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  • Results 1 to 17 of 17

    Thread: Another downside of knife valves

    1. #1
      kimini is offline Senior Member
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      Another downside of knife valves

      If your pond uses sand filters, it'll eventually get back into the pond, where it'll get drawn into the bottom drain and work its way to a knife valve if you use them. The problem is that some sand ends up sitting on the slotted O-ring which serves as the seal when the knife valve is closed, pushing that sand into the slot where it's stuck forever. This causes the knife to no longer close completely and it'll leak a little forever after. This may or may not be an issue, and though I knew they weren't perfect, they were cheap and they wouldn't get used much, so figured they'd be fine. Didn't think about sand jamming them open though...

      I finally bit the bullet and ordered true union ball valves, which are much easier to service, way more expensive, and more reliable, I assume, if they can handle the sand.

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    2. #2
      tbullard is offline Senior Member
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      Here is a good option for an affordable true union ball valve. https://www.supplyhouse.com/Red-Flag...ion-Ball-Valve

    3. #3
      kimini is offline Senior Member
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      I should have mentioned that those are the ones I ordered. Still expensive, but they're the only way to have a reliable system.

    4. #4
      john s is offline Supporting Member
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      With or without sand, eventually a knife valve will clog if you need to press down to close it. To avoid, place the valve in a horizontal position. Even better, place the valve under the pipe, pull down to open, press up to close. Either way, I have not had a problem in many years. Not so with the press down to close. - John

    5. #5
      MCAsan is offline Senior Member
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      The 4" knife valves I put on our BD drain pipes are still working just fine with more than 15 years of service. Perhaps the problem is not the knife valve but rather the use of sand filters.

    6. #6
      mplskoi is offline Supporting Member
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      All of my ball valves got to be very hard to close. Eventually the handles broke and I keep a pipe wrench in the pump house.
      One upside for me of knife valves is that they fit better in tight spaces. I have swapped mine out during switching for one RDF to another and it was easier to fit the knife valves in.

    7. #7
      kimini is offline Senior Member
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      That's a good point about orientation, though it can't help in my installation. I'm surprised that this isn't popping up more often, given the popularity of sand filters and their propensity for losing sand to the pond. That, and knife valves are probably used more often than union valves due to their expense.

      Regarding ball valves becoming hard to turn, was this on a gray PVC full union valve, or the Home Depot white PVC valves? I've definitely had that exact trouble with the latter.

    8. #8
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      I'm confused...how does sand get to the pond if you always plug the port leading to the pond while blowing out the filter?

    9. #9
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    10. #10
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      Stay away from white pvc valves they are made cheap and have been difficult to turn

      Sent from my XT1575 using Tapatalk

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    11. #11
      icu2's Avatar
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      I'd just add that color isn't always a good indicator of good or bad ball valves...
      I've used a large number of these valves and even after over 10 years of use, they turn as easily as day one.



      https://www.lowes.com/pd/AMERICAN-VA...lve/1000339525

      But I know the ones you mean and you're right, some become impossible to turn in short order.
      --Steve



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    12. #12
      icu2's Avatar
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      Quote Originally Posted by MCAsan View Post
      The 4" knife valves I put on our BD drain pipes are still working just fine with more than 15 years of service. Perhaps the problem is not the knife valve but rather the use of sand filters.
      I've always heard people discourage others from using sand filters on their ponds. I've seen a lot of threads on converting them to
      bead type filters, but I thought sand filters would clog quickly on a pond as opposed to a swimming pool.
      --Steve



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    13. #13
      coolwon is offline Senior Member
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      Quote Originally Posted by icu2 View Post
      I've always heard people discourage others from using sand filters on their ponds. I've seen a lot of threads on converting them to
      bead type filters, but I thought sand filters would clog quickly on a pond as opposed to a swimming pool.
      Very, very, very quickly.

      Garfield
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    14. #14
      kimini is offline Senior Member
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      Sorry, I should have said "SG" filter, forgetting that "sand filter" is overly generic, though in the context of this thread it doesn't matter. As for how sand got into the pond... well, all it takes is one time to forget to cap the outlet return before switching on the air.
      Last edited by kimini; 01-24-2020 at 08:27 PM.

    15. #15
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      I’ve had some issues with sand in the slot too. I’ve found if just before you remove the “90” from the waste line you close the knife valve half way or so. Doing this causes the waste water to flow faster through the valve and become more turbulent. In my case it helps clean the slot out better.

    16. #16
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      Quote Originally Posted by kimini View Post
      Sorry, I should have said "SG" filter, forgetting that "sand filter" is overly generic, though in the context of this thread it doesn't matter. As for how sand got into the pond... well, all it takes is one time to forget to cap the outlet return before switching on the air.
      If using 4" waste lines from the filters, that's a lot of money to keep from getting your hand wet.
      I just use test plugs.
      --Steve



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    17. #17
      kimini is offline Senior Member
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      That's why they're going into the 4" bottom drain pipes only.

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