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Pressure losses if going from a 1/2" pipe to 3/4" pipe?

  1. Apr 21, 2015 #1
    I am thinking of burying a 3/4" water line which is tied into my outdoor 1/2" house spigot line. This buried line would run about 100' to where I can better use it for washing a car or irrigating a flowerbed. My question is: by taking the new addition of water line from a 1/2" feed water line to 3/4" water line for 100' and then using a 5/8" hose to wash a car, would I be defeating my purpose by greatly decreasing water pressure and water volume? Since fluid is essentially not compressible, I think not, but need some advice. thanks,
     
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  3. Apr 21, 2015 #2

    Andrew Mason

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    Welcome to PF, DWDman!

    The pipe size change does not change the energy of the water. The speed of the water will decrease in going from the 1/2" to the 3/4" inch line and the pressure will increase. Bernoulli's principle.

    AM
     
  4. Apr 21, 2015 #3
    DWDman, what are you trying to compare? 100 feet of 3/4 inch pipe compared to 100 feet of 5/8 inch hose? The pressure drop through the pipe will be much lower than if you used hose; leaving you with more pressure & flow to wash the car or water the garden. On the other hand, it would take some work to bury the pipe. Depending on your climate you might need to think about freezing and the need to blow it clear or anti-freeze it for the winter months.
     
  5. Apr 21, 2015 #4

    sophiecentaur

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    Bernoulli is only part of what's going on. A pipe with a lot of water flowing through it will dissipate significant Energy due to friction and turbulence. A fatter pipe will involve lower velocity for a given volume rate. This means that, for a given drop in pressure over the length of the pipe, you will get more water through. That is not Bernoulli at work but it is the reason that supply companies use big pipes.
    It must be his sexy name that explains why it crops up so often when it may not be most relevant thing at work. (See all the arguments on PF about how aeroplanes fly, too.)
     
  6. Apr 22, 2015 #5

    Andrew Mason

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    Yes, but the question was whether he would "be defeating my purpose by greatly decreasing water pressure and water volume?" It seems to me that the answer to that question is that pressure would not decrease by using the larger line. Rather it will increase.The slower speed in the larger pipe reduces energy losses too. So, it would be a good idea to use the 3/4" line over that 100'.

    AM
     
  7. Apr 22, 2015 #6

    sophiecentaur

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    I don't think we can really be disagreeing (Quote "yes, but"). But there are conflicting requirements when using a hose for different applications. Plant watering will probably need as high a volume flow as possible (= wide supply pipe). Car washing will need pressure with (probably) less actual flow. Either way, the fatter the main length of pipe, the better. It it will give only a small drop in pressure over its length. For the most Kinetic Energy per Second, delivered in the form of a car-washing jet, you want as much pressure as possible just before the nozzle. A nozzle will, of course, lower the pressure (Bernoulli) but the pressure will still be higher because of the fat pipe and the velocity of the water will be higher (more KE) without losing energy on the way there.

    No risk of "defeating my purpose" with wide supply pipes.
     
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