Michael, Thanks for posting an accurate compression constant for liquid water. I should have known better than to grab something from the internet for a quick post (1 part in 5x10^7). Nothing like being off by a few orders of magnitude! I've since found a decent site on water, if you're curious: http://www.lsbu.ac.uk/water/explan2.html
Take care and God bless. Michael Fitz wrote: > Rodney, > > I deal with these problems all the time. The sprinkler system MUST have > some air in it or an expansion tank otherwise the pressure will "jack" up to > high pressures. We see this problem every time a check valve is installed > in a domestic water system where there is a water heater downstream. Cold > water fills the system and is then heated, say from 50 to 130. The > volumetric expansion of water is very high when compared to the linear > expansion of the copper water pipes (or the steel pipes of a sprinkler > system) and the water will drain out of the PRV. If it doesn't have one, > something breaks as explained below. > > As with most real systems, it can be extremely complicated to exactly model > and calculate precisely what happens. Assume that water expands when > heated, albeit a small amount. This amount is about .000207/degree C at 68F. > For a 1% change in volume and assuming the factor is constant (it isn't), we > would need to heat the water 48 degrees C or about 87 F. > > Also assume that water is somewhat compressible. Table 5.4 of Eshbach's > Handbook of Engineering Fundamentals, 4th Edition, lists the compressibility > (K or Beta) of water at 68F as 318,000 PSI where dP=dV*K This means for a > 1% change in volume, the pressure would go up or down 3,180 PSI. > > IF we assume a completely sealed rigid container full of water, not too > unlike a sprinkler system, then we just need to equate the thermal expansion > of water to the compressibility of water. This way, the change in volume > cancels out. In other words, if we heated the water 87F, then the pressure > would rise 3,180 PSI. This rough calculation indicates that the pressure > would theoretically increase about 36 PSI per degree F. > > Now in real life, the pipes also expand because of the temperature rise (2X > the linear expansion rate) AND because the pressure rise of the water > stresses the material so it also elastically expands. If there are any > gaskets that allow the pipes to move a little in the groove, that changes > things a little. So you can see the actual pressure rise is difficult to > calculate but what happens is that the actual pressure rise in a system is a > lot less than that rigid container calculation shown above. This is how we > select expansion tanks, see > http://www.watts.com/pro/divisions/watersafety_flowcontrol/learnabout/learna > bout_thermexpansion.asp It's a good explanation why it is so very dangerous > to overfill a propane tank. The liquid will expand until the tank goes > "hydraulic" and then it burps over the PRV with a very little change in > temperature. > > General rules of thumb: Water compressibility 318,000 PSI, Glycerin = > 630,000 PSI. Glycerin can exert twice the force assuming the same > temperature change, another reason to have the right expansion tanks on an > antifreeze system. > > Air (assuming ideal) P2 = P1*V1/V2. Halve the volume, double the absolute > pressure, quite compressible. > > P2 = P1*T2/T1 Double the absolute temperature and you double the absolute > pressure. As an example, for a temperature rise of air in a rigid container > from 68F to 98F and a system pressure of 75 PSIA, the pressure would > increase to 75*(98+460)/(68+460) or 79.3 PSIA. For those that read gauges, > essentially from 60 to 64 PSI. > > Hope this clears up why trapped air pockets DON'T cause significant pressure > increases with temperature and why air free systems "dangerously" increase > in pressure with rises in temperature. > > Michael > > Michael M. Fitz, P.E., C.F.I. > MDE Inc. > 700 S. Industrial Way > Seattle, WA 98108 > www.MDE.com > 206-321-1804 > _______________________________________________ Sprinklerforum mailing list http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum For Technical Assistance, send an email to: [email protected] To Unsubscribe, send an email to:[email protected] (Put the word unsubscribe in the subject field)
