Water Pressure 1

[WaterGIS]

Hi all
I have conducted a water pressure test using digital pressure logger.
The pressure gage gave me about 16-17 bar, though when I open the water to go out free it doesn't rise more than 1 meter. It really confused me. The pressure gage is calibrated and we did tests in different places and gave out the accurate result.
Dose any one knows what might be the problem.
Regards,

 

[JStephen]

Can you give us some more detail on what you're actually measuring the pressur on? And when water only rises one meter- are you saying you squirt it in the air and it only goes up one meter? Or is that in a tube of some kind?

 

[WaterGIS]

Hi
We are measuring water in the distributing network the diameter of the pipe is 3/4 " galvanized pipe, we place the pressure gage on a taping point. The water rise in the air freely without any tube but it is very weak. As I told you before (less than one meter).
Regards,

 

[25362]

Question: when you release the water, do you first stop pressurizing the line ?

If you do, consider that the energy is stored in the tested piping system in two ways: spring-like (longitudinal and circumferential) expansion of the piping, and compression of the liquid.

The isothermal compressibility of water (1/V)([∂]V/[∂]P) at 25^oC is about 0.00045/MPa.
At 1.7 Mpa (=17 bar), the volume change [Δ]V/V, would be 1.7x0.00045=0.00077. This would give a rough idea of how much water would be released upon opening a vent, when knowing the volume V of water in the pressurized system.

 

[WaterGIS]

No I don't stop pressurizing the line... it is the same pressure. the area is having water from a tank elevated 330 m and the point is in the end of the surface area it is elevated 165 m

 

[katmar]

How close is the pressure gage to the point where you are getting the "weak rise"? Does the pressure decrease when you open the vent to test the rise? Is the entire network (from the 330m source) 3/4" pipe? What size is the vent where you got the "weak rise"?

 

[quark]

You didn't account for velocity head. If you apply Bernoulli's equation to your process,

P1/w + V1²/2g + H1 = P2/w + V2²/2g + H2

The height to which the liquid will rise is then,

H2-H1 = P1/w- V2²/2g

If P1 is gauge pressure then P1 is zero(atmospheric) and V1 is zero.

Generally your observation is correct because I did observe the same thing but never thought of it.

Regards,

 

[katmar]

A theoretical calc based on velocity head gives a height way more than 1m. Think of a fire hose pointed vertically upwards, or one of the dancing fountains often seen at casinos.

The only solution I can see is that the vent releases water at a rate faster than the source can supply it (because of a restriction somewhere) and the pressure goes down as the vent opens.

The quickest way to check this is to install the pressure gage as close to the vent as possible and see what happens to the pressure when the vent is opened.

 

[BobPE]

I think katmar may be on to something. Static pressure is what you were recording with your meter. Residual pressure may be something completely different and may be substantially less when you system is flowing. It is this residual pressure that drives flow (and height) in your water column (I simplified the concept of residual pressure for ease of discussion, it is far more complex than this).

Look to understand your system where the gage is connected. Small diameter pipes, blockages, partially closed valves, devicies in-line, etc.

You do not say anyting about the flow rate in your post, so I am assuming that you are just letting it flow wide open, driving the solution to the worst case in terms or residual pressure.

BobPE

 

[WaterGIS]

Thank you all
The network varies from 4" from the source to 3/4" where the home connection is, the source is 2000 m3 tank. The point where I measure the pressure is the same where I open it free to go out, the size of the open is 3/4.
Another thing that I observed that there is air goes out with water.
In another area we have the gage reading about 9 bar and the water goes high (more than 6m) freely.

Regards,

 

[JStephen]

When water is not flowing, you get a uniform pressure throughout the system (with variations due to elevation), regardless of the size, length, or characteristics of the lines.

When water is flowing, you get pressure drop due to friction, which will vary depending on where you are in the system.

The height to which the water will shoot upwards depends on the friction through the system, and also on the nozzle configuration. A larger nozzle will let more water through, but that in turn gives a greater pressure drop in the system, which reduces the height to which it will shoot. It might make more sense to just check flow rate wide open rather than testing the height of spray (big bucket and a stopwatch could tell you that).

If you're wondering why you aren't getting enough water through the lines, it might be a design problem, just inadequate line sizes, etc. But I remember seeing in college some samples of 2" pipe that had mineral deposits built up until the hole in the middle was maybe 1/4" diameter. You could be having problems like that.

 

[WaterGIS]

i will check the flow rate...
regarding the mineral deposit , i dont think we have becuse we have a low TDS.

 

[25362]

Air+water in a two-phase flow may introduce a considerable friction drop. Following Lockhart and Martinelli as shown in Perry VI:

If the square root of the ratio of the estimated friction drops for a given pipe length for only water to that of only air, is, say, 0.02, the friction drop for the two-phase flow may be 5000 times that estimated for only water. If the ratio is 1, one could expect this factor to be about 20.

 

[bimr]

I agree with Katmar. The only explanation is that you have a valve closed and leaking or a restriction in the line such that you are only measuring static head. When you open the open, you are releasing the static head and water then is only petering through the valve or restriction.

 

[WaterGIS]

Hi
I think we have some illegal use in this area; also the tank that feeds the area is getting empty very fast.