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pumping away PONPC

Mike MMike M Member Posts: 20
While reading  Classic Hydronics, the PONPC, as noted, I cant visualize (page 58-60). (ie) If the pressure doesnt change there is no flow as has been stated. I wrote down what I see on pressure changes in the tank if I were water where I would go... see attachment) am I missing something. gigven these pressure changes i visualize are small but necessary the Bernoulli effect on water.


dont know if you can enlarge this enough to see the numbers


  • Dan HolohanDan Holohan Moderator Posts: 11,877

    If the pipe is already filled, how can water leave the tank and enter the pipe? Can you compress water?

    Putting on my Socratic hat here.
    Site Administrator

    Hug your kids.
  • Jamie HallJamie Hall Member Posts: 6,869
    It can be a little confusing...

    but the key point -- which is easy to miss -- is that the expansion tank volume changes only with change in the temperature of the water, not with the flow -- and the change is relatively slow and small.  Thus the flow into or out of the expansion tank is, for all practical purposes, zero, and the pressure at the point where the expansion tank connects to the piping is, again for all practical purposes, constant and at the value set by the precharge in the tank.

    The whole point of pumping away from the expansion tank -- and hence from the point of no pressure change -- is to ensure that the pressure at the entry to the pump is at, or very close to, the static system pressure, and thus that the suction pressure at the pump inlet is positive -- significantly  above atmospheric.  The reason for this is to remove any possibility that cavitation will occur in the pump, which is noisy at best and which, at worst, can destroy a pump impeller remarkably quickly.  Cavitation can only occur if the local pressure in the impeller is below the boiling point of the water.

    If one were to pump towards the expansion tank, conversely, the pressure at the pump inlet would always be less than the system static pressure when the pump was running, by the amount of head loss around the loop.  This could well be low enough to cause cavitation (or even, in cases, to suck in air through bad seals!).  This we don't want.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • billbill Member Posts: 429
    edited November 2013
    Always a tough explanation

    This PONPC thing is so simple but tough to explain. I've read about Henry's law, Boyle's law and Dan's soda pop bottle in his book Pumping Away. It's all still tough to understand.

    Then I had an awakening. An expansion tank is identical to a pressure tank on a well system which is the same as a city water line.

    Now say you had a closed loop in your house with a circ on it. Forget about a boiler there. If you put the city pressure right at the discharge spot of the circ. the suction side would see a reduction of pressure. Now put the city line on the return and a increase in pressure will occur at the discharge. Now the benefit here is to snag all the air out there and bring it home to be released.

    Now could that circulator control expansion tank or pressure tank or city water pressure? The pump isn't going to pull water out of nowhere and shove it up the tank.

    Pump operation can't affect incoming pressure. With it's modest function the expansion tank packs a lot of importance.

    Always a tough explanation.
  • Mike MMike M Member Posts: 20
    pressures as I see them

    attached is a diagram of the pressures I visualize as the pump starts up...
  • Dan HolohanDan Holohan Moderator Posts: 11,877

    the pipe is already completely filled with water. The tank can't add another drop to a pipe that's already filled.
    Site Administrator

    Hug your kids.
  • Jamie HallJamie Hall Member Posts: 6,869
    Give it another shot here...

    Think of your system with the pump off, and assume, for convenience, that the pump inlet and the expansion tank are closely connected (that just makes the analysis easier).  The system is, we hope, full of water (which is essentially incompressible) -- it will be if it's operating properly.

    Now turn the pump on.  What happens?  The pressure rises at the outlet.  This pressure increase makes its way around your piping system at something very close to the speed of sound (it isn't exactly that, but it's close) -- let's say, for argument, at something like 2,000 feet per second.  Typical piping loop -- say a twentieth of a second for the pressure increase to get around to the pump inlet.  In the meantime, the pressure at the inlet has dropped, and this pressure drop is moving at the same speed in the opposite direction.  But note: this is a pressure change, not a volume change, when that pressure drop gets to the expansion tank, it can go no further -- since the tank is held at a constant pressure -- and at this point the pressure rise from the pump outlet compensates for the drop at the inlet.  No change.  No water flows either into or out of the tank  So our pressure rise from the pump does only one thing: it causes the water in the pipes to start moving, and because it is moving, there are friction losses -- and the end result is that in a very very short time the moving water friction losses come to exactly balance the pressure rise caused by the pump.  In fact, with a pump, the pump starts sufficiently slowly that you never see a pressure spike at all, unless you have ultra sensitive high speed pressure recorders.

    This is much more confusing to describe as above than it is to simply remind folks that the piping system is full of water at all times, and that therefore no water can move either into or out of the tank.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Mike MMike M Member Posts: 20

    as the way you have explained it, the expansion tank serves no purpose.  You could easily use a gate valve here to shut off the tank. Pressure is applied by force of the water.  This force requires directional movement, in this case, from the tank water to the return line water,  higher to lower as Dan states frequently. So tell me if I were to measure the exact weight of water in grains, of a piped system @100F, with the system off, (to include the expansion connected), and start the system up, and the expansion tank pressure applied to the system, then a ball valve between the expansion tank and the return line shut off, then the circulator shut off.  Would the system have an increase in water volume of even 1 grain of water from the expantion tank? If not, how can pressure be applied to the return water without movement? We know pressure MOVES higher to lower.  Now either the the pipes expand out, or the water moves to the lower pressurized area, the return line water cavity, which ever has the least resistance. How ever small the amount is....if the pressures were equal, why wouldn't the 12# pressure in the tank apply its pressure temporarily to the return line with its volume of 12# water verses 6# water displacement till the circulator shut off and then when the circulator shuts off, the water volume in excesss of 12# would re-enter the expansion tank to balance the system when the circulator shuts off.  The increase of pressure on the supply side has to come from somewhere....namely, the return side, and transfers it to the supply side, this transfer of water pressure unbalances the system pressure, and causes movement higher to lower. If the tank is higher it will fill this drop in pressure, in order to do work it requires movement, however small.

  • Dan HolohanDan Holohan Moderator Posts: 11,877

    pressure isn't volume and that may be where you're getting stuck. We call it a "circulator" because it's not a positive-displacement machine. It's a pressure-differential machine.

    Forget pressure for a moment and keep going back to this: If the circulator were able to move water from the pipe into the tank, that would leave behind a hole, void of any matter, because there's only a finite amount of water in the system. You can't move it from here to there because nature forbids that in a closed system.

    In a similar way, you can't move water out of the tank and put it into the pipe because the pipe is already filled and water isn't compressible.

    Meaning that the circulator can neither add nor remove water from the tank, and since it can't do either, it can't compress (or decompress) the air in the tank. Boyle's Law applies here so the tank becomes the point of reference for the differential-pressure machine we call a circulator.

    In other words, the circulator isn't in charge; the tank is.

    Back to my first point: Pressure isn't volume. A circulator can move water by creating a pressure difference that's either higher or lower than the static fill pressure of the system.

    I know it's tough but try to make it visual. Keep thinking about that finite amount of water.
    Site Administrator

    Hug your kids.
  • Mike MMike M Member Posts: 20
    don't give up!!

    now you state that with the pump on the return you get only about a 1 # increase in pressure on the outlet side of the pump to the expansion tank and the 5# balance is on the return side so that the water feed trys to feed the pressure drop but in your own words, it can't, because water is still in the pipes. so how would this change anything, you said it cant add water to a system that is full.
  • hot rodhot rod Member Posts: 7,208
    a drawing

    From John Siegenthaler's 3rd edition of Modern Hydronic Heating Possibly based on some old B&G schematics, John added the color which helps sort out the concept.

    In these cases the circ creates a 8 psi, ∆P, or pressure difference. When pumping away from the PONPC that ∆P is seen added to the fill pressure.

    Fill pressure "static" of 10psi, circ adds 8, for a 18psi gauge reading at the discharge flange of the circ.

    The system piping and fittings, etc "scrub away" or use up the added head energy. But the PONPC never drops below the 10 fill pressure.

    When you pump at the PONPC that ∆P is seen as the 8psi subtracted from the 10psi fill or 2 psi at the suction side of the circ.

    Consider if this were a high head circ able to create a 10- 15 psi or more difference.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • ZmanZman Member Posts: 3,558

    Hot Rod,

    I was trying to remember where I had seen that drawing. That was definitely the "ah ha" moment for me.

    It also took a while for me to see pressure as a unit of force rather than a measurement of compression(change in volume).

    To the OP, I hope this helps. I think this is a hard one to get your head around.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Mark EathertonMark Eatherton Member Posts: 5,592
    OK, let me give it a shot....

    Mike, as Dan said, the "pump" or "Circulator" (synonamous terms except that a pump has the ability to actually overcome atmospheric pressures, but forget that for now) is just a pressure differential machine. It doesn't care whether it creates POSITIVE pressure, or negative pressure, all it wants to do between its flanges is create a pressure differential. Got it? Just a pressure differential machine. Nothing more, nothing less.

    Now, wether it creates POSITIVE pressure across its flanges, or negative pressure across its flanges, or splits its differential half and half (half POSITIVE, half negative) all has to do with the pumps location in relation to the expansion tank connection to the system piping, which we call the point of no pressure change, or PONPC for short.

    Let's say, as in your case the "pressure differential machine" has a maximum capacity of 6 PSI. Lets also say that they system is filled to 12 PSI. If the pump is pumping away from the PONPC, then it will ADD its pressure differential to the static fill pressure of the system. So, if you had a gage on the outlet of the pump, with the pump turned on, working against a major resistance of the piping circuit, it would read 18 PSI (12 + 6 = 18).

    Under this same restrictive piping consideration, if the pump were pumping TOWARDS the PONPC, it would SUBTRACT its differential potential away from the static fill, so a gage on the inlet to the pump would show 6 PSI (12 - 6 = 6).

    Now how much actual flow you will see in the system is dependent upon how much restriction there is. If it is big bore pipes (1-1/2" or greater in a residential app) then the pressure differential will be low and the pressure differential machine will move a LOT of Gallons per minute (GPM). If it is real restrictive, (1/2" pipe) then the pressure differential will be MORE, but the flow will be less. Look at the manufacturers performance curves and you will see this.

    The location of the water make up valve is critical. If it is placed on the pumps inlet, AND the pump is pump towards the PONPC, AND the system has either a high pressure drop or a high head pump, every time the pump turns on, the make up valve "sees" a low pressure condition being created by the pump, and pressure reducing valve starts sending water into the system. I can see your next question coming from a mile away... "Where does the water GO if the system is already full?" It goes into the expansion tank where there is a compressible gas. And it only does this until the system fill pressure reaches the pressure relief valves threshold, which is typically 30 PSI in a residential setting. Then suddenly, WHOOSH, the relief valve cuts loose with a WHOLE bunch of water that is coming out of the expansion tank until the pressure drops low enough for the relief valve to (hopefully) reset its self. And then this vicious cycle starts all over again... It has been the death of many systems due to fresh water induction...

    Under ideal conditions, the pump/circulator/pressure differential machine SHOULD pump away from the PONPC, thereby allowing it to produce ONLY positive pressure. Then, regardless of WHERE the make up valve is located, the pump can not create a low enough pressure condition that would cause the make up to send water into the system.

    Now, lets talk about what happens with air in the system. Have you ever left a glass of water next to your bed, and when you wake up the next morning there are a million miniature bubbles on the inside walls of the glass? That is oxygen. You can make it come out of suspension even faster under negative pressure. In fact, you can make 70 degree F water boil if you apply enough negative pressure to it. Now, if you are water, and you are near the top of a 2 or 3 story building, and the pump is pumping towards the PONPC, when it turns on, you "see" negative pressure, and all that dissolved oxygen you've been carrying around on your body comes out of suspension and starts gathering together to make a large bubble. This creates a condition called air binding. Pumps can't move water against a solid blockage of air, and consequently no water flows.

    Again, under this same scenario, with the pump pumping away from the PONPC, when it turns on, you see only positive pressure, which causes the oxygen bubbles to decrease in size, not accumulate and turn into a large bubble, stay in suspension and be pushed around with the water to the air separator or air eliminator where it can't cause any problems.

    The static fill pressure (non pumped pressure) is the pressure required to raise the water to the top of the system, and overcome atmospheric air pressure. So, in a building that is say 3 stories tall, you have 24 feet of vertical elevation. Each vertical foot requires .434 PSI in order to overcome atmospheric pressure. Lets round it up to 1/2 PSI just for ease of math. So, 24 vertical feet times .5 PSI/foot means you need 12 PSI just to get the water to the top of the system. To this, you want to add 5 PSI to avoid air coming out of suspension and some other nasty scenarios that can and do happen if the pressure is too low on the top of the system. That static pressure is different than the dynamic pressure created by the pressure differential machine, but they have to work together.

    If you are using a high head pump, say one that can generate 40' of head, AND you have a high pressure drop system AND you have a pump pumping towards the PONPC, AND you have low fill pressure AND you have automatic air vents in your system, it is entirely possible to create enough negative pressure on the top of the system that the pump will actually suck air into the device (auto vent) that was meant to let air out. You will then have a very noisy problematic system.

    Will a system work with a pump on the return pumping towards the PONPC? Heck yeah! That has been proven for over 100 years. Many older pumps that had a rag seal for the shaft seal were intentionally set up on the return to give the pump the lowest possible fluid temperature, thereby increasing the seals life expectancy. Obviously it will work. But most of those types of circulators are a low head variety pump, so they don't create too many problems.

    Will it work better if the pump is pumping away from the PONPC? You bethcha!! Proven by 1000's contractors who have taken older systems that were regularly requiring air to be bled out of their distribution systems, moved the pump so it pumps away from the PONPC,and haven't needed to be touched since they were set up to pump away from the PONPC.

    Don't let the physics clog up your mind. As I use to tell my students, just make sure that ALL pumps pump away from the PONPC, and your life and the lives of those people living with your systems will be MUCH better, quieter and less maintenance (read bleeding air on a regular basis) intensive.

    Hopefully that clears it up.

    Now, if you have a pump that is showing 6 PSI differential, you can convert that PSI into feet of head, and using the manufacturers performance curves, determine how many GPM it is moving under THAT condition. Just remember, THAT condition can be constantly changing. Take the pressure differential in PSI, and multiply it times 2.307 and that will equal feet of head. Using the manufacturers performance curve, go along the left hand side of the graph until you come to the feet of head you calculated, and then move to the right until you intersect the pumps performance curve, then drop straight down and rad the GPM along the bottom of the chart.

    Now, you probably know a lot more about pumps and circulators than you really wanted to, but so do a LOT of other people reading this thread, and that's what happens when you hang out here at The Wall.

    Just remember to ALWAYS Pump Away, have a nice day and buy as many Holohan books as you can :-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • thanks to Dan

    Learned this way back when and which is why I seen many good boilers are piped wrong due to the ' lobster trap' theory and the shipping engineering dept.
  • billbill Member Posts: 429
    Are you sure Mark?

    I wonder what Stephen Hawking has to say about all this:)?
  • Mark EathertonMark Eatherton Member Posts: 5,592
    Toughest concept to explain and understand...

    Easiest one to apply :-)

    Mr Hawkings might have said it a LITTLE differently.
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • EmpireEmpire Member Posts: 2,343
    HONESTLY Where?

    Where else can you go in this world now a day's and get sooooooooo many people trying to convey a thought, or theory if you will and have it answered by people that at one point scratched their but's on that same question...?  HeatingHelp.Com that's where!  Thank you Dan for all you do and to give us a forum to help and learn.  

    Now.............can anyone help with my nephew inserting a disc shaped cotton candy CD in my Blue Ray?????  lol

    Peace gentlemen;

    Mike T. 
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