Question: circ in closed vs open system

[varadhammo]

Hi guys,

I'm trying to solve a problem with my injection setup. Before I go into all the details, I want to know this:

Suppose we have a circulator in a closed loop 20' elevation below a closed pressurized storage tank. The circulator only has to overcome the friction in the piping to move fluid in that loop, right? The ferris wheel thing. OK.

Now suppose the tank is open to atmospheric pressure. Same piping, same circ. Clearly the static pressure at the pump will be bit lower since pressure is atmospheric at the top of storage rather than say 15 psi. Other than that, is there any difference at all in the pump's operating dynamics? Does it have to overcome any more static head pressure than the closed/pressurized version? If the pump in the atmospheric version was being ramped down by a triac control like a tekmar injection controller, would it have any increased tendency to stall as it is being ramped down, as compared to the pressurized version?

Here's a drawing to make it clearer.

 

[brant2000]

No, besides the increased static pressure (as you noted), which is negligible for your purposes, these pumping systems would act exactly the same. As far as the pump is concerned, it is a closed loop.

 

[DaveBP]

What is pressurizing the storage tank to 15PSI?

 

[heaterman]

What is pressurizing the storage tank to 15PSI?

Probably a fill valve or by manually filling to that pressure?

 

[varadhammo]

Well, the air in an expansion tank bladder I guess. The pressurized tank was just hypothetical, my storage in unpressurized, GARN tank. Here's another newbie question: if you have only water in a horizontal closed loop with no bladder or trapped air, can there be any static pressure? No, right? Or yes because water isn't truly 100% incompressible? Or do the dissolved gasses compress?

 

[varadhammo]

Yeah, so say the fill valve/pressure regulator fills a hypothetical system with NO expansion tank, and shuts off. Is there any pressure in the piping?

Edit: well assume that you manually cut off the autofill, so there is no street pressure pushing on the water in the piping. Does the pressure drop to zero?

 

[heaterman]

Not understanding what you are asking here......

 

[varadhammo]

OK, sorry. Basically, if you fill a closed loop with street pressure until there is no air left in it (and no air bladder), then turn all the valves off, can there be any static pressure in that closed loop, or does there have to be air in there somewhere in order to have any static pressure?

 

[maple1]

Yeah, so say the fill valve/pressure regulator fills a hypothetical system with NO expansion tank, and shuts off. Is there any pressure in the piping?

Edit: well assume that you manually cut off the autofill, so there is no street pressure pushing on the water in the piping. Does the pressure drop to zero?

If you fill a system just to the point of fill - say just until water comes out a fill vent or open valve up top - then you will have zero at the top, and 0.4psi fer foot of drop at the bottom.

But no idea if that's what you're asking.

 

[brant2000]

OK, sorry. Basically, if you fill a closed loop with street pressure until there is no air left in it (and no air bladder), then turn all the valves off, can there be any static pressure in that closed loop, or does there have to be air in there somewhere in order to have any static pressure?

Theoretically, yes in a completely rigid system with no gases or devices that would allow for expansion, I believe there would be no way to increase static pressure. The reality is that this is only hypothetical. Even in order to measure pressure, the gauge has to allow for some expansion, as will everything else in your system.

 

[varadhammo]

@maple1: Yeah, that's what I thought. OK anyway, I've posted my actual problem on a new thread. Please take a look guys. https://www.hearth.com/talk/threads/injection-issues.138877/

 

[varadhammo]

@brant2000 : ok that answers my question. i was wondering because I've filled a loop with street pressure until there's "no" air in it and shut off the loop, and there was still pressure. I was confused as to where the pressure was coming from. Thanks.

 

[brant2000]

I admire the attention to details here, but think you're over-thinking it. If you're doing fluids homework, you may need to worry about these things; but as far as systems using 1/25 HP pumps, don't sweat the details.

 

[varadhammo]

Yeah, what can I say, I like to know the theory, and then I like the know the factors that make reality different than the theory

 

[Clarkbug]

To look at this from the beginning, the difference between your two drawings is really just if the circulator will see the extra 15 PSI at the suction side. Open systems dont see the pressure in the system, just the static head pressure from your water column. But as you have it drawn, there isnt anything that would make your pump "stall" from a pressure standpoint.

I know its just a theoretical system, but your atmospheric system system curve would be different than a closed system curve, if thats what you were asking.

As far as your second question, I think what you are asking is if the water in a pipe that is horizontal has any static pressure on it? Yes, a gauge would read zero, assuming that you have not pressurized the piping. (Im imagining you would cap one end, fill the rest with a garden hose, and then cap the other end.) That would be GAUGE pressure, there is still the pressure of the atmosphere on it.

 

[Clarkbug]

Yeah, what can I say, I like to know the theory, and then I like the know the factors that make reality different than the theory

Location of your pump and expansion system would make a much bigger difference