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Thread: Inlet pressure to the filter decreases

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    stev32k's Avatar
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    Inlet pressure to the filter decreases

    This technical side conversation has been split off of this topic. JasonLion

    When the inlet pressure to the filter decreases it means the flow has increased. That sounds counter intuitive, but that is what happens. The inlet pressure at the filter is a measure of the pump discharge pressure and as the pump flow increases the pressure decreases. If you had a valve on the pump discharge you would see that closing the valve would make the pressure rise and opening the valve would make the pressure go down. That's how centrifugal pumps operate.

    As the flow though the filter increases the differential pressure across the filter will increase - the differential pressure is the inlet pressure minus the outlet pressure, and this difference will increase if the flow is increased or if the filter gets dirty. The reverse is also true. If you reduce the flow though the filter the difference between inlet and outlet will get smaller and smaller as the flow goes down. If there is no flow though the filter the inlet and outlet pressures will be the same.
    20' x 40' IG with vinyl liner volume approx. 35,000 gal.1.5 H.P. main pump, Polaris 280 cleaner W/ 3/4 H.P. booster pump
    Hayward sand filter, 3.14 sq ft, 62 gpm. Stenner 45 MPH10 chlorine feed pump. 1 micron final filter bags (home made and very effective)

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    mas985's Avatar
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    Re: Filter pressure a direct relationship to filtration time

    Quote Originally Posted by stev32k
    When the inlet pressure to the filter decreases it means the flow has increased.
    Not always. That is only true if the reason the the pressure decreases is because the return head has decreased like when the filter is cleaned.

    Filter pressure can also decrease when suction head increases as well as with a clogged impeller. In both cases, filter pressure can drop with lower flow rate.
    Mark
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    stev32k's Avatar
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    Re: Filter pressure a direct relationship to filtration time

    Quote Originally Posted by mas985
    Quote Originally Posted by stev32k
    When the inlet pressure to the filter decreases it means the flow has increased.
    Not always. That is only true if the reason the the pressure decreases is because the return head has decreased like when the filter is cleaned.

    Filter pressure can also decrease when suction head increases as well as with a clogged impeller. In both cases, filter pressure can drop with lower flow rate.
    The discharge pressure from a centrifugal will always decrease with increasing flow. So if the inlet pressure on a filter decreases and the pump suction is not plugged and the impeller is not damaged or clogged it can only mean the flow has increased.

    Yes, an increase in suction pressure caused by plugging, or a drop in water level will result in a decreased discharge pressure. However, in this case the pressure drop is caused by running the booster pump which lowers the discharge pressure and results in higher flow and higher DP. If both inlet and outlet pressures make a sudden drop at the same time it is a sure indication that the filter has broken though and is channeling.

    If the discharge pressure drops because of increased suction pressure you would most likely be able to hear cavitation in the pump and/or see air bubbles in the suction strainer. If the impeller gets clogged or worn down all you can do is take the pump apart to see what the problem is.
    20' x 40' IG with vinyl liner volume approx. 35,000 gal.1.5 H.P. main pump, Polaris 280 cleaner W/ 3/4 H.P. booster pump
    Hayward sand filter, 3.14 sq ft, 62 gpm. Stenner 45 MPH10 chlorine feed pump. 1 micron final filter bags (home made and very effective)

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    JasonLion's Avatar
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    Re: Inlet pressure to the filter decreases

    The OP has a Polaris 360, which doesn't have a booster pump. It is a pressure side cleaner and is normally installed to take water from before the filter. Presumably the cleaner line is reducing the pressure side head, which will increase the flow rate through the pump.

    However, some of your statements are too broad. A suction side cleaner would decrease the filter pressure while reducing flow rate, a very common situation that your statements appear to deny. Also, what really matters in regards to the original question is the amount of water flowing through the filter, which is indeed reduced in this particular situation. The flow rate through the pump increases, but more water than the increase is going through the cleaner and not getting filtered. Thus total pump run time needs to increase to still run the same amount of water through the filter.

    If the cleaner line taped off after the filter then the situation you describe would hold, and pump run time could be decreased.
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    stev32k's Avatar
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    Re: Inlet pressure to the filter decreases

    OK, I stand corrected. I was not aware the line to the cleaner came off before the filter. In that case I see how the filter flow would be reduced.
    20' x 40' IG with vinyl liner volume approx. 35,000 gal.1.5 H.P. main pump, Polaris 280 cleaner W/ 3/4 H.P. booster pump
    Hayward sand filter, 3.14 sq ft, 62 gpm. Stenner 45 MPH10 chlorine feed pump. 1 micron final filter bags (home made and very effective)

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