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Thread: Air Block in Suction line

  1. #1
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    Default Air Block in Suction line

    Hello ,




    Let me explain the scenario. I live in a building. We have a water pipe supplying water from the city mains running parallel to my building. This horizontal water pipe is shared between my neighbour and me. We both have water pumps that suck water and fill our overhead storage tanks.




    Please refer to the key in the diagram to understand the shapes .

    If the figure does not load, please click : http://img36.imageshack.us/i/pumpdiagram.jpg/


    I have a check valve just about half an inch from the begining of the vertical pipe. Off late, whenever I start my pump,after say a gap of 20 hours, I get a very shrill sound from the pump for 2-4 seconds and I do not get any water. If I leave the pump on after this sound, about 4-7 minutes later, I will get a perfectly normal flow of water.

    Alternatively, I can open the priming tap( which is connected to the suction line of the pump) and pour some water in the suction line, a shrill sound comes once again lasting about 2-4 seconds and then I get the normal flow of water.

    Another thing, I've noticed is if we switch off the pump after its run for 30-45 minutes and after the water started flowing, then restart it after 15- 30-45-60 minutes ( never tried beyond 60 minutes), I would never have to do any of the things I have mentioned above. The water would just flow normally.

    We have not had any problem with this setup all these years, however this problem started the day my neighbour extended the main pipe & took a seperate connection from the mains till his house. Earlier we had a mutual understanding where one pump ( placed in my house ) would supply water to both houses.

    Now since we have seperate pumps, we start pumps one after the other, never together.

    When my neighbour did this seperation I saw that I had a leaking check valve which was leaking like say 30 drops a minute or less. I did not bother changing it that day.

    I did another test because I felt that even if it was leaking, since it was connected so close to the source it would not matter. I filled the suction line completely using the priming tap near the pump and then monitored it for 12 hours, I did not notice any noticeable drop in the level of water. Can this method be used to check for a leak in a check valve ?

    I still have not replaced the check valve.

    I feel its a air leak somewhere, but I cannot figure it out.

    I hope I have explained the problem well enough and I hope we can find a solution to this daily priming activity.

    Thanking you

    Danny
    Last edited by danny25; 09-30-2009 at 07:16 AM. Reason: addition

  2. #2
    Moderator valveman's Avatar
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    If the check valve is leaking water out, it will even more easily leak air in. You have a suction leak that is letting air in, and causing the pump to loose prime.

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    Quote Originally Posted by valveman View Post
    If the check valve is leaking water out, it will even more easily leak air in. You have a suction leak that is letting air in, and causing the pump to loose prime.
    This is what I find most confusing, if the horizontal water line contains water where does air enter it, so that it can enter the vertical line.
    Last edited by danny25; 09-30-2009 at 06:32 AM. Reason: Correction

  4. #4
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    The same place that leaks water out under pressure, will let air leak in when the pump creates a vacuum.

  5. #5
    Plumber Cass's Avatar
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    The red line should be suction, if I am reading the diagram right...and the air is being sucked in somewhere on that line...

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    Quote Originally Posted by Cass View Post
    The red line should be suction, if I am reading the diagram right...and the air is being sucked in somewhere on that line...
    No the black line is the suction, the red is the delivery line.

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    Quote Originally Posted by danny25;
    I did another test because I felt that even if it was leaking, since it was connected so close to the source it would not matter. I filled the suction line completely using the priming tap near the pump and then monitored it for 12 hours, I did not notice any noticeable drop in the level of water. Can this method be used to check for a leak in a check valve ?
    Does this give me correct results ? Or is it incorrect ?

    Quote Originally Posted by valveman View Post
    The same place that leaks water out under pressure, will let air leak in when the pump creates a vacuum.
    Correct me if I am wrong : if the check valve is leaking water when the pump is off, then air enters the vertical line through the same place where water is leaking, but what causes confusion in my mind is how does air manage to get below the check valve when there is water supposed to be in that horizontal line, also how does the air cross the check valve and enter the vertical line when that line is full with water.

    I'm sorry for the pestering.
    Last edited by danny25; 09-30-2009 at 07:06 AM. Reason: addition

  8. #8
    Moderator valveman's Avatar
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    Water flowing past a leak in the pipe can draw in air just like a bubler in a hot tub. And if there is a vacuum created, air comes in much easier than water. You need to fix the leak.

  9. #9
    Plumber Cass's Avatar
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    OK then it is somewhere on the black line...between check valve and pump...

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    Quote Originally Posted by valveman View Post
    Water flowing past a leak in the pipe can draw in air just like a bubler in a hot tub. And if there is a vacuum created, air comes in much easier than water. You need to fix the leak.
    I have checked the pipes visibly for any leakages & there is no leakage of water through the pipe.

  11. #11
    DIY Senior Member Bob999's Avatar
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    Is your neighbor having the same problem or only you?

    If only you the leak must be in that part of the system unique to you. If both are having the problem the leak is in parts of the system that are common to you both.

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    Quote Originally Posted by Bob999 View Post
    Is your neighbor having the same problem or only you?

    If only you the leak must be in that part of the system unique to you. If both are having the problem the leak is in parts of the system that are common to you both.
    Sadly, its only me

  13. #13
    Porky Cutter,MGWC Porky's Avatar
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    Default You're getting air in the suction!

    Valveman is right. Any water leaking the suction line at rest will cause air to enter the suction line when it's pumping. The pump may pump air and water while it's running but when it's at rest the air bubbles in the water will rise and settle in or near the pump. Then when the pump starts again after setting for a time the air in the pump causes the pump to be partially air logged. Repairing all the leaks should solve your problem.

    Porky

  14. #14
    Master Plumber nhmaster's Avatar
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    Put a vacuum gauge on it and start checking for a leak.

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    Quote Originally Posted by Porky View Post
    Valveman is right. Any water leaking the suction line at rest will cause air to enter the suction line when it's pumping. The pump may pump air and water while it's running but when it's at rest the air bubbles in the water will rise and settle in or near the pump. Then when the pump starts again after setting for a time the air in the pump causes the pump to be partially air logged. Repairing all the leaks should solve your problem.

    Porky

    Hello,

    Well, I checked the pipes for visible leakages and I could not find any leakage from where water is leaking. I can see all the pipes .

    But, what I do know is the check valve does not shut 100% tight so it leaks water into the pipe below the check valve. Can this be the problem ?

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