We are needing to perform a triple hydrant C-Factor test on a 14” main line but we are not allowed to close the valve downstream of the flowing hydrant. Immediately down stream of the flowing hydrant, the DIP pipe has an aerial ditch crossing (pipe is exposed). My question is, if we put a digital flow meter on this pipe and obtained the flow in the pipe while simultaneously performing the triple hydrant flow test, would this combined flow (hydrant discharge added with flow in exposed pipe using the ultrasonic flow meter) work as the “Q” in the Hazen-Williams equation to find “C”? My only hesitation is with the residual pressures and not being sure if they are the true residual pressures used to calculate the pressure drop if you have ongoing flow before the start of the test (I.e. not closing the valve after the flowing hydrant).
I have read thru the post below and the pages 191-197 reference in the below post.
Just to add to what Ben said, you can calcualte C-factor if you flow a hydrant and have two residual hydrants in series with pressure gages. For this to be successful
1. There must be only one flow path between the residual gages and the flow hydrant
2. The flow out of the hydrant needs to exactly the same as the flow between the two gages
3. The gages and their elevations must be known accurately
4. It is best to have constant diameter pipe between the two gages
5. The velocity must be sufficiently high to ensure that the head loss is much greater than error in measurement
You usually need to close some isolating valves in the area to obtain these kinds of conditions.
You can read more about such testing in Advanced Water Distribution Modeling and Management from Bentley, pages 191-197.
Thank you,
John W
You need to know the flow in the test section between the two pressure hydrants. Number 2 above is not the only way.
If you can put an accurate flow meter on the exposed pipe, you may not even need to open the third hydrant. The problem is that you need to get sufficient flow through the test section to get a significant head loss. You may be able to open a hydrant downstream of the exposed pipe (but you wouldn't need to measure the flow out of that hydrant since the flow in the test section is already measured).
Run your model to get an idea of how much flow you will get in the test section with and without that downstream hydrant open. Even if it isn't fully calibrated, it will give you some idea of what to expect. Try to get at lest 20 ft of head loss in the test section with no side inflows/outflows).
Tom,
I want to thank you for the quick and very informative response. This information was invaluable as we were out of the country and on the job site when we reached out. So your quick response and great information is very much appreciated and led us to sucessful testing.
John
Mr. Walski,
I have one more question if you dont mind. For our project, we have several locations that only have two hydrants on a dead end run within a residential street. Assuming the residentail demands are negligable and off peak, would it be possible to flow the dead end hydrant and put one pressure guage on other residual hydrant and for a second residual reading, put the second pressure guage on a house hose bibb to read the second residual pressure?
As long as there is no flow from the main to the lateral pipe that serves the hose bib, the second residual pressure could be used. You need to know the exact elevation of that gauge, not ground elevation.
As before, you need to have a large head loss between the two pressure readings which can be difficult in short cul-de-sacs.