Blowing out skimmer and main drain

[ship of fools]

Hello everyone ... 2 year lurker and first time poster here.

I bought a house with an existing pool two years and stumbled on this site and have learned enough that i believe next year i will take the plunge and become a BBB convert. I started off using Sustain and have had good results but as you all know it is a wee tad pricey

In the meantime i have a quick question. Two years ago i paid a guy to close the pool and watched him. Last year my brother and i did it with him directing and me doing the work (he owned a pool for many years) and had no trouble.

This year i have been planning on doing it myself... ran through several online guides etc and feel very comfortable of the steps and process to get things done.

Of course i have an issue ...... when i am blowing out the main drain and close the valve it appears that the bubbles continue to flow out even though the valve is closed before i shut the compressor off ... it appears that air is still coming out of the skimmer when i blow it out ....... i was under the impression that once i shut the valve there would be an air lock ... but if i still see bubbles coming out wouldnt that mean water could leak back in?? Is it possible the valves are old and shot?? .. It is a 30 year old pool and i did have intermittent issues getting good suction with the vacuum this year.

Outside of this i am very comfortable and confident doing the rest .... but these could obviously cause some major problems so i need to get some answers or call a pro.

Thanks in advance

 

[Durk]

I'm no expert, but I think I can respond to this. When you blow the main pipe, it is is full of air that has displaced water and is flowing up through the drain. When you close the valve, some air will continue to the surface because it is past the lowest point in the plumbing, and is free to go to the surface though the drain. This is what you see. Water then enters the plumbing to replace this air and traps the air that remains on the other side of the low point and further rises until the trapped air in the pipe is compressed to a pressure equal to that of the column of water in the pool above the level of the of the water in the drain pipe. This will still be below the frost line and will remain liquid through the winter.

 

[Bama Rambler]

Durk did a very good job of explaining what happens. As long as the water remains below the forst line there's nothing to worry about.