air lock in gravity flow pipes 1

[mikeellsmore]

We use gravity flow to transfer leachate (contaminated water) from a tank to an open pond on different elevations. Head difference, between the level in the tank and the dischage pipe end into pond, is 4-6 metres. Horizontal distance bewteen the tank and the pond is about 1000 metres. Pipe diameter 63 mm poly. Pipe goes through a valley between the tank and the pond that is about 2 metres below the discharge point. System suffers from air locks each time we go to use. (Pipe is drained between uses as it forms part of another system). Bleeding the air from the low point restores the flow.

Can someone explain the fluid mechanics of why an air lock stops flow in this type of situation, what head difference would stop this occuring?

 

[d23]

mikeellsmore

I can’t validate this, but I would guess it has something to do with density of water and air. You would require a small “siphon effect” to get past the dip. Do you have some way to level the line with pipe stands?

I would guess the elevation or head you have is just enough to overcome the friction loss. To operate you system in its current configuration you may need to install a small priming pump to get fluid movement started...

Good Luck

 

[RWF7437]

Have you considered installing one or more air/vacuum relief valves at or near the low point ? If you fill the line slowly no air lock should form but the valves would help ensure that. They're not very expensive and usually work just fine even on raw wastewater lines.

 

[PUMPDESIGNER]

Air locks are nasty to deal with, but simple to understand.
STATIC CONDITIONS - Air rises, water falls.
DYNAMIC CONDITIONS - Water is pushed through the pipe. Air will attempt to stay at the top. The result is that water will go underneath the air to get by. Sufficient velocity will cause the water to demand more space, so air is pushed along. Insufficient velocity and the air will not be pushed through, water continues to go under the air pocket.

BEST SOLUTION - Do not install air locks.
WORKING SOLUTION - Not sure but you can try installing a specific and sort of rare type of air vent at each high spot. ARI in Israel makes one. It has a rolling seal with a check valve built into the discharge of the valve. Air and even low pressure air is pushed out of the valve by rising water. We used one on a pump intake with a horrible air lock (installed by someone not us), fixed it instantly. PUMPDESIGNER

 

[RWF7437]

I have no idea what PUMPDESIGNER means by " don't install air locks".

Air / vacuum valves may be found at this website:

http://www.gaindustries.com/html/04b_airvacuumvalves.htm

Russ

 

[PUMPDESIGNER]

I am referring to gravity flow and pump intake lines. Low velocity lines have major problems when air locks are allowed by design.

One caution. When using vacuum relief valves in this application be careful of pressure surges. Air release and sudden closure against water creates incredible pressure surges. PUMPDESIGNER

 

[RWF7437]

If I recall correctly, there are no pumps in this system. The original post stated that this was a gravity line. As described, it is an inverted siphon ! There are no pumps ! If there are no pumps, how can they "shut down quickly" ??

 

[mikeellsmore]

Thanks RWF437. Yes, my original query relates to an inverted siphon. Logic says it should just flow (subject to friction head equalling the static head difference) because the discharge point is lowerer than the level in the head tank but it does not unless we break the pipe at the low point first to establish flow? Why is it so? It is as if there is an air lock at the low point (I would have imagined any air bubble would just make its way up the pipe but it doesn't seem to). Any comments?

 

[quark]

As the highest point in the system is the tank, initially when pipe is empty and you start filling it with water air tries to escape in a direction opposite to that of water (towards highest point). Try installing an airvent (a simple pipe) at the discharge end with a height more than that of elevational difference. This may solve your problem.

 

[aerospacedesign]

what is basic difference between advection and convection.
and diffusion and conduction.

 

[25362]

I believe quark hit the nail on the head. Whatever air displacement takes place by the first water run, air will tend to go up and plug up the line. Try to pour water into a funnel with its end submerged, and you may get water, pushed by air bubbles, spattering back at your face.

To aerospacedesign, I think these expressions belong to Meteorology: atmospheric air currents that move horizontally do it by advection, or vertically, by convection. I may be wrong.

 

[25362]

I've read that vertical pipes where the liquid Froude Number is less than 0.3 would be self-venting.

 

[Tremolo]

In a comment related to that of Chemical, the Froude number can be used to determine if the liquid velocity is sufficient to push the gas bubble ahead of it to clear the pipeline.

For fluid flow in pipes, the Froude number is easily calculated as,

Fr = u/sqrt(g*D)

where u is the fluid velocity, g is gravity, D is the pipe diameter, and the "sqrt" represents the square root.

First, consider horizontal pipes. If the liquid velocity is too low, then a separated flow geometry will be present with the liquid flowing underneath the gas. At high liquid velocities, the liquid will flow as a plug and push the gas in front of it to clear the line. As the liquid Froude number approaches or exceeds unity, the liquid velocity will be sufficient to obtain the plug flow geometry and push the gas in front of it.

For the pipe describe by mikeellsmnore, the diameter is .063 m and gravity is 9.81 m/s. Thus, to obtain a Froude number > 1, the liquid velocity must be greater than,

u > Fr*sqrt(g*D) = 1.0*sqrt(9.81*.063) = 0.79 m/s, or 2.6 feet per sec.

Next, for a downward flowing vertical pipe, the gas bubble will tend to rise due to buoyancy. To avoid the collection of gas at the piping high point (and the potential for vapor lock), the liquid velocity should be greater than the velocity of the rising gas bubble. In other words u liquid > u bubble

The bubble rise velocity can be estimated by,

u bubble = 0.345*sqrt(g*D)

If this expresion is rearranged, we get back the expression for Froude number:

(u bubble)/sqrt(g*D) = Fr = 0.345

So, to clear the bubble from the vertical pipe, the Froude number should exceed 0.3. This is similar to the statement made by chemical.

In your case, if you are starting from a zero flow condition, then the question is what is required to start the fluid moving?

To start the fluid moving, the pressure in the tank plus the static head of the water between the tank and the gas pocket (i.e., the driving pressure) must be greater than the pressure in the gas pocket. The pressure in the gas pocket will be equal to the pressure at the discharge end of the pipe (I assume the pipe discharges above the surface of the pond water?) plus the static head of the water column between the gas pocket and the pond (i.e., the back preesure).

At first glance, the piping geometry you described should provide enough driving pressure to get the system flowing. Hoevever, the problem may be that the gas pocket extends beyond the "valley" and is present farther up the piping upstream of the valley. In this case, the static head of water between the tank and the gas pocket will be reduced and the driving pressure will be insufficient to overcome the back pressure.

I am not familiar with the valving procedure you use when refilling the pipe system. Perhaps the procedure can be modified to reduce the penetration of the gas pocket between the tank and the valley, thereby maximizing the driving pressure.

 

[BobPE]

Tremelo:

I agree you will have a separated flow geometry, but that is not gravity flow.

BobPE

 

[RWF7437]

Lots of interesting discussion here and way more calculations than I even knew were possible ! Still, a simple vent sounds like the best solution and need not be hard to install.

BobPE, just a thought. Because this is an inverted siphon it is clearly not "gravity" flow since there is no free water surface within the pipe. Which makes me think Tremolo's analysis seems to make the most sense.

Sure hope all this is helping mikellsmore with his original problem.

 

[BobPE]

RWF7437:

If you read the post you will see where it breaks down due to assumption of gravity flow. It is a very good post though, please don't get me wrong....

BobPE

 

[smckennz]

Somewhere along your line you have one or more local high points. Air is collecting in them and the compressibility of air is stopping them getting blown out by the water. Look at almost any manual on air venting and you will see a diagram explaining this. If you still have a problem with visualising what is going on, experiment with a length of plastic tube and some water. You will find that if the profile is a smooth U it will always flow provided outlet is lower than inlet. If the profile is wavy, you will see pockets of air formed at the high points. You will also see by the water interface level difference, that each of these pockets adds a backperssure to the system.
If your 63 poly is laid on the ground, you may be able to eliminate the high points by rerouting the pipe slightly, perhaps with a bit of digging. Each local high point only needs to rise a few metres to cause a problem.
If you cant get rid of the high points, air eliminators can be used. However I would try to avoid using them as they can be troublesome with dirty liquid.

Cheers

Steve McKenzie

 

[mikeellsmore]

Thanks all for the suggestions. It would be great if you could post a simple sketch with questions in eng-tips to avoid any confusion. Can this be done? I will post details and success of our modifications to resolve our problem in the next couple of weeks. Thanks again all for your comments.

 

[milldude]

Mikeellsmore:

Sounds like you have a similar problem to what we had
here at our mill. We had problems with the volume of
flow from an elevated lagoon thru a HDPE line to a lower
lagoon. The flow was strictly gravity through the line
but the line always is under continuous flow. We looked
for a localized "high spot" within the length of the run
of pipe. By "localized high spot" what I mean is wherever
the invert of the pipe (bottom) rises up in grade above the
obvert of the pipe (top) you have a potential for an airlock at that location. We found such a spot and put
a saddle on the pipe and an air vent line so that when fluid
flows thru this location, it can force the air out the vent
line. Did it work? Hard to tell as the measurement of flow
thru this line is very difficult (48" line). What we
eventually did is put a small submersible pump in at the
elevated lagoon and ran a pipe from it into the existing
HDPE gravity line and we run it periodically when we need
more flow. Works like a charm.
As well, from my experience from municipal watermain
installations, during the pressuretesting of new lines we
would fill the watermain, making sure that at all "high points", as I have defined above, that we installed a temporary 3/4" copper service line to the surface to allow
air to escape. Without it, you could never pressurize the
line. That's why, in your post when you say the line goes
through a valley before it gets to the discharge point, I'm
picturing a "high point" in the line, which will only allow
you flow when you bleed the line at what you call the "low
point" in the line, which in fact, is probably a "localized
high point".

Make any sense?

Milldude