Fluid Mechanics - Need Help on a Water Pipeline Project

[ariley1663]

TL;DR Summary

I have a water supply pipeline with minimal slope and no pump. I want to increase my pipeline discharge rate without upsizing my entire pipeline. I am hoping that I can alter the inlet side of my system to achieve my desired outlet discharge.

My current pipeline is open ended on each side meaning I raise water level in a creek to create natural flow to the inlet and the outlet discharges from my pipe directly into a surface ditch. The surface ditch is well cleaned so that the top of the pipe is above water level at discharge meaning the pipeline shouldn't have any back pressure on it. My current pipeline is 1,198 lineal feet of 10" PVC with 3' of fall from inlet to outlet. The max flow rate that I seem to be able to achieve out of this system if 1.2 CFS, I need to move 3 CFS. My hope is that I can calculate a combination of larger pipe sizes and lengths at my inlet to allow this flow rate. For example, start the inlet with 22' of 18" diameter pipe then swage down to 15" diameter pipe for 44' then swage down to 12" pipe for 66' and then connect to the existing 10" pipe for the remaining 1066 ft of pipeline. Is anyone able to help me with this project?

 

[Baluncore]

Welcome to PF.

You need to make a long trumpet shaped adaptor, without steps, at the inlet end of the pipeline. That will eliminate the vena contracta, that is now reducing the effective diameter of your pipeline at the input.
https://en.wikipedia.org/wiki/Vena_contracta

 

[ariley1663]

Thank you Baluncore. How would I calculate the required dimensions of this adaptor?

 

[Baluncore]

How would I calculate the required dimensions of this adaptor?

We need some idea of the geography of the creek at the inlet.

I would start by looking at the depth of the raised water in the creek, and the possible width of the inlet. Next, consider the direction that the pipeline takes away from the creek. If the creek is flowing, arrange for the flow to be directly towards the wider inlet, rather than flowing across it.

Are there any sharp bends in the pipeline that might be swept or straightened?

If there are dips in the pipeline, where sediment might accumulate, they may need cleaning. If the pipeline has been in operation for some time, determine if there is biological growth on the inside of the pipe. You may benefit from cleaning that chemically with a biocide, or physically, with a "pig".
https://en.wikipedia.org/wiki/Pigging

I expect there will be more replies to your question during the next 24 hours. You may be lucky and get an expert. Don't rush, give people time to think about the problem.

 

[ariley1663]

Thank you sir. I will take some photos of the inlet layout and the diversion this afternoon. I have copied an image below to show the layout of the line.

 

[jrmichler]

The entrance loss for a 10" ID pipe at 1.2 CFS is 0.12 feet of head. You have total head loss of 3.0 feet. Since your entrance loss is small relative to the total head, any changes to the inlet will have a negligible effect on flow.

Can you lower the discharge end of the pipe? If you could get the discharge end of the pipe under water and discharging downstream, the total head would be measured from the pipe entrance to the level of the surface of the water at discharge.

Your flow of 1.2 CFS is 540 GPM. My Cameron Hydraulic Data shows head loss of 0.16 feet per 100 feet of smooth 10 inch ID pipe. Since you have 1200 feet of pipe, the calculated head loss is 12 X 0.16 = 2.0 feet of head. For a flow of 3.0 CFS, or 1350 GPM, the calculated head loss for different diameter pipes are:

10" ID - 0.88 ft/100 ft
12" ID - 0.39 ft/100 ft
14" ID - 0.24 ft/100 ft
16" ID - 0.13 ft/100 ft
18" ID - 0.07 ft/100 ft

Your total head loss of 3.0 feet is 3.0/12 = 0.25 ft/100 ft. Any biological growth inside the pipe, bends, or other obstructions have their own head loss that is added to the pipe friction head loss.

 

[Baluncore]

@ariley1663 There is an "edit" button under your post.

The existing pipe could be "folded" double, to run half of the distance in parallel, with twice the capacity. Then you need to buy 600 ft of larger pipe.

If you cannot replace half of the pipeline with a larger pipe, then you should consider an open channel, say a ditch lined with black polythene sheet.

An open channel could follow a contour for more than half the distance.

 

[ariley1663]

Unfortunately, I am pretty stuck with my inlet and outlet elevations. To my knowledge, it used to be an open channel ditch but they had erosion issues so they put that section of line into the 10" pipeline. I can lay a secondary line of 12" next to the 10" to get my desired flow. I was just hoping I could have a cheaper option by altering my inlet.