hi I'm Mike Crowley and today at fluid
mechanics i'm going to explain waterfall
flow in pipes. Waterfall flow occurs when
you get gas such as air trapped in the
top of pipes which restricts the flow
usually this is going to be occurring in
the high points of the pipe.
it's a special case of two phase flow.
Two phase flows when you've got gas and
liquid traveling along a pipe in the case
of waterfall flow some of the gas
is stuck stationary usually at the high
points.
This can lead to noise pressure loss in
the system and certain systems that can
lead to unbalance flow. In this lesson
today I'm going to explain when this is
likely to occur and how to calculate
its effect. And I'll also explain some
solutions to the potential problem. I
will explain this phenomenon shortly
at Fluid Mechanics.
In most hydraulic systems there will
always be some gas, of course the most
common type of gas is air in water.
I suppose that's 90% or at least over half
the cases this is
where waterfall flow will occur. The gas
will collect in the high points forming
pockets of gas, these pockets cause the
restriction.
This reduces the flow area and increases
the pressure loss and increases noise. In
this lesson I will show you how to
identify when there is a potential
problem with waterfall flow and how to
calculate the pressure drop. This sketch
shows a typical hydraulic system or
fluid system we have a tank with fluid
in it with a pipe connected to a pump
and then the pipe goes along
horizontally then up, horizontal, down,
horizontal and then up to a header tank.
At this point here when the flow starts
to go down the actual flow has
got an a negative flow gradient so in other
words the flow is going into downwards
and it's a negative gradient. It's at
this point here that the there's
potential for gas to be trapped at that
at that point there. Where can the gas
come from, the gas can be in the in the
pipework before you fill up up the
system, so in other words when you first
prime the system that could be gas in
there, and you haven't got rid of it all.
Or alternatively you could get gas
entrained in the system coming out of
the fluid so if you've got a tank here
and the suction to the tank you could
actually be sucking gas in
from the air or
alternatively if you've got a
hydrocarbon, it could be oil you could have
gas or hydrocarbons in
there and and hydrocarbon gases they
could come out of solution. If you've got
high temperatures, water could actually
vaporized I suppose that that's another
another type of gas bubble that could
actually form at the top of the the pipe.
Or alternatively if the pipe is very
high then the pressure drops
and gas can come out of solution
there's lots of reasons why the gas
could come out of solution and what
you'll find is there is potential for
the gas to get trapped of these high
points here. Now in this sketch I've
shown orthogonally drawn
pipes going up, across and down but even
if the pipes are very shallow angles
there is still a high point there is
still a negative flow gradient and at
the high points there is potential for
gas to get trapped. So if we move on a
bit,
look at the next slide. And what I'm
trying to show here is, what pressure or
head you're going to require to drive the
fluid along the pipe. So typically
for a system like this,
the tank where you're taking the fluid
is at one level here, and it's pumping
the fluid up to a higher level here and
the head difference between the two
tanks is h1. For a given flow rate along
that pipe there will also be a friction
head loss along along the pipe and
if we come down to this formula here the
total head that the pump has to deliver
or the system has to deliver, to pump a
given flow along the pipe, H is h f plus
h1. Now normally with water
hydraulic systems you talking terms of
head for other types of people they
might be wanting to talk in terms of
pressure. Pressure or head is much the same
thing, it's just
different types of units. If we did this
in terms of pressure when the pressure
would be the friction pressure loss due
to friction okay plus ρ g h in other
words, the density of the fluid x the
gravitational constant x h1 and for
water the density would be around about
a thousand the gravity is 9.81.
So moving on a bit further to the next
slide and we can then see what happens
when we actually get an air bubble in
the system. Initially and i'm showing the
the air bubble just there, I say air
bubble, gas whatever the
gas is, so for air, any gas its
same effective thing. We have an air bubble
there, and it's collected at the the high
point. At the start of the negative
pressure gradient. So what determines
whether the bubble will stopped there or
be moved along by
the fluid. So in other words as the flow
is coming along the pipe and tries
to push it down the the negative
gradient the bubbles got its buoyancy
in its trying to come back up
ok. So there is a balance between the
rate at which the fluid is going down
the pipe and the rate at which the
bubble wants to come up and if the rate
at which the bubble wants to come up is
greater than the fluid going down then
the bubble is going to stay up at the
top. If you got high enough velocity the
bubble just be pushed pushed on and you
won't get this phenomenon. So what
things affect whether you're going to
get the bubble there,
well obviously the first thing is the
velocity of the fluid in in the pipe.
The next thing is the viscosity of the
fluid obviously we've got a higher
viscosity fluid the rise rate of the
bubble is going to be lower so it's
going to be move along. In this
particular case here I've showed a
vertical pipe going down but the angle
of the pipe is obviously very important
and vertical pipe is the worst-case
scenario but of course you could have us
a very shallow angle pipe. It could still
rise up to the top even in a very
shallow raised and pipe but it's less
likely to. Other things affect the
bubble is the
density of the fluid water is usually
one of the more density fluid
and least viscous so that's
why is probably more likely to
happen in water
pipes than any other type of pipe. The
other thing that affects it is
the pipe size. Trying to
model or determine beforehand whether
you're going to get this bubbles coming
collecting at that point is actually
quite difficult it is a bit down to
experience and knowing what the velocity
is down here. So what what people should
try and do at the design stage is first
of all trying to avoid this type of
negative flow graders or try and make
sure that the pipework is always on the
rise. Now that's easier said than done.
If this was a water main going over a hill
or something,
what you could
try and do is have a gradual
incline so instead of going up and over
try and go straight across, easier said
than done. And in lots of cases you just
have no choice you have to go up and
down. So the next thing to ask yourself
is, is it a problem. I mean if you're
only dropping a short distance well
maybe you can just live with that, and
i'll explain shortly how
to calculate what the pressure losses is
due to the bubble. Now if the air
bubble is stable there, what's going to
happen is that the
air or gas bubble is going to
start to grow. So if i move on to the
next slide what will happen is that
you have the bubble and it will grow. So
as more gas comes along it
will start to collect at the at the high
point and the bubble will start to grow
the air bubble will continue to grow until
the bubble is large enough to get to the
bottom here to this position and then it
will stop growing. Any excess air
or gas that comes along then
well then just be swept along and
effectively this is as big as the bubble
can get. So now the term waterfall
flow or why they use the word
waterfall flow is more obvious. What's
actually happening is you got the air
bubble there and the flow is dropping
down there, effectively that's why it's
called
waterfall flow. And the energy loss in
the system is actually the head
difference from there to there. This
energy loss or head loss and and the
fact that it's called waterfall flow is
true even if the pipe is inclined at an
angle. If the pipe is inclined at an
angle and there is a bubble of air on
top of the the flow as it runs down the
incline the energy is lost and you still
lose that amount of head. So
always in waterfall flow cases
the energy loss is effectively the
level change, whether it's inclined pipe
or whether it's a vertical pipe like
I've shown in this in this example. If we
now look at the formulas that we had
previously for the
pressure and the head
there's a new term to add to the head
formula, basically the head now has this
additional term added,
h2 or in terms of the pressure you've
added this additional term which is ρ
g h2. This increase in head or pressure,
it all depends on how the system is
designed but if you had a centrifugal
pump there what that would mean is that
you get slightly less flow and slightly
increased pressure
it all depends on the type of pump and
the type of system but it will
definitely, introduce another
energy loss in the system, and it's
likely to increase the pressure loss and
reduce the flow. So moving on to the next
sketch. What can we do about this is air
bubble or how can we mitigate it. As I
said we could try and design it out. But
if we have got an air or a
gas bubble that we need to control,
the only real way to do is you've got to
basically put a valve at the high point
and bleed the gas off. Commonly this two
ways of doing this I've shown two types
of valve one on top of each other but
the first valve there is just a
simple isolation valve and basically you
just bleed
gas out with a manual valve. This is
similar to the sort of thing
that you might see on a radiator a bleed
valve these types of valves are very
useful when you're trying to prime a
system, where it's the first time and
then once you've got the air out you
know no more air will be introduced
into the system but if there's a
continual supply air coming into the
system, well then that's not much good.
And if you need to get rid of the air
and you can't live with the problem, then
what you have to do is put in an
automatic air vent valve and this is
shown on top. Now normally when you
haven't an automatic air vent valve in
front of that you have an isolation
valve as well because sometimes these
can be these type of valves can be
problematic and start to leak. So they
usually have an isolation valve and then
an air vent valve.
inside the air vent valve
there there's a little float and when
there's gas in the chamber at the top
there,
the float sinks and the gas can escape
and then when the chamber is
full of fluid the float
rises and blocks off the flow.
These air vent valves are
not suitable in all cases it is possible
that the pressure inside the pipe there
is lower than the external atmospheric
pressure in which case when you open the vent,
air and will come in
rather than go out. So you just got to be
a bit little bit careful of using these
type of valves. In terms of living with
this situation you got to ask yourself
well you know maybe the pressure lost
you to h2 is not significant.
So therefore what we just allow the pump
and design the system
to cope with it.
and just accept the fact that you've got
a pressure loss at that part of the
system.
So if I show the next slide to finish
off. What i'm trying to show here is the
long pipe work you might get for a
water main or something like that. Or an oil
main and this pipeline could be
tens of kilometers long and what you'll
tend to find is the pipe may have to go
over many inclines or small hills.
Following the contours of the
land and so effectively the pipe will go
up and then go back down again and this
increasing level here, could be 10m or 20m.
And of course if you've got a
number of these hills
that's going over, if you don't do
something about the about the gas that's going
to collect in the top of the pipe then
you're going to have head losses
added onto head losses, added onto
head losses. So generally in those type
of situations people
usually do put in automatic air
vent valves. In water mains typically the
flow velocity is actually a relatively
low. In long mains they
try to keep the pressure
losses down, so the flow velocities are low
and there is significant chances of gas
collecting at the top, and
the diameters are quite large as well. So
in summary trying to avoid counter
gradients or negative flow gradients
where you can. If you can't avoid them
well then maybe you can just live with
them. Or you need to calculate the
pressure loss due to the at the
waterfall flow or the counteragent and
I've shown in the video how to actually
calculate that. If you have any general
questions on this please leave a comment
on my website and I will get endeavor to
answer them on my blog that is. I
won't be able to answer any general
questions directly but I will endeavor
to answer them if you leave comments on
my website and my block. If you need
detailed hydraulic advice on a
consultancy basis then please contact me
directly.
My contact details are again given on my
website.
That's it today from fluid mechanics
thank you for listening