Point of no pressure change

Zetch

I've read pumping away and understand the premise that a circulator cant change pressure at the expansion tank but I cant seem to understand why it cant put water in or out of the tank? is it because of the diaphragm moving with the pump turning on maintaining the constant pressure in the tank?

Jamie Hall

Well, sort of yes -- although a diaphragm to separate the air and the water is useful but not necessary. There is a fixed amount of air in the tank. There is also a very nearly constant volume. If something outside the tank reduces the volume of water in the system -- and thus tries to remove water from the tank, the air in the tank will expand and, necessarily, the pressure will drop accordingly. Similarly, if something increases the volume of water in the system and thus tries to push water into the tank, the air will compress and its pressure will increase accordingly.

So, strictly speaking the tank is not a constant pressure.

However, a pump cannot change the volume of water in the system. All it does is move the water. The only thing that can change the volume of water in the system is a change in temperature of the water, making the water expand or contract. Therefore, with regard to the action of the pump, the pressure tank is indeed a point of no pressure change.

Zetch

I apologize if this seems like a stupid question but suppose you ignore any thermal expansion and assume the pressure inside the system is at a constant and the only influence is the circulator, if pumping towards the point of no pressure change the entire PSI differential on the pump would appear on the inlet of the pump making the average system pressure lower than the static fill pressure, vice versa if you pumped away from the point of no pressure change the entire PSI differential would be added making the overall system pressure higher than the static fill, what I don't understand is how solely moving water and not changing the volume of the system or mass of water in the system can change the overall pressure, wouldn't the diaphragm have to push or pull in some way to change the volume and in turn increase or decrease the average system pressure in each of these scenarios

Jamie Hall

If there is no volume change of the water -- no expansion -- there can be no volume change of the air, and the air -- and the water in the tank -- will remain at a constant pressure.

Now you are quite correct in one way. If a pump is in use, it will increase the pressure on the outlet side of the pump relative to the pressure on the inlet side. If the constant pressure point is on the inlet, then the increased pressure will be on the outlet and the average system pressure will increase. Conversely, if the constant pressure point is on the outlet, the average system pressure will decrease. Either way, the pressure added by the pump will be lost to friction as the water circulates in the system.

The pump, be it noted, does not change the volume of water --- just tries to make it move.

Further, if the lowest pressure is at the inlet to the pump, it may well be low enough (in a heating system, for instance) to cause the water to boil in the inlet. This leads, at best, to noise and inefficiency and, at worst, to rapid destruction of the pump.

To look at it slightly differently, one needs to look at it from the standpoint of the total energy of the water. The total energy is the sum of the energy seen as pressure, the energy seen as velocity, and the energy seen as temperature. The last we assume to be constant. Now the pump adds energy. Thus the total energy of the water leaving the pump must increase, and the way it does that is in increased pressure. However, as the water moves through the system some of that energy is lost to friction (it actually shows up as heat and a slight temperature increase in the water and the pipes -- but usually very slight). That lost energy does show up as a drop in pressure since, if the pipe sized is constant, the velocity does not change as the water moves through the system. One might add that if the pipe size changes, the velocity will change -- and the pressure will also change in the opposite direction, to keep the total energy in the water constant at that point.

Now if the flow of water is completely blocked, then what happens depends on the design of the pump. In most of the pumps heating people play with, you get no flow and the pressure on the outlet side will increase to what is called the shutoff head, while the pressure on the inlet side will decrease. There will be recirculating flow in the pump itself which is undesirable and may, in time, damage the pump. Some types of pumps (such as many hydraulic pumps) are positive displacement pumps, and they will attempt to move the fluid. Since there is no flow, the fluid will not move but compress (even water compresses a little!) while the pressure rises; if there is no relief provided, either the pump will stall or something will break.

Zetch

So the pump is adding energy to the system as kinetic energy giving the water velocity and it shows up as pressure and that pressure drops due to friction?

ch4man

how can a pump pull water out of the x-tank? where is it going to put it? the boiler and pipes are already full.

Zetch

this might sound stupid but if you aren't removing or adding water from the tank and just pumping a fixed mass and fixed volume in a circle how does simply flipping the direction of the pump change the systems average pressure?

Jamie Hall

Zetch said:

this might sound stupid but if you aren't removing or adding water from the tank and just pumping a fixed mass and fixed volume in a circle how does simply flipping the direction of the pump change the systems average pressure?

Presumably you mean changing the system so the expansion tank -- which we agree has no pressure change at constant temperature -- is near the outlet to the pump rasher near the inlet?

Consider that for the pump to move the water, the pressure at its outlet must always be greater than the pressure at its inlet. In the case with the expansion tank near the inlet, that will be the location of the lowest pressure in the system when the pump is running, since the pump adds pressure. Therefore the average pressure in the system will be greater than the pressure at the expansion tank. In the case where the expansion tank is near the outlet of the pump, again the pump will be adding pressure, so the pressure at its inlet must be lower. Thus the pressure at the outlet -- and expansion tank -- will be the location of the highest pressure in the system, and the average pressure in the system will be lower than the pressure at the expansion tank.

Zetch

I think its starting to click its just so abstract it takes some time hard to visualize

hot_rod

Here are a couple graphics that show how delta P, head added at the circulator, adds or subtracts from the static fill

HomerJSmith

The reason that it is called the Point of No Pressure Change is because the pressure is the same whether the pump is running or not running.

Zetch

Appreciate the help its just difficult to wrap your head around a pump creating a change in pressure on only one side, not like a blower or compressor or anything i'm used to thinking about where the differential is spread between the inlet and outlet

STEVEusaPA

https://youtu.be/tZRuqiQkukE?si=HaupF9B7joAnahsQ

Jamie Hall

Zetch said:

Appreciate the help its just difficult to wrap your head around a pump creating a change in pressure on only one side, not like a blower or compressor or anything i'm used to thinking about where the differential is spread between the inlet and outlet

If there were no expansion tank to stabilise the pressure at one point, the pump would indeed create a lower pressure on the intake and a higher one on the outlet -- just like the blower you are thinking of. But the expansion tank prevents that from happening.

EdTheHeaterMan

Took me some time to understand the concept. I took a one day seminar where Dan Holohan was the instructor and he was able to drive the concept home. He used the diagram that Bob hot_rod Rohr posted above. There is a video of that seminar and if you look here https://www.youtube.com/watch?v=awxeWBxC4Aw at time stamp 13:00. I would watch it from the beginning but you can start at 13:00. If you like to hear the history then keep going. If you get board with the history, then fast forward to minute 22:00 for the explanation of how the PONPC will be affected by the circulator location.