In the last two videos we’ve looked at phenomena
that cause high pressure spikes in pipes. But a lot of people rightly pointed out that
very low pressure in pipes can be equally as dangerous. Hey I’m Grady and this is practical engineering. On today’s episode, we’re revisiting the
water hammer video to take a look at negative pressures. 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 just 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 downstream of the valve. 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 indication
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 kilopascals 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! This video is sponsored by viewers like you. Practical Engineering is not a high budget
enterprise, but it does cost money to buy these materials and hire animators to make
these fancy graphics. These are the folks making this channel possible
through Patreon, and I just want to give them a big thanks for giving me the coolest hobby
ever: making videos that hopefully give the whole world a glimpse into the fascinating
world of engineering. I’ve got a lot of exciting projects planned
for 2018, so make sure you subscribe to the channel if you haven’t already. Thank you for watching, and let me know what
you think!
Anybody else disappointed by the lack of a collapsing pipe in the video?
r/firefighting