We're testing a new copper line for leaks.
The line was purged of air, filled with water at 116 psi, and monitored
with a pressure gauge.
After first night, pressure dropped to 102 psi.
After second night, pressure dropped to 92 psi.
This leads me to believe that there's a leak, however, there are no
signs of humidity whatsoever.
Is it possible for a line that shows no humidity (i.e. a line that
presumably has no leaks) to keep loosing pressure?
Is there a logical explanation for this?
Thanks in advance.
It's hard to say since most of the line is not exposed.
I haven't, but this morning we refilled the line and purpuosefully
opened the faucet to experience a similar drop in preassure. The water
lost was about a third of a mug.
We are actually testing 3 lines in this fashion: 1 cooper line for
water and 2 copper lines for gas.
The good news is that the line for water has stabilized at 76 psi for
the last 72 hrs. so I feel pretty comfortable with this.
For the gas lines I'll start another thread.
116 psi!!! Where did you get WATER at that presure? I always thought
leaks were tested with air presure. Water is not compresible, and
changes in temperature will change the presure, I think. --Phil
Phil Munro Dept of Electrical & Computer Engin
mailto: firstname.lastname@example.org Youngstown State University
It's been a while so I may not be completely correct but here goes: Water
is compressible and that is the crux of why pressure can change in a fixed
volume system such as the copper lines. Water, like almost any fluid,
contains air in solution (entrained air) and it is this air that gets
compressed (or expands).
Take the expression:
Delta P=(Delta V)/V*BM
where Delta P=change in pressure; Delta V =change in volume; V=Volume under
pressure (copper lines); BM=Bulk Modules
As the temperature of the fluid changes so does its density (lbs/cubic ft).
Since there is no change in the weight of the fluid it's volume will change
via compression/expansion of the entrained air.
The temperature (density) does not change enough to account for a pressure
change of approx 20% (116 to 92) so look for a leak somewhere.
As a side note: the compressibility of water (or any fluid, like hydraulic
oil) plays a significant role in high response control systems. Too much
entrained air and the end result is poor response and in many cases an
Youngstown State University
Again, no experience, and I thought I would learn about this here.
Some people here seem to have written as if using water is the normal
procedure, but others hint that it's not, or say to use air.
I offer this for the OP's consideration.
Water is only compressible to a slight degree, and the major thing
they could be compressing is the slight amount of air that may be left
in the pipes when they are is filled with water. Because that volume
is small, tThat means that it would only take the loss of a little bit
of air or a little bit of water (giving more room for the air), to
multiply the available space significantly and make the pressure drop
Also, the air can get absorbed intio the water. This happens much
faster at high pressures, and iiuc when the air is absorbed in the
water, it takes up no space (amazingly) and thus the pressure would
Look at the water prssure arrangements in 6-story buildings. (In nyc.
There may be a different height where this applies in other places.)
I think the same thing applies with houses that have wells and
Four or 5 stories and less, city water pressure is enough, Seven or 9
stories or more, they put a water tank on the roof.
At 6 stories they pump the water into a tank that is about 1/3 air
after the air in the tank is compressed, and they use the air pressure
to pump the water to the sixth floor (and the other floors, but for
the sixth it's essential) After a while the air in the compression
tank has been absorbed into the water, the pressure drops, and a
separate air pump has to be used to add more air. Here the water is
being replaced with new water every time someone uses any water,
unlike the OP's sealed pipe, but there is still absorption to the
extent the original water will take it.
Remove NOPSAM to email me. Please let
me know if you have posted also.
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