Sewage check valve not working

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Colin Blamire, Sep 26, 2020.

  1. Colin Blamire

    Colin Blamire New Member

    Joined:
    Sep 26, 2020
    Location:
    Ontario, Canada
    Hi,

    Installed a new sewage pump in my pit. 2" ABS, straight vertical rise 8', a 90 deg bend with a 5'run, then another 90 deg bend to straight sloped run of 50' to the septic.
    A foot above the pit lid I installed a sewage rated flapper check valve vertically with flow correct. Seems all good.
    Discharge goes well, but then backflow runs through the check valve back into the pit. Seems there is a lot of negative pressure running back to the pump.
    I've tried 2 weeping holes on the pipe between the thread and check valve, one below the water line and then one above. I've covered each separately, and then both. Each time I remove the check valve to check for debris there is none...and the valve works for one time only after I replace it. I've also tried 2 different check valves to rule out damage. Is this an air flow issue? I've tried so many times the water in the pit is probably drinkable. What coul be keeping the flapper from closing?
    Any ideas?
    Colin
     
  2. Reach4

    Reach4 Well-Known Member

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    Sep 25, 2013
    Location:
    IL
    The pit is vented to the roof?
     
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  4. Colin Blamire

    Colin Blamire New Member

    Joined:
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    Location:
    Ontario, Canada
    Yes
     
  5. Colin Blamire

    Colin Blamire New Member

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    Sep 26, 2020
    Location:
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    Even when the lid is off it still does the same thing.
     
  6. Martin Boring

    Martin Boring In the Trades

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    May 9, 2020
    Location:
    Malcolm Ne.
    You may have to try a little bigger weep hole under the water line to get the flapper to close quicker. Had to do that on one that was acting that way a while back after a pump and check valve change out. I like to keep the hole close to the pump connection.
     
  7. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    I would try a different brand with different construction. Maybe one with a ball.
     
  8. Colin Blamire

    Colin Blamire New Member

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    Sep 26, 2020
    Location:
    Ontario, Canada
    I've got a new PVC check/ball valve combo on the way that is higher quality. I'm just hoping this isn't a venting issue on the septic side...venting currently through the bed.
     
  9. Reach4

    Reach4 Well-Known Member

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    Location:
    IL
    I don't think pressure there should exist, and if it did the check valve should stop the flow anyway.

    Venting for the septic tank is typically through the pipes, which eventually get connected to the roof. I presume there are various vented plumbing fixtures sharing that septic tank.
     
    Last edited: Sep 28, 2020
  10. hj

    hj Master Plumber

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    Occupation:
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    Location:
    Cave Creek, Arizona
    The "hole" has nothing to do with the operation of the check valve, it is to prevent "air lock" if the pump sucks all the water out of the pit. The vent is also irrelevant as far as the check valve is concerned.
     
  11. Bannerman

    Bannerman Well-Known Member

    Joined:
    Mar 19, 2014
    Location:
    Ontario, Canada
    This is the cause. Although drain flow will be forced while the pump is operating and adding more fluid, the fluid already within the line will need to rely on gravity to flow to the septic tank once the pump is turned off. The gravity flow through the 5' horizontal and sloped 50' sections is creating suction behind which is pulling the check valve open.

    Replace the 90 elbow with a 'T' fitting where the vertical joins the 5' run. Install either an AAV or an extended length of vertical pipe into the 'T' which will function as a vent to allow air to enter to break the suction from that point forward. The check valve will then remain closed and the effluent above the check valve will continue to remain in that vertical section up to the 'T' when the pump is not running.

    As there is now a hole into the discharge line above the pit's liquid level, air entering that hole will cause the effluent between the pump and the check valve to continue to drain back into the pit after the pump has shut off.
     
    Last edited: Sep 28, 2020
  12. Colin Blamire

    Colin Blamire New Member

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    Sep 26, 2020
    Location:
    Ontario, Canada
     
  13. hj

    hj Master Plumber

    Joined:
    Aug 31, 2004
    Occupation:
    Plumber
    Location:
    Cave Creek, Arizona
    1. there would only be "suction" on the check valve if it were HIGHER than the pipe's outlet
    2. IF there is suction, it would be "pulling' the water OUT of the pipe, not letting it backflow into the pit.
    3. The water above the check valve forces it closed, (sometimes quite loudly), so something is preventing it from closing.
     
    breplum and Reach4 like this.
  14. Bannerman

    Bannerman Well-Known Member

    Joined:
    Mar 19, 2014
    Location:
    Ontario, Canada
    Water is commonly siphoned out from a fixture trap when the trap arm downstream from the trap is not properly vented.

    The OP stated his sewage pump is discharged through an 8' vertical rise which transitions to a 5' horizontal run then to a 5o' sloped run of 2" pipe which discharges into the septic tank. Although the fittings utilized were specified, there was no mention of venting the drain line itself.

    A sewage pump will produce a large slug of rapid flowing effluent which will fill the 2" pipe diameter. Once the pump is shut off, the volume of effluent flowing within the 5' + 50' unvented run to the septic tank will create negative pressure behind it.

    Negative pressure will cause the check valve flap to open slightly which will allow air to be drawn through the hole above the pit's liquid level or thorough the pump itself since there will be little liquid remaining in the pit. The air drawn upward through the check-valve will break the siphon and will displace some liquid so not all liquid will be siphoned from the vertical pipe section. Since effluent is heavier than air, any liquid remaining in the vertical section will drop back into the pit through the partially open check valve which will continue to remain open until the flow eventually enters the septic tank and the negative pressure situation is eliminated.

    Venting the drain line at its highest elevation will ensure negative pressure on the check-valve will not occur and will also allow the effluent within the long pipe to flow more smoothly to the septic tank.

    The OP has already tested 2-check valves with each performing similarly. Installing an AAV (air admittance valve) at the highest elevation will be a simple and low-cost method to determine if a vent will resolve the problem.
     
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