"Paul Mathews" wrote in
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.
Think pressure causing flow, and open allowing flow.
- and my first guess is a switching T designed to empty one tank before
the second empties (used for automatically switching into reserve tanks), a
collapsing line (wrong or under-designed hose), or less likely, the hansen
If I have the facts correct:
1) the hose between the tanks is the same diameter,
2) the pump intake hose has a smaller diameter than than the hose connecting
the two tanks
3) The connection between the tanks is - stinger pipe, hose, T, hose,
4) The tanks siphon properly, and the fluid seeks the same level when the
pump is not operating.
5) The tanks do not siphon or seek the same level when the pump is running.
6) When the pump is running, the tanks are drained one at a time.
For flow to happen in a T, there must be pressure differences at each
end of the T, and open ports.
But here there is no flow on one port, and then there is - and what changes
the flow condition is the flow from another port. So pressure changes, and
possibly ports are blocked/restricted.
I would first look at the outside of the T and check that it is a simple T
and not an unbalanced T.
If it is indeed a simple T (longitudinal axes to tanks, transverse port to
pump, length of tank port legs of the same length), then:
Second, I would then look at the lines as the pump is running, especially at
bends. If there is anywhere the line is not perfectly round DURING pumping,
the hose is collapsing and pinching off flow. (Especially "transparent"
line, which to my knowledge is not rated for any suction to speak of.
If the lines are all perfectly round during operation and no fluid is
leaving the tank connected by the lines:
Third, I would look inside the T and see if it is actually a shuttle T
(ball/vane inside) rather than a simple T.
- if it has a ball or a vane, it is doing its job in draining one tank
first. - if it is a simple casting, then:
Fourth, a hose can be blocked by the inner wall of the hose going inside the
fitting path when the hose is connected to a fitting.
When fluid flows into the fitting at that port, the separated inner wall
is moved back out of the way of the flow. When other fluids enter the
fitting form other ports and/or the flow is reversed out that port, the
split-off inner wall is pushed into the path and blocks the flow. Acts like
a hidden check valve.
Since the hose inner wall being inside the fitting, it reduces the
diameter of that port relative to other ports, creating an inherent pressure
differential in the multi-port fitting.
In your case, theory says that since the line is larger from the unblocked
side, the mass from the flow will force the inner wall into its port
opening, creating a pressure difference and blocking the flow from that port
until the mass no longer is present and the pressure difference shifts.
(I call it the hansen effect because when I first had it as a problem, I
was using 3000-10000 psi hansen outer/inner sleeved fittings.)
or something like that.
(And if this problem wasn't metric but rather more innately human-friendly ,
the solution would be more easily spotted. Wrap your mind around 18 and wrap
your mind around 1. Esoteric process former, instinctual process latter.)
This is not some academic
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.
"Aldo" wrote in
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
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.
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
James M Driscoll Jr
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
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.
Obviously you have not told us about something important. As someone
else observed, If the 2 tanks are connected by a hose where both ends
are always under liquid and no air is in the lines then the 2 tanks
should always equalize in level when the pump is not running due to
siphoning. Since you are filling vehicles I assume the pump is not
running a lot of the time. Therefore something in the piping from one
tank to the other is preventing siphoning - what is that something?
The same pressure but 2 different volumes in each pipe length.
the larger volume in the longer pipe would simply contain more
Could be but like I am trying to say, one pipe definitely has a higher
potential of energy.
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
If it is important to you for more than curiosity.
Why not swap the hoses on the T then swap the hoses on the tank etc etc.
As you have 2 of almost everything, the old fault finding technique of
SAWP TILL YOU DROP should sort it out for you.
Here is a more detailed description of the plumbing. All threads and
bore diameters are NPT (tapered US std pipe threads):
Stinger: 36" length 3/4" pipe
Elbow/nipple: PVC 3/4" female right angle to 1" hose barb
Hose: rigid 1" ID PVC
Nipple: straight 1" hose barb to 1" NPT male
T: cast iron, 1" x 1" x 1"
riser: 1" PVC 18" length
pump inlet: 1" female
pump: vane type, 0.5kw DC motor, rated 15 gpm, outlet 3/4"
meter: impeller type, inlet/outlet 3/4"
hose: 3/4", 10 ft long
nozzle: standard auto-shutoff fuel dispensing type
all metal parts Ni-plated for compatibility with neat biodiesel
all exposed gaskets/seals replaced with biodiesel-compatible materials
As far as I can tell, there is virtually no flow in Tank A until Tank B
is quite low. So, theories about flow resistance cannot be correct.
You sure there isn't a check valve in the piping? In particular, a check
valve in tank A? If the check valve took a certain pressure differential
to pull it off its seat, it might not open until tank B got low. It
would also prevent siphoning. Otherwise, if something isn't completely
obstructing the flow from tank A to B then siphoning should equalize the
2 tank levels over night.
Dear Paul Mathews:
Well, they *can*, but...
Do you have a picture of the setup? If you don't have any
webspace, you can email picture(s) to me, and I'll post back with
a link to it/them so everyone can see. My email address is:
snipped-for-privacy@D.T, with M = dlzc, D = aol, and T = com
David A. Smith
I think I've been open-minded about any explanations, and I don't have
any hypotheses of my own, so I don't understand the comment. I
appreciate troubleshooting suggestions, and we'd like to have these
tanks empty in unison, so I'll eventually get to a thorough diagnosis.
However, I'm procrastinating for these reasons:
a) PVC hoses must be inserted into boiling water to soften them enough
to get them over hose barbs. This fueling station is solar-powered,
remote from mains power, and getting boiling water on site is a hassle.
b) biodiesel is quite messy to work with, so I try to avoid such tasks
The plumbing really is as I've described in detail. I'll work on
getting some photos.