Pump mystery

This situation puzzles me: I have two 1000-liter tanks containing biodiesel. A vane pump and metering system draws fuel from both tanks simultaneously, or should.
Each has a kind of 'stinger' pipe inserted through the top opening, with hose going from stinger to a T-fitting, thence to the pump. The pump is mounted closer to one tank, call it Tank A, than the other, tank B. Consequently, the hose from Tank B is significantly longer than the hose from Tank A. If it matters, there are about 2.5m of 25mm diam hose from Tank B, and about 1.5m of the same type hose from Tank A. For both tanks, the fuel flow is up through the stinger, over and down through the hose to the T-fitting, thence up through the vane pump and meter, thence out the vehicle fuel tank through 3m of 18mm diam hose and a conventional fuel nozzle.
Here's the mystery: Tank B, the tank more distant to the pump, empties first! This seems completely counter-intuitive to me. I expected both tanks to empty at the same time, or, perhaps, Tank A to empty slightly faster than Tank B. Can anyone explain? This is not some academic exercise, this is a description of what has happened repeatedly since we began using this system 7 months ago.
Paul Mathews
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When you say a "stinger", I am thinking you are meaning that basically no tank pressure is pushing the fluid through the pipe at all and the pump is doing the "sucking" to get the flow going to begin with? Are both tanks at the same height on the ground? Are both tanks under or above the pump itself?
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Yes, the pump draws the fluid up through the stinger pipe, through an ell fitting and hose barb, into the hose, though another hose barb into the T, into 25mm pipe, thence into the pump. Paul
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I forgot to address your other questions: Yes, both tanks at same height. These tanks are about 1m high. The pump inlet is at about 80cm, so, sometimes the pump is below the liquid level(s) and sometimes above it, i.e., when both tanks are 80% full, their liquid levels are at the same height as the pump inlet. However, what always happens is that Tank B gets nearly emptied before Tank A even begins to subside. Paul
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Well, once the flow is higher from one tank, the other tank can't really force it's flow to enter the faster flow.
Anyways, it sounds like a great physics problem for schools :)
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wrote:

Years ago, there was a short-lived trend to fluidic logic. Something like a tee could serve as an 'either/or" arrangement.
Brian Whatcott Altus OK
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message

I remember those things Brian. They were bloody unreliable. You had to filter the air to just about medical quality if they were to go a month without choking up. I had the satisfaction of dumping two of them.
Tom Miller
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On Wed, 15 Mar 2006 00:15:05 +1100, "Tom Miller"

Such a cute idea though. And slow, and fussy!
Brian Whatcott Altus OK
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Another probable explination may be the flow within the T itself, as there will be a turbulence issue with the convergence of the flows from the tanks inside the T. One flow path may become dominant within the T, due to slight differences in the intersections of the 3 flow paths. What does the inside of the T look like? If there is a sharp ridge on once side, and a smoother edge on the other, the difference in resistance of flow from one inlet to the other may explain it. This application would probably be better with a Y instead of a T.
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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I wondered about this myself, and I may yet change the T to a Y, but I reasoned (to myself) that the T might make a lot more difference if it weren't for the fact that the pump is sucking the fuel up from the T. It seems to me that this should guarantee that the flow takes a kind of Y shape, regardless of the fact that the fitting itself is a T. However, the T is iron pipe, and it's a classic T shape. Paul
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It doesn't matter to the flow what is causing the flow. What dominates is the local geometry and the state of the incoming flow caused by near upstream geometry. A Y will likely help noticeably, but as I said in my other response, it is surprisingly difficult to make a passive pumping system that will evenly empty two tanks. If you will be happy with, more or less, even then:
1. Change out the T to a Y. 2. Make sure the lines going into the Y are relatively straight for the last 10-40 diameters, the longer the better. 3. Swap out the short line and make it the same length as the long line. 4. Plum an always primed siphon line with a diameter several times larger than the pumped line from tank A to tank B and never let the fluid level in the tanks go low enough to break the siphon.
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Charly Coughran
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Dumb question, If we say the letter "T" as shown in this sentence is your fitting, do the flows from both tanks enter the fitting from the right and left with the exit flow out the bottom? If not, then you'd get a lot more resistance from the stream that had to turn 90 deg than the one that went straight through.

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Let's take the easy case first. Thru flow in a tee is when the flow is just flowing along the long leg (of the tee not the letter) with the branch entry valved off. Branch flow is when the flow enters the short leg and turns 90 degrees to exit the tee with one of the thru entries valved off. In this case, there will be 3 or 4 times more head loss with branch flow than thru flow. There is more going on, however, than just having to turn the corner. A standard 90 degree elbow will have only 1.5 to 2 times the head loss compared to the thru flow tee, much less than the branch flow tee. The thru flow tee will have 5 to 10 times the loss of a similar length of pipe.
The additional flow loss in the branch tee over the elbow and the thru tee over the pipe is caused by the more complex flow allowed by the more complex shape of the fittings. When you add to that the more complex flow due to the interaction of two distinct entering flows when all entries are open in the tee, things get harder to predict.
It is true that with all the tee ports open, you would expect branch input and thru output to be less even than thru input and branch output. Even meaning here, equal incoming flow rates. Relatively small perturbations in the incoming flows, however, can dramatically change the situation by choking off and restricting flow from one incoming branch. Whether you could actually get to the point where branch output actually had more head loss than thru output is an open question. I have seen set ups that had pretty simple problems that nobody really noticed that dramatically altered the flow from what people expected.
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Charly Coughran
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Just to clarify: The T-fitting is positioned so that it is symmetrical with respect to the tanks, i.e., the tank hoses enter horizontally from opposing ports. Flow is thence upward to the pump inlet.

can make a lot of difference. However, it's still puzzling to me that the tank located at the greatest distance empties first. Paul
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It still sounds like there is simply more head pressure on the longer run. (more mass/ higher energy potential) So therefore the higher flow rate will be from such higher head pressure first since it can easily take over from the lower head pressure.
-- James M Driscoll Jr Spaceman
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Yes, but how did it get that higher pressure relative to the other?
More pressure one one or more resistance on the other...

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The length of the pipe differences.
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symmetrical
from
that
If I understand what you are saying, that pressure difference of which you refer is the difference in pressure due to an equal flow in different lengths: same flow in a long pipe gives as greater pressure drop than it would in a short pipe. - but no flow in the short leg initially means that there is no "pressure from flow".
Here, there is the same pressure: from atmosphere at the two fluid surfaces relative to the common pump intake.
So the delta P from fluid to pump is the same, and any difference in flow is caused by differences in resistance in the path.
But for what happens here, the shorter pipe clearly has more resistance initially, even though there is no flow in the shorter pipe.

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The same pressure but 2 different volumes in each pipe length. the larger volume in the longer pipe would simply contain more potential energy.

Could be but like I am trying to say, one pipe definitely has a higher potential of energy.

Yes, I would love to see the total setup to really find out more "stuff" about it all. :) Fun physics project for a school if the pipes have no difference in restriction. :)
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And Paul - you are sure that no part of the hose on the short leg is partly closing (flattening or the inner wall collapsing) when the pump is first running?

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