How to estimate water volume flowing out of hydrant

[bstrom1953]

Hi, I am the system operator for the water system in our small town and I need to flush the T-mains by opening the 4 inch hydrants around town. I must estimate the amount of water dumped on the ground so I can accurately calculate the percent of water lost by the system. I purchased a pitot tube type of measuring device that reads out from 0 to 100 p.s.i. I am wondering how to calculate the gallons per minute flowing by using the p.s.i. reading from the pitot tube out of a 4 inch fire hydrant outlet. Thanks, Bill

[admin]

Here you can find calculator for Prandtl probe and Pitot tube:
http://www.pipeflowcalculations.com/prandtl/

With Pitot tube you are measuring total pressure in some point of flow stream. That value (total pressure) is your reading on the instrument. Than, you should also know static pressure in the pipe (measured with gauge), and the difference between total and static pressure will give you dynamic pressure. Dynamic pressure is directly related to flow velocity like:
pd = rho * v^2 / 2
Velocity multiplied with flow stream area will give you flow rate - q = v * D^2 * pi / 4

All of this is included in calculator with link above.

Also, here is the text that should explain what total, static and dynamic pressure is:
http://www.pipeflowcalculations.com/pip ... essure.php

Please take care that flow velocity close to pipe wall will be much lover than in the axis of pipe, so you should try to place probe as far as possible from the wall.

I you find it convenient, your results and experience would be very welcomed!

[bstrom1953]

Hi, thanks for your reply. I have more information and a few questions.

My goal is to get the water moving in the 6 inch transmission mains at 5 feet per second. If one foot of 6 inch pipe has 1.47 gallons of water in it, at 5 feet per second, that's 7.35 gallons per second or 441 gallons per minute. I'm hoping to use the pitot tube to help me calculate the gallons per minute being discharged. If I don't achieve 441 gal/min, then I have not reached a velocity of 5 ft./sec in the mains. I will also know the approximate volume of water that has been discharged during the flushing process.

Question:

1. Using the Prandtl Probe Calculator, the total, static and dynamic pressures are all accurate to the millionths place. How do I get readings that are accurate to the millionths place? Below is a calculation I made with hypothetical values that demonstrate values of pt and p pressures resulting in a pd of 0.0002 psi. My pitot tube has a pressure gauge that measures from 0 - 100 psi. , not the accuracy needed for the Calculator.

Calculation report

Prandtl Probe Calculator

1. total pressure (pt):
pt : 15.228426 psi

2. static pressure (p):
p : 15.228226 psi

3. dynamic pressure (pd):
pd : 2.0055294E-004 psi

4. density (ρ):
ρ : 4.3711017E-005 lb/in3

5. diameter (D):
D : 4 in

6. velocity (V):
V : 4.957172 ft/s

7. volumetric flow rate (q):
q : 194.28946 gpm US

8. mass flow rate (w):
w : 0.0010161677 slug/s


2. You mentioned static pressure: "Then, you should also know static pressure in the pipe (measured with gauge)". Is that taken at the hydrant with all caps in place, no water flowing? I would need to make a special cap with a gauge mounted in it to read the pressure, correct?

Thanks for your assistance, Bill

[admin]

Static pressure should be measured while water flowing, with gauge that is placed on the pipe wall not interrupting flow stream. When entering pressures always use absolute values (atmospheric pressure being added to the gauge pressure).

Please also check value of density and enter one for the water and check "calculate for liquid" on the right side of Pitot calculator.

Regarding big number number of decimals - just ignore them.

[bstrom1953]

I entered 62.4 for pounds per cubic foot and checked "calculate for liquid". Then I entered hypothetical values that resulted in pd of just 1...see below:

Prandtl Probe Calculator

1. total pressure (pt):
pt : 30 psi

2. static pressure (p):
p : 29 psi

3. dynamic pressure (pd):
pd : 1 psi

4. density (ρ):
ρ : 62.41 lb/ft3

5. diameter (D):
D : 4 in

6. velocity (V):
V : 12.178366 ft/s

7. volumetric flow rate (q):
q : 477.31412 gpm US

8. mass flow rate (w):
w : 66.35738 lb/s

The velocity is getting down into the range I am after...that's good.

My pitot tube gauge has graduations of 2 psi so I can only estimate plus or minus 1 psi. I need pd to be around 0.2 before the resulting velocity is around 5 feet/sec which is my goal. My gauges aren't that accurate.

If the pressure at the hydrant is around 50 psi, what should I expect to read on the pitot tube gauge? It seems like it should be much less if I am discharging water into a diffuser that is essentially an open pipe.

Do I need to get gauges designed for lower pressures?

Thanks , Bill

[admin]

Pitot tube measures total pressure in one point of stream line - so your readings should be 50 + 0.2 psi = 50.2 psi - if pressure where you place Pitot tube is 50 as you say.