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What is Air Lock? — Practical Engineering

What is air lock?

Air lock is a constriction in flow that happens when a gas gets trapped in a pipe.

Where does the gas come from?

Gasses, like air, in liquid pipelines is somewhat inevitable. Sometimes they sneak in by being dissolved into the liquid. 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. 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.

How does gas get trapped?

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. 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. 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.

Why should I care?

Air takes up space. 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

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. 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.

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. 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. The other option is to bleed the gas through a valve. 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.

Conclusion

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.