Water Maker Output - Suction OK?

Discussion in 'Boat Design' started by Scott Carter, Jun 21, 2010.

  1. Scott Carter
    Joined: Oct 2006
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    Scott Carter Senior Member

    Haven't heard back from the manufacturer on this question yet so I thought I'd see if one of y'all have any insight on this. I'm plumbing a fresh water system and adding a water maker. I need to know if there exists any potential adverse affect on the performance of a water maker if its fresh water outlet (product) is plumbed in series with the inlet side of the fresh water system pump (small Jabsco vane pump). The owner's request is that no more holes get cut into his fresh water tank, which would have been my choice for supplying the water maker product to the tank. So I need a way for the water maker's water to get into the tank. The only available port is the one the fresh water pump is already drawing from, near the bottom of the tank. So, in theory, some of the pump suction that compels the water out of the tank will also be seen by the water maker line filling the tank via the Tee fitting I'll insert into the line just as it exits the bottom of the tank. So the water maker and tank are in parallel, and the two of them in series with the pump.
    Who knows about this? Speak up!
    Thanks in advance,
    Scott

    p.s. I didn't know where else to put this post, so I opted for design. Sorry to the moderator if it is mis-placed.
     
  2. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    Put the water from the water maker into the top of the tank via the tank vent line, just insert a T piece there.
    The tank will still vent as the water goe into the tank, and no more holes are required.
     
  3. Scott Carter
    Joined: Oct 2006
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    Scott Carter Senior Member

    Landlubber, that's a great idea (which we'd considered briefly), except that I did not mention that neither the vent nor the deck fill tube are readily accessible without removal of some finished cabinetry. So, our preferred options are to fill by the water maker through the tank's draw hose, or in distant second place to make a new hole in the tank. So I'm still wondering if anyone knows whether moderate suction (by the fresh water system pump inlet) on the fresh water output line of a typical osmotic water maker could prove detrimental. As to how much suction, it's hard to determine, because the pump will be drawing water out of the tank at the same time that it is putting suction on the water maker's line. So there will be some residual vacuum on the line, it's just tricky to know how much.
    In case anyone is tempted to ask, I don't have the specs on the water maker yet, so I don't know about the membranes, vale configuration, etc. I'm still hoping to get a firm answer out of the manufacturer for this, but I thought perhaps one of you might have run into this before.
     
  4. Scott Carter
    Joined: Oct 2006
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    Scott Carter Senior Member

    Answering my own question here for the benefit of any reader who has an interest: According to Sea Recovery (water maker's manufacturer) a slight suction on the product line is not necessarily detrimental to the water maker. But, it does have the potential to actually pull brine solution through the membrane, and thus into your fresh water system, when the unit is not in operation. So it's not advisable to plumb the system in this manner if it's possible to avoid it.
    I was thoroughly warned as well that the opposite situation is even more undesirable. That is, to place any sort of resistance, slowdown or obstruction to the product flow is a big no-no. A complete blockage (perhaps a ball valve in the product line) while the unit is in operation will instantly and permanently damage/destroy a membrane. A valve is the most commonly seen mistake, but even if the the water maker is situated lower than the tank it's filling and it's not filling from the top (dumping into atmospheric pressure) but rather from the bottom this is a problem. The static head pressure exerted on the membrane through the fill line can be enough to damage the membrane.
    The absolute recommended way is to plumb the product line into a top filling port.
     
  5. yipster
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    yipster designer

  6. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Ive never seen water added to the suction side . You could try the suction side, but it Sounds like a bad idea for various reasons.

    Before you try make sure the suction side is not fitted withn a non return valve.
     
  7. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Is there enough vertical to fit a duck bill vented riser loop? (hooked up as you first wanted to.)
     
  8. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The performance of the water maker will deteriorate with pressure fluctuations at its output. If the pump doesn't run, there is positive static pressure equal to the water level in the tank, so the osmosis will slow down or stop completely. With a negative pressure brine is pulled through the membrane and that also is not what your customer wants.
    Once the owner understands this he will probably change his mind about the extra hole.
     

  9. michael pierzga
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    michael pierzga Senior Member

    Have a look at the Product dump solenoid valve. Is it closed .... in product water dump overboard position.... when the watermaker is turned off ? An additional solenod valve to close off the product water out could be usefull
    Remember even a tiny leak into the feed water column of your fw system pressure pump will cause it to loose its prime
     
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