What is Air Lock?

Engineering nearly always involves
assumptions and simplifications there are just too many variables in the real
world to keep track of them all so we simplify we neglect the variables that
don't matter and we make assumptions about the variables we can't measure or
predict but what happens when one of those assumptions is wrong one of the
most basic assumptions made by engineers who design pipelines is that those
pipelines only carry the fluid that's intended but that's not always the case
hey I'm Grady and this is practical engineering on today's episode we're
talking about airlock in pipe systems this video sponsored by hellofresh
America's number one meal kit is it hell of fresh calm a use code practical 10
for 10 free meals more on that leader put simply airlock is a constriction and
flow that happens when gas gets trapped inside a pipe that's the answer to the
title of this video but it's not very satisfying on its own in fact if you're
as curious as I am it just leads to more questions the first three that come to
mind are where does the gas come from how does it get trapped and why should I
care we don't normally get to see inside pipelines and observe how they work so I
built a little model here in my garage we can use to talk about air lock how it
happens and why it matters the first question is where does the gas come from
it might surprise you to learn that getting gases like air in liquid
pipelines is somewhat inevitable sometimes they sneak in by being
dissolved into the liquid like carbon dioxide is dissolved in a coke most
liquids have at least some dissolved gases even the water coming out of the
tap often has a certain amount of dissolved air this gas can come out of
solution when the fluid is warmed or agitated or if it goes through a
chemical reaction another potential source of gases in liquid is leaks
through damaged areas or loose fitting joints if these occur in an area of the
pipe with a pressure below the ambient air pressure air can leak from outside
the pipe into the line but I haven't mentioned the most obvious source of air
after all when you buy a pipe from a manufacturer it doesn't come pre filled
with liquid it starts out empty or more accurately
it starts out full of air when you add liquid to a pipe that's full of air
whether it's for the first time or after the pipe was drained for maintenance
that's a perfect opportunity for it to be trapped which leads me to the second
question how does gas get trapped this one's a little easier to answer because
gases are so much less dense than liquids they almost always float that
means any high spot in a pipe is susceptible to trapping bubbles and
unfortunately avoiding these Heights is often easier said than done take the
example of an irrigation line on a farm these lines can't be buried because they
need to be moved from time to time so they sit on the surface of the ground
and as such follow the natural contours with low spots and valleys and high
spots over hills and embankments these high spots are perfect traps for air
even if the pipes can be buried like water or petroleum pipelines it's not
always feasible to avoid undulations after all the deeper you dig the higher
the cost often it just makes sense to follow a ridge or hill up and back down
rather than going straight through in buildings and houses fresh water and
heating lines have to avoid all sorts of obstacles which often means routing them
in ways that create high spots which can trap air bubbles the same is true in
industrial settings for a wide variety of types of pipelines
you might be thinking big deal air gets trapped where it's not supposed to all
the time that's why we have burps and farts and
bleed valves on brake lines but the thing you have to remember is that air
takes up space it doesn't necessarily seem like it out
in the open but when it's trapped in a pipe it's taken up cross-sectional area
that could otherwise be used by flow it's a constriction just like a kink in
a rubber hose which means it can cause a serious reduction in flow rate pipes can
be expensive and the bigger they are the more they cost so engineers try to use
the smallest pipe possible to meet the specific need if you've got a bunch of
air trapped in your pipe that's taking up valuable space without any
contribution to the flow rate designing pipes is an exercise in managing energy
the fluid starts at one end with a certain amount of it and the flow rate
depends on how much energy gets lost as it makes its way to the other end
Engineers use a graphical tool called the hydraulic grade line to show this
visually the line represents the potential energy available in the fluid
at any point along the pipe it's also the level the liquid would reach if you
were to tap in a vertical stand pipe at any location along the pipe the
hydraulic grade line slopes downward along pipes as the fluid loses energy to
friction it also drops steeply at sharp bends and vows which can cause
turbulence in the flow and you know it also causes a loss of energy airlock in
fact as the bubble grows and grows in the pipe you end up with a condition
called waterfall flow you can see why it's called that in the demonstration in
this case you lose the energy equivalent to the height of the waterfall which is
easy to see on the hydraulic grade line unlike friction or turbulence in the
pipe this doesn't depend on flow and it adds up every undulation in a pipe with
a trapped bubble of air is going to rob the fluid of this energy and if the
hydraulic grade line drops below the outlet of the pipe you won't get any
flow at all that's the definition of airlock also called vapor lock a pipe
that doesn't flow is not very useful so we've come up with a bunch of ways to
deal with this problem the simplest but not necessarily the
cheapest is to deal with the airlock with a bigger pump you can be okay
knowing you'll always have trapped gas in your pipe if you can use more
pressure to overcome the energy losses associated with airlock that's not
always feasible though consider a long pipeline with lots of undulations if you
use a single pump to overcome all that air lock the pressure rating of the pipe
near the pump will have to be enormous the second option is just to design
pipelines that don't trap air if the flow of the fluid in your pipeline is
fast enough trapped air will just be blown out and if there aren't any high
spots in your pipes there won't be anywhere for it to be trapped in the
first place again that's not always feasible consider a pipeline moving
water from one end of a hill to another drawing a straight line between points a
and B is easy but digging a trench this deep to install the pipe or worse
tunneling it is not an inexpensive endeavor the other option is to bleed
the gas through a valve pretty simple in some cases but not necessarily in all
cases cities don't want to send out technicians to bleed the air out of
pipelines every day so many pipelines are equipped with automatic air release
valves these are a simple but clever solution for releasing air from the high
points without any human intervention I built an example of this by gluing the
float to a check valve when there's no air in the pipe the float holds the
clothes but when a big enough air bubble grows in the pipe the float acts like a
weight and pulls the valve open venting the air from the pipe
keep an eye out for these type of valves when you're perusing the constructed
environment and now you'll know how they work the job of an engineer is to take
the signs and knowledge we have and apply that to design completely new and
sometimes untested systems it almost always involves making assumptions and
if you make bad assumptions you get bad answers and ultimately bad designs
that's certainly true for airlock where if you assume that gases don't get into
pipes or that they can't constrict the flow you might design a pipeline that
doesn't work luckily for engineers this is a well-known phenomenon in pipe
systems it's just one of the many complexities that come with the job and
we've developed a lot of creative ways to overcome it
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