in C-D equalled the friction of the flow rising in D plus the friction of flow rising from C and running through the length of pipe C-D, and so on.

I'd think the initial flows from D would be around 2 times that of C, which is maybe 4 times that of B... but I'm guessing.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Seems it's a slow week at Eng-tips, so here is something to add to the discussion.

1. Fit a valve to each of the inlet lines and throttle each valve until the the total head loss of each one is equal, this should result in equal drawndown in each tank.

2. Increase the pipe size on all inlet pipes and the header to the point where the friction loss thru the lines in so low it is inconsequential - as an added improvement, in lieu of end connecting the the supply line to the engine centre connect the supply line.

Option 2 is preferred as it is not subject to flow variations.

 

[mvujica]

Have you seen the Schütz drawings? It is factory kit for connecting multiple tanks, but there is no return line from generator to tanks as we have. What do you thik about my idea to put separate piping and connecting the tanks so they will level naturaly also than I can remove three inlet and return pipes and pump fuel just from the last tank (D).

 

[BigInch]

I don't know why you would need a return line. The generator should not be receiving more fuel than it needs. There should be nothing to return.

Do you have a constant speed fuel pump at the generator?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mvujica]

All our standby generators have fuel return line, the one we are talking is caterpillar C32 1000kVA generator. I belive all have constant speed fuel pump so there must be return. As I said in first post I belive that about 200l/h of fuel is returned to the tanks at our current load of 120kW as oposed to 800kW full load when engine is consuming aroud 230l/h.

 

[BigInch]

Quite strange that return line. Do you have day tanks at the genset that you want to flush back to these plastic tanks?

Yes. Connect near the bottoms and feed the main fuel line from a low point in the connection piping. I thought that's what I was talking about in my second post.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[MikeHalloran]

Big, all Diesels of which I am aware return fuel to the tank. Under most conditions, they return a lot more than they actually use. The excess/recirculated fuel is used to cool and lubricate the injection pump.



Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

But only to the day tanks, no?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mvujica]

Where ever, our tanks are located just below the gen-set outdoors so there is no need for day tank.

 

[hydtools]

When the fuel level in any one of the tanks goes below the end of the pickup pipe air will be drawn into the system and the flow of fuel will soon stop.
Maybe each tank needs a float valve to close the pickup pipe when the tank level reaches some level near empty so that air cannot be drawn from that 'empty' tank and fuel continues to be drawn from another tank.

Ted

 

[MikeHalloran]

Systems I've seen in boats use a two-gang selector valve, so the engine's supply and return are both switched to the same tank. I'm not aware of ever having seen an automated one.

I can't figure out how that German system works.

The tanks would be self- leveling with bottom connections, but it may not be legal to do that, or practical, given the difficulty of connecting to plastic tanks.






Mike Halloran
Pembroke Pines, FL, USA