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exspansion tank

Kevin_52 Member Posts: 8
When you pump Towards in a fhw system, you decrease the system pressure, which increases the size of air bubbles(created by water giving off its dissolved gases when it is heated)making it harder to break them down and move them to the air collector: the relationship is inverse when pumping away.


  • larry_34
    larry_34 Member Posts: 2
    exspansion tank

    why in laymans does the pump show negative pressure when you pump into it and positive when you pump away also does it have any relation to the flow rate by how much fill pressure is in the tank and why is it more critical with higher heads?
  • Brad White_164
    Brad White_164 Member Posts: 18
    Let me try

    Wherever the expansion tank connects is the point of no pressure change (PONPC), accept that as a given. When you pump towards it, the discharge pressure by definition cannot change. Any pressure developed is absorbed by the expansion tank and the pressure cannot change, is a short-hand explanation, over-simplified.

    A circulator (pump) has a total dynamic head or just plain "head", the height of a column of water in feet that can be forced at the discharge side, balanced by the height of a column of water on the suction side.

    Picture a U-Tube with the pump at the bottom and the pump off. The fluid levels will be equal/level. Turn the pump on and the suction is drawn down while the discharge is forced up. The difference in these levels is the total dynamic head. (Just to illustrate the head concept, I described this in an open system but the principles are the same in a closed system as far as pressure is concerned.) The term "feet of head" or PSIG are simply units of measurement, it should be understood.

    To give made-up numbers or values to this, say the system at rest has a static pressure of 15 PSIG at the pump suction and the expansion tank is properly installed there. Turn on the pump and the suction may drop by 2 PSIG settling at 13 PSIG. The discharge pressure shoots up to 18 psig. The difference is 5.0 PSIG (which corresponds to 11.55 feet of head by the way).

    Now, with the negative suction and positive discharge pressures evident, what would happen if you pumped towards the point of no pressure change? The pump has a 5.0 PSIG potential pressure differential it can generate. But the discharge cannot increase! Therefore the suction must decrease! The total pressure will not be denied. No sir.

    So now you have a discharge pressure of 15 PSIG -unchanged from the static pressure) and 10 PSIG on the suction side. All of the pressure potential is transferred to the suction side.

    Now, why is this a problem or not and why do higher head and/or higher fill pressures make a difference?

    The atmospheric pressure at sea level is 14.696 call it 14.7 PSIA ("A" for absolute relative to a vacuum). In the example above, the lowest pressure in the system is 10 PSIG ("G" for gauge pressure) and also 24.7 PSIA. Thus, this example will not be a problem for the absolute pressure inside the piping is higher than the air outside of it. Air cannot leak in causing God-Knows-What mischief.

    If a high head pump is used in our system, say a 35 foot head pump (29.85 PSIA) and the initial fill is only 8 PSIG (22.7 PSIA). The pump turns on an immediately the suction pressure drops by 29.85 PSIA, a full 7.15 PSIG of vacuum. This will draw in air through any leak, gasket, vent, any path that it can because the external air pressure is higher than the suction pressure of the pump. Movement is from high to low pressure always.

    Increasing the fill pressure by at least 7.15 PSIG, one hopes more, will rectify this.

    The long and short of it though, is, always pump away from the expansion tank and save yourself potential aggravation and of understanding the chancy dynamics of your system in operation. At least you will have eliminated the variable of drooping suction pressures and save your talents for other areas of creative benefit!

    Over-simplified but the general principles hold.

  • terrence hill
    terrence hill Member Posts: 1
    new question how do i size expansion tanks??????

  • Brad White
    Brad White Member Posts: 2,398
    You should

    post this as a new question so that you get your own responses specific to your needs.

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • ALH_4
    ALH_4 Member Posts: 1,790

    As Brad said, the expansion tank is essentially the "point of no pressure change". For illustration purposes, think of it as having a relatively constant pressure whether the pump is on or off.

    When the expansion tank is on the discharge of the pump, you are "pulling" the fluid through the system.

    When the expansion tank is on the suction side of the pump, you are "pushing" fluid through the system.

    You always want to push the fluid through the system, because pulling hard enough can cause the fluid to vaporize. Hence high head pumps are more prone to problems in this arrangement.
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