Siphon Effect in diesel fuel pipeline.
Siphon Effect in diesel fuel pipeline.
(OP)
I am installing an explosion vent safety device (per NFPA) on a 12000 gallon diesel fuel tank. The tank is fed by a 30000 gallon storage tank up on a hill with a submersed centrifugal pump. The difference in elevation is 33 feet.
I do not think that a siphon effect can happen with this change in elevation. However, I would like to know for sure it will not happen.
Can anyone give me a little help on how to calculate whether or not a siphon effect will occur? Is it just as simple as looking at the potential energy?
Thanks for any help!
I do not think that a siphon effect can happen with this change in elevation. However, I would like to know for sure it will not happen.
Can anyone give me a little help on how to calculate whether or not a siphon effect will occur? Is it just as simple as looking at the potential energy?
Thanks for any help!





RE: Siphon Effect in diesel fuel pipeline.
The actual analysis is fairly complicated and involves keeping the Froude number below about 0.31. The theory is well described by PD Hills, Chem Eng, Sept 1983, pgs 111-114
For rough estimates, if the flowrate in US GPM is less than 1.2 x (Diam^2.5) no siphon will form. The diameter is in inches in this relationship.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Siphon Effect in diesel fuel pipeline.
If your being feed by a tank on a hill, presumedly the piping is sloping down at all times.
The only place you might have a siphon that I can imagine is if you had an inverted "U" pipe running over the top edge of the tank on the hill and connecting to your feed line to your 12000 gal tank down at the bottom. Is this what you have?
You can get a siphon effect in the inverted "U", if the distance between the top of the inverted "U" and the liquid level of the tank is around 41 feet or less, because diesel has about the same vapor pressure as water, but is only about 85% as heavy.
With a diesel vapor space at the top of the inverted "U", at normal type ambient temps, there would be about 0.5 psi of saturated diesel vapor there, so let's call that zero.
You will have atmospheric pressure at the surface of the diesel in the tank, 14.7 psia
The pressure difference of atmospheric at one end and 0 or 0.5 psi at the top can lift a column of diesel by,
H = (14.7-0) psi * 144 lbs/in2 /0.85 /62.4 pcf = 40 ft
If you pump into that inverted "U", as soon as the diesel gets to the top and flows over the neck, you've got a siphon started, and if the outlet of the inverted "U" outside that tank, is below the liquid level in the tank, you're virtually assured to quickly get some siphon action.
RE: Siphon Effect in diesel fuel pipeline.
Like BigInch, I assumed the pipe from the submerged pump ran over the top edge of the tank wall.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Siphon Effect in diesel fuel pipeline.
You could theoretically develop a vacuum in the uphill tank and suck it back up into that tank. In real life, with that much elevation difference, you'd probably collapse the upper tank before that happened.
If you're siphoning over a wall or to the top of the tank or whatever, that's a different issue, but you give no information concerning that.
RE: Siphon Effect in diesel fuel pipeline.
RE: Siphon Effect in diesel fuel pipeline.
DB
RE: Siphon Effect in diesel fuel pipeline.
If anyone wants to prove its impossible, put a pump at the high point of the 1/U at the lip of the higher tank and flowing into the high tank, feed the pump from the lower tank with a diesel surface level 44 feet below that pump's top of volute elevation and do a NPSH calculation using NPSHR of 0.5 feet. You will find that NPSHA is less than NPSHR and the pump will sit there and cavitate rather than pump diesel.
RE: Siphon Effect in diesel fuel pipeline.
RE: Siphon Effect in diesel fuel pipeline.
RE: Siphon Effect in diesel fuel pipeline.
I always thought a siphon worked off gravity not surface pressure. The principle will work even if both vessels are in a vacuum. So if pressure is developed inside a vessel and that pressure forces its contents out, I wouldn't call that a siphon action whether the fluid is forced up hill or down. It's just fluid being forced from a pressurized vessel.
On the other hand, if inginear's concern is with fluid siphoning from the upper tank to the lower tank, if the pipe connecting them is full and is not large enough to allow air to break the siphon, I don't see why it wouldn't occur.
Why not use the high point vent with a floating roof?
DB
RE: Siphon Effect in diesel fuel pipeline.
To start a siphon, you must have a differential pressure to raise the fluid to the top of the 1/U. To do that, you must fight gravity for the run up the riser. Once you get to the top and start to go over, gravity helps on that bit as it starts to move down in the downcomer, but there's still a lot of fluid in the riser, where gravity opposes the flow direction. "If you stop sucking, the gas goes back into the tank". At that point you've only cancelled out a little bit of gravity. As the fluid on the downcomer side gets lower, gravity difference between the riser and the downcomer is more balanced and when the fluid in the downcomer finally gets all the way down to the level of the surface inside the tank (ambient pressure on both liquid surfaces (inside the tank and the liquid surface at the interface inside the pipe, being equal), gravity and pressure are completely balanced. "stop sucking now (almost)." If the pipe ended at that point and was completely open to gravity, the fluid in the downcomer would simply tend to fall out. As it fell out, it would evacuate the downcomer which would tend to reduce the pressure at the top of the "1/U", that was just before completely balanced. That slight pressure difference across the top of the U would allow a little bit of fluid in the riser to flow over the top into the downcomer. The faster the liquid fell out of the downcomer, the faster the fluid would also move over the top. The fluid in the riser is no longer balanced because its lost some of the weight of the fluid at its top, so the pressure in the tank is again sufficient to push a little more fluid into the bottom of the riser and force the remaining fluid in the riser to move slightly higher, once again balancing pressure at the top of the "1/U".
Take the example you mention for the evacuated tank. Think in terms of absolute pressure. If internal pressure is 5 psia, and the siphon is open to atmosphere somewhere, you've got 15 psi pushing in and only 5 pushing out. Let's say we ditch the 1/U for a, /\/\ where the left "/\" is full of diesel and the right "/\" is full of air, and we let the fluid oscillate back and forth for a minute or two and see what happens. Eventually, the "\/" in the middle will reach a point where it has apx 21 more feet of diesel in its left leg than it has in the right leg, oscillations will stop and all flows will halt, because the 10 psi differential pressure would be balanced by the 21 feet of differential head of the diesel in the left leg over that of the right leg. You'd have to connect a "vacuum pump" to the right leg's outlet to get any more diesel out of that tank.
Take a look at Bernoulli's equation and see if it makes sense now. Flow Energy at any point = elevation + Pressure head + velocity^2/2g - Head loss due to flow. If not, see if you can find out how a mercury barometer works. A water barometer is the same except instead of 760 mm, its 33 ft, and a diesel barometer needs 40 ft.
And one more thing... remember that in a closed tank, with just one drop of fluid remaining inside, the absolute pressure in that tank is equal to the vapor pressure of that fluid and that's as much vacuum as you can ever get in that tank at that temp.
Don't feel you have to make excuses. I can't understand how to wire a 2-switch-at-each-door-going-to-the-one-light-in-the-middle-of-the-hallway to save my life.
RE: Siphon Effect in diesel fuel pipeline.
DB
RE: Siphon Effect in diesel fuel pipeline.
RE: Siphon Effect in diesel fuel pipeline.
RE: Siphon Effect in diesel fuel pipeline.
DB