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 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 carry only the fluid that’s intended. But, that’s not always the case. Hey I’m Grady and this is Practical Engineering. Today, we’re talking about air lock in pipe systems.

Put simply, air lock is a constriction in flow that happens when a gas gets trapped in a pipe. That’s the answer to the title, 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: (1) Where does the gas come from? (2) How does it get trapped? And (3) 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 airlock, 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 gasses, like air, in liquid pipelines is somewhat inevitable. Sometimes they sneak in by being dissolved into the liquid just like carbon dioxide is dissolved in a coke. Most liquids have at least some dissolved gasses. 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 gasses in liquid pipelines 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 pipe from a manufacturer or supplier, 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 gasses 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 high spots 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, following the natural contours with low spots in 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 hill or ridge 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 taking up cross sectional area that could otherwise be used for flow. It’s a constriction, just like a kink in a rubber hose, which means it can cause a serious reduction in the 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 that the liquid would reach if you were to tap in a vertical standpipe 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 valves which cause turbulence in the flow. And, you know what also causes a loss of energy? Air lock. 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 or vapor lock.

A pipe that doesn’t flow is not very useful, so we’ve come up with a bunch of ways of dealing with this problem. The simplest, but not necessarily the cheapest, is to just deal with the airlock with a bigger pump. You can be okay knowing that you’ll always have trapped gasses in your pipe if you can just 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 airlock, 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, this is not always feasible. Consider a pipeline moving water from one end of a hill to the other. 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 their pipelines every day. So, many pipelines are equipped with automatic air release valves. These are a simple but clever solution for releasing air from high points without any human intervention. I built an example of this by gluing a float to a check valve. When there’s no air in the pipe, the float holds the valve closed. But when a big enough 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 types 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 science 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 air lock, where, if you assume that gasses 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 complexities that come with the job and we’ve come up a with a lot of creative ways to overcome it. Thank you, and let me know what you think!