Calculating Water Flow Rate 4

[callawaygolfer]

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.

 

[RWF7437]

You're on the right track. You also need to know the differences in elevation between various parts of the system. It would best to draw a sketch of the whole system and then draw the Hydraulic Grade Line ( HGL). Draw your sketch to scale with different scales horizontal and vertical to exagerate the vertical changes.

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.

 

[rbcoulter]

You want to pull 1000 gpm out of a line that is running 1550 gpm? You will probably significantly draw the pressure down in the main line , maybe below the point where the utility is comfortable. To calculate the water flow in your pipe you need to know about the utility system - there pipe sizes, pumps, tanks, etc.

Is this an emergency system?

 

[BRIS]

1) take note from rbcoulter - you cannot do this without involvingtheutility and working this flow through the utility system.

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.

 

[callawaygolfer]

I made sure to involve the utility from the get go. They told me the flow rate is 1550 gpm and the pressure in the line is 78 psi.

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?

 

[rbcoulter]

The utility may or may not be able to keep the pressure constant. That depends on the means they use to maintain the pressure (tanks or pumps or some combination) and the size & lengths of their supply lines.

This can be calculated but probably goes beyond what can be discussed here.

 

[bimr]

There is no way for you to calculate the pressure in the utility's system unless you know the particulars of the utility's distribution system. You should contact a knowledgeable person at the utility to obtain the information.

Just what is the emergency use for this water?

 

[callawaygolfer]

The facility that will ultimately use the water is for cooling towers at a power station. The water that we were getting from the city was effluent from their waste water treatment facility, but it's way out of spec. Our treatment facility can't keep up with the demand so we are adding this new line to supplement the water demand.

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?

 

[rbcoulter]

Crane Techical Paper No. 410 "Flow of Fluids Through Valves, Fittings, and Pipe"

 

[bimr]

It seems that you have a very unusual application. Most power plants would not use municipal potable water because the water is too expensive from the utility. A large plant will use 4,000 gpm.

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.

 

[callawaygolfer]

This is for emergencies only. We are getting effluent water from the city's water treatment facility. This water is way out of spec and our facility can not keep up with the water demand from the power plant. We've been in contact with the water facility director of operations from the get go.

This is for an existing plant. All of the water treatment requirements are already in place.

 

[rconner]

I guess depending on any control valving and location(s) etc., an "11.4 fps" flow velocity and what can happen in systems with such high flow rates could conceivably present problems particularly for the 6" pvc piping and maybe even the utility's upstream piping (you may wish to check with your supplier to confirm the suitability of the pipe for that service).

 

[BRIS]

All you can do is to calculate the pressure you require at the water main to supply 1000gpm and ask the utility to supply 1000gpm at that pressure.