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 <Clipped> In summary, I guess I am simply trying to state that trapped air pockets will create a more significant pressure increase or decrease in a closed sprinkler system than water when the temperature changes. I will absolutely admit that I cannot explain mathematically why we see such drastic pressure increases in the numerous wet sprinkler systems where we have identified and resolved these issues (based strictly on Charles's Law, the pressure increases should not be so extreme), but I can state that the excessive pressure problem is completely resolved after we bleed off a significant amount of the trapped air pockets. This is not a theory, this is absolute real world action with undeniable results. Some systems relieve air for 20-40 minutes through multiple sprinkler heads (loosened enough to hear and feel the air escape from the system at the various high points). After we remove a significant amount of the trapped air, the problem is resolved. And to address the obvious question, the problem is resolved even without a 175 psi PRV (we do not always provide the 175 psi PRV, but we typically recommend them for gridded systems; some clients agree to provide them, but some decide against paying for the 175 psi PRV). Rodney K. Hamm, P.E. President/Owner Falcon Fire Protection Office (478) 953-1677 Cell (478) 396-6988 __________ Information from ESET Smart Security, version of virus signature database 3952 (20090320) __________ The message was checked by ESET Smart Security. http://www.eset.com _______________________________________________ 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)
