How to Collapse a Pipe by Closing a Valve
100 Comments


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!

100 thoughts on “How to Collapse a Pipe by Closing a Valve

  1. @2m34s: "…more than 30 inches of mercury…" Ah yes, the often theorized "super vacuum" where the pressure goes slightly greater than one atmosphere below one atmosphere absolute. Perhaps it was a very nice day.

  2. If big amounts of water are flowing, the ,,water hammer'' inside the pipe (before the water is flowing through the valve) has much kinetic energy. So, if you use a hydrant as a fire fighter, you have to close valves slowly. The hose after the hydrant won't be affected by a short vacuum, but the pressure increase in the pipe itself can break it in worst case.

  3. In vaccum, water does not stay in it's liquid form. Instead it vaporizes and dissolves into its components (h and o). Its cooking basically and thats where the bubbles come from.

  4. Surely the water-hammer problem is exacerbated by converting from old gate valves — that you had to turn, turn, turn, turn… — to newer quarter-turn ball valves. Not only is the shutoff process "easier" now, it's also much, much "faster!"

  5. I always say, never close a valve in the direction after a pump, but before a pump. In a vacuum, your pressure maxes out at zero, but pressure can go to infinity. As long as you can rely on your material to boil under a vacuum, and that your material can withstand this absence of pressure than you would be fine.

  6. The bubbles are not dissolved gases; it's actually steam. The boiling point of water drops with pressure, so under vacuum, water will actually boil. That's what causes cavitation – areas of low pressure cause water to vaporize, but the surrounding high pressure causes the bubbles to collapse.

  7. a local man to my area died because a pipe beneath a damn got clogged. but the problem from that was it created a low pressure vacuum which once cleared caused a massive suction that killed the diver.

  8. I was away from my phone, but just by listening i could visualize the whole thing and i can't believe i never thought of this phenomenon.
    Thank you for the information, now i know why old piping shakes sooo weird in old buildings

  9. i dont know if anyone else mentioned it, but when you said that the vacuum exceeded 100 kpa thats actually the dial needle overshooting the actual presuure because of the needle momentum. the vacuum pressure can never exceed 1 bar bellow zero

  10. I'm disappointed. I wanted to see a pipe collapse from doing this to it. Still gets a like, but yeah, do it again and show us what ACTUALLY happens when this is a problem.

  11. Very interesting! Thanks a lot! Does the neg. pressure also happens with pipes down the stream full of gas instead of fluid or are gases compressible enough to compensate this? What about Steam?

  12. As a Water Distribution Employee for a Water Utility we have 26 different Fire Departments within our system. Education is the key. Both Paid and Volunteer Firefighters love to open fire hydrants as fast as possible and in turn slam them closed. Every year we have to remind them over and over please open them slowly and close slowly if you have water flowing. They can create a water hammer at their pumps just as easily as they can at the hydrants. We even have to teach them what the pressure relief valve is for on their Fire Engine pump panels. Since my job is repairing the breaks, I see this problem never going away I will be including this video along with the positive water hammer video to all the Fire Depts hopefully they will watch and remember why slow and steady is best. Some of our infrastructure dates back to 1880. We only use USA made products and materials. Sadly it’s becoming difficult to find products. They are considering now using products made in Israel and Australia. Thanks again for making great videos

  13. Great videos. Its great to get the masses into engineering. I recommend a video on concrete dyes and concrete colouring. Enough with plain boring grey colour fair face.

  14. I've not watched the whole video, but one must understand that under NO circumstance can the negative pressure be greater than an absolute value of not more than 14.696 PSI.
    Possible positive pressure is potentially unlimited. Therefore engineers have only to be concerned with "hammering" as a consequence of negative pressure and its ensuing harmonics as the wave transgresses from the high pressure to the low pressure of any given vessel.

  15. Thank you for explaining what my back yard garden hose is doing when I close the valve (and then water bursts from the valve/hose connection). I found empirically that closing the valve slowly mitigates the issue.

  16. Years ago, we used to "water hammer" our house via its upstairs bathroom lever-action faucet. We never popped the pipes, but it was fun. Cheers!

  17. So why don't dish washers and clothed washers either close more slowly or incorporate anti-hammer in their device (since it's their device causing the problem)?

  18. I am an volunteer firefighter from germany. Once we've made an exercise: we closed the valves from four waterhouses on the exactly same time, the house next to oure fireengine explodes! (before we closed the valves 2.050 litre/min on 12 bar pressure were gone out of the houses). On testings from the producer the houses held an minimum of 25 bar pressure.

    Greetings from germany
    (Please excuse my horrible english)

  19. Practical Engineering is engaging in the "Number 1 No-No" that a plumber can do with plastic potable water pipe. Or just plastic pipe in general.

    DO NOT USE OR INSTALL IT UNDER DIRECT SUNLIGHT!!!

    For that matter do not even leave it on the back of your truck WITHOUT COVERING IT UP!!!

    Don't leave it out on the lawn as seen at 3:54. Or on the roof. Nor anywhere that direct sunlight can reach it's naked walls.

    Plastic in general breaks down in sunlight. It is exactly these kinds of videos which create potential water emergencies. Things like water mains left out in the sun that burst when the homeowner is away down in Disneyland.

    Boy did I make good money that weekend! Not only got paid for fixing his pipes. But for drying out tons of his personal items.

  20. Is it possible that some of the gas close to the inlet is actually steam? The low pressure would in turn lower the boiling point of the water which could potentially create steam. I haven't done the math on this, just a thought.

  21. the light haze and bubbles that occur when the valve is closed is caused by the water coming to the boiling temperature. the boiling temperature of water drops at very low pressure. the same phenomenon occurs with a ship propeller which also creates a low pressure when turning at high rotation speed

  22. Video titled How to collapse a pipe by closing a valve…no visual of such a thing occurring…very misleading title to say the least…

  23. Today I noticed a process feed pump running at negative 2.5 psi vacuum.
    So I immediately turned the control valve in a non-counterclockwise direction

  24. so if my friends piss me off I can go into their bathrooms and just turn the water on and off as fast as I can?

  25. The you tube video "Obere Wasserschlosskammer" demonstrates the amount of water that needs to dissipate to stop water hammer in a very large dam. The pressures involved with such a large system must be absolutely massive and your demonstration shows why it happens. Even though its on a small scale, it demostrates how much pressure or vacuum is actually created.

  26. I see this too much working on hydraulic equipment when people jam the valve controls back and forth and you watch 1/2 the system jump and the lines on the other 1/2 shrink.
    then they wonder why things have to be in the shop so much…..

  27. I stumbled across this vid & I'm no engineer but found it really interesting.

    I do enjoy taking things apart to see how they work tho.

  28. the gauge is tricking you. you literally can't pull more than 30 inhg of vacuum as that's literally a perfect vacuum.

  29. It was very fasinating to watch a moving bubble towards close valve when we were kids now i am engr and knows why.

  30. Well, I was a little disappointed we didn't actually get to see a pipe collapse… But to answer your final question, isn't the best solution simply to avoid quarter turn valves in favor of the sort that have to turn and turn and turn to open or closed? I forget what those are called — a screw and seat valve.

  31. I learned something today.

    Another pointless thing that'll never ever be involved in my life and most human lives too. Learning meaningless things.

  32. Wait so is using a spray handle on a garden hose the same as closing the valve since everytime you let off the handle it shuts off the water suddenly?

  33. 2:30 With 100 kPa (1 bar) of atmospheric pressure (= Zero on the gauge), how would a pressure of less than -100 kPa on the gauge (= 0 kPa absolute) be possible!? You cannot have less pressure than vacuum?!

  34. I live in an old UK Terraced house that has a lead water main feeding 4 properties. I have started experiencing water hammer on my cold water pipe since a new neighbour moved in. I can eliminate my other 2 neighbours as it was not present when my old neighbour lived next door. When it occurs it will hammer every 15 seconds exactly, sometimes from early in the morning all the way through to late at night, stop for a bit and then start again. It is really p155ing me off. It is as though my new neighbour has a solenoid valve on the cold main somewhere in their house, every 15 seconds, i experience a sudden drop in pressure if I am running a tap. It only lasts for a fraction of a second but its every 15 seconds it will do it again. Im almost convinced they have an auto irrigation system running in their house (its rented). Should I contact the water authority and express my concerns? This can't be good for old lead system terminating into copper where it enters my home.

  35. When I was working I seen new 8"water line jump out of the ground and blow a part thanks to a city working doing a job he shouldn't be doing..the city had a hydraulic gate wrench and he was told to just cracking the valve so we could flush the line before we could test it and he just opened up the main line as fast as that hydraulic gate wrench could go and it looked just like when you snap a garden hose to free a kink.

  36. Just make it fool proof and build a pipe system using valves that take longer to open or close. Not an expert with names, but the valve handle that looks like a gear that you would turn 360 degrees 5 times is a good example

  37. On board ships, the sea water pumps are located below the floor deck and the water is delivered higher. So every time we stop pumps, the non return valve of the pumps' discharge closes making the boom noise…

  38. I'm a firefighter and this is the exact truth and pretty much part of firefighting 101 when turning on and shutting off a hydrant.

  39. That's something i was observing lately when i was closing the faucet and the hose started making suckling noises at the nipple as the water was creating a vacuum at a higher negative pressure than what the hose clamp can handle without leak.

  40. Can't you just make the pipes be able to deal with one atm of crushing pressure? Your plastic pipe could, so it's surely not hard.
    Or does it get harder to do with thicker pipes?

  41. Great material , i got the same problem in the real life ,water puryfing instalation , the water booster pump lose his (absorbing) pressure in the tank behind the pneumatic absorbing membrane , this cause a strong hits "like hammer" directly to the sealing glue in the connections when the pump electric motor stops , this cause a massive leakage in the installation , chain reaction starts , leakage was masive , so pump try to achieve defined pressure , in my situation it was 4,5 bars , pump cant get the pressure because the leakage was masive , so it work constantly lead to pump seizure. Conclusion, its very important to check water booster pneumatic pressure at least 4-7 months i think , to avoid failure . Mine booster loose his absorbing pressure after 1,5 years of very intensive usage ( about 3000 liters /24H 792 gal/24h)

  42. Valtec produces special valves with planetar reductor. It allowes to close valve turning holder not just for 90° but for 360°

  43. In principle, just like an inductance in a DC circuit. First it hinders the inrush current and when switched off, a voltage peak arises, because the inductance wants to maintain the current flow.

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