Drain/Winterize Large Diameter Water Line with Compressed Air

[Ghost006]

I'm currently in the middle of a discussion between another engineer and the operator of an agricultural irrigation system on how to winterize the system. The pipeline is predominantly 14" PVC installed to grade so that water will drain to the designed low points. The discussion is over what to do at those low points. The simple solution is to install a riser to the ground surface with the ability to remove a cap and pump out each low point/drain in the fall.

The other proposed option is actually what was installed on the operator's previous system (lease of the pump site was ending and needed to move to a different site requiring new main pipeline). This option is to install a saddle on the large diameter pipe with a 2" brass pipe coming off the bottom, running horizontally to the side with a curb stop installed followed by another 2" section of pipe that runs to the surface with a cap on it. The theory behind this is to allow for compressed air to be used to force the water through that 2" pipe and up to the surface.

Honestly, both of these systems make sense to me. The other engineer is of the mind that air will not force the water through the 2" pipe. The resources they've shown me, however, are in relation to home/golf irrigation systems where they were not installed to grade and are attempting to force the water up through the nozzles. This process requires that air move fast enough to suspend water particles and force them up through the nozzles. I'm not sure this applies to what we're discussing on this project, however, as the drain detail would simply require the air pressure be enough such that it could force the water through the required elevation head to the surface.

I'm hoping that someone here can provide some clarity on this discussion, preferably with some references or other resources (other engineers here respond better to technical resources). Any information would help though.

I've attached the drain detail from the design of the previous system for reference.

Thanks!

 

[LittleInch]

Well based off your sketch you will need about 0.1 bar min ( say 2 psi) to lift the water out of the pipe.

Basically static head (density x g x height) plus a bit to get it moving up the pipe. It really is that simple. your key difference is that you're taking the drain tapping off the bottom of the pipe. Off the top - won't work.

The issue will be when the air/ water interface gets close to the hole you will get blow-by and hence some air / spurting which could be an issue depending on what's there.

The higher the pressure used the more the quantity of air bypassing the water / likelihood of it happening.

The issue will be locating the low points accurately.

You could probably do it from one end by adding air and gradually going along the line opening the drains until air starts coming out then moving to the next one.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ghost006]

LittleInch - Those are my thoughts exactly. The issue with the blow-by when the water surface approaches the inlet of the 2" pipe is handled a couple of different ways. The one detail shows a second curb stop that would then be opened to allow whatever water is left at that point to drain to a gravel sump. The other detail does not have that second valve but instead operates under the assumption that the low points in the system would be below the frost line and complete evacuation of water from the line would not be necessary.

The low points are accurately located during installation with survey grade GPS.

Apparently they winterized the prior system exactly as you are describing with gradually moving from drain to drain.

Thanks for the response...confirms that I just might not be going crazy after all!

 

[dhengr]

Ghost006:
Given that the 14” PVC pipe a below the frost line, or even if it does freeze a bit, a few inches of standing water, a few yards long, in the bottom of the main PVC pipe shouldn’t hurt anything. But, all that small dia. copper hardware and pipe at a lower point (elev.) must drain completely or you could have serious trouble, where a few cups of free water might freeze and bust copper hardware. I like the plumbing of the left detail, but the sump thinking of the right detail. Isn’t there an on-off valve, like the self-draining valve on a fire hydrant, where the drain is only opened when you close the primary supply valve, then it drains the small volume of pipe system to ground. Mount this valve at the bot. tee on the left curb stop riser, and provide the capped vert. pressure drain riser too, but no need for a second curb stop riser. Make some volume (a function of the water in the main pipe) below this plumbing pit your crushed rock sump, all in a smaller footprint drilled pit. Of course, this assumes that the virgin soil drains reasonably well, so the entire pit doesn’t become a wet-well.