Can wrong check valve placement or air trapped by fittings cause cavitation?

Discussion in 'Pumps and Tanks Well Forum & Blog' started by big_e, Jul 1, 2012.

  1. big_e

    big_e New Member

    Messages:
    1
    Location:
    Michigan
    I have a flowtec 3/4 HP jet pump with thermoplastic housing. The well is a sand point type of 2 inch PVC construction, 21 feet deep with water at 17 feet. The pump's only use is for irrigation. When working properly, I estimate it's output is about 6-7 gpm at 50 psi. Although the pump is brand new the well itself is about 20-30 years old and had sat unused for over a decade.

    The pump's inlet is plumbed as follows. The 2" PVC inch well --> 90 degree PVC elbow --> 2 inch PVC insert to 1 1/4 inch FIP reducer fitting --> 1 1/4 inch galvanized nipple --> galvanized union --> galvanized nipple --> 1 1/4 inch brass check valve from the home depot --> 1 1/4 inch galvanized close nipple --> pump inlet. On the outlet there is simply a priming tee and connection to two hose bibs. There is no pressure tank.

    The problem I am having is the pump will run good for about 10 minutes, then eventually the sprinklers start spitting air and the pressure will slowly drop down over time from 50 psi to about 35 psi in the span of an hour. If I stop the pump for a minute or two and restart, the pump will briefly reprime itself the pump will reach full pressure, but it will lose pressure rather rapidly compared to if the well hasn't been used for a day.

    According to flowtec's FAQs, the check valve is supposed to be at least 2 inches away from pump inlet. I used a close nipple, so it is right up to the pump inlet. Does a water passing though a check valve create enough turbulence to disrupt pump operation if its too close to the inlet? Another idea I had is that the 90 degree elbow with a reducer on it is trapping air. The worst possibility is that the well simply isn't deep enough and the pump is drawing down the water table too much and air is entering through the point.

    I've already had to clean out the check valve once to free it from plant debris that was stuck in the valve seat. If the jet were to be partially blocked, would that generate enough air to disrupt the pump? I've replaced the union and redoped all the pipe threads with Rectorseal #5, so unless I botched the PVC glue joints I have no idea where the air could be coming from. Any ideas on what could be the problem?
  2. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,049
    Location:
    NW Ontario, Canada
    Only God can make air. The well is not deep enough and is getting pumped dry.
  3. valveman

    valveman Moderator Staff Member

    Messages:
    4,482
    Location:
    Lubbock, Texas
    Yeah it sounds like the water is getting so deep, making the pump is suck so hard, it is pulling the air right out of the water. Has to do with your atmospheric pressure, but it happens between 17’ and 24’. You will need a two pipe jet pump or a submersible.
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