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Intermediate High Point on 2" raw water line

Intermediate High Point on 2" raw water line

(OP)
I'm designing a new 20 GPM deep groundwater well in an area with shallow bedrock. I'm comfortable with design of the well development (I'm relying on a senior hydrogeologist for that), but the topography and bedrock has presented an interesting challenge for the 1000ft long, 2 inch diameter raw water pipe from the well to the treatment facility. Between the well and the treatment facility there is a peak. The topo ranges as much as 20 ft.

The hydraulic grade line should stay well above the pipeline, but I'm not sure how best to eliminate concerns about air binding up the pipeline at the high point. One colleague is suggesting the line is small enough and the pressure high enough (60 psi) that any air that comes out of solution or is left during fill of the line would just get pushed along to the plant, which wouldn't be a bid deal. I'm not convinced, but I've never had to put such a small line in for this great of a distance with this much change in elevation. I've certainly never tried to put an ARV on a 2 inch line.

So I've looked at a few options - relocating the well, changing the alignment, decreasing/increasing the size of the pipe, horizontally drilling the pipe from well to plant with a continuous slope, etc. but it's bothering me that this is just a little line that we should be able to simply trench through with 3 ft of cover. Is there any basis in the assertion that air wouldn't collect and restrict flow?

I'd appreciate any thoughts.

RE: Intermediate High Point on 2" raw water line

2
You colleague is incorrect as it is velocity that is important, and not pressure.

Pumping Station Design has an extensive discussion of the velocity to flush air bubbles out. With such a small diameter pipe, there should not be any problem with flushing the air out, if you maintain sufficient velocity.

See the attachment. You have not mentioned the slope of the pipeline. However, the slope will determine the minimum velocity.

You should have at least one air release valve at the well pump.

RE: Intermediate High Point on 2" raw water line

bimr,
I have been looking for a table like that B-16 in your attachment for a long time. Can you confirm this is from "Pumping Station Design" by Garr M. Jones?
Thanks!

RE: Intermediate High Point on 2" raw water line

That is correct.

RE: Intermediate High Point on 2" raw water line

(OP)
Thank you, bimr. This was just what I needed. I'm going to look into adjusting pipe slope.

RE: Intermediate High Point on 2" raw water line

The best criterion for air removal is the Froude Number. For design purposes this would always be converted to a velocity, but the advantage of using the Froude No. is that the same value applies to all pipe sizes, whereas the velocity requirement changes with differing pipe diameters (as shown in bimr's table).

A very thorough discussion of this, with some very useful links, is in the following thread thread378-289211: Hydraulic Design 101 - Air at Highpoint

There is not a clear cut-off point where the air is removed. At a Fround No. of 0.35 the air will start being moved slowly forward and once the flow rate increases to Fr = 1.0 the air is removed at almost the same velocity as the water.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Intermediate High Point on 2" raw water line

The values listed in Pump Station Design are based on equations developed from empirical observations of removal of air from pipelines done by Wisner, Kouwen, and Mohsen. The Froude numbers used for the same purpose are also based on empirical observations.

Civil Engineers generally use the Froude number for different applications. In the immortal words from Streeter and Wylie, "It is useful in calculations of hydraulic jump, in design of hydraulic structures, and in ship design."

For Civil Engineers, pipes are designed for either flowing full or are designed for partial flow. Chemical Engineers on the other hand prefer to do calculations and modeling of two-phase flow with Froude numbers.

"In the context of this course, the Froude number is a dimensionless liquid flow rate term which can be used as a superficial volumetric comparator:



Where V is based on the assumption of full-pipe flow.

The Froude number is useful in sizing gravity flow piping because extensive observations have yielded empirical relationships which define the interdependency of Froude number, static heads, flow rates, and pipe size."

However, the Froude number methodology is likely to be conservative because the methodology does not take slope into account.


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