High point in a water transmission line 1

[PBW2]

What constitutes a high point in a water transmission line? For instance, a 24" water transmission line that is laid along relatively flat terrain, would a 12" elevation difference (peak) require an ARV, would 6", 24"? The water line in question was designed with a slope but in reality may have been installed with minor elevation changes (rise & dips) over an 800' section. Trying to locate literature on the subject and would appreciate any guidance or thoughts.

 

[bimr]

It is really a question of the fluid velocity, which you have not stated.

Maybe this will help:

The Water Pollution Control Federation Manual of Practice No. FD-4 "Design of Wastewater and Stormwater Pumping Stations" states "A minimum velocity of 1.2 m/s (4 ft/sec) is required in the pipeline to shear the bubble and keep it moving downgrade."

If you have a velocity of 2 ft/sec or less, the air will become trapped over time at a high point. However, a velocity above 3.5 ft/sec will push any air downstream and you do not need an air release. If you were to have significant downhill slope, then the velocity needs to be increased to push the air downstream.

The attached articles explain this in detail.

 

[rconner]

If this happens to be a new pipeline that has not yet been tested(as this sounds), it would seem these high points now not provided with air release quite likely contain air. Most venerable installation and testing standards for pipelines suggest or require that lines be filled slowly, and that air be removed at least for hydrostatic testing (if not by ARV at least by corporation stops etc. tapped into the top of the pipe, at known high points).

 

[PBW2]

bimr/rconner thanks for your replies.

A little additional information. The flow varies from .5 fps (minimum) to 4.4 fps (maximum) the average velocity is .7 fps. I am waiting to get an Asbuilt profile of the line but I suspect the asbuilt to show that the line was installed with dips and high points (worst case less than 2'). The contractor is arguing that the line should be accepted by the municipality as installed. The municipality is arguing that the pipe should have been installed according to the design drawings (straight with slight downslope). We are talking about 1000 lf of relocated pipe.

Thanks again.

 

[bimr]

Water lines are not generally installed with slope. What was the reasoning for the slope? Is the pipe operating at all times?

Water lines generally follow the slope of the ground surface. +/-2 feet over 800 feet is not generally something that one would be concerned with.

 

[cvg]

virtually every municipal waterline I have ever designed and most plans I have seen had a profile with slopes and invert elevations shown and required. so to say waterlines are generally not installed with slopes does not quite line up with industry practice that I have experienced. you will have persistent air pockets with your low velocities. you will need at least temporary air release in order to hydrotest. The waterline is probably acceptable. But my question is why was this construction change not reviewed or approved prior to construction?

 

[fel3]

bimr & cvg…

Here is my experience:
[a] For horizontal control, I prefer coordinates. Some designers prefer station/offset from street centerline (if available).
For large diameter transmissions main designs, I include station/coordinate/elevation tags at each vertical bend, tee, and cross, and station/coordinate/deflection angle tags at each horizontal bend. This is especially important for steel pipe (e.g. CML/C) because that information is needed for fabrication. Some designers also include slopes and some do not. One advantaqe to including slopes is this allows cross-checking the station/elevation data. Transmission mains don't follow every little rise and dip of the ground so as to reduce the need for air/vacuum valves and to simplify fabrication and construction. Some designers use top of pipe and some use invert and math is required to find the other. Specifying top of pipe generally insures adequate cover. Specifying invert gets you closer to the bedding grade required, but you still need to subtract the pipe wall thickness. The sompany I started with preferred top of pipe. One guy I know uses centerline (probably due to his extensive plant piping experience) even though the centerline is not a physical entity that the contractor can measure from in the field.
[c] For small diameter distribution mains, I typically specify the minimum cover over the top of the pipe as well as separations between the main and other underground utilities. Thus, the contractor can install the pipe paralleling the ground profile or he can straighten out the vertical profile as he chooses. Distribution mains usually have so many service connections that air/vacuum valves are rarely needed. Each service connection acts as a manual air release valve.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[PBW2]

Thanks for all of the replies!

We rec'd the as-built profile and as we thought the water line was installed with dips and rises. The contractor kept 4' cover and put the instrument away. Basically the profile shows that the new water line drops .9' in 34', rises 1.6' in 120', drops 1.2' in 120', rises 0.65' in 140', level for 70', then gradually drops down 2.6' over 220', it then rises 6' in 80' to the tie-in point with the existing line and an ARV. Although the water line was not installed per the design profile, it seems to me that the water line should function as installed, but that there is potential for air pockets. I read somewhere that if the elevation difference is less than the pipe diameter it shouldn't be a concern. Thoughts or comments?

 

[bimr]

If all of the air is removed during construction, there should not an issue. Again, the key to passing any air downstream, is velocity.

 

[greggy]

Whenever you have a main break, mass quantities of air are sometimes sucked into the main, travel down the main, and reside at these highpoints. Instead of an automatic air release, valve, and piping in an expensive vault, consider just tapping the main with a 1-inch copper line and bring it over to the side of the road into a valve box with a curb valve. Then if you suspect air in the line and want to check it, you open the curb valve manually and allow the main to exhaust. Since the possibility of having air in the main is small, this is a less expensive means to check the main and bleed any air once in a while as needed.