What is maximum PSI to blow water out of PVC pipes?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by 920codyroad, Jan 16, 2010.

  1. 920codyroad

    920codyroad New Member

    Messages:
    3
    Location:
    Texas
    Winterizing barn pipes, but not going as I had hoped. Help, please???

    We had some days where the temperature dropped to 18 degrees and the pipes in the barn froze and burst. I just got done replacing them. The pipes are 3/4 PVC. I installed a pneumatic coppling in the line inside the house where it is fed from. The run is about 75 feet from the house to the barn.

    I tested for water leaks and all is good. I hooked up the compressor to it and blew 70 PSI of air through. I would have thought the water would come rushing out when I turned on the spicket outside the barn. But, instead it just dribbled out. Once It stopped I would have thought I would hear the air rushing out of the spicket, but I don't. If I shut the spicket and let it sit closed for a few minutes and then open it slowly, the water and air start coming out. But, very slowly. :confused:

    What is the maximum PSI I can blow through 3/4" PVC? Am I doing anything wrong?
    Last edited: Jan 16, 2010
  2. gardner

    gardner DIY Senior Member

    Messages:
    222
    Location:
    Ontario
    Personally I have had bad luck blowing pipes out. The problem with blowing the lines is that bubbles of air can travel freely through the liquid water, without moving much or any of it along the pipe. Water can easily flow back down the pipe to the low spot after you stop blowing air through.

    I prefer to design the lines so that that they pitch towards a low point, possibly in several places, and then place boiler drains at the low points. You should be able to drain the system dry by opening up all the valves.
  3. 920codyroad

    920codyroad New Member

    Messages:
    3
    Location:
    Texas
    Unfortunately, that isn't an option. The barn is on the same ground level as the house, the pipes to the barn are buried, both going to the barn and under the slab.

    The pipes coming out of the slab burst this week. I was lucky in that they burst and split about 6" above the slab. so, I had enough pipe to install a coupling and new pipes.

    But, I need to figure out a way to winterize these pipes. I would really like to have a blowout but don't know the maximum PSI I can push through without bursting pipes.
  4. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,250
    Location:
    Land of Cheese
    3/4" schedule 40 PVC pipe has a bursting pressure of over 1500 psi.
    That's not to say your joints or fixtures won't pop well before.
  5. 920codyroad

    920codyroad New Member

    Messages:
    3
    Location:
    Texas
    So, if I open the farthest spicket, I can pump 135 PSI (maximum my 60 gallon compressor goes) through without worrying about damage?

    What's the best way to get more CFM out of my compressor? I think it's volume that I'm after, not PSI.
  6. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,250
    Location:
    Land of Cheese
    That will be fine as long as your system is in good shape.

    The actual compressor pump is where the CFM rating comes from. Bigger tank(s) and bigger line(s) will give your more CFM, but the pump will never keep up with a wide open hose.
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,874
    Location:
    Cave Creek, Arizona
    air

    All you need is about 5 psi to push the water out of the pipes, but you DO need enough volume to keep it moving. A compressor with a tank will usually have the volume needed.
  8. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    75' of 3/4" ID is about 400 cu. in., about 1.7 gallons. Doesn't alcohol mix with water and prevent freezing? Or make all of it evaporate? Then you flush the pipe later?

    Is there some other liquid that is non-toxic with a low freezing point that doesn't react with the pipe material?

    http://wiki.answers.com/Q/What_is_the_freezing_point_of_rum
    Last edited: Jan 17, 2010
  9. gtmtnbiker

    gtmtnbiker New Member

    Messages:
    30
    Location:
    Massachusetts
    What about using RV anti-freeze? That's what my in-laws use to winterize pipes in a summer house in VT.
  10. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    suck it out

    A big shop vac or electricians vac will SUCK the water out and not just tickle the water with bubbles passing through.

    You could BLOW the water out using a "pig" - maybe a cotton ball or whole wheat bread ball, but only in a relatively straight line with out branches that lead to a shower valve or washer or toilet..... Read up on "pigs" online before attempting this creative approach.

    I would go for the vac unless you have a large diesel air compressor around the house. Volume is the key.

    When you replaced the pipes you should have used black 160psi poly. We go to about 10' here and they do not break.
  11. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    Something is wrong...with 70 PSI, and I am assuming you used a compressor, you should have had the water flying out and when the water was pushed out there should have been tons of air...is there a check valve somewhere on the line????
  12. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,250
    Location:
    Land of Cheese
    I was more under the impression that there is probably still ice in the line.
  13. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    A strong possibility.

    I use my compressor to blow out the sprinkler lines each Fall. The procedure calls for a 50 psig regulator setting. I have a reasonably large portable tank and moderate size compressor but it still depletes the tank several times. As the heads clear of liquid the volumetric rate of air uptake shoots up and the pressure begins falling.

    Don't know about it's affect on the plastic pipe, but propylene glycol is the base for non-toxic anti-freeze used in cars these days.
  14. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Vodka is fairly non toxic and you can use it to celebrate the spring thaw if you save it when you refill the lines.
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