Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Water Pressure in Closed System
Name: Bryan
Status: Other
Grade: Other
Location: MN
Country: United States
Date: March 2006

Hi. 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?

Hi Bryan,

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 water flows.

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 self regulate.



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. Probably automaticly. 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.

Jim Swenson

Click here to return to the Engineering Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory