Water Pressure in Closed System
Country: United States
Date: March 2006
My question concerns water pressure systems and the vertical
movement of water.
It looks like water pressure systems [for example - water tower to
home faucets] are "closed" systems. If that is true, is it possible
to have water enter the system [other then at the tower] without
disrupting the water pressure system?
If yes, how would one measure how much to compensate [the water
pressure] for this entry of this water or does the secondary [water]
entry point negate the entire water pressure system?
It was difficult to understand exactly what you were asking. I think I
understand and will try and respond.
Yes, a municipal water system is a closed water system and it is under
pressure. That is how the water is delivered to you home. Since the
pressure is greater than atmospheric, when you open the faucet, the
Cities and towns typically use a combination of pumps and water towers
to maintain a constant and steady flow and pressure of water. Water
systems are operated at a pressure controlled by the height of the water
towers. The pumps, your secondary water entry point, can operate over a
range of pressures. When usage is light, the pumps will cause a higher
system pressure and that will cause water to flow into the tower. When
usage is heavier, flow and pressure is still maintained by water flowing
out of the towers.
There is really not a need to measure how much to compensate. You set
the system up to maintain a certain pressure and the hydraulics tend to
Water-towers are always only partly full.
There is an airspace over the water's surface up there.
If closed, it could be pressurized air, or partial vacuum, or
anything in between.
But this is a time-honored primitive device; it is simple:
the airspace is vented to air outside.
It's just a pool in the sky, with a casual roof to stay clean.
When water goes down to a faucet, air comes in at the top.
When new water is injected,
it must be pumped up to that height by pressure in its pipe.
And as it enters the reservoir at the top, air goes out.
Imagining three pipes: water in, water out, and air-vent,
You an make a 3-way sum:
water_out + water_in + air_vent = 0
I guess it is not a closed system.
That would be hard, and public utilities happen much sooner
when they do not involve doing hard things.
I think you can presume they have a water-level gauge up there,
and to manage that level they have a valve
with which they turn on and off incomming water.
At any moment the tank can be 20% full or 80% full; it does not matter.
Customers will not notice the pressure change
until the level is well below 0%, down into the pipe below.
Click here to return to the Engineering Archives
Update: June 2012