What is Water Hammer?


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!

100 thoughts on “What is Water Hammer?

  1. In short it's conservation of momentum. And it's actually pretty easy to deal with, if you apply an accumulator to the plumbing, ie, a short section of pipe that has an air pocket in it which resides above the plumbing system "water table". That space acts as a shock absorber, allowing the in-rushing water somewhere to go while the transience subsides. In industrial systems, pressurized bladder are often used to deal with these transient effects. It's one thing when you're dealing with water in a home and the pipes vibrate a bit after the water is shut off, it's another when you have a system delivering fluid with pressures up to thousand of pounds of pressure and the flow is suddenly shut off… as in the lack of an accumulator can cause a life threatening catastrophe (I've worked with test systems operating at well over 10K PSI and substantial flow rates that require 3" diameter hoses)

  2. It’s not often that I comment on YouTube, but I strongly believe in giving credit when credit is due. Your videos are excellent. Not only are they thorough, they are engaging and encourage a greater understanding of the world. Keep up the good work.

  3. Referring to your annotated equation at 3:52; the "dv" should be labelled as "change in velocity" rather than as "velocity", so that the "dv by dt" (of which it is part) represents acceleration (acknowledging that these are partial derivatives taken in the appropriate direction from the three dimensional reference frame). The reduction in the transient pressure spike, which follows your reduction of starting velocity, follows from the reduction in rate of change of velocity (with respect to time) rather than reduction of absolute initial velocity. The rate of change of velocity is less because the velocity falls to zero (over the same period) in both cases but from a lower starting value in one of them. Great video though 🙂

  4. I get a repetitive hammering when I have my bathroom faucet running – – not when I shut it off but while it's been running about a minute or so.

  5. Fella you seem to be discussing the inertia and later loss of inertia translated into force or motion . My theory is that you can fill pipes up with spring attached fixed soft balls to reduce the sock wave as it would be absorbed by the material and negative interference would happen too .

  6. What hammer can also be caused by lack of water in the pipes, and then suddenly turning the water on full blast. Our MUD (Municipal Utility District) was fixing a water main at 2:00 in the morning and turned on a water pump after the repair. The water pressure destroyed our sediment filter and water softener!

  7. There's this pressure gauge at the water supply line to an autoclave in the hospital where I work as a biomedical engineer. This pressure gauge breaks every couple of months because of the same effect explained on this video. Every time the water supply valve shuts off you can see the gauge's needle suddenly jumps to maximum and backs to normal pressure. The gauge is screwed to a coiled piece of pipe that's supposed to absorb the water shock effect but I guess the coil is too short.

  8. I've been teaching apprentices this theory for 40 years (or trying to) but these days very few engineers understand the importance of pipe sizing and the important result of velocity reduction to a maximum limit. That just leaves the two of us who seem to care. Keep up these brilliant videos. Thank you.

  9. This guy is so irritating me and talk so much, stop showing your ugly face and show us the water hammer process in short term.

  10. I love when I don't understand something and find your videos at the top of the results. It's usually the last video I need to watch, even though I'm no engineer.

  11. Excellent, Grady! Thank you for helping us understand this and all of the other concepts you've made videos for.

  12. this has GOT to be a pretty big concern when developing oil/crude/natural gas pipelines.
    on those scales, it must be pretty difficult to manage water hammer. especially as the materials of the pipes degrade over time.

    one more reason to not like oil pipelines lol

  13. Water hammer in a high temperature-high pressure pipeline system connected to high pressure pumps is a different story all together.

  14. I worked in a high rise that had a sort of shock absorber to accept the excess back pressure of a water hammer situation. On various floors there was a tee in the pipeline with the absorber mounted on this tee within the wall of the building. Problem was that they were made in 1962 and with age the plastic deteriorated and became brittle. The back pressure would force a diaphragm against a spring compressing it within this cylinder. Eventually the housing would crack wide open flooding the building. Preventative maintenance dictates replacing these before the end of service life. However the blueprints of the building which showed exactly where these were within those walls had been lost. So they actually had to wait until one blew to find and replace it. The new units were made of stainless steel. Glad I don't work there any more.

  15. I don't really think that the nursery level "music" adds anything to this video. What is its purpose other than to irritate??

  16. The most interesting example of this that I've come across is in sliding boat lifts that work by moving a wedge of water up a sloped concrete channel. At the top of such a system there has to be a lock gate to keep the upper channel contained. Water hammer from the moving water section can damage this, so ridged channels running off to the sides are used to dissipate the water's energy when it reaches the top. As you can imagine, there are quite a lot of other problems to be managed in such a system.

  17. Okay. This was totally nerdy. Yet, I was fascinated enough to watch it through and actually learn a few things. Heck, I might even name my next mighty World of Warcraft character Water Hammer! Now, I feel all … nerdified.

  18. Great video, I always wanted to know what water hammer was and you explained it in simple easy to understand terms, many thanks.

  19. I live in England and work for Essex and Suffolk water company and we are taught about operating valves and pressure spikes. We have a test rig to simulate pressure surges when valves are operated and are taught that pressure shocks can damage the main if it is isolated too quickly. Good video.

  20. Tighten the valves and make it very hard for someone to open them. Not only someone will not be able to close the valve quickly but also it will make it less likely for accidents to happen

  21. Just curious now why oil is used in hydraulics rather than water? Surely it can’t be because of corrosion, or can it?

  22. The last part of this video talking about the water towers reminds me of how a Ram Pump is able to work!

  23. While watching your clip, I just recalled my graduate course of Chemical engineering, Fluid Dynamics where I learned and observed the phenomenon of Water Hammer. I do remember, our lab engineer urged us to pay special attention while working in the process industries.

  24. I would tell my wife there was a man working under our house when the pipe would knock, at 3 in the morning.

  25. I would love to discuss this but I have no where near your education. I did service plumbing before i got sick, i estimated that i worked in over 50k homes.
    The code officials wanted backflow preventors & sometimes check valves. That takes away from the pressure going back into a water tower or even just back from the house to the city main. .
    80s when i started they wanted air chambers at the fixtures 18" of dead pipe aftre the tee coming out of the wall, or along side of the shower valve. Then they said( 10+ years later) if you do work and see these dead ends, that they need to come out because ot causes stagnant water. They came up with the little air hammer arresters (little blatter device) they work ok but need to be exposed for replacing.
    Meanwhile the city pipes are well over 100 years old, they were never sized for the population wich some palaces increased 10+ times. Their solutions was increase the city pressure. Some places I saw 160psi. To me it's crazy to expect the lines to keep holding. We were also told max safe house pressure should be 75 psi. What would the street size need to be if the line was actually kept a constant 75 psi, so that every single resident wouldn't need the PRV??

  26. Ya know, I never even heard of water hammering till now. But no doubt, I can see the importance of closing valves slower.
    I have a feeling that pipes that have suffered from hammer due to rapidly closing of vales have bulged out valves so that even when theyre closed slowly, the bulge is going to slam it shut the rest of the way. Am I right on that?

  27. All this was taught to us during firefighter training when using hydrants imagine the weight of water moving through an 18 inch town mains pipe ,could be catastrophic…..🇪🇸

  28. Where is the best place to install a water hamer? Behind the ball valve, are does it really matter.? Residential-Rigid line.

  29. We had one, someone stealing water, slammed a ball valve, blew apart four 8 inch 90's. The whole town heard the thump.

  30. I work in the fire sprinkler industry and on rare occasions I have seen unexplainable damage occur to sprinkler heads. I suspected that a water hammer was contributing to this issue, however unable to prove it. Based on your data and under the right conditions it appears a water hammer can easy exceed 500 psi, which exceeds the factory testing requirements for fire sprinklers. If you could please attach a link of the pressure transducer and associated equipment your using in this video. Thank you !

  31. what kind of digital pressure gauge did you use? I am in search of one to combine with my raspberry pi…

  32. No it is NOT 80 million Kg it's 80 thousand tonnes. Why do you Americans always insist on using small units for big values? You use lbs instead of tons, feet instead of yards or miles – it's rediculous!

  33. First thing thank you for taking your time and making a video I just started Plumbing three months in you have helped my understanding with the water hammer

  34. Another perfect example of the way to control water hammer in power production by dams on rivers, is by providing a surge chamber to absorb the "stop shock".
    this is a large vertical open column which allows water to rise in the column and take up the shock when pen-stocks are closed. Thanks for the clip, very interesting!

  35. Note for " SimMaster". I absolutely agree with you! every advertisement on radio and TV is accompanied by blasts of so called music, so as to make sense of any speech is almost impossible, No it does not help to sell anything, quite the opposite, it really pisses me off!

  36. What about valve chatter? I bought a new valve for an outside faucet and it chatters until I open the valve wide open. My other two outside faucets have ever done this.

  37. (Not so) Fun fact:
    an attempt to prevent water hammer also contributed to the TMI reactor incident. By draining the pressurizer in a effort to prevent it from „going solid“, too much coolant ended up being drained, which further aggravated an already dangerous situation.

  38. I believe most residential well systems use a pressure tank containing a bladder with the water on one side and the (compressed) air on the other. The purpose is to prevent the pump from cycling too often, but apparently it would have the side effect of absorbing pressure spikes.

  39. You can have a water line at 5psi and snap it shut to create 15psi. Do this in a chamber with basic sprung valves and you have a simple reliable pressure amplifier for irrigation.

  40. Soooo what if you organized it so you had a friend on every faucet in a huge building and all of you run the water fully open then all at once, everybody snaps the faucets closed. Did engineers prepare for that?

  41. Excellent explanation thank you. I’m getting water hammer now from hot or old upstairs or down. The only think we’ve changed is re-plastering the kitchen. Didn’t touch the hot or cold system at all apart from a pipe carrying cold to a washing machine: I fastened that one tighter to the wall and to a redundant hw pipe(turned off).

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