fredrosse wrote:From a previous post within this topic:
“A few simple facts;
Water at any pressure at a temperature corresponding to it's boiling point at that pressure, when released into atmospheric pressure will expand as it flashes into steam at a ratio of 1600:1- ie. on pint of water at 100psi at boiling point will flash into 1600 pints of steam at atmospheric pressure.”
Not to downplay the importance of the potential explosive energy in heated water suddenly exposed to atmospheric pressure, but the numbers quoted above are off by nearly one order of magnitude.
One volume of saturated water at 100 PSIG (337F), if suddenly released at atmospheric pressure, will turn into only 200 volumes of resulting steam & water at 212F, not 1600 volumes.
At higher boiler pressures, the expanded volume is somewhat higher, 150 PSIG yields an expansion ratio of 240.
I don't think there is even a need (other than for accuracy) to debate these numbers as it misses the point. The point is that unless there is a sudden (catastrophic) drop in pressure the heated and pressurized water inside of the boiler will happily stay in it's current liquid state. Look at it this way, blowing-off is done from the bottom, liquid portion of the boiler contents. When blowing-off do we see a sudden rise on the pressure gage? I didn't think so. As the heated, pressurized water leaves the boiler and enters an area of atmospheric pressure it flashes to steam but the liquid contents of the boiler remain liquid. There may be some additional evaporation as the pressure of the boiler decreases but that is slowed as the additional steam increases the pressure to the new equilibrium point. The addition is less than the amount of pressure removed by the blow-off, therefore there is a reduction in gage pressure. The water can only flow through the blow-off valve so fast. It's this resistance that prevents the contents of the boiler from seeing the atmospheric pressure, therefore no flashing of the boiler contents to steam.
At some point an opening to atmosphere could be so large that it would cause the contents of the boiler to see atmopheric pressure at a rate fast enough to cause the shock wave needed to destroy a boiler. As I noted, this would need to be caused by a plate failure or by a valve so large as to be considered ridiculous. The code backs this up as I can find no mention of maximum size of safeties.
Not to say that's a good idea to open valves too quickly or have oversized safety valves. The concerns are mostly due to the fact that oversized safeties fail prematurely at best, or could leave you with a dry boiler due to excessive venting at worse. One should also open valves slowly to allow the down stream pressure to equalize and so not to rip pipes apart from the pressure surge.
As I noted previously, I think the oversize safety causing boilers to explode is an old wives tale created by those who had a little portion of fact, but stretched it well beyond it's reasonable limit.
Cheers
Ken
