Re: water hammer - why air in pipes?
Posted by Moore on December 06, 1999 at 22:54:41:
In response to Re: water hammer - why air in pipes?
As water is delivered to you from the water company, it is pumped under pressure. When everybody in the City stops using water the pump shuts off. Some cities store lots of water in a large tank high on the hill, and only pump very large volumes of water for short periods of time. The water high on the hill adds gravity pressure to pipes to your house, and the large tank allows air entrapped in the aereated high pressure water to escape into the tank, and then to be vented off. Water that hasn't been "decanted" still has air bubbles trapped in it as it is delivered under pressure, Main line pressure needs to be very high to meet the city needs, so the pressure is normally reduced at a pressure reduction valve at each houses water meter, or at the service connection. Where the pressure is reduced, the water is squeezed through a smaller opening than the pipe supplying the main line pressure. Air molecules bump against the water molecules and cause friction. Water is one part hydrogen two parts oxygen, Air is oxygen. The water releases the oxygen after it passes the pressure reduction valve serving your house, and is pushed through the cold water pipe to the cold faucets, but also to the hot water heater. The cold water enters the hot water heater and circulates downward through a heat exchanger until it gets hot and is ready to be delivered to the hot water faucet at the tub abd sinks. The hot water heater keeps cooking that water as it sits in the heater until the thermostat tells it to shut off. The heated water gets to a hot state almost to boiling, and gives off air (Releases oxygen molecules). The oxygen is trapped inside the pipe. There's nowhere for it to go. When you open a faucet on the lower floore, the air is lighter than water and goes to the highest elevation in the piping system. A water air arrestor tries to give the air someplace higher to go to before it gets to the ninety degree elbow just before the valve that gets slammed shut. If there's too much air in the line the air gets to the restriction at the sharp bend in the piping and develops a hammer inside the pipe. Sometimes a banging that sounds like a water hammer isn't a true air in the line hammer at all. Sometimes the piping inside the wall is very close to a stud or to the drywall, and isn't fastened down, so as the water surges through and makes a corner, it shakes the pipe, and causes it to rattle against the framing members inside the drywall. Does the hammer noise come from inside the wall at the same location all the time? Can you actually feel the vibration of the hammer against the wall, or inside a closet? If so, poke a silver dollar sized hole in the drywall and look in with a penlight to see if the piping is shaking.

Hope this helps you sleep at night. Hope Santa finds you cheery and bright.


: : I have water hammer and I am trying to understand it better. Thanks to those who have helped me already.
: : I have reviewed everything I can find on water hammer including the last two years of this wonderful site.

: : My 2 questions are:

: : if water hammer is caused by air trapped in the pipes, how does the air get there? I know when repairs are done, air will be in the pipes until it is expelled through a faucet valve or whatever. After that, shouldn't the system stay closed with no air getting in?

: : Could I have some problem which is causing air to get into the plumbing system?

: : I am working with the plumbing contractor who built my home. We have already added arrestors and a new pressure reducer valve. I still have water hammer.

: : We still need to check the for faulty washers.

: : Thanks,

: : Debbie
: : debbie_salierno@yahoo.com
: Debbie, I would check the pressure after the prv to see if your pressure is below 80lbs. Does it hammer no matter what fixture you are useing? Is the plumber useing a manufactured arrestor or is he makeing his own?You may not have the arrestors installed in the right places. At the wash machine and the stools. These arrestors should be near the fixtures. Again i would check to see if the prv is set right. Check the pressure at your hrdrant. Dale Peck LMP




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