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water thermal expansion & pressure

5 gal water in a sealed container [boiler]. temp of boiler water rises from 50*f to 200*f.......i assume the 50* pressure would be 0? what pressure @ 200*f?

what pressure @212*f? thanx


  • Paul FredricksPaul Fredricks Member Posts: 1,542

    I don't know. But  I get this feeling that it depends on the size and shape of the container. This may be bad science, but: A one cubic foot container, that is a cube, holes just over 7 gallons of water. The outside surface area is 864 square inches. That same 7+ gallons fits in a container that is 24" x 12" x 6" (I think) which is 1872 square inches. I don't know if any of that matters, but we are talking PSI, pounds per square inch. Does the pressure change because the force is distributed over a larger area?
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    i assume the 50* pressure would be 0? what pressure @ 200*f?

    You must make a lot of assumptions to answer that. If the boiler is infinitely rigid and has no pressure relief valve, no expansion tank, and the supply and return connections are capped off, it is easily to answer that. But that is so unrealistic that there is no point in answering the question.

    If there is an expansion tank in working order, the pressure increase will be less. If the boiler is not infinitely rigid, it will expand a little until it breaks, or something (such as the pressure relief valve) leaks.
  • Jamie HallJamie Hall Member Posts: 6,708
    Thermal expansion

    Shape of the container doesn't matter.  Nor, curiously, does the initial volume (although one might think it would -- it doesn't).  If we assume an infinitely rigid container, then you have two coefficients to consider (neither of which I seem to be able to find at the moment... sorry!): first, the volumetric thermal expansion coefficient, which gives you a percent change in volume for a given change in temperature.  The second is the volumetric elasticity, which gives you the percent change in volume for a given change in pressure (absolute pressure now, rather than gauge).  Again, if we are assuming that infinitely rigid container, the percent change in volume must be zero -- and hence the percent change in volume from the temperature change must be exactly the same as the percent change in volume from the pressure change, but in the opposite direction.

    Since I don't have the coefficients for water available, as I say I can't give you the relationship.  What I can tell you, though, from experience, is that if you raise the temperature of the water in a boiler with no free space (air or steam -- a gas, in other words) from 50 to 200, the boiler, or some fitting associated with it, is going to break, as the required pressure to make the volume change zero is going to be in the tens of thousands of psi.  Now if there is something weaker associated -- such as a copper pipe -- you may get lucky and the pipe will simply yield; if it has no weak spots it won't burst.

    Since you are still below the boiling point of water at atmospheric pressure, you won't get an explosion.  If you continue above the boiling point, though, you will launch the boiler, or parts of it, a considerable distance and you don't want to be there...

    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
  • jpjp Member Posts: 1,935
    volume change vs pressure

    I don't know the answer either, but I do know at 55,000psi you can compress water by 10%.   another words, compress 10 gallons of water down to 9 gallons of water.
  • thanx

    just realizing why ive read posts that say not to turn the boiler off & on just to make hot water........pressure variations?

    i built my system years ago & do so on a regular basis with no problems cast American Standard boiler ithink & it never developed much pressure but now the expansion tank [galvanized non bladder] has a leak & dripsy.

    So the q becomes= Would oversizing the expansion tank of any system help it last longer if constantly turned on & off so to use only as a water heater?
  • Jamie HallJamie Hall Member Posts: 6,708

    now if you have an expansion tank, then the pressure change is very small, relatively speaking, assuming that the expansion tank is set up properly (if it's leaking, you need a new one, correctly sized for your system).

    The main reason for not turning the main boiler off and on (summertime) has nothing to do with pressure cycling, but with overall efficiency.  Firing up a great big boiler for a cup of tea is a somewhat doubtful proposition, at least in my view...

    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
  • posts i'd read

    specifically mentioned boilers leaking due to constant on off & i duuno what type of boiler [steel or cast] though it seems a cold start boiler would do so on a regular basis?
  • Glen AspenGlen Aspen Member Posts: 53
    use this formula

    P1 x T2 = T1 x P2, state the pressures and temperature in terms absolute. I get 19 psia. Charles, Boyle & Guy Lussac all had a lot to say about temp, pressure & volume.
  • Jamie HallJamie Hall Member Posts: 6,708
    Sorry me lad...

    Boyle's Law is perfectly valid -- for a gas.  Not for a liquid or a solid.  So it doesn't apply here...

    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
  • Mark EathertonMark Eatherton Member Posts: 5,584
    Show me a pipe or vessel that can withstand 55,000 PSI, and I'll...

    show you one HEAVY pipe/vessel.

    Must be where the term that water is virtually nearly incompressible came from eh...

    For MOST purposes, it is not compressible to a degree that we normal mortal hydronic human beings need worry about it...

    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.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Water is incompressable...

    I had a friend who had a gold seal high pressure steam boiler license (with an endorsement for steam locomotives as well). His day job was running such boilers at a military installation. He said water was compressable, and this did not agree with what I learned in high school physics. Yet my friend was not pulling my leg and really believed it.

    As evidence, he pointed out that if he had a steam boiler running at high pressure, and released the pressure, the water level in the boiler came up even though the feedwater was disabled. He said that showed that the water expanded in the boiler and that caused the water level to increase.

    I had no answer to him at the time. Since I still believe water to be nearly incompressable (we were talking only 100 to about 250 psi), I needed a better explanation. My current view to account for the facts as he presented them is that the high pressure expanded the volume of the boiler, slightly lowering the water level, and when he reduced the pressure, the boiler volume decreased closer to its unpressurized volume, raising the level.

    Trouble with this explanation is that the boilers in question are fire tube boilers and the shells of those boilers are several inches thick steel alloy; likewise the boiler tubes are special steel alloy specifically designed for use as boiler firetubes. Not the stuff you get at your local plumbing supply place. So even with a couple of hundred psi pressure, could the boiler really expand enough to decrease the water level enough to see?

    Am I right? If I am wrong, what other explanation could there be for the facts as presented?
  • VictoriaEnergyVictoriaEnergy Member Posts: 126
    Awnser for the expansion

    The expansion factor for water from cold to hot is roughly 4%

    You have to factor in the volume of all the water in the system, not just the boiler when figuring expansion tank sizing.  4% of the total volume of water would be the minimum "acceptance volume" spec you're looking for in a tank.
    Home Owners Please Note:

    You are receiving advice from some very skilled pros completely free of charge. One of the reasons I participate is to sharpen my own troubleshooting skills. So; did we get it right? I would be grateful if you extend this courtesy back by posting the final outcome of the issue you are inquiring about. Thanks
  • jpjp Member Posts: 1,935
    edited April 2011
    life outside the boiler box.....

    ME, the wall thickness of the pipes  were a couple of times the ID.  somewhere i have a piece of it laying around.

    life does exist outside your pex world, sometimes it comes in handy knowing all the facts, you're not so likely to go off course somewhere.   graph comes from btw.

    JDB, high school physics is a great start, but a lot of times you first learn simple approximations then later learn why those approximations aren't totally valid.

    but you seems to agree water is compressible away.......  it either is or isn't.

    if you change the density of a sample of water, its volume will change.  we all know water expands(ice) why is it so hard to believe it doesn't contract?
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Oh! I believe it expands and contracts.

    Water is pretty amazing, actually. It certainly expands with temperature, and contracts with temperature. But around 4C it expands again as it is about to freeze. Hence ice floats on water.

    Looking at your chart, I see that if I increase the pressure 200x, its density increases 1%. So perhaps it is more compressable than steel (I do not happen to know), but that is not very compressable.

    So getting back to my friend with the gold seal steam boiler license, he says it compresses enough to see the water level change when lowering the pressure on a boiler. If he drops the pressure from 250 psi to 25 psi (I do not know what pressures he used), I infer from your chart that the volume would expand perhaps about 0.1%. Would that be visible on the water gauge on the front of the boiler? I doubt it. Yet he sees it.

    It seems to me there is more to this than I understand, or that you have shown.
  • edited April 2011

    having read other sites i now accept 5% as an acceptable figure....vvv though I piped the expansion tank WAY OVERSIZED from the bottom of the water jacket......there is is a relief valve above the water jacket/boiler
  • jpjp Member Posts: 1,935
    edited April 2011
    need more info

    first off, i do agree water is not that compressible, but it can compress and that's good information to know, used or not.

    as far as the steamer, I would ask:

    what was the temperature change?  

    what was the volume change?

    what was the change in volume ? percent wise?

    was the change transient or stable? 

    seems a lots going to happen going from 250psi to 25 psi.


    if you go here you can see that volume expansion is also dependent  on temperature, as temp increase water will expand even more and more, its not linear, or proportional.  look down on at the chart for water.
  • Brad WhiteBrad White Member Posts: 2,393
    edited April 2011
    4%, 5%

    Close enough. I usually use 4% when giving preliminary estimates, that is the volume increase from 50 degrees F. to 200 degrees F.  The pressure/temperature curve is a rapidly rising one, for sure.

    And yes water *is* compressible, all substances are to a degree. But as ME said, it is to a degree none of us should really worry about it. Anyone that makes too much of a deal about that should take up knitting, basically needs a hobby.

    As for an over-sized expansion tank, I would say, "there is no such thing", meaning, there is no harm in having one too large, save the initial expense. You will still have pressure variations with temperature changes, but the larger the tank, the less extreme they will be.  I normally size my expansion tanks for a 20% allowed increase in pressure and not more than 80% of the relief valve setting in any case. Thus a system with a 30 psig relief valve would not want to be over 24 psig in my book, which means the initial cold fill pressure should not be more than 20 psig. Yes, my tank volumes tend to be a bit large. :)
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
  • jpjp Member Posts: 1,935
    good point brad

    "... Anyone that makes too much of a deal about that should take up knitting,..."

    you're right, technical trivia is only going to confuse bubba the plummer/heeting pro. little extra knowledge is for the birds.....

    lets keep it simple, R10 works anytime and all the time for slabs, short cycling a fact of life no way around it, 2 boilers work better than 1.........  gotta keep things at old school for sure....

  • LOL John Paul is exhuminating 4 sainthood

    & imagine the energy used. jp!
  • Tom BlackwellTom Blackwell Member Posts: 126
    Drum level

    In large boilers the drum level will rise on a drop in pressure-not because the water has expanded, but because steam bubbles in the tubes expand and displace the water. This is why process boilers are so prone to trips on low water. When there is a large draw of steam, the pressure will fall and the drum level will rise-then the combustion controls catch up and the pressure rises again and away the drum level goes. Water in a closed non-expandible vessel will exert infinite pressure when it expands due to temperature rise. Of course there is no such thing as an infinitely rigid pressure vessel. Synthetic quartz is made in just such vessels and the expansion of heated water drives the pressure up above 80,000 psig. In most cases in hydronics, there is enough air trapped in nooks and crannies to limit hydraulic pressure to more reasonable levels.
  • Mark EathertonMark Eatherton Member Posts: 5,584
    My suggestion...

    jp, now that you have offended myself and 95% of the other people who frequent this web site, maybe it would serve you and us best if you found someplace else to profess your worldly knowledge of everything, physical or otherwise.

    I've gotten along just fine for my 36 year tenure as a licensed master professional, without your intervention...

    I rarely respond to your posts because you are SO condescending. I responded to this post with tongue firmly planted in cheek, and you blew it all out of proportion, referring to myself and others as "plummer/heeting pro's" ...

    I don't know what your problem is, nor do I care. I am here to share my wealth of knowledge, accumulated over my 36 year career, and don't need any help/arguments from the likes of yourself, nit picking about whether water is compressible. or not, and to what degree it affects the daily operation of our systems...

    Please, find some place else to hang out and criticize/argue with the participants. I reserve the right to NOT read any of your further posts, and will exercise my rights to do so, beginning now.


    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.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Thanks, Tom B.

    I think your explanation is on the right track. I suppose you are thinking of water-tube boilers, and I was thinking about fire-tube boilers (because of my fondness for steam railroad locomotives). I have no idea what kind of boilers they had at Ft. Dix, and my friend has left this mortal coil, so I cannot ask him. They were originally coal fired.
  • GordanGordan Member Posts: 891
    Perhaps you could educate us.

    Under what circumstances would compressibility of water matter (that is, not be of a magnitude vastly smaller than other phenomena) in the context of hydronic heating?
  • Brad WhiteBrad White Member Posts: 2,393

    In hydraulics (where oil is used, not water anyway), the application of pressure to a fluid over one area to induce a greater force over a smaller area, that comes to mind. My dad was a hydraulic engineer, so I should probably ask him first. Any fluid is compressible to even small degrees we agree, so there is a bit of "slippage" between force applied and net force output. I imagine that depends entirely on the fluid used.

    Beyond that, the only applications I can think of  are in marine engineering and submarine design, whereby compressed air is used to blow the tanks (imagine the use of a highly compressible fluid to move an essentially incompressible one, just occurred to me....).


    In marine engineering, what comes to mind is not so much compressibility but volume and buoyancy at temperature and density. All seagoing ships have what are called Plimsoll marks which adjust the load waterline basis for differences in salinity and density due to temperature. For example, tropical sea water tends to be less buoyant/less dense than colder arctic waters. Hence, a ship in tropical seas would tend to ride lower in the water. There are marks for summer, winter, fresh water, salt water to name a few.

    So density yes, pressure? No idea.
    "If you do not know the answer, say, "I do not know the answer", and you will be right!"

    -Ernie White, my Dad
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