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So I was thinking about pumps....

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delta T
delta T Member Posts: 884
Commenting on another thread involving a dripping relief valve and two expansion tanks and I started to wonder what can actually happen in a system with two expansion tanks? Lets say we have two identical expansion tanks, each with the same charge on them, and one is connected directly behind the pump, and the other directly in front of it. What happens if you only have one expansion tank, but the connection branches (lets say for the sake of argument that the branch piping is of equal length and size) and one branch connects behind, the other in front of the pump? I don't think I've ever had cause to think about this....

Lets say for sake of argument in the case of a single expansion tank that it is a bladder style and not a compression tank.

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  • Gordy
    Gordy Member Posts: 9,546
    edited December 2018
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    I’ve tried it with a compression tank, it doesn’t work you lose the pumping away benefit. Should not matter what type of tank.
  • delta T
    delta T Member Posts: 884
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    I'm wondering what happens to the pressures in other parts of the loop, obviously this is not something I would ever purposely install, just curious about the physics. The pump wants to create a delta p, but can't change the static pressure at the expansion tank connections, and if it is a single tank, we know the pressure is exactly equal on both sides. The pump can't possibly just stop pumping can it? but on the other hand, no delta p, no force to move the water.....

    Assuming the pump moves water, what would you see if you had pressure gauges very 6" all along the loop?

    @Gordy what was your set up? single tank? connected how?
  • Gordy
    Gordy Member Posts: 9,546
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    The differential ends up on the suction side. Path of least resistance.
    delta T
  • Gordy
    Gordy Member Posts: 9,546
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    single compression tank. Hose bib drain fitting in one end went to a drain bib at suction side of pump.

    Air troll in other end piped to boiler on discharge side of pump.

    Close the airtrol end pump away from Ponpc, Close the drain end pump into ponpc. Open both and the differential went to suction side of pump.
    delta T
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    On the situation where the pump is short circuited -- the same expansion tank connected to both inlet and outlet -- it will merrily churn away at its maximum flow rate, and there will be a very small pressure differential between the inlet and outlet. Not very good for the pump...

    If there are two expansion tanks, one on each side, but not interconnected, they will respond just as they normally would in their respective locations.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    delta TSolid_Fuel_Man