Calculating Water Flow Rate
Calculating Water Flow Rate
(OP)
I need help in calculating the water flow rate in pipe line project.
Given:
1. Tapping into the city's 8" water main.
2. Measured flow rate of 1550 gpm and 78 psi just upstream of the tap point, measured at a fire hydrant. By my calculation you have 181.52 feet of head available at the tap point.
3. Adding a new 6" water line to feed the building water service requirements. Using 6" cement lined ductile-iron pipe below grade outside the building and 6" PVC pipe inside the building.
4. The 6" line will dump into a well that feeds our cooling water system. Water flow rate needs to be at least 1000 gpm. This system will only be used rarely, so the 11.4 fps flow velocity isn't much of a concern.
5. Including pipe bends the total length of ductile iron pipe is 450 feet and the PVC it's 375 feet.
6. Head loss through valves & back flow preventers is about 70 feet.
7. I've been using a Hazen-Williams constant (C) of 140 for both pipes.
I know to use Bernoulli's equation, but I keep getting the velocity at the well is much greater than at the water main, which to me doesn't make since unless the pipe is no longer flowing full.
Not finding much on the internet to help either.
I'm assuming the pressure head at the well is 0 feet.
This is out of my area of expertise and it's been awhile since I studied fluid mechanics in school, plus I've seen to have misplaced (or had stolen) my fluid mechanics reference books.
Any help would be appreciated.
Given:
1. Tapping into the city's 8" water main.
2. Measured flow rate of 1550 gpm and 78 psi just upstream of the tap point, measured at a fire hydrant. By my calculation you have 181.52 feet of head available at the tap point.
3. Adding a new 6" water line to feed the building water service requirements. Using 6" cement lined ductile-iron pipe below grade outside the building and 6" PVC pipe inside the building.
4. The 6" line will dump into a well that feeds our cooling water system. Water flow rate needs to be at least 1000 gpm. This system will only be used rarely, so the 11.4 fps flow velocity isn't much of a concern.
5. Including pipe bends the total length of ductile iron pipe is 450 feet and the PVC it's 375 feet.
6. Head loss through valves & back flow preventers is about 70 feet.
7. I've been using a Hazen-Williams constant (C) of 140 for both pipes.
I know to use Bernoulli's equation, but I keep getting the velocity at the well is much greater than at the water main, which to me doesn't make since unless the pipe is no longer flowing full.
Not finding much on the internet to help either.
I'm assuming the pressure head at the well is 0 feet.
This is out of my area of expertise and it's been awhile since I studied fluid mechanics in school, plus I've seen to have misplaced (or had stolen) my fluid mechanics reference books.
Any help would be appreciated.





RE: Calculating Water Flow Rate
A Hazen-Williams C=140 seems a bit high. You may want to be conservative to allow for roughening of the pipe with age. HWC=100 is commonly used.
Also, how low can you reduce the pressure in the main. Most Cities would only allow you to draw it down to about 40 psig except in an emergency, such as a fire. The you might be able to go as low as 20 psig.
The exit loss at the "well" is usually taken to be = to the velocity head (v^2/2g).
good luck.
RE: Calculating Water Flow Rate
Is this an emergency system?
RE: Calculating Water Flow Rate
2) I am not sure you are on the right track -Bernoulli's equation is not a lot of interest to you !.
What you can do is take the head difference from the main to your tank and calculate the flow OR take your design flow and calculate the head loss from the tank to the main.
You are trying to do both at once that is why you are ending up with a residual head and excess velocity at your tank. You need to either calculate flow from head difference or head difference from flow.
If the flow you want is 1000 gpm then calculate the head losses for this flow starting from zero at the tank. If the head at the main is less than 181.52 feet then you can meet the flow and you will need to throttle the flow with a valve.
Alternatively take the head at the main of 181.5 feet and the head at the exist to thetank of zero feet- calculate the head difference across the system and the flow that this will give (probably greater than 1000 gpm). (the losses you quote at valves etc should relate to flow and not fixed at 70 feet).
However unless you work back into the utility system you are wasting your time because you have no idea what the head will be in the system when you pull out your 1000 gpm.
RE: Calculating Water Flow Rate
This is for emergency use only.
How do I calculate the head in the system after we pull out 1000 gpm? So instead of having a 78 psi pressure head at the main it will be something less? Can't the utility keep the main pressure fairly constant? The utility hasn't asked me how far I expected the pressure draw down will be?
RE: Calculating Water Flow Rate
This can be calculated but probably goes beyond what can be discussed here.
RE: Calculating Water Flow Rate
Just what is the emergency use for this water?
RE: Calculating Water Flow Rate
What kinds of questions do I need to raise with the utility? Right now I've assumed the pressure the utility gave me is constant.
Also, I'm looking for a good fluid mechanics reference to buy. Any suggestions?
RE: Calculating Water Flow Rate
RE: Calculating Water Flow Rate
That is why power plants are located adjacent to rivers, oceans, etc. Recycled water is suitable for cooling use although if it is being used in a cooling tower, the water should be disinfected and filtered before use. The concentration of phosphorus in recycled water is typically too high as well.
If in fact, you are going to use potable water for this application, you should not be that concerned about the pressure since you will be dumping it into a cooling basin at atmospheric pressure anyway.
You are talking about a major use of water here. Generally, one would negotiate such a water purchase with the upper levels of management at the water utility. You don't just tap into the water supply and turn the faucet on.
The water utility will generally want some type of contract that they can use to obtain the funds to invest in the infrastructure that will supply that water to you.
You just need to have the rainmaker at your organization who is setting up this project to contact the utility. He just needs to tell them that he wants X gpm of water @ X psig and then await the response.
You also should be aware that you are only solving one piece of the puzzle. The supply of water to a power plant is a systems problem that must take into account other power plant factors such as the water quality, the water treatment requirements, the water evaporation in the cooling tower, the type of cooling tower, the type of power plant (peaker or base load), the chemical treatment program, NPDES discharge requirements, etc.
RE: Calculating Water Flow Rate
This is for an existing plant. All of the water treatment requirements are already in place.
RE: Calculating Water Flow Rate
RE: Calculating Water Flow Rate