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 thanks to hello fresh for supporting practical engineering this is not a cooking channel and I'm an engineer not a chef but one of my wife's and my favorite thing to do is spend time together in the kitchen hello fresh delivers seasonal recipes and pre-measured ingredients right to your door it turns the chore making dinner into something that we look forward to all day oh my gosh it's like somebody's been stirring in that pot with a fork the pre-portion ingredients means there's less prep and less food waste I want you to do mine in the shape of a heart and our favorite part is that the recipes are things that we either haven't tried before or wouldn't have chosen on a normal meal plan things are getting here if that sounds like something you'd like to try hellofresh is offering a special discount to fans of the channel visit hellofresh calm and use code practical 10 a check out for 10 free meals including free shipping yeah pour it really slow and seriously if you sign up try to get this bibimbap recipe because it is one of the best meals that has ever come out of this kitchen oh they're so pretty again that's hellofresh calm and use code practical ten thank you for watching and let me know what you think

Channel: Practical Engineering

Views: 1,923,800

Rating: 4.9289756 out of 5

Keywords: air lock, pipes, piping, water pipes, valve, air release valve, irrigation, gas, trapped gas, petroleum pipes, hydraulic grade line, bleed valve, pump, aeration, kinetic energy, hydropower, hydrology, fluid, air pressure, Practical Engineering, Civil Engineering, Engineer, Grady

Id: zdkp9N3qfkI

Length: 9min 45sec (585 seconds)

Published: Tue Feb 11 2020

Reddit Comments

[u/Leaf_Rotator]

Grady is the best!

[u/redditadminsguzlecum]

What about viscous fluids where the ball on the float valve would get stuck?

[u/jovejq]

As an electrician, I'm wondering what, if any, the flowing of liquids in pipes has in common with the flow of current in a conductor. Is there such a thing as, an airlock in the flow of electrons.

[u/pastiehands]

Ham Grady

[u/ratingus]

The concept of the hydraulic gradeline is simliar to the link budget of a telecommunications system. Always interesting to find these similar ideas in all areas of engineering.

[u/americanmiddleclass2]

Try that garbage in a sewer Force main