You might know that most liquids are incompressible
(or least barely-compressible), which means no matter how much pressure you apply, their
volume doesn’t change. This can be really useful, like in hydraulic
cylinders, but that lack of “springiness” can also lead to catastrophic failure of pipe
systems. Hey I’m Grady, and this is Practical Engineering. On today's episode, we’re talking about
hydraulic transients, also known as Water Hammer. It’s easy to forget how heavy water is,
since we hardly ever carry more than a few ounces at a time. But if you add up the water in the pipelines
of your city or even just the pipes in your house, it makes up quite a bit of mass. And, when all that water is moving through
a pipe, it has quite a bit of momentum. If you suddenly stop that movement—for example,
by quickly closing a valve—all that momentum has nowhere to go. Since water isn’t compressible or springy,
it can’t soften the blow. You might as well be slamming concrete into
the back of the valve and the walls of your pipe. Instead of being absorbed, that sudden change
in momentum creates a spike in pressure that travels as a shockwave through the pipe. Sometimes, you’ll even hear this shockwave
as banging in your walls when you close a faucet or run the washing machine, hence the
superhero-esque nickname, Water Hammer. Banging pipes inside your walls can sound
a bit spooky, but for large diameter pipelines that can be hundreds of kilometers long, that
surge in pressure from a change in momentum can cause major damage. Let’s do a quick calculation: if you have
pipeline carrying water that is 1 meter in diameter and runs for 100 kilometers (a fairly
average-sized pipeline), the mass of water in the pipe is about 80 million kilograms. That’s a lot of kilograms. In fact, it’s the equivalent of about 10
freight trains. Imagine you’re an operator at the end of
this pipeline in charge of closing a valve. If you close it quickly, you’ve essentially
slammed those trains into a brick wall. And the pressure spike that results from such
a sudden change in momentum can rupture the pipe or cause serious damage to other parts
of the system. There’s actually another term for when a
large spike in pressure ruptures a sealed container: a bomb. And water hammer can be equally dangerous. So, how do engineers design pipe systems to
avoid this condition? Let’s build a model pipeline and find out. Here’s my setup. I’ve got about 100 feet (30 meters) of PVC
pipe connected to the water on one end and a valve on the other. I also have an analog and digital gauge so
we can see how the pressure changes and a clear section of pipe in case anything exciting
happens in there. I mean civil-engineering-exciting, not like
actual exciting. Watch what happens when I close this valve. It doesn’t look like much from the outside,
but look at the data from the pressure gauge. The pressure spikes to over 2,000 kilopascals
or 300 psi. That’s about 5 times the static water pressure. It’s not enough to break the pipe, but way
more than enough to break this pressure gage. You can see why designing a pipeline or pipe
network can be a little more complicated than it seems. These spikes in pressure can travel through
a system in complicated ways. But we can use this simple demonstration to
show a few ways that engineers mitigate the potential damage from water hammer. This is the equation for the pressure profile
of a water hammer pulse. We’re not going to do any calculus here,
but the terms of this equation show the parameters that can be adjusted to dial back these damaging
forces. And, the first one is obvious: it’s the
speed at which the fluid is moving through the pipe. Reducing this is one of the simplest ways
to reduce the effect of water hammer. Velocity is a function of the flow rate and
the size of the pipe. If you’re designing a pipeline, the flow
rate might be fixed, so you can increase the size of your pipe to reduce the velocity. A smaller pipe may be less expensive, but
the flow velocity will be higher which may cause issues with water hammer. In this case, my pipe size is fixed, but I
can reduce the flow rate to limit the velocity. Each time I reduce the velocity and close
the valve, the resulting spike in pressure decreases. Next, you can increase the time over which
the change in momentum occurs. One common example of this is adding flywheels
to pumps so they spin down more slowly rather than stopping suddenly. Another example is just to close valves more
slowly. If I gently shut the valve rather than allowing
it to snap shut, the pressure changes are more subtle. On large pipelines, engineers not only design
the components, but develop the requirements for operation of the equipment. This will almost always include rules for
how quickly valves can be opened or closed to avoid issues with water hammer. The final parameter we can adjust is speed
of sound through the fluid, also known as the wave celerity. This describes how quickly a pressure wave
can propagate through the pipe. The wave celerity is an indirect measure of
the elasticity of the system, and it can depend on the compressibility of the fluid, the material
of the pipe and even whether or not it’s buried in the ground. In a very rigid system, pressure waves can
reflect easily without much attenuation. I’ve got flexible PVC pipe sitting on the
ground free to move which is already helping reduce the magnitude of the water hammer. I can increase the flexibility even more by
adding an anti-surge device. This has an air bladder that can absorb some
of the shock and reduce the pressure spike even further. Anti-surge devices are very common in pipe
systems, and they can be as simple as a spring-loaded valve that opens up if the pressure gets too
high. In water distribution systems for urban areas,
water towers help with surge control by allowing the free surface to move up and down, absorbing
sudden changes in pressure. Plumbing is one of the under-acknowledged
innovations that has made our modern society possible. When you harness the power of water by putting
it in pipes, it’s easy to forget about that power altogether. Water can be as hard as concrete when confined,
and if you bang two hard things together, eventually something’s going to break. If you’re an engineer, your job is to make
sure it’s not the expensive infrastructure you designed. Part of that means being able to predict surges
in pressure due to water hammer and design systems that can mitigate any potential damage
that might result. Thank you for watching, and let me know what
you think! Thanks to Blue Apron for sponsoring this video. Blue Apron delivers all the fresh ingredients
you need, right to your doorstep, in exactly the right proportions to create delicious
recipes at home. We are really loving it at our house, and
having a lot of fun cooking these meals together (not to mention eating them). If that sounds like something you'd be interested
in, the first 100 people that click the link in the description will get 3 meals free with
their first order. Again, thank you for watching, and let me
know what you think!
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Title should be "What is Water Hammer." Your titles' use of "a" implies an object, not a phenomenon; and made me expect something like this water hammer.
Something i sort of knew intuitively, but never actually sat and thought about. Never would have thought it could be an issue.
Man, there is so much that goes into things we use on a daily basis. Fascinating. Thank you, OP.
Great channel
Big scale: Surge chamber of a dam being flodded because 60 tonnes of water per second need to be stopped due to a routine switch of of the three turbines.
It's fascinating that out society has people smart and diligent enough to study things like math, engineering etc.
sobs in stupid
I'm not sure, but I've heard the Children of the Forest could tell you what the Hammer of the Waters is, if you can find them.
I have seen large diameter piping blow open at the flanges due to water hammer during ramp up. Huge clean up, lots of yelling at each other, lawsuits to decide who was going to pay for the fuck up.
All and all though it wasn't too bad, no injuries, the fluid was essentially a slurry of ore and some nasty stuff that might give you a mild chemical burn.
Furnaces going positive or running out are scarier. The scariest is still high pressure gas. Incompressible fluids are an engineer's friend.
Well... I just learned the purpose of water tower. Fascinating