In the last two videos, we’ve looked at phenomena that cause high-pressure spikes in pipes. But a lot of people pointed out that very low pressure in pipes can be just as dangerous.
If you watched the water hammer video I made a few months back, you’ll know that slamming a valve shut on a flowing pipe can cause a huge spike in pressure. That’s because the fluid inside a pipe has a lot of momentum, and fluids aren’t compressible enough to absorb sudden changes in velocity. Spikes in pressure aren’t always bad, but they can be dangerous if a pipe bursts or expensive by requiring stronger pipes with higher pressure ratings.
But in that video, I didn’t talk about what happens on the other side of the valve. So, I’m revisiting that demonstration with a few modifications so we can get the full picture. Here’s the setup: valve, clear pipe, pressure gage, more clear pipe, 50-foot garden hose, tree. The tree’s not important but I don’t want anyone to think I’m wasting this water. You won’t be surprised to learn that flowing fluid in a pipe downstream of a valve also has momentum, and that fluid also has a hard time stopping without a big fluctuation in pressure. But, unlike upstream where the momentum is carrying the fluid toward the valve, on the downstream side, the fluid is trying to flow away from it. So, the spike in pressure is negative - in other words, it creates a vacuum.
You may have noticed something different about this pressure gage. It only measures pressures that are below atmospheric - it’s a vacuum gauge. Watch what happens when I slam this valve shut. We get a very strong vacuum in the pipe, and then some fluctuations as the pressure wave propagates back and forth through the pipe. The momentum of the fluid in the water hose is pulling away from the valve. That fluid tension sharply lowers the pressure in the pipe. This trapped bubble gives a pretty good indicator of what’s happening as well. This is pretty far from a laboratory setting (no offense to the backyard scientist), but I’m seeing a peak of more than 30 inches of mercury or 100 kilo-pascals below atmospheric pressure. That’s a lot of vacuum. In fact, it’s enough to pull dissolved gas out of the water.
Take a look at the spot just downstream of the valve when I slam it shut. A spontaneous cloud of fine bubbles forms as the vacuum pulls. This is dissolved gases coming out of solution with the water. When the pressure returns, the bubbles shrink, but they don’t immediately go back into solution with the water, so you can still see a light haze in the water, especially when I turn the valve back on. Very cool in this demonstration, but bad news if your pipe wasn’t designed to withstand these types of pressures. Just like positive pressure spikes from water hammer, this phenomenon has caused numerous failures of pipe systems from implosions due to vacuum.
So, how can this be avoided? If the risk of failure is significant, like for very large pipelines or costly equipment, engineers will specify vacuum relief valves that will allow air into the pipe if the pressure gets too low, reducing the vacuum to protect the equipment. But, the simplest solution is the same as discussed in the other water hammer video: avoid sudden changes in velocity. Ask any firefighter and they’ll tell you: you gotta close valves slowly. You still get a vacuum downstream, but much less of one. Hope you liked this quick follow up. Thank you for watching, and let me know what you think!