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
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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 thanks to hello fresh for supporting
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